An Early Look at the New Baseball

I guess it wouldn’t be baseball in these trying times without a debate about the state of the ball. This year’s rendition started in February when Eno Sarris and Ken Rosenthal of The Athletic reported that they obtained an internal memo Major League Baseball had sent outlining changes to the baseball that would “reduce offense slightly in the 2021 season.” Specifically, Rawlings loosened the tension of the ball’s first wool winding, reducing the weight and bounciness of the ball as measured by COR, or the coefficient of restitution.

How would the new ball affect the league’s offensive environment? At that point, we could only speculate. Included in Sarris and Rosenthal’s article is a cautionary tale from the Korean Baseball Organization, which experienced a crash in league-wide offense after minor reductions to the ball’s COR. But the article also cited Dr. Meredith Willis, who believed that because MLB intended to reduce the ball’s COR along with its weight – the KBO actually increased the weight of its ball by one gram – the effects would be less severe. As for MLB, its memo included an independent lab that found minor decreases in fly ball distance with the new ball.

Then came spring training, and along with it the first uses of the new baseball. As March closed out, Rob Arthur and Ben Lindbergh published an article for The Ringer entitled “The New Baseball Still Seems Juiced.” Using data from spring training games, they made two key observations: (1) home run per contact rate had increased, not decreased, from last spring, and (2) the new ball seemed to have a high drag coefficient. “Higher drag should translate to less carry and fewer home runs,” Arthur and Lindbergh wrote. “Yet the higher-drag balls also have a higher home run rate on contact, because they have a substantially higher exit speed.” If the ball’s COR was really reduced, they added, the opposite phenomenon should occur.

And now, here we are, approximately one week into the 2021 season. It’s far too early to draw conclusions about individual players or even teams, but taking a glance at league-wide offensive changes won’t hurt. As of writing, major league hitters have put 4,727 balls into play. By dividing the number of home runs hit so far by the number of balls in play, then comparing it to that of other years, we can see if home run rates are down or up. This is identical to Arthur and Lindbergh’s HR per contact metric; the wording is just different. Here’s how league-wide home run rates have fluctuated throughout the Statcast era:

The results are a bit surprising. Having read The Ringer article, you might have expected to see the home run rate increase or remain stagnant. Yet, as far as home runs are concerned, league-wide offense has declined. If this rate remains constant throughout the remainder of the season, it would represent a return to around 2016 or ’17, seasons with environments slightly in favor of hitters but manageable for pitchers nonetheless. It also aligns with MLB’s goal of limiting offense some but not so much that the league is turned upside down.

So are Arthur and Lindbergh wrong? Not necessarily. It’s undeniable that MLB introduced a higher-drag ball during spring training, but several factors outside of their control may have contributed to the discrepancy. For one, the level of competition in spring games is different than that of actual major league games. Five teams reportedly added humidors to their stadiums this offseason, which could explain why the home run rate has been suppressed beyond what one might expect. Also, batted ball distance is influenced by the lower temperatures of April, as Arthur explained. When our sample is relatively small, even minor factors can have an outsized influence.

But there’s a twist. The rate of home runs is down, yes, but the higher exit speeds indicative of higher-drag balls are still there. They’re most notable in line drives, which I’ve defined as balls hit at launch angles between 10 and 25 degrees. Going back to 2018, here’s how they’ve fared in a few key metrics:

Line Drives, 2018-21
Year wOBA EV (mph) Dist (ft)
2018 .719 92.7 270
2019 .731 92.9 272
2020 .729 92.9 267
2021 .726 94.1 266
SOURCE: Baseball Savant

An uptick of 1.2 mph? That’s almost certainly a change caused by a new baseball and not home run-happy hitters, who’ve hit line drives with roughly the same might until now. If we compare line drives to fly balls (between 25 and 40 degrees) using the same metrics, more pieces of the puzzle begin to emerge:

