Home runs are on the rise. We all know this. The number of homers per game is at an all-time high in 2019, and has increased by about 36% just since 2015:
What we do not know is exactly why.
Commissioner Manfred recently suggested that the current baseballs have less drag through the air, caused by the more perfect “centering of the pill” (the innermost part of the ball). It has basically become an operational fact that there is something going on with the baseballs. Manfred’s explanation implies that the flight of the baseball is the key difference.
To look at this closer, I considered the distance traveled by balls in the air as a function of the exit velocity and launch angle at contact. If the average distance on similarly struck balls has increased over time, it would suggest that the ball itself is more aerodynamically efficient.
Pitch-by-pitch data for the 2015-2019 seasons was collected from Baseball Savant via the Statcast Search page. Two random forest models were built for each year, one using all fly balls and one using home runs. To account for a possible difference in flight due to the warm air in the summer months, only data through June of each year was used. (At the end of the season, the analysis can be applied to the full data set). In both cases, the distance the ball traveled is the response variable and the exit velocity and launch angle are the explanatory variables. The models are applied to a test data set of various exit velocity/launch angle combinations.
The following plots display the predicted distance of the test points for fly balls by year.
Notice that the 2019 points (in dark blue) are not consistently higher than the other years. In fact, no particular year sees a consistently low or high prediction. The same is true for the home run models:
Similarly struck balls are not flying farther in 2019.
I am not a physicist, but I have to imagine that if the decreased drag of the baseball were the reason for the increase in homers, the results of this analysis would have been different. The balls hit at the same velocity and angle should travel a greater distance if they were cutting through the air more efficiently. Since this is not the case, it suggests that the “centering of the pill” is not driving the power surge. Whatever is happening, it seems like it is maybe happening at or before contact, not after.
Many pitchers have raised concerns over the seams of the baseball, and the inability to grip it. A difference of this kind would appear more plausible, as it could result in more pitching mistakes, and thus more balls hit out of the park. It would then not be that homers are sneaking over the wall where in previous years they would not have, but rather that hitters are more likely to square one up.