Humidity Effects on the Moment of Inertia in Solid Wood Baseball Bats

Engineering of Sport 15 - Proceedings from the 15th International Conference on the Engineering of Sport (ISEA 2024)

A recent surge in the home run rate in Major League Baseball has led to deeper scrutiny of all factors that might affect the properties that contribute to batted ball performance. For example, a special committee found that small changes in seam height, which were still well within acceptable manufacturing tolerances, led to a lower drag coefficient and increased home run rates for the same exit velocity and launch angle. While much of the increased scrutiny in baseball has focused on the balls, small changes in bat performance could have a similar effect. For example, Drane and Sherwood found a 1% increase in wood bat performance for bats conditioned in very humid air. For an exit velocity of 100 MPH and a launch angle of 30 degrees, batters can expect a 0.578 batting average and a 44.9% home run rate. A 1% increase in bat performance yields an exit velocity of 101 MPH, which increases the expected batting average to 0.661 with a 55.7% home run rate. In this work we investigate specifically the effect of various humidity conditions while storing wooden bats made from ash on their weight and inertia. This builds on previous work by Drane and Sherwood to decouple changes in bat performance from changes in the bat’s mass, balance, and moment of inertia.