RACKET

Abstract

A racket 2 includes a frame 4, a grip 6, a grommet 10, and a string 12. In the racket 2, an in-plane stiffness index Gi, which is a ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm), is 1.40 or greater; an out-of-plane stiffness index Go, which is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm), is from 60000 to 85000; a moment of inertia Mi about an axis of the racket 2 is from 13500 g.Math.cm.sup.2 to 15000 g.Math.cm.sup.2; and an index of inertia Ii is 0.150 or greater, which is calculated by the following mathematical formula: Ii=Mi/(Wr.Math.Lc) (where Wr is a mass (g) of the racket, and Lc is a distance (mm) from a grip end to a center of gravity of the racket).

Claims

1. A racket, wherein: an in-plane stiffness index Gi, which is a ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm), is greater than or equal to 1.40; an out-of-plane stiffness index Go, which is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm), is greater than or equal to 60000 and less than or equal to 85000; a moment of inertia Mi about an axis of the racket is greater than or equal to 13500 g.Math.cm.sup.2 and less than or equal to 15000 g.Math.cm.sup.2; and an index of inertia Ii is greater than or equal to 0.150, which is calculated by the following mathematical formula: $\mathrm{Ii}=\mathrm{Mi}/\left(\mathrm{Wr}.Math.\mathrm{Lc}\right)$ (where Wr is a mass (g) of the racket, and Lc is a distance (mm) from a grip end of the racket to a center of gravity of the racket).

2. The racket according to claim 1, wherein the mass Wr is greater than or equal to 285 g and less than or equal to 305 g.

3. The racket according to claim 1, comprising a frame, wherein: a material of the frame is a fiber reinforced resin including a plurality of reinforcement fibers; and a ratio of the reinforcement fibers with an inclination angle having an absolute value of greater than or equal to 40 and less than or equal to 50 to all of the plurality of reinforcement fibers included in the frame is greater than or equal to 8% by mass and less than or equal to 30% by mass.

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 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 an enlarged sectional view taken along line IV-IV of FIG. 1.

[0010] FIG. 5 is a perspective view showing part of a manufacturing process of the racket of FIG. 1.

[0011] FIG. 6 is an enlarged view showing part of a prepreg for the tennis racket of FIG. 4.

[0012] FIG. 7 is an enlarged view showing part of another prepreg for the tennis racket of FIG. 4.

[0013] FIG. 8 is an enlarged view showing part of yet another prepreg for the tennis racket of FIG. 4.

[0014] FIG. 9A is a front view showing a method of measuring a top pressure stiffness value of the tennis racket of FIG. 1, and FIG. 9B is a side view thereof.

[0015] FIG. 10 is a front view showing a method of measuring a side pressure stiffness value of the tennis racket of FIG. 1.

[0016] FIG. 11A is a plan view showing a method of measuring a throat stiffness value of the tennis racket of FIG. 1, and FIG. 11B is a front view thereof.

[0017] FIG. 12A is a plan view showing a method of measuring a ball-hitting face stiffness value of the tennis racket of FIG. 1, and FIG. 12B is a front view thereof.

[0018] FIG. 13 is a front view showing a method of measuring a moment of inertia of the tennis racket of FIG. 1.

DETAILED DESCRIPTION

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

[Elements of Tennis Racket]

[0020] FIGS. 1 to 4 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, 2, and 4, 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. 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 resin 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 in the circumferential direction of the head 14.

[0026] The grip 6 is wound around the shaft 18. The grip 6 is formed by a tape. The grip 6 suppresses slip between a hand of a player and the racket 2 when the racket 2 is swung by the player.

[0027] 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. The material of the grommet 10 is typically 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.

[0028] 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.

[0029] 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. The face 20 may be formed by two or more strings 12.

[Manufacturing Method]

[0030] Hereinafter, one example of a method of manufacturing the tennis racket 2 is described with reference to FIG. 5. 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. 5 shows a tubular prepreg 34p and a sheet-shaped prepreg 34s. In FIG. 5, the illustration of the mandrel and the tube is omitted.

[0031] 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. As a result of the winding, a layered body 36 is obtained. In FIG. 5, an arrow A1 represents the longitudinal direction of the layered body 36. Another prepreg 34 is further wound around the layered body 36 if necessary.

[0032] 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.

[0033] 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. The racket 2 includes a plurality of fiber reinforced layers.

