Welcome to baseball, circa 2019. Balls are flying, and records are falling. Players are on pace to hit 6,618 home runs, 1,000 more than last season’s 5,585 and 500 more than 2017’s all-time high of 6,105. At least 13 of Major League Baseball’s 30 teams are in a position to break franchise records. Twenty-one players are expected to hit more than 40 home runs — including Brewers outfielder Christian Yelich, who is projected for 61 — and the number who have already exceeded last season’s home run totals currently stands at 36 (and has probably gone up since this writing).
Balls are also traveling farther, with Rangers outfielder Nomar Mazara hitting a 505-foot homer this past Friday and tying the Statcast era record. In 2018, only 82 home runs surpassed 450 feet. This season, that number is already up to 84, with a projected total of 178 — an increase of 117 percent. And it’s not just long distances. Despite the fact that fly ball rates are up less than 0.5 percent from 2017 and 2018, home run rates are up 7.8 percent and 17.8 percent.
So, what’s going on?
Yes, the ball is different
As Jayson Stark recently demonstrated, it’s the ball. On Thursday, commissioner Rob Manfred even issued a statement acknowledging that “we do think it’s a drag issue.” Rob Arthur provided evidence of this as early as the first week in April, showing that this season’s ball had a significantly lower drag coefficient than that of 2018. With such a small sample, he determined the aerodynamics were comparable to those of the 2017 home run surge. By now, it’s clear that Arthur’s findings were an underestimate.
Last year, MLB commissioned a Home Run Committee to figure out the reason behind the 2017 home run surge. The committee found the only possible cause was a decrease in the ball’s drag coefficient; however, the group was unable to identify the specific source of the change. Performing my own independent studies, I determined the decrease in drag could be traced back to an increase in lace thickness, which inadvertently produced a rounder baseball. The introduction of thicker laces also corresponded to a marked increase in pitcher blister injuries, suggesting that as a possible factor.
To be clear though, the 2019 baseball is not the 2017 baseball. Not only is it different from those of late 2015-2018, it’s different from balls going back to at least 2000.
Even hitters and managers aren’t quite sure what’s going on. During an April 27 postgame interview, Astros pinch-hitter Tony Kemp said of his walk-off home run, “I think I surprised myself. … I wasn’t imagining myself hitting a home run in that situation, but I’ll take a home run right there.” When asked about a home run he hit on June 2, Dodgers first baseman David Freese answered, “I don’t know. I don’t really know how that went out, but it did.” Similarly, an “excuse-me” home run hit by Mets third baseman Todd Frazier on June 6 surprised managers on both sides. Mets manager Mickey Callaway commented, “You don’t anticipate (a hit like that) going out. You usually read the swing, and it just kept on going.” Meanwhile, Giants manager Bruce Bochy said, “I thought it was a fly ball. I was surprised by how far it went.”
Pitchers, in particular, are saying the ball has changed. Mets starter Noah Syndergaard described the baseballs as “(feeling) like ice cubes,” referencing a video of a dog trying to pick up an ice cube. Nationals closer Sean Doolittle told USA Today’s Steve Gardner, “(the baseballs) feel really slick. … There’s been several times where I’ve gotten one and I haven’t even thrown a pitch with it. It just really didn’t feel right in my hand, and so I just threw it out.” He also described the seams as feeling “lower but wider, if that makes sense,” and pointed out that, “(for guys) that spin the ball, it seems to be a little bit different.” Cubs starter Jon Lester told reporters, “… (the balls are) not rubbed up like they used to be. You get balls back and they’re basically white. It seems like they’re right out of the box.”
Statements like these suggest a baseball with smoother leather (which would not retain mud as effectively) and possibly with lower seams. Those properties would also produce a ball with less drag, leading to more home runs.
A detailed study of this season’s baseballs shows that is indeed the case.
What exactly has changed?
While the 2017 home run surge seems related to thicker laces, that does not mean the 2019 ball has undergone the same changes, as there are a number of ways to improve aerodynamics. Three effects can cause a ball to travel farther: It displaces less air, it “wobbles” less, or it creates less turbulence. The first requires a smaller ball; the second relies on a more centered core; the third occurs with either a smoother surface or a more spherical ball.
