Carlos Rodon’s hot streak coincides with reintroduction of sinker

Nicky Delmonico may have something to say about this, but I think the most important development for the White Sox over the last month has been the resurgence of Carlos Rodon. Rodon has been on a tear, and it’s pretty easy to pinpoint the July 30 start against Cleveland as the point when he started to turn his season around. In fact, we can divide his season into two five-game segments that couldn’t look more different:

Really, the only thing that’s held constant is his ability to rack up strikeouts, and that’ll happen when you have a slider like Rodon’s. Nearly everything else — including control, pitch efficiency, and batted ball profile — have significantly changed over the last five games. The biggest difference in Rodon’s repertoire between those the two time periods is that his two-seam sinker has featured prominently during the more recent starts. Brooks Baseball can show us his pitch usage by game and from the below, it’s pretty easy to see an increased effort to get the sinker into the mix:

For those who haven’t read much on the physics of “vertical movement” on pitches, here’s a crash course. With the exception of the curveball, all baseball pitches are generally thrown with backspin, which causes them to stay elevated by more than what gravity would dictate on a spin-less pitch. This gives them “positive” vertical movement. A sinker, somewhat counter-intuitively, also falls into this group; the spin on the ball naturally causes it to fight gravity. What makes a sinker effective is that it’s thrown with less backspin than the standard fourseam fastball at roughly the same velocity. This season, Rodon has generated roughly a 2.5-inch “spread” between the vertical movement on his regular fastball and that of the two-seam sinker, which isn’t a lot. However, when the two pitches are used in tandem, hitters face the challenge of figuring out whether that fastball is going to stay up or whether it’s going to dip by roughly the width of a bat barrel. That usually isn’t enough to make a difference on whether the hitter makes contact, but it can have a big say in the quality of the contact.

Before Rodon began mixing in that sinker, hitters were slugging .509 on his fastballs. Since July 30, that number has dropped to .405. The .405 number is deceptively good because generally — and especially in the case of Rodon — hitters have a harder time squaring up breaking pitches. Therefore, you typically expect the slugging percentage on fastballs to look high relative to the aggregate number. The reduced slugging comes from a couple different aspects of his batted ball profile:

This paints a picture of hitters that have been getting caught in-between the two pitches. We’re seeing more grounders because hitters are swinging on top of the sinker. That leads to fewer extra-base hits and more double plays; three of the five recent double-plays above have come on the two-seam. Similarly, all four pop-ups have come on the fourseam because batters are getting too far underneath it. Double-plays get you two outs on a single pitch and pop-ups are just strikeouts that don’t require a minimum of three pitches, so you can see how both of these developments have helped to make Rodon more efficient and work deeper into games of late. He’s recorded at least one out in the seventh inning in each of his last five starts.

In 2015 and 2016, the two-seam fastball was an important piece of Rodon’s attack, so it’s nothing new. From an outsider’s perspective, it’s unclear whether his increased reliance on it lately is due to added comfort in a wider pitch variety as he regains his old command or simply some sage advice from Don Cooper. Regardless of the cause, Rodon’s looked the part of a front-line starter since the sinker’s return to prominence. If he can keep inducing weaker contact while continuing to strike hitters out as we all know he can, Rodon’s going to be tough to beat.