GOLF BALL

This application claims priority on Patent Application No. 2008-14839 filed in JAPAN on Jan. 25, 2008. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. In particular, the present invention relates to the dimple patterns of golf balls.

2. Description of the Related Art

Golf balls have numerous dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball surface shift backwards leading to the reduction of a drag. The turbulent flow separation promotes the displacement between the separating point on the upper side and the separating 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 the drag and the enhancement of the lift force are referred to as a “dimple effect”.

The United States Golf Association (USGA) has established the rules about symmetry of golf balls. According to the rules, the trajectories during PH (pole horizontal) rotation and the trajectories during POP (pole over pole) rotation are compared with each other. A golf ball having a large difference between these two trajectories, that is, inferior aerodynamic symmetry, does not be conformed to the rules. A golf ball with inferior aerodynamic symmetry has a short flight distance because the aerodynamic characteristic of the golf ball for PH rotation or for POP rotation is inferior. The rotation axis for PH rotation posseses through the poles of the golf ball, and the rotation axis for POP rotation is orthogonal to the rotation axis for PH rotation.

The dimples can be arranged by using a regular polyhedron that is inscribed in a phantom sphere of a golf ball. In this arrangement method, the surface of the phantom sphere is divided into a plurality of units by division lines obtained by projecting the sides of the polyhedron on the spherical surface. The dimple pattern of one unit is developed all over the phantom sphere. According to this dimple pattern, the aerodynamic characteristic in the case where a line passing through a vertex of the regular polyhedron is a rotation axis is different from that in the case where a line passing through a center of a surface of the regular polyhedron is a rotation axis. Such a golf ball has inferior aerodynamic symmetry.

JP-A-S50-8630 discloses a golf ball having an improved dimple pattern. The surface of the golf ball is divided by an icosahedron that is inscribed in the phantom sphere thereof. Based on this division, dimples are arranged on the surface of the golf ball. According to this dimple pattern, the number of great circles that do not intersect any dimples is 1. This great circle is identical with an equator of the golf ball. The region near the equator is a unique region.

Generally, a golf ball is formed with a mold having upper and lower mold halves. The mold has a parting line. A golf ball obtained with this mold has a seam at a position along the parting line. Through this forming, spew occurs along the seam. The spew is removed by means of cutting. By cutting the spew, the dimples near the seam are deformed. In addition, the dimples near the seam tend to be orderly arranged. The seam is located along the equator of the golf ball. The region near the equator is a unique region.

A mold having a corrugated parting line has been used. A golf ball obtained with this mold has dimples on the equator thereof. The dimples on the equator contribute to eliminating the uniqueness of the region near the equator. However, the uniqueness is not sufficiently eliminated. This golf ball has insufficient aerodynamic symmetry.

U.S. Pat. No. 4,744,564 (JP-A-S61-284264) discloses a golf ball in which the dimples near the seam are greater in volume than the dimples near the poles. This volume difference contributes to eliminating the uniqueness of the region near the equator.

A golf ball disclosed in U.S. Pat. No. 4,744,564 eliminates, by the volume difference of dimple, the disadvantage caused by the dimple pattern. The disadvantage is eliminated not by modification of the dimple pattern. In the golf ball, the potential of the dimple pattern is sacrificed. The flight distance of the golf ball is insufficient.

Research has been conducted to determine the causes of the uniqueness of the region near the equator, and the consequent insufficient symmetry and flight distance. However, the causes have not been cleared yet, and a general theory for the improvements has not been established. In the conventional development of golf balls, design, experimental production, and evaluation are conducted through trials and errors.

An objective of the present invention is to provide a golf ball having excellent aerodynamic symmetry and a long flight distance. Another objective of the present invention is to provide a method for easily and accurately evaluating the aerodynamic characteristic of a golf ball.

SUMMARY OF THE INVENTION

The inventors of the present invention have found, as a result of thorough research, that aerodynamic symmetry and a flight distance depend heavily on a specific parameter. Based on this finding, the inventors have completed a method for evaluating a golf ball with high accuracy. In addition, by using the evaluation method, the inventors have completed creating a golf ball having excellent aerodynamic symmetry and a long flight distance.

An evaluation method according to the present invention comprises:

a calculation step of calculating a data constellation, regarding a parameter dependent on a surface shape of a golf ball having numerous dimples on its surface, based on a surface shape appearing at a predetermined point moment by moment during rotation of the golf ball; and

a determination step of determining an aerodynamic characteristic of the golf ball based on the data constellation.

Preferably, at the determination step, the aerodynamic characteristic of the golf ball is determined based on a fluctuation range of the data constellation. Preferably, at the calculation step, the data constellation is calculated throughout one rotation of the golf ball. Preferably, at the calculation step, the data constellation is calculated based on a shape of a surface near a great circle orthogonal to an axis of the rotation.

Preferably, at the calculation step, the data constellation is calculated based on a parameter dependent on a distance between an axis of the rotation and the surface of the golf ball. At the calculation step, the data constellation may be calculated based on a parameter dependent on a volume of space between a surface of a phantom sphere and the surface of the golf ball.

Another evaluation method according to the present invention comprises:

a first calculation step of calculating a first data constellation, regarding a parameter dependent on a surface shape of a golf ball having numerous dimples on its surface, based on a surface shape appearing at a predetermined point moment by moment during rotation of the golf ball about a first axis;

a second calculation step of calculating a second data constellation, regarding a parameter dependent on the surface shape of the golf ball, based on a surface shape appearing at a predetermined point moment by moment during rotation of the golf ball about a second axis; and

a determination step of determining an aerodynamic characteristic of the golf ball based on comparison of the first data constellation and the second data constellation.

Preferably, the aerodynamic characteristic determined at the determination step is aerodynamic symmetry.

A golf ball designing process according to the present invention comprises:

a step of determining positions and shapes of numerous dimples located on a surface of a golf ball;

a calculation step of calculating a data constellation, regarding a parameter dependent on a surface shape of the golf ball, based on a surface shape appearing at a predetermined point moment by moment during rotation of the golf ball,;

a determination step of determining an aerodynamic characteristic of the golf ball based on the data constellation; and

a step of changing the positions or the shapes of the dimples when the aerodynamic characteristic is insufficient.

A golf ball according to the present invention has values Ad1 and Ad2 which are obtained by the following steps (1) to (18):

(1) assuming a line connecting both poles of the golf ball as a first rotation axis;

(2) assuming a great circle which exists on a surface of a phantom sphere of the golf ball and is orthogonal to the first rotation axis;

(3) assuming two small circles which exist on the surface of the phantom sphere of the golf ball, which are orthogonal to the first rotation axis, and of which an absolute value of a central angle with the great circle is 30°;

(4) defining, among the surface of the phantom sphere, a region sandwiched between the two small circles by dividing the phantom sphere at the two small circles;

(5) determining 30240 points arranged at an interval of a central angle of 3° in a direction of the first rotation axis and at an interval of a central angle of 0.25° in a direction of rotation about the first rotation axis;

(6) calculating a length L1 of a perpendicular line which extends from each point to the first rotation axis;

(7) calculating a total length L2 by summing 21 lengths L1 calculated based on 21 perpendicular lines arranged in the direction of the first rotation axis;

(8) determining a maximum value and a minimum value among 1440 total lengths L2 calculated along the direction of rotation about the first rotation axis, and calculating a fluctuation range by subtracting the minimum value from the maximum value;

(9) calculating the value Ad1 by dividing the fluctuation range by a total volume of dimples;

(10) assuming a second rotation axis orthogonal to the first rotation axis assumed at the step (1);

(11) assuming a great circle which exists on the surface of the phantom sphere of the golf ball and is orthogonal to the second rotation axis;

(12) assuming two small circles which exist on the surface of the phantom sphere of the golf ball, which are orthogonal to the second rotation axis, and of which an absolute value of a central angle with the great circle is 30°;

(13) defining, among the surface of the phantom sphere, a region sandwiched between the two small circles by dividing the phantom sphere at the two small circles;

(14) determining 30240 points arranged at an interval of a central angle of 3° in a direction of the second rotation axis and at an interval of a central angle of 0.25° in a direction of rotation about the second rotation axis;

(15) calculating a length L1 of a perpendicular line which extends from each point to the second rotation axis;

(16) calculating a total length L2 by summing 21 lengths LI calculated based on 21 perpendicular lines arranged in the direction of the second rotation axis;

(17) determining a maximum value and a minimum value among 1440 total lengths L2 calculated along the direction of rotation about the second rotation axis, and calculating a fluctuation range by subtracting the minimum value from the maximum value; and

(18) calculating the value Ad2 by dividing the fluctuation range by the total volume of the dimples. The values Ad1 and Ad2 are equal to or less than 0.009 mm⁻².

