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An Astrophysicist Explains The Challenges Of De-Juicing The Baseball

A human cadaver positioned as if throwing a baseball at "Bodies...The Exhibition".
Photo: Timothy A. Clary/Getty

Major League Baseball is in the era of the dinger. Batters slugged 6,776 dingers in the last full regular season (2019), which is notable for being about 1,200 more dingers than the year before, and about 600 more dingers than 2017, which was at the time the dinger-est year in MLB history. MLB hitters socked 1.28 dingers per game across the truncated 2020 season, a rate that is topped only by the absurd 1.39 dingers a game socked in 2019. If you like dingers, baby, they've got the dingers.

The lure of the dinger, and the effort players put into chasing it, have side effects. The stolen base, for example, has been in decline at a rate that is roughly inversely proportional to the rise of the dinger. Triples, too, have been in steady decline, to the point where batters hit fewer per game in 2020 than in any season in MLB history. It makes sense, even if it's not particularly fun: If home runs are a reasonably likely outcome, it's dumb baseball to risk outs with daring base-running. Directional slap-hitting, bunt singles, sacrifices, the hit-and-run—any baseball play that risks an out that could otherwise be spent swinging for the fences is, over the course of a season, a low-percentage play.

I have been pretty unhappy about this trend, because the stuff I like best about baseball is the daring, full-speed stuff, the stuff that involves multiple players moving at the same time, and dirty jerseys, and a guy chugging around second base like a hungry lion is on his heels. Also, importantly, I like for dingers to be rare, special events, far less likely to happen in a game than a double to the gap. A commonplace dinger is a bummer, to me.

So I would like to see something done about this. Back in October, on this very blogsite, I argued that MLB should make physical changes to the balls they use so that they will bounce off the bat a little less enthusiastically and fly a little shittier, and produce fewer home runs. My thinking, there, is that a juiced ball has been blamed for the explosion of power numbers, and so a de-juiced ball would naturally produce the opposite effect.

I am not a baseball manufacturer and I am even less a physicist—I somehow got my shoelaces knotted together trying and failing to explain one extremely simple physics concept in my damn blog—and so to help me understand the realities of de-juicing a baseball, I spoke with Dr. Meredith Wills, a sports data scientist with SportsMEDIA Technology and a holder of a Ph.D. in physics. Dr. Wills has also put her interest in baseball and her knowledge of extremely fancy astrophysics to use over the years studying the flight of baseballs, and in particular looking at the characteristics and behaviors of the juicy balls of the dinger era. She knows her stuff.

We wound up talking about chaotic flight trajectories, rumors about the dirty rotten Yankees, and cats. This conversation has been lightly edited for length and clarity.

You will have noticed in my blog that I misunderstood what is meant by Coefficient Of Restitution (COR). Can you explain COR? What is it?

Oh, sure. It basically just has to do with how the ball... well, OK, we’ll stick with COR, but there’s a separate acronym that’s not coming to mind that specifically has to do with a round bat and a round ball. COR assumes a flat surface, as a physics term. It has to do in this case with how hard the ball comes off the bat. The ball will squish when it hits the bat, and the less the ball squishes—it has to do with conservation of momentum, conservation of energy—if the ball squishes more, it ends up losing energy, and therefore it just doesn't come off the bat as fast, so you have lower exit velocities. If you have a higher coefficient of restitution, then you end up with the ball not compressing as much, it retains more of its energy and it comes off the bat faster, with a higher exit velocity.

That’s why COR is important. It has nothing to do with the carry of the ball, like once the ball is in the air at a given exit velocity, COR no longer affects its flight. But it’s easier to get up to a higher exit velocity if the ball has a higher COR. The lower the COR, the more the ball squishes against the bat. A good comparison is if you’ve ever tried to dribble an underinflated basketball. A properly inflated basketball has a higher COR.

Ah, I get it. And I had stupidly assumed that COR referred to aerodynamics, but I guess the term you would use to refer to the ball’s aerodynamic characteristics is “drag,” right?

