Going Deep: Contextualizing Curveball Spin Rate

(Photo by Samuel Stringer/Icon Sportswire)

In the first half of the 2018 season, Anaheim Angels starting pitcher Garrett Richards leads baseball in average curveball spin rate. Without regard to the injuries that cut short 2016 and 2017, if things hold up, it will be the second time (first in 2015) in a row he’d be league-best in terms of spin.

But does that matter?

First, you can’t talk about spin without discussing what is known as the Magnus Force. Curveballs (along with other top-spin pitches) break the way they do because of the force of air pressure pulling the ball down (gravity) and because air is a very heavy thing. When a ball with top-spin travels forward, regardless of velocity, the air is going to force the ball downward. Because of friction on the spinning ball, the air above is pulled down and around the front of the ball. The airflow below the ball is met by the force of the air above it coming around the ball. If you know physics, you can reference Newton’s Third Law in the sense that when the air is deflected downwards, there is also an equal, opposite force pushing air upwards.

Right. Enough with the scientific babble. But, you need to be aware of that to understand why heavy spin isn’t always a good thing for a curveball.

With fastballs, backspin is also affected by the Magnus Force; heavy spin fastballs thrown at high velocity appear to rise only because they drop more slowly. You expect a small amount of break, perhaps at a certain point in its trajectory, but when it doesn’t it gives the illusion of rising, even though it’s really not.

Because fastballs (mainly four-seams) stay in the air longer, you’re more likely to have a fly ball because the batter is expecting that break and when it doesn’t occur when expected, they swing a bit lower and get under the ball. And high spin isn’t a necessity with fastballs because lower spin achieves more break and the opposite happens; more grounders.

Yet it doesn’t work that way with curveballs. High spin influences ground balls while low spin is going to cause more contact to stay in the air. Garrett, our example in this article, typically induces 50% or more ground ball contact overall (it’s down a bit below 50% in 2018) and somewhere around 60%+ on his curveball alone.

Be aware that a higher spin rate curve with low velocity will break much earlier because the Magnus Force is causing the air to pull it down earlier during the ball’s flight path. A lower spin rate with a higher velocity will get to the batter faster but also take more time to break. I’ll get to how that applies in a minute.

Let’s look at two different curveballs from Richards that differ in spin rate by about 500 RPM. The first having 3633 RPM at 84 MPH and the second with 3127 RPM at 81 MPH.

What you see on the bottom gif is a slightly flatter trajectory as opposed to the obvious bend of the higher spun curveball. With the first gif, the pitch was released earlier to accommodate the heavier spin; the 3 MPH difference doesn’t bear much significance in this case. Since there was a lot of late break on the pitch, one could assume that the batter would have swung over the pitch and either missed or drove it into the ground.

We now know that successful curveballs are not necessarily dependent on the spin rate; high or low spin can be beneficial under the right circumstances for a pitcher. However, velocity is a factor and that’s where Bauer Units come in.

Bauer Units were developed by Driveline Baseball as an attempt to measure curveball effectiveness (keep in mind the level of spin and velocity ‘rules’ I stated before). How it works is the formula takes the spin rate of a pitch and divides it by the velocity. Richards’ BU comes to and even 40 (3252 RPM/81 MPH) with a standard deviation of 1.6.

Is 40 a good score? Well, let’s look at a scatter chart of Richards’ velocity versus his spin rate. We see a positive correlation which is a good thing because (generally speaking) if you’re going to put more spin on the curve, you want to make sure that you’re throwing it harder. So, we could conclude that 40 is probably a really good BU for a pitcher, regardless of the (positive or negative) direction of the trendline.

For Richards, the high spin rate is good because he’s able to compensate with higher velocity. Other pitchers may find success with lower spin rate and lower velocity. It all depends on what the strategy is for the pitcher during a given at-bat. Being somewhere in the middle, or zero correlation means your BU score is going to be low and are probably hanging a lot of curveballs the harder you throw them.

We can’t exactly use this formula as concrete proof of effectiveness. We talked about Richards’ BU score of 40 and presumed it was a great score. Taking a pitcher like Seth Lugo of the New York Mets who has comparable spin rate and a BU score of 39.5 (1.1 standard deviations). Richards BAA/BABIP is .065/.111 while Lugo’s curveball has produced a .171/.230. Obviously both great numbers but for two pitchers with a BU difference of .5, those averages are very different.

Does spin rate matter? Yes, but more when taking velocity into account. What is a successful curve? BUs hope to measure that. What’s more important than spin rate, BU, or velocity is the pitcher’s ability to work the curve into their repertoire and unleash it at the right times. You can have an average curveball but if you know how/when to use it, it can become a powerful weapon.

Michael Augustine

I've Contributed to Bronx Baseball Daily, Rant Sports, Beyond the Box Score, Fansided, Community Research on Fangraphs and Rotoballer. I hated any and all math in school so I am self-taught in statistical analysis. I also assist with the development of Out of the Park Baseball (18 and 19). Come check out my pitching gifs: https://gfycat.com/@Augustine_MLB




Big fan of the term ‘Magnus Force’. Going to try to incorporate that into my everyday lexicon!

I did a bit of back of the envelope research on spin rate v. whiff percentage and some other things, and it seems like it’s only really relevant on four seam fastballs.

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