Two forces that change the path a pitch travels are gravity and Magnus force. Gravity causes a pitch to accelerate downward, which causes the impact to grow as the pitch approaches the plate. Magnus force is movement of a baseball caused by transverse spin of the baseball. These two forces result in a theoretical expected path for the baseball. However, on the field unexpected motion happens, especially with elite pitchers.
Some pitchers have a cutter, sinker, or changeup that starts to break as they cross the plate. This unexpected movement makes the ball hard to hit. What causes that movement? One possible answer is seam shifted wake.
What is seam shifted wake?
Let’s build a foundation to better understand the situation.
A pitch causes air to be displaced on the way to the plate. From a scientific viewpoint, air flows around the baseball. As air flows around the baseball, it slows the baseball – this can be called drag.
A sphere has much less drag than a flat plate (source: Shane Maxemow paper). Very little drag happens at the front (batter’s side) of the baseball. Most drag happens on the sides of the baseball as the flow of air separates from the baseball.
As the air separates from the baseball, a viscous wake is created behind the baseball (pitcher’s side of the baseball). A large drag creates a large turbulent wake, while a small drag creates a thin wake.
“Very small changes to shape and surface roughness [seams for example] of bounding surfaces can result in very different flows.” — Wikipedia, Reynolds Number
Baseballs are smooth except for the seams. When a seam is located where the air flow separates from the baseball, the drag will be greater and the wake will be more turbulent. Looking at the wake, when the seams are positioned so that only the top or only the bottom of the baseball has seam-created drag, the baseball will break in the direction away from the drag.
A blog post looked at many images to find zones on the side of a traveling baseball that create the most drag. To make a break happen (either upward or downward), the baseball needs to sustain an orientation with one side (top or bottom) having seams in the green, and the other side having no seams in the green. If the seam is in the red zone it has little impact. If the seam is in front of the green zone, it has a subtle impact of moving the separation point (called ‘tripping the boundary layer’).
Why is seam shifted wake important?
When the break happens near the plate, it causes an unexpected motion, making the pitch very hard to hit.
“The seam-shifted pitches had almost the exact same velocity and vertical movement as the overall population, but they had more horizontal movement, and they got more whiffs and they allowed fewer big hits.” — Enos Sarris
His statement was supported by the following chart that compared results for pitchers who were 2020 leaders in seam shifted wake (SSW) and those who were not.
Changeup leader board includes Zac Gallen and Luke Weaver.
Glenn Healey’s study quantified baseball movement caused by drag, magnus effect, and seam shifted wake (also called side force movement). For each type of pitch, the following chart shows the percentage of total movement caused by seam shifted wake.
Let’s focus on changeups. Diamondback starters are among the league-leaders for movement caused by seam shifted wake. Zac Gallen’s changeup had the 4th highest side force movement. Luke Weaver’s changeup had the 7th highest side force movement. Details follow:
Let’s compare Gallen and Weaver to league-leading pitchers - GB%, whiff%, and SLG for changeup pitches. In 2019 and 2020, Zac Gallen and Luke Weaver had much higher whiff rates (about double). It seems their changeups are deceiving batters. While Gallen had a higher GB%, Luke Weaver did not. It seems that Zac Gallen’s changeups are harder to hit. As far as SLG, Gallen and Weaver were much better in 2019, and were slightly below the average leaderboard pitcher in 2020. Overall, their high rankings reflect excellent pitching results. In the following chart green shaded results were better than the average leaderboard pitcher, while red shaded were worse than the average leaderboard pitcher.