Seam Shifted Wake Effect: What Is It and Why Does It Matter?

Pitch trajectory and movement are the results of forces acting on the ball. Until very recently, we understood those forces to be drag, Magnus (because of spin), and gravity. However, spin induced movement and the actual movement aren’t the same, even in a controlled environment. The additional force acting on the ball is due to the seams and is from the Seam Shifted Wake (SSW) effect. 

SSW Overview 

Seam Shifted Wake (SSW) is a term that comes from the Utah State University Experimental Fluid Dynamics Laboratory where they conducted Particle Image Velocimetry (PIV) studies on baseballs in flight. Figure 1 shows a PIV image detailing the aerodynamics behind SSW. 

Figure 1: PIV image showing seams creating an asymmetrical pressure distribution on the baseball, leading to the SSW effect.

Figure 1 shows the velocity field of a non-spinning baseball moving to the left at 90 mph. The red and blue colors in the image show vorticity (rotating air). The front of the ball is the point of highest pressure, then the pressure decreases as you move around to the top and bottom of the ball (minimum pressure), and it then increases as you move around the backside of the ball until the flow separates.  

Once the flow separates, the whole wake is at the same pressure, which is near atmospheric pressure. This is higher than the very low-pressure region but lower than the high-pressure region at the front of the ball (since the pressure is greater at the front of the ball than in the wake, this is where the drag force comes from). 

In Fig. 1 we see the seam on the top of the ball is causing the flow to separate earlier than it normally would (as seen on the bottom of the ball). This causes an asymmetric wake. Since there is a larger low-pressure region on the bottom of the ball than on the top, there is a force pushing the ball down (the pressure is higher on the top of the ball vs the bottom). 

SSW occurs in both the non-Magnus (perpendicular to Spin Direction) and Magnus directions (Spin Direction). The research to date shows the SSW effect goes away at higher spin rates. SSW in the Magnus direction, while it does exist, would be very hard to control since the seams are constantly rotating with the ball and they only shift the wake if the seams are in a small range of angles. 

SSW is much easier to control in the non-Magnus direction, because you can orient the ball so there are seams in a certain area for most of the rotation of the ball. 

SSW Break 

Estimating SSW Break is made possible by the PRO 3.0 seeing the near full trajectory of the pitch (except for about 4 feet while the ball travels above the unit), in addition to being a camera-based technology device that sees how the ball is spinning. Since the PRO 3.0 sees both how the ball actually moved and can compute how the ball should have moved based on how it was spinning, we are able to determine a good estimation for how much of the total break is due to SSW. 

It is important to note there are many factors that influence both the SSW and Magnus Effects, such as atmospheric properties, pitch velocity, spin rate, and seam height. The research of SSW is very young, so there is still much we do not know. However, it is important to remember that on a macro level, SSW manifests as how the ball moves differently than how it should move purely based on spin. However, “spin break” is a theoretical concept, and can’t be isolated. Therefore, it is important to have increased accuracy in the physics of our spin break estimations to provide the best SSW Break numbers.  

The way we increase accuracy in our spin break estimation is by considering more closely the properties of the air the baseball is traveling through. To account for properties of air such as density and viscosity, the elevation, temperature, barometric pressure, and relative humidity are taken into our models at the start of a pitch session. Doing so allows us to better distinguish how balls travel in different environments. For example, it is well known that hit balls travel farther in Denver than they do in Los Angeles, and pitched balls move less in Denver than they do in Los Angeles due to the differences in air properties. 

By showing SSW Break in addition to Total Break, pitchers can see if they are getting increased movement or decreased movement due to SSW. By pairing Rapsodo’s Seam Orientation data with the SSW Break data, pitchers and coaches will be able to home in on the orientations that will get the best movement for them.