GOLF BALL WITH IMPROVED SYMMETRY

Abstract

The present invention is directed to a golf ball comprising a substantially seamless design and a plurality of modified dimples, which synergistically improve the golf ball's symmetric flight performance. The modified dimples may be altered in any number of ways including, but not limited to, by means of changing dimple coverage, dimple diameter, dimple depth, dimple edge angle, dimple volume, dimple cross-sectional shape, and dimple plan shape. All such modifications are designed to produce golf balls wherein dimples positioned around the equator are balanced by dimples at the poles and elsewhere, thereby resulting in a ball that flies consistently regardless of orientation.

Description

FIELD OF THE INVENTION

The present invention relates to golf balls, and more particularly, to golf balls having modified dimples that improve symmetric performance.

BACKGROUND OF THE INVENTION

Golf balls generally include a spherical outer surface with a plurality of dimples formed thereon. The dimples on a golf ball improve the aerodynamic characteristics of a golf ball and, therefore, golf ball manufacturers have researched dimple patterns, shape, volume, and cross-section in order to improve the aerodynamic performance of a golf ball. Determining specific dimple arrangements and dimple shapes that result in an aerodynamic advantage requires an understanding of how a golf ball travels through air.

As illustrated in FIG. 1, when a golf ball travels through the air, the air surrounding the ball has different velocities and, thus, different pressures. The air develops a thin boundary layer adjacent to the ball's outer surface. The air exerts maximum pressure at stagnation point, B, on the front of the ball. The air then flows over the sides of the ball and has increased velocity and reduced pressure. The air separates from the surface of the ball at points S1 and S2, leaving a large turbulent flow area called the wake that has low pressure. The difference in the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag, which is the air resistance that acts on the golf ball in the direction opposite the ball's flight direction.

The dimples on a golf ball cause the thin boundary layer to flow in a turbulent manner. Rather than flowing in smooth, continuous layers (i.e., a laminar boundary layer), this turbulent boundary layer has a microscopic pattern of fluctuations and randomized flow. It is the circumference of each dimple, where the dimple wall drops away from the outer surface of the ball, which actually creates the turbulence in the boundary layer. The turbulence energizes the boundary layer and helps move the separation points S1 and S2 further backward, so that the layer stays attached further along the ball's outer surface. As a result, there is a reduction in the area of the wake, an increase in the pressure behind the ball, and a substantial reduction in drag.

The circumference of each dimple is also important in optimizing lift, which is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the air flow that results from the ball's backspin. Due to the backspin, the top of the ball moves with the airflow, which delays the air separation point S1 to a location further backward. Conversely, the bottom of the ball moves against the air flow, which moves the separation point S2 forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air underneath the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball.

By using dimples to decrease drag and increase lift, almost every golf ball manufacturer has increased their golf ball flight distances. However, a golf ball must meet certain standards in order to be included on the official Conforming Golf Balls List (the “List”) produced by the United States Golf Association and The Royal and Ancient Golf Club of St. Andrews, Scotland, the two ruling bodies for the game of golf. Inclusion on the List is important for the commercial success of a golf ball, because it is a requirement for use in competitive golf, and because, even for recreational golf, most serious players won't use a ball unless it appears on the List.

One of the standards, commonly referred to as the “Symmetry Rule,” specifies that a ball must fly essentially the same distance and for essentially the same amount of time regardless of how it is oriented when struck by the golf club. It is important for a ball to have this property not only for inclusion on the List, but also to ensure consistent performance in use. If a ball flies farther when oriented in a certain way, it would cause the golfer to hit the ball farther than intended if the ball happened to be oriented that way before being struck. Commercial golf balls may fly differently in particular orientations, mostly due to asymmetry in the dimple pattern resulting from the inclusion of a straight dimple-free path around the equator of the ball. This path, or “parting line” or “great circle” was necessary to provide a place for the two halves of the mold to separate during the molding process. The effect was worsened by abrasive buffing that was performed on the parting line to remove flash and other molding artifacts. It was discovered that the effect could be minimized or eliminated by altering a group of dimples on each hemisphere away from the equator of the ball, usually by making them shallower.

