Effect of Surface Hardness of Wooden Baseball Bat on Restitution Performance

Abstract

The ball-bat coefficient of restitution (BBCOR) and surface hardness of amateur wooden baseball bats available in the market were measured to clarify the effect of the surface hardness on restitution performance. As the impact velocity of the ball increased, the BBCOR decreased, and the rate of decrease differed depending on the bat. Therefore, when evaluating the BBCOR ratio at an impact velocity of 180 km/h compared to 120 km/h, the bats were divided into two groups: one in which the BBCOR decreased monotonically and one in which the BBCOR did not decrease up to 180 km/h. The relationship between the BBCOR ratio and the surface hardness or coating hardness is discussed. The Leeb hardness was measured as the surface hardness and the nanoindentation hardness was measured as the coating hardness. Except for two bats with significantly different coating hardnesses, it was found that the BBCOR ratios were related to the surface hardness. The bats with high and low coating hardness had the highest and lowest BBCOR ratios, respectively. Furthermore, when the cross sections were observed using a microscope, one of the bats with high surface hardness and BBCOR ratios underwent deformation by physical external pressure. These findings suggest that the BBCOR of wooden bats is related to surface hardness and coating hardness, and compression of the wood surface could affect BBCOR. In the industrial production of wooden bats, it would be useful to evaluate not only the density and defects such as knots, but also the surface hardness of the wood and coating hardness.