RACKET

Abstract

A racket includes a frame 4. The frame 4 includes a head 14. The head 14 includes a first high-elasticity layer 37a and a second high-elasticity layer 37b. Each of these high-elasticity layers 37 is positioned on an inner side in the head 14 in a thickness direction of the head 14. Each high-elasticity layer 37 includes straight-type reinforcement fibers. In the frame 4, a ratio (G2/G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20. In the frame 4, a ratio (Tf/Wf) of a thickness Tf of the frame 4 to a width Wf of the frame 4 is preferably greater than or equal to 2.0.

Claims

1. A racket comprising a frame including a head, wherein in the frame, a ratio (G2/G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20.

2. The racket according to claim 1, wherein the head includes a high-elasticity layer that is positioned on an inner side in the head in a thickness direction of the head, the high-elasticity layer including straight-type reinforcement fibers.

3. The racket according to claim 2, wherein the high-elasticity layer is present in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head.

4. The racket according to claim 1, wherein in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head, a ratio (Tf/Wf) of a thickness Tf of the frame to a width Wf of the frame is greater than or equal to 2.0.

5. The racket according to claim 1, wherein the ratio (G2/G1) is greater than or equal to 3.55.

6. The racket according to claim 5, wherein the ratio (G2/G1) is greater than or equal to 3.90.

7. The racket according to claim 6, wherein the ratio (G2/G1) is greater than or equal to 4.10.

8. The racket according to claim 1, wherein the side pressure stiffness value G1 is less than or equal to 90 kgf/cm.

9. The racket according to claim 1, wherein the ball-hitting face stiffness value G2 is greater than or equal to 100 kgf/cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a front view showing a tennis racket according to one embodiment.

[0007] FIG. 2 is a right-side view showing the tennis racket of FIG. 1.

[0008] FIG. 3 is an exploded view showing part of the tennis racket of FIG. 1 in an enlarged manner.

[0009] FIG. 4 is a perspective view showing part of a manufacturing process of the racket of FIG. 1.

[0010] FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1.

[0011] FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5.

[0012] FIG. 7 is an enlarged view showing part of a prepreg for first fiber reinforced layers of a head of FIG. 6.

[0013] FIG. 8 is an enlarged view showing part of a prepreg for second fiber reinforced layers of the head of FIG. 6.

[0014] FIG. 9 is a perspective view showing part of a high-elasticity layer of the head of FIG. 6.

[0015] FIG. 10 is an enlarged view showing part of a prepreg for the high-elasticity layer of FIG. 9.

[0016] FIG. 11 is a graph showing a relationship between a side pressure stiffness value and a ball-hitting face stiffness value of a frame of the tennis racket of FIG. 1.

[0017] FIG. 12 is a front view showing a method of measuring the side pressure stiffness value of the frame of the tennis racket of FIG. 1.

[0018] FIG. 13A is a plan view showing a method of measuring the ball-hitting face stiffness value of the frame of the tennis racket of FIG. 1, and FIG. 13B is a front view thereof.

DETAILED DESCRIPTION

[0019] Hereinafter, preferred embodiments are described in detail with reference to the drawings as necessary.

[0020] FIGS. 1 to 3 each show a tennis racket 2. The racket 2 includes a frame 4, a grip 6, an end cap 8, a grommet 10, and a string 12. The racket 2 can be used in regulation-ball tennis. In FIGS. 1 and 2, an arrow X represents the width direction of the racket 2; an arrow Y represents the axial direction of the racket 2; and an arrow Z represents the thickness direction of the racket 2. In FIG. 2, the illustration of the grommet 10 and the string 12 is omitted.

[0021] The frame 4 includes a head 14, a first throat 16a, a second throat 16b, and a shaft 18. The head 14 forms the contour of a face 20 (the face 20 will be described below in detail). The front view shape of the head 14 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket 2. The minor axis direction of the ellipse coincides with the width direction X of the racket 2. In FIG. 1, reference sign Ch indicates the center of the head 14. The first throat 16a extends from the head 14. The second throat 16b extends from the head 14. The second throat 16b merges with the first throat 16a at a position away from the head 14. The shaft 18 extends from the position where the two throats 16 merge together. The shaft 18 is continuous with the throats 16. A portion of the head 14, the portion being positioned between the two throats 16, is a yoke 22. The frame 4 is hollow.

