APPARENT COEFFICIENT OF RESTITUTION OF IMPACTS IN PADDLE RACKETS WITH DIFFERENT CHARACTERISTICS
Keywords: paddle racket, restitution, impact velocity, grip support
AbstractThe aim of this paper is to study the influence of racket characteristics, type of grip support and pre-impact ball velocity on the apparent coefficient of restitution (ACOR). The ACOR was determinate for 14 paddle rackets, of three different groups(conventional, hollow and experimental). The measurements were made for two impact velocities (30 and 17 m/s), and two different types of grip support (free and with the player arm), with the Peak Performance motion analysis system, based on 180 Hz video cameras and a software for automatic tracking. Elliptical paraboloids were adjusted to the experimental data using the least square method as explained in another paper. For studying the influence of the factors on the ACOR in the points of frequent impact for the drive (smash and volley), the following decomposition was made: ... , using a impact velocity j (j=H: high, j=L: low) with grip support k (k=F: free, k=A: supported by the arm); m is a general average; i R is the increase (or decrease) due to the racket i; k b is the increase (or decrease) due to the impact velocity; k g is the increase (or decrease) due to type of grip support. k j i r are random variables that are supposedly not correlated with mean value zero and variance 2 s . The results of this study show that for the drive impact point, the average ACOR of the 8 conventional rackets is equal to 0.374, while the average for the three hollow rackets equaled 0.426. This represents an increase of 14%. For the smash and volley points, the increases were 16% and 13% respectively, percentages high enough to produce a detectable difference in the performance of the hollow rackets under playing conditions. The impact velocity has an important influence on the ACOR: an increase of 17 m/s to 29 m/s (70%) produces a decrease of 0.10 (25%). The difference in ACOR due to the type of grip support is 0.016 (4%), which is less important than the influence of impact velocity. In two rackets measurements were made with the racket grip clamped, in this case increasing the ACOR about 25% with respect to the free racket. However, this latter result is not considered of relevant practical importance, because the experimental condition is not close enough to the real grip support by a player's arm.
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