Seam-shifted wake

Seam-shifted wake (SSW) is an aerodynamic phenomenon involving baseballs. The term was coined in 2019 by Andrew Smith during his work on the phenomenon with Barton L. Smith (no relation) at Utah State University (USU). Nazmus Sakib and John Garrett also contributed to the early work.

The USU group showed that Major League Baseball (MLB) baseball seams, when in specific locations relative to the direction of the ball, force the boundary layer to separate earlier (closer to the front of the ball) than it normally would. If this occurs on one side of the ball and not on the opposite side, a net force is produced. This force can be similar in magnitude to that caused by spin (i.e. the Magnus effect).

In the particle image velocimetry image shown, the seam on the top of the ball is causing separation (wake formation) on the top closer to the front of the ball than normal. The separation on the bottom of the ball is at the normal location.

The resultant movement from this effect had been noted by others before and named the "laminar effect" based on a mistaken notion that smooth portions of the ball caused a laminar boundary layer more prone to separation than a turbulent boundary layer. This was most often discussed with respect to 2-seam fastballs. Data from MLB on pitch spin and movement, which were not available in 2019, now make it clear that SSW effects are present in most pitches. Seam effects cause two-seam fastballs and changeups to develop addition arm-side movement as well as sink. Four-seam fastballs can gain extra vertical ride as well as glove-side movement. Certain sliders will "sweep" (or move arm-side) due to seam effects. Some claim benefits from SSW pitches in terms of batter outcomes

Evidence of seam-shifted wake pitches for MLB pitchers can be found by comparing spin-based movement to observed movement on MLB's Spin Leaderboard. These two values being equal indicates no seam-shifted wake, while differences between them (called deviation) are a sign of seam-shifted wake.

Several extensive explanations of how SSW works and what it does to pitches are available.