The performance of the baseball is primarily driven by two factors: the coefficient of restitution (“COR”), which impacts the exit velocity of the ball off the bat, and the drag coefficient, which impacts how far a batted ball carries once struck. All else equal, an increase in COR will lead to greater exit velocities, while an increase in drag will decrease the carry distance. Together, these two properties have a significant impact on the run environment in Major League Baseball and on the home run rate, in particular.
While COR is best measured in laboratory conditions, the drag coefficient of an individual baseball can be estimated using pitch tracking data from Statcast. The chart below shows the average drag coefficient of four-seam fastballs for each regular season date going back to the start of the 2016 season using a method developed by Alan Nathan and David Kagan, which adjusts for environmental conditions. Here we include only four seam fastballs to better account for drag variability based on the spin rate of the pitch.
As a general rule of thumb, a decrease in the drag coefficient of 0.01 will increase the distance of a batted ball with an exit velocity of 100 mph by approximately 5 feet (and an increase of 0.01 will decrease batted ball distance by 5 feet). However, while drag fluctuates from year to year, these changes are small relative to the ball-to-ball variation observed within an individual season. This finding was originally made in the Home Run Committee Report and is reflected by the variance in day-to-day drag measurements in the chart above.
To illustrate differences in intra-season and season-to-season variation, the drag coefficient distribution for the 2020 and 2021 seasons are shown overlaid in the chart below. The distributions of drag coefficient values align closely: the mean drag coefficient for all four seam fastballs in 2020 was 0.3410, in 2021 it was 0.3411. Relative to this small difference, the standard deviations of the drag coefficients are large (0.02576 in 2020 and 0.02584 in 2021). This intra-season variability in drag is attributable, in part, to the fact significant parts of the baseball are constructed by hand.
While more difficult to measure with Statcast data—exit velocities vary greatly based on the quality of contact, hitter ability, training philosophies, etc.—COR has also changed over this period. In 2021, MLB informed Clubs that after several years of COR approaching the higher end of the allowable range (0.53 to 0.57), Rawlings had made a small change to its manufacturing process (loosening the tension of the first wool winding) to re-center the COR within the specification range. This re-centering of the COR distribution follows from recommendations of the Home Run Committee to re-evaluate the size of the specification range of COR. During the 2021 season, re-centered balls were mixed with existing inventory due to production shortages caused by the COVID-19 pandemic. In 2022, re-centered balls will be used exclusively.
Storage conditions, particularly the humidity level, also have a significant impact on the performance of the baseball. As the humidity level increases, the weight of the ball will increase slightly, and, as noted in the Home Run Research Committee’s 2018 Report, COR will decrease.
To reduce variability in storage conditions and ensure that the ball feels and performs more consistently, all 30 Clubs will use humidors during the 2022 season (in 2021, 10 Clubs—the Astros, Blue Jays [Rogers Centre only], Cardinals, Diamondbacks, Mariners, Marlins, Mets, Rangers, Red Sox, and Rockies—used humidors). The humidor in Coors Field is set to 65% relative humidity, and the humidors in all other parks are set to 57% relative humidity. All humidors are set at 70° F. Because humidity levels vary over the course of the year—generally starting lower in the cold, early months of the season and rising over the course of the summer—and by region, humidors will impact gameplay differently from park to park. For example, a humidor in a dry, cold climate, will likely decrease COR relative to a pre-humidor baseline, particularly early in the season. Conversely, in a warm locale, a humidor may increase COR, particularly in the summer months.