[Prepreg]

[0034] FIG. 6 shows a first prepreg 34a. 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. 6, 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 20 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 20 and less than or equal to 60 is referred to as a bias-type reinforcement fiber.

[0035] FIG. 7 shows a second prepreg 34b. 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. 6) in which each first reinforcement fiber 42a is inclined. In FIG. 7, 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 20 and less than or equal to 60. Each second reinforcement fiber 42b is a bias-type reinforcement fiber.

[0036] FIG. 8 shows a third prepreg 34c. 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 A1. 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.

[0037] The frame 4 includes fiber reinforced layers including bias-type reinforcement fibers and fiber reinforced layers including straight-type reinforcement fibers.

[In-Plane Stiffness Index Gi]

[0038] The tennis racket 2 has a proper in-plane stiffness index Gi. The in-plane stiffness index Gi is the ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm). The in-plane stiffness index Gi is calculated by a mathematical formula shown below.

[00001]$\mathrm{Gi}=\mathrm{Git}/\mathrm{Gis}$

[0039] FIG. 9 shows a method of measuring the top pressure stiffness value Git. In FIG. 9, the tennis racket 2 is fixed to a support 44. The support 44 includes a spacer 46. The yoke 22 is placed on the spacer 46. The width direction X of the racket 2 coincides with the horizontal direction. The axial direction Y of the racket 2 coincides with the vertical direction. A plate 48, which is a rigid body, is in contact with the top of the racket 2. The plate 48 is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the plate 48 is measured from when the load is 25 kgf to when the load is 50 kgf. The top pressure stiffness value Git (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The top pressure stiffness value Git is measured in a state where the string 12 is removed from the frame 4.

[0040] In light of repulsion performance, the top pressure stiffness value Git is preferably greater than or equal to 60 kgf/cm, more preferably greater than or equal to 70 kgf/cm, and particularly preferably greater than or equal to 80 kgf/cm. In light of control performance, the top pressure stiffness value Git is preferably less than or equal to 110 kgf/cm, more preferably less than or equal to 100 kgf/cm, and particularly preferably less than or equal to 90 kgf/cm.

[0041] FIG. 10 shows a method of measuring the side pressure stiffness value Gis. In FIG. 10, the tennis racket 2 is placed on a base 50, 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 52, which is a rigid body, is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the plate 52 is measured from when the load is 25 kgf to when the load is 50 kgf. The side pressure stiffness value Gis (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The side pressure stiffness value Gis is measured in a state where the string 12 is removed from the frame 4.

[0042] In light of repulsion performance, the side pressure stiffness value Gis is preferably greater than or equal to 45 kgf/cm, more preferably greater than or equal to 50 kgf/cm, and particularly preferably greater than or equal to 60 kgf/cm. In light of control performance, the side pressure stiffness value Gis is preferably less than or equal to 100 kgf/cm, more preferably less than or equal to 90 kgf/cm, and particularly preferably less than or equal to 80 kgf/cm.

[0043] Preferably, the in-plane stiffness index Gi is greater than or equal to 1.40. The tennis racket 2 with the in-plane stiffness index Gi being greater than or equal to 1.40 has excellent spin performance. It is presumed that the reason why this racket 2 has excellent spin performance is that upon impact with a tennis ball, deflection of the head 14 in the axial direction Y is suppressed. It is presumed that in the case of the head 14 with less deflection in the axial direction Y, the real length of the longitudinal threads 32b in the axial direction upon impact with a tennis ball is greater. Upon impact with a tennis ball, the longitudinal threads 32b deform and then restore, and it is presumed that the deformation and the restoration of the longitudinal threads 32b impart great rotational force to the tennis ball. In light of spin performance, the in-plane stiffness index Gi is more preferably greater than or equal to 1.45, and particularly preferably greater than or equal to 1.50. The upper limit of the range of the in-plane stiffness index Gi of the tennis racket 2 suitable for practical use is 1.80.

[Out-of-Plane Stiffness Index Go]

[0044] The tennis racket 2 has a proper out-of-plane stiffness index Go. The out-of-plane stiffness index Go is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm). The out-of-plane stiffness index Go is calculated by a mathematical formula shown below.

