Sunday, February 24, 2019
Statcast Lab: Active Spin Percentage
Knucklers, footballA knuckleball will move in haphazard ways with almost no spin. You can thank the seams of the baseball for that. Otherwise, a spinless smooth ball will not move beyond its straight line trajectory other than for gravity.
A perfect spiral football will also not move left or right. You can thank the axis being pointed along its direction. What is the axis of a football? First, remember that a ball spins around its axis. And we are all familiar with how a football spins. If you hold a football, your thumb will be pointed toward you, near the back tip of the football. So, the axis of the football is aligned with the direction the football is going in, which is exactly opposite to where your thumb is pointing. Thumb points toward you, football goes exactly in the opposite direction. And so, such a football has no left/right movement.
Fastball, curveballA pure fastball is thrown with your thumb (i.e., the axis of a baseball) perpendicular to the direction of motion. Your fingers point in the direction of motion, while your thumb points exactly opposite to the spin axis.
The backspin of the ball is counteracting part of the effect of gravity, and so will end up higher than it otherwise would be without backspin. That's an upward deflection. A curveball is thrown with topspin, adding to gravity with additional downward deflection. When thrown with pure top or backspin, then the entire movement of these deflections can be estimated based on the number of revolutions the ball makes toward the plate. In other words, 100% of the spin contributes to movement. We call the spin that contributes to movement as Active Spin. You can also throw a pitch with some side-action so that some of the deflection will go up or down, and some will go side to side. All of this is Active Spin.
Interlude: In other literature, you will see terms like "useful spin" or "spin efficiency". In my view, these are not the best terms. The opposite of useful is useless, and a pitch with not high spin efficiency will have low efficiency. Both these terms would imply, to a lay user, that useless spin and low efficiency is bad. By instead using the term Active Spin, the opposite of Active is Inactive. Neither word implies anything good or bad. Hence, Active Spin provides a meaningful word without being ambiguous.
How do you get Inactive Spin? That's a football. For a football, none of the spin contributes to movement, and so, a football has 100% Inactive Spin. In order for a baseball to have Inactive Spin, you would twist your arm slightly so that the baseball is thrown more like a football. The more the baseball is thrown like a football, the more the spin of a baseball will go from Active Spin to Inactive Spin.
Active Spin PercentageThe technology currently used in MLB parks and elsewhere to track the trajectory of the pitch also measures the spin rate--or RPM--of the pitch, which allows us to easily convert that into number of revolutions in its flight. We can also infer the amount of movement of a ball based on its estimated trajectory of measured points along that trajectory.
Alan Nathan was generous enough to provide the physics-based math equations that allow us to estimate how much movement we'd expect for each pitch based mostly on the amount of spin. By comparing the calculated movement to the estimated max-movement, we end up with an estimated Active Spin Percentage: the percentage of a pitch's spin that contributes to movement. In other words, a pitch that is 100% Active Spin is thrown with pure backspin or pure topspin, or any spin as long as the axis (your thumb) is perpendicular to the direction of motion. And a pitch that is 0% Active Spin is essentially thrown like a football, with your thumb parallel to the direction of motion.
Sample playersVerlander throws his fastball with essentially 100% Active Spin. When we calculate it pitch by pitch, we get a range of around 85 to 115%, which is what happens when we compare estimated values with inferred values. So, there's a margin of error we have to appreciate, of which most of it goes away when we are talking about 1000+ pitches.
Interlude: if one SD (standard deviation) is say about 5% of error, then if you have 100 pitches, it chops that down to 0.5%. That's because you can reduce the amount of error by the square root of the number of pitches: square root of 100 is 10, so divide 5% by 10, and you get 0.5%. Similarly, if you have 2500 pitches (of which the square root is 50), it chops it down to 0.1% (5 divided by 50).
Patrick Corbin for example throws his excellent slider with 22% of Active Spin. Is that good or bad? Given that he might have the best slider in MLB, it's good. Would 24% or 20% be better? I don't know. It's likely that 22% is perfect. When you are the best at something, then you've probably figure it out.
Interestingly, we have his curveball at 17%, which is a FAR CRY from almost all other curveballs. Garrett Richards for example is at 97%. It's an almost certainty that Corbin is throwing a "slow" slider, and not a curveball. But in the world of nomenclature, we live with what we've got. The reality is that, analytically, you can't include Corbin's 73mph "curve" with Richards' 81mph curve or Wainwright's 73mph curve.
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A special thanks to Alan Nathan for a review of this post. Anything that the reader disagrees with, he is disagreeing with me.
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