[0022] The main material of the frame 4 is a fiber reinforced resin. The fiber reinforced resin includes a resin matrix and a large number of reinforcement fibers. The frame 4 includes a plurality of fiber reinforced layers. The fiber reinforced layers will be described below in detail.

[0023] Examples of the base resin of the frame 4 include: thermosetting resins such as epoxy resin, bismaleimide resin, polyimide, and phenolic resin; and thermoplastic resins such as polyether ether ketone, polyether sulphone, polyether imide, polyphenylene sulfide, polyamide, and polypropylene. Epoxy resin is a particularly suitable resin for the frame 4.

[0024] Examples of the reinforcement fibers of the frame 4 include carbon fibers, metal fibers, glass fibers, and aramid fibers. Carbon filament fibers are particularly suitable fibers for the frame 4. Multiple types of fibers may be used in combination as the reinforcement fibers.

[0025] As shown in FIGS. 2 and 3, the head 14 includes a groove 24. The groove 24 is recessed from the outer peripheral surface of the head 14. The groove 24 is formed over substantially the entire periphery of the head 14, except the yoke 22. The head 14 further includes a plurality of holes 26. The plurality of holes 26 are arranged over substantially the entire periphery of the head 14.

[0026] The grip 6 is formed by a tape wound around the shaft 18. The grip 6 suppresses slip between a hand of a player and the racket 2 when the racket 2 is swung by the player. As shown in FIG. 3, the grommet 10 includes a base 28 and a plurality of pipes 30. The base 28 is belt-shaped. Each pipe 30 is integrated with the base 28. Each pipe 30 rises from the base 28. A typical material of the grommet 10 is a synthetic resin that is softer than the frame 4. The tennis racket 2 may include a plurality of grommets 10. The number of pipes 30 of each grommet 10 may be one.

[0027] The grommet 10 is attached to the head 14. In a state where the grommet 10 is attached to the head 14, the base 28 is accommodated in the groove 24. The base 28 may partly protrude from the groove 24. Further, in the state where the grommet 10 is attached to the head 14, the pipes 30 extend through the respective holes 26.

[0028] As shown in FIG. 1, the string 12 is stretched on the head 14. The string 12 is stretched in the width direction X and the axial direction Y. The string 12 extends through the pipes 30. The string 12 forms a large number of threads 32. Of the string 12, portions extending in the width direction X are referred to as transverse threads 32a. Of the string 12, portions extending in the axial direction Y are referred to as longitudinal threads 32b. The face 20 is formed by a plurality of transverse threads 32a and a plurality of longitudinal threads 32b. The face 20 generally extends along an X-Y plane. FIG. 1 shows part of the face 20. The face 20 may be formed by two or more strings 12.

[0029] Hereinafter, one example of a method of manufacturing the tennis racket 2 is described with reference to FIG. 4. In this manufacturing method, a mandrel, a tube, and a plurality of prepregs 34 are prepared. Each prepreg 34 is made from a plurality of reinforcement fibers arranged in parallel and a matrix resin. In this manufacturing method, first, the mandrel is inserted into the tube. The prepregs 34 are sequentially wound around the tube. As a result of the winding, the prepregs 34 have a tubular shape. FIG. 4 shows a tubular prepreg 34p and a sheet-shaped prepreg 34s. In FIG. 4, the illustration of the mandrel and the tube is omitted.

[0030] By rotating the mandrel, the prepreg 34s is wound around the prepreg 34p. As a result of the winding, the prepreg 34s has a tubular shape, and thus a layered body 36 is obtained. Another prepreg 34 is further wound around the layered body 36 as necessary. A plurality of sheet-shaped prepregs 34s may be layered one on top of another, which may be then wound around the mandrel or the prepreg 34p. In FIG. 4, an arrow A1 represents the longitudinal direction of the layered body 36.

