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Aaron Civale vs. Zach Plesac: A tale of two spin rates

Somehow the Indians found two pitchers on either end of the spin spectrum

Cleveland Indians v Oakland Athletics Photo by Ethan Miller/Getty Images

Two rookie pitchers broke in with the Indians in 2019, both with wonderful debuts that made one dream of a beautiful post-Kluber/Bauer future.

Zach Plesac gave the Tribe 115.2 innings of 3.81 ERA ball, striking out 18.5% of the batters he faced while walking 8.5%, and dancing with the Devil a bit with a .255 BABIP. It was solid, though some of the peripherals (the 4.94 FIP, the middling 39.1% groundball rate, the league average 38% hard-hit rate) make one hope that he knows what he needs to work on in advance of the 2020 season.

Next to him was Aaron Civale. I wrote about him the other day, I’m unabashedly a fan. Civale posted a sterling 2.34 ERA and topped Plesac in both strikeout rate (20.3%) and walk rate (7%) but in just 57 innings, and his FIP was a full run above his ERA at 3.40. Both pitchers showed us things to like and obvious flaws of youth, both filling fans with images of shutouts, Cy Youngs, and division titles.

The pair of them also shared a certain extreme — both sat at the very edge of the league when it came to curveball spin rate. Civale averaged a 2984 RPM on his curve, which put him in the 96th percentile of major league pitchers. As spin rates go, that’s pretty much as good as you can get. Plesac sat as opposed to that as possible, with a 1910 average RPM, placing him in the 1st percentile. If you want a baseline of where major league curve spin rates start, just watch Plesac. Baseball, so rarely a game of extremes, has given us a clear look at it in these two.

This is not nothing. These two pitchers are markedly different in how they threw their curveball last year. It’s also not some kind of referendum or firm statement of value on either Civale or Plesac. Spin rate is not all that matters when it comes to pitchers. You can read more about that Driveline Baseball, but essentially the idea is that not all spin is created equal. You need the rotation of the ball to actually be exerting the Magnus effect on the pitch, where the spin causes the ball to move in a certain direction based on how the spin bothers the air around it. I don’t have evidence of one of these guys having more “useful spin” than the other.

That said, here are those two pitchers’ curves, roughly normalized to location, side by side.

At first glance they do seem pretty similar, but Civale’s does have a harsher bite, a more obvious parabolic arc where all of a sudden the pitch just falls off a table like a good curve should. Last year this resulted in by Civale drawing a swinging strike on 14.7% of curves he threw, while Plesac drew one on 6.4% of curves.

It must be that Civale is simply throwing a pitch that moves more, because he’s getting more swinging strikes, right? Well, that’s what’s interesting. According to Baseball Savant, Civale averages 1.85 feet of vertical movement and .13 feet (about an inch and a half) of arm-side run on his curve. Plesac is at 1.82 feet of vertical movement, and about a fifth of a foot (roughly 1.5 inches) of glove-side run.

If Baseball Savant is to be believed, the movement of the two pitches is pretty similar. A few inches of horizontal run could be the difference, but Civale got a swinging strike 15 times on 97 curves, compared to 15 on 197 for Plesac. That's twice as often. It could be mere deception, a trick of the eye. In the documentary Fastball, along with at Driveline and other pitch research places, they mention that the power of the high spin fastball is that it doesn’t drop as much as the eye or brain thinks it “should”, which is why players swing underneath it for lazy fly balls more often.

Does the high spin make the curveball do the same thing, trick the hitter into expecting a different path? Or maybe it causes the ball to move the same amount, but later, more abruptly. That would make sense, that the high spin would allow for a sharper bite, a higher loft initially then a similar bottom. If the overall movement is tracked from the hand you would get similar numbers off of Statcast because the result would be the same, but it would make sense that Civale would be able to give it a bit more bend overall.

These two young pitchers have a lot of talent, and a good deal of hope of the fan-base resting on their shoulders. If there’s one thing Indians fans expect at this point, it’s consistent, competitive pitching. Having them at the polar opposite ends of the curve spin spectrum simply gives us a little fun science experiment to watch as the season wears on. With this right in front of us all year, it will be a nice little instruction as to the power of spin rate, how it matters, and what it does for a pitcher.