[00002]$\mathrm{Go}=\mathrm{Gos}\mathrm{Goh}$

[0045] FIGS. 11A and 11B show a method of measuring the throat stiffness value Gos. In this measurement, a first bar 54a, a second bar 54b, and a third bar 54c are prepared. The material of these bars 54 is steel. Each bar 54 has a circular cross-sectional shape having a radius of 5.0 mm. Each bar 54 extends in the width direction X. The distance between the first bar 54a and the third bar 54c in the axial direction Y is 100 mm, and the distance between the third bar 54c and the second bar 54b in the axial direction Y is 100 mm. The position of the first bar 54a is shifted toward the head 14 side from one end P1 of each throat 16. The position of the second bar 54b is shifted toward the grip 6 side from the other end P2 of each throat 16. The racket 2 is placed on the first bar 54a and the second bar 54b. The width direction X and the axial direction Y of the racket 2 coincide with the horizontal direction. The third bar 54c is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the third bar 54c is measured from when the load is 25 kgf to when the load is 50 kgf. The throat stiffness value Gos (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The throat stiffness value Gos is measured in a state where the string 12 is removed from the frame 4.

[0046] In light of repulsion performance, the throat stiffness value Gos is preferably greater than or equal to 350 kgf/cm, more preferably greater than or equal to 370 kgf/cm, and particularly preferably greater than or equal to 400 kgf/cm. In light of control performance, the throat stiffness value Gos is preferably less than or equal to 480 kgf/cm, more preferably less than or equal to 460 kgf/cm, and particularly preferably less than or equal to 440 kgf/cm.

[0047] FIGS. 12A and 12B show a method of measuring the ball-hitting face stiffness value Goh. In this measurement, a first bar 56a, a second bar 56b, and a third bar 56c are prepared. The material of these bars 56 is steel. Each bar 56 has a circular cross-sectional shape having a radius of 10.0 mm. Each bar 56 extends in the width direction X. The distance between the first bar 56a and the third bar 56c in the axial direction Y is 170 mm, and the distance between the third bar 56c and the second bar 56b in the axial direction Y is 170 mm. The first bar 56a is positioned at the top Pt of the head 14. The racket 2 is placed on the first bar 56a and the second bar 56b. The width direction X and the axial direction Y of the racket 2 coincide with the horizontal direction. The third bar 56c is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the third bar 56c is measured from when the load is 25 kgf to when the load is 50 kgf. The ball-hitting face stiffness value Goh (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The ball-hitting face stiffness value Goh is measured in a state where the string 12 is removed from the frame 4.

[0048] In light of repulsion performance, the ball-hitting face stiffness value Goh is preferably greater than or equal to 100 kgf/cm, more preferably greater than or equal to 110 kgf/cm, and particularly preferably greater than or equal to 120 kgf/cm. In light of control performance, the ball-hitting face stiffness value Goh is preferably less than or equal to 170 kgf/cm, more preferably less than or equal to 160 kgf/cm, and particularly preferably less than or equal to 150 kgf/cm.

[0049] The out-of-plane stiffness index Go is preferably greater than or equal to 60000 and less than or equal to 85000. The tennis racket 2 with the out-of-plane stiffness index Go being greater than or equal to 60000 has excellent repulsion performance. In light of this, the out-of-plane stiffness index Go is more preferably greater than or equal to 65000, and particularly preferably greater than or equal to 70000. The tennis racket 2 with the out-of-plane stiffness index Go being less than or equal to 85000 has an excellent feel at impact and excellent control performance. In light of this, the out-of-plane stiffness index Go is more preferably less than or equal to 83000, and particularly preferably less than or equal to 80000.

[Moment of Inertia Mi]

[0050] The tennis racket 2 has a proper moment of inertia Mi about the axis Y. FIG. 13 shows a method of measuring the moment of inertia Mi. In this measurement, a part of the racket 2, the part being positioned 15 mm away from the grip end, is fixed to a cord 58. The racket 2 is suspended by the cord 58. The axial direction Y of the racket 2 coincides with the vertical direction. The racket 2 is rotated about the vertical direction Y. period Tc (sec) of the rotation is measured, and the moment of inertia Mi (g.Math.cm.sup.2) is calculated based on a mathematical formula shown below.

[00003]$\mathrm{Mi}=254458.Math.{\left(\mathrm{Tc}/\right)}^2-8357$

[0051] The moment of inertia Mi is measured in a state where the string 12 is removed from the frame 4.