Preferably, an absolute value of a difference between the values Ad1 and Ad2 is equal to or less than 0.005 mm⁻².

Another golf ball according to the present invention has values Ad3 and Ad4 which are obtained by the following steps (1) to (16):

(1) assuming a line connecting both poles of the golf ball as a first rotation axis;

(2) assuming a great circle which exists on a surface of a phantom sphere of the golf ball and is orthogonal to the first rotation axis;

(4) defining, among the phantom sphere, a region sandwiched between the two small circles by dividing the phantom sphere at the two small circles;

(5) assuming 120 minute regions by dividing the region at an interval of a central angle of 3° in a direction of rotation about the first rotation axis;

(6) calculating a volume of space between the surface of the phantom sphere and a surface of the golf ball in each minute region;

(7) determining a maximum value and a minimum value among the 120 volumes calculated along the direction of rotation about the first rotation axis, and calculating a fluctuation range by subtracting the minimum value from the maximum value;

(8) calculating the value Ad3 by dividing the fluctuation range by a total volume of dimples;

(9) assuming a second rotation axis orthogonal to the first rotation axis assumed at the step (1);

(10) assuming a great circle which exists on the surface of the phantom sphere of the golf ball and is orthogonal to the second rotation axis;

(11) assuming two small circles which exist on the surface of the phantom sphere of the golf ball, which are orthogonal to the second rotation axis, and of which an absolute value of a central angle with the great circle is 30°;

(12) defining, among the phantom sphere, a region sandwiched between the two small circles by dividing the phantom sphere at the two small circles;

(13) assuming 120 minute regions by dividing the region at an interval of a central angle of 3° in a direction of rotation about the second rotation axis;

(14) calculating a volume of space between the surface of the phantom sphere and a surface of the golf ball in each minute region;

(15) determining a maximum value and a minimum value among the 120 volumes calculated along the direction of rotation about the second rotation axis, and calculating a fluctuation range by subtracting the minimum value from the maximum value; and

(16) calculating the value Ad4 by dividing the fluctuation range by a total volume of dimples. The values Ad3 and Ad4 which are equal to or less than 0.008.

Preferably, an absolute value of a difference between the values Ad3 and Ad4 is equal to or less than 0.003.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;

FIG. 3 is an enlarged front view of the golf ball in FIG. 1;

FIG. 4 is a plan view of the golf ball in FIG. 3;

FIG. 5 is a schematic view for explaining an evaluation method according to one embodiment of the present invention;

FIG. 6 is a schematic view for explaining the evaluation method in FIG. 5;

FIG. 7 is a schematic view for explaining the evaluation method in FIG. 5;

FIG. 8 is a graph showing an evaluation result of the golf ball in FIG. 3;

FIG. 9 is a graph showing another evaluation result of the golf ball in FIG. 3;

FIG. 10 is a schematic view for explaining an evaluation method according to an alternative embodiment of the present invention;

FIG. 11 is a schematic view for explaining the evaluation method in FIG. 10;

FIG. 12 is a graph showing an evaluation result of the golf ball in FIG. 3;

FIG. 13 is a graph showing another evaluation result of the golf ball in FIG. 3;

FIG. 14 is a front view of a golf ball according to a comparative example;

FIG. 15 is a plan view of the golf ball in FIG. 14;

FIG. 16 is a graph showing an evaluation result of the golf ball in FIG. 14;

FIG. 17 is a graph showing another evaluation result of the golf ball in FIG. 14;

FIG. 18 is a graph showing another evaluation result of the golf ball in FIG. 14; and

FIG. 19 is a graph showing another evaluation result of the golf ball in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

Golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6. On the surface of the cover 6, numerous dimples 8 are formed. Of the surface of the golf ball 2, a part except for the dimples 8 is a land 10. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 6 although these layers are not shown in the drawing. A mid layer may be provided between the core 4 and the cover 6.

The golf ball 2 has a diameter of 40 mm or greater and 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of the air resistance, the diameter is more preferably equal to or less than 44 mm, and particularly preferably equal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g or greater and 50 g or less. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g, and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Illustrative examples of the base rubber for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. Two or more types of rubbers may be used in combination. In light of resilience performance, polybutadienes are preferred, and high-cis polybutadiene is particularly preferred.

In order to crosslink the core 4, a co-crosslinking agent can be used. Preferable examples of co-crosslinking agent in light of resilience performance include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. Preferably, the rubber compound includes an organic peroxide together with a co-crosslinking agent. Examples of suitable organic peroxide 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.

The rubber composition for the core 4 may include various additives, such as a sulfur compound, a filler, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant at an adequate amount as needed. The rubber composition may include a crosslinked rubber powder or a synthetic resin powder.

The core 4 has a diameter of preferably 30.0 mm or greater, particularly preferably 38.0 mm or greater. The core 4 has a diameter of preferably 42.0 mm or less, and particularly preferably 41.5 mm or less. The core 4 may be formed with two or more layers.

One example of suitable polymer for the cover 6 is ionomer resin. Examples of preferable ionomer resin include binary copolymers formed with α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Other examples of preferable ionomer resin include ternary copolymers formed with 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. In the binary copolymer and ternary copolymer, preferable α-olefin is ethylene and propylene, while preferable α,β-unsaturated carboxylic acid is acrylic acid and methacrylic acid. In the binary copolymer and ternary copolymer, a part of carboxyl groups is neutralized with a metal ion. Some of the metal ion for neutralization are sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion.

Other polymer may be used instead of or together with ionomer resin. Examples of the other polymer include thermoplastic polyurethane elastomers, thermoplastic styrene elastomers, thermoplastic polyamide elastomers,thermoplastic polyester elastomers, and thermoplastic polyolefin elastomers.

A coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material and a fluorescent brightener are blended into the cover 6 at an adequate amount as needed. For the purpose of adjusting specific gravity, powder of a metal with a high specific gravity such as tungsten and molybdenum may be blended with the cover 6.

The cover 6 has a thickness of preferably 0.3 mm or greater and particularly preferably 0.5 mm or greater. The cover 6 has a thickness of preferably 2.5 mm or less and particularly preferably 2.2 mm or less. The cover 6 has a specific gravity of preferably 0.90 or greater and particularly preferably 0.95 or greater. The cover 6 has a specific gravity of preferably 1.10 or less and particularly preferably 1.05 or less. The cover 6 may be formed with two or more layers.

