Do Metal Bats Offer More Bang for the Buck?

Baseball today is known as the home run era. Testing for performance-enhancing drugs is taking place, and talk about juiced balls and the dimensions of new ballparks is rampant.

But what kinds of numbers would sluggers like Barry Bonds post if they were using metal bats like college baseball players? Perhaps not as high as one might think. Rice’s Michael Carroll and others have turned the matter of athletics into a matter of academics by applying mathematics, mechanical engineering, and the laws of physics to the study of baseball bats. As a result, limits have been set on the performance of metal bats.
Carroll, the Burton J. and Ann M. McMurtry Professor of Engineering in Mechanical Engineering and Computational and Applied Mathematics, is a member of the National Collegiate Athletic Association (NCAA) Baseball Research Panel that was appointed in 1999 to investigate metal bats.

The NCAA approved the use of metal bats in the mid-’70s for economic reasons. The metal bats didn’t break like wooden bats did, yet they were as cheap as wood and performed as well as—but not better than—wooden bats.

Over the years, however, as engineers designed better aluminum alloys, metal bats began to surpass wooden ones in performance and expense.
The improvements have given rise to a couple of concerns, though. The first involves the safety of the pitcher. The metal bats had the potential of hitting the ball faster, allowing the ball to reach the pitcher quicker than the one-third to one-half second it takes for the pitcher to react. The second concern was that the newer metal bats were hurting the integrity of the game by shifting the competitive balance between offense and defense. “Players were getting higher batting averages, more home runs, and higher scores,” Carroll says.

The Baseball Research Panel decided to address both concerns by specifying manufacturing standards that would ensure metal bats were no more effective than wooden bats. “We realized there is always a risk of the pitcher getting hit by the ball,” Carroll says, “but we didn’t want the risk to be greater than what was deemed acceptable in the wooden-bat era.”

Carroll and the other panel members began analyzing what happens when a baseball strikes a wooden bat and comparing the results with those of a metal bat. According to NCAA specifications, a baseball must be formed by yarn wound around a small core of rubber and/or cork and covered by two pieces of white horsehide or cowhide tightly stitched together. This structure allows the ball to squash upon impact with the bat and then spring back into its spherical shape. This temporary deformation results in loss of energy.

When struck by a wooden bat, the ball squashes in diameter by 50 percent. The change isn’t observable by the naked eye, but film viewed in slow motion reveals that the shape of the ball becomes elliptical when a wooden bat is used.

The deformation is less when the ball is hit by a metal bat because the thin metal wall can “dimple,” absorbing part of the impact so that the ball doesn’t squash as much. Because the ball does not lose as much shape, it also does not lose as much energy; consequently, it returns faster than it does when hit by a wooden bat.

Wooden bats also deform slightly on impact with the ball, but not to the extent that metal bats do.

Because of such variability, researchers took a series of measurements with metal and wooden bats of various lengths, noting changes in both the bat and ball on impact at different speeds. They developed a coefficient of restitution (COR), which measures the difference between the ratio of speed at which the ball and bat come together and the ratio of the speed at which they separate after impact. To set a uniform standard, the NCAA decided that the COR of a baseball cannot exceed 0.555.

The panel then took a series of measurements to develop manufacturing standards that would keep metal bats in line with wooden ones. After bats of various lengths and diameters were tested, Carroll and colleagues developed regulations that would prevent metal bats from being more effective than wooden bats. “The length of the bat in inches minus the weight in ounces can’t be greater than three units,” Carroll says. “And the ball exit speed ratio [BESR, a measurement of how fast the ball moves after it hits the bat] can’t be more than 0.728.” If the BESR exceeds that number, a metal bat cannot be certified.
A member of the National Academy of Engineering, Carroll never imagined he would one day apply his love of math and mechanical engineering to baseball.

He grew up in Ireland, where they “hit a ball with a stick,” and he didn’t know much about the sport until he moved to the United States in 1960 and became a Boston Red Sox fan. He remembers that year fondly because of the exciting World Series games between the Pittsburgh Pirates and the New York Yankees and the interesting political debates between John F. Kennedy and Richard Nixon.

“What a great game,” Carroll recalls thinking. “What a great country.”

—B. J. Almond