GOLF BALL

Abstract

A golf ball includes a core, one or more mid layers, a cover, and dimples. A central hardness Ho and a surface hardness Hs of the core, a hardness Hm(min) of a layer having a lowest hardness among the mid layers, and a hardness Hc of the cover satisfy the following mathematical formulas (i) to (iv). Hs−Ho>15  (i) Hc−Hm(min)>20  (ii) −10<Hm(min)−Ho<15  (iii) 5<Hc−Hs<20  (iv)

Description

This application claims priority on Patent Application No. 2016-242294 filed in JAPAN on Dec. 14, 2016. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a core, a mid layer, a cover, and dimples.

Description of the Related Art

The face of a golf club has a loft angle. When a golf ball is hit with the golf club, backspin due to the loft angle occurs in the golf ball. The golf ball flies with the backspin.

The greatest interest to golf players concerning golf balls is flight distance. Golf players particularly place importance on flight distances upon shots with drivers. There have been various proposals for improvement of flight performance. JP2010-188199 discloses a golf ball including a core having a high hardness at the surface thereof and a low hardness at the central point thereof. When the golf ball is hit with a driver, the rate of backspin is low.

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.

There have been various proposals for dimples. JPH4-109968 discloses a golf ball in which the dimple pattern of each hemisphere can be divided into six units. JP2004-243124 (US2004/0157682) discloses a golf ball in which the dimple pattern near each pole can be divided into four units and the dimple pattern near the equator can be divided into five units. JP2011-10667 (US2010/0326175) discloses a golf ball in which a parameter dependent on the shapes of dimples falls within a predetermined range.

Golf players desire golf balls having excellent flight performance upon a shot with a driver. There is room for improvement of flight performance.

In golf, a golf ball is hit with a wood type club, an iron type club, a hybrid type club (utility), a putter, or the like. The feel at impact upon hitting is an interest to golf players. Generally, golf players desire golf balls having soft feel at impact.

Upon hitting with a wood type club, an iron type club, or a hybrid type club, the frequency of a missed shot is high. Therefore, golf players are insensitive to the feel at impact when hitting golf balls with these clubs.

Meanwhile, upon putting, a golf ball is often hit at the sweet spot of a putter. Golf players are sensitive to the feel at impact upon putting. Golf players desire golf balls that provides soft feel at impact upon putting.

An object of the present invention is to provide a golf ball having excellent flight performance upon a shot with a driver and excellent feel at impact upon putting.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core, one or more mid layers positioned outside the core, and a cover positioned outside the mid layers. A Shore C hardness Ho at a central point of the core, a Shore C hardness Hs at a surface of the core, a Shore C hardness Hm(min) of a layer having a lowest hardness among the mid layers, and a Shore C hardness Hc of the cover satisfy the following mathematical formulas (i) to (iv).

Hs−Ho>15  (i)

Hc−Hm(min)>20  (ii)

−10<Hm(min)−Ho<15  (iii)

5<Hc−Hs<20  (iv)

The hardness Hc of the cover is higher than a Shore C hardness Hm(max) of a layer having a highest hardness among the mid layers. The golf ball further includes a plurality of dimples on a surface thereof. A minimum value of 15 peak values obtained by executing steps (a) to (h) for each of 15 axes Ax is not less than 95 mm, when spherical polar coordinates of a point that is located on a surface of a phantom sphere of the golf ball and has a latitude of 0 (degrees) and a longitude of ϕ (degrees) are represented by (θ, ϕ) the 15 axes Ax being

• • (1) a first axis Ax1 passing through a point Pn1 coordinates of which are (75, 270) and a point Ps1 coordinates of which are (−75, 90),
  • (2) a second axis Ax2 passing through a point Pn2 coordinates of which are (60, 270) and a point Ps2 coordinates of which are (−60, 90)
  • (3) a third axis Ax3 passing through a point Pn3 coordinates of which are (45, 270) and a point Ps3 coordinates of which are (−45, 90),
  • (4) a fourth axis Ax4 passing through a point Pn4 coordinates of which are (30, 270) and a point Ps4 coordinates of which are (−30, 90),
  • (5) a fifth axis Ax5 passing through a point Pn5 coordinates of which are (15, 270) and a point Ps5 coordinates of which are (−15, 90),
  • (6) a sixth axis Ax6 passing through a point Pn6 coordinates of which are (75, 0) and a point Ps6 coordinates of which are (−75, 180),
  • (7) a seventh axis Ax7 passing through a point Pn7 coordinates of which are (60, 0) and a point Ps7 coordinates of which are (−60, 180),
  • (8) an eighth axis Ax8 passing through a point Pn8 coordinates of which are (45, 0) and a point Ps8 coordinates of which are (−45, 180),
  • (9) a ninth axis Ax9 passing through a point Pn9 coordinates of which are (30, 0) and a point Ps9 coordinates of which are (−30, 180),
  • (10) a tenth axis Ax10 passing through a point Pn10 coordinates of which are (15, 0) and a point Ps10 coordinates of which are (−15, 180),
  • (11) an eleventh axis Ax11 passing through a point Pn11 coordinates of which are (75, 90) and a point Ps11 coordinates of which are (−75, 270),
  • (12) a twelfth axis Ax12 passing through a point Pn12 coordinates of which are (60, 90) and a point Ps12 coordinates of which are (−60, 270),
  • (13) a thirteenth axis Ax13 passing through a point Pn13 coordinates of which are (45, 90) and a point Ps13 coordinates of which are (−45, 270),
  • (14) a fourteenth axis Ax14 passing through a point Pn14 coordinates of which are (30, 90) and a point Ps14 coordinates of which are (−30, 270), and
  • (15) a fifteenth axis Ax15 passing through a point Pn15 coordinates of which are (15, 90) and a point Ps15 coordinates of which are (−15, 270), the steps (a) to (h) being the steps of
  • (a) assuming a great circle that is present on the surface of the phantom sphere and is orthogonal to the axis Ax,
  • (b) assuming two small circles that are present on the surface of the phantom sphere, that are orthogonal to the axis Ax, and of which absolute values of central angles with the great circle are each 30°,
  • (c) defining a region, of the surface of the golf ball, which is obtained by dividing the surface of the golf ball at these small circles and which is sandwiched between these small circles,
  • (d) determining 30240 points, on the region, arranged at intervals of a central angle of 3° in a direction of the axis Ax and at intervals of a central angle of 0.25° in a direction of rotation about the axis Ax,
  • (e) calculating a length L1 of a perpendicular line that extends from each point to the axis Ax,
  • (f) calculating a total length L2 by summing 21 lengths L1 calculated on the basis of 21 perpendicular lines arranged in the direction of the axis Ax,
  • (g) obtaining a transformed data constellation by performing Fourier transformation on a data constellation of 1440 total lengths L2 calculated along the direction of rotation about the axis Ax, and
  • (h) calculating a peak value and an order of a maximum peak of the transformed data constellation. A minimum value of 15 orders obtained by executing the steps (a) to (h) is not less than 27. A maximum value of the 15 orders obtained by executing the steps (a) to (h) is not greater than 37. An average of the 15 orders obtained by executing the steps (a) to (h) is not less than 30 and not greater than 34.

The golf ball according to the present invention has excellent resilience performance when being hit with a driver. When the golf ball is hit with a driver, the spin rate is low. Furthermore, the dimple pattern of the golf ball has an excellent aerodynamic characteristic. The golf ball has excellent flight performance when being hit with a driver.

When the golf ball is hit with a putter, the shock is small. When the golf ball is hit with a putter, the feel at impact is soft.

The golf ball has both excellent flight performance when being hit with a driver and excellent feel at impact when being hit with a putter.

Preferably, a total thickness of the cover and the mid layer is not greater than 2.8 mm.

Preferably, an average of the 15 peak values obtained by executing the steps (a) to (h) is not less than 200 mm.

Preferably, a total volume of the dimples is not less than 450 mm³ and not greater than 750 mm³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according to an embodiment of the present invention;

FIG. 2 is an enlarged front view of the golf ball in FIG. 1;

FIG. 3 is a plan view of the golf ball in FIG. 2;

FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;

FIG. 5 is a schematic diagram for explaining an evaluation method for the golf ball in FIG. 2;

FIG. 6 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 7 is a schematic cross-sectional view for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 8 is a schematic cross-sectional view for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 9 is a graph showing an evaluation result of the golf ball in FIG. 2;

FIG. 10 is a graph showing another evaluation result of the golf ball in FIG. 2;

FIG. 11 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 12 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 13 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 14 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 15 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 16 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 17 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 18 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 19 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 20 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 21 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 22 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 23 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 24 is a schematic diagram for explaining the evaluation method for the golf ball in FIG. 2;

FIG. 25 is a front view of a golf ball according to Example 2 of the present invention;

FIG. 26 is a plan view of the golf ball in FIG. 25;

FIG. 27 is a front view of a golf ball according to Example 3 of the present invention;

FIG. 28 is a plan view of the golf ball in FIG. 27;

FIG. 29 is a front view of a golf ball according to Comparative Example 1;

FIG. 30 is a plan view of the golf ball in FIG. 29;

FIG. 31 is a front view of a golf ball according to Comparative Example 2;

FIG. 32 is a plan view of the golf ball in FIG. 31;

FIG. 33 is a front view of a golf ball according to Comparative Example 3;

FIG. 34 is a plan view of the golf ball in FIG. 33;

FIG. 35 is a front view of a golf ball according to Comparative Example 4; and

FIG. 36 is a plan view of the golf ball in FIG. 35.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The golf ball 2 has a large number of dimples 10 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 10 is a land 12. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the drawing. The golf ball 2 may include another layer between the core 4 and the mid layer 6. The golf ball 2 may include another layer between the mid layer 6 and the cover 8.

The golf ball 2 preferably has a diameter of not less than 40 mm and not greater than 45 mm. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm.

The golf ball 2 preferably has a weight of not less than 40 g and not greater than 50 g. In light of attainment of great inertia, the weight is more preferably not less than 44 g and particularly preferably not less than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably not greater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferable. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably not less than 50% by weight and particularly preferably not less than 80% by weight. A polybutadiene in which the proportion of cis-1,4 bonds is not less than 80% is particularly preferable.

The rubber composition of the core 4 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents in light of resilience performance are monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of flight performance upon a shot with a driver, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

In light of flight performance upon a shot with a driver, the amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably not less than 10 parts by weight and particularly preferably not less than 15 parts by weight. In light of soft feel at impact upon putting, the amount is preferably not greater than 50 parts by weight and particularly preferably not greater than 45 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

In light of flight performance upon a shot with a driver, the amount of the organic peroxide per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight, more preferably not less than 0.3 parts by weight, and particularly preferably not less than 0.5 parts by weight. In light of soft feel at impact upon putting, the amount is preferably not greater than 3.0 parts by weight, more preferably not greater than 2.8 parts by weight, and particularly preferably not greater than 2.5 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic sulfur compound. Organic sulfur compounds include naphthalenethiol compounds, benzenethiol compounds, and disulfide compounds.