FIG. 2 shows a partially enlarged cross-sectional view of the golf ball 2 in FIG. 1. In FIG. 2, a cross section along a plane passing through the center (deepest part) of the dimple 8 and the center of the golf ball 2 is shown. In FIG. 2, the top-to-bottom direction is the depth direction of the dimple 8. What is indicated by a chain double-dashed line in FIG. 2 is the surface of a phantom sphere 12. The surface of the phantom sphere 12 corresponds to the surface of the golf ball 2 when it is postulated that no dimple 8 exists. The dimple 8 is recessed from the surface of the phantom sphere 12. The land 10 agrees with the surface of the phantom sphere 12.

In FIG. 2, what is indicated by a double ended arrow Di is the diameter of the dimple 8. This diameter Di is a distance between two tangent points Ed appearing on a tangent line TA which is drawn tangent to the far opposite ends of the dimple 8. The tangent point Ed is also a edge of the dimple 8. The edge Ed defines the contour of the dimple 8. The diameter Di is preferably 2.00 mm or greater and 6.00 mm or less. By setting the diameter Di to be equal to or greater than 2.00 mm, great dimple effect can be achieved. In this respect, the diameter Di is more preferably equal to or greater than 2.20 mm, and particularly preferably equal to or greater than 2.40 mm. By setting the diameter Di to be equal to or less than 6.00 mm, fundamental feature of the golf ball 2 being substantially a sphere is not impaired. In this respect, the diameter Di is more preferably equal to or less than 5.80 mm, and particularly preferably equal to or less than 5.60 mm.

FIG. 3 shows an enlarged front view of the golf ball 2 in FIG. 1. FIG. 4 shows a plan view of the golf ball 2 in FIG. 3. In FIG. 3, when the surface of the golf ball 2 is divided into 12 units, kinds of the dimples 8 in one unit are indicated by the reference signs A to D. All the dimples 8 have a circular plane shape. The golf ball 2 has dimples A with a diameter of 4.20 mm, dimples B with a diameter of 3.80 mm, dimples C with a diameter of 3.00 mm, and dimples D with a diameter of 2.60 mm. The dimple pattern of this unit is developed all over the surface of the golf ball 2. When developing the dimple pattern, the positions of the dimples 8 are finely adjusted for each unit. The number of the dimples A is 216; the number of the dimples B is 84; the number of the dimples C is 72; and the number of the dimples D is 12. The total number of the dimples 8 is 384. The latitude and longitude of these dimples 8 are shown in the following Tables 1 to 5.

TABLE 1

Dimple Arrangement

Latitude
Longitude

Kind
(degree)
(degree)

1
A
85.691
67.318

2
A
81.286
199.300

3
A
81.286
280.700

4
A
75.987
334.897

5
A
75.987
145.103

6
A
75.303
23.346

7
A
71.818
100.896

8
A
65.233
133.985

9
A
65.233
346.015

10
A
65.189
39.055

11
A
65.060
75.516

12
A
61.445
158.091

13
A
61.445
321.909

14
A
61.070
252.184

15
A
61.070
227.816

16
A
60.847
108.080

17
A
57.147
58.461

18
A
55.279
288.525

19
A
55.279
191.475

20
A
54.062
211.142

21
A
54.062
268.858

22
A
54.041
350.081

23
A
53.504
126.971

24
A
53.069
307.598

25
A
53.069
172.402

26
A
49.772
228.202

27
A
49.526
107.190

28
A
49.456
249.324

29
A
47.660
15.660

30
A
47.244
67.559

31
A
46.729
50.974

32
A
46.350
323.515

33
A
46.350
156.485

34
A
45.673
34.636

35
A
44.933
339.633

36
A
44.933
140.367

37
A
44.882
295.495

38
A
44.882
184.505

39
A
44.242
359.196

40
A
42.196
120.253

41
A
40.522
237.865

42
A
36.705
73.432

43
A
36.500
11.475

44
A
36.079
45.962

45
A
35.806
193.343

46
A
35.806
286.657

47
A
35.713
250.884

48
A
35.005
131.984

49
A
34.833
177.642

50
A
34.833
302.358

51
A
34.560
207.408

52
A
34.560
272.592

53
A
33.900
86.867

54
A
30.252
359.718

55
A
30.080
119.572

56
A
29.307
239.817

57
A
26.977
337.630

58
A
26.967
217.628

59
A
26.522
53.578

60
A
26.233
313.918

61
A
26.233
166.082

62
A
25.945
77.590

63
A
25.668
199.232

64
A
25.668
280.768

65
A
25.588
40.979

66
A
23.737
107.042

67
A
22.987
91.662

68
A
20.802
269.276

69
A
20.537
29.857

70
A
19.971
149.439

71
A
18.932
325.930

72
A
18.877
118.043

73
A
18.548
209.356

74
A
17.974
1.141

75
A
17.973
241.141

76
A
16.138
138.223

77
A
15.811
220.861

78
A
15.723
161.053

79
A
15.558
340.213

80
A
15.057
54.091

TABLE 2

Dimple Arrangement

Latitude
Longitude

Kind
(degree)
(degree)

81
A
15.011
66.203

82
A
14.992
186.255

83
A
14.535
312.879

84
A
14.152
282.171

85
A
14.107
77.896

86
A
14.065
197.945

87
A
11.930
127.300

88
A
11.464
351.579

89
A
11.459
231.583

90
A
9.454
267.333

91
A
9.446
27.328

92
A
8.895
147.125

93
A
7.578
116.668

94
A
6.950
301.950

95
A
6.664
2.030

96
A
6.663
242.035

97
A
5.164
289.168

98
A
4.715
158.076

99
A
4.699
71.498

100
A
4.677
38.046

101
A
4.670
191.529

102
A
4.386
169.415

103
A
4.370
49.384

104
A
4.189
104.832

105
A
3.868
253.091

106
A
3.866
13.085

107
A
3.702
277.673

108
A
3.284
343.658

109
A
3.276
223.664

110
A
−1.138
263.313

111
A
−1.145
23.305

112
A
−3.156
296.805

113
A
−3.730
117.727

114
A
−5.028
98.222

115
A
−5.301
66.255

116
A
−5.320
186.266

117
A
−5.560
1.243

118
A
−5.562
241.252

119
A
−5.603
174.914

120
A
−5.608
54.904

121
A
−6.610
77.578

122
A
−6.651
197.586

123
A
−6.740
316.100

124
A
−9.310
219.881

125
A
−9.379
327.238

126
A
−9.834
338.778

127
A
−11.302
139.305

128
A
−11.465
304.650

129
A
−11.656
258.951

130
A
−11.661
18.940

131
A
−13.404
89.766

132
A
−13.611
208.915

133
A
−13.916
293.296

134
A
−14.848
128.252

135
A
−14.902
247.791

136
A
−14.902
7.778

137
A
−14.989
104.117

138
A
−15.045
116.532

139
A
−15.350
60.821

140
A
−15.357
180.810

141
A
−15.509
150.296

142
A
−15.563
30.304

143
A
−15.581
281.633

144
A
−16.386
269.878

145
A
−20.645
328.793

146
A
−21.042
311.017

147
A
−23.090
19.912

148
A
−23.809
172.748

149
A
−23.819
52.779

150
A
−24.625
69.349

151
A
−24.650
189.318

152
A
−25.075
261.401

153
A
−25.417
133.803

154
A
−25.453
156.111

155
A
−25.495
36.142

156
A
−25.836
276.531

157
A
−25.899
100.191

158
A
−26.295
4.604

159
A
−26.501
351.270

160
A
−26.527
248.419

TABLE 3

Dimple Arrangement

Latitude
Longitude

Kind
(degree)
(degree)

