Behind Corey Kluber's success is a truly devastating curveball

Behind Corey Kluber's success is a truly devastating curveball

Updated Mar. 4, 2020 2:13 p.m. ET

If you check out the FanGraphs leaderboard of the best pitchers so far, you've got a lot of awfully familiar names. Felix Hernandez and Clayton Kershaw are obvious. Chris Sale? Jon Lester? No shock. The guy that stands out -- the guy currently in second place in Wins Above Replacement -- is Corey Kluber.

People are gradually coming to terms with the reality of Kluber being a rotation ace, but there's still the question of how. It's uncommon to have an ace come completely out of nowhere, and a few years ago Kluber was a little-thought-of trade return for Ryan Ludwick. Even the Indians couldn't have seen all this coming.

The Corey Kluber story is complicated, as all of them are. He's extremely dedicated and focused off the field. He's changed the fastball that he throws. He's made all kinds of little tweaks and adjustments, and he's benefiting now from just having gotten an opportunity in the majors. But there is this one little signature of his that's never been as good as it is today. Recently, Baseball America polled big-league managers on the best tools in the league. One of the prompts concerned the owner of the best curveball in the American League. Justin Verlander came in third, and, yeah, his curveball is really good. Dellin Betances came in first, and he's really gotten comfortable in the bullpen. Corey Kluber came in second. That is, according to people who coach at the highest level, Kluber has one of baseball's true elite curveballs.

Which is interesting in part because Kluber doesn't even identify the pitch as a curveball. He just thinks of it as a breaking ball. It definitely doesn't move like an ordinary curveball -- it has an extreme amount of horizontal break. But, lest anyone think that Kluber is just getting the job done with smoke and mirrors, his repertoire is legitimate. He can pump it into the mid-90s, and he has this particularly unusual breaking ball that's catching everyone's attention.


You've probably heard of the PITCHf/x system, which tracks the location and movement of every pitch thrown in every game. I don't want to bore you with a lot of details, but it's an absolute fact that Kluber's breaking ball is of a rare variety. It's faster than most curveballs, it has more drop than most sliders, and it just flies off to the left, as opposed to breaking 12-to-6. The majority of curveballs break to the glove side, but Kluber's movement is extreme, and I went into the data trying to find a comparison. I wanted to know: does any other starter in the majors throw a breaking ball like Corey Kluber's breaking ball?

Using the Baseball Prospectus PITCHf/x leaderboards, I looked at every starter's curve and slider from between 2008-2014. I set a minimum of 100 thrown. I was left with a sample of 740 breaking balls. Of those, 225 were within two miles per hour of Kluber's 83.2 mph average. Of those, 35 were within two inches of Kluber's vertical movement. Of those, three were within two inches of Kluber's horizontal movement. So we have just three comparable pitches to Corey Kluber's breaking ball: Yu Darvish's slider, Jose Fernandez's breaking ball, and Marcus Stroman's curveball. Just missing the cut: Sonny Gray's curveball.

About that, then. Stroman's just a highly-touted rookie, so we'll see what he becomes. But, Darvish's slider is one of the best pitches in baseball. Fernandez's breaking ball is another one of the best pitches in baseball. He'd thrown it more than a third of the time. In that same Baseball America poll, Darvish was credited with having the AL's best slider, and Fernandez was credited with having the NL's third-best slider, behind Kershaw and a reliever. So, baseball people think Kluber's breaking ball is elite, and it compares very favorably to another two pitches that are undoubtedly elite.

Let's take a look at these things, for the sake of breaking up the text. Kluber's breaking ball:

Stroman's curveball:

Fernandez's breaking ball:

Darvish's slider:

Given the success of Fernandez's breaking ball, you wonder if Kluber could throw his almost twice as often as he does. Maybe he doesn't think he could, or maybe he doesn't want to. He also has a useful two-seamer, cutter, and change. But Kluber uses this pitch against both righties and lefties alike, and he uses it to great effect. He could probably use it more without being predictable.

There's an argument to be made that, this season, Kluber's breaking ball has been the most effective pitch in the whole league among starters. It's yielded a slugging percentage of .101, which is the lowest of any regular pitch. Nobody's hit it for a homer. FanGraphs tracks pitch values, which work as a measure of a pitch's effectiveness. Again, I'll skip the math, but for example, a single allowed is bad for a pitch value, and a called strike or a swinging strike is good for a pitch value. According to the numbers, Kluber's breaking ball has been worth 17 runs better than average, or 4.2 runs better than average per 100 thrown.

Over the past decade, only three pitches thrown often by starters have finished with a better pitch value per 100. The short list:

    Right there is the Fernandez breaking ball to which Kluber's breaking ball compares. Now, this doesn't necessarily mean Kluber's breaking ball is the best pitch in baseball. For one thing, we just need to see more of it. For another, every pitch is somewhat dependent on the other pitches thrown, and Kluber only uses his breaking ball once out of every six pitches or so. If he threw it more often, maybe it would get hit more often, because hitters would look for it more. As an example, we can't say Kluber's breaking ball is as good as or better than Fernandez's, because Fernandez has thrown his twice as much. He's had a similar amount of success despite the pitch getting so much more exposure, and that's to the pitch's credit.

    But we don't have to overstep. Corey Kluber's breaking ball might not be the best pitch in the league, but it would appear to be one of them. Managers agree. And it's pretty clear how the pitch has gotten better and more consistent over time. Kluber now is able to get a little more horizontal break. He's also able to get a little more vertical break. To show you how the pitch has improved, here's Kluber's year-to-year rates of two-strike breaking balls that have generated strikeouts:

    2012: 25%
    2013: 29%
    2014: 39%

    That's a step forward in terms of consistency. And that's all that separates anyone with decent stuff from stardom: consistency, in commitment and delivery. With consistent dedication, one can achieve consistent mechanics, and with consistent mechanics, the sky is the limit for anyone with an assortment of pitches. Kluber's got everything he could possibly need, and now it's up to all the rest of us to catch up.

    Corey Kluber's been one of the best pitchers in baseball. According to MLB managers, he's got one of the best breaking balls in baseball. Statistically, it profiles as one of the best breaking balls in baseball, and it compares favorably to two of the other best breaking balls in baseball. The Corey Kluber story isn't all about one individual pitch, but the story of that pitch happens to mirror the story of Kluber. With skill and consistency, one's able to reach some impossible heights.