How has the construction of MLB baseballs changed, and does it explain the modern home run boom?

MATT MORET: Welcome to Field Study, a podcast about sports and science from the Pittsburgh Post-Gazette. I’m Matt Moret.

Today’s episode is a bonus piece. It’s sort of a companion to episode 4, which was my story about baseball humidors and the lack of standardized storage in Major League Baseball. But this installment turns our attention to what happens before a baseball even leaves the factory.

Dr. Meredith Wills is a sports data scientist with a PhD in astrophysics. Over the past several years, Dr. Wills has been trying to explain a historic surge of home runs happening across MLB.

Her research has focused on how manufacturing differences have altered the shape of big league baseballs. These physical changes affect air drag and cause balls to fly further once hit. They also help explain why batters are getting hit by pitches more often than ever before.

For this episode, I spoke with Dr. Wills about her discovery and what MLB may or may not be doing to change the state of the game. I began by asking her how she found herself physically tearing apart baseballs in search of clues.

With that, here’s Dr. Wills.

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MEREDITH WILLS: The ball in 2017 — for people who aren’t familiar, we got a sudden increase in home runs, particularly in the rate that home runs were being hit. And [Major League Baseball] actually had a home run committee that went and looked at possible causes. Was it climate change? Was it launch angle? Was it something about strategy?

And ultimately they figured out that the only thing that could be causing the home run change was the ball. That was the only thing that had any kind of significant difference, particularly what’s called the “drag coefficient” — essentially the aerodynamics of the ball. It was very clearly different. But they didn’t know how the ball was different, they just knew it was. They couldn’t find the physical difference.

There were a bunch of us who were looking at it in different ways. Rob Arthur was one, he thought it may have to do with the core of the ball. I was looking at construction in general because in the past, when you’ve changed materials — for instance the interior yarn that’s used. There’s yarn wrapped around [a baseball’s] core. Ben Lindbergh uses this example I like. When we ran out of American wool because of World War I, we start importing wool from Australia and ended up with a bouncier ball that came off the bat better. And so suddenly the ball started flying — hence the Dead Ball era versus the Live Ball era. That’s one of the changes, there’s others.

So that change ended up being that the red laces on the outside of the ball were thicker. This was a clear difference, a measurable difference. And people tend to think that lace thickness is the same as seam height. It turns out that seam height is actually related to how much leather gets squashed under the stitches. So it’s not the thickness of the laces so much as how loose the cover was to begin with, and therefore, if you’re tightening it and the cover needs to fit, you get higher seams.

But on the other hand, the thicker laces essentially keep the roundness of the ball — what’s known as the spherical symmetry — more intact. So a ball that has thinner laces will actually, at the seams, end up deforming away from being completely round, more than a ball with thicker laces does.

That was the measurement that I discovered and it has to do with the manufacturing process. It turns out that when you get cotton wet, which you do when you pull it through wet leather — which they have to wet so that it conforms to the ball — think about breaking in a pair of shoes or something.

You end up pulling the cotton through wet leather. The cotton gets wet. It turns out when you air dry cotton stretched, it stays stretched. So thinner laces stretch more. The ball deviates from being round. The old ball stretched more, so it had more drag. New ball stretched less, so less drag.

That was up through last season.

This season, it’s completely different. It is, no pun intended, a whole new ballgame. Players — pitchers in particular — are describing the ball as different.

Noah Syndergaard [of the New York Mets] specifically said it was like holding an ice cube, and he was using the analogy of a YouTube video where there’s a dog trying to pick up an ice cube. The implication was that he just couldn’t grip the ball. Jon Lester [of the Chicago Cubs] has said similar things, and Sean Doolittle [of the Washington Nationals] has as well. And those are pitchers who just described the physical aspects of the ball.

Very clearly, there are issues with grip, because walks are up. Hits-by-pitches are up. So essentially, command and control seems to be suffering. Pitchers are having that issue. The ball is also flying.

The conclusion you can draw from that, assuming that it’s the only change, is that the leather is probably slicker. And that was actually something that was brought up by the Home Run Committee last year as, “This kind of change would lead to a ball with less drag.”

So that was something that they considered as a possibility for the 2017 ball, but they didn’t go in and test that precisely. This year, just based on the descriptions, it sounds like it’s probably that. I’m in the process of going through some 2019 balls now and I’m still coming to conclusions. That should be published in The Athletic fairly soon, let’s say the beginning of June.

