CARY, N.C.—Carter Stewart has worked hard at developing his curveball, which now tops TrackMan's spin rate leaderboard. At the Tournament of Stars this past week, Stewart threw 13 breaking balls in his lone outing, averaging an astounding 3,286 rotations per minute.
In order to understand the magnitude of Stewart's curveball feat, take a look at TrackMan's amateur leaderboard. His 3,286 average RPM is perched a mile above anyone else's, and one particular curve he threw had the fifth-highest spin rate ever tracked.
On a major league level, Stewart's average spin rate from last week would rank as the best in baseball, slightly ahead of Garrett Richards' 3,233. Moreover, his curve thrown with the highest spin rate from this tournament, at 3,474 RPM, would be tied for the 10th-highest spin rate in tracking history.
So just how does Stewart get so much spin on the ball?
He attributes it to his loose arm action, whipping his forearm as quickly as possible in his delivery, and the way the he grips the ball.
"I just hold it, almost like a two-seam. Hold it in the—really the—right seam with my middle finger and just kind of throw it. I just spin it like a normal curveball," Stewart said.
But the pitcher's breaking ball didn't always look this devastating. He admits he used to utilize a spike grip on the pitch, which didn't get as much action as he liked. Needing a better secondary weapon, Stewart worked hard on improving his curve as he got older.
"You have to figure something out that's going to work for you, so I decided to start holding it like the way I do and it just came to be better," Stewart said. "It was natural and something I love to do.”
Stewart's curve already works as an out pitch, but also projects at the next level because he's still adding velocity. He now throws the pitch in the upper 70s, and it isn't unusual to see him topping 80, which is a considerable jump up from where he sat in 2016. Across six different amateur events last summer and fall, Stewart's curve averaged 74.6 mph. The 6-foot-6, 195-pound righthander still has room to grow into his frame, so it wouldn't be a surprise to see his velocity grade out even higher next spring.
Stewart doesn't model the spiraling pitch after any one player but notes that his curveball grip probably helps him spin the ball more than most. His long fingers wrap around the ball up in-between the narrowest gap in the seams, where his index and middle finger rest together on the edge of the ball. When Stewart throws, he pushes those two fingers against the seam to generate spin. The main difference is that his fingers aren't pushed up against the end of the horseshoe and instead sit farther back on the ball in the narrower section of the seams to almost emulate a two-seam fastball.
Another potential influence is the position of his thumb, which sits parallel with the seams. Stewart's thumb pushes with them as if he were pushing his thumb up a zipper.
When shown Stewart's grip, an ACC pitching coach estimated that it factors into how he gets more spin, but that nothing is immediately apparent from the way he holds the ball.
"I do think there's some combination of having the right grip,” the pitching coach said, “with the right grip pressure, with an efficient arm action and slot, with at least a certain threshold of extension that you get—if you combine all those things well, I think you can create a high amount of spin on a pitch."
To the public (and possibly private) baseball industry, the secrets to achieving a higher spin rate remain a mystery. There are steps being taken to obtain firm specifications though, most notably by Seattle-based Driveline Baseball. What we do know; however, is that a curveball with more spin results in more groundballs and a higher swinging strike percentage.
When batters come into the box against Stewart, they don't often know what they're up against. As his curveball gains velocity, he's noticed batters are having a tougher time squaring it up. In fact, during the Tournament of Stars, seven of his 13 curves went for strikes and the only one hit into play was a weak groundout.
"I'm starting to throw it harder and harder and throw it in the low 80s, so the harder I throw it, the harder it is to pick up in my opinion. And it's really just come to if they can see it in time," Stewart said.
When asked whether he thinks there is a most effective grip for producing spin, Stewart could only say that this method suited him and he wouldn’t throw it any other way.
"It's just whatever feels best for you. In my opinion this feels best for me and I don't want to change it," Stewart said.