What's in baseballs, and can materials explain a spike in scoring?

The “juiced” baseball is one of the most persistent conspiracy theories in sports.

Players, pundits, and spectators have all accused Major League Baseball (MLB) officials of tampering with how the balls are built to make them fly farther. Such “lively,” or “juiced,” balls would lead to higher-scoring games. As the theory goes, more action means more fans, which translates to more money for the league.

In particular, juiced balls would ensure fans see more of one of the most exciting plays in baseball: the home run. When a player hits a home run, they send a baseball soaring out of the playing field and put points on the scoreboard.

“Every year when home runs go up, people say the ball is juiced,” says Kathy Smith-Stephens, director of quality assurance for the sporting goods giant Rawlings. “Well, it’s not.”

We now know, for example, that a slew of juiced-up hitters were taking steroids and other performance-enhancing drugs to help crank out dingers between the late 1980s and mid-2000s. Yet oddities in offensive output still pop up despite today’s tougher antidoping policies. For instance, home run rates have been increasing since roughly the middle of the 2015 baseball season.

Rawlings, which has been MLB’s sole baseball manufacturer for decades, asserts that its baseballs’ properties are tightly controlled and that they can’t be the source of higher scores. Independent researchers agree. Meanwhile, ball manufacturers are still developing ways to help their products perform more uniformly, even in different conditions.

The first baseballs, on the other hand, were not known for their uniformity. Teams used what they could get their hands on to make baseballs, which earned stones, socks, and walnuts a place in baseball materials history.

Today, MLB imposes stricter standards on the ball’s construction. The ball must be a sphere between roughly 142 and 149 g with a circumference of 23 cm, give or take a few millimeters. Balls must consist of a core made of cork and rubber, or similar material, which is wrapped in yarn and covered with cow- or horsehide. Save for the addition of the cowhide cover option in 1974, the ball composition guidelines have remained unchanged since 1955.

Rawlings and MLB have developed ball specifications that are more stringent than what appear in the rule book, says Smith-Stephens, who engineered aircraft engines before coming to Rawlings. “Our baseball specs today are detailed beyond any other product we have,” she adds. In addition to baseballs, Rawlings makes bats, gloves, helmets, and other sports gear.

The ball’s core, also called the pill, must meet certain weight, diameter, and composition requirements. Today’s MLB ball uses what’s called a cushioned cork, first introduced about 100 years ago, in which a cork orb is coated with two layers of rubber.

The yarn, which is mostly wool except for an outermost layer of a polyester-cotton blend, is wound around the core by machines to ensure balls stay taut and spherical. Then workers hand-stitch an alum-tanned leather cover to each yarny sphere.

These humdrum materials are unsurpassed in performance, according to Smith-Stephens. “There’s nothing like leather,” she says. It’s strong, it’s resilient, and it has memory, she explains. That means somebody can whomp it with a baseball bat and the ball will return to its original shape. “That’s a benefit of wool, too. It can get crushed and still rebound.”

MLB players can send baseballs screaming off their bats at nearly 200 km per hour. Under that type of abuse, no synthetic material that Rawlings has tested performs as well as what the company’s already using, Smith-Stephens says.

And performance is the primary concern, whether you’re a batter, a ball manufacturer, or even a juiced-ball conspiracy theorist. The key performance metric for ball aficionados is the coefficient of restitution, or COR, which people in the biz pronounce as “core.”

The COR for any object falls between 0 and 1, explains nuclear-physicist-turned-baseball-physicist Alan M. Nathan of the University of Illinois, Urbana-Champaign (UIUC). If you drop the object and it doesn’t bounce at all, it has a COR value of 0. If it returns to its original height, its COR is 1.

Baseballs are almost exactly in the middle, Nathan says, adding that bounce height scales with the square of the COR. Thus, a ball with a COR of 0.5 returns to 25% of its drop height. But baseball is not played by simply bouncing balls, and the internationally recognized standard for measuring COR—designated ASTM F1887—is a bit more aggressive.

In this test, an air cannon or pitching machine launches a baseball at a steel plate. Rawlings runs tests on balls from every lot it makes at two locations. The company runs tests at the balls’ birthplace in Costa Rica and again in the U.S. at a Rawlings facility in St. Louis.

For a ball to make it to the big leagues, its COR must land between 0.514 and 0.578. Juiced balls would have a COR exceeding that maximum value.

Amid the elevated home run rate, MLB recently shared COR values and other metrics for league baseballs measured by the independent Baseball Research Center at the University of Massachusetts, Lowell, over the past several years.

MLB provided the report to a dogged sports writer, Ben Lindbergh (who wrote about the data at bit.ly/lindbergh_report), but MLB also invited—and paid—UIUC’s Nathan to review ball data collected between 2012 and summer 2016. “There’s nothing in the data to suggest the ball has changed,” Nathan tells C&EN. “Quite frankly, I’m amazed at how well they have the whole thing controlled,” he adds, referring to the ball’s consistency across seasons.

Lloyd Smith, who directs the Sports Science Laboratory at Washington State University, is likewise impressed. The balls are so well policed he can’t imagine anyone getting away with adding a secret component to balls to make them livelier.

Rawlings has, however, developed baseballs not currently used in major league play that feature an extra layer beneath their leather cover. The layer is a flexible elastomer, about a millimeter thick, which is designed to keep moisture out of the ball, says Smith-Stephens of Rawlings—and that’s all she divulged on the barrier’s chemical makeup.

Baseballs’ woolly interiors absorb and retain water, making the balls’ behavior change with changes in humidity, Washington State’s Smith explains. As balls take on moisture at elevated humidity, they get heavier and their COR values drop, making home runs harder to hit.

For example, in 2002, the Colorado Rockies, an MLB team, decided to store their game balls in a humidor to combat excessive home run rates. The humidor stores balls at 50% relative humidity, higher than a typical summer’s day value in Denver of about 30%. Between 2002 and 2010, the team saw the number of home runs per game drop 25%. In 2011, Smith and Nathan collaborated on a peer-reviewed paper showing the humidor could indeed account for that reduction (Am. J. Phys., DOI: 10.1119/1.3554642).

Rawlings’s elastomer barrier layer effectively makes the ball indifferent to humidity, says Smith of the Sports Science Laboratory. “It’s a great example of how you can use materials to make a ball more uniform,” he adds. And materials engineering is present in softballs, too.

Like baseballs, softballs generally have leather covers, but their interiors are typically made from solid polyurethane. Polyurethane is also sensitive to humidity, but the synthetic material allows manufacturers to compensate for environmental differences as they produce their softballs, Smith explains.

For instance, he adds, companies can tailor how they cure, or harden, the polymer to tune its COR. By making livelier balls for Florida’s swampy climes and deader balls for drier locales, manufacturers can give players a more uniform playing experience by design.

And perhaps batters will be swinging at more polymer-based balls in the future. Rawlings filed a patent in 2015 for balls with polycarbonate cores “that allow a consistent performance of the ball in varying temperature and humidity environments.”

Although softball and some baseball leagues have embraced polymers—the official baseball used by college teams is made by Rawlings with the elastomeric layer—experts don’t anticipate MLB taking that step anytime soon. The league is slow to change, they say.

The irony, of course, is that change is part of the league’s current quandary. If the ball is the same as it was before, why are players hitting noticeably more home runs these days? “I would say that’s still a mystery,” Nathan concedes.