Scuffed Baseballs: Post 44

What happens to the flight of a baseball once it is scuffed? We decided to find out.

We started with a new 2019 MLB ball and fired it 12 times from our WSU cannon. The pitch distance is the standard 60.5 feet. The pitches were just below 90 mph and 1500 RPM. The cannon is amazingly consistent as you can see below in Figure 1. This is from the Rapsodo report. (thanks to @norton_training for the loan of the Rapsodo).

Then we took that pretty new ball and scuffed it. Like this:

Note that the scuff lies right on the spin axis. Next, we launched this ball 12 times at the same speed and spin and trajectory. The scuffed part of the ball does not touch the flextip, so there is no chance this ball released any differently.

I’m not going to show the Rapsodo report for these pitches because they flew so far outside that the unit no longer tracks them. But, the video in Figure 3 is representative of all the pitches. A clip with the new ball is first followed by the scuffed ball. Note the metrics. They are essentially the same in spite of the fact that the balls go to totally different places. Based on the dimensions of my poor Easton portable L-screen, there is more than 2 feet difference in horizontal break in these two pitches.

Did we expect this? Heck no, because I believe what I read in books. And I believe my own (apparently too simplistic, yet still complicated) explanations. Two books on baseball physics, Keep Your Eye On the Ball by Watts and Bahill and The Physics of Baseball by Adair claim that a scuffed ball will move toward the scuff since that side will be turbulent. And that’s how I saw it too before I decided to do this. In addition to getting the direction wrong, I never guessed we could generate 2 feet of break.

And, please allow me to digress. You can ask the baseball community any question about baseball aerodynamics with a simple answer (e.g. left or right?), and you will get both answers, mostly in equal numbers. So when you report the correct answer, half of those folks will say “everyone knows that!” Not picking on anyone. I will say that the experienced pitchers who weighed in were 100% correct.

Explaining What Happened

A hint on what might be going on comes from the field of Cricket. Rabindra Mehta has written extensively on the subject of how bowlers fool batters, and I’ve been hearing about it for a while. (Mehta’s article) He has discussed a phenomena called “reverse swing”, wherein transition to turbulence due to a roughened ball causes it to break (“swing”) in the opposite direction as the seams normally do. It took me a long time (today years) to appreciate what he is saying.

Before you can understand why this is happening, you have to understand why baseballs move in the first place. And I think anyone can without resorting to too much hand waving. Pressure applied to an area is a force. If you have a baseball and the pressure on all sides of it is the same, these forces cancel out. But, if the pressure on one side is larger than another, there is a net force. And Force = mass times acceleration. So if you apply a force to a baseball (which has a fixed mass) it will accelerate in the same direction as the force.

The pressure on the front of the ball is higher than the far away from the ball. Looking at the video in Figure 4, as the air “blobs” move around the front of the ball, their pressure goes down due to the Bernoulli effect. When they reach the fattest part of the ball (the vertical line), the pressure starts to increase again. If those blobs went all the way to the back of the ball, the pressure would return to the value on the front of the ball, and the ball would have no drag force (except skin friction drag) since the pressure on the front and back would be the same.

But, life doesn’t work that way. At some point, the blobs leave the surface of the ball and the wake of the ball forms.

(The colors in the picture mark the location of the wake)

Once this happens, the pressure stops changing, and since it happens before the blobs get to the rear of the ball, the pressure in the wake is smaller than on the front of the ball. This result in a drag force and slows the ball down.

Drag is boring. I want to talk about “lift”, which is a force 90 degrees from the direction the ball is moving (vertical or out of your screen in Figure 4). If one of those air blobs leaves the ball (which is what we mean when we say that the “boundary layer separates”) before the other, that means the pressure on the top and bottom of the ball is not the same. In that case, there is a force on the ball either pushing it up or down.

Why would that happen? I know of 3 reasons.

1. Magnus Effect
2. Seams
3. A scuff

How Does Magnus Effect Work?

Magnus effect is very well studied and understood. The ball in Figure 5 is moving to the left and not spinning. The separation points are at the same distance from the center of the ball and the wake is symmetric. There is no pressure force acting in the vertical direction, so no lift.

In Figure 6, the ball is spinning clockwise. The separation point on top moves back while the separation point on the bottom moves forward, and the wake tilts downward indicating a pressure force on the ball upward. We call this Magnus Effect.

In Figure 7, the rotation rate is increased, the separation points move more, the wake tilts more and the Magnus force is bigger.

This post is not about Magnus effect, but I wanted to include this discussion because any there are other reasons that the separation points might move, but no matter the reason, pressure force works the same way. If the separation point on one side of the ball is different than the other side, there is a lift force on the ball. If that force persists for most of a full rotation of the ball, the ball will deflect.

Most baseball tracking technologies assume that all baseball behavior can be explained by the Magnus effect. But those separation locations can change for other reasons too.

Non Magnus Effects

Much of this site is dedicated to the notion that seams can cause shifts in the boundary layer separation locations and thus the wake (click Seam Shifted Wake above).

A scuff is a possibility that does not get much play in baseball since it is illegal to doctor the ball and any damaged ball is quickly replaced. Scuffing used to happen intentionally, but is probably policed too carefully to go on today. MLB was not always as vigilante about replacing balls scuffed in play as they are now, and here’s a great clip of John Smoltz discussing that (at 2:52).