161
A
−28.009
338.630

162
A
−28.872
320.134

163
A
−29.656
216.752

164
A
−33.266
165.532

165
A
−33.289
45.587

166
A
−33.571
26.465

167
A
−34.810
121.946

168
A
−34.881
92.123

169
A
−35.921
70.481

170
A
−35.948
190.419

171
A
−35.969
106.249

172
A
−36.237
241.545

173
A
−36.677
269.561

174
A
−36.780
309.211

175
A
−38.058
3.003

176
A
−40.005
57.051

177
A
−41.376
295.414

178
A
−41.680
176.151

179
A
−42.945
217.442

180
A
−44.210
21.410

181
A
−44.278
258.399

182
A
−44.396
320.927

183
A
−44.500
159.270

184
A
−44.941
115.286

185
A
−44.961
279.798

186
A
−46.360
142.796

187
A
−48.437
243.048

188
A
−49.314
5.102

189
A
−49.778
68.092

190
A
−50.602
188.133

191
A
−52.599
226.337

192
A
−52.972
309.720

193
A
−52.982
127.612

194
A
−53.185
348.010

195
A
−53.519
169.798

196
A
−54.005
207.538

197
A
−54.153
290.081

198
A
−54.419
88.781

199
A
−54.511
328.756

200
A
−55.417
108.606

201
A
−56.454
49.583

202
A
−59.768
242.157

203
A
−60.664
3.667

204
A
−61.192
142.183

205
A
−61.580
72.132

206
A
−62.555
192.606

207
A
−63.591
27.254

208
A
−64.742
166.150

209
A
−71.117
239.508

210
A
−71.895
0.773

211
A
−73.954
321.276

212
A
−75.160
276.770

213
A
−75.592
156.215

214
A
−81.496
104.116

215
A
−83.209
358.182

216
A
−83.703
222.567

217
B
71.726
222.962

218
B
71.726
257.038

219
B
65.062
12.846

220
B
64.201
204.125

221
B
64.201
275.875

222
B
56.523
25.705

223
B
44.733
202.702

224
B
44.733
277.298

225
B
44.730
82.887

226
B
42.191
217.140

227
B
42.191
262.860

228
B
41.735
96.344

229
B
36.680
330.394

230
B
36.680
149.606

231
B
36.636
317.227

232
B
36.636
162.773

233
B
36.073
348.257

234
B
35.785
60.068

235
B
35.768
108.197

236
B
34.642
226.451

237
B
33.690
32.733

238
B
29.217
21.434

239
B
28.939
260.890

240
B
28.206
141.817

TABLE 4

Dimple Arrangement

Latitude
Longitude

Kind
(degree)
(degree)

241
B
26.112
65.597

242
B
26.015
292.775

243
B
26.015
187.225

244
B
24.460
250.577

245
B
24.459
10.579

246
B
24.275
130.633

247
B
24.145
349.181

248
B
24.139
229.180

249
B
15.512
293.264

250
B
15.320
173.775

251
B
14.775
41.979

252
B
13.715
99.702

253
B
8.740
331.201

254
B
8.205
212.585

255
B
6.028
60.110

256
B
6.022
180.144

257
B
5.563
136.285

258
B
4.862
93.872

259
B
4.358
82.630

260
B
4.307
202.659

261
B
3.795
313.779

262
B
0.913
323.942

263
B
−1.407
143.793

264
B
−4.880
163.968

265
B
−4.907
43.957

266
B
−5.030
284.024

267
B
−5.184
153.695

268
B
−5.231
33.684

269
B
−6.134
273.262

270
B
−6.841
230.478

271
B
−6.845
349.569

272
B
−15.871
235.789

273
B
−16.146
354.934

274
B
−18.714
79.067

275
B
−18.758
199.051

276
B
−23.971
288.774

277
B
−26.108
112.218

278
B
−26.223
236.362

279
B
−29.185
80.517

280
B
−29.232
200.478

281
B
−33.697
285.117

282
B
−34.334
228.527

283
B
−35.520
150.290

284
B
−36.149
330.142

285
B
−36.438
136.825

286
B
−41.409
35.857

287
B
−42.609
82.467

288
B
−43.798
200.849

289
B
−45.001
97.037

290
B
−45.076
336.769

291
B
−51.775
32.952

292
B
−63.684
311.963

293
B
−64.471
216.578

294
B
−64.482
96.287

295
B
−64.561
336.711

296
B
−64.843
263.144

297
B
−64.922
287.410

298
B
−72.192
77.689

299
B
−73.119
198.413

300
B
−74.983
38.997

301
C
74.657
63.484

302
C
71.768
190.178

303
C
71.768
289.822

304
C
62.942
179.469

305
C
62.942
300.531

306
C
56.191
7.848

307
C
55.053
77.053

308
C
54.553
41.717

309
C
53.846
333.327

310
C
53.846
146.673

311
C
51.471
92.182

312
C
43.387
308.955

313
C
43.387
171.045

314
C
39.782
24.035

315
C
30.483
99.122

316
C
28.904
324.540

317
C
28.904
155.460

318
C
25.096
177.021

319
C
25.096
302.979

320
C
19.173
19.184

TABLE 5

Dimple Arrangement

Latitude
Longitude

Kind
(degree)
(degree)

321
C
19.031
258.510

322
C
16.665
302.816

323
C
13.992
109.225

324
C
13.490
250.202

325
C
13.489
10.199

326
C
13.283
88.625

327
C
9.824
321.654

328
C
2.241
125.798

329
C
1.894
353.532

330
C
1.889
233.538

331
C
−0.688
333.972

332
C
−0.779
214.792

333
C
−1.916
306.499

334
C
−3.246
133.810

335
C
−3.817
86.960

336
C
−3.875
206.975

337
C
−5.619
108.070

338
C
−5.643
251.068

339
C
−5.645
11.059

340
C
−13.167
160.039

341
C
−13.201
40.044

342
C
−13.992
70.775

343
C
−14.020
190.767

344
C
−14.119
169.982

345
C
−14.134
49.990

346
C
−15.855
319.691

347
C
−18.820
342.978

348
C
−19.621
218.069

349
C
−20.962
227.066

350
C
−21.132
300.259

351
C
−23.321
88.424

352
C
−23.382
208.402

353
C
−24.157
122.583

354
C
−25.238
144.976

355
C
−30.175
296.333

356
C
−30.604
60.620

357
C
−30.611
180.571

358
C
−33.028
14.319

359
C
−35.296
253.537

360
C
−36.369
208.069

361
C
−37.100
342.734

362
C
−43.286
128.706

363
C
−43.365
231.100

364
C
−43.751
352.045

365
C
−46.901
46.162

366
C
−53.473
153.219

367
C
−54.282
257.158

368
C
−54.735
18.268

369
C
−57.211
273.655

370
C
−62.936
120.983

371
C
−66.376
49.500

372
C
−71.885
110.989

373
D
69.657
168.114

374
D
69.657
311.886

375
D
58.920
90.139

376
D
11.497
258.235

377
D
11.492
18.232

378
D
−5.801
126.695

379
D
−19.739
163.893

380
D
−19.766
43.912

381
D
−28.169
304.659

382
D
−35.660
351.929

383
D
−50.268
268.667

384
D
−69.514
132.796

From the standpoint that the individual dimples 8 contribute to the dimple effect, the average diameter of the dimples 8 is preferably equal to or greater than 3.5 mm, and more preferably equal to or greater than 3.8 mm. The average diameter is preferably equal to or less than 5.50 mm. By setting the average diameter to be equal to or less than 5.50 mm, fundamental feature of the golf ball 2 being substantially a sphere is not impaired. The golf ball 2 shown in FIGS. 3 and 4 has an average diameter of 3.84 mm.

Area s of the dimple 8 is an area of a region surrounded by the contour line when the center of the golf ball 2 is viewed at infinity. In the case of a circular dimple 8, the area s is calculated by the following formula.

