Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
INTRODUCTION: Wooden bats have been used in baseball since the founding of the game in nineteenth-century America. These bats are still exclusively used by professional players, and in the Olympic Games and World Championships. More durable metal bats were introducted into baseball in 1972, and are used in most non-professional and youth baseball leagues.
Metal bats have been the subject of recent research as concerns arise over increasing ball exit velocity and player safety. The National Centre for Catastrophic Sports Injury Research ranked baseball ninth in nonfatal injuries to US collegiate athletes in 1997. However, baseball had the highest fatal injury rate of the 13 men’s sports surveyed (0.63 fatal injuries per 100,000 participants), exceeding that of gridiron and ice hockey (NCAA News, June 8, 1998). The US Consumer Product Safety Commission reported 14 fatalities in children from blunt impact by a baseball to the head or chest between April 1994 and April 1995 (Van Amerongen, Rosen, Winnik & Horwitz, 1997).
As the closest infielder to the hitter, the pitcher is at greatest risk of being injured by the batted-ball. In 1998, 375 Division I collegiate pitchers were struck by linedrives (hard-hit, low-trajectory balls). Such impact injuries comprise about 3% of all injuries to pitchers, and this rate has remained relatively constant since 1972 (Dick, 1999). However, increasing sophistication in metal bat design indicates the risk to pitchers may be greater when facing hitters using metal bats. Ball exit velocity (BBV) for balls hit with metal bats has been demonstrated higher than from wood bats (Bryant, Burkett, Chen, Krahenbuhl & Lu, 1977; Elliott, 1979). BBV values obtained from wood bats ranged between 39.62 - 44.17 m/s (143 - 159 km/h). At a distance of 16.46 m, minimum movement time for a pitcher to complete a protective motion against a linedrive is approximately 400 ms (Cassidy & Burton, 1989). This represents a ’safe’ BBV of approximately 42 m/s (151 km/h). Greenwald, Penna & Crisco (2001) quantified average ball exit velocity from metal bats as 47.61 m/s (171 km/h), with even high-school hitters achieving BBV exceeding 160 km/h. It is evident ball exit speeds from metal bats present a particular danger to the pitcher.
Differences in elastic or vibrational characteristics of the bat constituent material have been proposed as factors in the superior performance of metal bats (Ashley, 1991). However during a high-speed impact, the baseball may be in contact with the bat for as little as 2 ms (Greenwald, Penna & Crisco, 2001). Adair (1994) estimated the impulse generated by a ball striking a bat held firmly in the hands may take up to 8 ms to propagate from the point of impact in the barrel, to the hands, and back to the impact point - by which time the ball will have already departed. These findings suggest factors beside bat material properties may affect ball exit velocity.
This study investigated the effect of bat design on linedrive speed. The selection of wood or metal materials promotes considerable design differences between bats. The greater density of aluminium alloys means the bat must be shaped as a hollow tube to maintain the same weight as a solid ash bat, whose mass is distributed throughout the implement, with a greater proportion of mass located in the hitting region (barrel). Resistance to angular acceleration is determined by the distribution of bat mass with respect to its axis of rotation. As a first class lever rotating about the hands against the resistance of the barrel weight, for a given torque the “end-heavy” wood bat will achieve less angular acceleration than a metal bat. Such a bat also requires greater impulse to produce a change in bat speed, resulting in decreased linear velocity of the barrel, thereby imparting less velocity to the ball (Noble, 1998; Hay; 1973).
STATEMENT OF THE PROBLEM: To develop effective standards for equipment and maximise player safety in baseball, the effect of bat design on ball exit velocity must be quantified. Metal bats are currently certified “safe” before commercial sale using robotic swing testing. The purpose of this study was to quantify the effect of bat weight distribution on ball exit velocity from metal and wood bats swung by high-performance hitters.
METHODS: Baseball bats: One metal bat and one wood bat were selected for analysis. Bat specifications are listed in Table 1 (expressed in both SI and empirical units as is traditional in baseball). Bats were selected as representative of the length and mass of bats used in high school and collegiate baseball. The metal bat was constructed from an alloy of heat-treated zinc, magnesium and aluminium. The wood bat was a solid northern white ash bat. Although sold as conforming to NCAA mass and length restrictions (which state the empirical length-to-weight differential must not exceed 3), the metal bat was approximately 30 g lighter than the certified weight. While the bats were virtually identical in length and similar in mass, the primary difference between the two bats was the location of the centre of mass (CM). The theoretical point around which the mass of the metal bat was distributed was located using a knife-edge balance technique (Hay, 1973), and was found to be approximately 5 cm closer to the handle than that of the wood bat.
Table 1: Specifications of wood and metal baseball bats used in this study.
Empirical length- weight differential
Bat centre of mass (balance point) (m)
Diameter at widest point of bat barrel (m)