Now, before getting back to Mehta’s mind blowing idea, let’s discuss the easy stuff about laminar and turbulent flow. It’s well known that laminar flow separates earlier than turbulent flow. So if you can one side of the ball to be laminar and the other turbulent, whammo, you have a tilted wake and a pressure force. That is hard to do with a baseball due to its seams, but it is possible on cricket balls due their stitches only being on the equator. One of Mehta’s examples is shown in Figure 8. Note that this is a wind tunnel test and the air is moving rather than the ball.

If a bowler can throw the ball so that its axis is perpendicular to the seams, the seams will remain on the front on the ball on one side and the flow on that side will become turbulent. Note how the wake is tilted upward because the separation on the bottom (which is turbulent due to the seams on the front of the ball) is later than on the top, which is laminar. This is called “swing”. And I get this.

But, Mehta goes one step further and asks, what if the flow is turbulent before it encounters the seams? He holds open this possibility when 1) the ball is thrown fast (flow is more likely to be turbulent at higher speeds) and 2) it is late in the match and the ball is scuffed. Note that in Cricket, the ball is not replaced. In some matches, 2 balls total are used.

Mehta claims that if both sides of the ball are turbulent, the smooth side may have a later separation than the side with the seams. All of a sudden, the ball “swings” the other way and we have “reverse swing.” Note that is still all about where the separation points are.

But, baseball fans don’t care about Cricket, right? I don’t. Back to the much more interesting baseball.

We put the ball through our PIV measurement machine in 4 orientations corresponding to a full rotation in the 4-seam configuration. The scuff is on top, so imagine that you are looking down from the ceiling in the pitches recorded by Rapsodo.

If you are new to our measurements and how to interpret them, we have a primer here.

What we expect from convectional wisdom is that the scuff mark on the top of the ball will make the flow turbulent and therefor delay separation. What we see is just the opposite.

The flow separates from the scuff. It appears to be a laminar separation (the blue streak is straight and not very diffuse). As strangely, the flow on the bottom, which has not encountered any seams in the image above, is turbulent causing a very delayed separation. Between the two effects, the wake is tilted upward meaning the ball is being pushed the other way.

What does an unscuffed ball do? In all of these configurations, the flow is symmetric top to bottom. Compare the unscuffed ball in Figure 13 below to the scuffed ball in the same position in Figure 9. Both boundary layers (top and bottom) are turbulent. They are turbulent because for some reason, they were able to make it past the pole without separating. I am not sure why this happens.

A few days after originally posting this, I received an insightful question from @JohnnyAsel asking why the seams cannot cause this laminar separation. Actually, they can when they lie near the pole of the ball as shown, in Post 35. I’ve added one of the figures from that post below in Figure 14. We found that how close to the pole the seam needs to be in order to cause a laminar separation depends on the height of the seams. This is why higher seam balls have more drag: a larger part of the rotation results in laminar separation and a massive wake. Also note that if you have a pitch spinning with a seam on top or bottom consistently, you will get a wake tilted toward that seam and a force on the ball toward the opposite side, assuming you don’t also have a seam on the opposite pole (as is the case below. Post 45 shows how to break that symmetry).

The good news is that fluid dynamics still works. The ball does move in the opposite direction of the wake and the tilt of the wake make sense given the separation locations. It’s just very surprising that

1. The scuff causes a laminar separation
2. The other side is turbulent, even when there is no seam upstream.

I cannot explain either of these effects. This does not fit Mehta’s explanation, which, we note, does not appear to be based on any measurements or flow visualization.

2-Seam vs 4-Seam Scuffs

It is common to teach baseball players to make sure to throw a ball in a 4-seam configuration.

I’ve had a theory for a while that a scuffed 2-seamer will run while a scuffed 4-seamer won’t, and most baseball players (infielders and catchers, anyway) throwing a ball are throwing a scuffed ball. So we decided to scuff the ball so the scuff would be on the axis of a 4-seam pitch.

This is the same ball that has a scuff on another side (visible in the poster frame of the video) but that scuff will now be spinning around the ball and it’s effect will average out. Ignore that.

Care to guess what that ball will do? Be honest.

No difference. Another good theory up in smoke. So why throw 4-seam? I have no idea. They’re afraid of accidentally throwing a “Laminar Express?”

Rapsodo Limitations

Note that Rapsodo is saying that these scuffed pitches have zero horizontal break while there is clearly a great deal of break. That is because Rapsodo relies on Magnus Effect, and the Magnus Effect is not generating and horizontal break for this pitch. The scuff is generating a ton, and Rapsodo knows nothing about that.

I’m not picking on Rapsodo. It’s an amazing system that I am really happy I was able to use. But it is important to understand it’s limitations, as I discussed earlier.

Here’s my take on what it does: it appears to know where the ball crossed the plate quite accurately and reliably, as long as the ball is near the zone. It uses radar to find the ball peak speed, and this always works. It usually picks up the spin of the ball, axis, RPM. It uses that information to try to guess where it came from and how much break there was. This is what doesn’t work for a scuffed ball or for a seam-shifted wake.