MORET: OK, so can we take a step back and talk a bit about how you researched the threads and how those have changed? What did you actually do with the baseballs and what type of baseballs were you looking at?

WILLS: Well, obviously they’re major league baseballs. If people aren’t aware, until this year, minor league baseballs are different from major league baseballs. All major league baseballs are made in Costa Rica. Minor League baseballs are made in China. They literally say, “Made in China” on them. The construction is actually not as high quality. I’ve torn apart both.

It turns out the ball started changing in the second half of 2015 until the end of 2018. So that’s kind of its own sample. And the ball turns out to be very consistent over that period, particularly with the laces. So for the study last year, I looked at the sample for 2016 and 2017. They were game balls, or they were batting practice. The ones I looked at from 2014 were just kind of from somebody — in fact, Ben Lindbergh — who did a study previously. I used the exact same baseballs, except in this case I took them apart.

Fortunately the construction methods are the same and it’s only at the very last minute that they have inspectors who decide, OK, this will be a game ball, this will be a batting practice ball, these will be something we put in a box and sell to fans. Obviously each of those are slightly lower quality, but the materials themselves are the same all the way through.

The things that differ are slight blemishes on the covers, if the stamps are slightly off-center, if the laces aren’t quite as pristine as you’d like. So they’re very subtle differences and they have nothing to do with the original materials themselves. It’s almost more aesthetics than anything, quite honestly.

MORET: So when you started breaking down the baseballs, did anything surprise you about it? Or were you pretty much expecting what you saw?

WILLS: Mostly it surprised me what the difference was, because I almost missed the measurement to find the lace thickness. It was quite literally the last thing I did and it was the one that was not on my list of things to measure. And the only reason I did it was because I started looking at the lengths of the laces. I had looked at all three layers of yarn, I looked at the length of the yarn, I looked at the weight, I looked at the size of the core — there were 15 different variables I was looking at, all of which were independent.

As I was going through and trying to measure the lengths of these laces, they felt wrong. And they looked wrong, between the two populations. And, funnily enough, this is where the knitting background helps me, I guess. There is a way to find out the thickness of yarn or thread or whatever, which is that you just wrap stuff around a dowel that has some kind of ruler along it. You see how many wraps you’re getting, usually per inch, but in this case it was thin enough that I did it per centimeter. So that’s what I looked at and, low and behold, the laces were thicker. I almost missed it, but I didn’t. Which is kind of cool!

And it turns out that they’ve been thicker — I looked at 2018 balls and it’s still thicker. This year, we’ll see.

MORET: I’d also like to talk a bit about the physics behind this, so could you talk a bit about air drag in general?

WILLS: In terms of the nature of drag, Neil deGrasse Tyson talked a bit about some of this related to lace thickness and used the analogy of race cars. The lower the drag, the more smoothly something travels through the air — the less air resistance you have. He pointed out that we don’t build race cars like cubes. We actually make them so that they’re really, really sleek and the airflow goes over them really easily. Hence, they go faster.

With a baseball, there’s a few ways to reduce drag. Ultimately what you would want is the lowest drag, you almost want it like a cue ball from a pool table. That’s going to have the least amount of drag because it’s very smooth and it’s perfectly round. The closer your baseball is to a cue ball, the more aerodynamic it’s going to be — hence, the further it will travel.

The things that the home run committee brought up as ways to essentially make the ball travel further — you know, decrease drag and make it more aerodynamic — there were three things.

One of them was that they thought the core may be slightly off-center. If the core was slightly off-center, what you would get would be that when [a baseball] spins in the air, it really will wobble. The way to think about it is that I am terrible at throwing footballs. I cannot throw a football that’s a spiral. If you can almost, but not quite, do it, you can picture that football moving in the air where it kind of jiggles back and forth. That’s the kind of effect you would get if the core was off-center.

Personally I would be surprised if that’s the case, because, if you find a video on YouTube of how they make a baseball, there is a very particular machine that they use to put the yarn on and it has a dowel that is almost like a gyroscope or a top or something — it really spins back and forth a lot in all these different directions. If that core was off-center enough to make a difference, the machine would shake itself apart probably before it finished its first baseball. And as far as I know, Rawlings does not want that to happen. So I would be surprised if the cores were systematically that off-center. And that’s actually an important thing to think about.

It’s not like it’s one or two baseballs are like this. The entire population of baseballs is what changes, and therefore you get that effect of the home run surge of of the pitchers saying the balls are different. It has to be systematic. So the idea of a systematic change to where the center of the ball is that somehow doesn’t destroy the equipment when you make it — I don’t think that’s a likely solution.