S=(Di/2)²*π

In the golf ball 2 shown in FIGS. 3 and 4, the area of the dimple A is 13.85 mm²; the area of the dimple B is 11.34 mm²; the area of the dimple C is 7.07 mm²; and the area of the dimple D is 5.31 mm².

In the present invention, the ratio of the sum of the areas s of all the dimples 8 to the surface area of the phantom sphere 12 is referred to as an occupation ratio. From the standpoint that sufficient dimple effect is achieved, the occupation ratio is preferably equal to or greater than 70%, more preferably equal to or greater than 74%, and particularly preferably equal to or greater than 78%. The occupation ratio is preferably equal to or less than 95%. According to the golf ball 2 shown in FIGS. 3 and 4, the total area of the dimples 8 is 4516.9 mm². The surface area of the phantom sphere 12 of the golf ball 2 is 5728.0 mm², and thus the occupation ratio is 79%.

In light of suppression of rising of the golf ball 2 during flight, the depth of the dimple 8 is preferably equal to or greater than 0.05 mm, more preferably equal to or greater than 0.08 mm, and particularly preferably equal to or greater than 0.10 mm. In light of suppression of dropping of the golf ball 2 during flight, the depth of the dimple 8 is preferably equal to or less than 0.60 mm, more preferably equal to or less than 0.45 mm, and particularly preferably equal to or less than 0.40 mm. The depth is the distance between the tangent line TA and the deepest part of the dimple 8.

According to the present invention, the term “dimple volume” means the volume of a part surrounded by the surface of the dimple 8 and a plane that includes the contour of the dimple 8. In light of suppression of rising of the golf ball 2 during flight, the sum of the volumes (total volume) of all the dimples 8 is preferably equal to or greater than 240 mm³, more preferably equal to or greater than 260 mm³, and particularly preferably equal to or greater than 280 mm³. In light of suppression of dropping of the golf ball 2 during flight, the total volume is preferably equal to or less than 400 mm³, more preferably equal to or less than 380 mm³, and particularly preferably equal to or less than 360 mm³.

From the standpoint that sufficient occupation ratio can be achieved, the total number of the dimples 8 is preferably equal to or greater than 200, more preferably equal to or greater than 250, and particularly preferably equal to or greater than 300. From the standpoint that individual dimples 8 can have a sufficient diameter, the total number is preferably equal to or less than 500, more preferably equal to or less than 440, and particularly preferably equal to or less than 400.

The following will describe an evaluation method for aerodynamic characteristic according to the present invention. FIG. 5 shows a schematic view for explaining the evaluation method. In the evaluation method, a first rotation axis Ax1 is assumed. The first rotation axis Ax1 passes through the two poles Po of the golf ball 2. Each pole Po corresponds to the deepest part of the mold used for forming the golf ball 2. One of the poles Po corresponds to the deepest part of an upper mold half, and the other pole Po corresponds to the deepest part of a lower mold half. The golf ball 2 rotates about the first rotation axis Ax1. This rotation is referred to as PH rotation.

There is assumed a great circle GC which exists on the surface of the phantom sphere 12 of the golf ball 2 and is orthogonal to the first rotation axis Ax1. The circumferential speed of the great circle GC is faster than any other part of the golf ball 2 during rotation. In addition, there are assumed two small circles C1 and C2 which exist on the surface of the phantom sphere 12 of the golf ball 2 and are orthogonal to the first rotation axis Ax1. FIG. 6 shows a partial cross-sectional view of the golf ball 2 in FIG. 5. In FIG. 6, the right-to-left direction is the direction of the first rotation axis Ax1. As shown in FIG. 6, the absolute value of the central angle between the small circle C1 and the great circle GC is 30°. Although not shown in the drawing, the absolute value of the central angle between the small circle C2 and the great circle GC is also 30°. The phantom sphere 12 is divided at the small circles C1 and C2, and among the surface of the phantom sphere 12, a region sandwiched between the small circles is defined.

In FIG. 6, a point P (α) is the point which is located on the surface of the golf ball 2 and of which the central angle with the great circle GC is α° (degree). A point F (α) is a foot of a perpendicular line Pe (α) which extends downward from the point P (α) to the first rotation axis Ax1. What is indicated by a arrow L1 (α) is 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 rotation 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 to obtain a total length L2 (mm). The total length L2 is a parameter dependent on the surface shape in the cross section shown in FIG. 6.

FIG. 7 shows a partial cross section of the golf ball 2. In FIG. 7, a direction perpendicular to the surface of the sheet is the direction of the first rotation axis Ax1. In FIG. 7, what is indicated by a reference sign β is a rotation angle of the golf ball 2. In a range equal to or greater than 0° and smaller 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. In other words, a data constellation, regarding a parameter dependent on a surface shape appearing at a predetermined point moment by moment during one rotation of the golf ball 2, is calculated. The data constellation is calculated based on the 30240 lengths L1.

FIG. 8 shows a graph plotting a data constellation of the golf ball 2 shown in FIGS. 3 and 4. In this graph, the horizontal axis indicates the rotation angle β, and the vertical axis indicates the total length L2. From this graph, the maximum and minimum values of the total length L2 are determined. The minimum value is subtracted from the maximum value to calculate a fluctuation range. The fluctuation range is divided by the total volume (mm³) of the dimples 8 to calculate a value Ad1. The value Ad1 is a numeric value indicating an aerodynamic characteristic at PH rotation.

Further, a second rotation axis Ax2 orthogonal to the first rotation axis Ax1 is determined. Rotation of the golf ball 2 about the second rotation axis Ax2 is referred to as POP rotation. Similarly as for PH rotation, for POP rotation, a great circle GC and two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the great circle GC is 30°. The absolute value of the central angle between the small circle C2 and the great circle GC is also 30°. For a region sandwiched between the small circles among the surface of the phantom sphere 12, 1440 total lengths L2 are calculated. In other words, a data constellation, regarding a parameter dependent on a surface shape appearing at a predetermined point moment by moment during one rotation of the golf ball 2, is calculated. FIG. 9 shows a graph plotting a data constellation of the golf ball 2 shown in FIGS. 3 and 4. In this graph, the horizontal axis indicates the rotation angle β, and the vertical axis indicates the total length L2. From this graph, the maximum and minimum values of the total length L2 are determined. The minimum value is subtracted from the maximum value to calculate a fluctuation range. The fluctuation range is divided by the total volume (mm³) of the dimples 8 to calculate a value Ad2. The value Ad2 is a numeric value indicating an aerodynamic characteristic for POP rotation.

There are numerous straight lines orthogonal to the first rotation axis Ax1. A straight line of which the corresponding great circle GC contains the most number of dimple centers substantially located therein is set as the second rotation axis Ax2. When there are in reality a plurality of straight lines of which the corresponding great circles GC each contain the most number of dimple centers substantially located therein, the fluctuation range is calculated for each of the cases where these straight lines are set as second rotation axis Ax2. The greatest fluctuation range is divided by the total volume of the dimples 8 to obtain a value Ad2.

The following shows a result of the golf ball 2 shown in FIGS. 3 and 4, calculated by the above evaluation method. Total volume of dimples 8: 325 mm³

PH rotation

Maximum value of total length L2: 425.16 mm

Minimum value of total length L2: 423.10 mm

Fluctuation range: 2.06 mm

Ad1: 0.0063 mm⁻²

POP rotation

Maximum value of total length L2: 425.37 mm

Minimum value of total length L2: 422.89 mm

Fluctuation range: 2.48 mm

Ad2: 0.0076 mm⁻²

Absolute value of difference between Ad1 and Ad2: 0.0013 mm⁻²

The following Table 6 shows values Ad1 and Ad2 calculated for commercially available golf balls.

