Magnus Effect – Seams and Roughness: Post 38

Magnus effect causes a force on a round spinning translating object that is perpendicular to the rotation axis and in the direction that the front surface of the ball is moving. These effects are critical to baseball pitching and also affects batted balls.

The question that I’d like to answer in this post is: How do the seams and surface condition (leather roughness) contribute to the Magnus effect. For instance, do the lower seams on the regular season 2019 MLB ball result in less movement on pitches?

The surprising answer is “no,” and I will get to why. This is surprising because it has been showed that seam height does impact drag in a way that is significant to batted ball trajectory. I have written about it in Post 35 here and Rob Arthur had some really great articles based on MLB pitch data. Eno Sarris showed that comparing pitchers in 2019 to their 2018 performance showed no significant difference. This is a hint that seams do not affect Magnus effect.

This discussion will be based on a paper by Jeff Kensrud and Lloyd Smith in the Journal of Sports Engineering and Technology titled “Drag and lift measurements of solid sports balls in still air” in 2018. Magnus forces is perpendicular to the direction of motion and is thus called a lift force, while drag is defined as a force in the direction of motion. They made lift measurements on 6 balls of the same size at 40 m/s (89.5 mph). The balls were low and high seam baseballs, plus a polished ball, a roughened ball, a dimpled ball and a grooved ball. Their smooth ball findings were in line with previous findings and they extended the smooth ball findings to several other balls.

The measurements are of balls in still air–no wind tunnel (I am a fan of this approach). One downside is the data have a lot of random variations to them. This is probably not an issue with the lift measurement, but is rather due to the fact that the lift of these balls varies in time and they are sampled for a short duration.

The data in this paper are presented “dimensionlessly,” which is required in a scientific publication but often confusing to lay persons. Rather than Spin factor and Lift Coefficient, since these balls are generally the same size and are all traveling at the same speed in the same air according to the paper, the plot looks the same in dimensional quantities (Lift force vs RPM). I have also added curves that are weighted 80% to each data point to help interpret the noisy data.

In three of the four cases, lift increases at one slope at low RPM and a lower slope at larger RPM. The case that does not exhibit this behavior, the polished ball, probably has laminar boundary layers and behaves quite differently.

I have not plotted the baseballs. They were fed 2 and 4-seam, and the two different seam heights and orientations did not produce any appreciable difference in the lift. Their lift was about 15% less than the dimpled balls.

From all of this, I conclude:

  1. The surface matters. Roughness helps. Dimples help a lot. Note that dimples have been shown to generate streamwise vorticity and have a large impact on the flow. BUT, at this point, I don’t think we know what protrusions would do. A scuffed baseball should break more than a pristine one.
  2. A dimpled baseball with seams would move more than a normal baseball. Pitchers tell me that batting cage balls do not move well, but I suspect they are not able to spin it.
  3. Seams do not matter to Magnus effect. This is consistent with Sarris’s findings for 2019 breaking pitches.
  4. But, seams may have a large influence on RPMs.

A final note, based on a comment from Corey Frontin, it is possible that seam height affects spin decay, which would in turn affect lift. I have not yet seen any measurements on spin decay although Alan Nathan gave me a general estimate some time ago.

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6 thoughts on “Magnus Effect – Seams and Roughness: Post 38

  1. I agree largely with your arguments here, and would add that outside of the boundary layer (which you’ve shown to be rather small), incompressible, inviscid assumptions hold, so you would expect the seam height and roughness to have relatively little effect on the Magnus force. With that said, I think there’s a potential backdoor functional dependence of Magnus on seam height that may be neglected here.

    Seams could theoretically be delivering an angular deceleration moment, which- coupled with gyroscopic effects- may have a nontrivial effect on spin-related ball movement. If, say, seam height significantly effects the spin-down or spin-reorientation of the ball mid-flight, there could be an effect on the baseball’s flightpath, although admittedly, this would all be from a physical effect that we have been systematically neglecting in baseball for some time (via the constant spin assumption).

    Has anyone looked systematically at spin-down and spin-direction dynamics of balls in flight? Bart, you may be uniquely tooled to investigate the effect of rotational deceleration and it’s collusion with the Magnus force!

    1. Our system gets a single snapshot of the velocity field. In order to investigate what you are suggesting, I’d need 2, unfortunately.

      I think there is some spin decay work going on right now, though, either by Alan Nathan or Lloyd Smith.

      1. Diamond Kinetics has seen these effects via direct observation in both our ball and computer vision methods of analyzing balls in flight. We first saw it in our ball data and thought we must have something wrong. After about a year and a half of computer vision work, I think our ball data were right the whole time.

        Happy to share data for analysis.

          1. We’ve seen spin-reorientation as Cory called it. We should be able to measure spin decay, but we weren’t looking for that specifically. I’ll connect with you over email.

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