Examples of naphthalenethiol compounds include 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol compounds include benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide compounds include diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, bis(2,3,5,6-tetrachlorophenyl)disulfide, bis(2,3,4,5,6-pentachlorophenyl)disulfide, and bis(2,3,4,5,6-pentabromophenyl)disulfide.

In light of flight performance upon a shot with a driver, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight and particularly preferably not less than 0.2 parts by weight. In light of soft feel at impact upon putting, the amount is preferably not greater than 1.5 parts by weight, more preferably not greater than 1.0 parts by weight, and particularly preferably not greater than 0.8 parts by weight. Two or more organic sulfur compounds may be used in combination. A naphthalenethiol compound and a disulfide compound are preferably used in combination.

Preferably, the rubber composition of the core 4 includes a carboxylic acid or a carboxylate. The core 4 including a carboxylic acid or a carboxylate has a low hardness around the central point thereof. The core 4 has an outer-hard/inner-soft structure. When the golf ball 2 including the core 4 is hit with a driver, the spin rate is low. With the golf ball 2 having a low spin rate, a large flight distance is obtained. Examples of preferable carboxylic acids include benzoic acid. Examples of preferable carboxylates include zinc octoate and zinc stearate. The rubber composition particularly preferably includes benzoic acid. The amount of the carboxylic acid and the carboxylate per 100 parts by weight of the base rubber is preferably not less than 1 parts by weight and not greater than 20 parts by weight.

The rubber composition of the core 4 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is accomplished. The rubber composition may include various additives, such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like, in an adequate amount. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The core 4 preferably has a diameter of not less than 38.0 mm. The golf ball 2 including the core 4 having a diameter of not less than 38.0 mm has excellent flight performance upon a shot with a driver. In this respect, the diameter is more preferably not less than 38.5 mm and particularly preferably not less than 39.5 mm. From the standpoint that the mid layer 6 and the cover 8 can have sufficient thicknesses, the diameter is preferably not greater than 41.0 mm and particularly preferably not greater than 40.5 mm.

The core 4 has a weight of preferably not less than 10 g and not greater than 40 g. The temperature for crosslinking the core 4 is not lower than 140° C. and not higher than 180° C. The time period for crosslinking the core 4 is not shorter than 10 minutes and not longer than 60 minutes. The core 4 may have two or more layers. The core 4 may have a rib on the surface thereof. The core 4 may be hollow.

In the golf ball 2, the difference (Hs−Ho) between a hardness Hs at the surface of the core 4 and a hardness Ho at the central point of the core 4 exceeds 15. In other words, the golf ball 2 satisfies the following mathematical formula (i).

Hs−Ho>15  (i)

The core 4 that satisfies the mathematical formula (i) has a so-called outer-hard/inner-soft structure. When the golf ball 2 including the core 4 is hit with a driver, the spin is suppressed. When the golf ball 2 including the core 4 is hit with a driver, a high launch angle is obtained.

Upon a shot with a driver, an appropriate trajectory height and appropriate flight duration are required. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high spin rate, the run after landing is short. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle, the run after landing is long. In light of flight distance, the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle is preferable. The core 4 having an outer-hard/inner-soft structure can contribute to a high launch angle and a low spin rate as described above. The golf ball 2 including the core 4 has excellent flight performance.

In light of flight performance, the difference (Hs−Ho) is preferably not less than 17 and particularly preferably not less than 19. In light of ease of producing the core 4, the difference (Hs−Ho) is preferably not greater than 50 and particularly preferably not greater than 45.

In light of flight performance upon a shot with a driver, the central hardness Ho is preferably not less than 40, more preferably not less than 43, and particularly preferably not less than 46. In light of spin suppression and in light of feel at impact upon putting, the hardness Ho is preferably not greater than 60, more preferably not greater than 57, and particularly preferably not greater than 54.

The hardness Ho is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the central point of the cross-section of a hemisphere obtained by cutting the golf ball 2. The measurement is conducted in an environment of 23° C.

In light of spin suppression, the surface hardness Hs is preferably not less than 70, more preferably not less than 72, and particularly preferably not less than 74. In light of durability of the golf ball 2, the hardness Hs is preferably not greater than 90, more preferably not greater than 88, and particularly preferably not greater than 86.

The hardness Hs is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the surface of the core 4. The measurement is conducted in an environment of 23° C.

The mid layer 6 is positioned between the core 4 and the cover 8. The mid layer 6 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferable. Ionomer resins are highly elastic. The golf ball 2 that includes the mid layer 6 including an ionomer resin has excellent flight performance upon a shot with a driver.

An ionomer resin and another resin may be used in combination. In this case, in light of flight performance upon a shot with a driver, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably not less than 50% by weight, more preferably not less than 70% by weight, and particularly preferably not less than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferable ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of durability and flight performance upon a shot with a driver are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

The resin composition of the mid layer 6 may include a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferable. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of flight performance upon a shot with a driver, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably not less than 10% by weight, more preferably not less than 12% by weight, and particularly preferably not less than 15% by weight. In light of feel at impact upon putting, the content is preferably not greater than 50% by weight, more preferably not greater than 47% by weight, and particularly preferably not greater than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include an alloy of an olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with another base polymer. The alloy can contribute to the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferable. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferable.

Specific examples of polymer alloys include trade names “RABALON T3221C”, “RABALON T3339C”, “RABALON SJ4400N”, “RABALON SJ5400N”, “RABALON SJ6400N”, “RABALON SJ7400N”, “RABALON SJ8400N”, “RABALON SJ9400N”, and “RABALON SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “SEPTON HG-252” manufactured by Kuraray Co., Ltd.

In light of feel at impact upon putting, the proportion of the styrene block-containing thermoplastic elastomer to the entire base polymer is preferably not less than 5% by weight, more preferably not less than 15% by weight, and particularly preferably not less than 20% by weight. In light of flight performance upon a shot with a driver, the proportion is preferably not greater than 70% by weight, more preferably not greater than 60% by weight, and particularly preferably not greater than 55% by weight.

The resin composition of the mid layer 6 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The resin composition may include powder of a metal with a high specific gravity. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the mid layer 6 is accomplished. The resin composition may include a coloring agent, crosslinked rubber powder, or synthetic resin powder. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The mid layer 6 preferably has a hardness Hm of not less than 40 and not greater than 90. The golf ball 2 that includes the mid layer 6 having a hardness Hm of not less than 40 has excellent flight performance upon a shot with a driver. In this respect, the hardness Hm is more preferably not less than 50 and particularly preferably not less than 55. The golf ball 2 that includes the mid layer 6 having a hardness Hm of not greater than 90 has excellent feel at impact upon putting. In this respect, the hardness Hm is more preferably not greater than 85 and particularly preferably not greater than 83. In the case where the golf ball 2 includes two or more mid layers 6, each mid layer 6 preferably has a hardness within the above range.

The hardness Hm is measured according to the standards of “ASTM-D 2240-68”. The hardness Hm is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the mid layer 6, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

In the present embodiment, the number of mid layers 6 is one. Therefore, a Shore C hardness Hm(min) of the layer having the lowest hardness among the mid layers 6 is equal to the above-described hardness Hm. A Shore C hardness Hm(max) of the layer having the highest hardness among the mid layers 6 is equal to the above-described hardness Hm. In the present embodiment, the hardness Hm(min) is equal to the hardness Hm(max).

The mid layer 6 preferably has a thickness Tm of not less than 0.3 mm and not greater than 2.5 mm. The golf ball 2 that includes the mid layer 6 having a thickness Tm of not less than 0.3 mm has excellent feel at impact upon putting. In this respect, the thickness Tm is more preferably not less than 0.5 mm and particularly preferably not less than 0.8 mm. The golf ball 2 that includes the mid layer 6 having a thickness Tm of not greater than 2.5 mm has excellent flight performance upon a shot with a driver. In this respect, the thickness Tm is more preferably not greater than 2.0 mm and particularly preferably not greater than 1.8 mm. The thickness Tm is measured at a position immediately below the land 12. In the case where the golf ball 2 includes two or more mid layers 6, each mid layer 6 preferably has a thickness within the above range.

The cover 8 is the outermost layer except the mark layer and the paint layer. The cover 8 is formed from a resin composition. Examples of a preferable base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferable. Ionomer resins are highly elastic. The golf ball 2 that includes the cover 8 including the ionomer resin has excellent flight performance upon a shot with a driver. The ionomer resins described above for the mid layer 6 can be used for the cover 8.

An ionomer resin and another resin may be used in combination. Examples of the resin used in combination with the ionomer resin include polyurethanes, polyesters, polyamides, polyolefins, and polystyrenes. In this case, in light of flight performance upon a shot with a driver, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably not less than 50% by weight, more preferably not less than 60% by weight, and particularly preferably not less than 70% by weight.

The resin composition of the cover 8 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like in an adequate amount. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

In light of flight performance upon a shot with a driver, the cover 8 has a Shore C hardness Hc of preferably not less than 76, more preferably not less than 79, and particularly preferably not less than 82. In light of feel at impact upon putting, the hardness Hc is preferably not greater than 97, more preferably not greater than 95, and particularly preferably not greater than 93.

The hardness Hc of the cover 8 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hc is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the cover 8, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

In light of flight performance upon a shot with a driver, the cover 8 has a thickness Tc of preferably not less than 0.5 mm, more preferably not less than 0.7 mm, and particularly preferably not less than 0.8 mm. In light of feel at impact upon putting, the thickness Tc is preferably not greater than 2.0 mm, more preferably not greater than 1.5 mm, and particularly preferably not greater than 1.0 mm. The thickness Tc is measured at a position immediately below the land 12.

For forming the cover 8, known methods such as injection molding, compression molding, and the like can be used. When forming the cover 8, the dimples 10 are formed by pimples formed on the cavity face of a mold.

The golf ball 2 preferably has an amount of compressive deformation Sb of not less than 2.5 mm and not greater than 4.5 mm. The golf ball 2 having an amount of compressive deformation Sb of not less than 2.5 mm has excellent feel at impact upon putting. In this respect, the amount of compressive deformation Sb is preferably not less than 2.7 mm and particularly preferably not less than 2.8 mm. The golf ball 2 having an amount of compressive deformation Sb of not greater than 4.5 mm has excellent flight performance upon a shot with a driver. In this respect, the amount of compressive deformation Sb is more preferably not greater than 4.0 mm and particularly preferably not greater than 3.8 mm.

For measurement of the amount of compressive deformation Sb, a YAMADA type compression tester is used. In the tester, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured. A moving speed of the cylinder until the initial load is applied is 0.83 mm/s. A moving speed of the cylinder after the initial load is applied until the final load is applied is 1.67 mm/s.

In the golf ball 2, the difference (Hc−Hm(min)) between the hardness Hc of the cover 8 and the hardness Hm(min) of the layer having the lowest hardness among the mid layers 6 is greater than 20. In other words, the golf ball 2 satisfies the following mathematical formula (ii).