[0052] The moment of inertia Mi is preferably greater than or equal to 13500 g.Math.cm.sup.2 and less than or equal to 15000 g.Math.cm.sup.2. In the case of the tennis racket 2 with the moment of inertia Mi being greater than or equal to 13500 g.Math.cm.sup.2, a change in the angle of the face 20 that occurs when a tennis ball collides with the face 20 on a position that is not the sweet spot is small. In other words, the racket 2 with the moment of inertia Mi being greater than or equal to 13500 g.Math.cm.sup.2 has excellent face stability. In light of this, the moment of inertia Mi is more preferably greater than or equal to 13800 g.Math.cm.sup.2, and particularly preferably greater than or equal to 14000 g.Math.cm.sup.2. With the tennis racket 2 with the moment of inertia Mi being less than or equal to 15000 g.Math.cm.sup.2, a sharp feel at impact can be obtained. In light of this, the moment of inertia Mi is more preferably less than or equal to 14900 g.Math.cm.sup.2, and particularly preferably less than or equal to 14800 g.Math.cm.sup.2.

[Index of Inertia Ii]

[0053] The tennis racket 2 has a proper index of inertia Ii. The index of inertia Ii is calculated by a mathematical formula shown below.

[00004]$\mathrm{Ii}=\mathrm{Mi}/\left(\mathrm{Wr}.Math.\mathrm{Lc}\right)$

In this mathematical formula, Wr is the mass (g) of the racket 2, and Lc is the distance (mm) from the end of the grip 6 to the center of gravity of the racket 2.

[0054] The index of inertia Ii is preferably greater than or equal to 0.150. The tennis racket 2 with the index of inertia Ii being greater than or equal to 0.150 has excellent face stability. In light of this, the index of inertia Ii is more preferably greater than or equal to 0.152, and particularly preferably greater than or equal to 0.153. The upper limit of the range of the index of inertia Ii of the tennis racket 2 suitable for practical use is 0.180.

[Mass]

[0055] The mass of the tennis racket 2 is preferably greater than or equal to 260 g and less than or equal to 320 g. The racket 2 having a mass of greater than or equal to 260 g has excellent repulsion performance. In light of this, the mass of the tennis racket 2 is more preferably greater than or equal to 270 g, and particularly preferably greater than or equal to 285 g. The racket 2 having a mass of less than or equal to 320 g has excellent spin performance. In light of this, the mass of the tennis racket 2 is more preferably less than or equal to 310 g, and particularly preferably less than or equal to 305 g. The mass is measured in a state where the string 12 is removed from the frame 4.

[Performance of Tennis Racket]

[0056] The tennis racket 2 achieves all of the following: the in-plane stiffness index Gi being greater than or equal to 1.40; the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000; the moment of inertia Mi being greater than or equal to 13500 g.Math.cm.sup.2 and less than or equal to 15000 g.Math.cm.sup.2; and the index of inertia Ii being greater than or equal to 0.150. This tennis racket 2 has an excellent balance among repulsion performance, spin performance, and face stability.

[0057] As previously described, the tennis racket 2 includes bias-type reinforcement fibers. The bias-type reinforcement fibers can contribute to achieving the in-plane stiffness index Gi being greater than or equal to 1.40 and the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000. In particular, the reinforcement fibers 42 with an inclination angle (a or b) having an absolute value of greater than or equal to 40 and less than or equal to 50 can contribute to the in-plane stiffness index Gi and the out-of-plane stiffness index Go. The ratio of the reinforcement fibers 42 with an inclination angle having an absolute value of greater than or equal to 40 and less than or equal to 50 to all of the reinforcement fibers 42 included in the frame 4 is preferably greater than or equal to 8% by mass, more preferably greater than or equal to 10% by mass, and particularly preferably greater than or equal to 12% by mass. This ratio is preferably less than or equal to 30% by mass, more preferably less than or equal to 25% by mass, and particularly preferably less than or equal to 20% by mass.

[0058] The in-plane stiffness index Gi being greater than or equal to 1.40 and the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000 can be achieved through adjustments of the material, thickness, density, etc. of the reinforcement fibers 42.