[0031] After the mandrel is removed from the tube, the tube and the layered body 36 are set in a mold. In the mold, gas is injected into the tube, thereby inflating the tube. The prepregs 34 are pressed against the cavity surface of the mold by the inflation. The prepregs 34 are heated to cure the matrix resin. A molded article is obtained by the curing. The molded article has a reverse shape of that of the cavity surface.

[0032] The holes 26 are drilled in the molded article. The molded article is further subjected to treatments such as surface polishing and painting, and thereby the frame 4 is obtained. Components such as the grip 6 and the grommet 10 are attached to the frame 4. Further, the string 12 is stretched on the frame 4, and thus the tennis racket 2 is completed.

[0033] FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1. A cross section shown in FIG. 5 extends along a plane that is perpendicular to the axial direction of the frame 4 and that passes the center Ch of the head 14. The head 14 includes a first high-elasticity layer 37a and a second high-elasticity layer 37b (see also FIG. 2). Each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction.

[0034] FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5. FIG. 6 shows the head 14. The head 14 includes a plurality of fiber reinforced layers 38. In the present embodiment, the head 14 includes a plurality of first fiber reinforced layers 38a, a plurality of second fiber reinforced layers 38b, and a plurality of third fiber reinforced layers 38c. In the present embodiment, the number of first fiber reinforced layers 38a is five; the number of second fiber reinforced layers 38b is five; and the number of third fiber reinforced layers 38c is four. The first fiber reinforced layers 38a and the second fiber reinforced layers 38b are arranged alternately in the thickness direction of the head 14 (i.e., the vertical direction in FIG. 6). The first high-elasticity layer 37a is formed by the plurality of third fiber reinforced layers 38c. The first high-elasticity layer 37a is positioned on the inner side in the head 14 in the thickness direction (in FIG. 6, the lower side). Although not illustrated, the shape of the second high-elasticity layer 37b is mirror-symmetrical to the shape of the first high-elasticity layer 37a.

[0035] FIG. 7 shows a first prepreg 34a for the first fiber reinforced layers 38a. The first prepreg 34a includes a matrix 40 and a plurality of first reinforcement fibers 42a arranged in parallel. Each first reinforcement fiber 42a is inclined relative to the longitudinal direction A1. In FIG. 7, an arrow a represents an inclination angle (absolute value) of the first reinforcement fiber 42a relative to the longitudinal direction A1. The inclination angle a is greater than or equal to 30 and less than or equal to 60. In the present specification, a reinforcement fiber 42 having an inclination angle of greater than or equal to 30 and less than or equal to 60 is referred to as a bias-type reinforcement fiber. The first fiber reinforced layers 38a include bias-type reinforcement fibers.

[0036] FIG. 8 shows a second prepreg 34b for the second fiber reinforced layers 38b. The second prepreg 34b includes the matrix 40 and a plurality of second reinforcement fibers 42b arranged in parallel. Each second reinforcement fiber 42b is inclined relative to the longitudinal direction A1. The direction in which each second reinforcement fiber 42b is inclined is opposite to the direction (shown in FIG. 7) in which each first reinforcement fiber 42a is inclined. In FIG. 8, an arrow Ob represents an inclination angle (absolute value) of the second reinforcement fiber 42b relative to the longitudinal direction A1. The inclination angle Ob is greater than or equal to 30 and less than or equal to 60. Each second reinforcement fiber 42b is a bias-type reinforcement fiber. The second fiber reinforced layers 38b include bias-type reinforcement fibers.

[0037] FIG. 9 shows part of the high-elasticity layer 37. As previously described, the number of third fiber reinforced layers 38c included in the high-elasticity layer 37 is four. These third fiber reinforced layers 38c are obtained by folding a sheet-shaped third prepreg 34c in such a manner that the sheet-shaped third prepreg 34c is wound around itself.

