Major League Baseball concluded that reduced aerodynamic drag of the baseballs has contributed to a league-wide spike in home runs since the second half of the 2015 season. In other words, the physical properties of the balls themselves appear to have changed, reducing air resistance and allowing a ball to “carry” farther.
A team of 10 academics with expertise in engineering, physics and statistics conducted the analysis using Statcast data and high-tech testing of the balls themselves. The 84-page report released on Thursday determined that the change in drag was a key factor in the home run increase, although physical testing could not pinpoint a specific manufacturing alteration driving that change.
“So there is indirect evidence that the ball has changed, but we don’t yet know how,” theoretical physicist Leonard Mlodinow concluded in the accompanying executive summary.
As a result, the report’s recommendations were fairly vague: increase monitoring of temperature and humidity in ball storage locations; review manufacturing specifications; create aerodynamic tests for game balls; standardize the application of the mud rubbed on each ball before use; and form a scientific advisory council.
“I thank the committee for all of its hard work on this important issue,” MLB commissioner Rob Manfred said in a statement. “Based on the results of their study, I am accepting their recommendations immediately and look forward to their continued guidance in this area.”
Big league hitters slugged only 4,186 home runs in 2014, their lowest total since 1992. After the 2015 All-Star break, however, a pronounced uptick in home run rate was observed. In 2016, there were 5,610 home runs—the second-most in MLB history, trailing only the 2000 season at the height of the so-called Steroid Era.
In 2017, hitters blew away the previous record and finished with 6,105 home runs. That’s a rate of 1.26 home runs per team per game and a 47 percent increase over that 2014 nadir. So far the 2018 season has seen a small decline to 1.12 per game, slightly below the 2016 rate.
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The most dramatic finding was the change in drag coefficient. The reduction in drag was evident in physical testing at the Sports Sciences Laboratory at Washington State University as well as in statistical analysis of batted-ball data from games.
MLB found minimal variations in the baseballs’ seam height and no appreciable changes in size, weight or coefficient of restitution (COR), which measures the bounciness of a ball upon impact. Inspections at Rawlings’ ball-making factory in Costa Rica typically determines only about 55 percent of baseballs are appropriate for game play.
The stated range of what is an acceptable COR, however, is “unreasonably large,” Mlodinow wrote. The upper and lower boundaries permit a wide range of results. A baseball at the upper limit would travel 36 feet farther if struck in identical fashion as a ball at the lower limit. That was not believed to be a significant factor in the home run increase, though, because testing showed that game balls were far more precise and consistent than required.
The committee also studied whether hitters or pitchers had altered their approach in any significant way. By analyzing Statcast’s launch angle and exit velocity data for hitters and pitch type and usage for pitchers, the researchers found “no evidence that batter and pitcher behavior has changed in any manner that could account for the home run surge.”