EXAMPLES

Example 1

[0059] A tennis racket of Example 1 was fabricated. The frame of the racket was fabricated to include a plurality of fiber reinforced layers including reinforcement fibers. The ratio of the mass of the reinforcement fibers with an inclination angle (a or b) having an absolute value of greater than or equal to 40 and less than or equal to 50 to the total mass of the reinforcement fibers was 14%. The face area of the racket was 100 in.sup.2. The mass of the racket including the grip, the cap, and the grommet, but without the string, was 300 g. The overall length of the racket was 686 mm. A distance Lc from the grip end of the racket to the center of gravity of the racket was 320 mm. The in-plane stiffness index Gi, the out-of-plane stiffness index Go, the moment of inertia Mi, and the index of inertia li of the racket including the grip, the cap, and the grommet, but without the string, were measured. The in-plane stiffness index Gi was 1.57; the out-of-plane stiffness index Go was 75548; the moment of inertia Mi was 14713 g.Math.cm.sup.2; and the index of inertia Ii was 0.153.

Examples 2 to 4 and Comparative Examples 1 to 6

[0060] Tennis rackets of Examples 2 to 4 and Comparative Examples 1 to 6 were obtained. Tables 1 and 2 below show the specifications of these tennis rackets.

[Ball Speed and Spin Rate]

[0061] Two advanced-level tennis players did a rally by using each tennis racket. The speed (initial speed) and spin rate of a tennis ball immediately after being hit by the tennis racket were measured. The measurement was performed a plurality of times for each tennis racket, and average measurement values for each racket are shown in Tables 1 and 2 below.

[Feel at Impact]

[0062] The tennis players evaluated and rated the face stability of each tennis racket during the rally in accordance with the following grading system. [0063] A: Good [0064] B: Not bad [0065] C: Bad

[0066] The evaluation results are shown in Tables 1 and 2 below.

TABLE-US-00001 TABLE 1 Evaluation results Example 1 Example 2 Example 3 Example 4 Face area [sqin] 100 98 100 100 Mass [g] 300 305 285 270 Lc [mm] 320 315 325 330 Overall length [mm] 686 686 686 686 Gi 1.57 1.42 1.55 1.58 Go 75548 82311 69198 61309 Mi [g*cm.sup.2] 14713 14770 14436 13906 Ii 0.153 0.154 0.156 0.156 Ball speed [km/h] 120.6 121.0 120.2 120.0 Spin rate [rpm] 1260 1200 1290 1310 Face stability A A A A

TABLE-US-00002 TABLE 2 Evaluation Results Comp. Comp. Comp. Comp. Comp. Comp. Example Example Example Example Example Example 1 2 3 4 5 6 Face area [sqin] 100 100 105 100 110 100 Mass [g] 285 285 285 300 255 300 Lc [mm] 325 325 325 320 350 320 Overall length [mm] 686 686 692 686 686 686 Gi 1.22 1.48 1.11 1.63 0.83 1.49 Go 77173 59080 86243 62069 63760 60327 Mi [g*cm.sup.2] 13950 14348 14086 13417 16152 14115 Ii 0.151 0.155 0.152 0.140 0.181 0.147 Ball speed [km/h] 120.2 118.9 121.8 120.0 120.4 120.2 Spin rate [rpm] 1140 1250 1100 1270 1050 1230 Face stability A A A C B B

[0067] It is clear from Tables 1 and 2 that the tennis racket of each Example has an excellent balance among repulsion performance, spin performance, and face stability. These evaluation results clearly indicate the superiority of the racket of each Example.

[Disclosure Items]

[0068] The following items each disclose a preferred embodiment. [0069] [Item 1] [0070] A racket, wherein: an in-plane stiffness index Gi, which is a ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm), is greater than or equal to 1.40; an out-of-plane stiffness index Go, which is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm), is greater than or equal to 60000 and less than or equal to 85000; a moment of inertia Mi about an axis of the racket is greater than or equal to 13500 g.Math.cm.sup.2 and less than or equal to 15000 g.Math.cm.sup.2; and an index of inertia Ii is greater than or equal to 0.150, which is calculated by the following mathematical formula: Ii=Mi/(Wr.Math.Lc) (where Wr is a mass (g) of the racket, and Lc is a distance (mm) from a grip end of the racket to a center of gravity of the racket). [0071] [Item 2] [0072] The racket according to item 1, wherein the mass Wr is greater than or equal to 285 g and less than or equal to 305 g. [0073] [Item 3]

[0074] The racket according to item 1 or 2, including a frame, wherein: a material of the frame is a fiber reinforced resin including a plurality of reinforcement fibers; and a ratio of the reinforcement fibers with an inclination angle having an absolute value of greater than or equal to 40 and less than or equal to 50 to all of the plurality of reinforcement fibers included in the frame is greater than or equal to 8% by mass and less than or equal to 30% by mass.

[0075] 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.