Hc−Hm(min)>20  (ii)

When the golf ball 2 that satisfies the mathematical formula (ii) is hit with a driver, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a driver. In this respect, the difference (Hc−Hm(min)) is more preferably not less than 22 and particularly preferably not less than 24. In light of feel at impact upon putting, the difference (Hc−Hm(min)) is preferably not greater than 42, more preferably not greater than 40, and particularly preferably not greater than 38.

In the golf ball 2, the difference (Hm(min)−Ho) between the hardness Hm(min) of the layer having the lowest hardness among the mid layers 6 and the central hardness Ho of the core 4 exceeds −10 and is less than 15. In other words, the golf ball 2 satisfies the following mathematical formula (iii).

−10<Hm(min)−Ho<15  (iii)

When the golf ball 2 in which the difference (Hm(min)−Ho) exceeds −10 is hit with a driver, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a driver. In this respect, the difference (Hm(min)−Ho) is more preferably not less than −8 and particularly preferably not less than −6. The golf ball 2 in which the difference (Hm(min)−Ho) is less than 15 has excellent feel at impact upon putting. In this respect, the difference (Hm(min)−Ho) is more preferably not greater than 13 and particularly preferably not greater than 12.

In the golf ball 2, the difference (Hc−Hs) between the hardness Hc of the cover 8 and the surface hardness Hs of the core 4 exceeds 5 and is less than 20. In other words, the golf ball 2 satisfies the following mathematical formula (iv).

5<Hc−Hs<20  (iv)

When the golf ball 2 in which the difference (Hc−Hs) exceeds 5 is hit with a driver, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a driver. In this respect, the difference (Hc−Hs) is more preferably not less than 6 and particularly preferably not less than 7. When the golf ball 2 in which the difference (Hc−Hs) is less than 20 is hit with a driver, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a driver. In this respect, the difference (Hc−Hs) is more preferably not greater than 19 and particularly preferably not greater than 18.

The hardness Hc of the cover 8 is higher than the hardness Hm(max) of the layer having the highest hardness among the mid layers 6. When the golf ball 2 in which the hardness Hc is higher than the hardness Hm(max) is hit with a driver, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a driver. In this respect, the difference (Hc−Hm(max)) is preferably not less than 5, more preferably not less than 15, and particularly preferably not less than 20. In light of feel at impact upon putting, the difference (Hc−Hm(max)) is preferably not greater than 45, more preferably not greater than 40, and particularly preferably not greater than 38.

A total thickness TT of the mid layer 6 and the cover 8 is preferably not greater than 2.8 mm. The golf ball 2 in which the thickness TT is not greater than 2.8 mm has excellent feel at impact upon putting. In this respect, the thickness TT is more preferably not greater than 2.6 mm and particularly preferably not greater than 2.4 mm. In light of durability of golf ball 2, the thickness TT is preferably not less than 1.0 mm, more preferably not less than 1.4 mm, and particularly preferably not less than 1.6 mm. In the golf ball 2 including two or more mid layers 6, the thickness TT is the sum of the thickness of the cover 8 and the thicknesses of all the mid layers 6.

As shown in FIGS. 2 and 3, the contour of each dimple 10 is circular. The golf ball 2 has dimples A each having a diameter of 4.40 mm; dimples B each having a diameter of 4.30 mm; dimples C each having a diameter of 4.15 mm; dimples D each having a diameter of 3.90 mm; and dimples E each having a diameter of 3.00 mm. The number of types of the dimples 10 is five. The golf ball 2 may have non-circular dimples instead of the circular dimples 10 or together with the circular dimples 10.

The number of the dimples A is 60; the number of the dimples B is 158; the number of the dimples C is 72; the number of the dimples D is 36; and the number of the dimples E is 12. The total number of the dimples 10 is 338. A dimple pattern is formed by these dimples 10 and the land 12.

FIG. 4 shows a cross section of the golf ball 2 along a plane passing through the central point of the dimple 10 and the central point of the golf ball 2. In FIG. 4, the top-to-bottom direction is the depth direction of the dimple 10. In FIG. 4, a chain double-dashed line 14 indicates a phantom sphere 14. The surface of the phantom sphere 14 is the surface of the golf ball 2 when it is postulated that no dimple 10 exists. The diameter of the phantom sphere 14 is equal to the diameter of the golf ball 2. The dimple 10 is recessed from the surface of the phantom sphere 14. The land 12 coincides with the surface of the phantom sphere 14. In the present embodiment, the cross-sectional shape of each dimple 10 is substantially a circular arc. The curvature radius of this circular arc is shown by reference character CR in FIG. 4.

In FIG. 4, an arrow Dm indicates the diameter of the dimple 10. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the far opposite ends of the dimple 10. Each tangent point Ed is also the edge of the dimple 10. The edge Ed defines the contour of the dimple 10.

The diameter Dm of each dimple 10 is preferably not less than 2.0 mm and not greater than 6.0 mm. The dimple 10 having a diameter Dm of not less than 2.0 mm contributes to turbulization. The golf ball 2 having the dimples 10 has excellent flight performance upon a shot with a driver. In this respect, the diameter Dm is more preferably not less than 2.5 mm and particularly preferably not less than 2.8 mm. The dimple 10 having a diameter Dm of not greater than 6.0 mm does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the diameter Dm is more preferably not greater than 5.5 mm and particularly preferably not greater than 5.0 mm.

In the case of a non-circular dimple, a circular dimple 10 having the same area as that of the non-circular dimple is assumed. The diameter of the assumed circular dimple 10 can be regarded as the diameter of the non-circular dimple.

In FIG. 4, a double ended arrow Dp1 indicates a first depth of the dimple 10. The first depth Dp1 is the distance between the deepest part of the dimple 10 and the surface of the phantom sphere 14. In FIG. 4, a double ended arrow Dp2 indicates a second depth of the dimple 10. The second depth Dp2 is the distance between the deepest part of the dimple 10 and the tangent line Tg.

In light of suppression of rising of the golf ball 2 during flight, the first depth Dp1 of each dimple 10 is preferably not less than 0.10 mm, more preferably not less than 0.13 mm, and particularly preferably not less than 0.15 mm. In light of suppression of dropping of the golf ball 2 during flight, the first depth Dp1 is preferably not greater than 0.65 mm, more preferably not greater than 0.60 mm, and particularly preferably not greater than 0.55 mm.

The area S of the dimple 10 is the area of a region surrounded by the contour line of the dimple 10 when the central point of the golf ball 2 is viewed at infinity. In the case of a circular dimple 10, the area S is calculated by the following mathematical formula.

S=(Dm/2)²*π

In the golf ball 2 shown in FIGS. 2 and 3, the area of each dimple A is 15.20 mm²; the area of each dimple B is 14.52 mm²; the area of each dimple C is 13.53 mm²; the area of each dimple D is 11.95 mm²; and the area of each dimple E is 7.07 mm².

In the present invention, the ratio of the sum of the areas S of all the dimples 10 relative to the surface area of the phantom sphere 14 is referred to as an occupation ratio. From the standpoint of achieving sufficient turbulization, the occupation ratio is preferably not less than 78%, more preferably not less than 80%, and particularly preferably not less than 82%. The occupation ratio is preferably not greater than 95%. In the golf ball 2 shown in FIGS. 2 and 3, the total area of the dimples 10 is 4695.4 mm². The surface area of the phantom sphere 14 of the golf ball 2 is 5728 mm², so that the occupation ratio is 82.0%.

From the standpoint of achieving a sufficient occupation ratio, the total number N of the dimples 10 is preferably not less than 250, more preferably not less than 280, and particularly preferably not less than 300. From the standpoint that each dimple 10 can contribute to turbulization, the total number N of the dimples 10 is preferably not greater than 450, more preferably not greater than 400, and particularly preferably not greater than 380.

In the present invention, the “volume V of the dimple” means the volume of a portion surrounded by the surface of the phantom sphere 14 and the surface of the dimple 10. The total volume TV of the dimples 10 is preferably not less than 450 mm³ and not greater than 750 mm³. With the golf ball 2 having a total volume TV of not less than 450 mm³, rising of the golf ball 2 during flight is suppressed. In this respect, the total volume TV is more preferably not less than 480 mm³ and particularly preferably not less than 500 mm³. With the golf ball 2 having a total volume TV of not greater than 750 mm³, dropping of the golf ball 2 during flight is suppressed. In this respect, the total volume TV is more preferably not greater than 730 mm³ and particularly preferably not greater than 710 mm³.

The golf ball 2 according to the present invention has an excellent aerodynamic characteristic. In an evaluation method for the aerodynamic characteristic, the following steps (a) to (h) are executed:

(a) assuming a great circle that is present on the surface of the phantom sphere 14 and is orthogonal to an axis Ax;

(b) assuming two small circles that are present on the surface of the phantom sphere 14, that are orthogonal to the axis Ax, and of which the absolute values of central angles with the great circle are each 30°;

(c) defining a region, of the surface of the golf ball 2, which is obtained by dividing the surface of the golf ball 2 at these small circles and which is sandwiched between these small circles;

(d) determining 30240 points, on the region, arranged at intervals of a central angle of 3° in a direction of the axis Ax and at intervals of a central angle of 0.25° in a direction of rotation about the axis Ax;

(e) calculating the length L1 of a perpendicular line that extends from each point to the axis Ax;

(f) calculating a total length L2 by summing 21 lengths L1 calculated on the basis of 21 perpendicular lines arranged in the direction of the axis Ax;

(g) obtaining a transformed data constellation by performing Fourier transformation on a data constellation of 1440 total lengths L2 calculated along the direction of rotation about the axis Ax; and

(h) calculating the peak value and the order of the maximum peak of the transformed data constellation. The following will describe each step in detail.

FIG. 5 is a schematic diagram for explaining this evaluation method. FIG. 5 shows the phantom sphere 14 of the golf ball 2. In FIG. 5, reference character NP represents a north pole. The north pole NP corresponds to the top of a cavity face formed by an upper mold half for molding the golf ball 2. Reference character SP represents a south pole. The south pole SP corresponds to the deepest part of a cavity face formed by a lower mold half for molding the golf ball 2. Reference character Eq represents an equator. The phantom sphere 14 can be divided into a northern hemisphere NH and a southern hemisphere SH by the equator Eq.

The latitude of the north pole NP is 90° (degrees). The latitude θ of the equator Eq is zero. The latitude of the south pole SP is −90°. The counterclockwise direction when the phantom sphere 14 is seen from the north pole NP is a positive direction of longitude ϕ. The minimum value of ϕ is zero. The maximum value of ϕ is 360°. The spherical polar coordinates of a point present on the surface of the phantom sphere 14 are represented by (θ, ϕ). In FIG. 5, a point (0, 0) is located in the front.