TABLE 6

Marketed Products

A
B
C
D
E

Ad1 (mm⁻²)
0.00271
0.00468
0.00241
0.00506
0.00326

Ad2 (mm⁻²)
0.01135
0.01123
0.01324
0.01313
0.01248

Difference (mm⁻²)
0.00865
0.00656
0.01082
0.00806
0.00923

Ad3
0.00216
0.00526
0.00135
0.00484
0.00052

Ad4
0.01003
0.00929
0.01100
0.00913
0.01048

Difference
0.00787
0.00403
0.00965
0.00429
0.00997

As is clear from the comparison with the marketed products, the value Ad2 of the golf ball 2 shown in FIGS. 3 and 4 is small. According to the findings by the inventors of the present invention, the golf ball 2 with small values for Ad1 and Ad2 has a long flight distance. The detailed reason is not clear, but it is inferred that this is because transition of turbulent flow continues smoothly.

In light of flight distance, each of the values Ad1 and Ad2 is preferably equal to or less than 0.009 mm⁻², more preferably equal to or less than 0.008 mm⁻², much more preferably equal to or less than 0. 006 mm⁻², and particularly preferably 0.004 mm⁻². The ideal values of Ad1 and Ad2 are zero.

As is clear from the comparison with the marketed products, the difference between the values Ad1 and Ad2 of the golf ball 2 shown in FIGS. 3 and 4 is small. According to the findings by the inventors, the golf ball 2 with a small difference between the values Ad1 and Ad2 has excellent aerodynamic symmetry. It is inferred that this is because the similarity between the surface shape during PH rotation and the surface shape during POP rotation is high and hence the difference between the dimple effect for PH rotation and the dimple effect for POP rotation is small.

In light of aerodynamic symmetry, the absolute value of the difference between the values Ad1 and Ad2 is preferably equal to or less than 0.005 mm⁻², ore preferably equal to or less than 0.003 mm⁻², much more preferably equal to or less than 0.002 mm⁻², and particularly preferably equal to or less than 0.001 mm⁻². The ideal value of the difference is zero.

As described above, the golf ball 2 needs an appropriate total volume of the dimples 8. The fluctuation range of the total length L2 correlates with the total volume of the dimples 8. In a golf ball 2 with a small total volume of the dimples 8, the fluctuation range can be set small. However, even if the fluctuation range is small, the golf ball 2 with an excessively small total volume of the dimples 8 has a short flight distance. In the above evaluation method, the fluctuation range is divided by the total volume to calculate the values Ad1 and Ad2. The values Ad1 and Ad2 are numeric values obtained by taking the fluctuation range and the total volume into account. The golf ball 2 with appropriate values Ad1 and Ad2 has a long flight distance.

The absolute value of the central angle between the great circle GC and the small circle C1 and the absolute value of the central angle between the great circle GC and the small circle C2 can be arbitrarily set in a range equal to or less than 90°. As the absolute value of the central angle becomes smaller, the cost for calculation becomes lower. On the other hand, if the absolute value of the central angle is excessively small, accuracy of evaluation becomes insufficient. During flight of the golf ball 2, the region near the great circle GC receives large pressure from the air. The dimples 8 existing in the region contribute greatly to the dimple effect. In this respect, in the evaluation method, the absolute value of the central angle is set at 30°.

The dimples B close to the great circle GC contribute greatly to the dimple effect. On the other hand, the dimples 8 distant from the great circle GC contribute slightly to the dimple effect. In this respect, each of many obtained lengths L1 (α) may be multiplied by a coefficient dependent on the angle α to calculate the total length L2. For example, each length L (α) may be multiplied by sin a to calculate the total length L2.

In the evaluation method, based on the angles a set at an interval of an angle of 3°, many lengths L1 (α) are calcualted. The angles α are not necessarily set at an interval of an angle of 3°. The angles a are preferably set at an interval of an angle equal to or greater than 0.10 and equal to or less than 5°. If the angles a are set at an interval of an angle equal to or greater than 0.1°, the computer load is small. If the angles a are set at an interval of an angle equal to or less than 5°, accuracy of evaluation is high. In light of accuracy, the angles a are set at an interval of an angle more preferably equal to or less than 4° and particularly preferably equal to or less 3°.

In the evaluation method, based on the angles β set at an interval of an angle of 0.25°, many total lengths L2 are calculated. The angles β are not necessarily set at an interval of an angle of 0.25°. The angles β are preferably set at an interval of an angle equal to or greater than 0.1° and equal to or less than 5°. If the angles β are set at an interval of an angle equal to or greater than 0.1°, the computer load is small. If the angles β are set at an interval of an angle equal to or less than 5°, accuracy of evaluation is high. In light of accuracy, the angles β are set at an interval of an angle more preferably equal to or less than 4° and particularly preferably equal to or less 3°. Depending on the position of a point (start point) at which the angle β is first measured, the values Ad1 and Ad2 change. However, because the change range is negligibly small, the start point can be arbitarily set.

In the evaluation method, the data constellation is calculated based on the length L1 (α). The length L1 (α) is a parameter dependent on the distance between the rotation axis (Ax1 or Ax2) and the surface of the golf ball 2. Another parameter dependent on the surface shape of the golf ball 2 may be used. Examples of other parameters include:

(a) Distance between the surface of the phantom sphere 12 and the surface of the golf ball 2; and

(b) Distance between the surface and the center O (see FIG. 6) of the golf ball 2.

The golf ball 2 may be evaluated only based on a first data constellation obtained by rotation about the first rotation axis Ax1. The golf ball 2 may be evaluated only based on a second data constellation obtained by rotation about the second rotation axis Ax2. Preferably, the golf ball 2 is evaluated based on both the first data constellation and the second data constellation. Preferably, the aerodynamic symmetry of the golf ball 2 is evaluated by the comparison of the first data constellation and the second data constellation.

A data constellation may be obtained based on an axis other than the first rotation axis Ax1 and the second rotation axis Ax2. The positions and the number of rotation axes can be arbitrarily set. Preferably, based on two rotation axes, two data constellations are obtained. Evaluation based on two data constellations is superior in accuracy to that based on one data constellation. The evaluation based on two data constellations can be done in a shorter time than that based on three or more data constellations. When evaluation based on two data constellations is done, two rotation axes may not be orthogonal to each other.

As a result of thorough research by the inventors of the present invention, it is confirmed that when evaluation is done based on both PH rotation and POP rotation, the result has a high correlation with the flight performance of the golf ball. The reason is predicated as follow:

(a) The region near the seam is a unique region and PH rotation is most affected by this region;

(b) POP rotation is unlikely to be affected by this region; and

(c) By the evaluation based on both PH rotation and POP rotation, an objective result is obtained. The evaluation based on both PH rotation and POP rotation is preferable from the standpoint that conformity to the rules established by the USGA can be determined.

In a designing process according to the present invention, the positions of numerous dimples located on the surface of the golf ball 2 are determined. Specifically, the latitude and longitude of each dimple 8 are determined. In addition, the shape of each dimple 8 is determined. This shape includes diameter, depth, curvature radius of a cross section and the like. The aerodynamic characteristic of the golf ball 2 is evaluated by the above method. For example, the above values Ad1 and Ad2 are calculated, and their magnitudes are evaluated. Further, the difference between the values Ad1 and Ad2 is evaluated. If the aerodynamic characteristic is insufficient, the positions and the shapes of the dimples 8 are changed. After the change, evaluation is done again. In this designing process, the golf ball 2 can be evaluated without producing a mold.

