Seam Height ball-to-ball/year-to-year Variation: Post 32

Mike Fast of the Atlanta Braves generously gave us a whole crate of 2019 balls, 6 dozen total. I asked John measure each one on and off the seams in several places and record the average difference as the seam height. It’s a crude measurement compared to the laser scans happening at WSU, but is still capable of showing wide variations from ball to ball.

He found that the average of these balls had a seam height of 0.0314 inches (0.8 mm) and the standard deviation was about 10% of that, 0.0038 inches (0.08 mm). This standard deviation agrees well with Meredith Will’s observation, while our seam height is much larger. I think this is due to a difference is the way we measured the seam height. That may seem small, but that means if the distribution is Gaussian, we will find a few balls that are smaller than 0.0238″ and as large as 0.039″ (plus and minus 2-sigma). It turns out we found a ball with seams as large as 0.039 and another that is 0.025″. These two balls are shown in Figure 1. Note that have not been rubbed with mud, which may make the seams larger.

Figure 1: The ball with the highest seams from our collection of 2019 balls (left) and the one with the lowest seams (right).

With this large of a difference, it is obvious when you pick the ball up. You can also see it pretty well with your naked eye. To get a better idea of the whole collection, Figure 2 shows a histogram of all the seam heights.

Figure 2: Seam height histogram for 2019 MLB balls.

Note that these are not just both balls from the same year, they are from the same crate. I would imagine that they were manufactured on the same day. I would expect a wider distribution of the samples were more independent.

We have seen plenty in the lab to make us suspect that the drag on these two balls is significantly different. So, just inside the same crate of balls, you may have one that has significantly less drag than another. I’m not sure if pitchers are rejecting balls for low seams, but I think they should if they are concerned about giving up a home run.

A natural question to ask is why one ball has a higher seam than the other. What exactly is different? Meredith Wills has some theories on this question, and I believe, based on her most recent article, she would claim that the leather at the seam is thicker for the ball with the larger seam (I hope she’ll correct me if I have that wrong). It also seems to me like the high seam ball (on the left in Figure 3) has the two pieces of leather drawn more together.

Figure 3: closeup of our two most extreme examples, high seam on left, low on right.

An important takeaway here is that what is important to the 2019 Home Run results is the mean of this distribution. Has it changed? One data point: Meredith lent me a ball from the 2009 , and its seams are 0.045″ high, larger than any ball from our 2019 collection. I will have a post soon about how that ball behaves compared to the current ball.

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