1. How Thin Are a Baseball's Stitches Compared With a Coin?
In professional baseball in Japan, the specifications for game balls are strictly set. The ball has 108 stitches, which are about 0.8 centimeters (0.31 inches) wide and only about 0.9 millimeters (0.035 inches) high [1]. To picture this, 0.9 millimeters is much less than 9 millimeters. It is thinner than a 1-yen coin, which is about 1.5 millimeters (0.06 inches) thick, and you can barely feel the bump if you trace it with your finger [1]. The ball itself weighs between 141.7 and 148.8 grams (5 to 5.3 ounces), and its circumference is between 22.9 and 23.5 centimeters (9 to 9.25 inches) [1]. This makes the diameter about 7.4 centimeters (2.9 inches) [1].
Can such a tiny bump really change how the ball moves? To find out, we first need to look at the basic reason a ball curves.
2. How the Magnus Effect Pushes on a Spinning Ball
When an object moves while spinning, a force acts on it in a direction perpendicular to its path. This is called the Magnus effect, and it is said to have been studied by the German physicist Heinrich Magnus in 1852 [5].
Here is how it works. A thin layer of air sits right next to the surface of the ball. When the ball spins, differences in air flow and pressure appear on the left and right (or top and bottom) sides. One cause is that this air layer separates from the ball at different points on each side. This causes the air flow behind the ball (the wake) to curve, pushing the ball to the opposite side [6]. For a ball with backspin, this creates an upward force. This is why a standard straight pitch "resists falling" [6].
However, a forkball drops significantly even though it has spin. What is the reason for this?
3. Why Low Spin Alone Does Not Explain the Forkball's Drop
Traditionally, it was thought that a forkball drops because it has low spin [4]. If spin is low, the upward force from the Magnus effect is weak, so it seems natural that the ball would fall. But if there is even a little backspin, an upward force should still exist. It is explained that the tiny bumps of the stitches disturb the air flow, canceling out this upward force and even turning it into a downward force [4].
A research team including Tokyo Institute of Technology called this downward force the "negative Magnus effect." They announced in March 2021 that they had found it as a major factor causing the forkball to drop [2]. However, this does not mean that low spin is irrelevant.
So, how large is the effect of the stitches? Note that experts say low spin is not the only reason; it is best not to state things too definitively.
4. Two-Seam and Four-Seam Fastballs: Same Speed and Spin, 19 cm Apart
According to a press release from Keio University, a two-seam fastball and a four-seam fastball, thrown at the same speed of 151 km/h (94 mph) and same spin rate of 1,100 rpm, resulted in a 19-centimeter (7.5-inch) difference in drop at the batter’s hands just because the stitch orientation was different [3].
Let’s make these numbers easier to imagine. 151 km/h is about 42 meters (138 feet) per second, and 1,100 rpm is about 18 rotations per second. Since the ball’s diameter is about 7.4 centimeters (2.9 inches), a 19-centimeter difference is roughly the length of two and a half balls [3]. In the simulation, this difference was calculated.
However, this is not data from an actual pitch. It is the result of a simulation calculated using the supercomputer TSUBAME 3.0 [2]. What is this "force of the stitches" that is different from spin? It actually has a recent name.
5. The Seam-Shifted Wake: The Force From Stitches, Named in 2019
The force created by the stitches is called the "seam-shifted wake." It is introduced as having been named by Andrew Smith and others at Utah State University in 2019 [7]. Being named in 2019 does not mean the phenomenon was discovered that year.
The stitches cause the air layer to separate from the ball earlier than usual. If this happens on only one side of the ball, a force pushes the whole ball, separate from the spin [7]. The Magnus force acts as long as the ball is spinning. In contrast, it is thought that the seam force only acts when the stitches are in a specific area on the back of the ball, and it may disappear as the stitches pass by during rotation [8]. Since this explanation comes from a blog, it is safer to read the details as "it is thought that."
So, have actual game pitches shown movements that cannot be explained by spin alone?
6. What Real Pitch Data Shows Beyond the Magnus Effect
In 2020, Alan Nathan, Professor Emeritus at the University of Illinois, and others published research titled "Not Just About Magnus Anymore." They compared the spin axis measured by optical devices with the actual change in the ball's path. They reported that there were movements that could not be explained by the Magnus force alone, and that sinkers and changeups showed larger movements than predicted [9].
However, it has not been confirmed that this is definitely due to the force of the stitches [9]. The stitches are one strong candidate, and research to determine the cause is continuing. Calculations show a 19-centimeter difference based on stitch orientation, and actual pitch data shows deviations from spin-only predictions.
7. A Home Activity: Feeling the Raised Bump of a Seam
You can try this at home. If you have a baseball, trace the top of the stitches with your finger to feel the bump. Compare it to the thickness of a coin (about 1.5 mm); the stitch should feel thinner. You can measure the ball's circumference with a string and calculate the diameter (circumference ÷ 3.14). Try calculating how many balls make up 19 centimeters.
If you want to read more, Keio University’s press release shows the result of the 19-centimeter difference under the same conditions. Tokyo Institute of Technology’s announcement explains the negative Magnus effect.