[One of] the other two [home run committee theories is] that the leather is smoother — which, again, picture the cue ball. The closer that leather is to being very, very smooth, the more easily it will travel through the air. And then the third thing, which I brought up before, is that the rounder the ball is and the more spherically symmetric it is, it’ll travel further.

Those are kind of the three options that the home run committee threw out. There are other things related to baseball construction that would affect that as well. Like, if the ball was smaller, it’s going to travel further because it isn’t going to have to travel through quite as much air. If the seams are lower, that would do it, too. But in their studies, they didn’t actually find any statistically significant difference on things like size, or seam height or whatever. And those tend to be all over the place.

Every baseball is, in that way, very, very different. You can take balls going back however far and the odds of you finding a population of baseballs that have a systematically different seam height would be a surprise.

MORET: Also related to the actual production itself, you mentioned a machine that threads the baseballs. But in your article, you mention that there is a lot of variability between the individual balls. Could you talk a bit about that variability?

WILLS: Well the important thing to remember is that the cores, which are this manufactured rubber thing that I think has cork on the inside and is a few different layers, are all coming off an assembly line somewhere. Those are all the same.

And then the red laces are on these massive, massive spools — like 6 ft.-across sized spools. There’s actually a great picture in the home run committee report showing these massive spools. That stuff is bought from some kind of external supplier. Everything else, while it is probably still from a supplier, it’s very hard to get wool yarn — especially the type that they’re using — spun so that it is always the same thickness.

Similarly with the leather, they do scrape it down, they do their best to make it smooth, but depending on the part of the cow that it’s from, it’s going to be thicker, it’s going to be thinner. And so you end up with these very slight variations. If it’s thicker, it’s probably less easily stretched, and I’ve discovered this while taking the covers off. Sometimes the covers are really floppy, sometimes they just stay round, pretty much. And it seems to relate to the thickness of the leather or maybe to the tanning process as well.

My point is that, with the exception of the pill, everything on the ball is essentially organic. It’s all stuff that — and I’m not going to get the quote right — but it’s all stuff that [Major League Baseball Commissioner Rob] Manfred said were variations that were beyond their ability to control or eliminate. And those kind of variations, because of that organic nature, are just there. That’s not to say it’s impossible to eliminate them, but it would be a wonderful act of quality control on Rawlings’ part if they did it.

MORET: What is Rawlings’ actual role? They’re just the production people?

WILLS: They are the production people. Interestingly enough, right around when the home run committee report came out last May — on June 5 of last year — MLB announced that it had bought Rawlings. And specifically it had bought Rawlings because of recommendations in that home run committee report, to try to mitigate these variations and also to do things like physical quality control and also things like aerodynamic testing — which are not done, or have not been done.

Golf balls get aerodynamically tested, but, for some reason, baseballs do not. There’s no wind tunnel down there. Had this quality control been there, say, in 2015, they would have known before the balls were even distributed that they were different. So that was something the home run committee asked for, that was the purpose of MLB buying Rawlings, was to improve the standards and essentially make sure the baseballs were going to behave the same way population-to-population.

And Rawlings — I do know Buster Olney brought this up last week on the Baseball Tonight podcast, where he had been talking to Rawlings. And apparently they are experimenting with the ball, particularly to improve the grip for pitchers. So presumably they are aware of the fact that pitchers are having grip issues right this minute, and they said they are on their second prototype. That’s what they told Buster. So something is clearly being done with the ball and it looks like they’re trying to mitigate the problem pitchers are having now. I have to wonder about the first prototype. I wonder what they were going for there, for instance. Was it something that was developed in the offseason or … I don’t know.

It would be cool to know that stuff, but I can also understand why they don’t want to talk about it publicly. The players should know, but it’s OK if we, as the fans, aren’t necessarily told. Although I may still tear apart the baseballs to find out. *laughs*

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MORET: You can find Dr. Wills’ baseball breakdowns over at The Athletic, and her investigation of changes to the 2019 baseball is coming soon.

I want to thank her for her time and thank you for listening. This is the final episode of our first season, and I hope you enjoyed it as much as we have. We will return after a break, and I’m already excited about what we have in store.

Until then, please leave us a review and rating on iTunes. It makes a big difference.

I’m Matt Moret. Field Study is a production of the Pittsburgh Post-Gazette. And that’s all for now.