Yeah, and a good layman’s term for discussing drag is “carry.” How does the ball carry when it gets into the air? And the reason I think that there seems to be almost a disproportionate focus on COR is because the idea of quote-unquote “juicing” the ball has always been associated specifically with COR, the idea of how fast the ball comes off the bat. It wasn’t until MLB’s home run committee looked at the 2017 home run surge that anyone said, “No, it has something to do with drag, with the aerodynamics of the baseball.” That's literally the first time in history, as far as I can tell, that somebody took drag into account, which, considering the nature of a home run, seems almost insane.

It kinda makes sense, because we didn't have data to show drag characteristics until Statcast. You could sort of tell from video, but the idea of having enough data to get something that's statistically significant, that really required Statcast. 

Seems like COR would be easier than aerodynamic drag to eyeball in pre-technology times because you could just bounce a ball off the ground and note that this ball is springier than that ball.

Right. And when they have looked at the ball over time, there are three things they've looked at: They've looked at the weight of the ball, the size of the ball, and the COR. And they’re easy to measure! Any physicist, given the right materials, could do it.

Rob Arthur put out an article at the start of the postseason showing that exit velocities were going up during the postseason, which he said could be related to COR. And Alan Nathan pointed out that the one thing that Rawlings controls very, very well and has over a very long period of time, is in fact the COR. Which is interesting, when you think about it, because so much of the ball is made by hand. Only a few parts of the process are closely controlled, so maybe they contribute most directly to COR.

Are there any other physical properties that affect how a ball performs? I don’t suppose there’s, like, buoyancy?

No, not really. Those are really the two things that affect how the ball performs. One interesting thing about drag—and this is something that I think gets lost in some of the discussions, certainly those of the home run committee—is there seems to be this focus on finding all the sources of drag, but the problem is that the orientation of the baseball as it's coming off the bat heavily influences drag. It's what you would refer to as a semi-chaotic system. So I would argue that you can take the same baseball and send it out ten times at the same exit velocity, same launch angle, and it'll travel 10 different distances, because of the orientation of the ball.

Oh! Like the difference between a two-seamer and a four-seamer!

Exactly. And that’s one of the reasons why endeavoring to make all the balls the same, that doesn't actually mean they're all going to perform identically in every situation. Because if you have the seams—do you want me to grab a visual? I have some baseballs.

Yes, absolutely, by all means.

[some time passes while Dr. Wills retrieves a baseball]

You’ll like this. It doesn’t have any fancy stamps but this is a game ball from the NLCS, a foul ball. So, OK, for instance, if the ball is oriented this way [with the narrow seams aligned roughly horizontally] versus like that [tilted 45 degrees or so] you’re going to get a different kind of drag.

[here I am giggling in delight like a small child that has seen a frog for the first time]

And then the spin on the ball might be different. If it comes off the top of the bat it might end up spinning like this [showing backspin]. That’s sort of why the round bat is interesting, because it can affect the spin, because the bat is round instead of being flat. So the two factors are drag and coefficient of restitution, but COR according to Dr. Nathan is very well controlled. Drag, they do the best they can, but you can’t physically control it because even a cricket ball, which has just the one round seam on it, will behave erratically. This is actually fairly complicated!

[I am still laughing and clapping like there is a clown making balloon animals]

[a cat appears]

This is Opal.

Oh nice. Hi Opal! Does Opal like baseball?

She likes the laces! Oh my gosh, when I’m taking baseballs apart I can’t turn my back for a second. It’s terrible. How do I go to an editor? I have these results but a cat ate my data. She hasn’t managed to get much, but I have learned that every time I walk away I have to make sure that everything is put away. Or if I just turn my back.

Good cat! Hey, so, what is inside a baseball? When you dissect one of these baseballs, what’s in there?

First of all you have a leather cover (and that’s a huge source of variation, in terms of weight, you’ll easily get them going from 15 to 20 grams, which, a 30 percent variation is massive). Once you get inside the ball, first there’s an outer layer which is essentially cotton sewing thread. Inside that are three layers of yarn, all of different thicknesses. The outermost yarn layer is the thinnest, and is gray. Then the middle layer is white, and slightly thicker. Then the innermost yarn layer is the same kind of yarn as the outer layer, it’s just spun at a larger gauge. So the inner layer is the thickest yarn, the outer layer is the thinnest. It would be like having a finger weight yarn and an aran weight yarn, for anyone who knits.


