1. Field of the Invention
The present invention relates to a tennis racket and more particularly to a tennis racket having a frame portion, a shaft portion, the frame and shaft portions being integrally formed of a fiber-reinforced synthetic resin, a grip portion contiguous to the lower end of the shaft portion, and a throat portion provided at the inside lower end of the frame portion to define a ball striking area.
2. Description of the Prior Art
Recently, FRP tennis rackets having frame and shaft portions integrally formed of a fiber-reinforced synthetic resin have been becoming predominant in the market in place of wooden or aluminum rackets. In designing the tennis racket of this type, a so-called "pliancy" of the shaft portion at the time of striking a ball is an important factor as in the rackets of other materials. Various manufacturers now market several kinds of tennis rackets which are different in pliancy depending on the level of player.
Heretofore, the pliancy of the racket has been studied mainly in connection with the ball striking direction. That is, when a player strikes a ball while gripping the grip portion, the shaft portion deflects in a direction opposite to a ball shooting direction due to impact of the ball on strings which are stretched over the striking area, and at the next instant it deflects in the reverse direction. The ball return speed is increased by utilizing this vibration. On the other hand, recent researches have revealed that rackets, especially FRP rackets, vibrate at the time of impact not only in the aforesaid ball striking direction but also, at both side portions of the frame, in a direction toward and away from a center of the striking area. More particularly, with expansion of the strings at the time of impact, both side portions of the frame to which both ends of the shorter lateral strings are fastened deflect toward the center to approach each other, causing another vibration different from the vibration in the ball striking direction.
Such vibration at both side portions of the frame has heretofore been almost ignored, because it is rather small as compared with the vibration in the ball striking direction. However, as a result of earnest researches, the present inventor has found that a correlation between these two vibrations exerts an important influence on the performance of a racket. Experiments conducted by the inventor has shown that, in conventional rackets, the time elapsed until reaching an initial maximum amplitude value in the vibration in the ball striking direction is different not less than 15% from the time elapsed until reaching an initial maximum amplitude value in the vibration at both side portions of the frame. Consequently, the two vibrations interfere with each other to offset the energy transmitted to the ball.
The present invention has been accomplished on the basis of the above knowledge, and its object is to provide a tennis racket capable of correlating the vibration in the ball striking direction of the shaft portion and the vibration toward and away from the center of the striking area at both side portions of the frame with each other in an optimum manner, thereby permitting efficient transfer of energy to a ball.