In its report and executive summary, the Home Run Committee considered properties of the baseball relevant to such aerodynamic changes. The committee measured two of them directly:
- Smaller size
- Lower seam height
However, neither of those was sufficiently different to account for the decrease in drag. Therefore, they postulated three additional and as-yet-unmeasured sources:
- Smoother leather covers
- Greater spherical symmetry (i.e. a rounder ball)
- A more centered core (or “pill”)
While Manfred recently suggested pill-centering as a possible reason for the ball’s improved aerodynamics, this is the most difficult result to produce without significant manufacturing changes, since existing techniques make it hard to keep the pill from being centered to begin with. This 2011 Science Channel video shows how a baseball is wound. A baseball with a sufficiently off-center core would not only require different winding equipment, but its lopsidedness would cause the machinery to vibrate. (As an analogy, think of the thumping and jerking that occur during the spin cycle of an unbalanced washing machine.) Systematic production of such baseballs would lead to damaged equipment, and ball-winding equipment repairs or replacement appear nowhere on the Home Run Committee’s factory maintenance list. As baseballs are generally produced at least a half-year in advance, any major machinery updates affecting the 2019 ball would probably have been implemented before the report’s release last May. Therefore, it seems unlikely that pill-centering would explain a sudden change in drag; at the very least, we would be remiss not to also examine other possible sources.
Fortunately, each of the other four properties can be measured and quantified. Building on last year’s baseball construction research, I examined a sample of 39 balls from 2019, taking detailed measurements of each ball’s exterior and construction materials. I then compared my findings to samples used in previous studies: 12 balls from 2014, 14 balls from 2016-17, 12 balls from 2018, and 20 balls from 2000-2014. (Note: In a previous study, I identified these balls as covering 2010-2014, since that is what I was told by my source. I have since discovered that at least one ball dates to 2003, so I have expanded the presumed dates of the sample back to 2000, when then-Pacific Bell Park first opened.) In addition, I have expanded the 2018 sample to include six more balls and have added a 12-ball sample from the 2015 postseason. As a result, my complete pre-2019 data sets consist of 32 balls from before 2015 (13 with known years) and 44 late-2015-to-2018 balls.
The size of the ball
MLB’s official rules state the dimensions of the baseball must “measure not less than nine nor more than 9 ¼ inches (between 22.9 and 23.5 centimeters) in circumference.” Since a smaller ball would lead to lower drag, I measured the 2019 baseballs and compared their circumferences to samples from previous seasons. My results were similar to those of the Home Run Committee, in that I could find no systematic difference. If anything, this season’s ball seems slightly larger, though with such broad ball-to-ball variation that any systematic change would be indistinguishable.
The seams are lower
Since lower seams make for a smoother surface, an overall change in their height would influence the ball’s aerodynamics. To find seam height, I used digital calipers to measure first the average thickness of each leather cover and then the average thickness at its edges. Because the interior surface of each cover is smooth (i.e. the seams do not protrude inward), the difference between these two thicknesses constitutes the seam height. With this method, I determined seam heights for five of my samples: 2000-2014, 2014, 2015 postseason, 2018 and 2019. As each ball has two covers, this gave me twice the number of data points.
For the period of 2000-2018, my findings tallied with those of the Home Run Committee, in that seam height showed no meaningful or consistent seasonal change. However, my 2019 results were very different.
For the first time in at least 19 years, seam height had decreased to such an extent that, even taking uncertainties into account, the seams were demonstrably lower. (This is described by the term “statistically significant.”) In fact, when compared to the average from previous seasons, the seams on the 2019 balls are only 54.6 percent ± 15.0 percent as high. While these data cannot measure the extent of the effect, there is no doubt that lower seams would improve aerodynamics. These results are also consistent with anecdotal pitcher observations.
The leather is smoother
“Absolute smoothness” is difficult to quantify; the most precise techniques require lasers. However, finding “relative smoothness” is fairly simple. One way to do this is by measuring the “coefficient of static friction,” which amounts to the traction between two surfaces. Anybody who has tried walking on ice in different types of shoes has encountered the effects of static friction — if you’re wearing boots, you’re far less likely to slip than if you’re wearing dress shoes.
Quantifying static friction is remarkably straightforward. All that is required is a uniformly smooth board and the object being measured (in this case, a leather cover). One places the object on the board and raises one end; when the object loses traction and slides, the angle between the board and floor provides all of the information needed to calculate the coefficient of static friction. Therefore, as long as the same board is used, it becomes possible to measure leather smoothness.