[0038] FIG. 10 shows the third prepreg 34c for the high-elasticity layer 37. The third prepreg 34c includes the matrix 40 and a plurality of third reinforcement fibers 42c arranged in parallel. Each third reinforcement fiber 42c extends in the longitudinal direction Al. Each third reinforcement fiber 42c has a zero inclination angle (absolute value) relative to the longitudinal direction A1. Each third reinforcement fiber 42c may be slightly inclined relative to the longitudinal direction A1. In the present specification, a reinforcement fiber 42 having an inclination angle (absolute value) of less than or equal to 10 relative to the longitudinal direction A1 is referred to as a straight-type reinforcement fiber. The third fiber reinforced layers 38c include straight-type reinforcement fibers. In other words, the high-elasticity layer 37 includes the straight-type reinforcement fibers.

[0039] A graph in FIG. 11 shows a relationship between a side pressure stiffness value G1 and a ball-hitting face stiffness value G2 of the frame 4. In this graph, a straight line denoted by reference sign S1 is expressed by a mathematical formula shown below.

[00001]$G2=3.2.Math.G1$

In the case of the tennis racket 2 plotted on the straight line S1 or plotted not on the straight line S1 but above the straight line S1, a ratio (G2/G1) is greater than or equal to 3.20. This racket 2 satisfies a mathematical formula (1) shown below.

[00002]$\begin{array}{cc}G23.2.Math.G1& \left(1\right)\end{array}$

In the case of the tennis racket 2 satisfying the above mathematical formula (1), the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large.

[0040] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the side pressure stiffness value G1 is relatively small. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose side pressure stiffness value G1 is relatively small has excellent repulsion performance.

[0041] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose ball-hitting face stiffness value G2 is relatively large has excellent repulsion performance.

[0042] As previously described, each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction. At the time of measuring the ball-hitting face stiffness value G2 of the tennis racket 2, force in the thickness direction (Z-direction) is applied to the head 14. The force causes the head 14 to bend relative to the shaft 18 in the thickness direction. Since the third reinforcement fibers 42c are straight-type reinforcement fibers, due to the bending, a great tensile stress occurs on the third reinforcement fibers 42c of the high-elasticity layer 37. The third reinforcement fibers 42c suppress the bending. The high-elasticity layer 37 contributes to achieving a large ball-hitting face stiffness value G2.

[0043] At the time of measuring the side pressure stiffness value G1 of the tennis racket 2, force in the width direction (X-direction) is applied to the head 14. The force causes the head 14 to bend inward in the width direction. A stress that occurs on the third reinforcement fibers 42c due to the bending is small. The third reinforcement fibers 42c do not hinder the bending deformation. The tennis racket 2 including the high-elasticity layer 37 can achieve a small side pressure stiffness value G1.

[0044] FIG. 12 shows a method of measuring the side pressure stiffness value G1. In FIG. 12, the tennis racket 2 is placed on a base 44, which is a rigid body. The width direction X of the racket 2 coincides with the vertical direction. The axial direction Y of the racket 2 coincides with the horizontal direction. A plate 46, which is a rigid body, is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the plate 46 is measured from when the load is 25 kgf to when the load is 50 kgf. The side pressure stiffness value G1 is calculated by dividing the load difference 25 kgf by the displacement (cm). The side pressure stiffness value G1 is measured in a state where the string 12 is removed from the frame 4.

[0045] In light of repulsion performance, the side pressure stiffness value G1 is preferably less than or equal to 90 kgf/cm, more preferably less than or equal to 80 kgf/cm, and particularly preferably less than or equal to 75 kgf/cm. The side pressure stiffness value G1 of the tennis racket 2 suitable for practical use is greater than or equal to 20 kgf/cm.

[0046] FIGS. 13A and 13B show a method of measuring the ball-hitting face stiffness value G2. In this measurement, a first bar 48a, a second bar 48b, and a third bar 48c are prepared. The material of these bars 48 is steel. Each bar 48 has a circular cross-sectional shape having a radius of 10.0 mm. Each bar 48 extends in the width direction X. The distance between the first bar 48a and the third bar 48c in the axial direction is 170 mm, and the distance between the third bar 48c and the second bar 48b in the axial direction is 170 mm. The first bar 48a is positioned at the top of the head 14. The racket 2 is placed on the first bar 48a and the second bar 48b. The width direction X and the axial direction Y of the racket 2 coincide with the horizontal direction. The third bar 48c is lowered, and thereby a load is applied to the tennis racket 2. A displacement (cm) of the third bar 48c is measured from when the load is 25 kgf to when the load is 50 kgf. The ball-hitting face stiffness value G2 is calculated by dividing the load different 25 kgf by the displacement (cm). The ball-hitting face stiffness value G2 is measured in a state where the string 12 is removed from the frame 4.