So-called “seamless” balls were developed which used a corrugated or staggered parting line that weaved around the dimples to disguise its presence and minimize the disruption to the dimple pattern. By removing the visual seam at the equator, it was believed that this type of parting line would also improve symmetry of flight. U.S. Pat. Nos. 6,849,007 and 7,422,529, which are both incorporated herein by reference in their entireties, disclose seamless golf balls. However, it has been found that such balls do not always display satisfactory symmetry of flight performance.

Hence, there still remains a need in the art for a seamless golf ball having improved symmetrical flight performance.

SUMMARY OF THE INVENTION

The present invention comprises a golf ball comprising a substantially seamless appearance, a generally spherical surface, a plurality of dimples formed on the surface, and at least one subset of modified dimples on each hemisphere. In a first embodiment, the modified dimples are preferably altered by changing the dimple edge angle and/or depth and/or volume, or by moderately changing the cross-sectional profile. Said modified dimples are located between about 0° to about 45° latitude (where 0° is at the pole and 90° is at the equator). The modified dimples retain essentially the same appearance as the unmodified dimples. In a second embodiment, the modified dimples are altered by changing the diameter and/or plan shape and/or size, or by substantially changing the cross-sectional profile. In one particular aspect of the embodiment, some dimples may actually be removed from the pattern. In this embodiment, the modified dimples are visually different from the unmodified dimples. In a third embodiment, said modified dimples are located in a band between latitudes of about 30° and about 65°. In all cases, it is preferred that the same modifications are performed on both hemispheres of the ball.

All such dimple modifications are designed to produce golf balls wherein dimples positioned around the seamless parting line are balanced by the subset of modified dimples, thereby resulting in a ball that flies more consistently regardless of orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings which form a part of the specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

FIG. 1 illustrates the air flow on a golf ball in flight;

FIG. 2 is an equatorial view of a golf ball having a substantially seamless design according to the present invention;

FIG. 3 is a polar view of a golf ball having an arrangement of modified dimples according to an embodiment of the present invention;

FIG. 4 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 5 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 6 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 7 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 8 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 9 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 10 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 11 is a polar view of a golf ball having another arrangement of modified dimples according to an embodiment of the present invention;

FIG. 12 is a polar view of a golf having modified dimples around a mid-latitude; and

FIG. 13 is a schematic drawing illustrating a dimple half-profile.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a golf ball comprising a substantially seamless design and a plurality of modified dimples, which synergistically improve the golf ball's symmetric flight performance. The modified dimples may be altered in any number of ways including, but not limited to, by means of changing dimple coverage, dimple diameter, dimple depth, dimple edge angle, dimple volume, dimple cross-sectional shape, and dimple plan shape. All such modifications are designed to produce golf balls wherein dimples positioned around the equator are balanced by dimples at the poles and elsewhere, thereby resulting in a ball that flies consistently regardless of orientation.

FIG. 2 illustrates an exemplary golf ball 10 comprising a substantially seamless appearance, which is produced when the dimple pattern of one hemisphere interdigitates with the dimple pattern of the other hemisphere. As defined herein, the term “substantially seamless” means that a golf ball's equator, parting line, or seam is substantially indiscernible or hidden. Reference numeral 12 indicates the location of the golf ball's equator, and reference numeral 14 indicates a staggered wave parting line. U.S. Pat. No. 7,422,529 B2, which was previously incorporated by reference in its entirety, discusses in further detail the design of substantially seamless golf balls and the molds used to make such golf balls. The cosmetic or aesthetic appearance of a golf ball is improved by producing a golf ball comprising a substantially seamless design and substantially uniform distribution of dimples.

