Sizing up the the mystery of the mound at Tropicana Field

Chris Sale delivers a pitch during his Sept. 1 start against the Rays at Tropicana Field.Chris O'Meara/Associated Press

As a COVID-19 outbreak swept the Red Sox in early September, members of the Red Sox arrived at Tropicana Field for the conclusion of their four-game series against the Tampa Bay Rays on Sept. 2 with considerable concern about . . . the mound?

Marc Topkin, who covers the Rays for the Tampa Bay Times, noted the Red Sox twice appeared to measure the mound using a string tied to bats. Outfielder Hunter Renfroe and designated hitter J.D. Martinez were among those engaged in the project.

Why?

After outings, pitchers review data about their physical, mechanical, and performance details. Among the factors they’ll examine: Velocity, spin, release point, pitch movement, and extension toward the plate.

Statcast data — collected using the Hawk-Eye system, which employs 12 high-speed cameras around the park, including five that are focused on tracking pitches — has made such information easily accessible. And after Chris Sale’s start Sept. 1, the normal postgame review left several Red Sox with raised eyebrows.

Chris Sale calls the pitch sequence during the fourth inning of his start against the Rays in Tampa on Sept. 1.Chris O'Meara/Associated Press

On average, Sale’s extension on his four-seam fastball — meaning how far in front of the rubber toward the plate he released the ball — from that start in Tampa Bay was calculated at nearly 6 feet, 10 inches toward the plate, roughly a half-foot more than his average release point in all but one start he’d made to that point in his Sox career.

Sudden growth spurt?

“Don’t think I grew,” a puzzled Sale said.

To some degree, Sale saw the improved extension as a sign of health following Tommy John surgery.

“I was repeating my mechanics better than I have in a very long time,” he said in the days following that start. “I can straighten my elbow out . . . When I wasn’t pitching well, it was because that extra little pop, I was kind of reserved a little bit because my arm was broken.”

Still, Sale recognized his extension in that start in Tampa Bay had considerably exceeded that of any other start in a way that his teammates found curious.

“I don’t know what it is. I’ve heard some rumblings about stuff going on with Hawk-Eye and maybe some other scenarios,” Sale said. “I don’t know if there are any conspiracies out there regarding that as well.”

Sale isn’t alone in experiencing a major uptick in extension at Tropicana Field this year. Fourteen Sox pitchers threw at least 10 four-seamers both in and away from the Trop this year. Every one of those pitchers measured as having significantly more extension at the Trop, ranging from 2 extra inches (Yacksel Ríos) to more than 5 inches (Sale). Rays pitchers, meanwhile, have measured about 2½ inches more extension toward the plate at home than on the road — the largest home/road split in MLB.

Adding to the intrigue: The Rays had a 2.93 ERA at home — best in the majors — and a middle-of-the-pack 4.43 road mark. The 1.50 run gap was the largest in MLB.

Of course, it’s not unusual for Tampa Bay to pitch better at home than on the road. In their 24 seasons, the Rays have had a better home ERA than road ERA 21 times. This is the 18th year in which the Rays have had an ERA that is at least a half-run better at home, and the seventh in which they’ve been at least a full run better.

Pitchers have described feeling different on the Trop mound while identifying only a certain je ne sais quoi about its distinctiveness. Some compare the Trop mound to standing atop a mountain. Hitters have complained of the difficulty of picking up the ball there.

But upon learning of the pitcher extension data, Sox hitters wondered whether anything about the mound might have been in play, particularly given that the home/road extension differences had only started to show up in 2020. As they took their measurements, there was particular curiosity about whether the rubber was sloped in a way that would increase leverage for pitchers to drive toward the plate, thus potentially increasing extension.

By rule, the rubber is a flat slab 10 inches above the plate both in the front and back. It’s not allowed to be tilted. The mound slope is required to decline 1 inch per foot from 6 inches in front of the rubber to 6 feet in front of it. The Sox wanted to make sure the Rays mound met specs.

Here's a look at the specifications required for an MLB pitching mound. (Home plate is not to scale.)John Hancock/Major League Baseball/John Hancock, Globe Illustration

MLB, according to multiple major league sources, has examined the issue, using a laser level to examine the height and slope of the mound, finding no evidence to suggest the mound in Tropicana Field doesn’t meet specifications.

On Wednesday, Red Sox manager Alex Cora said he was unfamiliar with the particulars of his team’s interest in the mound in September.

“I wasn’t part of that. I was inside dealing with [the team’s COVID-19 outbreak]. I heard about it because somebody tweeted and showed a picture,” Cora said. “I think MLB is going to measure [the mound], like they always do. They’ll measure the bases, like they always do before each game, and everything will be legal.”

Still, the curiosity remains: Why do pitchers feature so much more extension at the Trop than elsewhere — a development that became pronounced starting in 2020?

Some questions have been raised about the mound at Tropicana Field.Douglas P. DeFelice/Getty

In the 2020 season, MLB switched its Statcast tracking from the radar-based TrackMan system to the optical Hawk-Eye system, better known from its longtime use for tennis reviews. Analysts with multiple teams said there had been calibration issues with Hawk-Eye that produced park variations in extension measurements — a conclusion the Rays themselves have reached.

“I’ve asked our analysts about it. It’s a park correction calibration. Not every place is the same,” Rays pitching coach Kyle Snyder said. “We do have analysts that before the data is submitted make sure it’s reliable so I’m not out there holding up an extension cord trying to determine the height of the mound.

“I feel bad for Dan Moeller, our head of the groundskeeping crew. He’s fantastic at what he does,” Snyder added, noting that in a dome, the mound never has to be rebuilt. “I just felt bad for him about the publicity about the mound potentially being off. It wasn’t. It was just a park correction that needed to be taken into account.”

So why do Tampa’s pitchers have such pronounced home/road splits?

It’s not uncommon for teams that play indoors (or with retractable roofs) to pitch drastically better at home. It could be a product of worse lighting indoors, more forgiving home dimensions (the Trop features both larger dimensions and more foul territory than other AL East park), teams (especially defenses) constructed to cover the home real estate, cooler summer temperatures thanks to air-conditioning, and more.

Perhaps it is a credit to the Rays — who went 52-29 at home — that their success has created questions the factors underlying their success. Regardless, both Red Sox ALDS Game 1 starter Eduardo Rodriguez and Rays counterpart Shane McClanahan will stand on the same hill Thursday night with a shared goal of excellence.