[0047] In light of repulsion performance, the ball-hitting face stiffness value G2 is preferably greater than or equal to 100 kgf/cm, more preferably greater than or equal to 200 kgf/cm, and particularly preferably greater than or equal to 250 kgf/cm. The ball-hitting face stiffness value G2 of the tennis racket 2 suitable for practical use is less than or equal to 500 kgf/cm.

[0048] In FIG. 2, an arrow Lh represents the length of each high-elasticity layer 37 in the axial direction. In the present embodiment, the length Lh is a distance in the axial direction from a top Pt of the head 14 to an end Ed of the high-elasticity layer 37. The length Lh is preferably greater than or equal to 170 mm, more preferably greater than or equal to 250 mm, and particularly preferably greater than or equal to 340 mm for the reason that a large ball-hitting face stiffness value G2 can be achieved with such setting of the length Lh.

[0049] In FIG. 5, an arrow Tf represents the thickness of the frame 4, and an arrow Wf represents the width of the frame 4. A ratio (Tf/Wf) of the thickness Tf to the width Wf is preferably greater than or equal to 2.0. If the ratio (Tf/Wf) of the tennis racket 2 is within this range, a large ratio (G2/G1) can be readily achieved. In light of this, the ratio (Tf/Wf) is more preferably greater than or equal to 2.2, yet more preferably greater than or equal to 2.4, and particularly preferably greater than or equal to 2.8. The ratio (Tf/Wf) of the tennis racket 2 suitable for practical use is less than or equal to 4.0.

[0050] The thickness Tf is preferably greater than or equal to 20.0 mm, more preferably greater than or equal to 26.0 mm, yet more preferably greater than or equal to 28.5 mm, and particularly preferably greater than or equal to 33.0 mm for the reason that a large ratio (G2/G1) can be readily achieved with such setting of the thickness Tf. The thickness Tf of the tennis racket 2 suitable for practical use is less than or equal to 40.0 mm.

[0051] In a graph of FIG. 11, a straight line denoted by reference sign S2 is expressed by a mathematical formula shown below.

[00003]$G2=3.55.Math.G1$

[0052] In the case of the tennis racket 2 plotted on the straight line S2 or plotted not on the straight line S2 but above the straight line S2, the ratio (G2/G1) is greater than or equal to 3.55. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance.

[00004]$G23.55.Math.G1$

[0053] In the graph of FIG. 11, a straight line denoted by reference sign S3 is expressed by a mathematical formula shown below.

[00005]$G2=3.9.Math.G1$

[0054] In the case of the tennis racket 2 plotted on the straight line S3 or plotted not on the straight line S3 but above the straight line S3, the ratio (G2/G1) is greater than or equal to 3.90. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance.

[00006]$G23.9.Math.G1$

[0055] In the graph of FIG. 11, a straight line denoted by reference sign S4 is expressed by a mathematical formula shown below.

[00007]$G2=4.10.Math.G1$

[0056] In the case of the tennis racket 2 plotted on the straight line S4 or plotted not on the straight line S4 but above the straight line S4, the ratio (G2/G1) is greater than or equal to 4.10. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has extremely excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has extremely excellent repulsion performance.

[00008]$G24.1.Math.G1$

Evaluation

Sample 1

[0057] A tennis racket model for simulation was fabricated. The specifications of the tennis racket model were as shown below. [0058] The width of each of the first prepreg and the second prepreg: 250 mm [0059] The thickness Tf of the frame: 29.9 mm [0060] The elastic modulus of the reinforcement fibers in the high-elasticity layer: 80 tf/mm.sup.2 [0061] The thickness of the high-elasticity layer: 0.825 mm

Samples 2 to 120

[0062] Tennis racket models of samples 2 to 120 were fabricated in the same manner as the sample 1, except that the specifications of the tennis racket models were varied as shown in Tables 1 to 6 below.

