1. Field of the Invention
This invention relates to a golf ball.
2. Description of the Related Art
A golf ball dimple plays two roles in flight of a golf ball. In one role, the dimple promotes turbulent transition on boundary layer of the golf ball with disturbing air flow around the golf ball, causes turbulent separation, and reduces pressure drag with separation point of turbulence moved backward thereby. In the other role, the dimple increases lift of the golf ball by promotion of locational difference between an upper separation point and a lower separation point to a flying direction of the golf ball. That is to say, the more a dimple disturbs air flow around a golf ball, the more advanced aerodynamically the dimple becomes.
In view of the roles of dimples described above, combinations of dimples of various configurations have been conventionally proposed to disturb air flow around a golf ball. For example, a golf ball having dimples of which configuration is pentagonal or hexagonal, a golf ball having dimples of which cross-sectional configuration is formed with two curved surfaces having different curvatures (i.e. double-radius), and a combination of circular dimples and non-circular dimples, are proposed.
In conventional products described above, however, effect of disturbing air flow around a golf ball is not sufficient, and desired flying distance is not obtained.
A golf ball is generally formed with a pair of upper and lower half mold, and dimples can not be disposed on the parting line which is the junction line of the mold. Therefore, on the surface of the golf ball, a great circle path (great circle zone) which does not cross dimples is formed.
And, a golf ball flies with back spin in case of being hit by a golf club. It is preferable that the golf ball always flies similarly wherever a rotational axis of the back spin is situated in the golf ball. That is to say, it is preferable that height of trajectory, flying time, and flying distance are same wherever a rotational axis of the back spin is situated in the golf ball. With the great circle path, however, effect of dimples in case of that the rotational axis corresponds to the above mentioned great circle path (in case of so-called seam-hitting) is different from effect of dimples in case of that the rotational axis does not correspond to the great circle path (in case of so-called pole-hitting).
As described above, if the discrepancy of flying ability of the golf ball is caused by the difference of the rotational axis based on the difference among hitting points on the golf ball, the golf ball has bad aerodynamic symmetrical quality, and can not reflect players' skill accurately.
It is therefore an object of the present invention to provide a golf ball which has sufficient effect of disturbing air flow around the golf ball, and can obtain desired flying distance. And it is another object of the present invention to provide a golf ball which has good aerodynamic symmetrical quality, and has no discrepancy of trajectory between in seam-hitting and in pole-hitting.