Key takeaways from MLB study of HR rates

May 24th, 2018

Baseball's home run surge of recent seasons is attributable not to a bouncier -- or "juiced" -- baseball, but rather to better carry resulting in longer fly-ball distances, a committee of experts has concluded.
In a report of findings released by Major League Baseball on Thursday and available at, the independent committee chaired by Alan Nathan, professor emeritus of physics at the University of Illinois at Urbana-Champaign, concluded its research had achieved "partial success," in that it had discovered that reduced drag on the baseballs was responsible for the rising home run rate but had not found any changes in properties of the balls that could account for the reduced drag.
Official news release on report
The 10-member committee -- which featured professors from the likes of Stanford, M.I.T., and Caltech -- was assembled by Commissioner Rob Manfred in response to questions raised by players, fans and members of the media regarding the rise in home run rate that began in the second half of the 2015 season and led to a record-high number of homers in 2017. In addition to professors in the fields of engineering, physics and mathematics, the committee also included renowned baseball researcher Dan Brooks, who is the founder of
Read the Committee's complete report on home run rates (PDF)
The committee's 84-page report was the culmination of a research process that began last August. The group conducted tests at UMass-Lowell, Washington State, and a group of them -- led by Dr. Nathan -- took a trip to the Rawlings factory in Costa Rica.
"A lot of people had reached conclusions that sort of suggested some kind of conspiracy theory," Nathan said. "We believe in science, and we took a scientific approach to this whole thing. I think it was important to do that, if for no other reason than to satisfy ourselves and what it is we know and don't know and to address some of the theories that were out there as to why there was such an increase."
Read the executive summary of Committee's findings and conclusions (PDF)
As a byproduct of the findings, Nathan's committee has made recommendations to MLB and to Rawlings, the sport's official ball manufacturer, for how best to monitor the baseball's parameters moving forward, and testing to isolate the reasons for the reduced drag is ongoing.
One of those recommendations actually came at the conclusion of the group's initial report at the end of 2017, suggesting all clubs store the baseballs in a controlled environment that does not change over time. MLB responded to this recommendation prior to the 2018 season by mandating that all clubs store the balls in an air-conditioned and enclosed room with climate sensors that measure temperature and humidity. That data will be used to determine whether a humidor is necessary in individual storage rooms in 2019 and beyond.
The committee also recommended that MLB standardize the application of mud on the baseballs, given that surface texture affects drag. The league has even developed a prototype baseball with a tackier texture that some teams experimented with during Spring Training drills. This could eventually also be a means of limiting pitchers' use of foreign substances to achieve better grips on the ball.
Finally, the committee recommended that MLB re-evaluate its specifications on parameters of the baseball (such as size, weight and COR), relax specifications on what it deemed "unimportant" features (such as the color of the pill) and require Rawlings to do impact testing at speeds more similar to game conditions than the low-speed drop test currently performed.
But the released report concluded that many of the oft-repeated theories of explanation for the homer surge -- including the "bounciness" of the balls, changes in the size, weight or seam height of the balls, changes to batter or pitcher behavior, or changes in average game temperature -- did not correlate with the improved carry.
Here are nine key takeaways from the committee's findings:
1. The ball is not "juiced".
The committee's research team tested 15 dozen unused Major League baseballs manufactured from 2013 to 2017 and 22 dozen game-used baseballs from 2012-17 to measure their physical characteristics. The testing involved firing balls at a cylindrical surface (similar in shape to a bat) at 120 mph to test the ball's coefficient of restitution (COR) -- or, in layman's terms, its bounciness.
No substantial changes in COR measurement between balls from the various time periods were detected (all data supporting the committee's assertions is available in the full report).
"The bottom line is that there is some variation from year to year and within any given samples of baseballs, just from normal manufacturing process and measurement uncertainty and things like that," Nathan said. "But there's no evidence whatsoever that the kind of change you would need to account for the home runs that we see actually exists."
2. But the ball is carrying farther.
To clear the wall, a batted ball must have a high enough exit velocity, fall within the right range of launch angles and, of course, travel far enough to clear the fence.
After studying batted ball data generated by Statcast™, the committee concluded that, in the 2016 and 2017 seasons, there was not a substantial change in the percentage of batted balls that fell within the right ranges of exit velocity and launch angle to create a home run, but there was a substantial change in the rate of home runs themselves.
What this points to is better carry. Though the study did not discover meaningful changes in the ball's lift, it found that the drag coefficient of MLB balls has decreased since 2015. The researchers used a physics model to calculate that if the change in home run rate was attributable entirely to changes in drag, one would expect the drag coefficient to have decreased by approximately 0.012. The exact change in drag coefficient in the time period studied -- if you're scoring at home -- was 0.0153.