In FIG. 5, reference character Loa represents a first longitude line. The longitude ϕ of the first longitude line Loa is 0° and also 360°. The phantom sphere 14 has numerous longitude lines. A longitude line that contains the maximum number of dimples 10 that centrally intersect the longitude line is defined as the first longitude line Loa. At a dimple 10 that centrally intersects a longitude line, the longitude line passes through the area center of gravity of the dimple 10.

In this evaluation method, a first axis Ax1 is assumed. The first axis Ax1 passes through a point Pn1 and a point Ps1. The point Pn1 and the point Ps1 are present on the surface of the phantom sphere 14. The point Pn1 is present on the northern hemisphere NH. The coordinates of the point Pn1 are (75, 270). The point Ps1 is present on the southern hemisphere SH. The coordinates of the point Ps1 are (−75, 90). The first axis Ax1 is tilted relative to the earth axis. The angle of the tilt is 15°. The earth axis is a line passing through the north pole NP and the south pole SP.

In this evaluation method, a first great circle GC1 that is present on the surface of the phantom sphere 14 of the golf ball 2 is assumed. The first axis Ax1 is orthogonal to the first great circle GC1. In other words, the first axis Ax1 is orthogonal to the plane including the first great circle GC1. In FIG. 5, the first great circle GC1 is tilted relative to the equator Eq. The angle of the tilt is 15°. The great circle is a circle that is present on the surface of the phantom sphere 14 and has a diameter equal to the diameter of the phantom sphere 14.

The golf ball 2 rotates about the first axis Ax1. During this rotation, the circumferential speed of the first great circle GC1 is high. Therefore, the surface roughness of the golf ball 2 at and near the first great circle GC1 greatly influences the flight performance of the golf ball 2.

In this evaluation method, two small circles C1 and C2 that are present on the surface of the phantom sphere 14 and are orthogonal to the first axis Ax1 are assumed. FIG. 6 shows these small circles C1 and C2. Each small circle is parallel to the first great circle GC1.

FIG. 7 schematically shows a partial cross section of the golf ball 2 in FIG. 6. FIG. 7 shows a cross-section passing through the center O of the golf ball 2. The right-left direction in FIG. 7 is the direction of the first axis Ax1. As shown in FIG. 7, the absolute value of the central angle between the small circle 01 and the first great circle GC1 is 30°. Although not shown, the absolute value of the central angle between the small circle C2 and the first great circle GC1 is also 30°. The golf ball 2 is divided at the small circles C1 and C2, and of the surface of the golf ball 2, a region sandwiched between the small circles C1 and C2 is defined. Since the circumferential speed of the first great circle GC1 is high, the dimples 10 present in this region greatly influence the aerodynamic characteristic of the golf ball 2.

In FIG. 7, a point P(α) is the point that is located on the surface of the golf ball 2 and of which the central angle with the first great circle GC1 is α° (degrees). A point F(α) is the foot of a perpendicular line Pe(α) that extends downward from the point P(α) to the first axis Ax1. An arrow L1(α) represents the length of the perpendicular line Pe(α). In other words, the length L1(α) is the distance between the point P(α) and the first axis Ax1. For one cross section, the lengths L1(α) are calculated at 21 points P(α). Specifically, the lengths L1(α) are calculated at angles α of −30°, −27°, −24°, −21°, −18°, −15°, −12°, −9°, −6°, −3°, 0°, 3°, 6°, 9°, 12°, 15°, 18°, 21°, 24°, 27°, and 30°. The 21 lengths L1(α) are summed, thereby obtaining a total length L2 (mm). The total length L2 is a parameter dependent on the surface shape in the cross section shown in FIG. 7.

FIG. 8 shows a partial cross section of the golf ball 2. In FIG. 8, a direction perpendicular to the surface of the sheet is the direction of the first axis Ax1. In FIG. 8, reference character p represents a rotation angle of the golf ball 2. In a range of equal to or greater than 0° and less than 360°, the rotation angles β are set at an interval of an angle of 0.25°. At each rotation angle, the total length L2 is calculated. As a result, 1440 total lengths L2 are obtained along the rotation direction. These total lengths L2 are a data constellation calculated through one rotation of the golf ball 2. This data constellation is calculated on the basis of 30240 lengths L1.

FIG. 9 shows a graph plotting the data constellation, for the first axis Ax1, of the golf ball 2 shown in FIGS. 2 and 3. In this graph, the horizontal axis represents the rotation angle β, and the vertical axis represents the total length L2. Fourier transformation is performed on the data constellation. By the Fourier transformation, a frequency spectrum is obtained. In other words, by the Fourier transformation, a coefficient of a Fourier series represented by the following formula is obtained.

$F_k=\sum _{n=0}^{N-1}\left(a_n\cos 2\pi \frac{\mathrm{nk}}{N}+b_n\sin 2\pi \frac{\mathrm{nk}}{N}\right)$

The above mathematical formula is a combination of two trigonometric functions having different periods. In the above mathematical formula, a_n and b_n are Fourier coefficients. The magnitude of each component to be combined is determined depending on these Fourier coefficients. Each coefficient is represented by the following mathematical formula.

$a_n=\frac{1}{N}\sum _{k=0}^{N-1}F_k\cos 2\pi \frac{\mathrm{nk}}{N}$ $b_n=\frac{1}{N}\sum _{k=0}^{N-1}F_k\sin 2\pi \frac{\mathrm{nk}}{N}$

In the above mathematical formulas, N is the total number of pieces of data of the data constellation, and F_k is the kth value in the data constellation. The spectrum is represented by the following mathematical formula.

P_n=√{square root over (a_n²+b_n²)}

By the Fourier transformation, a transformed data constellation is obtained. FIG. 10 shows a graph plotting the transformed data constellation. In this graph, the horizontal axis represents an order, and the vertical axis represents an amplitude. From this graph, the maximum peak is determined. Furthermore, the peak value Pd1 of the maximum peak and the order Fd1 of the maximum peak are determined. The peak value Pd1 and the order Fd1 are numeric values representing the aerodynamic characteristic during rotation about the first axis Ax1. In the present embodiment, the peak value Pd1 is 270.2 mm, and the order Fd1 is 33.

FIG. 11 also shows the phantom sphere 14 of the golf ball 2. FIG. 11 shows the equator Eq and the longitude line Loa having a longitude ϕ of zero. In FIG. 11, the point (0, 0) is located in the front. In FIG. 11, reference character Ax2 represents a second axis. The second axis Ax2 passes through a point Pn2 and a point Ps2. The point Pn2 and the point Ps2 are present on the surface of the phantom sphere 14. The coordinates of the point Pn2 are (60, 270). The coordinates of the point Ps2 are (−60, 90). The second axis Ax2 is tilted relative to the earth axis. The angle of the tilt is 30°.

FIG. 11 shows a second great circle GC2 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the second axis Ax2 is orthogonal. The second great circle GC2 is tilted relative to the equator Eq. The angle of the tilt is 30°.

For rotation about the second axis Ax2, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the second axis Ax2, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the second great circle GC2 is 30°. The absolute value of the central angle between the small circle C2 and the second great circle GC2 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the second axis Ax2 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd2 of the maximum peak and the order Fd2 of the maximum peak are determined. The peak value Pd2 and the order Fd2 are numeric values representing the aerodynamic characteristic during rotation about the second axis Ax2. In the present embodiment, the peak value Pd2 is 177.9 mm, and the order Fd2 is 37.

FIG. 12 also shows the phantom sphere 14 of the golf ball 2. FIG. 12 shows the equator Eq and the longitude line Loa having a longitude ϕ of zero. In FIG. 12, the point (0, 0) is located in the front. In FIG. 12, reference character Ax3 represents a third axis. The third axis Ax3 passes through a point Pn3 and a point Ps3. The point Pn3 and the point Ps3 are present on the surface of the phantom sphere 14. The coordinates of the point Pn3 are (45, 270). The coordinates of the point Ps3 are (−45, 90). The third axis Ax3 is tilted relative to the earth axis. The angle of the tilt is 45°.

FIG. 12 shows a third great circle GC3 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the third axis Ax3 is orthogonal. The third great circle GC3 is tilted relative to the equator Eq. The angle of the tilt is 45°.

For rotation about the third axis Ax3, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the third axis Ax3, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the third great circle GC3 is 30°. The absolute value of the central angle between the small circle C2 and the third great circle GC3 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the third axis Ax3 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd3 of the maximum peak and the order Fd3 of the maximum peak are determined. The peak value Pd3 and the order Fd3 are numeric values representing the aerodynamic characteristic during rotation about the third axis Ax3. In the present embodiment, the peak value Pd3 is 150.2 mm, and the order Fd3 is 37.

FIG. 13 also shows the phantom sphere 14 of the golf ball 2. FIG. 13 shows the equator Eq and the longitude line Loa having a longitude ϕ of zero. In FIG. 13, the point (0, 0) is located in the front. In FIG. 13, reference character Ax4 represents a fourth axis. The fourth axis Ax4 passes through a point Pn4 and a point Ps4. The point Pn4 and the point Ps4 are present on the surface of the phantom sphere 14. The coordinates of the point Pn4 are (30, 270). The coordinates of the point Ps4 are (−30, 90). The fourth axis Ax4 is tilted relative to the earth axis. The angle of the tilt is 60°.

FIG. 13 shows a fourth great circle GC4 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the fourth axis Ax4 is orthogonal. The fourth great circle GC4 is tilted relative to the equator Eq. The angle of the tilt is 60°.

For rotation about the fourth axis Ax4, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the fourth axis Ax4, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the fourth great circle GC4 is 30°. The absolute value of the central angle between the small circle C2 and the fourth great circle GC4 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the fourth axis Ax4 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd4 of the maximum peak and the order Fd4 of the maximum peak are determined. The peak value Pd4 and the order Fd4 are numeric values representing the aerodynamic characteristic during rotation about the fourth axis Ax4. In the present embodiment, the peak value Pd4 is 316.4 mm, and the order Fd4 is 34.

FIG. 14 also shows the phantom sphere 14 of the golf ball 2. FIG. 14 shows the equator Eq and the longitude line Loa having a longitude ϕ of zero. In FIG. 14, the point (0, 0) is located in the front. In FIG. 14, reference character Ax5 represents a fifth axis. The fifth axis Ax5 passes through a point Pn5 and a point Ps5. The point Pn5 and the point Ps5 are present on the surface of the phantom sphere 14. The coordinates of the point Pn5 are (15, 270). The coordinates of the point Ps5 are (−15, 90). The fifth axis Ax5 is tilted relative to the earth axis. The angle of the tilt is 75°.

FIG. 14 shows a fifth great circle GC5 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the fifth axis Ax5 is orthogonal. The fifth great circle GC5 is tilted relative to the equator Eq. The angle of the tilt is 75°.

For rotation about the fifth axis Ax5, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the fifth axis Ax5, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the fifth great circle GC5 is 30°. The absolute value of the central angle between the small circle C2 and the fifth great circle GC5 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the fifth axis Ax5 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd5 of the maximum peak and the order Fd5 of the maximum peak are determined. The peak value Pd5 and the order Fd5 are numeric values representing the aerodynamic characteristic during rotation about the fifth axis Ax5. In the present embodiment, the peak value Pd5 is 190.0 mm, and the order Fd5 is 27.

