Why does corking a bat work

They shot the balls at a steel plate or a wooden bat at 60, 90, and miles per hour and measured their bounciness after a collision—what physicists call the coefficient of restitution.

The result? That means more hits and more home runs. For the first seven seasons at Coors Field, there were 3. The idea was that higher humidity reduces the bounciness of the ball and slightly increases its weight. Indeed, the average number of home runs at Coors Field dropped 25 percent from through To test the theory, the authors placed several dozen balls in conditions ranging from 11 percent to 97 percent relative humidity for weeks, and temperatures from the 30s to nearly degrees, then fired them against metal cylinders that approximate bats.

Again measuring the coefficient of restitution, they found that the colder and moister a ball was, the less bounce it had.

Not long ago, Nathan says, a reporter in Arizona contacted him and told him that the Arizona Diamondbacks were considering installing a humidor at their stadium, too. Nathan did the math—this time starting at the desert-air base-line of 20 percent relative humidity, and conditioning balls to 50 percent relative humidity.

The Diamondbacks later put those plans on hold. Everybody, it seems, likes at least a few homers between their peanuts and Cracker Jack. Again, Newton's Laws dictate that in this case only a fraction of the bat's energy can actually be transferred to the ball. Because of these factors, the ideal bat weight should actually be somewhat greater than the weight of the ball. Some models predict that an ideal bat should weigh about five times as much as the ball, or about 25 ounces but this weight is significantly less than the weights of bats traditionally used in professional baseball.

In any event, a few ounces more or less may not make a significant difference in bat dynamics. Some batsmen may prefer lighter bats simply because they are easier to wield, allowing the batter more time to react to an incoming pitch and adjust his swing.

The distance gained by using a corked bat may be very small, if you also assume that you hit the ball at the right place and time. Lightening the bat does increase the probability that you will hit it properly, however.

This helps home run sluggers and singles hitters alike. Indeed, there has been a tendency for major league batters to adopt lighter bats in recent years. In these models, wood is typically removed from the handles, making them quite thin and easily breakable. Of course, another way to make a bat lighter is to drill a hole in the end, insert a piece of cork deep into the hole, and then carefully plug, seal, and cover the hole.

The weight of the bat would thereby be reduced, and if expertly done the bat would not be significantly weakened. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Physicist Alan Nathan of the University of Illinois and mechanical engineer Lloyd Smith of Washington State University wanted to put some of the biggest baseball myths out there—corked bats hitting farther, juiced balls and the effect of humidity on baseballs—to the test.

Because collecting measurements on a live batter is next to impossible, Nathan says, the team turned to an advanced testing lab that Smith designed at Washington State. Developed in , Smith's testing system measures the burst of a ball off the bat with three key instruments. There's a foot air cannon delivery system including a sabot—a plastic carriage that guides the ball down the tube—a box of three light screens to measure ball speed and a bat pivot to hold the bat in place.

First, the scientists place a baseball onto a polycarbonate sabot and load it into the cannon. Since the bat is stationary and there's no batter swinging it, Smith's setup must accelerate the baseball to a speed that combines both the velocity of a major league pitch and the bat speed of a major league hitter: about miles per hour. The sabot fits precisely within the cannon's tube, and even at that crazy speed it can keep a baseball along a straight path with little spin or deviation.

The stability the sabot provides is the key to hitting the sweet spot and accurately testing the pop in a bat, Smith says. The air cannon blast ejects both the sabot and the baseball from its barrel, so Smith designed an arrestor plate to stop the sabot and allow the ball to fly free. To soften the blow of the sabot, the plate rides on four pneumatic shocks. Yet even this wasn't enough, he says, so he designed the arrestor plate to be moving backward when it catches the sabot, cradling it to soften the impact.

The baseball then flies free into the light box, which contains three light screens designed to measure the ball's speed. As the ball passes through each screen, the time is recorded.

With these three separate times, Nathan and Smith can calculate the speed of the ball both as it enters the box and also after it hits the bat and rebounds backward. Those speed measurements allow the researchers to calculate what they call the coefficient of restitution COR.