So the reduced drag would appear to be sufficient enough to account for the surge.
"That was, to me, an unexpected but very, very satisfying result of this study," Nathan said. "There were lots of different things that pointed us in that direction, and they were all more or less consistent with each other."
3. The weather is not a factor.
It is well-known that higher temperatures result in lower air density, reduced drag and, ergo, better carry. Though global climate change is very much measurable, it has not had a dramatic effect on batted baseballs in the time period in question.
The committee's research found that the differences in home run rate persist even at fixed temperature values for both open-air and domed stadiums.
"If you look at some narrow range of temperatures, like 70 degrees or 75 degrees," Nathan said, "we could still see differences [in home run rate] between 2016 and 2017 [in the same conditions]."
4. It's not the seams.
The drag and lift of a baseball is almost certainly affected by the seams, which are still sewn manually as part of Rawlings' production process. But the committee's research found that the impact that height would have on the ball's drag was statistically insignificant when considered in relation to the larger drag changes observed.
"That's one of the things that is controlled by Rawlings," Nathan said. "They measure the seam height. This is part of their quality control. Other things, like the weight of the ball and the size of the ball, were also controlled by Rawlings, and our measurements state that those weren't factors, either."
5. It's not a core issue.
Nathan said the committee does not dispute the findings of a FiveThirtyEight report, released in March, in which X-rays revealed changes to the density and composition of the baseball's core (the newer "pill" has a wider diameter yet the same weight as the previous pill). But here, too, the committee did not find any change in the size, weight or performance of the ball that would be attributable to a change in materials.
"One of the things we asked Rawlings is to tell us every single change you made to the manufacturing process since 2014," Nathan said. "One of the changes was to the mold they use to manufacture the pill [in May 2015], because the old mold had worn out. We thought long and hard about this and concluded it had no effect whatsoever on any of the parameters we can measure."
6. The specs are all right.
A common criticism of MLB's response to questions about the balls is that saying their specs "fall within the range" of what is allowable is meaningless, given the width of the allowable parameters. The committee concluded that the range of ball specifications is, in some areas, unreasonably large. For example, a ball at the upper limit of the league's current COR specifications would have nearly 36 more feet of projected distance than a ball at the lower limit.
But the committee found that Rawlings' process had achieved greater precision in weight, size and COR than the specifications allow.
"Rawlings is able -- with whatever spec you're talking about, be it COR or seam height, etc. -- to manufacture the ball well within those specs," Nathan said. "The distribution is much, much narrower than the actual spec [range]."
7. The "launch angle" factor is overrated.
Increased attention to exit velocity and launch angle has been said to have created a "launch angle revolution" in the game, and it's true that players like Josh Donaldson, , J.D. Martinez and found great success by focusing on hitting the ball hard in the air.
But in analyzing Statcast™ data from the measurement tool's 2015 inception through 2017, the committee found no evidence that batter behavior, en masse, has been a contributing factor toward the homer surge. In fact, exit velocities decreased slightly from 2016 to 2017, spray angles from the time studied were stable and a small increase in launch angles was attributable primarily to, as the study refers to them, "players with lesser home run talents."
Basically, the long-ball surge was global, affecting players from all spectrums of homer-hitting ability and irrespective of their approach.
"Going into this, I thought that was going to be the magic bullet, the smoking gun," Nathan said. "But it wasn't."
8. Don't look to the pitchers.
The research group analyzed 32 categories of pitches -- eight different pitch types (fastball, cutter, splitter, changeup, curve, sinker, slider and knuckleball) each in four quadrants of location (up and in, up and out, down and in and down and out). The study found no evidence that any change in pitching strategies over the time period studied had been meaningful enough to account for the homer surge.
9. They don't know why the ball is carrying farther.
Further testing focused on the ball's surface roughness and center of gravity is ongoing. But the report does not rule out the possibility that manufacturing advances have contributed to the reduced drag by creating a more spherically symmetrical ball with a more properly centered pill (which would in theory lead to a lower drag). Nathan said the tools available to the researchers are not precise enough to properly determine one way or another whether that was the case.
"Rawlings is always trying to improve the manufacturing process to make it more uniform," Nathan said. "So the interesting question that comes up is whether the goal should be to improve the manufacturing process or to keep the ball performing exactly the way it is, regardless of whether it's improved or not."
Nathan said he understands that there will be people unsatisfied with the committee's incomplete answers.
"As a scientist, it is what it is," he said. "We don't want to claim more than we can legitimately claim. To admit that there are things that we don't know, we don't like to have to admit that. But that's what we have to admit."