Simulation

[0063] The side pressure stiffness value G1 and the ball-hitting face stiffness value G2 of each sample were calculated by simulation. Further, a tennis ball was brought into collision with each sample, and the speed of the tennis ball when rebounding off the sample was calculated by simulation. The results are shown in Tables 1 to 6 below.

TABLE-US-00001 TABLE 1 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 1 250 33.8 80 0.825 120 28 4.29 5966 Sample 2 250 33.8 80 1.650 123 29 4.24 5962 Sample 3 375 33.8 40 1.650 182 44 4.14 5953 Sample 4 375 33.8 80 0.825 192 45 4.27 5964 Sample 5 375 33.8 80 1.650 203 45 4.51 5986 Sample 6 500 31.2 80 1.650 250 61 4.10 5950 Sample 7 500 33.8 40 0.825 240 58 4.14 5953 Sample 8 500 33.8 40 1.650 255 60 4.25 5963 Sample 9 500 33.8 80 0.825 264 59 4.47 5983 Sample 10 500 33.8 80 1.650 282 61 4.62 5996 Sample 11 625 31.2 80 1.650 297 72 4.13 5952 Sample 12 625 33.8 40 0.825 289 70 4.13 5952 Sample 13 625 33.8 40 1.650 310 72 4.31 5968 Sample 14 625 33.8 80 0.825 315 71 4.44 5979 Sample 15 625 33.8 80 1.650 341 73 4.67 6000 Sample 16 750 31.2 80 1.650 345 84 4.11 5950 Sample 17 750 33.8 40 0.825 339 82 4.13 5953 Sample 18 750 33.8 40 1.650 365 84 4.35 5971 Sample 19 750 33.8 80 0.825 367 84 4.37 5973 Sample 20 750 33.8 80 1.650 400 86 4.65 5998

TABLE-US-00002 TABLE 2 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 21 250 33.8 40 0.825 107 27 3.96 5938 Sample 22 250 33.8 40 1.650 110 27 4.07 5948 Sample 23 375 31.2 80 0.825 173 44 3.93 5935 Sample 24 375 31.2 80 1.650 181 45 4.02 5943 Sample 25 375 33.8 40 0.825 173 44 3.93 5935 Sample 26 500 31.2 80 0.825 237 59 4.02 5942 Sample 27 625 31.2 80 0.825 279 71 3.93 5935 Sample 28 750 31.2 80 0.825 321 82 3.91 5933

TABLE-US-00003 TABLE 3 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 29 250 31.2 40 1.650 101 28 3.61 5906 Sample 30 250 31.2 80 0.825 110 29 3.79 5923 Sample 31 250 31.2 80 1.650 113 30 3.77 5920 Sample 32 375 28.6 80 0.825 156 44 3.55 5901 Sample 33 375 28.6 80 1.650 163 45 3.62 5908 Sample 34 375 31.2 40 0.825 157 43 3.65 5910 Sample 35 375 31.2 40 1.650 164 44 3.73 5917 Sample 36 375 33.8 0 0.825 128 36 3.56 5902 Sample 37 375 33.8 0 1.650 128 36 3.56 5902 Sample 38 500 28.6 80 0.825 211 59 3.58 5904 Sample 39 500 28.6 80 1.650 221 60 3.68 5913 Sample 40 500 31.2 40 0.825 216 58 3.72 5917 Sample 41 500 31.2 40 1.650 227 59 3.85 5928 Sample 42 625 28.6 80 1.650 259 71 3.65 5910 Sample 43 625 31.2 40 0.825 257 70 3.67 5912 Sample 44 625 31.2 40 1.650 271 71 3.82 5925 Sample 45 625 33.8 0 0.825 241 66 3.65 5910 Sample 46 625 33.8 0 1.650 241 66 3.65 5910 Sample 47 750 28.6 80 1.650 297 82 3.62 5908 Sample 48 750 31.2 40 0.825 298 81 3.68 5913 Sample 49 750 31.2 40 1.650 316 83 3.81 5924 Sample 50 750 33.8 0 0.825 293 79 3.71 5915 Sample 51 750 33.8 0 1.650 293 79 3.71 5915

