What is the relationship between spin axis and the backup slider? Alan Nathan knows the answer. He also knows why fastballs move more than curveballs and why split-finger fastballs drop. A physics professor emeritus at the University of Illinois, Nathan is an expert not only on nuclear physics, he is the man behind The Physics of Baseball
Nathan on velocity and break: “In principle, it would be possible to throw a curveball 100 mph. It’s not the physics that prevent you from doing that. What’s preventing you is that it is biomechanically hard to do. You’re trying to get on top of the ball and put topspin on it, and to do that you’re probably sacrificing speed. But there’s no reason, in principle, that if you were able to spin a 100-mph pitch with topspin you would get even more downward movement.
“You actually see more movement on fastballs than you do, typically, on curveballs. Defining movement as how much the ball deviates from a straight line with the effect of gravity removed, you get a lot of movement from fastballs. People don’t normally think of there being a lot of movement from fastballs, because the upward movement you’d get from backspin on a fastball means the ball doesn’t drop as much as it would just purely from gravity. But there’s a significant amount of upward movement. Rapidly spinning balls moving at high speeds have a lot of movement on them.
“All fastballs in fact, drop. Look at PITCHf/x data; look at the release point versus where it crosses the front of home plate. It’s always lower when it crosses the front of home plate, but it would be even lower if there were no backspin. Backspin keeps the ball up in the air longer. We call that movement, even though the batter doesn’t really perceive it as movement. It looks more like a straight line to a batter, but the ball is falling and it would fall even more if there were less spin on the ball. That’s exactly what a split-finger fastball is doing. A split-finger fastball is holding the ball in such a way as to reduce the amount of backspin. The ball doesn’t fall because of any action of the spin, it falls because of gravity.”*
On why two-seam fastballs sink and four-seam fastballs don‘t: “It has to do with how the spin axis is oriented. With a four-seam fastball, the ball has almost perfect backspin, and maybe a little bit of sidespin that makes the ball tail. With a two-seam fastball, the spin axis is tilted in such a way that there’s more sidespin, so there’s more tail on it, less backspin, and therefore it sinks. It sinks because there’s less backspin.
“It’s not because of the seams per se. I think it has to do with the finger action of the pitcher’s hand on the ball that orients that spin axis the way it does. I don’t know exactly how they do it, but I do not believe it’s the seams that are causing that ball to move differently, It really has to do with how the pitcher applies pressure with his middle and forefinger, which are the last two fingers to touch the ball as he releases it to orient that spin axis.
“Once again, I’m relying on laboratory experiments that seems to show that all other things being equal, a four-seam pitch and a two-seam pitch, if the spin axis were oriented exactly the same, would break exactly the same.”
On if fastballs can rise; “A fastball could rise in principle, It could actually rise if you could get enough spin on it. The easiest way to show that is take a Styrofoam ball or a ball that has the same size as a baseball but is much lighter, and throw it with backspin, and you could make it rise. The reason why you can make that rise is because it weighs a lot less than a baseball — gravity isn’t not pulling down on it as hard. With a baseball, you would have to get much more spin on it in order to make it rise. Probably, it’s physically impossible. From a physics standpoint it’s possible, but biomechanically it’s hard for a pitcher to do.”
On the impact of seams: “The NCAA baseball has higher seams, and the higher seams certainly affect the aerodynamic properties of a ball. Laboratory experiments have been done where you set the two balls spinning at the same rate and look at how much movement there is. The data show that there’s more movement with the high-seam ball. Moreover, a pitcher can get a different grip on the high-seam ball. I think you can grip the ball better and probably put more spin on it as a result. For both these reasons, there will be more movement with a high-seam ball.
“It is also true is that there is less air resistance on the flat-seam ball, on the major-league ball. That is fairly well known, but it’s only been well know recently, at least from laboratory experiments. Straying a little bit from your question, there is a move afoot among NCAA coaches who are lobbying to change the NCAA baseball from a raised-seam to a flat-seam ball in order to get back some of the home runs that were lost when they went to the BBCOR bats.”
On scuffed baseballs: “The seams do matter for movement, but the direction of the movement doesn’t depend too much on the seams. If the ball is spinning rapidly, the seam orientation is changing so rapidly that whatever forces arise due to the air flowing over the seams sort of averages out to zero because they’re so rapidly changing directions.
“The key to throwing a scuffed ball is to have the scuff on the spin axis. If you think of the North Pole of the Earth, the Earth is spinning around and the North Pole remains fixed. If you look at the North Star, which is located right on the spin axis of the Earth, the North Star doesn’t move. It’s always in the same location even though the Earth is spinning, because it’s sitting right on the spin axis. The idea of a scuffed ball is to orient the scuff so it’s right on the spin axis.
“For example, if you were throwing a four-seam fastball — a perfectly overhand four-seam fastball — you would want to have the scuff mark on one side of the ball, either the left side or the right side depending on which way you want the ball to break. The physics that govern the movement of a scuffed ball are exactly the same physics that lead to the movement of a knuckleball. In the case of a knuckleball, it’s the air flowing over the seams, but since the ball is not rotating rapidly — it might be rotating very, very slowly — unlike ordinary pitches where the ball is rotating rapidly and the effect of the seams average out, the knuckleball does not. In the case of a scuffed ball, where the scuff is located right on the rotation axis, it also does not.”
On bullet spin and back-ups sliders: “There are certain types of pitches that can be thrown in which the ball can break in an unexpected way, not because of a scuff, but simply because of how the spin axis is oriented and relative to the seams. There is a famous case that was looked at pretty carefully a few years ago, Freddy Garcia’s split-finger fastball. There were several cases of this where the ball broke in the wrong direction. It was thrown with the same kind of orientation as a two-seam fastball, so you’d expect it to tail. But instead of tailing, it actually broke in the opposite direction. A lot of effort was put into trying to understand why it did what it did. I think we sort of understand it, at least qualitatively. It had to do with how the seams were oriented for this particular pitch. It seems to be not one that’s thrown very often, and iit could have even been a scuff on the ball, but there wasn’t obviously a scuff on the ball.
“When a slider is thrown, the spin axis is oriented in the forward direction, unlike other pitches where it’s perpendicular to the forward direction. A pure gyroball is one that is thrown with so called bullet spin. A gyroball basically doesn’t break, In fact, there’s no movement to it.
“A slider is oriented in such a way that the spin axis actually passes right through a seam. If the spin axis is already slightly forward and it passes through a seam and the batter sees this red dot. That is the red dot skilled hitters — at least ones with good eyes — can see. They know the pitch coming at them is a slider.
“If the ball is not released properly so that the spin axis is oriented much too much in the forward direction, there will be very little movement on the pitch. That’s a backup slider, or at least this is what people have told me. It’s called a backup slider because the catcher moves over to try to catch it. He’s expecting it to break, and has to back up because he sees it is not breaking, Generally speaking, it is a mistake pitch. It’s a slider that doesn’t slide.
“The reason why it doesn’t slide is not because it’s not spinning. It’s because the spin axis has been oriented too far forward, probably due to the fact the pitcher released the ball a little bit later than he should have. Because of the motion of his fingers, the spin axis ended up being like that of a spiral football.”
David Laurila grew up in Michigan's Upper Peninsula and now writes about baseball from his home in Cambridge, Mass. He authored the Prospectus Q&A series at Baseball Prospectus from December 2006-May 2011 before being claimed off waivers by FanGraphs. He can be followed on Twitter @DavidLaurilaQA.