Cliff Notes Seam Shifted Wake: Post 65

If you are new to this and you don’t feel like sifting through the last 64 posts or watching a 90 minute Zoom explanation, this post is for you!

For more than a decade, baseball has worked under the assumption that pitched balls move primarily due to the competing forces of gravity and the Magnus Effect. My group has been able to prove that a there is an additional force on the ball due to what we call the Seam Shifted Wake.

We study baseball flight two different ways. One is to visualize the flow of air around the ball using a technique called PIV and the other is to throw balls across our lab with a pitching cannon.

Here is a PIV picture of a baseball that is not spinning and has no forces other than gravity and drag. PIV makes the colors marking the wake on the back of the ball. All we care about here is that the wake is straight, which means there is no sideways force on the ball.

Here’s one that IS spinning clockwise (like a fastball). Note that the wake is shifted down, indicating that there is a force upward. This is the Magnus force.

So far, there is nothing new other than a way to visualize the flow of air around the ball to confirm what we already knew. The spin of the ball makes the wake form early on one side (the bottom, in this case) and late on the other, thereby shifting the wake.

The New Shit is this: Seams can also change when the wake forms on one side of the ball. In fact, when they are in the right place, they are pretty good at it. Here’s a nice example on a ball signed by the great Fergie Jenkins. This ball is NOT spinning. The seam on the bottom of the ball is causing the wake to form early on that side, tilting the wake downward.

The one below belongs to my friend Doug, and it’s signed by Jack Morris, Alan Trammel and Sparky Anderson! The seam on top is causing the wake to tilt upward.

Now, you may be saying to yourself, what about all the other seams? Here’s the thing:

  • Seams in the wake don’t do anything.
  • Seams on the front of the ball don’t do anything.
  • But, seams near the middle of the ball do. Here’s a map of where seams matter. If a seam is in the green area, they are making the wake form early. The red area is where the wake forms normally.

So, the trick to a Seam Shifted Wake pitch is to:

  1. Get a seam in the green on one side of the ball and not the other
  2. Keep it there as the ball spins

That’s a tall order, but it can be done. I don’t say it as often as I should, but while the seam causes and effect anywhere in the green area, the front of the ball generates a bigger effect than the back.

Here’s an example using Trip Somer’s pitch spin simulator to show what a typical SSW “Discoball” changeup looks like from the pitcher’s point of view. Note that there is a seam on the top side of the ball that is located in the green region about 3/4 of the time. That will push the ball downward.

Until about 2 months ago, I had to wave my hands a lot to prove that, it is very clear today based on the Hawkeye data that became available for the 2020 season. I’ll get to that in a second, but first, feast your eyes on some great SSW pitches.

Message: this is happening all over the place. In fact, we now know that THE VAST MAJORITY of sinkers are SSW. Here’s how we know.

You can predict where a baseball will go based on a model of the Magnus effect that takes velocity, spin and axis into account. The Trackman measurement system used by MLB until 2020 measured the velocity, spin rate and movement of pitches. It was common to use a Magnus model to determine the axis of the pitch based on that information, assuming that the only forces affecting the movement of the pitch were gravity and Magnus force. This is called an Inferred Axis, or IA.

MLB’s new measurement system, HawkEye, measures the axis of the pitched ball directly. We call this Observed Axis or Optical Axis (OA). It also measures movement, so one can also compute IA. If OA does not equal IA, there is a Seam Shifted Wake Effect.

Back in November, I published an article in Baseball Prospectus on these findings (along with Alan Nathan and Harry Pavlidis). Some highlights from that article:

The most non-SSW pitch you can get is a high-efficiency 4-Seamer. A great example is Garret Cole.

Note how his pitches are centered on the line for which OA = IA. Now let’s look at “The Professor”, Kyle Hendricks and his 2S sinker.

For this pitch, IA is much larger than OA. In effect, the ball moves as if his arm slot is lower than it is. In some cases, the pitch also moves more.

Looking league wide, it is common for sinkers and changeups to not lie on the IA = OA line. It is also not uncommon for 4S fastball to not lie on the line, but note that the shift is to the opposite side of the line compared to sinkers! I’ve got a short Youtube video that explains why.

So, this is what is happening by accident. Well, that’s probably not quite right, it’s happening without pitchers understanding why it’s happening. Here’s to what we can do when we start doing it on purpose!

Lastly, note that this is a condensed explanation. The BEST explanation I have is the one in the Zoom from November. Or peruse this website for all the new shit.

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