TABLE-US-00004 TABLE 4 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 52 250 28.6 40 0.825 91 28 3.25 5875 Sample 53 250 28.6 40 1.650 93 29 3.21 5871 Sample 54 250 28.6 80 0.825 101 29 3.48 5895 Sample 55 250 28.6 80 1.650 104 30 3.47 5894 Sample 56 250 31.2 40 0.825 98 28 3.50 5897 Sample 57 375 26 80 1.650 146 45 3.24 5875 Sample 58 375 28.6 40 0.825 141 43 3.28 5878 Sample 59 375 28.6 40 1.650 147 44 3.34 5883 Sample 60 500 26 80 0.825 187 58 3.22 5873 Sample 61 500 26 80 1.650 196 59 3.32 5881 Sample 62 500 28.6 40 0.825 192 58 3.31 5880 Sample 63 500 28.6 40 1.650 201 59 3.41 5889 Sample 64 500 33.8 0 0.825 190 54 3.52 5899 Sample 65 500 33.8 0 1.650 196 54 3.52 5899 Sample 66 625 26 80 1.650 227 69 3.29 5879 Sample 67 625 28.6 40 0.825 225 69 3.26 5876 Sample 68 625 28.6 40 1.650 237 70 3.39 5887 Sample 69 625 28.6 80 0.825 245 70 3.50 5897 Sample 70 625 31.2 0 0.825 216 66 3.27 5877 Sample 71 625 31.2 0 1.650 216 66 3.27 5877 Sample 72 750 26 80 1.650 257 80 3.21 5872 Sample 73 750 28.6 40 0.825 258 80 3.23 5873 Sample 74 750 28.6 40 1.650 273 81 3.37 5886 Sample 75 750 28.6 80 0.825 279 81 3.44 5892 Sample 76 750 31.2 0 0.825 259 78 3.32 5881 Sample 77 750 31.2 0 1.650 259 78 3.32 5881

TABLE-US-00005 TABLE 5 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 78 250 26 0 0.825 53 24 2.21 5783 Sample 79 250 26 0 1.650 53 24 2.21 5783 Sample 80 250 26 40 0.825 83 29 2.86 5841 Sample 81 250 26 40 1.650 86 30 2.87 5841 Sample 82 250 26 80 0.825 93 30 3.10 5862 Sample 83 250 26 80 1.650 95 31 3.06 5859 Sample 84 250 28.6 0 0.825 58 23 2.52 5811 Sample 85 250 28.6 0 1.650 58 23 2.52 5811 Sample 86 250 31.2 0 0.825 63 22 2.86 5841 Sample 87 250 31.2 0 1.650 63 22 2.86 5841 Sample 88 250 33.8 0 0.825 67 21 3.19 5870 Sample 89 250 33.8 0 1.650 67 21 3.19 5870 Sample 90 375 26 0 0.825 93 39 2.38 5799 Sample 91 375 26 0 1.650 93 39 2.38 5799 Sample 92 375 26 40 0.825 126 43 2.93 5847 Sample 93 375 26 40 1.650 131 44 2.98 5851 Sample 94 375 26 80 0.825 140 44 3.18 5869 Sample 95 375 28.6 0 0.825 106 39 2.72 5828 Sample 96 375 28.6 0 1.650 106 39 2.72 5828 Sample 97 375 31.2 0 0.825 118 38 3.11 5862 Sample 98 375 31.2 0 1.650 118 38 3.11 5862 Sample 99 500 26 0 0.825 133 54 2.46 5806 Sample 100 500 26 0 1.650 133 54 2.46 5806 Sample 101 500 26 40 0.825 168 57 2.95 5848 Sample 102 500 26 40 1.650 177 58 3.05 5858 Sample 103 500 28.6 0 0.825 154 54 2.85 5840 Sample 104 500 28.6 0 1.650 154 54 2.85 5840 Sample 105 500 31.2 0 0.825 172 54 3.19 5869

