How Does Pitching Work?: Post 39

I’d like to have a discussion between various the stakeholders of pitching. The three groups I am thinking of are pitchers, analytics folks, and physicists. I hope no one will complain if I put myself (a Mechanical Engineer) in the last category. This will be mostly (or maybe completely) about aerodynamics and will include little or no biomechanics. This article is based on my understanding today. I absolutely welcome anyone telling me I am wrong about any part of it and taking me to school. In that event, I will update the post so that it always reflects my current understanding. I’d prefer to discuss on twitter @NotRealCertain (please link this post), but you can also reach me by email: Here’s a list of topics I hope to clarify:
  1. What is Gyro?
  2. What is a Cut Fastball?
  3. Are Cut and Gyro the same thing?
  4. Does Gyro spin lead to movement?
  5. What are wake effects?
  6. What is a 2-seam fastball?
  7. What is a sinker?
I’ve spent a lot of time over the last year discussing pitching with all three groups and, in spite of a lot of effort on my part, I remain confused on many topics. There may be truth out there and there may be consensus that I am unaware of masked by differences in language and terminology. I also think there is a lot of misunderstanding out there that gets repeated often. Pitchers describe pitching mostly by the result. They talk about grip and where the ball goes. The more cerebral among them may describe spin axis in 2 dimensions and cut. And that makes sense, but an aerodynamicist wants to describe the spin axis as the spin axis in 3D, or the line that the ball spins around. In order to apply a model of Magnus Effect, that line must be known. So the first barrier to be overcome is language. Since I am trying to bring together these three groups, I will translate any term that I deem confusing into as many languages as necessary. Baseball pitches move, in most (95%) of cases, due to the Magnus effect. Magnus is complicated, but its impact can be boiled down to this: A translating, spinning ball will have a force on it in the direction that the front (as defined by the direction of translation) is moving due to rotation. Imagine that the ball below is moving straight toward you. The Magnus effect will generate a force up and to the left since the leather on the front of the ball is moving in that direction. Note that the Magnus force direction is 90 degrees from the axis of rotation.
Figure 1

Axis of Rotation Language

Michael Augustine and Trip Somers provided a lot of input on this section, and basically saved me. I was really out to lunch. And the first point of confusion is in describing the axis that the ball is spinning on. Driveline wrote an article on this topic, and I cannot improve much upon it except maybe explaining the motivation for some of the different ideas. The article explains that pitch spin is usually described only in 2 dimensions while there are 3. I think the problem began when pitchers thought that only “true spin”, “active spin”, or the spin that contributes to efficiency, mattered. We are smarter than that now but our descriptions have not caught up. Two terms can characterize the axis based on the direction the ball moves. “Spin direction” is described in degrees while Tilt is related to the hands on a clock. From the Driveline article: Using Tilt for “…accurately determining a pitch’s axis of rotation in two dimensions ended up becoming much more intuitive for most people. As a result, Tilt quickly became the primary metric for coaches and players to describe Spin Direction.” Note that websites such as use spin axis. You can convert from one to the other by noting that 10 minutes on the clock is 5º. In some cases, one may want to describe the axis of rotation, or the rod through my spinning ball. This is called the “Spin Axis”. So for the pitch shown in Figure 2, with the camera directly behind the pitcher, the spin axis is about 220 degrees. The Tilt is about 1:17 and spin direction is 122. The spin axis may look like it could also be 45 degrees, but this ambiguity is removed by the pitching version of the “Right Hand Rule.” If you wrap your right fingers around the ball so your fingers point in direction of the spin (NOT the way you’d hold the ball to generate that spin, which tends to be the exact opposite) and stick your thumb straight out, your thumb would point to 220 degrees. The white arrow indicates the direction that the ball will be pushed by Magnus force. In the Driveline article, rather than accounting for the fact that the direction of motion is 90 degrees from the spin axis, they rotate the polar coordinates 90 degrees relative to the clock. That took me a minute, but I think this is a common way of applying Magnus effect.
Figure 2: Image of a pitch from behind indicating Spin Axis and Tilt. The pitcher is @BigSkyBoiler, who I am sure if very surprised to see his hand here.
Tilt is the language of pitchers, and they are the bottom line here. It’s also used by Rapsodo. And I get that lay persons (e.g. pitchers) don’t often think about x-y-z axes. Engineers and physicists spend at least 4 years in school getting comfortable with 3-space, We have two choices to to understand pitch flight. Understand the spin axis in 3D or speak separately about tilt and gyro. A textbook fastball thrown from an over-the-top arm slot results in a horizontal spin axis. The ball has backspin, so the Tilt is 12:00. The pitch drops less than a non-spinning ball would. A curveball thrown from the same slot has topspin and the same axis, but a 6:00 Tilt (and the ball is pushed downward). A very good example is shown in Figure 3. I don’t hear this said often, but in my mind, a curveball is the opposite of a fastball in the sense that the axis is similar but the spin direction is opposite.
Figure 3: Clayton Kershaw’s 12-6 Curveball. Video from Rob Friedman @pitchingnija.
Here’s the thing: Pitching machines throw the ball with a 2-D spin axis, but humans usually do not. That 3rd dimension gets in there, and in some cases, it does good things. In others, it is undesirable. This takes us to Gyro spin.