In addition to cosmetic advantages, a substantially seamless design, such as the one depicted in FIG. 2, may correct aerodynamic asymmetry arising from parting lines inherent in the dimple arrangement or from parting lines associated with the manufacturing process. However, the seamless golf ball designs are not always fully symmetrical. As can be seen in FIG. 2, the icosahedron dimple pattern therein comprises offset vertices, i.e. the three vertex dimples marked “A” would ideally be positioned at the vertices of a regular icosahedron that defines the layout. However, it can be seen that in this particular dimple pattern the vertex dimples are located significantly closer to the equator. As a result, the triangular groupings of dimples along the equator, which are characteristic of an icosahedron dimple pattern, are not neatly defined by the vertex dimples and are not the same as the triangular groupings around the poles. Such an asymmetrical arrangement may lead to asymmetrical flight performance, in spite of the seamless appearance. As explained further below, various schemes may be used to compensate for the asymmetry of the pattern, improving the symmetry of the flight performance.

FIG. 3 illustrates a seamless golf ball 10 comprising modified dimples 20, i.e., the dimples shaded in gray, that improve symmetric performance. As used herein, “modified” means altered from the configuration of same-sized dimples 30 in other parts of the ball, or altered from the expected configurations based on the overall pattern of dimples on the ball. The term “dimple” may include any texturizing on the surface of a golf ball, e.g., depressions and projections, which may have a variety of planform shapes, including but not limited to circular, polygonal, oval, or irregular shapes, and a variety of cross-sectional shapes, including but not limited to circular, catenary, elliptical, or conical shapes.

The modified dimples 20 could be altered in any way, including by means of changing their diameter, depth, volume, edge angle, cross-sectional shape, or even plan (top view) shape. However, from a cosmetic standpoint it is usually preferable to modify dimples 20 in a manner that is not visually obvious. For example, a slight adjustment of dimple depth is often sufficient to improve symmetrical performance. Usually, flight symmetry can be improved by altering the modified dimples 20 in such a way as to make them less aggressive aerodynamically, such as by reducing dimple diameter, depth, volume or edge angle. Such reductions make the local boundary layer less turbulent, balancing out the effects caused by asymmetry in the dimple pattern or by buffing of the dimples in the equator region.

In other cases, flight symmetry may be improved by altering the modified dimples 20 in such a way as to make them more aerodynamically aggressive, such as by means of a steeper edge angle, greater volume, or adding sub-dimples, i.e. dimples within a dimple. Such modifications further agitate or energize the local turbulent flow over the dimples, balancing out the effects caused by asymmetry in the dimple pattern or by buffing of the dimples in the equator region. Further discussion of the aerodynamic advantages of sub-dimples can be found in U.S. Pat. No. 6,569,038, which is incorporated herein by reference in its entirety.

The pattern of modified dimples 20 can vary substantially. Generally, in a first embodiment, the modified dimples are preferably altered by changing the dimple edge angle and/or depth and/or volume, or by moderately changing to the cross-sectional profile. The modified dimples retain essentially the same appearance as the unmodified dimples. The modified dimples 20 should be confined to latitudes of about 0° to about 45°, wherein the pole P is at 0° and the equator is at 90°, as illustrated in FIGS. 3-12. For circular dimples, the term “latitude” refers to the latitude at the center point, and for non-circular dimples it refers to the latitude of the centroid of the dimple. In a second embodiment, the same dimples are altered by changing the diameter and/or plan shape and/or size, or by substantially changing the cross-sectional profile and/or the depth and/or the volume. Some dimples may actually be removed from the pattern by reducing their volume by about 100% to about zero. In this embodiment, by virtue of the types or magnitudes of the changes, the modified dimples are visually different from the unmodified dimples.

FIGS. 3-11 illustrate different exemplary pattern arrangements of modified dimples 20, shown looking straight down on the pole (P) of ball 10. In each case, the underlying dimple pattern arrangement is based on an icosahedron pattern comprising 332 dimples. Also, in each case, about sixteen (16) modified dimples 20 at each pole, shaded in gray, are altered. However, the number of modified dimples 20 is not limited to sixteen but may be any suitable number. Similarly, the dimple pattern is not restricted to 332 dimple icosahedron patterns, but may include other polyhedron patterns including, but not limited to, octahedrons (8-sided polyhedrons), dodecahedrons (12-sided polyhedrons), icosidodecahedrons (polyhedrons with twenty triangular faces and twelve pentagonal faces), and various dipyramids (polyhedrons formed from two n-agonal pyramids placed symmetrically base-to-base), as well as patterns that are not polyhedron based.

