Study points to seam height in MLB's homer surge, but leaves much unexplained

SAN DIEGO — For the second time in two years, a group of scientists hired by Major League Baseball to conduct an independent study on the composition of the baseballs to determine what caused the recent home run spikes has concluded not much at all. To their credit, they have a little more information than last time. (Maybe it was the seams all along.) But then again, who doesn’t? 

According to the new report released by MLB on Wednesday morning at the winter meetings, 60% of the jump to 2019’s record-setting 6,776 homers is due to an increase in carry — in other words, the drag on the balls decreased. The other 40% it attributes to launch conditions — which, in the absence of other evidence, is believed to be a result of batter behavior.

Of the 60% that can be attributed to the ball, 35% of that was concluded to be the result of changes in the seam height, although it measured 2019’s average seam height as being smaller than that of 2018 “by less than 0.001 inches.” Despite not being able to explain the other 65% change in the aerodynamics of the ball, the researchers ruled out a meaningful change to a number of other physical properties — roundness, surface roughness, lace thickness. 

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Wednesday’s report was authored by Drs. Alan Nathan, Jim Albert, Peko Hosoi and Lloyd Smith, and again found no evidence that the balls were intentionally altered or “juiced.” The committee was first formed in August 2017, a season that had been a historic high-water mark in baseball for home runs. It capped off a steady trend of balls flying further that started following the All-Star break in 2015, spurring the league to commission the independent study to figure out what exactly happened. The following May, 10 scientists announced they had no idea.

The second study of Major League Baseball's homer surge issued by an independent commission attributed part of the shift to changes in the seam height on the baseball, but left much of the record power spike unexplained. (Brian Rothmuller/Icon Sportswire via Getty Images)
The second study of Major League Baseball's homer surge issued by an independent commission attributed part of the shift to changes in the seam height on the baseball, but left much of the record power spike unexplained. (Brian Rothmuller/Icon Sportswire via Getty Images)

The physical ball was different — there was a demonstrably lower drag coefficient — and the increased aerodynamics was causing balls hit with the same launch properties (angle and force) to fly further. But they could not determine what about the ball had changed. Or why. They ruled out batter behavior and bounciness. Although they conceded small fluctuations in these specifications year-to-year, they also concluded that the overall trend in home run rates was not correlated with changes in the size, weight, or seam height of the baseball.

“There is no smoking gun in there at all,” Nathan, a professor emeritus of physics at the University of Illinois and the chairman of the study, said at the time. Also: “We do admit that we do not understand this.”

Baseball was simply entering the Three True Outcomes Era, and it included many home runs, or so it seemed. And then 2019 happened and the home run-rate records set just two seasons prior were shattered amidst a growing sense the ball was again to blame.

And it was, according to research done by members of the media. The details were debated and measured, the league was questioned and cautioned, and in September, commissioner Rob Manfred announced that he was reconvening the same group of scientists to once again investigate the baseballs. The report said it ruled out “various hypotheses suggested in the media,” which had sprung from studies of the balls’ other physical properties.

After October saw a drop-off in home run rate — indicating a reverse-trend increase in drag — the league pushed back the publication date for the report so that it could include an analysis of what changed between the regular season and the postseason. 

This new report was completed using “improved experimental measurements” for studying seam height that helped identify the correlation between that variance eventually to produce the finding that 35% of the increased carry can be pinned on the seams.

It found that the postseason balls did exhibit increased drag, but did not detect a change in seam height from the regular-season balls. The committee wrote that the reason for the change in drag is not known, and pointed out that there was a much smaller sample size of games and baseballs from which to draw conclusions.

Astros pitcher Justin Verlander has repeatedly expressed frustrations with fluctuations in the baseball, and during the postseason said the ball 'dictated' the game. (Bob Levey/Getty Images)
Astros pitcher Justin Verlander has repeatedly expressed frustrations with fluctuations in the baseball, and during the postseason said the ball 'dictated' the game. (Bob Levey/Getty Images)

It also made six recommendations focused on tracking and monitoring the baseballs, their storage conditions and the atmospheric conditions of the ballparks. Specifically, the report called for further studies on how the mud rubbed on the baseballs affects drag, and on the viability of using humidors in all 30 ballparks.

The report released by Major League Baseball specifically concludes that there was “no evidence” that the changes in the baseball that contributed to the recent home run surges are “due to anything intentional on the part of Rawlings or MLB and were likely due to normal manufacturing variability.” This is good and useful (and, for what it’s worth, totally believable) for corroborating Manfred’s prior insistence that the league did not “juice the ball” and for removing any prior culpability.

However, in doing so the report takes pains to explain that “the year-to-year variation of mean values are significantly smaller than the variation within any given year” — in other words, for as variable as the ball has seemed on a season-wide scale, it’s even more so within seasons, series, and games. Even if this is a matter of less than one-thousandth of an inch, it has clearly proven significant. If individual at-bats are manifesting the effects of the ball’s variability, that would undermine the oft-cited defense that everyone is playing with the same ball.

The changes in the seam height — and whatever else is impacting the carry of the baseball — do not have to be intentional to be the result of something. This study largely confirms what truly independent researchers (those unaffiliated with the league) have long suspected and studied based on the available data: It would behoove MLB to not shy away from thoroughly, meaningfully, and exhaustively investigating not just what is different about the ball — but why.

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