1. What is Gyro?

Gyro spin refers to spin like an American football or a bullet. For a ball moving in the same direction as a gyro spin axis, this spin does nothing to the flight of the ball. Going back to the video in Figure 1, if this ball is moving directly toward you, it has a small gyro component that is evident in that the axis (the steel rod) is somewhat closer to you on the top than it is on the bottom. The axis is tilted toward home plate.
Figure 4: A capture from a video of Trevor Bauer 2-seamer taken from above from six years ago (I’m curious if he knows it is still out there, but I’ve never seen anything like it).
In Figure 4, which is taken from above, I’ve drawn an axis (green) on the ball. A pitch with this axis, with the one side (right) closer to home plate than the other side, has a small amount of gyro spin. If it were pure gyro spin, the green line would be straight up and down. Note that “Spin Efficiency” is the opposite of gyro. The term is used to describe how much of the spin contributes to the Magnus Effect for a ball traveling parallel to the ground.

2. What is a Cut Fastball?

Here is where I start to get sketchy. Most explanations of the cutter that I see focus on the grip and the movement. The word “Cut” if often used to describe glove-side movement. From what I can gather, A cutter is a fastball thrown with the hand not directly behind the ball (like a normal fastball), resulting in a substantial gyro component. The pitch has less lift than a fastball because gyro spin does not contribute to the Magnus Force while the ball is moving horizontally. Since most fastballs have a bit of tilt (something like 1:30 for a right hander), the normally run arm side, so a cutter will also have less run. I think it took a while for anyone to understand why this pitch is so effective, but Marino Rivera’s success (note it’s the only pitch he threw) made it clear that it was effective. Gyro has subtle properties that I will discuss farther down in 4.

3. Are Cut and Gyro the Same Thing?

Gryo describes the spin axis while Cut describes the ball movement. But I believe these are the same thing. Or perhaps cut relies on a certain gyro angle. Please tell me I’m wrong and what the right answer is.

4. Does Gyro Spin Lead to Movement?

I think it does, and in order to understand why, I had to take a bit of a more rich view of a pitch trajectory. I’ve tended to think of fastballs traveling horizontally in a straight line. If that were true, gyro would do nothing. But they don’t. Baseballs drop, and once they start dropping, gyro becomes side-spin and contributes to Magnus force. This is one theory on why the cutter appears to move armside. I met David Kagan (@DrBaseballPhD) at the Saber Seminar in 2019 and he laid this knowledge on me. It really changed my thinking. He has a very nice article on it at the Hardball Times. The beauty of this is that the movement does not begin until the ball starts moving downward. Thus, it creates the former unicorn known as “late break.” In that same video that Figure 4 is taken from, you can hear Trevor say “we know that late break doesn’t exist.” I am sure he has a different view now, as do I and probably many other people. For late break to exist, something has to change during the flight of the ball, and in the case of a cutter, it is the relative angle between the spin axis and the direction of motion. Whenever I have watched cutters, I don’t see much happening except the batter getting sawed off and looking confused. I don’t think the movement shows up very well on television because it is subtle. Too subtle to fool a batter unless it happens at the last second, and I think that is what happens. A different theory, explained to me by Mark Lowy, is that the lack of arm-side run of a cutter is perceived as glove side “cut.” Sliders often have a lot of gyro component and should enjoy the same benefit. And, you may have noticed, a lot of pitchers have very effective sliders. Gyro can also be useful for the Seam Shifted Wake or Laminar Express discussed next.

