The Right Stuff Revisited: Another Attempt to Quantify Pitch Quality

A few years ago, I introduced Stuff+ on these digital pages to help power the pitcher analysis in the series previews and elsewhere. It was an attempt to measure the raw stuff of a pitcher’s arsenal using the physical characteristics of their pitches. Back in 2019, this type of pitch evaluation was still a new frontier. Eno Sarris, Jeff Zimmerman, and a handful of others had dipped their toes into creating arsenal scores like this.

Fast forward two years and the amount of research into what makes individual pitches successful has grown exponentially. The biggest leap forward has been a much deeper understanding of how the spin imparted on a given pitch affects its flight path. In 2020, MLB updated it’s tracking system from Trackman to Hawk-Eye, giving us much more detailed information about how a pitch travels from the hand to the plate. The result has been a huge leap forward in pitch research.

It’s almost comical to look back on the components that were included and the way Stuff+ was calculated. I was using the data that was publicly available at the time, but as is the case with many things in baseball, we just didn’t fully understand the minutiae of the game. With more public data in hand, I figured it was time to update the way I calculate my Stuff+ scores to reflect the current research.

Spinning Stuff+ 2.0

The biggest change to Stuff+ is the way I approach spin rate. Back in 2019, we knew that spin rate was highly correlated with velocity and movement and, generally, high-spin pitches resulted in high whiff rates. With Hawk-Eye cameras installed across baseball, we now have the ability to directly measure the spin axis of a given pitch. Before 2020, we had been inferring spin axis based on the movement of the pitch, but it turns out that pitches don’t often move like we expect them to based on the rate and direction of their spin when leaving the pitcher’s hand.

This effect has been dubbed “seam-shifted wake” and the research into it has been gaining plenty of steam over the last year or so. Essentially, the idea is that seam-shifted pitches introduce a different type of spin to a pitch that affects the ball’s flight. A pitch’s spin direction will look one way out of the pitcher’s hand but the actual movement of the pitch when it crosses the plate suggests that the original spin direction was different.

There’s still so much we don’t know about this effect and the type of movement it creates, but one thing is apparent, not all spin is created equally. Some pitches rely solely on Magnus spin — back- or top-spin that pushes pitches up or down vertically, think four-seam fastballs and curveballs — while other pitches utilize a combination of Magnus and seam-shifted spin to generate the movement profiles. Tom Tango has done some initial research into which pitch types benefit from the different types of spin. Sinkers, cutters, and sliders in particular seem to have a positive relationship with seam-shifted wake. When we measure spin direction, we can compare the spin direction out of the hand and the inferred spin direction based on the movement of the pitch when it crosses home plate. The deviation between these two measurements is a good proxy for measuring the effect of seam-shifted wake on a pitch.

Baseball Savant has introduced very handy graphics that show this deviation. Here’s the inferred (left) and observed (right) spin direction of Yusei Kikuchi’s arsenal.

My Stuff+ scores now take this new seam-shifted research into account. They’re also using active spin rates — the amount of spin that’s actually contributing to movement rather than raw total spin.

Commanding Stuff+ 2.0

The other change I’ve made to the way I calculate these scores is how I quantify “command.” This was a major drawback of Stuff+ 1.0. It’s extremely difficult to measure a pitcher’s command using publicly available data. The biggest missing piece is intent. We have location data but we just don’t know if a pitcher meant to locate their pitch in any given area based on where it ended up. The best we can do is understand which areas of the strike zone are beneficial for the pitcher and which areas are best to avoid. That’s essentially what Max Bay has done in this short Twitter thread:

Using Statcast’s attack zones, I queried which regions of the zone possess negative run values per pitch type, and adjusted my command component to take into account pitchers who locate in those regions frequently. Essentially, it’s important to throw strikes with fastballs while avoiding the middle of the plate, breaking balls are surprisingly effective up in the zone and obviously effective down, and offspeed pitches must be located down in the zone to be effective.

Here are the Stuff+ scores for the Mariners starters for use as a reference in the series previews. Until mid-May, I’ll present the Stuff+ scores for the Mariners starting rotation using 2021 data. I’ll update the Mariners data monthly so that it can be referenced throughout the season.

Unsurprisingly, Robbie Ray’s fastball and slider grade out particularly well by Stuff+. He started leaning on those two pitches so often, it’s no wonder he found so much success in Toronto last year.

June Update: Ray’s stuff is clearly diminished as both his fastball and slider are sitting around two miles per hour slower than what he was averaging last year.

July Update: The addition of a sinker to Ray’s repertoire has paid dividends for his effectiveness over the last month. It doesn’t grade out very well by Stuff+ but it has the third highest whiff rate among all sinkers thrown at least 100 times this season. That’ll play.

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Logan Gilbert’s fastball grades out extremely well per this metric. He gets a ton of ride on the pitch and his elite extension (which this model does not account for) make it one of the better fastballs in baseball. He worked extensively on his secondary offerings during the offseason so it’s likely they’ll end up looking very different when I update the model with 2022 data.

June Update: Gilbert’s repertoire has been transformed. He’s all of his secondary offerings harder with the same general shapes. Both of his breaking balls look a lot better and they’re seeing better results stemming from the changes. His changeup is still a work in progress, however. It doesn’t possess the elite velocity differential or vertical movement that it had last year and he still can’t command it well.

July Update: Gilbert has improved his Stuff+ ratings on all of his pitches with better command and a slight bump in velocity across his arsenal.

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Even though Marco Gonzales is one of the softest throwing pitchers in the majors, he’s been so successful because his control of his entire arsenal is impeccable. His changeup is his only pitch that grades out well per Stuff+.

June Update: Gonzales has completely revamped his changeup. He’s now throwing it a third of the time and has increased it’s horizontal break by over four inches, the fourth highest horizontal movement among all changeups thrown in the majors. He’s also now placing it in ideal locations along with the steady pinpoint command of his sinker.

July Update: Gonzales has continued to lean heavily on his changeup and his command of his cutter and curveball has improved recently.

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Chris Flexen’s arsenal is a good example of a bunch of slightly above average characteristics combining to make a pretty good repertoire of pitches. He doesn’t have an overpowering fastball but commands it pretty well. His secondary pitches each have one standout and enough average characteristics to grade out pretty well per Stuff+.

June Update: Flexen’s changeup has also improved, though his fastball has also taken a step back with a slight drop in velocity. He’s also stopped throwing his curveball, leaning on his other three pitches the majority of the time.

July Update: Flexen has recently added a slider to his pitch mix, essentially replacing the usage of his curveball. The slider is a little harder than his curve and it’s been a lot more effective at generating whiffs.

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George Kirby’s stuff hasn’t disappointed in his first taste of the majors. His fastball relies primarily on it’s velocity since it’s shape isn’t all that impressive. His slider looks like it could be a great weapon for him with a ton of seam-shifted wake effects giving it added deception.

July Update: We finally have Stuff+ grades for Kirby’s changeup and curveball and they look excellent. They both have excellent movement profiles and really pair well with his fastball-slider combo that he’s leaned heavily on during his first taste of the big leagues.

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Unsurprisingly, Matt Brash’s entire arsenal was a Stuff+ darling. All three of his pitches featured elite horizontal movement. His biggest drawback was obviously his command, particularly with his fastball.