X-ray of baseballs reveals possible cause of home run surge

Baseballs used in MLB games have changed since mid 2015, and now there’s scientific proof. (AP Photo/Lynne Sladky)
Baseballs used in MLB games have changed since mid 2015, and now there’s scientific proof. (AP Photo/Lynne Sladky)

The home run surge of 2017 (and before) has been a heck of a lot of fun, but there are many outstanding questions about what’s really behind it. And now we might have the most compelling answer yet. Rob Arthur and Tim Dix of FiveThirtyEight published the findings of CT scans and other experiments that had been done on new and game used baseballs from before and after the official start of the surge (the 2015 All-Star Game), and what they’re reporting is undeniable: the core of the baseball has changed.

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Arthur and Dix reported the findings of experiments commissioned by ESPN Sports Science. A CT scan was done on eight different baseballs: one purchased new from Rawlings, four game-used balls from April 2014 through May 2015 (before the surge began), and three game-used balls from August 2016 through July 2017 (after the surge started). The CT scan revealed that the core of the newer baseballs are approximately 40% less dense than the older balls. Further testing of the chemical composition of the less dense baseballs revealed that Rawlings used a more porous substance to seal the core, which led to the decrease in density.

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A less dense core means a lighter baseball, and the newer baseballs are an average of 0.5 grams lighter than the old ones. Arthur and Dix admit that a lighter baseball on its own wouldn’t cause a huge jump in homers. But when added to existing research on the new baseballs done by Arthur at FiveThirtyEight and Ben Lindbergh of The Ringer, it turns into something significant.

Combine all these factors together — a lighter, more compact baseball with tighter seams and more bounce — and the ball could fly as much as 8.6 feet farther.

What’s insane is that even the new home run friendly baseball doesn’t account for the entire home run spike. According to physicist Alan Nathan, all those changes together would only increase home runs by 25 percent. Since home runs increased by about 46 percent between 2014 and 2017, Arthur and Dix believe that a change in hitting philosophy is responsible for the other 21 percent. More players are focusing on launch angle and hitting the ball upward to increase their chance of getting the ball in the air, which increases the likelihood of hitting a home run.

It will shock no one that MLB has nothing new to say about these findings. Commissioner Rob Manfred, who has consistently denied that anything significant has changed about the baseballs, formed a committee in November to investigate any changes to the baseball. The full results have yet to be released to the public, but what they found (or did not find) probably won’t surprise you.


According to Alan Nathan, one of the physicists on the commission, the task force found that all the characteristics that MLB regularly measures, including the weight, circumference, seam height and bounciness of the ball, were within ranges that meant variations in the baseballs were unlikely to significantly affect home run rates. MLB declined to provide the data supporting these assertions.

But Arthur and Dix have presented scientific evidence that directly refutes those unsupported findings. The core, as well as other aspects of the baseball, has changed. The biggest question now is whether Manfred has an explanation for that, or if he’ll even bother to recognize it at all.

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Liz Roscher is a writer for Yahoo Sports. Have a tip? Email her at lizroscher@yahoo.com or follow her on twitter! Follow @lizroscher