For FIGS. 3-11, the degree of modification required will vary depending upon the number, pattern, and density of modified dimples 20, as well as the type or types of modifications performed. For a simple, dense patch of dimples 20, as shown in FIG. 3, a simple change, i.e., decrease or increase, in edge angle of about 1-3°, from a typical edge angle e.g., about 14°-16°, might be sufficient. Similarly, the dimple depth may be reduced or increased by at least about 5%, or at least about 10%, or at least about 15%. Likewise, the dimple volume may be reduced or increased by at least about 9%, at least about 27%, or at least about 39%. However, other types of patches, as shown in other figures, might require more or less adjustment. Thus, flexibility is afforded to the designers. For a given patch configuration, various amounts and types of modifications in accordance with the present invention can be tried until satisfactory symmetry of flight performance is achieved.

FIG. 3-11 each illustrates an arrangement of modified dimples comprising one modified dimple 20 at about 0° latitude (forming a pole P) and fifteen other modified dimples 20. More particularly, the arrangement in FIG. 3 further comprises five modified dimples 20 at about 10° latitude (forming a pentagon), and ten modified dimples 20 at about 18° to 21° latitude (forming a pentagon). The arrangement in FIG. 4 further comprises five troikas formed from ten modified dimples 20 at about 30° latitude and five modified dimples 20 at about 40° latitude. The arrangement in FIG. 5 further comprises five troikas formed from five modified dimples 20 at about 18° latitude and ten modified dimples 20 at about 30° latitude. The arrangement in FIG. 6 further comprises fifteen modified dimples at about 30° to 34° latitude (forming a pentagon). The arrangement in FIG. 7 further comprises five modified dimples at about 21° latitude (forming a pentagon) and ten modified dimples 20 at about 30° latitude (forming a set of five pairs). The arrangement in FIG. 8 further comprises five modified dimples 20 at about 10° latitude (forming a pentagon) and ten modified dimples at about 30° latitude (forming a set of five pairs). The arrangement in FIG. 9 further comprises five modified dimples 20 at about 18° latitude (forming, a circle), five modified dimples 20 at about 34° latitude (forming a circle), and five modified dimples 20 at about 40° latitude (forming a circle). The arrangement in FIG. 10 further comprises five modified dimples at about 18° latitude (forming a circle) and ten modified dimples at about 42° latitude (forming a circle). The arrangement in FIG. 11 further comprises five modified dimples at about 10° latitude, five modified dimples at about 21° latitude, and five modified dimples at about 34° latitude (collectively forming a star).

According to one particular strategy for improving flight symmetry, the shaded modified dimples 20 shown in FIGS. 3-11 may have reduced diameter to be less aerodynamically aggressive than the unshaded dimples 30. Smaller dimples 20 are less aerodynamically effective in creating turbulence than larger dimples 30. Thus, by selectively positioning multiple small dimples 20 around the pole, one can balance the aerodynamic effect around the equator with the aerodynamic effect at the polar regions. Conventional sized dimples 30 have a diameter that typically ranges from about 0.100 inches to about 0.180 inches. According to the present invention, modified dimples 20 may be reduced or increased in diameter by at least about 3%, or at least about 10%, or at least about 15% to achieve the desired balance, i.e., a selective decrease or increase in turbulence.

The dimple modifications are not limited to latitudes of 0° to 45° like the pattern configurations depicted in FIGS. 3-11. In another embodiment, dimples located in a band from about 30° to about 65° latitude are modified in a similar fashion.

