Gyro Spin and Spin Efficiency

Many pitch systems measure how much ball spin contributes to the movement. There are a few names for this. pitchLogic uses “Spin Efficiency”, and others (including MLB) use the equivalent term “Active Spin". For the pitch pictured, a Spin Efficiency of 96% means that 96% of the 2045 RPM of this pitch contributes to the movement.

Why doesn’t all the spin contribute to break? Spin that doesn’t cause break is called Gyro Spin or rifle spin. The pitchLogic app indicates this spin component pictorially on the 3D Clock tab with the blue hoop around the ball, and the rifle spin and efficiency numbers. If this pitch had zero efficiency and 90º of gyro spin, the hoop would lie flat on the page. As you might guess, a bullet has no break due to spin and 0% efficiency (bullets are spun to make them stable, not to make them break).

The pitchLogic ball measures backspin, sidespin, and rifle spin equally well. The app uses these numbers to compute the spin direction (which is a combination of side and backspin) and the spin efficiency. Spin Direction and the amount of Gyro Spin are indicated in the clock graphic in blue.”

Some pitches typically have very high efficiency, especially 4S fastballs. High efficiency is also desirable for curveballs. In most cases, the vertical break on these pitches is due to spin, and more efficiency means more break.

Sinkers usually have lower efficiency than a 4S fastball, leading to less vertical break. Other pitches have very low efficiency, especially sliders that spin like a bullet. You can see the average efficiency of MLB pitchers by going to the spin leaderboard on baseballsavant.com and clicking on “Active Spin,” which is what MLB calls spin efficiency.

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Let’s discuss some example gyro/efficiency combinations. Say a high-arm-slot right-hander throws a fastball with good backspin. This has no rifle spin and has 100% spin efficiency. The app indicates this with the hoop around the ball, which we look at straight on in this case. The ball spins on a line that goes straight through the middle of the hoop.

A pitch with some gyro (and thus lower efficiency) has rifle spin and is shown next. Again, the ball is spinning around a line that goes through the center of the blue hoop. When viewing a video of a pitch like this, you may notice you can see the ball spinning around a point on the left side of the ball from the batter’s perspective, but the right side is hidden, as indicated by the hoop.

Also, notice that the force produced by this pitch (and thus the break) is smaller than the 100% pitch above. This is the main reason for seeking higher efficiency. The 64% efficiency pitch only has 64% of its 2000 RPM contributing to break.

Our final example is a 0% efficiency pitch. This spins like a bullet, so the rifle spin matches the total spin. Notice that this pitch produces no force at all. It will fly essentially like a rock. Many sliders have low efficiency, and to your eye, it may look like they break downward. What you see is just the effect of gravity, but your eyes are very accustomed to watching the flight of high-efficiency fastballs that resist gravity. Compared to those, the low-efficiency slider appears to break downward.

There are two effects that are not included in the discussion above. One is Seam Shifted Wake (or the effect of seams on the flight of the ball). The other is the fact that gyro angle and efficiency change as a ball starts to move more downward. These would each require much more space!