We used to think of the steroid era as the heyday of home run hitting, but our mental image of a juiced-up behemoth crushing the ball needs updating. This year is poised to easily be the most homer-happy on record — and not because of altered player physiques, but rather thanks to physical alterations to certain batches of baseballs that make them fly more freely through the air. What’s more, recent data shows that these less-air-resistant balls probably make up a larger share of all MLB balls today than they did even earlier this season, meaning the game is now consistently being played with a ball that travels farther than usual.
MLB has denied making any intentional changes to the baseball, noting that testing finds the baseball still within established standards. But those specifications are extremely wide, allowing for massive variation in fly ball distance and corresponding shifts in home run rates. If MLB only rejects balls when they lie outside of the standards, there could be wild differences in air resistance between pairs of baseballs.
And we can measure how much air resistance is exerted on a given ball using MLB’s pitch tracking system. By measuring the loss in velocity from a pitcher’s hand to home plate, I calculated the drag coefficient of each pitch thrown in baseball since 2008. Drops in air resistance coincided with jumps in home runs, with drag especially falling for the average pitch this year. My study and a follow-up have demonstrated that this reduced drag could be to blame for some of MLB’s recent home run surge.
Large variations in drag between different batches of baseballs used to be commonplace. I found that individual baseballs could differ by 20 or 30 feet of fly ball carry, more than enough to turn a harmless fly ball into a deep bomb. And my pitch-tracked measure of air resistance fluctuated wildly from month to month over the period from 2008 to 2015. These variations could really wreak havoc on individual players’ stats — just ask reliever Jake McGee, who had his best numbers in 2014 while throwing the highest-drag pitches in the league,1 and his worst stats two years later, when the air resistance on his average fly ball dropped — but they also meant that neither hitters nor pitchers were locked into using a given type of ball on a given night.
Lately, though, the baseballs have become a lot more consistent. The standard deviation (a measure of variation) in drag coefficient between individual baseballs has dropped drastically in the last few years.pooled standard deviation across all parks. I did so because individual ballparks have different shipping schedules for baseballs, receiving new lots at different times. I excluded months with partial data (March, October) from the analysis, and examined only fastballs.">2
That’s the continuation of a roughly decade-long trend toward more uniform baseballs. The trend accelerated this year — just before the publication of two articles with definitive evidence for the ball being juiced.
As tempting as it is to imagine a conspiracy, that timing is more likely to be a coincidence. Rawlings, MLB’s baseball supplier, manufactures the balls months in advance, so it’s unlikely that they could have made any rapid alterations in response to juiced-ball allegations. But it doesn’t seem as likely to be a coincidence that the baseball became so much more consistent this year. MLB commissioner Rob Manfred has admitted that the standards are broad, and in interviews conducted around the all-star break in early July, Manfred even said he was considering tightening the specifications.
He’s also made his preference for this era of baseball known, emphasizing that fans love strikeouts and home runs. As pitchers throw ever harder and batters accrue ever more of their value with dingers, Manfred must be happy with both the current state and long-term direction of the game. But, as sabermetrician Joe Sheehan pointed out in a recent article, if manufacturing variation just happened to produce a batch of homer-prone balls in the last few years, 2017’s home run surge could disappear just as suddenly as it began. All it takes are the same random variations to turn baseball back toward 2014’s pitching-dominant era.
Fortunately (at least from Manfred’s perspective), the balls seem less variable than ever before. Without more information on how the balls are made, it’s difficult to know whether the current trend toward more consistent baseballs will last and prevent the home run rate from shifting again. And for their part, MLB denied in a statement to FiveThirtyEight that any changes had occurred “[i]n terms of manufacturing, shipping, storage and preparation for use … to the balls that are used in games throughout the season.”
Making the balls more consistent might further affect MLB’s overall home run rate, but it also means pitchers and hitters should end up with a more even playing field. Such consistency won’t help every player, but at least they’ll have a better idea of what they’re working with — a ball that travels farther and flies faster than it used to.