They kinda fit into those. Or, I suppose it’s sport and aran weight. It’ll make sense to a knitter.


I should also point out that the inner layer, with the thickest yarn, is also by far the thickest layer. I don’t have the heart to slice a baseball open right now, someone else will have to do that for you. I just can't bring myself to do it.

That innermost yarn layer weighs more than twice as much as the outermost. It's roughly 54 grams for the inner layer, 14 grams for the middle, and 24 grams, 23 grams for the outer. You can tell I've looked at a lot of these. And the very center is the pill, which is rubber around a kind of cork center. And usually when people talk about juicing a ball they go straight for the pill, which I find a bit weird, because it’s pretty far in there and there’s a lot of stuff in between. But, yeah, that’s the interior of a baseball.

So if you were de-juicing the ball, and you were specifically going after COR, you’d start with the outer layer?

The outer layer? Not necessarily. People have tried changing the pill—Rawlings patented a different pill for softballs, I want to say in 2016, and around then Rob Arthur and Tim Dix did an article for FiveThirtyEight where they X-rayed some baseballs and found some differences with the cores. But if you're going to deaden a ball—and I suspect this is probably how the KBO did it—is the thicker yarn (because of the nature of yarn, which is mostly wool or wool-polyester blends now), if you've got a thick layer versus a thin one, that thick layer is just, you can squish it down. The thicker the wool yarn, the more compressible it is. If you wanted to make a ball more compressible, you would probably add to the layer of thicker yarn and maybe take away some of the thinner yarn.

Once I realized that, it struck me, you could effectively tune the COR. And that might’ve actually been how the KBO did it. And it might actually be why the KBO came after Skyline [Ed. note: KBO baseballs are manufactured by a company called Skyline Sports] and said, “No, you haven’t gotten this right.” It’s because I suspect it’s literally trial-and-error, trying to figure out how much to add of the most compressible yarn, how much to get rid of the least compressible yarn, etc. And you should be able to tune that fairly easily. 

Would you say that replacing the outermost layer of yarn with shoelaces would be going too far?

[awkward moments tick by]

What matters is your timeline. People have this idea that the ball is suddenly juiced. You have to remember that they manufacture baseballs all year round. The production timeline for a given season starts during the previous season. The baseballs for 2020, they were manufacturing them while 2019 regular-season play was still going on. This is just how it works. So you can’t just suddenly decide to have a ball that is more dead, or more juiced. You’d have to do what they did during the 2020 postseason, which is mix in a second population of balls, which in this case happened to be more dead, and were from a different season.

When you think of what the KBO was doing, it was on a much shorter timeline. They had a batch that Skyline made for them, which didn’t meet the COR requirements—and I remember they gave Skyline a fine or something like that—and then Skyline was able to very quickly produce another batch, which did meet the requirements. Which says that it had to be something that is easily changed in the manufacturing process.

So not Play-Doh.

Right. And to go back to Dr. Nathan’s point, Rawlings is very good at controlling the COR. and the winding of the yarn is the part of the manufacturing process that is purely automated. And the reason that I described those layers in terms of their weight is the machines themselves have a weight trip. Once the ball reaches a certain weight, the winding of that particular layer shuts off. And then there is an employee who moves the ball along to the next machine, but it’s not like there’s somebody watching a dial or something and then shutting it off manually. The machine shuts itself off. With MLB baseballs, the leather is cut using a die cut but a human being pushes it into the leather. The only automated thing, where the machine not only runs but shuts itself off, is the winding process. So that would make it the most tuneable thing, I would say.

So if this is done by weight, but you replace a certain amount of yarn with a thicker, squishier yarn, does that necessarily produce a larger ball?

It does. Or, I shouldn’t say “it does,” it’s more like, “it could.” It could. If you just introduced a squishier yarn, and didn’t change anything else, then you would end up with a slightly larger ball. Interestingly enough, the KBO did end up with a very slightly larger ball, So, again, I think it’s that tuning-type thing, to get to the right COR. But, yeah, if you just had a squishier yarn, you would end up with a larger ball.

And I assume, all on its own, a larger ball does not fly quite as well as a smaller ball.

Oh yes. And actually size is both easiest to measure and also frankly on average will dominate over things like seam height, or whatever. 