The following will describe another evaluation method according to the present invention. In the evaluation method, similarly as in the aforementioned evaluation method, a first rotation axis Ax1 (see FIG. 5) is assumed. The first rotation axis Ax1 passes through the two poles Po of the golf ball 2. The golf ball 2 rotates about the first rotation axis Ax1. This rotation is referred to as PH rotation. In addition, a great circle GC, a small circle C1, and a small circle C2 which are orthogonal to the first rotation axis Ax1 are assumed. The absolute value of the central angle between the small circle C1 and the great circle GC is 30°. The absolute value of the central angle between the small circle C2 and the great circle GC is also 30°. The above phantom sphere 12 is divided at the small circles C1 and C2, and among the phantom sphere 12, a region sandwiched between the small circles is defined.

This region is divided at an interval of a central angle of 3° in the rotation direction into 120 minute regions. FIG. 10 shows one minute region 14. FIG. 11 is an enlarged cross-sectional view of the minute region 14 in FIG. 10. For the minute region 14, the volume of spaces between the surface of the phantom sphere 12 and the surface of the golf ball 2 are calculated. This volume is the volume of parts hatched in FIG. 11. The volume is calculated for each of the 120 minute regions 14. In other words, 120 volumes along the rotation direction when the golf ball 2 makes one rotation are calculated. These volumes are a data constellation regarding a parameter dependent on a surface shape appearing at a predetermined point moment by moment during one rotation of the golf ball 2.

FIG. 12 shows a graph plotting a data constellation of the golf ball 2 shown in FIGS. 3 and 4. In this graph, the horizontal axis indicates the angle in the rotation direction, and the vertical axis indicates the volume for the minute region. From this graph, the maximum value and the minimum value of the volume are determined. The minimum value is subtracted from the maximum value to calculate a fluctuation range. The fluctuation range is divided by the total volume (mm³) of the dimples 8 to calculate a value Ad3. The value Ad3 is a numeric value indicating an aerodynamic characteristic at PH rotation.

Further, a second rotation axis Ax2 orthogonal to the first rotation axis Ax1 is determined. The rotation of the golf ball 2 about the second rotation axis Ax2 is referred to as POP rotation. For POP rotation, similarly as for PH rotation, a great circle GC and two small circles C1 and C2 are assumed. The absolute value of the central angle between the small circle C1 and the great circle GC is 30°. The absolute value of the central angle between the small circle C2 and the great circle GC is also 30°. Among the phantom sphere 12, a region sandwiched between these small circles is divided at an interval of a central angle of 3° in the rotation direction into 120 minute regions 14. For each minute region 14, the volume of spaces between the surface of the phantom sphere 12 and the surface of the golf ball 2 is calculated. FIG. 13 shows a graph plotting a data constellation of the golf ball 2 shown in FIGS. 3 and 4. In this graph, the horizontal axis indicates the angle in the rotation direction, and the vertical axis indicates the volume for the minute region. From this graph, the maximum and minimum values of the volume are determined. The minimum value is subtracted from the maximum value to calculate a fluctuation range. The fluctuation range is divided by the total volume of the dimples 8 to calculate a value Ad4. The value Ad4 is a numeric value indicating an aerodynamic characteristic for POP rotation.

The following shows a result of, the golf ball 2 shown in FIGS. 3 and 4, calculated by the above evaluation method.

Total volume of dimples 8: 325 mm³

PH rotation

Maximum value of volume for minute region 14: 3.281

Minimum value of volume for minute region 14: 1.396 mm³

Fluctuation range: 1.885 mm³

Ad3: 0.0058

POP rotation

Maximum value of volume for minute region 14: 3.511 mm³

Minimum value of volume for minute region 14: 1.171 mm³

Fluctuation range: 2.340 mm³

Ad4: 0.0072

Absolute value of difference between Ad3 and Ad4: 0.0014

The above Table 6 also shows values Ad3 and Ad4 calculated for the commercially available golf balls.

As is clear from the comparison with the marketed products, the value Ad4 of the golf ball 2 shown in FIGS. 3 and 4 is small. According to the findings by the inventors of the present invention, the golf ball 2 with small values for Ad3 and Ad4 has a long flight distance. The detailed reason is not clear, but it is inferred that this is because transition of turbulent flow continues smoothly.

In light of flight distance, each of the values Ad3 and Ad4 is preferably equal to or less than 0.008, more preferably equal to or less than 0.007, much more preferably equal to or less than 0.006, and particularly preferably 0.005. The ideal values of Ad3 and Ad4 are zero.

As is clear from the comparison with the marketed products, the difference between the values Ad3 and Ad4 of the golf ball 2 shown in FIGS. 3 and 4 is small. According to the findings by the inventors, the golf ball 2 with a small difference between values Ad3 and Ad4 has excellent aerodynamic symmetry. It is inferred that this is because the difference between the dimple effect for PH rotation and the dimple effect for POP rotation is small.

In light of aerodynamic symmetry, the absolute value of the difference between the values Ad3 and Ad4 is preferably equal to or less than 0.003, more preferably equal to or less than 0.002, and particularly preferably equal to or less than 0.001. The ideal value of the difference is zero.

As described above, the golf ball 2 needs an appropriate total volume of the dimples 8. The fluctuation range of the volume for the minute region 14 correlates with the total volume of the dimples 8. In a golf ball 2 with a small total volume of the dimples 8, the fluctuation range can be set small. However, even if the fluctuation range is small, the golf ball 2 with an excessively small total volume of the dimples 8 has a short flight distance. In the above evaluation method, the fluctuation range is divided by the total volume of the dimples 8 to calculate the values Ad3 and Ad4. The values Ad3 and Ad4 are numeric values obtained by taking the fluctuation range and the total volume of the dimples 8 into account. The golf ball 2 with appropriate values Ad3 and Ad4 has a long flight distance.

In the evaluation method, the region is divided at an interval of a central angle of 3° in the rotation direction into the 120 minute regions 14. The region is not necessarily divided at an interval of a central angle of 3° in the rotation direction. The region is divided at an interval of a central angle preferably equal to or greater than 0.1° and equal to or less than 5°. If the region is divided at an interval of a central angle equal to or greater than 0.1°, the computer load is small. If the region is divided at an interval of a central angle equal to or less than 5°, accuracy of evaluation is high. In light of accuracy, the region is divided at an interval of a central angle preferably equal to or less than 4° and particularly equal to or less than 3°. Depending on the position of a point (start point) at which the central angle is first measured, the values Ad3 and Ad4 change. However, because the change range is negligibly small, the start point can be arbitarily set.

In the evaluation method, the data constellation is calculated based on the volumes for the minute regions 14. Another parameter dependent on the surface shape of the golf ball 2 may be used. Examples of other parameters include:

(a) Volume of the minute region 14 in the golf ball 2;

(b) Volume of an area of between a plan including the edge of each dimple 8 and the surface of the golf ball 2 in the minute region 14;

(c) Area between the surface of the phantom sphere 12 and the surface of the golf ball 2 in front view of the minute region 14;

(d) Area between a plan including the edge of each dimple 8 and the surface of the golf ball 2 in front view of the minute region 14; and

(e) Area of the golf ball 2 in front view of the minute region 14.

(a) The region near the seam is a unique region and PH rotation is most affected by this region;

(b) POP rotation is unlikely to be affected by this region; and

In a designing process according to the present invention, the positions of numerous dimples located on the surface of the golf ball 2 are determined. Specifically, the latitude and longitude of each dimple 8 are determined. In addition, the shape of each dimple 8 is determined. This shape includes diameter, depth, curvature radius of a cross section and the like. The aerodynamic characteristic of the golf ball 2 is evaluated by the above method. For example, the above values Ad3 and Ad4 are calculated, and their magnitudes are evaluated. Further, the difference between the values Ad3 and Ad4 is evaluated. If the aerodynamic characteristic is insufficient, the positions and the shapes of the dimples 8 are changed. After the change, evaluation is done again. In this designing process, the golf ball 2 can be evaluated without producing a mold.