FIG. 15 also shows the phantom sphere 14 of the golf ball 2. FIG. 15 shows the equator Eq and a longitude line Lob having a longitude ϕ of 90°. In FIG. 15, a point (0, 90) is located in the front. In FIG. 15, reference character Ax6 represents a sixth axis. The sixth axis Ax6 passes through a point Pn6 and a point Ps6. The point Pn6 and the point Ps6 are present on the surface of the phantom sphere 14. The coordinates of the point Pn6 are (75, 0). The coordinates of the point Ps6 are (−75, 180). The sixth axis Ax6 is tilted relative to the earth axis. The angle of the tilt is 15°.

FIG. 15 shows a sixth great circle GC6 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the sixth axis Ax6 is orthogonal. The sixth great circle GC6 is tilted relative to the equator Eq. The angle of the tilt is 15°.

For rotation about the sixth axis Ax6, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the sixth axis Ax6, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the sixth great circle GC6 is 30°. The absolute value of the central angle between the small circle C2 and the sixth great circle GC6 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the sixth axis Ax6 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd6 of the maximum peak and the order Fd6 of the maximum peak are determined. The peak value Pd6 and the order Fd6 are numeric values representing the aerodynamic characteristic during rotation about the sixth axis Ax6. In the present embodiment, the peak value Pd6 is 270.2 mm, and the order Fd6 is 33.

FIG. 16 also shows the phantom sphere 14 of the golf ball 2. FIG. 16 shows the equator Eq and the longitude line Lob having a longitude ϕ of 90°. In FIG. 16, the point (0, 90) is located in the front. In FIG. 16, reference character Ax7 represents a seventh axis. The seventh axis Ax7 passes through a point Pn7 and a point Ps7. The point Pn7 and the point Ps7 are present on the surface of the phantom sphere 14. The coordinates of the point Pn7 are (60, 0). The coordinates of the point Ps7 are (−60, 180). The seventh axis Ax7 is tilted relative to the earth axis. The angle of the tilt is 30°.

FIG. 16 shows a seventh great circle GC7 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the seventh axis Ax7 is orthogonal. The seventh great circle GC7 is tilted relative to the equator Eq. The angle of the tilt is 30°.

For rotation about the seventh axis Ax7, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the seventh axis Ax7, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the seventh great circle GC7 is 30°. The absolute value of the central angle between the small circle C2 and the seventh great circle GC7 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the seventh axis Ax7 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd7 of the maximum peak and the order Fd7 of the maximum peak are determined. The peak value Pd7 and the order Fd7 are numeric values representing the aerodynamic characteristic during rotation about the seventh axis Ax7. In the present embodiment, the peak value Pd7 is 177.9 mm, and the order Fd7 is 37.

FIG. 17 also shows the phantom sphere 14 of the golf ball 2. FIG. 17 shows the equator Eq and the longitude line Lob having a longitude ϕ of 90°. In FIG. 17, the point (0, 90) is located in the front. In FIG. 17, reference character Ax8 represents an eighth axis. The eighth axis Ax8 passes through a point Pn8 and a point Ps8. The point Pn8 and the point Ps8 are present on the surface of the phantom sphere 14. The coordinates of the point Pn8 are (45, 0). The coordinates of the point Ps8 are (−45, 180). The eighth axis Ax8 is tilted relative to the earth axis. The angle of the tilt is 45°.

FIG. 17 shows an eighth great circle GC8 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the eighth axis Ax8 is orthogonal. The eighth great circle GC8 is tilted relative to the equator Eq. The angle of the tilt is 45°.

For rotation about the eighth axis Ax8, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the eighth axis Ax8, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the eighth great circle GC8 is 30°. The absolute value of the central angle between the small circle C2 and the eighth great circle GC8 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the eighth axis Ax8 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd8 of the maximum peak and the order Fd8 of the maximum peak are determined. The peak value Pd8 and the order Fd8 are numeric values representing the aerodynamic characteristic during rotation about the eighth axis Ax8. In the present embodiment, the peak value Pd8 is 150.2 mm, and the order Fd8 is 37.

FIG. 18 also shows the phantom sphere 14 of the golf ball 2. FIG. 18 shows the equator Eq and the longitude line Lob having a longitude ϕ of 90°. In FIG. 18, the point (0, 90) is located in the front. In FIG. 18, reference character Ax9 represents a ninth axis. The ninth axis Ax9 passes through a point Pn9 and a point Ps9. The point Pn9 and the point Ps9 are present on the surface of the phantom sphere 14. The coordinates of the point Pn9 are (30, 0). The coordinates of the point Ps9 are (−30, 180). The ninth axis Ax9 is tilted relative to the earth axis. The angle of the tilt is 60°.

FIG. 18 shows a ninth great circle GC9 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the ninth axis Ax9 is orthogonal. The ninth great circle GC9 is tilted relative to the equator Eq. The angle of the tilt is 60°.

For rotation about the ninth axis Ax9, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the ninth axis Ax9, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the ninth great circle GC9 is 30°. The absolute value of the central angle between the small circle C2 and the ninth great circle GC9 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the ninth axis Ax9 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd9 of the maximum peak and the order Fd9 of the maximum peak are determined. The peak value Pd9 and the order Fd9 are numeric values representing the aerodynamic characteristic during rotation about the ninth axis Ax9. In the present embodiment, the peak value Pd9 is 316.4 mm, and the order Fd9 is 34.

FIG. 19 also shows the phantom sphere 14 of the golf ball 2. FIG. 19 shows the equator Eq and the longitude line Lob having a longitude ϕ of 90°. In FIG. 19, the point (0, 90) is located in the front. In FIG. 19, reference character Ax10 represents a tenth axis. The tenth axis Ax10 passes through a point Pn10 and a point Ps10. The point Pn10 and the point Ps10 are present on the surface of the phantom sphere 14. The coordinates of the point Pn10 are (15, 0). The coordinates of the point Ps10 are (−15, 180). The tenth axis Ax10 is tilted relative to the earth axis. The angle of the tilt is 75°.

FIG. 19 shows a tenth great circle GC10 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the tenth axis Ax10 is orthogonal. The tenth great circle GC10 is tilted relative to the equator Eq. The angle of the tilt is 75°.

For rotation about the tenth axis Ax10, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the tenth axis Ax10, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the tenth great circle GC10 is 30°. The absolute value of the central angle between the small circle C2 and the tenth great circle GC10 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the tenth axis Ax10 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd10 of the maximum peak and the order Fd10 of the maximum peak are determined. The peak value Pd10 and the order Fd10 are numeric values representing the aerodynamic characteristic during rotation about the tenth axis Ax10. In the present embodiment, the peak value Pd10 is 190.0 mm, and the order Fd10 is 27.

FIG. 20 also shows the phantom sphere 14 of the golf ball 2. FIG. 20 shows the equator Eq and a longitude line Loc having a longitude ϕ of 180°. In FIG. 20, a point (0, 180) is located in the front. In FIG. 20, reference character Ax11 represents an eleventh axis. The eleventh axis Ax11 passes through a point Pn11 and a point Ps11. The point Pn11 and the point Ps11 are present on the surface of the phantom sphere 14. The coordinates of the point Pn11 are (75, 90). The coordinates of the point Ps11 are (−75, 270). The eleventh axis Ax11 is tilted relative to the earth axis. The angle of the tilt is 15°.

FIG. 20 shows an eleventh great circle GC11 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the eleventh axis Ax11 is orthogonal. The eleventh great circle GC11 is tilted relative to the equator Eq. The angle of the tilt is 15°.

For rotation about the eleventh axis Ax11, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the eleventh axis Ax11, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the eleventh great circle GC11 is 30°. The absolute value of the central angle between the small circle C2 and the eleventh great circle GC11 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the eleventh axis Ax11 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd11 of the maximum peak and the order Fd11 of the maximum peak are determined. The peak value Pd11 and the order Fd11 are numeric values representing the aerodynamic characteristic during rotation about the eleventh axis Ax11. In the present embodiment, the peak value Pd11 is 270.2 mm, and the order Fd11 is 33.

FIG. 21 also shows the phantom sphere 14 of the golf ball 2. FIG. 21 shows the equator Eq and the longitude line Loc having a longitude ϕ of 180°. In FIG. 21, the point (0, 180) is located in the front. In FIG. 21, reference character Ax12 represents a twelfth axis. The twelfth axis Ax12 passes through a point Pn12 and a point Ps12. The point Pn12 and the point Ps12 are present on the surface of the phantom sphere 14. The coordinates of the point Pn12 are (60, 90). The coordinates of the point Ps12 are (−60, 270). The twelfth axis Ax12 is tilted relative to the earth axis. The angle of the tilt is 30°.

FIG. 21 shows a twelfth great circle GC12 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the twelfth axis Ax12 is orthogonal. The twelfth great circle GC12 is tilted relative to the equator Eq. The angle of the tilt is 30°.

For rotation about the twelfth axis Ax12, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the twelfth axis Ax12, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the twelfth great circle GC12 is 30°. The absolute value of the central angle between the small circle C2 and the twelfth great circle GC12 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the twelfth axis Ax12 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd12 of the maximum peak and the order Fd12 of the maximum peak are determined. The peak value Pd12 and the order Fd12 are numeric values representing the aerodynamic characteristic during rotation about the twelfth axis Ax12. In the present embodiment, the peak value Pd12 is 177.9 mm, and the order Fd12 is 37.

FIG. 22 also shows the phantom sphere 14 of the golf ball 2. FIG. 22 shows the equator Eq and the longitude line Loc having a longitude ϕ of 180°. In FIG. 22, the point (0, 180) is located in the front. In FIG. 22, reference character Ax13 represents a thirteenth axis. The thirteenth axis Ax13 passes through a point Pn13 and a point Ps13. The point Pn13 and the point Ps13 are present on the surface of the phantom sphere 14. The coordinates of the point Pn13 are (45, 90). The coordinates of the point Ps13 are (−45, 270). The thirteenth axis Ax13 is tilted relative to the earth axis. The angle of the tilt is 45°.

FIG. 22 shows a thirteenth great circle GC13 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the thirteenth axis Ax13 is orthogonal. The thirteenth great circle GC13 is tilted relative to the equator Eq. The angle of the tilt is 45°.

For rotation about the thirteenth axis Ax13, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the thirteenth axis Ax13, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the thirteenth great circle GC13 is 30°. The absolute value of the central angle between the small circle C2 and the thirteenth great circle GC13 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the thirteenth axis Ax13 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd13 of the maximum peak and the order Fd13 of the maximum peak are determined. The peak value Pd13 and the order Fd13 are numeric values representing the aerodynamic characteristic during rotation about the thirteenth axis Ax13. In the present embodiment, the peak value Pd13 is 150.2 mm, and the order Fd13 is 37.