To go back to the tuning, the KBO specifically said one gram heavier, one millimeter larger (at the time I meant to go back and see if they meant a millimeter in, say, diameter). And that one-gram difference, think back to that five-gram variation in just the weight of the leather covers. Figuring out how to accurately change a batch of balls by a single gram each, you’re forced to look at, OK, what’s automated? I suspect that you actually can tune the winding process so that you can change, say, one layer by one gram. 

I did look at a selection of baseballs that I had, to see what was reasonable, and discovered that if I had a center that was one gram different, the circumference was one millimeter different. So that actually says a lot about what the KBO found, because that’s a very small change in terms of raw size, but in the aggregate it does wind up having an effect.

However, I should mention that I have not found evidence that the manufacturing process measures or controls for the size of the baseballs. The weight is much easier to control. I’m not even sure how you would control the size, because the yarn—it’s not perfect, it may be minimal but it will change in thickness. It isn’t entirely uniform. The weight is something that can be controlled with a lot more precision than anything else in the process.

How important is it that the ball stays ball-shaped after contact? Like, you couldn’t replace the interior of the ball with memory foam, right?

In fact you do get some variation! That's actually something that I've found is that if I'm looking at balls that haven't been used versus ones that have, you can often tell, I have one or two where literally you could find a flattened spot where the bat has hit it.

But it depends. In 2019, you couldn’t do that. It really just depends on how much the leather can move around, and in 2019 the leather wasn’t going anywhere. But for other seasons, if you do hit the ball hard enough, yeah, you basically end up deforming the leather such that it stretches in that one spot where the bat hit it.

If you look in three dimensions, on average you will get what you expect. But you can get these huge differences, where the ball is no longer properly round. It’s better than it used to be. Particularly with practice balls. I don't think teams order practice balls much. I don't know if I have any with me, but typically a practice ball is stamped to indicate that it is for practice. Having talked to pitchers who've been around long enough, in the past practice balls were legitimately not great, and they could tell after a while that they were throwing something that wasn't round.

I read that during the Dead-Ball Era MLB was very cheap, and would reuse the same balls to death, even making fans throw foul balls back onto the field so they could be reused. Do you think allowing the baseballs to take a little bit more punishment before they’re cycled out would meaningfully affect their flight characteristics?

I could see that. I’ve seen with balls that have been hit hard a number of times, in a lot of those cases you will actually see that the leather stretches, so that you almost get these holes, it almost rips along the laces. I can see that as essentially lowering the COR, it’s just that the ball is not going to hold together very well. The scuffing might make it easier to throw? The reason they changed over to—I’m blanking on the name—a batter was hit in the head with a heavily used ball and was killed, and so they started switching balls out.

Ray Chapman! That was like 1920, which was right at the end of the Dead-Ball Era, right?

Right. And it was his death that was one of the main reasons why MLB changed their practice of changing out the balls. Because by the time you get deep into a game, the ball is just very dirty.

Yeah, and he couldn’t see it?

Right, exactly. And I remember seeing a Rawlings prototype, I did see one in person finally, when they tried to replace the mud they rub on the balls, they wanted to make it just built-in. And the complaints that players had—the pitchers didn’t have any complaints but the batters and fielders did—is the ball was just too dark. So it’s not necessarily the flight characteristics of the ball so much as the ball’s visibility, it’s a safety issue.

Tough to deal with a ball you can’t see being hurled at you at 100 mph. That’s tough.

I have also heard, apparently the Yankees are notorious for really mudding up their baseballs. I can’t speak to that because I don’t actually have any baseballs that I know for a fact are Yankees game balls. I have one that is a game ball from somebody who may have gotten it from Yankee Stadium, and it is so much darker.

These damn Yankees!

I remember first seeing it and thinking, “Wow.” So there may be something to that!

Wow. Wow. Shifting gears: Do the drag characteristics that make a ball carry a long way for a batter also affect the performance of pitchers? Do pitchers derive a benefit from a ball with less drag?