FIG. 12 illustrates this additional strategy for promoting symmetric aerodynamic performance. Unlike the aforementioned strategies depicted in FIGS. 3-11, the modified dimples 20 are not confined to latitudes between about 0° and about 45° but they may traverse the mid-latitudes as depicted in FIG. 12. The mid-latitudes may be defined as ranging from about 30° to about 65°, or from about 35° to about 60°, or from about 40° to about 55°. FIG. 12 shows fifteen modified dimples 20 at a mid-latitude of about 65°.

The flight symmetry strategy, underlying the dimple arrangement of FIG. 12, seeks to balance the aerodynamic effect around the equator with the aerodynamic effect at the mid-latitudes. The modified dimples 20 in FIG. 12 may be altered by any of the means already described above including, but not limited to, changing dimple diameter, depth, volume or edge angle. As noted above, such changes alter the amount of turbulence induced into the boundary layer, increasing or decreasing the aerodynamic effect at the locations of these modified dimples 20. Alternatively, modified dimples 20 can be altered by including sub-dimples, in order to increase the aerodynamic effect at select mid-latitudes and longitudes.

Whichever configuration is selected for improving the symmetry performance of a golf ball, it is preferred that the same configuration be used on both hemispheres of the ball. This both eases manufacture and prevents new performance asymmetries from being introduced into the design.

Generally, it may be difficult to define and measure a dimple's diameter and edge angle due to the indistinct nature of the boundary dividing the ball's undimpled land surface from the dimple depression itself. FIG. 13 shows a dimple half-profile 34, extending from the dimple centerline 31 to the land surface outside of the dimple 33. Due to the effects of the paint and/or the dimple design itself, the junction between the land surface and the dimple sidewall is not a sharp corner and is therefore indistinct. This makes the measurement of dimple edge angle and dimple diameter somewhat ambiguous. To resolve this problem, the ball phantom surface 32 is constructed above the dimple as a continuation of the land surface 33. A first tangent line T1 is then constructed at a point on the dimple sidewall that is spaced 0.003 inches radially inward from the phantom surface 32 T1 intersects phantom surface 32 at a point P1, which defines a nominal dimple edge position. A second tangent line T2 is then constructed, tangent to the phantom surface 32, at P1. The edge angle is the angle between T1 and T2. The dimple diameter is the distance between P1 and its equivalent point diametrically opposite along the dimple perimeter. Alternatively, it is twice the distance between P1 and the dimple centerline 31, measured in a direction perpendicular to centerline 31. The dimple depth is the distance measured along a ball radius from the phantom surface of the ball to the deepest point on the dimple. The dimple volume is the space enclosed between the phantom surface 32 and the dimple surface 34 (extended along T1 until it intersects the phantom surface).

The dimple patterns of the present invention can be used with any type of golf ball with any playing characteristics. For example, the dimple pattern can be used with conventional golf balls, solid or wound. These balls typically have at least one core layer and at least one cover layer. Wound balls typically have a spherical solid rubber or liquid filled center with a tensioned elastomeric thread wound thereon. Wound balls typically travel a shorter distance, however, when struck as compared to a solid ball. The cores of solid balls are generally formed of a polybutadiene composition. In addition to one-piece cores, solid cores can also contain a number of layers, such as in a dual core golf ball. Covers, for solid or wound balls, are generally formed of ionomer resins, balata, or polyurethane, and can consist of a single layer or include a plurality of layers and, optionally, at least one intermediate layer disposed about the core.

While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives of the present invention, it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Additionally, feature(s) and/or element(s) from any embodiment may be used singly or in combination with other embodiment(s) and steps or elements from methods in accordance with the present invention can be executed or performed in any suitable order. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments, which would come within the spirit and scope of the present invention.

Claims

1. A golf ball comprising: wherein the subset of dimples is selected from the group consisting of

a substantially seamless appearance;

a generally spherical surface;

a plurality of dimples formed on the surface; and

a subset of modified dimples located between about 0° to about 45° latitude;

(i) modified dimples comprising a modified dimple depth; (ii) modified dimples comprising a modified dimple diameter; (iii) modified dimples comprising a modified dimple edge angle; (iv) modified dimples comprising a modified dimple volume; (v) modified dimples comprising a modified dimple cross-sectional shape; and (vi) modified dimples comprising a modified dimple plan shape.