TABLE-US-00006 TABLE 6 Evaluation Results Prepreg Thickness High-elasticity layer Stiffness width Tf Elastic modulus Thickness G2 G1 Speed mm mm tf/mm.sup.2 mm kgf/cm kgf/cm G2/G1 mm/s Sample 106 500 31.2 0 1.650 172 54 3.19 5869 Sample 107 625 26 0 0.825 162 64 2.53 5812 Sample 108 625 26 0 1.650 162 64 2.53 5812 Sample 109 625 26 40 0.825 195 67 2.91 5845 Sample 110 625 26 40 1.650 206 68 3.03 5856 Sample 111 625 26 80 0.825 214 68 3.15 5866 Sample 112 625 28.6 0 0.825 189 65 2.91 5845 Sample 113 625 28.6 0 1.650 189 65 2.91 5845 Sample 114 750 26 0 0.825 191 75 2.55 5813 Sample 115 750 26 0 1.650 191 75 2.55 5813 Sample 116 750 26 40 0.825 221 77 2.87 5842 Sample 117 750 26 40 1.650 234 78 3.00 5853 Sample 118 750 26 80 0.825 240 78 3.08 5860 Sample 119 750 28.6 0 0.825 225 77 2.92 5846 Sample 120 750 28.6 0 1.650 225 77 2.92 5846

[0064] In the case of the samples shown in Table 1, the ratio (G2/G1) is greater than or equal to 4.10. The speed of the ball rebounding off each sample in Table 1 is greater than or equal to 5950 mm/s.

[0065] In the case of the samples shown in Table 2, the ratio (G2/G1) is greater than or equal to 3.90 and less than 4.10. The speed of the ball rebounding off each sample in Table 2 is greater than or equal to 5930 mm/s and less than 5950 mm/s.

[0066] In the case of the samples shown in Table 3, the ratio (G2/G1) is greater than or equal to 3.55 and less than 3.90. The speed of the ball rebounding off each sample in Table 3 is greater than or equal to 5900 mm/s and less than 5930 mm/s.

[0067] In the case of the samples shown in Table 4, the ratio (G2/G1) is greater than or equal to 3.20 and less than 3.55. The speed of the ball rebounding off each sample in Table 4 is greater than or equal to 5870 mm/s and less than 5900 mm/s. In the case of the samples shown in Tables 5 and 6, the ratio (G2/G1) is less than 3.20. The speed of the ball rebounding off each sample in Tables 5 and 6 is less than 5870 mm/s.

[0068] These evaluation results clearly indicate the superiority of the tennis rackets with a large ratio (G2/G1).

Disclosure Items

[0069] The following items each disclose a preferred embodiment.

Item 1

[0070] A racket including a frame including a head. In the frame, a ratio (G2/G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20.

Item 2

[0071] The racket according to item 1, wherein the head includes a high-elasticity layer that is positioned on an inner side in the head in a thickness direction of the head, the high-elasticity layer including straight-type reinforcement fibers.

Item 3

[0072] The racket according to item 2, wherein the high-elasticity layer is present in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head.

Item 4

[0073] The racket according to any one of items 1 to 3, wherein in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head, a ratio (Tf/Wf) of a thickness Tf of the frame to a width Wf of the frame is greater than or equal to 2.0.

Item 5

[0074] The racket according to any one of items 1 to 4, wherein the ratio (G2/G1) is greater than or equal to 3.55.

Item 6

[0075] The racket according to item 5, wherein the ratio (G2/G1) is greater than or equal to 3.90.

Item 7

[0076] The racket according to item 6, wherein the ratio (G2/G1) is greater than or equal to 4.10.

Item 8

[0077] The racket according to any one of items 1 to 7, wherein the side pressure stiffness value G1 is less than or equal to 90 kgf/cm.

Item 9

[0078] The racket according to any one of items 1 to 8, wherein the ball-hitting face stiffness value G2 is greater than or equal to 100 kgf/cm.

[0079] The racket as described above is suitable also for use in, for example, soft tennis, squash, padel, and badminton. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention.