Patent Publication Number: US-7217203-B2

Title: Tennis racket

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2004-055463 filed in Japan on Feb. 27, 2004, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a tennis racket. More particularly, the present invention relates to a lightweight tennis racket for regulation-ball tennis having improved restitution performance, ball controllability, and vibration-damping performance. 
     The so-called “thick racket” which is thick in the out-of-plane direction of the racket frame is commercially available. Female and senior tennis players require the “thick racket” because they desire the tennis racket to have highs ball rebound performance, even though they hit the ball with a small amount of power. That is, they demand a tennis racket that is light in weight and has a high, ball rebound performance. Therefore a fiber reinforced resin is mainly used as the material for the tennis racket because the fiber reinforced resin has a light weight, has a high specific strength, and provides a high degree of freedom when designing the tennis racket. 
     However, the light weight tennis racket has a the problem that in the collision between the tennis racket and the ball, the coefficient of restitution of the ball becomes low according to the law of energy conservation. That is, to make the tennis racket lightweight causes the rebound performance to deteriorate. To solve this problem, it is possible to enhance the moment of inertia of the tennis racket in the swing direction by placing the center of gravity thereof at a position a little closer to the top of the racket frame. However, when the moment of inertia of the tennis racket in the swing direction is large, the player feels that the tennis racket is heavy and thus its operability deteriorates. 
     The light weight tennis racket causes the impact applied thereto when the ball is hit to be readily transmitted to a player&#39;s hand, which causes the player to suffer from tennis elbow. Thus, both female and senior tennis players who participate in competitions requires a tennis racket which has a high face stability and excellent controllability and is light weight. 
     To solve these problems, the present applicant proposed a tennis racket disclosed in Japanese Patent Application Laid-Open No. 2003-175134 (patent document 1). The present applicant developed a tennis racket whose rebound performance, operability, and face stability are improved in a favorable balance by enhancing the rigidity of the racket frame and setting the ratio between the swing-direction moment of inertia affecting the rebound performance thereof and the center-direction moment of inertia affecting the face stability thereof to a predetermined range. 
     However, in the tennis racket shown in patent document 1, attention was not paid to an improvement of its vibration-absorbing performance. 
     In Japanese Patent Application Laid-Open No. 2000-300698 (patent document 2), as shown in  FIG. 19 , the string protection member  2  is constructed of a vibration-damping member  3  in which the cylindrical portion  3   a  through which strings are inserted and the belt-shaped portion  3   b  connected with the cylindrical portion  3   a ; and the belt-shaped protection member  4  covering the periphery of the belt-shaped portion  3   b  of the vibration-damping member  3 . The weight member  5 , made of a material having a specific gravity of not less than 1.5 and the vibration-damping member  3  are mounted on the racket frame  1  by holding down the weight member  5  and the vibration-damping member  3  with the protection member  4 . Accordingly, the tennis racket exhibits improved rebound performance, face stability, and vibration-absorbing performance. 
     However, the tennis racket shown in patent document  2  is not constructed to increase the deformation amount of the string protection member  2 . Thus the rebound performance of the racket frame cannot be effectively improved, and its ball-flying performance cannot be enhanced. Another problem with this tennis racket is that the number of component parts increases which makes it difficult to achieve a light weight. Thereby the operability of the tennis racket may deteriorate. 
     In addition to the means disclosed in the above patent documents, the following rebound performance-improving means are conceivable: 
     (1) The area of the face of the racket frame is increased to widen the string-movable range. 
     (2) The in-plane rigidity of the frame is increased. 
     (3) The elasticity of the frame is made high. 
     However, means (1) has the problem that because the area of the face is increased, the weight and the moment of inertia of the tennis racket is increased and hence its operability deteriorates. Means (2) has the problem that the moldability deteriorates due to the alteration of the sectional configuration of the frame caused by the formation of a layered construction or a reinforcing portion. Means (3) has a the problem that the strength of the frame deteriorates. 
     Patent document 1: Japanese Patent Application Laid-Open No. 2003-175134 
     Patent document 2: Japanese Patent Application Laid-Open No. 2000-300698 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view the above-described problem. Therefore, it is an object of the present invention to provide a tennis racket that has a high vibration-damping performance and a high rebound performance without making the tennis racket heavy, and has a high degree of controllability due to an improvement in face stability. 
     To achieve this object, there is provided a tennis racket including a racket frame having a weight not less than 100 g nor more than 270 g. A string protection member is provided on at least one portion of a peripheral surface of a head part surrounding the ball-hitting face of the racket frame. The string protection member has a plurality of cylindrical portions through which strings are respectively inserted and a belt-shaped portion. Supposing that a midpoint of a maximum length of the ball-hitting face of the racket frame is set as a center thereof and that an intersection of a longest line of the ball-hitting face and an upper part of the ball-hitting face is set as a 0-degree position, the string protection member is mounted on at least one portion of the head part in a range from a clockwise 45-degree position to a clockwise 135-degree position and in a range from a clockwise 225-degree position to a clockwise 315-degree position by interposing the viscoelastic member between the string protection member and the racket frame. A moment (Is) of inertia of the tennis racket in a swing direction is set to not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2 , when the strings are not tensionally mounted thereon. A moment (Ic) of inertia of the tennis racket in a center direction is set to not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 , when the strings are not tensionally mounted thereon. 
     As described above, by mounting the viscoelastic member on at least one portion of the head part in the range from the 45-degree position to the 135-degree position and in the range from the 225-degree position to the 315-degree position, it is possible to enhance the moment of inertia in the swing direction and the center direction in a favorable balance, making it possible to improve the rebound performance and controllability of the tennis racket. 
     That is, when the string protection member is mounted on the above-described range, the weight thereof is applied to the outer side of the tennis racket with respect to its axis passing through the axis of the grip part. Therefore the moment of inertia in the center direction increases and the tennis racket has difficulty in its rotation on its axis. Thereby the tennis racket has face stability. However, when the string protection member is mounted on the top side of the racket frame disposed upward from the 45-degree position and the 315-degree position, the center of gravity of the tennis racket is disposed a little nearer to the top position of the racket from its center. Consequently the moment of inertia of the tennis racket in the swing direction is large, whereas the moment of inertia thereof in the center direction is not large. Thus the rebound performance of the racket frame is enhanced but its operability and face stability deteriorate. When the string protection member is mounted on the lower side of the racket frame disposed downward from the 135-degree position and the 225-degree position, neither the moment of inertia of the tennis racket in the center direction, nor the moment of inertia thereof in the swing direction is large. Thus neither the rebound performance of the racket frame nor its face stability is improved. 
     It is necessary to mount the string protection member on at least one portion of the above-described angular range and possible to extend the mounting-range of the string protection member from the above-described angular range. 
     It is favorable to mount at least one portion of the string protection member on the head part in the range from a 60-degree position to a 120-degree position and the range from a 240-degree position to a 300-degree position and more favorable to mount at least one portion of the string protection member on the head part in the range from a 75-degree position to a 105-degree position and the range from a 255-degree position to a 285-degree position. It is particularly favorable to mount one string protection member on the head part with the center of the string protection member disposed at a 90-degree position and a 270-degree position. The line connecting the 90-degree position and the 270-degree position with each other forms the longest width of the racket frame. This is because the above-described ranges increase the moment of inertia in the swing direction and the center direction in a favorable balance. Thereby it is possible to realize a high rebound performance, face stability, and operability. 
     The string protection member is mounted favorably in only the range from a 35-degree position to a 145-degree position and the range from a 215-degree position to a 325-degree position, more favorably in only the range from a 50-degree position to a 130-degree position and the range from a 230-degree position to a 310-degree position, and most favorably in only the range from a 65-degree position to a 115-degree position and the range from a 245-degree position to a 295-degree position. This is because if the string protection member is mounted in a range other than the above-described angular range, the tennis racket is heavy and its operability is low. 
     The angular difference between a start angular position of the string protection member and a termination angular position thereof is set to not less than 10 degrees, favorably not less than 15 degrees, and more favorably not less than 20 degrees. The angular difference between the start angular position of the string protection member and the termination angular position thereof is set to not more than 60 degrees, favorably not more than 40 degrees, more favorably not more than 30 degrees, and most favorably not more than 20 degrees. 
     The reason the angular difference between the start angular position of the string protection member and the termination angular position thereof is set to less than 10 degrees nor more than 60 degrees is as follows: If the mounting range of the string protection member is too long, the tennis racket is so heavy that its operability is low. If the mounting range of the string protection member is too short, the effect of enhancing the rebound performance of the racket frame and its face stability is insufficient. 
     The reason the moment Is of the inertia of the tennis racket in the swing direction when the strings are not tensionally mounted on the racket frame is set to not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2  is as follows: If the moment of inertia of the tennis racket in the swing direction is less than 450,000 g/cm 2 , the tennis racket has a favorable operability but has a low rebound performance. If the moment of inertia of the tennis racket in the swing direction is more than 490,000 g/cm 2 , the tennis racket has an unfavorable operability. The moment of inertia of the tennis racket in the swing direction is set to favorably not less than 455,000 g/cm 2 , more favorably not less than 456,000 g/cm 2 , and most favorably not less than 460,000 g/cm 2 . The moment of inertia of the tennis racket in the swing direction is set to favorably not more than 480,000 g/cm 2 , more favorably not more than 476,000 g/cm 2 , and most favorably not more than 470,000 g/cm 2 . 
     The reason the moment Ic of the inertia of the tennis racket in the center direction when the strings are not tensionally mounted on the racket frame is set to not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2  is as follows: If the moment of inertia of the tennis racket in the center direction is set to less than 15,000 g/cm 2 , the tennis racket has an unfavorable face stability. If the moment of inertia of the tennis racket in the center direction is more than 19,000 g/cm 2 , the tennis racket has a large ball-hitting face or heavy. Thus the tennis racket has an unfavorable operability. The moment of inertia of the tennis racket in the center direction is set to favorably not less than 16,000 g/cm 2 , more favorably not less than 16,300 g/cm 2 , and most favorably not less than 16,400 g/cm 2 . The moment of inertia of the tennis racket in the center direction is set to favorably not more than 18,000 g/cm 2 , more favorably not more than 17,900 g/cm 2 , and most favorably not more than 17,300 g/cm 2 . 
     By interposing the viscoelastic member between the frame and the string protection member, the viscoelastic member restrains vibrations of strings from being transmitted to the frame, even though the racket frame has a high strength and elasticity, thereby effectively damping the vibrations of the frame. 
     As the viscoelastic member, rubber, elastomer, and resin having a low elastic modulus are preferable. Rubber only or rubber mixed with carbon black is particularly preferable. 
     The viscoelastic member has a hole through which a cylindrical portion of the string protection member is penetrated, is interposed between a belt-shaped portion of the string protection member and a peripheral surface of the head part of the racket frame; and is plate-shaped. Since the viscoelastic member has the above-described configuration, it is possible to mount the viscoelastic member on the peripheral surface of the head part in a certain length and reliably fix the viscoelastic member between the string protection member and the frame. 
     The thickness of the viscoelastic member is not less than 1 mm nor more than 5 mm. The complex elastic modulus of the viscoelastic member measured at a frequency of 10 Hz is not less than 2.0E+7 dyn/cm 2  nor more than 1.0E+10 dyn/cm 2  at temperatures in the range of 0° C. to 10° C. 
     If the thickness of the viscoelastic member is less than 1 mm, it is impossible to sufficiently improve the rebound performance and vibration-absorbing performance of the racket frame. If the thickness of the viscoelastic member is more than 5 mm, the weight of the racket frame increases and hence its operability deteriorates. If the complex elastic modulus of the viscoelastic member is less than 2.0E+7 dyn/cm 2 , concentration of a stress on the frame is generated and hence the frame is liable to be broken. If the complex elastic modulus of the viscoelastic member is more than 2.0E+7 dyn/cm 2 , the string is deformed to a low extent by a load applied thereto when a ball is hit. Consequently the viscoelastic member is incapable of obtaining a sufficient spring effect. Further the rebound performance of the racket frame cannot be improved and a non-resonance occurs. Thus the viscoelastic member does not function as a vibration damper. The complex elastic modulus of the viscoelastic member is favorably not less than 1.0E+8 dyn/cm 2 , and more favorably not less than 3.86E+8 dyn/cm 2  nor more than 2.72E+9 dyn/cm 2 . 
     In the above-described construction, because the viscoelastic member is mainly functioned as the vibration-damping member, the string protection member is not demanded to have a high vibration-damping function. Thus it is unnecessary to form the string protection member from a soft material. Thereby the cylindrical portion which contacts the strings and the string protection member having the cylindrical portion are capable of having have rigidity to some extent. Therefore it is possible to hold down the viscoelastic member with the viscoelastic member being covered with the string protection member. Further it is possible to improve the durability of the string protection member and prevent the strings from biting into the string protection member. Thereby it is possible to prevent a stress from being collectively applied to the frame. Therefore it is possible to enhance the strength and durability of the racket frame. 
     The string protection member is required to have the durability securely. Thus Shore D hardness is set to favorably not less than 50 nor more than 80 and more favorably not less than 55 nor more than 75. More specifically, it is preferable that the string protection member is formed by molding thermoplastic resin such as nylon 11, nylon 12, polyether block amide, polyamide resin, and the like. Thereby the string protection member has vibration-absorbing performance to some extent and rigidity to some extent. 
     By interposing the viscoelastic member between the frame and the string protection member, the string protection member can be deformed by utilizing the deformability of the viscoelastic member. Consequently the spring effect can be obtained. Thereby the ball rebound performance can be enhanced. 
     The viscoelastic member is lightweight and is capable of performing its function only by mounting it on a predetermined portion of the peripheral surface of the frame. Therefore the viscoelastic member is capable of complying with the demand for making the tennis racket lightweight. 
     It is preferable that a bumper made of fiber reinforced resin is interposed between the string protection member and the viscoelastic member. In this construction, since the bumper made of the fiber reinforced resin is rigid, the spring effect of the viscoelastic member can be displayed sufficiently, which is preferable. 
     The width of the belt-shaped portion of the string protection member is enlarged so that the string protection member has a configuration of covering both outer surfaces of the head part between which a string groove thereof is interposed. The string protection member is mounted on the head part by interposing the viscoelastic member between the belt-shaped portion the string protection member and the head part, with the viscoelastic member covering an entire lower surface of the belt-shaped portion. 
     As apparent from the foregoing description, the moment (Is) of inertia of the tennis racket in the swing direction is set to not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2 , when the strings are not tensionally mounted thereon. The moment (Ic) of inertia of the tennis racket in the center direction is set to not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 , when the strings are not tensionally mounted thereon. Therefore it is possible to enhance the moment of inertia in the swing direction affecting the rebound performance of the tennis racket and that in the center direction affecting the face stability in a favorable balance. Thereby the tennis racket of the present invention is capable of maintaining preferable operability and having improved ball rebound performance and controllability. 
     When the strings are tensionally mounted on the ball-hitting face with the strings in penetration through the string protection member, the viscoelastic member interposed between the string protection member and the frame absorbs vibrations of the strings generated when a ball is hit, thereby suppressing vibrations of the frame. Further the string protection member is capable of obtaining the spring effect by utilizing the deformability of the viscoelastic member. Thereby the ball rebound performance of the racket frame can be also enhanced in this respect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view showing a tennis racket of a first embodiment of the present invention. 
         FIG. 2  is an exploded perspective view showing main parts of the tennis racket shown in  FIG. 1 . 
         FIGS. 3A and 3B  are sectional views showing the procedure of mounting a string protection member and a viscoelastic member on the frame of the tennis racket shown in  FIG. 1 . 
         FIG. 4  is a front view showing a string-stretching part of the tennis racket shown in  FIG. 1 . 
         FIG. 5  is a front view showing a head part of a tennis racket of a second embodiment of the present invention. 
         FIG. 6  is a front view showing a head part of a tennis racket of a third embodiment of the present invention. 
         FIG. 7  is a front view showing a head part of a tennis racket of a fourth embodiment of the present invention. 
         FIG. 8  is a front view showing the head part of the tennis racket of the fourth embodiment of the present invention. 
         FIG. 9  is a front view showing a head part of a tennis racket of a sixth embodiment of the present invention. 
         FIG. 10  is a front view showing a head part of a tennis racket of a comparison example 4. 
         FIG. 11  is a front view showing a head part of a tennis racket of a comparison example 5. 
         FIG. 12  is a front view showing a head part of a tennis racket of a comparison example 6. 
         FIG. 13  is a front view showing a head part of a tennis racket of a comparison example 7. 
         FIG. 14  is a front view showing a head part of a tennis racket of a comparison example 8. 
         FIG. 15A  is an exploded perspective view showing a string protection member according to another embodiment of the present invention. 
         FIG. 15B  is a sectional view showing a state in which the string protection member shown in  FIG. 15A  and the viscoelastic member are mounted on a racket frame. 
         FIGS. 16A and 16B  are schematic views showing a method of measuring the moment of inertia of a racket frame. 
         FIG. 17  is a schematic view showing a method of measuring the rebound performance of a racket frame. 
         FIGS. 18A ,  18 B, and  18 C are schematic views showing a method of measuring the vibration-damping factor of a racket frame. 
         FIG. 19  is a sectional view showing a conventional tennis racket. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described below with reference to the drawings. The embodiments which will be described below are suitable for a racket frame for regulation-ball tennis. 
       FIGS. 1 through 4  show a first embodiment of the present invention. In a tennis racket  10 , one string protection member  21  is mounted at two portions on the peripheral side of a head part  12  surrounding a ball-hitting face F. Strings S are mounted on the racket frame, with a viscoelastic member  31  interposed between the string protection member  21  and the racket frame  11 , as shown in  FIG. 3B . 
     The racket frame  11  includes the head part  12 , a throat part  13 , a shaft part  14 , and a grip part  15 . These parts  12 ,  13 ,  14 , and  15  are continuously formed. One end of a yoke  17  is connected to the one-side throat part  13 , and the other end thereof is connected to the other-side throat part  13  so that the yoke  17  and the head part  12  form a string-stretching part G surrounding the ball-hitting face F. As shown in  FIGS. 2 and 3 , a groove portion  18  on which the viscoelastic member  31  and the string protection member  21  are mounted is circumferentially and continuously formed on the peripheral surface of the head part  12 . As shown in  FIG. 4 , a plurality of string holes  19  through which strings S respectively are inserted are formed in and penetrate through the frame  11  in a direction perpendicular to the frame (thickness) direction of the frame  11 . That is, the string holes  19  are formed on the frame  11  in its widthwise direction. 
     As shown in  FIG. 2 , the string protection member  21  has a plurality of cylindrical portions  22  in which string insertion holes  22   a  are respectively formed for inserting strings S therethrough. A belt-shaped portion  23  connects the cylindrical portions  22  to each other in such a way that the cylindrical portions  22  are projected inward. The belt-shaped portion  23  has a thickness of 1 mm and a large width corresponding to the thickness of the racket frame  11 . The belt-shaped portion  23  has a string groove  24  formed at its center in its widthwise direction. The string protection member  21  has a configuration of an assembly of a grommet formed integrally with a bumper. The string protection member  21  is made of fiber reinforced resin so that the string protection member  21  is rigid. More specifically an epoxy resin is added to carbon fiber (RC: 43%). 
     The viscoelastic member  31  has a thickness of 3 mm and is substantially flat with a sectional configuration corresponding to that of the peripheral surface of the head part  12 . A plurality of through-holes  32  through which the cylindrical portions  22  of the string protection member  21  are respectively inserted, extend through the viscoelastic member  31 . 
     The viscoelastic member  31  is formed by molding rubber having a lower elastic modulus than that of the fiber reinforced resin of the string protection member  21 . More specifically, the viscoelastic member  31  is formed by molding a vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber, 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The complex elastic modulus of the viscoelastic member  31  measured at a frequency of 10 Hz is not less than 2.0E+7 dyn/cm 2  nor more than 1.0E+10 dyn/cm 2  at temperatures in the range of 0° C. to 10° C. 
     With reference to  FIG. 1 , supposing that the midpoint of the maximum length of the ball-hitting face F of the racket frame  11  is denoted as a center O thereof and that the intersection of the longest line of the ball-hitting face F and the upper part of the ball-hitting face F is set as a 0-degree position, one string protection member  21  is mounted in a range A 1  disposed from a clockwise 35-degree position to a clockwise (“clockwise” is omitted hereinafter) 55-degree position and a range A 2  disposed from a 305-degree position to a 325-degree position. More specifically, supposing that the ball-hitting face F is regarded as a clock face, the string protection member  21  is mounted in the range A 1  in which a 1.5 o&#39;clock position is disposed at the central position and in the range A 2  in which a 10.5 o&#39;clock position is disposed at the central position. Therefore the ranges A 1  and A 2  form 20 degree respectively. Each string protection member  21  has a weight of 2 g. 
     When the string protection member  21  and the viscoelastic member  31  are mounted on the string-stretching part G of the racket frame  11 , the cylindrical portions  22  of the string protection member  21  are inserted into the through-holes  32  of the viscoelastic member  31  respectively. Thereafter the viscoelastic member  31  is mounted on the inner peripheral side of the string protection member  21 . 
     Thereafter all the cylindrical portions  22  of the string protection member  21  on which the viscoelastic member  31  has been mounted are inserted into the string holes  19  of the ranges A 1  and A 2  of the racket frame  11 . Thereby as shown in  FIG. 3B , the viscoelastic member  31  is interposed between the frame  11  and the string protection member  21  with the viscoelastic member  31  embedded in the groove portion  18  of the frame  11 . Finally the strings S are mounted crosswise on the frame  11 . Each viscoelastic member  31  has a weight of 3 g. 
     In the tennis racket  10  having the above-described construction, at least one portion of the range in which the string protection member  21  and the viscoelastic member  31  are mounted is included in the range from a 45-degree position to a 135-degree position and in the range from a 225-degree position to a 315-degree position. 
     The moment (Is) of inertia of the tennis racket  10  in the swing direction is set to is not less than 450,000 g/cm 2  nor more than 490,000 glcm 2 , when the strings S are not tensionally mounted thereon. The moment (Ic) of inertia of the tennis racket  10  in the center direction is set to be not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 , when the strings S are not tensionally mounted thereon. 
     By setting the moment of inertia to the above-described range, it is possible to maintain a high rebound performance and face stability. Thereby it is possible to improve the performance of the racket frame of repulsing a ball together with ball controllability thereof, in a favorable balance. 
     The total of the weight of the string protection member  21  and the viscoelastic member  31  is set to 5 g. Thus the tennis racket has an increase of only 10 g by the mounting of the string protection member  21  and the viscoelastic member  31  in the ranges A 1  and A 2 . Therefore a preferable operability can be maintained without making the weight of the racket frame heavy. 
     The viscoelastic member  31  interposed between the frame  11  and the string protection member  21  is made of rubber having a lower elastic modulus than that of the material of the string protection member  21 . Thus the viscoelastic member  31  is capable of effectively damping and absorbing vibrations of the strings generated when a ball is hit, which are transmitted to the frame  11 . Further the string protection member  21  can be deformed by utilizing the deformability of the viscoelastic member  31 . Thereby the racket frame is capable of enhancing the performance of repulsing the ball and improving the flight performance of the ball. 
     Because the string protection member  21  that contacts the string S directly is made of a fiber reinforced resin, the string protection member  21  has a certain degree of vibration-damping performance and yet has a necessary degree of rigidity. Therefore it is possible to increase the durability of the string protection member  21  and that of the frame  11 . 
       FIG. 5  shows a second embodiment of the present invention. In the second embodiment, the string protection member  21  and the viscoelastic member  31  are mounted in a range B 1  disposed from an 80-degree position to a 100-degree position and a range B 2  disposed from a 260-degree position to a 280-degree position. 
     That is, supposing that the ball-hitting face F is regarded as the clock face, the string protection member  21  is mounted in the range B 1  in which a 3 o&#39;clock position is disposed at the central position and in the range B 2  in which a 9 o&#39;clock position is disposed at the central position. Therefore the ranges B 1  and B 2  form 20 degrees respectively. 
       FIG. 6  shows a third embodiment of the present invention. In the third embodiment, the string protection member  21  and the viscoelastic member  31  are mounted in a range C 1  disposed from a 125-degree position to a 145-degree position and a range C 2  disposed from a 215-degree position to a 235-degree position. That is, supposing that the ball-hitting face F is regarded as the clock face, the string protection member  21  is mounted in the range C 1  in which a 4.5 o&#39;clock position is disposed at the central position and in the range C 2  in which a 7.5 o&#39;clock position is disposed at the central position. Therefore the ranges C 1  and C 2  form 20 degrees respectively. 
       FIG. 7  shows a fourth embodiment of the present invention. In the fourth embodiment, the string protection member  21  and the viscoelastic member  31  are mounted in a range D 1  disposed from a 50-degree position to a 70-degree position and a range D 2  disposed from a 290-degree position to a 310-degree position respectively. That is, supposing that the ball-hitting face F is regarded as the clock face, the string protection member  21  is mounted in the range D 1  in which a 2 o&#39;clock position is disposed at the central position and in the range D 2  in which a 10 o&#39;clock position is disposed at the central position. Therefore the ranges D 1  and D 2  form 20 degrees respectively. 
       FIG. 8  shows a fifth embodiment of the present invention. In the fifth embodiment, the string protection member  21  and the viscoelastic member  31  are mounted in a range E 1  disposed from a 110-degree position to a 130-degree position and a range E 2  disposed from a 230-degree position to a 250-degree position. That is, supposing that the ball-hitting face F is regarded as the clock face, the string protection member  21  is mounted in the range E 1  in which a 4 o&#39;clock position is disposed at the central position and in the range E 2  in which an 8 o&#39;clock position is disposed at the central position. Therefore the ranges E 1  and E 2  form 20 degrees respectively. 
     In each of the second embodiment through the fifth embodiment, the viscoelastic member  31  is formed by molding the vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber, 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The complex elastic modulus of the viscoelastic member  31  measured at a frequency of 10 Hz is not less than 2.0E+7 dyn/cm 2  nor more than 1.0E+10 dyn/cm 2  at temperatures in the range of 0° C. to 10° C. The tennis racket has an increase of only 10 g by the mounting of the string protection member  21  and the viscoelastic member  31  on the racket frame. 
     Since the constructions of the other parts are similar to those of the first embodiment, the same parts are denoted by the same reference numerals and description thereof is omitted herein. 
     At least one portion of the range in which the string protection member  21  and the viscoelastic member  31  are mounted is included in the range from the 45-degree position to the 135-degree position and in the range from the 225-degree position to the 315-degree position in each of the second embodiment in which the string protection member  21  and the viscoelastic member  31  are mounted on both side positions of the head part  12 , the third and fifth embodiments in which the string protection member  21  and the viscoelastic member  31  are mounted on lower positions of the head part  12 , and the fourth embodiment in which the string protection member  21  and the viscoelastic member  31  are mounted on upper positions of the head part  12 . The moment (Is) of inertia of the tennis racket in the swing direction is set to not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2 , when the strings S are not tensionally mounted on the racket frame. The moment (Ic) of inertia of the tennis racket in the center direction is set to not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 , when the strings S are not tensionally mounted on the racket frame. Thereby the frame  11  is capable of maintaining a high rebound performance and enhancing its face stability and hence improving its rebound performance and ball controllability in a favorable balance. The viscoelastic member  31  absorbs vibrations of the strings, thereby damping vibrations of the frame  11  sufficiently. 
       FIG. 9  shows a sixth embodiment of the present invention. In the sixth embodiment, the string protection member  21  and the viscoelastic member  31  are mounted in a range B 1 ′ disposed from a 70-degree position to a 110-degree position and a range B 2 ′ disposed from a 250-degree position to a 290-degree position. That is, supposing that the ball-hitting face F is regarded as the clock face, the string protection member  21  is mounted in the range B 1 ′ in which the 3 o&#39;clock position is disposed at the central position and in the range B 2 ′ in which the 9 o&#39;clock position is disposed at the central position. Therefore the ranges E 1  and E 2  form 40 degrees respectively. The string protection member  21  has a thickness of 2 mm and a weight of 4 g. The viscoelastic member  31  has a thickness of 5 mm and a weight of 5 g. 
     Since the constructions of the other parts are similar to those of the first embodiment, the same parts are denoted by the same reference numerals and description thereof is omitted herein. 
     In the sixth embodiment, the string protection member  21  and the viscoelastic member  31  are thick and disposed in a long range. Thus the total weight of the tennis racket increases by 18 g. However, the tennis racket is capable of displaying the effect of the present invention effectively. Therefore the moment (Is) of inertia of the tennis racket in the swing direction is not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2 , when the strings S are not tensionally mounted on the racket frame. The moment (Ic) of inertia of the tennis racket in the center direction is not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 , when the strings S are not tensionally mounted on the racket frame. Thereby the frame is capable of maintaining a high rebound performance and enhancing its face stability and hence improving its rebound performance and ball controllability in a favorable balance. 
     EXAMPLES 
     A tennis racket of each of the examples 1 through 10 and comparison examples 1 through 9 was prepared to evaluate the characteristics thereof by measuring the coefficient of restitution and the like of each tennis racket. 
     As shown in tables 1 through 3, the tennis rackets were prepared by differentiating the mounting position of the string protection member and the viscoelastic member; and the material, complex elastic modulus, and thickness of the viscoelastic member. The coefficient of restitution, sweet area, and vibration-damping factor of each tennis racket were measured. A ball-hitting test was also conducted. 
     The complex elastic moduli shown in tables 1 and 2 were measured by using a DVE-V4 produced by Leology Inc. at 5° C. under the conditions shown below. In the tennis racket of the examples 1 through 6, 8, 9 and the comparison examples 4 through 9, the complex elastic modulus of the viscoelastic member was not less than 2.0E+7 dyn/cm 2  nor more than 1.0E+10 dyn/cm 2  at temperatures in the range of 0° C. to 10° C.: 
     Specimen: 5 mm (width)×30 mm (length)×2 mm (thickness) 
     Length of deformed portion of specimen: 20 mm (both sides having length of 5 mm were supported) 
     Initial strain: 10% (2 mm) 
     Amplitude: 12 μm 
     Frequency: 10 Hz 
     Mode: Stretching mode 
     The “mounted position” shown in tables 1 and 2 means the position where the string protection member was mounted, with the viscoelastic member interposed between the string protection member and the frame. Each mounted position is indicated by an hour in the right-hand side of the head part in the range from 12 o&#39;clock to 6 o&#39;clock. The string protection member is mounted symmetrically in the left-to-right direction. Therefore when the mounted position is 3 o&#39;clock, the string protection member is mounted at a 3-o&#39;clock position and a 9-o&#39;clock position. 
     The total weight of the viscoelastic member and the string protection member means the sum of the weight of the viscoelastic member and the string protection member at the left-hand side and the weight thereof at the right-hand side. Thus when the viscoelastic member and the string protection member are mounted at the 3-o&#39;clock position and the 9-o&#39;clock position, the total weight of the viscoelastic member and the string protection member is described as 5 g×2=10 g. 
     The weight of the viscoelastic member is the weight of one viscoelastic member. The weight of the string protection member is the weight of one string protection member. 
     Table 3 shows a start position, a termination position, and a center position of the string protection member of each of the examples and the comparison examples. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 {circle around (1)} 
                 {circle around (2)} 
                 {circle around (3)} 
                 {circle around (4)} 
                 {circle around (5)} 
               
               
                   
               
               
                 Mounted position 
                 1.5 o&#39;clock 
                 3 o&#39;clock 
                 4.5 o&#39;clock 
                 3 o&#39;clock 
                 2 o&#39;clock 
               
               
                 Total weight of viscoelastic member and string protection 
                 10 g 
                 10 g 
                 10 g 
                 18 g 
                 10 g 
               
               
                 member 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Viscoelastic member 
                 Kind 
                 SBR + Carbon 
                 SBR + Carbon 
                 SBR + Carbon 
                 SBR + Carbon 
                 SBR + Carbon 
               
               
                   
                 Complex elastic modulus [dyn/cm 2 ] 
                 3.86E + 08 
                 3.86E + 08 
                 3.86E + 08 
                 3.86E + 08 
                 3.86E + 08 
               
               
                   
                 Thickness [mm] 
                 3 
                 3 
                 3 
                 5 
                 3 
               
               
                   
                 Weight [g] 
                 3 
                 3 
                 3 
                 5 
                 3 
               
               
                 String protection member 
                 Weight [g] 
                 2 
                 2 
                 2 
                 4 
                 2 
               
               
                 Weight of frame 
                 [g] 
                 240 
                 239 
                 240 
                 247 
                 239 
               
               
                 Balance 
                 [mm] 
                 363 
                 361 
                 359 
                 365 
                 362 
               
               
                 Moment of inertia 
                 Is (swing direction) [g · cm 2 ] 
                 460,000 
                 456,000 
                 450,000 
                 476,000 
                 458,000 
               
               
                   
                 Ic (center direction) [g · cm 2 ] 
                 16,300 
                 17,200 
                 16,400 
                 18,700 
                 16,800 
               
               
                   
                   
                 28 
                 27 
                 27 
                 25 
                 27 
               
               
                 Coefficient of restitution 
                 [−] 
                 0.416 
                 0.424 
                 0.418 
                 0.430 
                 0.420 
               
            
           
           
               
               
               
               
               
               
            
               
                 Sweet area (coefficient of restitution not less than 0.38) [cm 2 ] 
                 70 
                 68 
                 70 
                 76 
                 69 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Vibration-damping factor 
                 Primary out-of-plane [%] 
                 0.51 
                 0.50 
                 0.70 
                 0.55 
                 0.50 
               
               
                   
                 Secondary out-of-plane [%] 
                 0.50 
                 0.70 
                 0.45 
                 0.75 
                 0.60 
               
               
                 Evaluation by ball-hitting 
                 Operability 
                 4.0 
                 4.1 
                 4.2 
                 3.6 
                 4.0 
               
               
                   
                 Face stability 
                 3.8 
                 4.1 
                 3.8 
                 4.5 
                 3.9 
               
               
                   
                 Ball-flying performance 
                 3.8 
                 4.0 
                 3.6 
                 4.2 
                 3.8 
               
               
                   
                 Vibration-damping performance 
                 3.8 
                 4.0 
                 4.0 
                 4.0 
                 3.7 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 {circle around (6)} 
                 {circle around (7)} 
                 {circle around (8)} 
                 {circle around (9)} 
                 
                   
                 
               
               
                   
               
               
                 Mounted position 
                 4 o&#39;clock 
                 3 o&#39;clock 
                 3 o&#39;clock 
                 3 o&#39;clock 
                 3 o&#39;clock 
               
               
                 Total weight of viscoelastic member and string protection 
                 10 g 
                 10 g 
                 10 g 
                 10 g 
                 10 g 
               
               
                 member 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Viscoelastic member 
                 Kind 
                 SBR + Carbon 
                 silicon 
                 SBR 
                 PEBAX5533 
                 11-NYLON 
               
               
                   
                 Complex elastic modulus [dyn/cm 2 ] 
                 3.86E + 08 
                 1.41 + 07 
                 5.07E + 07 
                 2.72E + 09 
                 1.45E + 10 
               
               
                   
                 Thickness [mm] 
                 3 
                 3 
                 3 
                 3 
                 3 
               
               
                   
                 Weight [g] 
                 3 
                 3 
                 3 
                 3 
                 3 
               
               
                 String protection member 
                 Weight [g] 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 Weight of frame 
                 [g] 
                 239 
                 239 
                 239 
                 239 
                 239 
               
               
                 Balance 
                 [mm] 
                 360 
                 361 
                 361 
                 361 
                 361 
               
               
                 Moment of inertia 
                 Is (swing direction) [g · cm 2 ] 
                 453,000 
                 456,000 
                 455,000 
                 455,000 
                 456,000 
               
               
                   
                 Ic (center direction) [g · cm 2 ] 
                 169,000 
                 17,200 
                 17,300 
                 17,300 
                 17,200 
               
               
                   
                   
                 27 
                 27 
                 26 
                 26 
                 27 
               
               
                 Coefficient of restitution 
                 [−] 
                 0.421 
                 0.416 
                 0.421 
                 0.423 
                 0.414 
               
            
           
           
               
               
               
               
               
               
            
               
                 Sweet area (coefficient of restitution not less than 0.38) [cm 2 ] 
                 69 
                 59 
                 67 
                 68 
                 58 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Vibration-damping factor 
                 Primary out-of-plane [%] 
                 0.61 
                 0.30 
                 0.45 
                 0.45 
                 0.30 
               
               
                   
                 Secondary out-of-plane [%] 
                 0.60 
                 0.33 
                 0.60 
                 0.55 
                 0.35 
               
               
                 Evaluation by ball-hitting 
                 Operability 
                 4.1 
                 4.1 
                 4.0 
                 4.0 
                 4.1 
               
               
                   
                 Face stability 
                 3.9 
                 4.0 
                 4.1 
                 4.1 
                 4.1 
               
               
                   
                 Ball-flying performance 
                 3.9 
                 3.8 
                 3.9 
                 4.0 
                 3.7 
               
               
                   
                 Vibration-damping performance 
                 3.9 
                 3.0 
                 3.7 
                 3.6 
                 3.1 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                   
                 Comparison 
                 Comparison 
                 Comparison 
                 Comparison 
                 Comparison 
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 {circle around (1)} 
                 {circle around (2)} 
                 {circle around (3)} 
                 {circle around (4)} 
                 {circle around (5)} 
               
               
                   
               
               
                 Mounted position 
                 Not 
                 3 o&#39;clock 
                 3 o&#39;clock 
                 TOP 
                 TOP 
               
               
                   
                 mounted 
               
               
                 Total weight of viscoelastic member and string protection 
                   
                 10 g 
                 24 g 
                 5 g 
                 15 g 
               
               
                 member 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Viscoelastic member 
                 Kind 
                 Not 
                 Lead 
                 Lead 
                 SBR + Carbon 
                 SBR + Carbon 
               
               
                   
                   
                 mounted 
               
               
                   
                 Complex elastic modulus [dyn/cm 2 ] 
                   
                   
                   
                 3.86E + 08 
                 3.86E + 08 
               
               
                   
                 Thickness [mm] 
                   
                   
                   
                 3 
                 3 
               
               
                   
                 Weight [g] 
                   
                   
                   
                 3 
                 9 
               
               
                 String protection member 
                 Weight [g] 
                 Not 
                 Not 
                 Not 
                 2 
                 6 
               
               
                   
                   
                 mounted 
                 mounted 
                 mounted 
               
               
                 Weight of frame 
                 [g] 
                 230 
                 239 
                 240 
                 234 
                 244 
               
               
                 Balance 
                 [mm] 
                 355 
                 361 
                 360 
                 363 
                 375 
               
               
                 Moment of inertia 
                 Is (swing direction) [g · cm 2 ] 
                 434,000 
                 456,000 
                 450,000 
                 450,000 
                 500,000 
               
               
                   
                 Ic (center direction) [g · cm 2 ] 
                 14,300 
                 17,200 
                 19,200 
                 14,400 
                 14,400 
               
               
                   
                   
                 30 
                 27 
                 23 
                 31 
                 35 
               
               
                 Coefficient of restitution 
                 [−] 
                 0.400 
                 0.410 
                 0.412 
                 0.405 
                 0.407 
               
            
           
           
               
               
               
               
               
               
            
               
                 Sweet area (coefficient of restitution not less than 0.38) [cm 2 ] 
                 32 
                 53 
                 45 
                 44 
                 49 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Vibration-damping factor 
                 Primary out-of-plane [%] 
                 0.30 
                 0.32 
                 0.32 
                 0.40 
                 0.45 
               
               
                   
                 Secondary out-of-plane [%] 
                 0.29 
                 0.30 
                 0.33 
                 0.41 
                 0.50 
               
               
                 Evaluation by ball-hitting 
                 Operability 
                 4.5 
                 4.2 
                 4.0 
                 4.2 
                 3.0 
               
               
                   
                 Face stability 
                 3.0 
                 4.0 
                 4.3 
                 3.2 
                 3.3 
               
               
                   
                 Ball-flying performance 
                 2.9 
                 3.3 
                 3.3 
                 3.1 
                 3.2 
               
               
                   
                 Vibration-damping performance 
                 2.8 
                 3.0 
                 3.0 
                 3.5 
                 3.6 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Comparison 
                 Comparison 
                 Comparison 
                 Comparison 
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 {circle around (6)} 
                 {circle around (7)} 
                 {circle around (8)} 
                 {circle around (9)} 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Mounted position 
                 TOP 
                 3 o&#39;clock 
                 TOP 
                 Yoke 
                 3 o&#39;clock 
                 3 o&#39;clock 
               
               
                 Total weight of viscoelastic member and string protection member 
                 5 g 
                 10 g 
                 5 g 
                 5 g 
                 24 g 
                 6 g 
               
            
           
           
               
               
               
               
               
               
            
               
                 Viscoelastic member 
                 Kind 
                 SBR + Carbon 
                 SBR + Carbon 
                 SBR + Carbon 
                 SBR + Carbon 
               
               
                   
                 Complex elastic modulus [dyn/cm 2 ] 
                 3.86E + 08 
                 3.86E + 08 
                 3.86E + 08 
                 3.86E + 08 
               
               
                   
                 Thickness [mm] 
                 3 
                 3 
                 7 
                 1 
               
               
                   
                 Weight [g] 
                 3 
                 3 
                 7 
                 1 
               
               
                 String protection member 
                 Weight [g] 
                 2 
                 2 
                 5 
                 2 
               
               
                 Weight of frame 
                 [g] 
                 244 
                 240 
                 253 
                 235 
               
               
                 Balance 
                 [mm] 
                 372 
                 363 
                 370 
                 359 
               
               
                 Moment of inertia 
                 Is (swing direction) [g · cm 2 ] 
                 497,000 
                 470,000 
                 510,000 
                 443,000 
               
               
                   
                 Ic (center direction) [g · cm 2 ] 
                 17,900 
                 14,500 
                 19,200 
                 16,400 
               
               
                   
                   
                 28 
                 32 
                 27 
                 27 
               
               
                 Coefficient of restitution 
                 [−] 
                 0.427 
                 0.410 
                 0.438 
                 0.416 
               
            
           
           
               
               
               
               
               
            
               
                 Sweet area (coefficient of restitution not less than 0.38) [cm 2 ] 
                 70 
                 46 
                 88 
                 60 
               
            
           
           
               
               
               
               
               
               
            
               
                 Vibration-damping factor 
                 Primary out-of-plane [%] 
                 0.52 
                 0.70 
                 0.52 
                 0.50 
               
               
                   
                 Secondary out-of-plane [%] 
                 0.75 
                 0.42 
                 0.90 
                 0.50 
               
               
                 Evaluation by ball-hitting 
                 Operability 
                 3.0 
                 3.8 
                 2.9 
                 4.3 
               
               
                   
                 Face stability 
                 4.4 
                 3.3 
                 4.6 
                 3.8 
               
               
                   
                 Ball-flying performance 
                 4.0 
                 3.2 
                 4.5 
                 3.6 
               
               
                   
                 Vibration-damping performance 
                 4.0 
                 4.0 
                 4.3 
                 3.8 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Start position 
                 Termination 
                 Center 
               
               
                   
                 Number 
                 (angle) 
                 position (angle) 
                 position (angle) 
               
               
                   
                 of string 
                 of string 
                 of string 
                 of string 
               
               
                   
                 protection 
                 protection 
                 protection 
                 protection 
               
               
                   
                 members 
                 member 
                 member 
                 member 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparison 
                 1 
                 350 
                  10 
                  0 
               
               
                 Example 4 
               
               
                 Comparison 
                 1 
                 330 
                  30 
                  0 
               
               
                 Example 5 
               
               
                 Example 1 
                 2 
                  35 
                  55 
                  45 
               
               
                   
                   
                 325 
                 305 
                 315 
               
               
                 Example 2 
                 2 
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Example 3 
                 2 
                 125 
                 145 
                 135 
               
               
                   
                   
                 235 
                 215 
                 225 
               
               
                 Comparison 
                 3 
                 350 
                  10 
                  0 
               
               
                 Example 6 
                   
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Comparison 
                 2 
                 350 
                  10 
                  0 
               
               
                 Example 7 
                   
                 170 
                 190 
                 180 
               
               
                 Example 4 
                 2 
                  70 
                 110 
                  90 
               
               
                   
                   
                 290 
                 250 
                 270 
               
               
                 Comparison 
                 2 
                  65 
                 115 
                  90 
               
               
                 Example 8 
                   
                 295 
                 245 
                 270 
               
               
                 Comparison 
                 2 
                  80 
                 100 
                  90 
               
               
                 Example 9 
                   
                 280 
                 260 
                 270 
               
               
                 Example 7 
                 2 
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Example 8 
                 2 
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Example 9 
                 2 
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Example 10 
                 2 
                  80 
                 100 
                  90 
               
               
                   
                   
                 280 
                 260 
                 270 
               
               
                 Example 5 
                 2 
                  50 
                  70 
                  60 
               
               
                   
                   
                 310 
                 290 
                 300 
               
               
                 Example 6 
                 2 
                 110 
                 130 
                 120 
               
               
                   
                   
                 250 
                 230 
                 240 
               
               
                   
               
            
           
         
       
     
     The racket frames  11  of the examples 1 through 10 and the comparison examples 1 through 9 were made of fiber reinforced resin and hollow. The racket frames had the same configurations and had a thickness of 28 mm and a width of 13 to 16 mm. The area of the ball-hitting face F was 115 square inches. The weight of each racket frame and the balance thereof were set as shown in table 1. 
     More specifically, prepreg sheets (CF prepreg (T300, T700, T800, M46J manufactured by Toray Industries Inc.) composed of thermosetting resin reinforced with carbon fiber were layered one upon another on a mandrel (φ14.5 mm) covered with an internal-pressure tube made of nylon 66 was fitted. Thereby a cylindrical laminate was formed. The prepreg sheets were layered one upon another at angles of 0°, 22°, 30°, and 90°. After the mandrel was removed from the laminate, the laminate was set in a die. After the die was clamped, the die was heated at 150° C. for 30 minutes, with an air pressure of 9 kgf/cm 2  kept applied to the inside of the inner-pressure tube. 
     In each of the racket frames of the examples 1 through 10 and the comparison examples 1 through 9, the string protection member  21  was formed by molding a mixture of carbon fiber and epoxy resin. 
     Example 1 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the first embodiment. That is, the viscoelastic member  31  was formed by molding a vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber (SBR), 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. One string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges A 1  and A 2 . The tennis racket  10  had a weight of 240 g. 
     The moment Is of inertia of the tennis racket in the swing direction was set to 460,000 g/cm 2 , and the moment Ic of inertia thereof in the center direction was set to 16,300 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 28). In measuring the moment of inertia, the strings were not mounted on the racket frame. 
     Example 2 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the second embodiment ( FIG. 5 ). That is, the example 2 is different from the example 1 in that one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1  and B 2 . The tennis racket  10  had a weight of 239 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 456,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Example 3 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the third embodiment ( FIG. 6 ). That is, the example 3 is different from the example 1 in that one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges C 1  and C 2 . The tennis racket  10  had a weight of 240 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 450,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 16,400 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Example 4 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the sixth embodiment ( FIG. 9 ). That is, the example 4 is different from the example 1 in that one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1 ′ and B 2 ′. The viscoelastic member  31  had a thickness of 5 mm and a weight of 5 g. The string protection member  21  had a thickness of 2 mm and a weight of 4 g. The tennis racket  10  had a weight of 247 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 476,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 18,700 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 25). 
     Example 5 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the fourth embodiment ( FIG. 7 ). That is, the example 5 is different from the example 1 in that one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges D 1  and D 2 . The tennis racket  10  had a weight of 239 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 458,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 16,800 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Example 6 
     The thickness, weight, and position of the string protection member  21  and the material, complex elastic modulus, thickness, weight, and position of the viscoelastic member  31  were all identical to those of the fifth embodiment ( FIG. 8 ). That is, the example 6 is different from the example 1 in that one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges E 1  and E 2 . The tennis racket  10  had a weight of 239 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 453,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 16,900 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Example 7 
     The material of the viscoelastic member  31  of the example 2 was varied to form the viscoelastic member  31  of the example 7. More specifically, one string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1  and B 2 . The viscoelastic member  31  was formed by molding silicone rubber. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 1.41E+07 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. The tennis racket  10  had a weight of 239 g. 
     The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 456,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Example 8 
     The material of the viscoelastic member  31  of the example 2 was varied to form the viscoelastic member  31  of the example 8. More specifically, the viscoelastic member  31  was formed by molding a vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber (SBR) and 1.5 parts by weight of sulfur. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 5.07E+07 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. One string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1  and B 2 . The tennis racket  10  had a weight of 239 g. 
     The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 455,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,300 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 26). 
     Example 9 
     The material of the viscoelastic member  31  of the example 2 was varied to form the viscoelastic member  31  of the example 9. More specifically, the viscoelastic member  31  was formed by molding PEBAX5533 (produced by ATOCHEM Inc.). The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 2.72E+09 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. One string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1  and B 2 . The tennis racket  10  had a weight of 239 g. 
     The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 455,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,300 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 26). 
     Example 10 
     The material of the viscoelastic member  31  of the example 2 was varied to form the viscoelastic member  31  of the example 10. More specifically, the viscoelastic member  31  was formed by molding nylon 11. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 1.45 E+10 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. One string protection member  21  and one viscoelastic member  31  were disposed in each of the above-described ranges B 1  and B 2 . The tennis racket  10  had a weight of 239 g. 
     The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 456,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Comparison Example 1 
     Neither the string protection member  21  nor the viscoelastic member  31  was mounted on the racket frame  11 . The tennis racket had a weight of 230 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 434,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 14,300 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 30). 
     Comparison Example 2 
     Let it be supposed that the 0-degree position of the frame  11  is the 12 o&#39;clock position of a clock. Five grams of lead was mounted on the 3 o&#39;clock position (90-degree position) and the 9 o&#39;clock position (270-degree position). The tennis racket had a weight of 239 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 456,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Comparison Example 3 
     The weight of the frame  11  was reduced by 14 g. Twelve grams of lead was mounted on the 3 o&#39;clock position and the 9 o&#39;clock position. The tennis racket had a weight of 240 g. The moment Is of inertia of the tennis racket in the swing direction when strings were not mounted on the racket frame was set to 450,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 19,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 23). 
     Comparison Example 4 
     As shown in  FIG. 10 , one string protection member  21  and one viscoelastic member  31  were mounted in a range H forming 20 degrees in the range from a 350-degree position to a 10-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the top position of the head part  12  of the frame  11 . The viscoelastic member  31  was formed by molding a vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber (SBR), 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. The tennis racket had a weight of 234 g. 
     The moment Is of inertia of the tennis racket of the comparison example 4 in the swing direction when strings were not mounted on the racket frame was set to 450,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 14,400 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 31). 
     Comparison Example 5 
     As shown in  FIG. 11 , one string protection member  21  and one viscoelastic member  31  were mounted in a range H′ forming 60 degrees in the range from a 330-degree position to a 30-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the top position of the head part  12  of the frame  11 . The viscoelastic member  31  was formed by molding the vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber (SBR), 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The viscoelastic member  31  had a thickness of 3 mm and a weight of 9 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 6 g. The tennis racket had a weight of 244 g. 
     The moment Is of inertia of the tennis racket of the comparison example 5 in the swing direction when strings were not mounted on the racket frame was set to 500,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 14,400 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 35). 
     Comparison Example 6 
     As shown in  FIG. 12 , one string protection member  21  and one viscoelastic member  31  were mounted on each of the above-described range B 1 , the above-described range B 2 , and the above-described range H forming 20 degrees in the range from the 350-degree position to the 10-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the top position of the head part  12  of the frame  11 . The viscoelastic member  31  was formed by molding the vulcanized rubber composition consisting of 100 parts by weight of styrene-butadiene rubber (SBR), 1.5 parts by weight of sulfur, and 40 parts by weight of carbon black. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. The tennis racket had a weight of 244 g. 
     The moment Is of inertia of the tennis racket of the comparison example 6 in the swing direction when strings were not mounted on the racket frame was set to 497,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 17,900 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 28). 
     Comparison Example 7 
     As shown in  FIG. 13 , one string protection member  21  and one viscoelastic member  31  were mounted on each of the above-described range H forming 20 degrees in the range from the 350-degree position to the 10-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the top position (12 o&#39;clock position) of the head part  12  of the frame  11  and a range I forming 20 degrees in the range from a 170-degree position to a 190-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the 6 o&#39;clock position of the head part  12 . The viscoelastic member  31  was formed by molding the vulcanized rubber composition consisting of 100 parts by weight of the styrene-butadiene rubber (SBR), 1.5 parts by weight of the sulfur, and 40 parts by weight of the carbon black. The viscoelastic member  31  had a thickness of 3 mm and a weight of 3 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. The tennis racket had a weight of 240 g. 
     The moment Is of inertia of the tennis racket of the comparison example 6 in the swing direction when strings were not mounted on the racket frame was set to 470,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 14,500 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 32). 
     Comparison Example 8 
     As shown in  FIG. 14 , the range in which the string protection member  21  and the viscoelastic member  31  were disposed was set longer than that of the example 4. The thickness of each of the string protection member  21  and the viscoelastic member  31  was also set larger than that of the example 4. More specifically, one string protection member  21  and one viscoelastic member  31  were mounted on each of a range B 1 ″ forming 50 degrees between a 65-degree position to a 115-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the 3 o&#39;clock position of the head part  12  of the frame  11  and a range B 2 ″ forming 50 degrees in the range from a 245-degree position to a 295-degree position, with the center of the string protection member  21  and the viscoelastic member  31  disposed at the 9 o&#39;clock position of the head part  12 . The viscoelastic member  31  was formed by molding the vulcanized rubber composition consisting of 100 parts by weight of the styrene-butadiene rubber (SBR), 1.5 parts by weight of the sulfur, and 40 parts by weight of the carbon black. The viscoelastic member  31  had a thickness of 7 mm and a weight of 7 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 2.5 mm and a weight of 5 g. The tennis racket had a weight of 253 g. 
     The moment Is of inertia of the tennis racket of the comparison example 8 in the swing direction when strings were not mounted on the racket frame was set to 510,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 19,200 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Comparison Example 9 
     The thickness and weight of the viscoelastic member  31  of the comparison example 9 were set smaller than those of the viscoelastic member of the example 2. More specifically, one string protection member  21  and one viscoelastic member  31  were mounted on each of the above-described ranges B 1  and B 2 . The viscoelastic member  31  was formed by molding the vulcanized rubber composition consisting of 100 parts by weight of the styrene-butadiene rubber (SBR), 1.5 parts by weight of the sulfur, and 40 parts by weight of the carbon black. The viscoelastic member  31  had a thickness of 1 mm and a weight of 1 g. The complex elastic modulus of the viscoelastic member  31  measured in the above-described condition was 3.86E+08 dyn/cm 2 . The string protection member  21  had a thickness of 1 mm and a weight of 2 g. The tennis racket had a weight of 235 g. 
     The moment Is of inertia of the tennis racket of the comparison example 6 in the swing direction when strings were not mounted on the racket frame was set to 443,000 g/cm 2 . The moment Ic of inertia thereof in the center direction when strings were not mounted on the racket frame was set to 16,400 g/cm 2  (the ratio of the moment Is of inertia to the moment Ic of inertia: about 27). 
     Measurement of Moment of Inertia 
     As shown in  FIG. 16(A) , each tennis racket  10  was hung with an instrument for measuring the moment of inertia thereof, with the grip  15  thereof located uppermost to measure a swing period Ts thereof. The moment of inertia thereof in the swing direction (moment of inertia in out-of-plane direction on grip end) was computed by the following equations. 
     As shown in  FIG. 16(B) , each tennis racket was hung with the instrument for measuring the moment of inertia thereof, with the grip  15  thereof located uppermost to measure the center period Ts thereof. The moment of inertia thereof in the center direction (moment of inertia on axis of grip) was computed by the following equations. 
     Calculation of Moment of Inertia 
     
         
         
           
             Swing direction: Is (g·cm 2 )
 
 Is=M×g×h ( Ts/ 2/π) 2   −Ic  
 
             Center direction: Ic (g·cm 2 )
 
 Ic= 254458×( Tc/π)   2 −8357
 
             Around center of gravity: Ig
 
 Ig=Is−m (l+2.6) 2  
 
Where M=m+mc, h=(m×l−mc×lc)/m+2.6, mc: weight of tennis racket, l: balance point of tennis racket, mc: weight of chuck, lc: balance point of chuck.
 
Measurement of Coefficient of Restitution
 
           
         
       
    
     As shown in  FIG. 17 , strings were tensionally mounted on a tennis racket  10  of each of the examples and the comparison examples at a tensile force of 60 pounds applied thereto longitudinally and 55 pounds applied thereto widthwise. The grip part  15  was fixed at a weak force to allow the tennis racket  10  to be free, with the tennis racket set vertically. A tennis ball driven by a ball-launching machine collided with the ball-hitting face of the tennis racket at a constant speed V1 (30 m/sec) to measure a speed V2 of the rebound tennis ball. The coefficient of restitution is obtained by computing the ratio of the launched speed V1 to the rebounded speed V2. The higher the coefficient of restitution is, the longer the tennis ball was rebounded. 
     Measurement of Primary Out-of-Plane Vibration-Damping Factor 
     As shown in  FIG. 18A , the upper end of the head part  12  of the racket frame  11  of each of the examples and the comparison examples was hung with a string  51 . An acceleration pick-up meter  53  was fixed vertically to the ball-hitting face at one connection point between the head part  12  and the throat part  13 . In this state, as shown in  FIG. 18B , the other connection point between the head part  12  and the throat part  13  was hit with an impact hammer  55  to impart vibration to the racket frame  11 . An input vibration (F) measured with a force pick-up meter installed on the impact hammer  55  and a response vibration (α) measured with the acceleration pick-up meter  53  were inputted to a frequency analyzer  57  (manufactured by Hewlett Packard Corp., dynamic single analyzer HP 3562A) through amplifiers  56 A and  56 B to analyze the input vibration (F) and the response vibration (α). A transmission function in a frequency region obtained by the analysis was determined to obtain the frequency of the tennis racket. The vibration-damping ratio (ζ) was computed by using the following equation to obtain the primary out-of-plane vibration-damping factor. Table 1 shows the average value of the primary out-of-plane vibration-damping factor of the racket frame of each of the examples and the comparison examples.
 