2. The golf ball of claim, wherein the subset of dimples is modified dimples comprising a modified dimple depth, and wherein the dimple depth is reduced or increased by at least about 5%.

3. The golf ball of claim 2, wherein the dimple depth is reduced or increased by at least about 10%.

4. The golf ball of claim 2, wherein the dimple depth is reduced or increased by at least about 15%.

5. The golf ball of claim 1, wherein the subset of dimples is modified dimples comprising a modified dimple diameter, and wherein the dimple diameter is reduced or increased by at least about 3%.

6. The golf bail of claim 5, wherein the dimple diameter is reduced or increased by at least about 10%.

7. The golf ball of claim 5, wherein the dimple diameter is reduced or increased by at least about 15%.

8. The golf ball of claim 1, wherein the subset of dimples is modified dimples comprising a modified dimple edge angle, and wherein the dimple edge angle is reduced or increased by about 1-3°.

9. The golf ball of claim 1, wherein the subset of dimples is modified dimples comprising a modified dimple Volume, and wherein the dimple volume is reduced or increased by at least about 9%.

10. The golf ball of claim 9, wherein the dimple volume is reduced or increased by at least about 27%.

11. The golf ball of claim 9, wherein the dimple volume is reduced or increased by at least about 39%.

12. The golf ball of claim 9, wherein the dimple volume is reduced by about 100%.

13. The golf ball of claim 1, wherein the subset of dimples is modified dimples comprising a modified dimple cross-sectional shape, and wherein the cross-sectional shape is selected from the group consisting of circular, catenary, elliptical, and conical shapes.

14. The golf ball of claim 1, wherein the subset of dimples is modified dimples comprising a modified dimple p,an shape, and wherein the plan shape is selected from the group consisting of circular, polygonal, oval, and irregular shapes.

15. A golf ball comprising: wherein the subset of dimples is selected from the group consisting of

a substantially seamless appearance;

a generally spherical surface;

a plurality of dimples formed on the surface; and

a subset of modified dimples located between about 30° to about 65° latitude;

(i) modified dimples comprising a modified dimple depth; (ii) modified dimples comprising a modified dimple diameter; (iii) modified dimples comprising a modified dimple edge angle; (iv) modified dimples comprising a modified dimple volume; (v) modified dimples comprising a modified dimple cross-sectional shape; and (vi) modified dimples comprising a modified dimple plan shape.

16. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple depth, and wherein the dimple depth is reduced or increased by at least about 5%.

17. The golf ball of claim 16, wherein the dimple depth is reduced or increased by at least about 10%.

18. The golf ball of claim 16, wherein the dimple depth is reduced or increased by at least about 15%.

19. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple diameter, and wherein the dimple diameter is reduced or increased by at least about 3%.

20. The golf ball of claim 19, wherein the dimple diameter is reduced or increased by at least about 10%.

21. The golf ball of claim 19, wherein the dimple diameter is reduced or increased by at least about 15%.

22. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple edge angle, and wherein the dimple edge angle is reduced or increased by about 1-3°.

23. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple volume, and wherein the dimple volume is reduced or increased by at least about 9%.

24. The golf ball of claim 23, wherein the dimple volume is reduced or increased by at least about 27%.

25. The golf ball of claim 23, wherein the dimple volume is reduced or increased by at least about 39%.

26. The golf ball of claim 23, wherein the dimple volume is reduced by about 100%.

27. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple cross-sectional shape, and wherein the cross-sectional shape is selected from the group consisting of circular, catenary, elliptical and conical shapes.

28. The golf ball of claim 15, wherein the subset of dimples is modified dimples comprising a modified dimple plan shape, and wherein the plan shape is selected from the group consisting of circular, polygonal oval, and irregular shapes.