When A Cutter Stops Cutting

There are two types of Major League pitchers. Some pitchers succeed by chucking high-velocity, high-spin, high-efficiency heaters past hitters. These are Gerrit Cole’s of the world, the ones who can dominate by pure, awesome force. They light up radar guns, go viral on Twitter, and top 95 mph routinely.

But not every pitcher is blessed with these God-given gifts. There are other hurlers that rely on a different kind of fastball, utilizing the pitch as a means to induce soft contact. Despite a lack of strikeouts, this caliber of fastball can still be plenty effective. Royals pitcher Brad Keller’s fastball falls into this category.

Keller, 25, anchored the Royals’ rotation during the condensed 2020 season, posting a 2.47 ERA/3.43 FIP. His underlying metrics weren’t as strong. but they still portrayed the hurler as above-average. He was stingy with walks, induced plenty of ground balls, and was worth 1.3 fWAR, good for 26th in the Majors.

Some may attribute his success to Small Sample Size, but there was a particular method behind Keller’s success last season. He utilized a fastball, that defied conventional norms, 40% of the time

Keller’s fastball wasn’t particularly notable in the velocity department; its 93 mph velocity only ranked in the 47th percentile. Its 18 inches of drop was 23% below average, compared to fastballs of similar quality, while the lateral movement on the pitch was -140% below average. The spin rate on the pitch was below average, as well (53rd percentile).

Bring it all together and Keller’s 2020 fastball had subpar velocity, a subpar spin rate, and a subpar spin efficiency. And yet, it was plenty effective. Batters only posted a .245 wOBA against the offering, with both Fangraphs and Baseball Savant grading it as an above-average pitch.

Most effective fastballs contain a high amount of active spin. Having a large amount of active spin gives the illusion that the pitch is rising, which helps it perform better in the upper half of the strike zone. Fastballs of this standard generate more strikeouts and whiffs.

Keller’s the exact opposite. His fastball resembles a cutter, more than a modern fastball. This kind of fastball isn’t thrown to generate whiffs, rather, it’s a pitch designed to induce soft contact.

Quick physics lesson. Most fastballs have two forces that impact ball flight: gravity and Magnus force. But if the pitch is thrown with more gyro spin, a third force can be exerted: seam-shifted wake.

Keller’s success with his fastball can be attributed to seam-shifted wake, a term that’s been making the rounds lately. Seam-shifted wake, also called SSW, is a force that is imparted on air-borne baseball under a specific set of conditions. For a pitcher to add seam-shifted wake to his pitches, he needs to alter the seams correctly, which causes a change in the location of wake formation. A delta in the wake formation imparts a force on the ball, which results in the unexpected movement in a different direction. Seam-shifted wake usually occurs on pitched balls that have more gyro spin.

TL;DR: pitched balls that have SSW give it additional late movement, which helps with their effectiveness. However, this late, effective movement can only occur if there isn’t much spin-based movement and if certain criteria is met upon release. Cutters and changeups are two of the most common pitch types that benefit from SSW.

Ever see the late movement of Stephen Strasburg’s trademark changeup? That’s SSW at work.

In 2020, Keller’s sinker, his slider, and his fastball all benefited from seam-shifted wake effects. Opposing hitters only mustered a collaborative .237 wOBA against the three deliveries.

Alec Lewis of The Athletic detailed Keller’s 2020 season and explored how he used SSW to his advantage to generate more ground-balls. As a result, Keller’s 52.8% ground-ball percentage ranked 12th league-wise last year. (min 50 innings)

Here’s a great thread about Keller and how he’s been using seam-shifted wake by baseball physicist Barton Smith.

This season, I expected Keller to continue his stellar, SSW-aided breakout. But the opposite has happened. Keller’s spin efficiency has risen on his fastball, changeup, and slider. His ERA has spiked to 6.34 and his Baseball Savant page isn’t pretty.

This season, Keller’s spin efficiency on his fastball has increased to 66%, an 11% year-over-year increase from last season. In turn, this has limited the seam-shifted wake effect of the pitch. It’s not only his fastball that’s in the red, his changeup and slider have also gained more active spin.

Usually, a fastball gaining more spin efficiency is a net positive, but for a fastball with cutting action, it could lead to ineffectiveness. Increasing the amount of active spin on the pitch prevents SSW from occurring, making it straighter and easier to hit. Because his pitches are missing the late break that they had last season, they’re becoming more hittable, to the detriment of both Keller and his team.

Last season, Braves’ skipper Brian Snitker raved about the natural cut of Keller’s fastball after his strong outing against his team. However, his natural cut all goes for naught if the pitch’s active spin is rising and SSW isn’t occurring. The key behind Keller’s success with his fastball lies with his usage of SSW.

Last season, his fastball had a -3 Run lValue, per Baseball Savant. This season, it’s ballooned to 11, the fourth-worst mark in the league. A similar trend follows his breaking and offspeed pitches. His slider, arguably his best pitch last year, hasn’t fared as well; it currently has -1 Run Value.

This is just my speculation, but could Keller be purposefully increasing the efficiency on his changeup and slider while the efficiency of his fastballs is increasing as a result? For his sake, I surely hope that’s not the case.

Granted his slider has performed worse, so maybe it’s not that explanation. Maybe the explanation isn’t so straightforward.

As Robert Arthur of Baseball Prospectus recently pointed out, the seam-shifted wake effect has been lessened this season. Perhaps that’s the byproduct of MLB’s new ball.

Whatever Keller’s trying, it’s not working out this season. His fastball has been tattooed while his slider has fared considerably worse. It’s a pity for a pitcher that looked to be a solid building block for the Royals as recent as last season. Here’s to hoping Keller can refine his arsenal in the nearby future.

Picture Credit: The Kansas City Star

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