Why spin rate matters for fastballs

We’re now two full seasons into the Statcast™ era, and “spin rate” has become an everyday term in the baseball world. Yet since it’s still so new to so many, it’s important to remind fans what it means, and how that can be different from pitch to pitch. Is high spin good? Sure … sometimes. It depends on what you’re throwing, and what you’re trying to do. What does a spin rate of 2,400 rpm? What about 2,800?

These are all good questions, so let’s provide some answers, and today we’ll focus only on four-seam fastballs. That’s an important point, because spin means different things for different pitches, since not every pitch spins in the same direction. For four-seam fastballs, high spin can be a very powerful thing… but so can low spin. That’s not necessarily true for other pitches.

Let’s get started by simply showing the distribution of spin, which should help show the range of rpm numbers. In 2016, there were over 255,000 four-seam fastballs thrown, and the Major League average four-seam spin rate was 2,264 rpm. As you’d expect, a huge majority of four-seam fastballs are clustered around that point, in the 2,100 rpm to 2,500 rpm range.

In the same way that baseball had a 4.19 ERA and a .255 batting average across the sport last year, 2,264 rpm is your starting point for average four-seam fastball spin rate. Lots of pitches were above, many were below, and a ton were extremely close to average. So, what does that mean?  

The reason why we care about spin in the first place is because it helps to define how a pitch can move. For example, high spin on a fastball helps the ball defy gravity for slightly longer than a pitch with average spin, and this is often referred to as a “rising fastball.” That’s a somewhat misleading name, because the ball doesn’t really rise, it just falls more slowly than a hitter expects. But even if that’s a difference of only a few inches, that can make all the difference as far as whether the batter makes square contact or misses entirely.

Because the ball stays up and hitters are more likely to miss it or make contact with the bottom of the ball, the high-spin fastballs are positively correlated with swinging strikes and fly balls, as these images makes clear:

Let’s take a look at the average spin leaders, among the 495 pitchers who threw 100 four-seamers.

Highest four-seam fastball spin rate in 2016, minimum 100 pitches 2,674 rpm — Andrew Bailey 2,659 rpm — Carl Edwards Jr. 2,565 rpm — Justin Verlander 2,551 rpm — Matt Bush 2,550 rpm — Max Scherzer 2,546 rpm — Aroldis Chapman 2,523 rpm — Jose Leclerc 2,517 rpm — Cody Allen 2,511 rpm — Yu Darvish 2,509 rpm — Dellin Betances

This is a fascinating mix of names, starting with the fact that eight of the 10 spin leaders here were very, very good last year. (Yes, that includes Edwards, who we’ve been talking about for months, and Bush, who made his Major League debut after five years away from …

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