A golf ball includes a spherical core and a cover. The core includes an inner core, a mid core, and an outer core. A hardness H(C) is equal to or greater than a hardness H(B). A hardness H(E) is equal to or greater than a hardness H(D). An angle α is calculated by (Formula 1) from a thickness Y (mm) of the mid core, the hardness H(C), and the hardness H(D). An angle β is calculated by (Formula 2) from a thickness Z (mm) of the outer core, the hardness H(E), and the hardness H(F). Each of the angle α and a difference (α−β) between the angles α and β is equal to or greater than 0°. The golf ball has excellent flight performance upon a shot with a driver. α=(180/π)* a tan [{ H ( D )− H ( C )}/ Y] (Formula 1) β=(180/π)* a tan [{ H ( F ) −H ( E ) }/Z] (Formula 2)
1. A golf ball comprising a spherical core and a cover positioned outside the core, wherein
the core includes an inner core, a mid core positioned outside the inner core, and an outer core positioned outside the mid core,
a JIS-C hardness H(C) at a point C present outward from a boundary between the inner core and the mid core in a radius direction by 1 mm is equal to or greater than a JIS-C hardness H(B) at a point B present inward from the boundary between the inner core and the mid core in the radius direction by 1 mm,
a JIS-C hardness H(E) at a point E present outward from a boundary between the mid core and the outer core in the radius direction by 1 mm is equal to or greater than a JIS-C hardness H(D) at a point D present inward from the boundary between the mid core and the outer core in the radius direction by 1 mm, and
when an angle (degree) calculated by (Formula 1) from a thickness Y (mm) of the mid core, the hardness H(C), and the hardness H(D) is defined as an angle α and an angle (degree) calculated by (Formula 2) from a thickness Z (mm) of the outer core, the hardness H(E), and a JIS-C hardness H(F) at a point F located on a surface of the core is defined as an angle β:
α=(180/π)* a tan [{ H ( D )− H ( C )}/ Y] (Formula 1); and
β=(180/π)* a tan [{ H ( F )− H ( E )}/ Z] (Formula 2),
the angle α is equal to or greater than 0°, and
a difference (α−β) between the angle α and the angle β is equal to or greater than 0°,
wherein the inner core, the mid core, and the outer core comprise different thermoset rubber compositions.
2. The golf ball according to claim 1 , wherein the angle β is equal to or greater than −20° but equal to or less than +20°.
3. The golf ball according to claim 1 , wherein
a ratio (Y/X) of the thickness Y of the mid core relative to a radius X of the inner core is equal to or greater than 0.5 but equal to or less than 2.0, and
a ratio (Z/X) of the thickness Z of the outer core relative to the radius X is equal to or greater than 0.5 but equal to or less than 2.5.
4. The golf ball according to claim 1 , wherein
a ratio (S 2 /S 1 ) of a cross-sectional area S 2 of the mid core relative to a cross-sectional area S 1 of the inner core on a cut surface of the core that has been cut into two halves is equal to or greater than 1.0 but equal to or less than 8.0, and
a ratio (S 3 /S 1 ) of a cross-sectional area S 3 of the outer core relative to the cross-sectional area S 1 on the cut surface of the core is equal to or greater than 2.5 but equal to or less than 12.5.
5. The golf ball according to claim 1 , wherein
a ratio (V 2 /V 1 ) of a volume V 2 of the mid core relative to a volume V 1 of the inner core is equal to or greater than 2.5 but equal to or less than 20.0, and
a ratio (V 3 /V 1 ) of a volume V 3 of the outer core relative to the volume V 1 is equal to or greater than 10.0 but equal to or less than 57.0.
6. The golf ball according to claim 1 , further comprising a mid layer between the core and the cover.
7. The golf ball according to claim 6 , wherein the mid layer includes an inner mid layer and an outer mid layer positioned outside the inner mid layer.
8. The golf ball according to claim 1 , wherein the cover includes an inner cover and an outer cover positioned outside the inner cover.