I would say, there’s a little bit in there. And this is some of the stuff that Rob Arthur looks at, he specifically looks at pitches within a particular range to figure out how drag has changed things. But the ball is only traveling, well, less than 60 feet six inches, because the release point is well forward of the rubber. Whereas a home run or any long fly ball is traveling several hundred feet, and the drag can make a much bigger difference. Where it might affect the pitcher would be something like in 2019, where the seam height was noticeably lower and some pitchers were forced to change their grip.

I can also attest to the fact that you can easily differentiate a 2019 ball from other balls because the 2019 ball will roll right off your table. It was Chris Townsend, who does A’s Cast, who pointed this out to me. And he’s right! You’ll have different baseballs on the table and if one is a 2019 ball you have to keep your eye on it, because it will roll away. The ball is rounder and the seams are lower, so the seams don’t hold it in place. And that did force some pitchers to change their grips. And you will discover—pitchers will discover this, because they hold the ball very loosely—that the 2019 ball will slip right out of your hand.

So if you were gonna go the opposite way from the 2019 ball, and increase the drag, would seam height be the place to start?

Seam height is significant! In fact, this is where Barton Smith’s stuff is interesting, because what he found is something called seam-shifted wake. But if you have higher seams—and, again, this depends on the angle of the seams—you absolutely are more likely to create drag. Even the 2019 MLB home run committee found that seam height accounted for a significant amount of drag, lower seams were in fact related to that season’s home run surge.

The issue is that because of that chaotic effect of the seam angle, you end up with a situation where ball-to-ball it is more difficult to measure or control. Sometimes the leather is stretchier, and stretchier leather will give you higher seams. Seam height is probably more dependent on how much leather is squished under the laces than the material of the laces themselves.

The 2019 home run committee found that lace thickness was unrelated to drag, which frankly I could’ve told them. Because what I found for the pre-2019 ball was, the only reason lace thickness mattered, it had to do with essentially the way that thinner laces stretched more, so the ball became less round. Where the leather comes together is like a fault line, and the seams tug on the leather and bunch it up, and that, and not the thickness of the laces, is what creates seam height. And so this idea that lace thickness was correlated with home runs—it was before 2019, but not in 2019, since they changed production in such a way that lace thickness became irrelevant. A whole section of my article about the 2019 balls talks about this!

For a layperson, which I am, I do find that those are tricky things to keep separate in my mind, the concept of seam height versus lace thickness. It’s easy for me to imagine dip-chewing MLB types conflating those two things and reaching a wrong conclusion.

I was just watching this yesterday, I guess it was a StarTalk thing with Neil deGrasse Tyson and Bill Nye, and Bill Nye went through the whole thing, and you could tell that he was assuming that lace thickness and seam height are the same thing. The whole time. Like, I need to get this tattooed across my forehead: Lace thickness does not equal seam height!

As a person who thinks about baseball a lot, how do you feel about both the performance of the modern ball and the condition of the game? Do you feel that MLB should look at making changes, in one direction or another?

Well, in terms of the way guys play, they’re gonna do what they do. And I guess we could look this up, I think it might’ve been Tom Tango who showed, statistically, that it’s better to try to hit a home run and strike out than to just put the ball in play. And teams have worked towards that, and in recent years we have had balls that have had more carry, lower drag, whatever. So you do have this compounding effect. And I think it was Rob Arthur, again, who found that partway through the 2019 season guys started to figure out that the ball was traveling farther, and started modifying their swings so they could hit more home runs. Which makes sense.

Right, so long as that’s a realistic, achievable goal. But if things were tweaked so that you were less likely to be rewarded with a home run, presumably over a long enough period that would incentivize players to approach batting differently. Optimization is this thing that is coursing through American sports, and it’s informed by analytics and is finer-tuned than maybe it used to be. Which is why I think I keep coming back to making mechanical changes, because it seems like if you don’t change the likely outcomes, then best practices will continue to pursue this narrow range of best outcomes, and that’s something that can affect every layer of a sport like baseball. 

Well, I will admit, I feel like home runs, the number of home runs is for people who aren't, like, really rabid fans. You know, they tune in occasionally if there's something on ESPN, or they'll watch the postseason, but they're not like diehard fans of a particular team. And they don’t watch a lot of games. And after a while, frankly, you know—think back to the World Series and how excited everybody got about a squeeze! Everybody was so psyched about it!

