Effect of grip-enhancing agents on sliding friction between a fingertip and a baseball

Effect of grip-enhancing agents on sliding friction between a fingertip and a baseball
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On June 15, 2021, Major League Baseball (MLB) announced enhanced enforcement of the rules that prohibit the use of foreign substances (i.e., sticky substances) to balls, except for rosin, a mixture of magnesium powder and pine resin. Enforcement of the ban on grip-enhancing agents had a significant impact on pitching performance, as observed by a 4% decrease in spin rates on fastballs. However, it is currently unknown how applied substances alter the friction coefficients and the potential effect of these changes on baseball pitching.

Here, we examined the impact of rosin and sticky substances on the friction coefficient between fingertips and MLB balls (Fig. 1). Additionally, we compared the friction coefficient between MLB and Nippon Professional Baseball Organization (NPB) balls which are often thought to be less slippery than MLB balls. Our study aimed to determine how grip-enhancing agents affect friction between the ball and fingertips. As there are no regulations on ball friction in either the MLB or NPB’s official baseball rules, the results of this study could be used as a reference for discussions addressing the impact of friction-altering agents on pitching.

Fig. 1: Measurement of friction between fingertip and baseball leather sheet.

a, b Experimental setup for MLB ball leather sheet with seam (a) and MLB ball leather sheet without seam (b). c–e Finger conditions: no application (c), rosin application (d), and sticky-substance application (e). f An example of a time series of the normal force (Fz), the horizontal force (Fy), and the friction coefficient µ (Fy/Fz) during a single trial in which an index fingertip is slid with five different levels of the normal force. g The relationship between horizontal (Fy_mmax) and normal forces (Fz_mmax) at the maximum friction coefficient µmax. Using this plot, we calculated the mean µmax as the slope of the least square regression square line (e.g., 0.778 in g). h The relationship between µmax and Fz_mmax. Using this plot, we evaluated the dynamic change in friction across the applied normal force. The data in g and h were collected from three trials at five force levels (15 slides) under no application conditions for an MLB ball without a seam.

The application of rosin powder increased the friction coefficient between a finger and a baseball (27.0% with seam and 23.9% without seam), and sticky substance further increased it (54.9% with seam and 61.0% without seam) (Fig. 2a).

Fig. 2: Comparison of mean and CV values of friction coefficient for MLB ball between application conditions.

a Mean µmax value under each application condition for MLB balls. b Within-participant CV values of µmax under each application condition for MLB balls. c Between-participant CV values of mean µmax under each application condition for MLB balls. Error bar indicates the standard deviation. * p < 0.05, ** p < 0.01, *** p < 0.001.

Under no application condition, the friction coefficient showed variation among participants (Fig. 3a, d), which is due to differences in the moisture levels of the participants’ finger skin. The application of rosin drastically reduced between-participant (Fig. 2c, Fig. 3b, e). The reduction in variation in friction may be caused by the occurrence of shear within the rosin powder layers, which minimizes the effect of skin conditions such as moisture. Thus, the application of rosin increases finger–ball friction and potentially maintains friction consistency across pitchers. This could be beneficial for building a fair environment for baseball pitching. In addition, the application of rosin reduced the within-participant variation in friction (Fig. 2b). Thus, the friction coefficient remained relatively constant across the normal force (Fig. 3b, e), indicating that the friction force between a fingertip and baseball surface is highly proportional to the normal force. This constant friction coefficient facilitates precise control of the friction force between a fingertip and a ball by controlling the normal force during a pitching motion and across multiple pitches. The normal force must dominate over the maximum tangential force, which significantly affects the ball spin rate.

The sticky substance slightly reduced the between-participant variation (Fig. 2c) and increased the within-participant variation (Fig. 2b). The relationship between friction coefficient and normal force appears exponential (Fig. 3c, f) when the friction coefficient is substantial in the region with low normal force, which is due to the strong adhesion of the sticky substance at low normal force conditions. The friction coefficient in the low normal force region must correspond to the friction coefficient near the time of ball release. Therefore, using the sticky substance, pitchers can apply a greater tangential force to the ball during the release process, resulting in a higher ball spin rate than in conditions with no application and rosin application.

Fig. 3: Relationship between µmax and normal force at µmax for each participant (with different plot color) under each application condition under each application condition for MLB ball.

a No application with seam. b Rosin application with seam. c Sticky-substance application with seam. d No application without seam. e Rosin application without seam. f Sticky-substance application without seam.

We demonstrated that the friction coefficient of MLB balls is lower than that of NPB balls (16.6% with seam and 23.1% without seam) (Fig. 4a). Differences in thickness, tanning process, and chemicals in tanning between the cowhide leathers of MLB and NPB balls could affect the coefficient of friction. Before games, MLB balls are rubbed with mud and NPB balls are rubbed with sand to remove gloss, slimy materials, and wax around the seam, thereby improving the finger–ball grip. However, the purpose of the mud and sanding is not to increase friction, and no study has investigated their effects on friction coefficients. Therefore, we further investigated the effect of mudding and sanding on friction. MLB balls tended to have a lower friction coeffiction than NPB balls (13.9% with seams and 22.0% without seams) even after rubbing with mud or sand. We also found that the friction coefficient of NPB balls under no application condition for some participants with high moisture levels in the finger was higher than that of MLB balls with sticky-substance application. These findings indicate that there is room for improving friction in MLB balls.

Fig. 4: Comparison of mean and CV values of friction coefficient between MLB and NPB balls.

a Mean µmax value for MLB and NPB balls with and without seam. b Mean within-participant CV values of µmax for MLB and NPB balls with and without seam. c Between-participant CV values of µmax for MLB and NPB balls with and without seam. Error bar indicates standard deviation. ** p < 0.01, *** p < 0.001.

Sticky substances can increase friction, thus positively affecting pitching performance such as ball spin rate. However, the nonlinear relationship between normal force and friction force may be unstable for ball control precision.The application of rosin can help maintain constant friction during pitching and across pitchers, which could contribute to a level playing field in baseball pitching.

For more details on our work, please see: https://www.nature.com/articles/s43246-022-00317-4

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