EXAMPLES
Example

A rubber composition was obtained by kneading 100 parts by weight of polybutadiene (trade name “BR-730”, available from JSR Corporation), 30 parts by weight of zinc diacrylate, 6 parts by weight of zinc oxide, 10 parts by weight of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.5 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold having upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 18 minutes to obtain a core with a diameter of 39.7 mm. Meanwhile, a resin composition was obtained by kneading 50 parts by weight of ionomer resin (trade name “Himilan 1605”, available from Du Pont-MITSUI POLYCHEMICALS Co., LTD.), 50 parts by weight of another ionomer resin (Trade name “Himilan 1706”, available from Du Pont-MITSUI POLYCHEMICALS Co., LTD.), and 3 parts by weight of titanium dioxide. The above core was placed into a final mold having numerous pimples on its inside face, followed by injection of the above resin composition around the core by injection molding to form a cover with a thickness of 1.5 mm. Numerous dimples having a shape inverted from the shape of the pimples were formed on the cover. A clear paint including a two-component curing type polyurethane as a base was applied on this cover to obtain a golf ball of Example having a diameter of 42.7 mm and a weight of about 45.4 g. The golf ball has a PGA compression of about 85. The golf ball has the dimple pattern shown in FIGS. 3 and 4. The detailed specifications of the dimples are shown in the following Table 7.

Comparative Example

A golf ball of Comparative Example was obtained in the same manner as in Example except that the final mold was changed so as to form dimples whose specifications are shown in the following Table 7. FIG. 14 is a front view of the golf ball of Comparative Example, and FIG. 15 is a plan view of the golf ball. For one unit when northern hemisphere of the golf ball is divided into 5 units, the latitude and longitude of the dimples are shown in the following Table 8. The dimple pattern of this unit is developed to obtain the dimple pattern of the northern hemisphere. The dimple pattern of the southern hemisphere is equivalent to the dimple pattern of the northern hemisphere. The dimple patterns of the northern hemisphere and the southern hemisphere are shifted from each other by 5.98° in the latitude direction. The dimple pattern of the southern hemisphere is obtained by symmetrically moving the dimple pattern of the northern hemisphere relative to the equator after shifting the dimple pattern of the northern hemisphere by 5.98° in the longitude direction. The following shows the result of this golf ball calculated by the above evaluation method.

Total volume of dimples: 320 mm³

PH rotation

Maximum value of total length L2: 424.71 mm

Minimum value of total length L2: 424.20 mm

Fluctuation range of total length L2: 0.51 mm

Ad1: 0.0016 mm⁻²

Maximum value of volume for minute region: 2.024 mm³

Minimum value of volume for minute region: 1.576 mm³

Fluctuation range of volume: 0.448 mm³

Ad3: 0.0014

POP rotation

Maximum value of total length L2: 426.15 mm

Minimum value of total length L2: 422.95 mm

Fluctuation range of total length L2: 3.20 mm

Ad2: 0.0100 mm⁻²

Maximum value of volume for minute region: 2.784 mm³

Minimum value of volume for minute region: 0.527 mm³

Fluctuation range of volume: 2.784 mm³

Ad4: 0.0087

Absolute value of difference between Ad1 and Ad2: 0.0084 mm⁻²

Absolute value of difference between Ad3 and Ad4: 0.0073

TABLE 7

Specifications of Dimples

Diameter
Depth
Volume

Kind
Number
(mm)
(mm)
(mm³)

Example
A
216
4.20
0.1436
0.971

B
84
3.80
0.1436
0.881

C
72
3.00
0.1436
0.507

D
12
2.60
0.1436
0.389

Comparative
A
120
3.80
0.1711
0.973

Example
B
152
3.50
0.1711
0.826

C
60
3.20
0.1711
0.691

D
60
3.00
0.1711
0.607

TABLE 8

Dimple Arrangement of Comparative Example

Latitude
Longitude

Kind
(degree)
(degree)

1
A
73.693
0.000

2
A
60.298
36.000

3
A
54.703
0.000

4
A
43.128
22.848

5
A
34.960
0.000

6
A
24.656
18.496

7
A
15.217
0.000

8
A
14.425
36.000

9
A
5.763
18.001

10
B
90.000
0.000

11
B
64.134
13.025

12
B
53.502
19.337

13
B
44.629
8.044

14
B
30.596
36.000

15
B
24.989
6.413

16
B
15.335
12.237

17
B
5.360
5.980

18
B
5.360
30.020

19
C
70.742
36.000

20
C
49.854
36.000

21
C
34.619
13.049

22
C
14.610
23.917

23
D
80.183
36.000

24
D
40.412
36.000

25
D
33.211
24.550

26
D
22.523
29.546

[Flight Distance Test]

A driver with a titanium head (Trade name “XXIO”, available from SRI Sports Limited, shaft hardness: R, loft angle: 12°) was attached to a swing machine available from True Temper Co. Then, the golf ball was hit under the conditions of a head speed of 40 m/sec, a launch angle of about 13°, and a backspin rotation speed of about 2500 rpm, and the carry and total distances were measured. At the test, the weather was almost calm. The measurement was done 20 times for each of PH rotation and POP rotation, and the average values of the results are shown in the following Table 9.

TABLE 9

Results of Evaluation

Comparative

Example
Example

Front view
FIG. 3
FIG. 14

Plan view
FIG. 4
FIG. 15

Total number
384
392

Total volume (mm³)
325
320

Occupation ratio (%)
79
65.2

Graph of L2
FIG. 8
FIG. 16

(PH rotation)

Graph of L2
FIG. 9
FIG. 17

(POP rotation)

Ad1 (mm⁻²)
0.0063
0.0016

Ad2 (mm⁻²)
0.0076
0.0100

Difference between Ad1
0.0013
0.0084

and Ad2 (mm⁻²)

Graph of volume for
FIG. 12
FIG. 18

minute region (PH rotation)

Graph of volume for
FIG. 13
FIG. 19

minute region (POP rotation)

Ad3
0.0058
0.0014

Ad4
0.0072
0.0087

Difference between
0.0014
0.0073

Ad3 and Ad4

Carry
PH rotation
204.4
204.0

(Yard)
POP rotation
202.4
198.8

Difference
2.0
5.2

Total
PH rotation
212.8
214.0

(Yard)
POP rotation
212.1
204.3

Difference
0.7
9.7

While Ad1 and Ad2 of Example are greater than Ad1 of Comparative Example, they are smaller than Ad2 of Comparative Example. While Ad3 and Ad4 of Example are greater than Ad3 of Comparative Example, they are smaller than Ad4 of Comparative Example. The difference between Ad1 and Ad2 of Example is smaller than that of Comparative Example. The difference between Ad3 and Ad4 of Example is smaller than that of Comparative Example. As shown in Table 9, the flight distance of the golf ball of Example is greater than that of the golf ball of the Comparative Example. It is inferred that this is because in the golf ball of Example, transition of turbulent flow continues smoothly. Further, in the golf ball of Example, the difference between the flight distance at PH rotation and the flight distance at POP rotation is small. It is inferred that this is because the difference between the dimple effect for PH rotation and the dimple effect for POP rotation is small. From the results of evaluation, advantages of the present invention are clear.

By the evaluation method according to the present invention, the aerodynamic characteristic of a golf ball can be evaluated with high accuracy. By the designing process according to the present invention, a golf ball having an excellent aerodynamic characteristic can be obtained. The golf ball according to the present invention has excellent aerodynamic symmetry and a long flight distance.

The dimple pattern described above is applicable to a one-piece golf ball, a multi-piece golf ball, and a thread-wound golf ball, in addition to a two-piece golf ball. The above description is merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention.

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