FIG. 23 also shows the phantom sphere 14 of the golf ball 2. FIG. 23 shows the equator Eq and the longitude line Loc having a longitude ϕ of 180°. In FIG. 23, the point (0, 180) is located in the front. In FIG. 23, reference character Ax14 represents a fourteenth axis. The fourteenth axis Ax14 passes through a point Pn14 and a point Ps14. The point Pn14 and the point Ps14 are present on the surface of the phantom sphere 14. The coordinates of the point Pn14 are (30, 90). The coordinates of the point Ps14 are (−30, 270). The fourteenth axis Ax14 is tilted relative to the earth axis. The angle of the tilt is 60°.

FIG. 23 shows a fourteenth great circle GC14 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the fourteenth axis Ax14 is orthogonal. The fourteenth great circle GC14 is tilted relative to the equator Eq. The angle of the tilt is 60°.

For rotation about the fourteenth axis Ax14, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the fourteenth axis Ax14, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the fourteenth great circle GC14 is 30°. The absolute value of the central angle between the small circle C2 and the fourteenth great circle GC14 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the fourteenth axis Ax14 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd14 of the maximum peak and the order Fd14 of the maximum peak are determined. The peak value Pd14 and the order Fd14 are numeric values representing the aerodynamic characteristic during rotation about the fourteenth axis Ax14. In the present embodiment, the peak value Pd14 is 316.4 mm, and the order Fd14 is 34.

FIG. 24 also shows the phantom sphere 14 of the golf ball 2. FIG. 24 shows the equator Eq and the longitude line Loc having a longitude ϕ of 180°. In FIG. 24, the point (0, 180) is located in the front. In FIG. 24, reference character Ax15 represents a fifteenth axis. The fifteenth axis Ax15 passes through a point Pn15 and a point Ps15. The point Pn15 and the point Ps15 are present on the surface of the phantom sphere 14. The coordinates of the point Pn15 are (15, 90). The coordinates of the point Ps15 are (−15, 270). The fifteenth axis Ax15 is tilted relative to the earth axis. The angle of the tilt is 75°.

FIG. 24 shows a fifteenth great circle GC15 that is present on the surface of the phantom sphere 14 of the golf ball 2 and to which the fifteenth axis Ax15 is orthogonal. The fifteenth great circle GC15 is tilted relative to the equator Eq. The angle of the tilt is 75°.

For rotation about the fifteenth axis Ax15, an aerodynamic characteristic is evaluated by the same method as that for rotation about the first axis Ax1. Specifically, for rotation about the fifteenth axis Ax15, two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the fifteenth great circle GC15 is 30°. The absolute value of the central angle between the small circle C2 and the fifteenth great circle GC15 is also 30°. In the region, of the surface of the golf ball 2, sandwiched between these small circles, 1440 total lengths L2 are calculated. In other words, a data constellation for the fifteenth axis Ax15 is calculated. Fourier transformation is performed on this data constellation, thereby obtaining a transformed data constellation. From a graph plotting the transformed data constellation, the peak value Pd15 of the maximum peak and the order Fd15 of the maximum peak are determined. The peak value Pd15 and the order Fd15 are numeric values representing the aerodynamic characteristic during rotation about the fifteenth axis Ax15. In the present embodiment, the peak value Pd15 is 190.0 mm, and the order Fd15 is 27.

In this evaluation method, the steps (a) to (h) are executed for each of 15 axes Ax that are

(1) the first axis Ax1 passing through the point Pn1 the coordinates of which are (75, 270) and the point Ps1 the coordinates of which are (−75, 90),

(2) the second axis Ax2 passing through the point Pn2 the coordinates of which are (60, 270) and the point Ps2 the coordinates of which are (−60, 90),

(3) the third axis Ax3 passing through the point Pn3 the coordinates of which are (45, 270) and the point Ps3 the coordinates of which are (−45, 90),

(4) the fourth axis Ax4 passing through the point Pn4 the coordinates of which are (30, 270) and the point Ps4 the coordinates of which are (−30, 90),

(5) the fifth axis Ax5 passing through the point Pn5 the coordinates of which are (15, 270) and the point Ps5 the coordinates of which are (−15, 90),

(6) the sixth axis Ax6 passing through the point Pn6 the coordinates of which are (75, 0) and the point Ps6 the coordinates of which are (−75, 180),

(7) the seventh axis Ax7 passing through the point Pn7 the coordinates of which are (60, 0) and the point Ps7 the coordinates of which are (−60, 180),

(8) the eighth axis Ax8 passing through the point Pn8 the coordinates of which are (45, 0) and the point Ps8 the coordinates of which are (−45, 180),

(9) the ninth axis Ax9 passing through the point Pn9 the coordinates of which are (30, 0) and the point Ps9 the coordinates of which are (−30, 180),

(10) the tenth axis Ax10 passing through the point Pn10 the coordinates of which are (15, 0) and the point Ps10 the coordinates of which are (−15, 180),

(11) the eleventh axis Ax11 passing through the point Pn11 the coordinates of which are (75, 90) and the point Ps11 the coordinates of which are (−75, 270),

(12) the twelfth axis Ax12 passing through the point Pn12 the coordinates of which are (60, 90) and the point Ps12 the coordinates of which are (−60, 270),

(13) the thirteenth axis Ax13 passing through the point Pn13 the coordinates of which are (45, 90) and the point Ps13 the coordinates of which are (−45, 270),

(14) the fourteenth axis Ax14 passing through the point Pn14 the coordinates of which are (30, 90) and the point Ps14 the coordinates of which are (−30, 270), and

(15) the fifteenth axis Ax15 passing through the point Pn15 the coordinates of which are (15, 90) and the point Ps15 the coordinates of which are (−15, 270). Accordingly, 15 peak values (Pd1 to Pd15) and 15 orders (Fd1 to Fd15) are calculated.

The minimums among the 15 peak values (Pd1 to Pd15) are Pd3, Pd8, and Pd13. The minimum value of the peak value Pd is 150.2 mm. According to the findings by the present inventor, the minimum value is preferably not less than 95 mm. In the golf ball 2 in which the minimum value is not less than 95 mm, a sufficient dimple effect can be achieved even during rotation about any axis Ax. When the golf ball 2 is hit with a driver, the flight distance is large. In this respect, the minimum value of the peak value Pd is more preferably not less than 120 mm and particularly preferably not less than 140 mm.

The maximums among the 15 peak values (Pd1 to Pd15) are Pd4, Pd9, and Pd14. The maximum value of the peak value Pd is 316.4 mm. According to the findings by the present inventor, the maximum value is preferably not greater than 500 mm. The golf ball 2 in which the maximum value is not greater than 500 mm has an excellent aerodynamic characteristic. When the golf ball 2 is hit with a driver, the flight distance is large. In this respect, the maximum value of the peak value Pd is more preferably not greater than 400 mm and particularly preferably not greater than 330 mm.

The average of the 15 peak values (Pd1 to Pd15) is preferably not less than 200 mm. The golf ball 2 in which the average is not less than 200 mm has an excellent aerodynamic characteristic. When the golf ball 2 is hit with a driver, the flight distance is large. In this respect, the average is more preferably not less than 210 mm and particularly preferably not less than 220 mm. The average is preferably not greater than 300 mm and particularly preferably not greater than 230 mm. In the present embodiment, the average is 220.9 mm.

The minimums among the 15 orders (Fd1 to Fd15) are Fd5, Fd10, and Fd15. The minimum value of the order Fd is 27. According to the findings by the present inventor, the minimum value is preferably not less than 27. The golf ball 2 in which the minimum value is not less than 27 has an excellent aerodynamic characteristic. When the golf ball 2 is hit with a driver, the flight distance is large.

The maximums among the 15 orders (Fd1 to Fd15) are Fd2, Fd3, Fd7, Fd8, Fd12, and Fd13. The maximum value of the order Fd is 37. According to the findings by the present inventor, the maximum value is preferably not greater than 37. The golf ball 2 in which the maximum value is not greater than 37 has an excellent aerodynamic characteristic. When the golf ball 2 is hit with a driver, the flight distance is large.

The average of the 15 orders (Fd1 to Fd15) is preferably not less than 30 and not greater than 34. The golf ball 2 in which the average falls within this range has an excellent aerodynamic characteristic. When the golf ball 2 is hit with a driver, the flight distance is large. In the present embodiment, the average is 33.6.

In this method, the golf ball 2 is evaluated by the 15 peak values Pd and the 15 orders Fd based on the 15 axes Ax. By this method, the aerodynamic characteristic of the golf ball 2 can be objectively evaluated.

EXAMPLES

Example 1

A rubber composition A was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 25.5 parts by weight of zinc diacrylate, 12 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.9 parts by weight of dicumyl peroxide, 0.5 parts by weight of diphenyl disulfide, 0.1 parts by weight of 2-naphthalenethiol, and 2 parts by weight of benzoic acid. This rubber composition A was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 160° C. for 20 minutes to obtain a core with a diameter of 38.6 mm. The amount of barium sulfate was adjusted such that a core having a predetermined weight was obtained.

A resin composition (a) was obtained by kneading 26 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 26 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 48 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “RABALON T3221C”), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The core was covered with the resin composition (a) by injection molding to form a mid layer with a thickness of 1.0 mm.

A resin composition (e) was obtained by kneading 55 parts by weight of an ionomer resin (the aforementioned “Himilan AM7329”), 45 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The sphere consisting of the core and the mid layer was placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face. The mid layer was covered with the resin composition (e) by injection molding to form a cover with a thickness of 1.05 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g. Dimple specifications I of the golf ball are shown in detail in Tables 4, 6, and 8 below.

Examples 2 and 3 and Comparative Examples 1 to 4

Golf balls of Examples 2 and 3 and Comparative Examples 1 to 4 were obtained in the same manner as Example 1, except the specifications of the dimples were as shown in Tables 10 and 11 below. The specifications of the dimples are shown in detail in Tables 4 to 9 below.

Examples 4 to 7 and Comparative Examples 5 to 10

Golf balls of Examples 4 to 7 and Comparative Examples 5 to 10 were obtained in the same manner as Example 1, except the specifications of the core, the mid layer, and the cover were as shown in Tables 11 to 13 below. The specifications of the core are shown in detail in Tables 1 and 2 below. The specifications of the mid layer and the cover are shown in detail in Table 3 below.

Example 8

A rubber composition B was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 22.5 parts by weight of zinc diacrylate, 12 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.9 parts by weight of dicumyl peroxide, 0.5 parts by weight of diphenyl disulfide, 0.1 parts by weight of 2-naphthalenethiol, and 2.0 parts by weight of benzoic acid. This rubber composition B was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 160° C. for 20 minutes to obtain a core with a diameter of 36.6 mm. The amount of barium sulfate was adjusted such that a core having a predetermined weight was obtained.

A resin composition (a) was obtained by kneading 26 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 26 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 48 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “RABALON T3221C”), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The core was covered with the resin composition (a) by injection molding to form a first mid layer with a thickness of 1.0 mm.

