Disco Ball Changeup Experiment: Post 42


I like to talk about “seam-shifted wake” pitches and I know of two examples, the “laminar express” 2-seamer and the “disco ball changeup.” The best example of the latter that I have seen is Strasburg’s, shown below.

Figure 1: A Strasburg Changeup. Video from @pitchingninja.

As I have noted before, about 10% of his changeups have a downward force exceeding gravity, despite that the pitch has a slight upward Magnus force. His teammate, Max Scherzer, has very similar results, as shown in the plot below in Figure 2. This is assuming we can believe the data on Baseball Savant, and I have been cautioned on that. Since Max and Stras play in the same stadium and use the same measurement system a lot, I’ve included 2 others who throw a similar pitch, Trevor Bauer and Anibal Sanchez. Note Sanchez also played for the Nationals last year.

First, let me explain the plot a bit. Savant provides 3 components of release point, velocity at the plate and what I believe is acceleration averaged over the pitch. The plot below is the vertical acceleration and it is normalized by the acceleration due to gravity, so you can read the numbers in g’s. Note that -1g means the ball is free-falling. If you throw this pitch with a vertical spin axis (3:00 tilt, and most of these are closer to 2:30), there is no Magnus force in the vertical direction and the ball should free fall.

Figure 2. Histogram of vertical acceleration in g’s. Data from baseballsavant.com.

The thing to note in Figure 2 is that Max and Stras each have about 10% of their pitches accelerating downward with more than gravity, like a weak curveball (Bauer’s average curveball is about -1.5 on this metric). That means that either the measurement is in error, or some other force is countering Magnus some of the time. I believe it is the latter and that Max and Stras generate a seam-shifted wake with their changeups.

Note that this affect requires gyro, and these pitches are very efficient. But they gain the correct sign of gyro as they fall. This means that the wake effect is only present for part of the pitch, the very last part, and that the acceleration numbers (averaged over the pitch) are likely underestimated.

As an aside, we made a video about this that you can view on Youtube here:

I am interested in this pitch because

  1. It is a seam-shifted wake and
  2. It does not require the ball to be thrown with gyro.

So we can do it in our lab.

I hope that served to wet your apatite.

The Experiment

Figure 3: Video of Experimental Setup.

Our setup, or most of it, is shown in the video in Figure 3. We will use:

  1. The WSU cannon with flextip that allows us to impart arbitrary spin on any 100% efficient axis we want
  2. A Rapsodo, lent to us by Norton Performance Training LLC (Thanks!)
  3. A Diamond Kinetics PitchTracker baseball (more on this below)
  4. A go-pro to monitor the launch.
  5. A scientific high speed camera

Each of these is pointed out in Figure 4, which is a capture from Figure 3.

Figure 4. The setup.

A Rapsodo capture is shown in Figure 5.

Figure 5. Rapsodo data.

The launch is shown in Figure 6. We use the streak formed by the ball to confirm that the initial vertical trajectory is the same for each shot.

Figure 6. GoPro video of the launch.

The high speed camera will image the ball as it arrives.

Figure 7.

We will get more than 10 photos at unique positions and rotations and we will use these to determine:

  1. The speed of the ball at the plate
  2. The spin rate of the ball at the plate
  3. The angle of descent

Of these, 3) is most important to the experiment (speed and spin rate will be reported by Rapsodo and the Diamond Kinetics ball). A seam-shifted wake should cause the ball to arrive at a steeper angle.

On top of all that data, we will have data from a ball with an IMU installed. Diamond Kinetics has agreed to collaborate on this project and will provide raw data from their ball (accelerations and rotations in 3-D) which will provide invaluable information. Thank you to Michael Ressler for making this happen.

If this unicorn pitch exists, we will find it and we will pin down how it must be thrown. If we are successful, you should see a lot more of these pitches in 2020.

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