As with seam height, I measured samples from 2000-2014, 2014, 2015 postseason, 2018 and 2019. Since the majority of the balls were either unused or batting practice balls, this meant the samples were generally unaffected by umpire-applied mud. To ensure a uniform shape, each cover was folded in half, with the two sides held together by neodymium magnets. Because the folding process puckers the upper side of the cover, I took care to measure the side that appeared smoothest, and some samples were eliminated due either to scuffs or excessive puckering. (In cases where covers are glued down very tightly, it is difficult to remove them without affecting smoothness.)
Each folded cover was placed on a piece of laminated pressboard, one end of which was then raised until the cover began to slide. The height of the raised end of the board was recorded, enabling calculation of the angle between the board and the floor. To keep the motion as uniform as possible, the board was moved along and against two laminated surfaces, with microfiber towels placed at each end.
Up through 2018, the baseballs showed the sort of ball-to-ball variation expected from a handmade construction process. However, the static friction for the 2019 balls is 27.6 percent lower, a statistically significant result demonstrating the leather covers are genuinely smoother. Like decreased seam height, this contributes to a lower drag efficient, making home runs even more likely. In addition, slicker leather can be expected to produce the sort of grip issues being experienced by at least some pitchers.
The ball is rounder
In a previous article, I found the 2017 home run surge may have been caused by improved spherical symmetry (i.e. ball roundness), which could be traced back to “bulging” along the seams; because this deforms the ball, more bulging produces greater drag. My results showed that pre-2015 balls had much more pronounced deformation than balls from 2018, meaning the rounder 2018 balls would travel farther.
Using the same techniques, I measured seam bulging on my 2019 baseballs. I also added six balls to my 2018 sample.
My findings were, to say the least, unexpected. While the 2000-2014 balls showed bulging of 0.66 percent ± 0.34 percent and the 2018 balls 0.28 percent ± 0.33 percent, those from 2019 deviated from spherical by only -0.04 percent ± 0.31 percent. Not only were the 2019 balls virtually round, what bulging they did show was slightly negative, suggesting the seams might be slightly “nestled” into the leather. In addition, while this change is only a trend when compared to the 2018 sample, the difference between the 2000-2014 and the 2019 samples is statistically significant.
Here, the effect on aerodynamics may actually be two-fold. Not only are the balls rounder (therefore producing less drag), but “nestled down” seams might decrease the impact of the already-lower seam height. This double-whammy would produce a ball that travels even farther.
The lace thickness has changed (again)
While the 2019 balls already show a number of changes that would affect drag, it would be remiss not to also look into what I believe is the source of the 2017 surge — that is, lace thickness. Since my original study, I have acquired a number of new baseball samples, enabling me to look for changes over time. As before, lace thickness is measured in “wraps per centimeter.” Since thinner laces will have more wraps, the larger the number, the thinner the laces.
The previous results — comparing 2014 to 2016-2017 balls — were as expected. In addition, balls from 2018 and the 2015 postseason had lace thicknesses comparable to those of 2016-2017. (Note the 2015 postseason results suggest the change in laces occurred partway through the 2015 season.) The spread in lace thicknesses over 2000-2014 is interesting, in that its uncertainty overlaps those of late 2015-2018; however, when one looks at the graph showing home run rates over time, the idea that lace thickness undergoes periodic changes does not seem unreasonable. While the effect was more dramatic over late 2015-2018, one of the thicker lace measurements came from a ball that could be definitively dated to 2003. Since the home run rate in 2003 was much higher than that of 2014, this may imply that trends or fluctuations in home runs may correlate — at least in part — with lace thickness. That being said, despite the larger uncertainties, the 2000-2014 laces are, on average, thinner than those from late 2015-2018.
As with the other measurements, the 2019 baseballs are markedly different — and once again in an unexpected way.
With statistical significance, the lace thickness has decreased to something comparable with pre-2015. This change could be encouraging for pitchers, given the increase in blisters during late 2015-2018.
However, the decrease in lace thickness also flies in the face of earlier findings concerning spherical symmetry.
Traditionally, the final manufacturing step involves air-drying damp, finished baseballs. Since the laces are made of cotton and stretched cotton stays stretched when air-dried, the supposition was that thinner laces would not only stretch more but dry stretched, producing a ball with more bulging along the seams. That theory appeared sound, and was validated by findings comparing pre-2015 and 2018 baseballs.
While it is possible the original seam-bulging supposition was incorrect, the fact the 2000-2014 and the 2019 balls show marked differences in spherical symmetry suggests that lace thickness is no longer affecting shape. Regardless, the improved roundness of the current ball would lead it to carry farther.