A resin composition (c) was obtained by kneading 43 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 14 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “RABALON T3221C”), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The first mid layer was covered with the resin composition (c) by injection molding to form a second mid layer with a thickness of 1.0 mm.

A resin composition (e) was obtained by kneading 55 parts by weight of an ionomer resin (the aforementioned “Himilan AM7329”), 45 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. The sphere consisting of the core, the first mid layer, and the second mid layer was placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face. The second mid layer was covered with the resin composition (e) by injection molding to form a cover with a thickness of 1.05 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 8 with a diameter of about 42.7 mm and a weight of about 45.6 g. Dimple specifications I of the golf ball are shown in detail in Tables 4, 6, and 8 below.

[Flight Test]

A driver (trade name “XXIO PRIME”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R, loft angle: 11.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 35 m/sec. The ball speed and the spin rate were measured immediately after the hit. Furthermore, the flight distance was measured. The flight distance is the distance between the point at the hit and the point at which the ball stopped. The average value of data obtained from 12 measurements is shown in Tables 10 to 13 below.

[Feel at Impact]

Twenty golf players hit golf balls with putters and were asked about feel at impact. The evaluation was categorized as follows on the basis of the number of golf players who answered, “the feel at impact was soft”.

A: 16 persons or more

B: 10 to 15 persons

C: 3 to 9 persons

D: 2 persons or less

The results are shown in Tables 10 to 13 below.

TABLE 1
Specifications of Rubber Composition
(parts by weight)
	A	B	C	D
Polybutadiene	100	100	100	100
Zinc diacrylate	25.5	22.5	28.5	35.5
Zinc oxide	12	12	12	12
Barium sulfate	Appro-	Appro-	Appro-	Appro-
	priate	priate	priate	priate
	amount	amount	amount	amount
Dicumyl peroxide	0.9	0.9	0.9	0.9
Diphenyl disulfide	0.5	0.5	0.5	0.5
2-naphthalenethiol	0.1	0.1	0.1	0.1
Benzoic acid	2.0	2.0	2.0	2.0
Crosslinking	160	160	160	160
temperature (° C.)
Crosslinking time	20	20	20	20
(min)

TABLE 2
Specifications of Rubber Composition
(parts by weight)
	E	F	G	H
Polybutadiene	100	100	100	100
Zinc diacrylate	24.0	26.0	25.0	31.5
Zinc oxide	12	12	5	5
Barium sulfate	Appro-	Appro-	Appro-	Appro-
	priate	priate	priate	priate
	amount	amount	amount	amount
Dicumyl peroxide	0.9	0.9	0.9	0.9
Diphenyl disulfide	0.5	0.5	0.5	0.5
2-naphthalenethiol	0.1	0.1	—	0.1
Benzoic acid	2.0	2.0	—	—
Crosslinking	160	160	140	160
temperature (° C.)
Crosslinking time	20	20	20	20
(min)

TABLE 3
Specifications of Resin Composition
(parts by weight)
	a	b	c	d	e	f	g
Himilan	26	25	43	26	—	—	50
AM7337
Himilan	26	25	40	40	55	46	50
AM7329
Himilan	—	—	—	—	45	47	—
1555
RABALON	48	51	14	31		9	—
T3221C
Titanium	4	4	4	4	4	4	4
dioxide
Light	0.2	0.2	0.2	0.2	0.2	0.2	0.2
stabilizer
Hardness	57	54	83	70	92	87	97
(Shore C)

TABLE 4
Specifications of Dimples
		Dm	Dp2	Dp1	CR	V
	Number	(mm)	(mm)	(mm)	(mm)	(mm3)
I	A	60	4.40	0.138	0.2506	17.61	1.051
	B	158	4.30	0.137	0.2445	16.94	0.996
	C	72	4.15	0.134	0.2341	16.13	0.908
	D	36	3.90	0.123	0.2114	15.52	0.736
	E	12	3.00	0.122	0.1743	9.28	0.432
II	A	30	4.60	0.135	0.2581	19.66	1.123
	B	66	4.50	0.135	0.2528	18.82	1.075
	C	84	4.40	0.135	0.2476	17.99	1.028
	D	30	4.30	0.135	0.2425	17.19	0.982
	E	48	4.20	0.135	0.2376	16.40	0.936
	F	60	4.00	0.135	0.2280	14.88	0.850
	G	6	2.70	0.135	0.1773	6.82	0.388
III	A	6	4.70	0.135	0.2635	20.52	1.172
	B	126	4.40	0.135	0.2476	17.99	1.028
	C	122	4.30	0.135	0.2425	17.19	0.982
	D	6	4.15	0.135	0.2351	16.01	0.914
	E	66	3.90	0.135	0.2234	14.15	0.808
	F	12	3.00	0.135	0.1873	8.40	0.478

TABLE 5
Specifications of Dimples
		Dm	Dp2	Dp1	CR	V
	Number	(mm)	(mm)	(mm)	(mm)	(mm3)
IV	A	30	4.60	0.135	0.2581	19.66	1.123
	B	68	4.50	0.135	0.2528	18.82	1.075
	C	92	4.40	0.135	0.2476	17.99	1.028
	D	74	4.30	0.135	0.2425	17.19	0.982
	E	38	4.15	0.135	0.2351	16.01	0.914
	F	14	3.85	0.135	0.2211	13.79	0.787
	G	8	3.60	0.135	0.2103	12.07	0.688
V	A	156	4.91	0.135	0.2766	22.39	2.609
	B	98	4.65	0.135	0.2620	20.09	2.217
	C	12	3.00	0.135	0.1878	8.40	0.663
VI	A	70	4.10	0.135	0.2336	15.63	1.538
	B	30	3.90	0.135	0.2242	14.15	1.336
	C	120	3.80	0.135	0.2197	13.44	1.243
	D	170	3.70	0.135	0.2153	12.74	1.155
	E	20	3.60	0.135	0.2110	12.07	1.072
	F	12	2.50	0.135	0.1716	5.85	0.422
VII	A	30	4.60	0.135	0.2581	19.66	1.123
	B	54	4.50	0.135	0.2528	18.82	1.075
	C	72	4.30	0.135	0.2425	17.19	0.982
	D	54	4.20	0.135	0.2376	16.40	0.936
	E	108	4.00	0.135	0.2280	14.88	0.850
	F	12	2.70	0.135	0.1773	6.82	0.388

TABLE 6
Aerodynamic Characteristic
	I	II	III
	Peak	Pd1	270.2	143.5	195.1
	value	Pd2	177.9	195.4	153.1
		Pd3	150.2	147.0	147.8
		Pd4	316.4	322.0	322.0
		Pd5	190.0	152.2	152.2
		Pd6	270.2	143.5	195.1
		Pd7	177.9	195.4	153.1
		Pd8	150.2	147.0	147.8
		Pd9	316.4	322.0	322.0
		Pd10	190.0	152.2	152.2
		Pd11	270.2	143.5	195.1
		Pd12	177.9	195.4	153.1
		Pd13	150.2	147.0	147.8
		Pd14	316.4	322.0	322.0
		Pd15	190.0	152.2	152.2
	Order	Fd1	33	31	31
		Fd2	37	31	31
		Fd3	37	33	33
		Fd4	34	36	36
		Fd5	27	29	29
		Fd6	33	31	31
		Fd7	37	31	31
		Fd8	37	33	33
		Fd9	34	36	36
		Fd10	27	29	29
		Fd11	33	31	31
		Fd12	37	31	31
		Fd13	37	33	33
		Fd14	34	36	36
		Fd15	27	29	29

TABLE 7
Aerodynamic Characteristic
	IV	V	VI	VII
	Peak	Pd1	116.0	245.2	181.3	206.0
	value	Pd2	93.2	204.6	117.2	302.6
		Pd3	174.6	317.5	87.3	190.4
		Pd4	440.9	336.5	296.0	420.1
		Pd5	151.1	134.7	146.3	112.6
		Pd6	207.7	147.7	225.3	196.5
		Pd7	177.0	230.7	329.8	155.2
		Pd8	165.9	458.1	347.1	281.5
		Pd9	257.7	778.5	259.2	358.3
		Pd10	157.5	244.8	165.7	89.7
		Pd11	187.0	524.8	181.3	206.0
		Pd12	146.3	284.0	117.2	302.6
		Pd13	263.3	184.0	87.3	190.4
		Pd14	383.1	282.7	296.0	420.1
		Pd15	146.1	185.4	146.3	112.6
	Order	Fd1	31	25	35	31
		Fd2	33	29	37	33
		Fd3	30	29	35	29
		Fd4	34	31	41	31
		Fd5	32	31	35	29
		Fd6	30	33	39	35
		Fd7	33	31	37	37
		Fd8	34	29	39	31
		Fd9	34	31	41	33
		Fd10	30	29	35	33
		Fd11	31	29	35	31
		Fd12	32	23	37	33
		Fd13	32	29	35	29
		Fd14	34	31	41	31
		Fd15	32	31	35	29

TABLE 8
Specifications of Dimples
	I	II	III
	Front view	FIG. 2	FIG. 25	FIG. 27
	Plan view	FIG. 3	FIG. 26	FIG. 28
	Total number N	338	324	338
	Total volume	564.6	579.0	574.3
	TV (mm3)
	Peak value	Max	316.4	322.0	322.0
	Pd	Min	150.2	143.5	147.8
		Ave	220.9	192.0	194.0
	Order	Max	37	36	36
	Fd	Min	27	29	29
		Ave	33.6	32.0	32.0

TABLE 9
Specifications of Dimples
	IV	V	VI	VII
Front view	FIG. 29	FIG. 31	FIG. 33	FIG. 35
Plan view	FIG. 30	FIG. 32	FIG. 34	FIG. 36
Total number N	324	266	422	330
Total volume	589.7	632.2	519.8	571.3
TV (mm3)
Peak value	Max	440.9	778.5	347.1	420.1
Pd	Min	93.2	134.7	87.3	89.7
	Ave	204.5	304.0	198.9	236.3
Order	Max	34	33	41	37
Fd	Min	30	23	34	29
	Ave	32.1	29.4	37.1	31.7

