Abstract des Autors

In seam bowling, the spin vector precesses rapidly into the torque vector, thereby crossing from one hemisphere to the other once torque is imparted onto the ball. However, the spin axis reaches the torque vector only at large times. If the spin axis does not coincide with the pole of the ball, the seam wobbles and introduces another roughness element. Thus, if the seam is angled toward the smooth side (for half a period per spin revolution) then the flow is turbulent on both sides of the ball and the aerodynamic side force decreases and vanishes, thereby disturbing the swing. The aim of this study was to investigate how accurate is the placement of the spin axis at release of the ball. A smart cricket ball instrumented with three high-speed MEMS gyroscopes was used for this purpose and the data of four spin bowlers were analysed. It was found that the spin axis can under- and overshoot the optimal position of the ball as well as deviate off the pole. The spin rates recorded were 15-20 rps produced by peak torques of 0.29-0.39 Nm. The deviation of the spin axis correlated with the torque, indicating that too much torque imparted onto the ball worsens the accuracy of bowling. Undershooting the optimal position is due to equal torques imparted by index and middle fingers. The optimal position of the spin axis can be achieved if the middle finger is closer to the seam than the index. This causes an overshooting torque vector and allows the spin axis to be placed at the pole of the ball at release.