When announcing the purchase of Rawlings in June 2018, MLB executive vice president Chris Marinak told reporters, “We are particularly interested in providing even more input and direction on the production of the official ball.” These goals were also advocated in the Home Run Committee’s recommendation that “MLB should re-evaluate the specifications on parameters of the baseball that affect the game,” specifying size, weight and other properties known to vary not only from season to season but from ball to ball. Such improvements to specifications and oversight would lead to baseballs that are more uniform and thus behave more predictably.
Despite Manfred’s early-season supposition that the increase in home run rate was due to “variation(s) that we don’t know how to eliminate,” the changes to the current ball show that variation has decreased, particularly leather smoothness. As baseball leather is skived (i.e. scraped down) by hand, increased smoothness could be the result of better quality control. Rawlings has even attempted to improve this process before, telling the Home Run Committee that they “eliminat(ed) wet shaving” in February 2017.
Other changes may also relate to better specifications. In their report, the Home Run Committee describes the final manufacturing step as follows: “The balls are rolled between grooved wooden platens after stitching (when the balls are moist) and again 24 hours later, to flatten the seams and maintain a spherical shape.” Before 2019, deviations from spherical symmetry were consistent with lace stretching due to air-drying. However, the fact that the new ball is rounder despite thinner laces suggests that perhaps the cotton laces are no longer being stretched during the finishing process. Since the only way to dry cotton without stretching is through heat and moisture removal (think of putting wet clothes in a dryer), one hypothesis is that balls are now being dried under hot air flow. Preventing lace stretching may also account for lower seams, since tighter laces could potentially “hold seams down.”
Putting it all together
The first recommendation of the Home Run Committee was that “MLB should work with Rawlings and/or independent test labs to develop methods to measure and monitor parameters of the baseball that affect the carry.” Independent aerodynamic testing is currently studying sources of drag variation, and according to Joel Sherman of the New York Post, internal testing is also being done, since MLB officials “privately acknowledge that its internal testing shows less average drag on the baseball than last year.”
In addition, Rawlings is actively experimenting with baseball construction. Buster Olney told “Baseball Tonight” listeners that, having spoken to Rawlings, “they’re just hoping that … they can develop a baseball with a tacky surface.” He also reported that “Rawlings now is in the second prototype of a ball with a consistent grip, one that was previewed with players in Spring Training. Rawlings is making some tweaks, (and) they hope to have a third prototype ready for laboratory testing sometime this summer.” Rawlings’ concern seems to be the leather, since the spring training prototype was intended to eliminate the need from pregame “rubbing-up” — a process designed to increase grip. While it is unclear if Rawlings’ interest in improving grip is related to or has been affected by the current baseball, we know that the company monitors leather smoothness as part of the manufacturing process and therefore is probably aware of this season’s issues. Since early season data found that walks, wild pitches, and hit-by-pitches were up — all issues that relate to grip — Rawlings’ efforts in this area will likely be welcomed by players.
In his recent statement, Manfred said “(Rawlings) hasn’t changed their process (or materials) in any meaningful way.” Given MLB’s internal tests showing lower drag, and Rawlings’ ongoing experimentation with the ball, this seems like a potential disconnect. However, it may simply be a question of semantics.
The Home Run Committee found that Rawlings regularly implements production improvements, including changes to the yarn (February 2014), the pill (March 2014, May 2015), the leather (June 2014, February 2017, August 2017) and the drying process (March 2016, February 2018). The Committee described these changes as “largely technical in nature and very unlikely to be in any way related to the (2017) home run increase.” That being the case, things like enhancing leather smoothness or drying baseballs more efficiently might not be considered “meaningful” to manufacturing.
While this may have been a reasonable attitude in the past, such enhancements now appear to have compounded, producing a more aerodynamic ball.
Since internal inquiries continue to be inconclusive, it might be beneficial for MLB to commission another report from the Home Run Committee, this time focusing on the 2019 baseball. Unlike the study of the 2017 home run surge, where the Committee looked at a number of potential causes besides the ball, this research would be much more straightforward. And while my study focuses on the construction differences themselves, the Home Run Committee is in a position to determine how much each attribute — lower seams, smoother leather, greater spherical symmetry — contributes to aerodynamics. Such information would prove invaluable to MLB’s goals of tightening specifications and improving quality control. It would also help Rawlings determine future production improvements.
After all, any one of these changes would cause the ball to fly farther; together, they have made the current home run surge inevitable.