TABLE 10
Results of Evaluation
				Comp.	Comp.
	Ex. 1	Ex. 2	Ex. 3	Ex. 1	Ex. 2
Core	A	A	A	A	A
Ho (Shore C)	48	48	48	48	48
Hs (Shore C)	80	80	80	80	80
Diameter (mm)	38.6	38.6	38.6	38.6	38.6
(First) Mid layer	a	a	a	a	a
Thickness (mm)	1.0	1.0	1.0	1.0	1.0
Hardness (Shore C)	57	57	57	57	57
Second mid layer	—	—	—	—	—
Thickness (mm)	—	—	—	—	—
Hardness (Shore C)	—	—	—	—	—
Cover	e	e	e	e	e
Thickness (mm)	1.05	1.05	1.05	1.05	1.05
Hardness Hc	92	92	92	92	92
(Shore C)
Dimple	I	II	III	IV	V
Pd Min (mm)	150.2	143.5	147.8	93.2	134.7
Fd Min	27	29	29	30	23
Fd Max	37	36	36	34	33
Fd Ave	33.6	32.0	32.0	32.1	29.4
Compression Sb (mm)	3.66	3.66	3.66	3.66	3.66
Hs − Ho	32	32	32	32	32
Hc − Hmin	35	35	35	35	35
Hmin − Ho	9	9	9	9	9
Hc − Hs	12	12	12	12	12
TT (mm)	2.05	2.05	2.05	2.05	2.05
Ball speed (m/s)	50.00	50.00	50.00	50.00	50.00
Spin rate (rpm)	2600	2600	2600	2600	2600
Flight distance (m)	160.0	159.6	159.8	159.1	159.0
Feel at impact	A	A	A	A	A

TABLE 11
Results of Evaluation
	Comp.	Comp.			Comp.
	Ex. 3	Ex. 4	Ex. 4	Ex. 5	Ex. 5
Core	A	A	B	C	D
Ho (Shore C)	48	48	46	50	58
Hs (Shore C)	80	80	78	82	90
Diameter (mm)	38.6	38.6	38.6	38.6	38.6
(First) Mid layer	a	a	a	a	b
Thickness (mm)	1.0	1.0	1.0	1.0	1.0
Hardness (Shore C)	57	57	57	57	54
Second mid layer	—	—	—	—	—
Thickness (mm)	—	—	—	—	—
Hardness (Shore C)	—	—	—	—	—
Cover	e	e	e	e	e
Thickness (mm)	1.05	1.05	1.05	1.05	1.05
Hardness Hc	92	92	92	92	92
(Shore C)
Dimple	VI	VII	I	I	I
Pd Min (mm)	87.3	89.7	150.2	150.2	150.2
Fd Min	34	29	27	27	27
Fd Max	41	37	37	37	37
Fd Ave	37.1	31.7	33.6	33.6	33.6
Compression Sb (mm)	3.66	3.66	3.96	3.37	2.60
Hs − Ho	32	32	32	32	32
Hc − Hmin	35	35	35	35	38
Hmin − Ho	9	9	11	7	−4
Hc − Hs	12	12	14	10	2
TT (mm)	2.05	2.05	2.05	2.05	2.05
Ball speed (m/s)	50.00	50.00	49.90	50.10	50.45
Spin rate (rpm)	2600	2600	2500	2700	3050
Flight distance (m)	159.0	159.0	160.5	159.6	158.0
Feel at impact	A	A	A	B	D

TABLE 12
Results of Evaluation
	Comp.		Comp.	Comp.
	Ex. 6	Ex. 6	Ex. 7	Ex. 8	Ex. 7
Core	B	F	B	G	F
Ho (Shore C)	46	49	48	60	49
Hs (Shore C)	78	81	80	70	81
Diameter (mm)	38.6	38.6	38.6	38.6	37.9
(First) Mid layer	a	a	c	a	a
Thickness (mm)	1.0	1.0	1.0	1.0	1.0
Hardness (Shore C)	57	57	83	57	57
Second mid layer	—	—	—	—	—
Thickness (mm)	—	—	—	—	—
Hardness (Shore C)	—	—	—	—	—
Cover	c	g	e	e	e
Thickness (mm)	1.05	1.05	1.05	1.05	1.40
Hardness Hc	83	97	92	92	92
(Shore C)
Dimple	I	I	I	I	I
Pd Min (mm)	150.2	150.2	150.2	150.2	150.2
Fd Min	27	27	27	27	27
Fd Max	37	37	37	37	37
Fd Ave	33.6	33.6	33.6	33.6	33.6
Compression Sb (mm)	3.66	3.66	3.66	3.66	3.57
Hs − Ho	32	32	32	10	32
Hc − Hmin	26	40	9	35	35
Hmin − Ho	11	8	35	−3	8
Hc − Hs	5	16	12	22	11
TT (mm)	2.05	2.05	2.05	2.05	2.40
Ball speed (m/s)	49.87	50.17	50.07	50.18	50.04
Spin rate (rpm)	2775	2500	2680	2960	2650
Flight distance (m)	157.8	161.7	159.6	157.6	159.7
Feel at impact	A	C	D	B	B

TABLE 13
Results of Evaluation
	Comp.	Comp.	Comp.
	Ex. 9	Ex. 10	Ex. 11	Ex. 8
Core	H	A	B	B
Ho (Shore C)	66	48	46	46
Hs (Shore C)	83	80	78	78
Diameter (mm)	38.6	36.9	36.9	36.6
(First) Mid layer	b	a	d	a
Thickness (mm)	1.0	1.4	1.4	1.0
Hardness (Shore C)	54	57	70	57
Second mid layer	—	—	—	c
Thickness (mm)	—	—	—	1.0
Hardness (Shore C)	—	—	—	83
Cover	f	c	e	e
Thickness (mm)	1.05	1.50	1.50	1.05
Hardness Hc	87	83	92	92
(Shore C)
Dimple	I	I	I	I
Pd Min (mm)	150.2	150.2	150.2	150.2
Fd Min	27	27	27	27
Fd Max	37	37	37	37
Fd Ave	33.6	33.6	33.6	33.6
Compression Sb (mm)	2.60	3.66	3.66	3.66
Hs − Ho	17	32	32	32
Hc − Hmin	33	26	22	35
Hmin − Ho	−12	9	24	11
Hc − Hs	4	3	14	14
TT (mm)	2.05	2.90	2.90	3.05
Ball speed (m/s)	50.34	49.99	49.99	49.99
Spin rate (rpm)	3350	3050	2850	2615
Flight distance (m)	154.7	155.9	157.7	159.8
Feel at impact	D	C	C	C

As shown in Tables 10 to 13, the golf ball of each Example has excellent flight performance upon a shot with a driver and excellent feel at impact upon putting. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention.

Claims

1. A golf ball comprising a core, one or more mid layers positioned outside the core, and a cover positioned outside the mid layers, wherein

a Shore C hardness Ho at a central point of the core, a Shore C hardness Hs at a surface of the core, a Shore C hardness Hm(min) of a layer having a lowest hardness among the mid layers, and a Shore C hardness Hc of the cover satisfy the following mathematical formulas (i) to (iv), Hs−Ho>15  (i), Hc−Hm(min)>20  (ii), −10<Hm(min)−Ho<15  (iii), and 5<Hc−Hs<20  (iv),

the hardness Hc of the cover is higher than a Shore C hardness Hm(max) of a layer having a highest hardness among the mid layers,

the golf ball further comprises a plurality of dimples on a surface thereof,

a minimum value of 15 peak values obtained by executing steps (a) to (h) for each of 15 axes Ax is not less than 95 mm, when spherical polar coordinates of a point that is located on a surface of a phantom sphere of the golf ball and has a latitude of θ (degrees) and a longitude of ϕ (degrees) are represented by (θ, ϕ), the 15 axes Ax being

(1) a first axis Ax1 passing through a point Pn1 coordinates of which are (75, 270) and a point Ps1 coordinates of which are (−75, 90),

(2) a second axis Ax2 passing through a point Pn2 coordinates of which are (60, 270) and a point Ps2 coordinates of which are (−60, 90)

(3) a third axis Ax3 passing through a point Pn3 coordinates of which are (45, 270) and a point Ps3 coordinates of which are (−45, 90),

(4) a fourth axis Ax4 passing through a point Pn4 coordinates of which are (30, 270) and a point Ps4 coordinates of which are (−30, 90),

(5) a fifth axis Ax5 passing through a point Pn5 coordinates of which are (15, 270) and a point Ps5 coordinates of which are (−15, 90),

(6) a sixth axis Ax6 passing through a point Pn6 coordinates of which are (75, 0) and a point Ps6 coordinates of which are (−75, 180),

(7) a seventh axis Ax1 passing through a point Pn7 coordinates of which are (60, 0) and a point Ps7 coordinates of which are (−60, 180),

(8) an eighth axis Ax8 passing through a point Pn8 coordinates of which are (45, 0) and a point Ps8 coordinates of which are (−45, 180),

(9) a ninth axis Ax9 passing through a point Pn9 coordinates of which are (30, 0) and a point Ps9 coordinates of which are (−30, 180),

(10) a tenth axis Ax10 passing through a point Pn10 coordinates of which are (15, 0) and a point Ps10 coordinates of which are (−15, 180),

(11) an eleventh axis Ax11 passing through a point Pn11 coordinates of which are (75, 90) and a point Ps11 coordinates of which are (−75, 270),

(12) a twelfth axis Ax12 passing through a point Pn12 coordinates of which are (60, 90) and a point Ps12 coordinates of which are (−60, 270),

(13) a thirteenth axis Ax13 passing through a point Pn13 coordinates of which are (45, 90) and a point Ps13 coordinates of which are (−45, 270),

(14) a fourteenth axis Ax14 passing through a point Pn14 coordinates of which are (30, 90) and a point Ps14 coordinates of which are (−30, 270), and

(15) a fifteenth axis Ax15 passing through a point Pn15 coordinates of which are (15, 90) and a point Ps15 coordinates of which are (−15, 270), the steps (a) to (h) being the steps of

(a) assuming a great circle that is present on the surface of the phantom sphere and is orthogonal to the axis Ax,

(b) assuming two small circles that are present on the surface of the phantom sphere, that are orthogonal to the axis Ax, and of which absolute values of central angles with the great circle are each 30°,

(c) defining a region, of the surface of the golf ball, which is obtained by dividing the surface of the golf ball at these small circles and which is sandwiched between these small circles,

(d) determining 30240 points, on the region, arranged at intervals of a central angle of 3° in a direction of the axis Ax and at intervals of a central angle of 0.25° in a direction of rotation about the axis Ax,

(e) calculating a length L1 of a perpendicular line that extends from each point to the axis Ax,

(f) calculating a total length L2 by summing 21 lengths L1 calculated on the basis of 21 perpendicular lines arranged in the direction of the axis Ax,

(g) obtaining a transformed data constellation by performing Fourier transformation on a data constellation of 1440 total lengths L2 calculated along the direction of rotation about the axis Ax, and

(h) calculating a peak value and an order of a maximum peak of the transformed data constellation,

a minimum value of 15 orders obtained by executing the steps (a) to (h) is not less than 27,

a maximum value of the 15 orders obtained by executing the steps (a) to (h) is not greater than 37, and

an average of the 15 orders obtained by executing the steps (a) to (h) is not less than 30 and not greater than 34.

2. The golf ball according to claim 1, wherein a total thickness of the cover and the mid layer is not greater than 2.8 mm.

3. The golf ball according to claim 1, wherein an average of the 15 peak values obtained by executing the steps (a) to (h) is not less than 200 mm.

4. The golf ball according to claim 1, wherein a total volume of the dimples is not less than 450 mm3 and not greater than 750 mm3.