I love squeezes, personally I’m always a fan of good defense, I love watching, for example, Ji-Man Choi. That’s my thing. As for home runs, if you’re going to hit multiple home runs every single game, then they become commonplace. They’re still exciting, but they’re more commonplace.

How would I put this? The way that the ball has changed, it has sort of led to more home runs. I feel like if they had a better understanding of what was going on they could make a ball that did what it used to do. And this year’s ball, so far as I can tell, was much more like the ball of 2018. So they did get away from that weird 2019 ball.

When I talk about consistency, it would be nice if balls from year to year could be more interchangeable in terms of behavior. And the 2019 ball—how do I put this? It did feel like it was part of a concerted effort to make a better baseball. All of these things that the home run committee published in 2018, that was right around the time that MLB bought Rawlings. Before people come back and say, “They only bought part of Rawlings”—I understand that. They bought the part of Rawlings that makes the baseball. Rawlings makes a lot of stuff other than the baseball. MLB specifically bought in with the goal of being involved in the process. That was the whole reason. And they made what would be considered a better baseball. There’s been this focus on smoother leather and lower seam height, and they did it! And it turns out that the perfect baseball isn’t really good for playing baseball. So now they’ve gone back to something that is more like a regular baseball. Normal, whatever.

So as a prescription for injecting more variety into the sport, instead of deadening the ball, would you say that just having lots of different balls with lots of different characteristics would be a reasonable solution?

Actually, I don’t think that’s a good thing. You want a level of predictability. If you remember watching the postseason from last year, the outfielders did eventually figure it out but you did absolutely have situations where fielders, but really everybody, was misreading the ball. When you had a mixed bag, literally a mixed bag of balls, where it could either be a 2019 ball or a deader ball from another batch. Think about Will Smith’s non-walkoff home run in Game 5 of the 2019 NLDS. Every single person in that stadium (except for Adam Eaton, who by then had figured out how to read the deader balls) thought that was a home run. The guys were literally jumping over the dugout fence to celebrate, and it ended up in Adam Eaton’s glove on the warning track. [Ed. note: Hell yes it did.]

You’ve seen this, where outfielders start to get tentative when they can’t rely on the ball. That ball-to-ball variation is fine, so long as it’s not too much. The last thing you want is all over the place. Variation is fine, as long as it's within some reasonable range. 

So, if what we’re trying to do is get more triples, more stolen bases, more squeezes, the occasional Baltimore Chop, the hit-and-run, a little more small ball, what would you, Dr. Meredith Wills, astrophysicist, do to get there?

If we’re assuming a consistent ball, I would just move the fences back everywhere. I think that’s actually a better idea than trying to change the ball or trying to change the way the players play. Just make it harder to hit a home run.

How much farther? Are we talking 500-foot fences, here?

No no no. Didn’t San Diego alter their park dimensions? Like there’s a few parks that have done it. Just move ‘em back out! I realize that some people will get upset because that will mean fewer home runs from Fernando Tatis Jr. But that’s really the way to do it.

And he’s plenty exciting even without dingers. Just running the bases!


Think about something Rob Manfred said, he told the press, if we change the ball, you’ll be the first to know. So they’re sort of committed to a transparent process. And the KBO did it! It’s doable. But we know so little about the manufacturing process, everything I’ve done has been through reverse-engineering. I’m not really sure that the way MLB works...I don’t expect that to change. Meanwhile, the teams themselves can move the fences!

OK, yeah, and I guess if you change the ball then you’ll need time to learn over a period of years how it plays out on the field, if or how it influences the way the game is played. 

Yeah that’s a good point. The lag time on any result would be very different. Changing the ball, the way that MLB has talked about it, would require input from the players and more of an intense process. And with the way this summer went and the really contentious situation we have between the players and the commissioner’s office, I’m not sure how that would go over.

Does Opal have a preference for one year’s baseball over the others?

I don’t think so? But one thing I did notice as I watched the postseason on my iPad is that Opal did watch the games occasionally! I did have a cat a number of years ago, when I had a bigger TV, who loved watching the outfielders. Green background, individual fielder, glued to the TV.

Hey, so the current style of play would’ve been great! Plenty of outfield action.

Yeah, nothing with too many players. But one guy on a green background, she’d pay close attention!

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