Abstract: According to the present invention, an output inductor, rather than being composed of a separate inductor element, is magnetically coupled to a third inductor and a fourth inductor constituting a second transformer. Thus, compared to the case in which the output inductor is composed of the separate inductor element independently of the third inductor and the fourth inductor, the cost as well as the volume of the DC-DC conversion device can be reduced.

Abstract: An auxiliary air chamber member includes: a lower surface arranged on the outer circumferential surface side of a well section; an upper surface arranged outward from the lower surface; an auxiliary air chamber formed between the upper surface and the lower surface; a pair of edges that join the lower surface and the upper surface on both sides in the width direction and that engage with the well section; and a plurality of joining sections that partially join the upper surface and the lower surface as a result of being sunk into the interior of the auxiliary air chamber from the upper surface and the lower surface between one edge and the other edge along the wheel width direction Y. The joining points of the individual joining sections are arranged on the strong axis of the cross-sectional main axis of the auxiliary air chamber member in an axial right section.

Abstract: A wheel width direction (Y) cross-section of a well portion (11A) includes a hump portion (H1) and a hump portion (H2) that protrude to the outside (Z1) in a wheel radial direction (Z), and a curved surface (11s) that connects hump portion (H1) and hump portion (H2) and is sunken to the inside (Z2) in the wheel radial direction (Z). The curved surface (11s) is formed as a series of circular arcs (11e, 11f, 11g) that have centers of curvature (O1, O2, O3) that are further to the outside (Z1) in the wheel radial direction (Z) than the curved surface (11s).

Abstract: According to the present invention, a wheel width direction (Y) cross-section of a well portion (11A) comprises: a hump portion (H1) and a hump portion (H2) that protrude to the outside (Z1) in a wheel radial direction (Z); and a curved surface (11s) that connects hump portion (H1) and hump portion (H2) and is sunken to the inside (Z2) in the wheel radial direction (Z). The curved surface (11s) is formed as a series of circular arcs (11e, 11f, 11g) that have centers of curvature (O1, O2, O3) that are further to the outside (Z1) in the wheel radial direction (Z) than the curved surface (11s).

Abstract: An auxiliary air chamber member includes: a lower surface arranged on the outer circumferential surface side of a well section; an upper surface arranged outward from the lower surface; an auxiliary air chamber formed between the upper surface and the lower surface; a pair of edges that join the lower surface and the upper surface on both sides in the width direction and that engage with the well section; and a plurality of joining sections that partially join the upper surface and the lower surface as a result of being sunk into the interior of the auxiliary air chamber from the upper surface and the lower surface between one edge and the other edge along the wheel width direction Y. The joining points of the individual joining sections are arranged on the strong axis of the cross-sectional main axis of the auxiliary air chamber member in an axial right section.