Fly Balls, 2018-21
Year wOBA EV (mph) Dist (ft)
2018 .522 90.1 327
2019 .575 90.5 332
2020 .589 90.8 334
2021 .560 90.9 327
SOURCE: Baseball Savant

For fly balls, the increase in exit velocity is negligible. A hypothesis test would tell you that the difference is due to mere variance. Instead, what’s intriguing is the change in hit distance, which is down seven feet from last season. And unlike line drives, fly balls have become far less valuable per wOBA. What gives? It turns out that distance has a much greater impact on fly balls compared to line drives. Intuitively, this makes sense. The difference between a home run and a warning track flyout is sometimes as small as a few feet. On the other hand, there’s probably not much difference between two line drives set apart by the same distance.

Some statistical tomfoolery backs up our intuition. I downloaded two groups of hitters from Baseball Savant. Group one consisted of hitters who hit at least 25 line drives in the 2020 season, and group two consisted of hitters who hit at least 25 fly balls in the same season. I then plotted the relationship between hit distance and xwOBA for each group’s hitters. The results are straightforward. Hit distance explains 57% of the variance in xwOBA on fly balls (r-squared = 0.57), but only explains 18% of the variance in xwOBA (r-squared = 0.18) on line drives. Assuming their exit velocities are equal, an increase in the same hit distance will likely have a more meaningful impact on a fly ball compared to a line drive. Is using the averages of hitters too simplistic of an idea? Possibly, but it’s enough for our purposes.

Based on what we’ve learned so far, it’s reasonable to conclude that the higher speed and shorter distances of higher-drag balls don’t apply themselves equally to every batted ball. Line drives and grounders are primarily influenced by the former characteristic, while fly balls are primarily influenced by the latter characteristic. If so, the drop in home run rate makes a whole lot more sense. The long ball is mainly a product of the 25 to 40 degree ranges of launch angle, and while line-drive home runs do happen, they’re nowhere near as common. I’m still not sure why line drive wOBA has hardly budged despite a higher exit velocity – maybe this is one aspect where we need more time to draw a conclusion.

The pressing issue, though, is that inflated exit velocities on batted balls force us to view the improvements of hitters with a rather healthy dose of skepticism. So far into the season, many players have surpassed their previous max EV highs; to my knowledge, 19 of them did so by a margin of 2 or more mph. But how can we tell which of those power surges are genuine? Did they make mechanical adjustments, or are they beneficiaries of the new baseball? The league’s rate of 110 mph-plus EVs per batted ball events had been fairly consistent since 2015, but this season, that no longer is true:

This dilemma persists even when hitters’ adjustments are easy to spot, as it’s still difficult to distinguish between what’s signal and what’s noise. Sure, there are the changes, but how in the world do you isolate the effects of the ball? It’s maddening to even think about.

Indeed, when the status quo is disrupted, it forces us to make sudden, uncomfortable adaptations. There have been fluctuations in batted ball numbers and offensive environments before, but the fact that we’re alerted to a change this early into the season, one with implications for how we evaluate players, is somewhat alarming. Hitters are under scrutiny, and so are pitchers. In theory, those with a penchant for allowing fly balls are aided by a ball that travels less. The specs could change down the road – this is an early look, after all – but there’s mounting evidence that we’re correct about the gist. Connor Kurcon, for example, has illustrated that the Top 1% of exit velocities around the league are up by around 1 mph regardless of batted ball type. Debates about the official ball have become a sort of annual tradition. But really, they shouldn’t be.





Justin is a contributor at FanGraphs. His previous work can be found at Prospects365 and Dodgers Digest. His less serious work can be found on Twitter @justinochoi.

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Brad Lipton
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Brad Lipton

Justin…this is a good, useful article that provides some perspective on the sss analysis that a few others on this site have been doing.

However, how does the new baseball impact the pitcher? There is another article posted on the site today that explores fastball velocities for pitchers, but does not comment on the ball. Why shouldn’t the new ball impact fastball velocity?