ζ=(½)×(Δω/ω n )
 
 To=Tn ×√{square root over (2)}
 
Measurement of Secondary Out-of-Plane Vibration-Damping Factor
 
     As shown in  FIG. 18C , the upper end of the head part  12  of the racket frame  11  of each of the examples and the comparison examples was hung with the string  51 . The acceleration pick-up meter  53  was fixed vertically to the ball-hitting face at one connection point between the head part  12  and the throat part  13 . In this state, to vibrate the racket frame  11 , the rear surface of the racket frame  11  was hit with the impact hammer  55  at the portion of the rear surface thereof opposite to the portion of the front surface thereof where the acceleration pick-up meter  53  was mounted. The vibration-damping factor was computed by a method equivalent to that used in computing the primary out-of-plane vibration-damping factor to obtain the secondary out-of-plane vibration-damping factor. Table 1 shows the average value of the secondary out-of-plane vibration-damping factor of the racket frame  11  of each of the examples and the comparison examples. 
     Evaluation of Tennis Racket by Hitting Ball 
     To examine the operability, face stability (controllability), rebound performance, and vibration-absorbing performance of each tennis racket, a questionnaire was conducted by requesting testers to hit tennis balls therewith. The questionnaire paper was marked on the basis of five (the more, the better). The operability, face stability (controllability), rebound performance, and vibration-absorbing performance of each tennis racket were evaluated on the basis of the average of marks given by 33 middle and high class players (who satisfied the condition that testers have more than 10 years&#39; experience of tennis and play tennis three or more days a week). 
     The frame of the comparison example 1 was more lightweight by about 15 g/5 mm than the conventional frame. As can be confirmed in table 1, the moment of inertia of the frame of the comparison example 1 was small in both the swing direction and the center direction. It was confirmed that the tennis racket of the comparison example 1 had a favorable operability, but had unfavorable ball-flying (ball rebound) performance, face stability and vibration-absorbing performance. In the tennis racket of the comparison example 2, the weight having five grams was mounted on the 3 o&#39;clock position and the 9 o&#39;clock position of the frame. The tennis racket of the comparison example 2 had a larger moment of inertia than that of the comparison example 1. Therefore the tennis racket of the comparison example 2 had improved rebound performance and face stability but had unfavorable vibration-absorbing performance. In the tennis racket of the comparison example 3, to increase the moment of inertia in the center direction and decrease the moment of inertia in the swing direction, the weight of the frame of the comparison example 3 was reduced by 14 g. The weight having 12 g was mounted on the 3 o&#39;clock position (90-degree position) and the 9 o&#39;clock position (270-degree position) of the frame. The tennis racket of the comparison example 3 had favorable operability and face stability but its ball-flying performance was equal to that of the tennis racket of the comparison example 2. 
     The position and length of the string protection member and the material and thickness of the viscoelastic member were examined. 
     Comparison is made between the tennis racket of the comparison example 2 having no viscoelastic member mounted thereon and the tennis racket of the example 2 having the viscoelastic member mounted thereon. The moment of inertia of the tennis racket of the comparison example 2 was equal to that of the tennis racket of the example 2. But the former had much improvement over the latter in the coefficient of restitution and vibration-absorbing performance thereof. This is attributed to the fact that the viscoelastic member was mounted on the 3 o&#39;clock position (90-degree position) and the 9 o&#39;clock position (270-degree position) of the frame of the former, which improved the secondary out-of-plane vibration-damping factor thereof. It has been found that the viscoelastic member mounted on the above-described positions improves not only the vibration-absorbing performance of the racket frame but also its rebound performance. 
     Comparison is made between the tennis rackets of the comparison examples 4 through 7 and the tennis rackets of the examples 1 through 3, 5, and 6. In the tennis racket of each of the examples 1 through 3, 5, and 6 and the comparison example 6, the viscoelastic member was mounted on at least one portion of the head part in the range from the 45-degree position to the 135-degree position and in the range from the 225-degree position to the 315-degree position. The tennis racket of each of the examples 1 through 3, 5, and 6 and the comparison example 6 had a high coefficient of restitution. The rebound performance, face stability, operability, and vibration-absorbing performance of the tennis racket of each of the examples 1 through 3, 5, and 6 were rated highly. In the tennis racket of each of the comparison examples 4, 5, and 7, neither the string protection member nor the viscoelastic member was disposed in the above-described range of the head part of the frame. Thus the moment of inertia of the tennis racket each of the comparison examples 4, 5, and 7 in the swing direction was more than 490,000 g/cm 2 , and the moment (Ic) of inertia thereof in the center direction was less than 15,000 g/cm 2 . Thus the ball-flying performance and face stability of the tennis racket of each of the comparison examples 4, 5, and 7 were rated low. 
     Comparison is made between the tennis racket of the example 2, the example 4, and the comparison example 8 is made. The string protection members of these tennis rackets were different in the length (angle) thereof. The tennis racket of the comparison example 8 having the 60-degree range in which the string protection member was disposed was rated more highly than the tennis racket of the example 4 having the 40-degree range in which the string protection member was disposed. The tennis racket of the example 4 having the 40-degree range in which the string protection member was disposed was rated more highly than the tennis racket of the example 2 having the 20-degree range in which the string protection member was disposed. The moment of inertia of the tennis racket each of the comparison example 8 in the swing direction was more than 490,000 g/cm 2 , and the moment of inertia thereof in the center direction was more than 19,000 g/cm 2 . Thus the operability of the tennis racket of the comparison example 8 was rated low. 
     The moment of inertia of the tennis racket of each of the examples 1 through 11 in the swing direction was not less than 450,000 g/cm 2  nor more than 490,000 g/cm 2 . The moment of inertia thereof in the center direction was not less than 15,000 g/cm 2  nor more than 19,000 g/cm 2 . Thus these tennis rackets were rated highly in the ball-flying performance, face stability, and operability. On the other hand, the moment of inertia of the tennis racket of each of the comparison examples 1 through 9 was out of the above-described range in the swing direction and in the center direction. Thus the tennis racket of each of the comparison examples 1 through 9 was rated low in the operability, rebound performance or face stability thereof or low in all of the operability, ball-flying performance, and face stability thereof. 
     Comparison is made between the tennis racket of the example 2 and the tennis racket (thickness of viscoelastic member: 7 mm) of the comparison example 8 and the tennis racket (thickness of viscoelastic member: 1 mm) of the comparison example 9. It has been found that the tennis racket of the example 2 in which the thickness of the viscoelastic member was not less than 1 mm nor more than 5 mm was superior to the tennis racket of the comparison examples 8 and 9 in the operability, ball-flying performance, face stability, and vibration-absorbing performance. 
     Comparison is made between the tennis racket of the example 2 and the tennis racket of the examples 7 through 10 in terms of the material of the viscoelastic member. The complex elastic modulus of the viscoelastic member used for the tennis racket of the examples 2, 8, and 9 measured at the frequency of 10 Hz was not less than 2.0E+7 dyn/cm 2  nor more than 1.0E+10 dyn/cm 2  at temperatures in the range of 0° C. to 10° C. Therefore the tennis racket of the examples 2, 8, and 9 had high vibration-absorbing performance. 
     The present invention is not limited to the above-described embodiments or examples. For example, as shown in  FIGS. 15A and 15B , the string protection member  21  may be constructed of a grommet part  21 A composed of the cylindrical portion  22  and the narrow belt-shaped portion  23  and a wide plate-shaped part  21 B, made of FRP, separate from the grommet part  21 A. A plurality of through-holes  25  through which the cylindrical portions  22  are inserted respectively is formed in penetration through the plate-shaped part  21 B.