Identifying the Modes Excited in a Tennis Racket by a Forehand Drive

Abstract

During an impact between a tennis racket and a tennis ball, mechanical oscillations are excited in the frame and stringbed. These vibrations continue after the ball has left the stringbed and have been shown to contribute greatly to the ‘feel’ of the racket. During this research, lightweight accelerometers were used to measure frame vibrations excited by a typical forehand groundstroke. The results indicated that a number of modes are excited mainly below 1,000 Hz, both parallel (in-plane) and normal (out-of-plane) to the plane of the stringbed. Although the natural frequencies and damping ratios could be calculated, it was not possible to identify the respective mode shapes. A full experimental modal analysis (EMA) of the tennis racket was conducted to reveal the 3-D modal behaviour. Comparison of the two sets of data revealed that the frequencies excited during a tennis impact are both frame and stringbed modes. The frame modes include in-plane and out-of-plane bending modes as well as torsional modes. The next stage in this research is to identify the mode shapes that are undesirable to the player. The results presented in this paper would then enable racket designers to engineer rackets with better ‘feel’ qualities.