Effect of Diameter and Thickness on Bat-Ball Coefficient of Restitution of Aluminum Alloy Baseball Bat

Recently, metal bats have been widely used in high school and other baseball games, but owing to their improved performance, these bats have led to increased amount of accidents. Pitchers are often untimely hit the ball, resulting in serious injuries. However, the National Collegiate Athletic Association evaluates the repulsion performance of bats and balls using a standardized value called BBCOR (Bat-Ball Coefficient of Restitution). Thus, only bats with a value of 0.50 or less can be used. The BBCOR can quantitatively indicate the restitution performance of a bat, and several designs for controlling the BBCOR of metal bats have been proposed in previous studies. However, most of these require the bat to be actually manufactured; therefore, a simple method for predicting the BBCOR using dimensions and other factors during bat design is needed. In this study, the compressive load and BBCOR was first determined using simple compression and ball impact tests on aluminum alloy baseball bats with varying outside diameters and plate thicknesses. Subsequently, the relationship between the BBCOR and spring constant of the bats was examined, and the effects of the outside diameter and plate thickness on the spring constant were investigated. Based on these results, the effects of the outer diameter and plate thickness on the BBCOR were examined, and a simple equation that can determine the BBCOR from the shape (outer diameter and plate thickness) was proposed.