Factors of unpredictable shots concerning new soccer balls

The configuration of new soccer balls has closely approached a perfect round shape by not only reducing the number of panels but also by eliminating roughness of the panel joints by utilizing thermal bonding process recently. Therefore, in a non-rotating or low-rotating moving shot, a soccer ball drops and curves, i.e. motion by the change of wake flow, which is called a knuckle ball effect. Factors of the knuckle ball effect on the new soccer balls were investigated in case of a non-rotating shot and a low-rotating one. A correlation between fluctuation of the wake and fluid forces was identified in a symmetric panel configuration of the ball. The asymmetric configuration of the ball panel was also investigated as a factor to bring unpredictable change of the ball flight path. The low-rotating ball changes its frontal area against the moving direction and it produces irregular lateral forces. Consequently, the factors which produced the knuckle ball effect were found to be different from the case of non-rotating shots and low rotating shots.