INERTIAL AND VIBRATIONAL CHARACTERISTICS OF SOFTBALL AND BASEBALL BATS: RESEARCH AND DESIGN IMPLICATIONS

The design of baseball and softball bats has been an ongoing process since the inception of the game in the early 19th century. Until the early 1970's, the only material to be used was wood and the potential for design improvements was limited because they involved changing the physical dimensions of the bat. Also, most of the changes were initiated by the player and were carried out by local craftsmen in wood shops. With the introduction of hollow-wall aluminum bats in the 1970's and other metal alloys as well as composite materials since then, the potential for applying relevant mechanical principles to improve the performance characteristics of bats has improved dramatically. When developing or evaluating a design feature, the following factors must be considered: (1) how is the bat swung and how forces are transmitted to the bat during the swing, (2) what are the constraints resulting from rules in the particular sport, and (3) what mechanical properties are relevant. Baseball and softball batting is a two-handed sidearm striking skill with the bat held at one hand and swung as a physical pendulum as the batter attempts to impart maximum velocity to the impacted ball in the desired direction by generating maximum linear velocity of the impacting part of the bat to the ball. The motion of the bat is predominantly in the horizontal plane and the rotation axis ranges from .15 to.20 m off the knob end of the bat toward the hitter’s body during the swing. Maximum bat velocity is usually achieved from .01 to .05 s prior to impact. The rules regarding baseball bat characteristics are different from those regarding softball bats. Also, rules are different for different genders and different levels of play. Until recently, most rules related to the bat’s composition, dimensions, and weight; however, a recent rule setting an upper limit on the coefficient of restitution (Bat Performance Factor) for bats at different levels and types of play is having a tremendous impact on the direction of bat design activity. Several inertial and vibrational properties of the bat are relevant to its effective use: (1) mass, (2) moment of inertia, (3) coefficient of restitution, (4) vibration frequency and node locations, and (5) center of percussion location. Mass and moment of inertia determine the amount of effort required to swing the bat while the latter three factors largely determine the bat’s effectiveness during impact in transferring maximum energy to the bat while transferring minimum energy to the human. When considering recent research and development activity and recent rules changes limiting the coefficient of restitution of baseball and softball bats, future improvements in bat design will most likely be related to coupling the bat’s flexibility with the batter’s swing.