We have long been aware of the fact that baseballs spin when traveling through the air and that Major League Pitchers are able to generate quite a lot of spin when they release their pitches (average spin rate on Major League pitches was around 2270 RPM’s last year).
It has been common knowledge for a while that the spin Major League pitchers are able to generate upon release can have large impacts on a ball's flight path as it travels towards home and this fact helps explain a large amount of the movement we see on pitches. Magnus force, which is defined as a phenomenon where a rotating object immersed in a flowing fluid sustains a force perpendicular to the line of its rotating motion, can be more easily understood as the force that causes objects to travel towards the direction the front of the object is spinning.
Since the implementation of the Statcast tracking system in all Major League stadiums prior to the 2015 season, spin rate has become an everyday metric utilized by the public in an attempt to better understand pitches and also attempt to evaluate their effectiveness or potential effectiveness. Due to improvements in tracking system technology over recent years, we have now been able to develop a much deeper understanding of why certain pitches move like they do.
One such discovery is that not all spin is created equal and that oftentimes not all the spin on a pitch is actually contributing to its movement. Spin efficiency provides us with a way to quantify this, in that it is a simple measurement of how much spin on a pitch is actually contributing to its movement. Though the terms spin efficiency and active spin are often used interchangeably, I generally believe it is easiest to think about in the following way: where true spin is the total amount of spin contributing to movement and thus spin efficiency is simply the ratio of true spin divided by raw spin. Raw spin is typically the number expressed when talking about spin rate (measured in revolutions per minute, RPM's).
While the connotations of the term efficiency imply higher is better, this is not necessarily always the case. The goal is not always to create more spin-induced movement on a pitch because a number of factors must be considered when determining the optimal spin efficiency for a pitch. Spin efficiency varies from pitcher to pitcher and also varies quite a bit across different pitch types, with pitches like sliders relying less on their raw spin to generate the movement they feature.
Pitch Type2020 Mean MLB Spin Efficiency 4-Seam Fastball89.8Cutter49.1Sinker89.0Changeup89.3Curveball68.7Slider35.9Values are approximate
Though the above is certainly interesting to consider, it shouldn't necessarily serve as an ultimate guide for pitchers to base their pitches after and compare themselves to. As previously mentioned, the desired amount of spin efficiency on a pitch depends on a number of factors, including the other pitch types in a pitcher's arsenal, and should be carefully considered when attempting to modify or even design new pitches. While the pitches that don’t feature a higher spin efficiency are lacking spin induced movement, they are still able to be influenced by a number of different forces as they travel towards home plate. Pitches thrown without a large amount of efficient spin have what is called gyro spin, or spin that doesn't directly contribute to movement. Pitches thrown with gyro are more susceptible to the influence of non-magnus forces as they travel through the air.
Consider Cincinnati Reds starter Luis Castillo, who has developed into an ace for the Reds over the last few seasons and is coming off his best year yet (though shortened) as he posted a 58 FIP- (42% better than a league average pitcher) in 70 innings pitched. While Castillo is well known for his 80-grade changeup, his slider is also a very intriguing pitch.
https://twitter.com/PitchingNinja/status/1308208460522311681?s=20
The point would perhaps better be illustrated by observing an overlay of all his pitches:
https://twitter.com/PitchingNinja/status/1235900786523156486?s=20
Though his slider spins at close to 2500 RPM's on average, very little of this is spin is actually contributing to the movement of the pitch. While this is a largely inefficient pitch (~14% efficiency last season), it was able to be a very effective pitch for him as he was able to use it to limit opponents to a .183 xwOBA and also use it to get swings and misses at a 43.4% clip. Castillo worked on his slider last offseason in order to create a better tunnel off his other pitches, and the changes he made appeared to have worked considering the lack of quality contact against his slider. A large part of the reason this pitch was able to be so successful for him last season was because of how well it plays off his other pitches and a higher efficiency slider likely wouldn't create the same effect.
Using BaseballCloud's BallR technology we can better visualize Castillo's slider. If this pitch was thrown with greater spin efficiency and less gyro, we would expect the pitch to feature more consistent, distinguishable horizontal movement than how it currently moves. Castillo's slider actually averaged 0 inches of horizontal movement last season according to Baseball Savant, though as Johnny Asel explains here, this number reported by Baseball Savant is a bit deceiving since this pitch doesn't actually feature zero horizontal movement. The pitch also featured an average horizontal approach angle of -3.4 degrees last season which tells us the pitch is generally moving horizontally away from right-handed batters at a fairly sharp angle as it crosses home.
https://videopress.com/v/UHtZniNe?autoPlay=true&loop=true&preloadContent=metadata
The recent release of the spin direction leaderboards at Baseball Savant has provided the public with a goldmine of information that wasn't previously accessible. Included in this new data are measurements of the observed and inferred movement for all pitches. The inclusion of this information will greatly facilitate in the public understanding of pitch movement and pitch design, while also helping us gain a better understanding of some of the non-magnus forces impacting pitch movement.
Sources
Wang, D., and L. S. Fan. “Particle Characterization and Behavior Relevant to Fluidized Bed Combustion and Gasification Systems .” Fluidized Bed Technologies for Near-Zero Emission Combustion and Gasification, 2013. Woodhead Publishing Series in Energy , doi:https://doi.org/10.1533/9780857098801.1.42.
All data and statistics courtesy of Baseball Savant and Fangraphs