Ball Smoothness, Roundness and Drag: Post 26

I’ve just been listening to Jonah Keri’s great interview with Dr. Meredith Wills on an Athletic podcast. They were discussing her recent Athletic article about the 2019 balls as well as her two early articles. This recent article has made a huge splash and I am happy that Dr. Wills has been able to get people talking about issues with the MLB ball. I have some issues with some of her claims that I’d like to discuss here. (She and I are on a first-name basis, so I sometimes call her Meredith).

Normally, scientific work is peer reviewed. These studies appear in the Athletic, which is obviously not a scientific venue and therefore does not have peer review. But other scientists should have an opportunity to weigh in on results, and that’s what I want to do here. I mean no disrespect to her important work. Rebuttal is not an attack. This is the normal process, although normally, a reviewer is anonymously appointed and the public would not be privy to the exchange.

Meredith understands the construction of a baseball more than I ever will, and I think her insight on how process changes may change the ball are really important. I also admire the simplicity of her methods, which the public obviously enjoys. And, she’s a huge baseball fan. I hope I can make a contribution in the area of aerodynamics, which is my background.

First, let me talk about what we agree about. As she says in that interview, the 2019 ball is visibly different. You can feel it. The seams are significantly lower, so much so that it exceeds the random variation in the seam heights. Anecdotally, I’ve found that I can hand a 2019 ball and an earlier ball to anyone and they can pick the 2019 ball as having the lowest seams by feel. I believe the seams in 2017 were lower than previous decades, but perhaps not as obviously so. As I pointed out in Post 24, seam height is directly related to baseball drag. I think this single issue explains everything about the home run surge. Meredith also says some of these changes may increase pitcher blisters, and I have no opinion on that.

The very low seam of a 2019 MLB baseball

There are two claims she has made that I disagree with. The first is that a smoother ball means less drag. This is not true and you can see Post 23 for and explanation. I don’t dispute the claim that the ball is smoother, but I don’t believe that contributes to home runs.

The second claim that I am uncomfortable with is that a less “round” ball is a less aerodynamic ball. I cannot prove that this is false, and there is also no proof that it is true. A baseball line drive is typically spinning rapidly. If the ball is, say, somewhat egg shaped, it will present a larger frontal area (and thus more drag) in some orientations and less in others. Since none of this will impact boundary layer separation, I believe these effects will average out and will have no impact. As I said, I have no proof of this, but I do spend a lot of time watching air flow around baseballs.

Lastly, the podcast had a lot of discussion of wind tunnel testing. Unfortunately, wind tunnel testing of baseball drag doesn’t work. This is probably just semantics and Meredith probably meant “laboratory testing.” You can read many articles by Lloyd Smith and his team about how baseball drag is measured.

Summarizing, I think Meredith is right. The seams are lower and its leading to lots more home runs. She has insight on how the ball may have ended up this way that seem on the money to me. She sees a series of distinct changes to the ball, where I see continuous variations and a current extreme. And I think the smoothness and roundness of the ball are not important to baseball drag.

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