Structural properties of golf balls: what is the Aai compression number really worth?

The hardness of golf balls is usually indicated by the Alii compression value. Four models of golf balls were subjected to compression testing to verify whether official Atti numbers are correct, and whether the hardness, or stiffness, of a golf ball can be expressed by a single value. The results show that golf balls can be compared to hard nonlinear springs, i.e., the stiffness increases with the load and deformation. Furthermore, the results obtained did not correspond to the official Ani values. We suggest indicating the stiffness of golf balls at 3 different loads, e.g., at I, 5, and 9 kN, as one single value can be misleading and inappropriate for nonlinear behaviour.