5. What are Wake Effects?

These are the newest ideas that I know of in baseball pitching. And, as far as I know, Trevor Bauer is the father of these ideas. His “Laminar Express” 2-Seam fastball is a great example. The video in Figure 5 shows Marcus Stroman’s sinker (left) compared to Bauer’s “Laminar Express” (right). The arm slot and spin are similar, but Bauer orients the seams of the ball in an important way that leads to the difference.
Figure 5: Marcus Stroman (left) throwing a sinker and Trevor Bauer (right) throwing the “Laminar Express.”
If you’ve been on this site before, you probably know that about half of it is dedicated to these effects, usually under the heading “Laminar Express.” I have come to see these effects more broadly that Bauer’s pitch, so I’ll lay it all out here. I think we all have a sense of what a wake is. It’s the region behind a moving object that is moving due to the moving object. The image below in Figure 6 is a PIV measurement of a special ball, signed by the late Sparky Anderson (as well as other ’84 Tigers including Jack Morris and Alan Trammel). [If you are new to our measurements, you may take a minute to read here about vorticity (the colors in our plots), boundary layer separation and wakes] The wake is the region where the air velocity vectors are big and there is color, whether red or blue. The pressure in the wake is low and it is high on the front of the ball. That difference is drag (there is also skin friction, but that is not very important).
Figure 6. PIV image of a ball moving 90 mph to the left.
The point where the wake begins forming is called the separation point. In Figure 6, the blue streak coming off the seam on the top of the ball is the separation point on that side. We have learned that seams tend to create the separation point when they are within a certain region on back of the ball (see Figure 7). Note that the red streak on the bottom of the ball comes off the leather, not a seam. The only seam in that vicinity (near the back of the ball) is too far back to form the separation point.
Figure 7. Map of the region where a seam will cause a separation point (Red) and the range where separation will happen without a seam (orange).
If a seam on one side of the ball can cause the separation point to form farther toward the front of the ball than on the other side, a pressure difference is formed between the two sides. This force will cause the ball to break, totally independent of the Magnus effect. Here’s the trick: how to you get a seam to hang out in one of these red locations on one side of the ball and not the other? That problem was solved before anyone knew about seams and separation points. It turns out that a seam in one of the red locations often means a lack of a seam on the opposite side on the front of the ball. This smooth region led many to conclude that the effect they were seeing was due to laminar flow on the other side of the ball. We have never found this to be the case in the lab. But, this notion led pitchers like Bauer to pursue the orientation that works. If you go all the way back to Figure 1, you can see that an axis exists such that when the ball spins around that axis, a seam is present in the same place about 3/4 of the time. It forms a red streak. That is the axis that is required. Now let’s talk about how to orient a ball that way and have a “seam shifted wake” The easiest pitch to understand that uses this effect is the “Discoball” Changeup. Strasburg’s is the best I have seen. I owe Michael Augustine for the Discoball term and Rob Friedman for pointing out that Strasburg uses it.
Figure 8. Stephan Strasburg’s Discoball Changeup.
The seam on the top rear of the ball is obvious. It may appear that the ball is more than falling, it’s being pushed down, and that is true, according to MLB data. I could go on and on, but instead, I will invite you to click “Laminar Express” in the menu above, and read the posts in reverse order.
This is not a figure. But click play.

6. What is a 2-Seam Fastball?

Thank you for continuing to read! This part is a bit more mundane, but I am including the last two topics because 1) they got me started on this whole thing and 2) I still find people don’t agree much on these terms. Again, thanks for input from Trip Somers. He filled me in that a 2-Seamer is a ball thrown in a 2-Seam orientation, period. 2-Seamers can be thrown in a way that they become sinkers (see 7) or or cutters, or can behave exactly the same as a 4-seamer. The “laminar express” is a special kind of 2-Seamer.

7. What is a Sinker?

A sinker is a fastball that has more tilt or gyro so that there is less ride and it falls more rapidly than a typical fastball. It is usually a 2-Seam fastball that has more tilt than a 4-Seamer, and maybe less efficiency. As a result, it has less ride and moves arm side. I don’t think that this is controversial, but what may be is how it gets that tilt. I have heard that some pitchers throw the ball in a 2-seam orientation and cause the ball to leave one finger before the other, thus generating more tilt. Once airborne, the behavior has nothing to do with the seams, unless there is a wake shifting effect going on or a scuff.


OK, that’s all I’ve got. Please question everything. Convince me I’m wrong and you’re right. Take me to school. Let me know if I spelled your name or anything else wrong.
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2 thoughts on “How Does Pitching Work?: Post 39

  1. One useful term to add to the definitions would be to talk about the “poles” of the rotation. I’ve seen people connect on that term when discussing seam asymmetry – one pole has a seam near it and the other is all smooth.

    Thanks as always for the great articles – I love connecting these three groups and getting on the same page definitionally!

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