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The Right Stuff Revisited: Another Attempt to Quantify Pitch Quality

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Pitching research has advanced by leaps and bounds over the last few years — Stuff+ needs to adapt.

Seattle Mariners v Boston Red Sox Photo by Kathryn Riley/Getty Images

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.

Source: Baseball Savant

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.

Next Steps

Since Hawk-Eye cameras were only first used during the 2020 season, the sample I’m working with is relatively small. Once we reach mid-season, I’ll likely update the data with pitches thrown in 2021. For now, I’ll present the Stuff+ scores for the Mariners starting rotation so that it can be used as a reference in the series previews this year. I’ll update the Mariners data monthly so that it can be referenced throughout the season.

LHP Marco Gonzales

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Active Spin & Differential Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Active Spin & Differential Control
Four-seam (6.1%) 55 88.1 (3) 17.5 (21) 7.8 (54) 2133 | 99% | 15 (56) 53.8% (80)
Sinker (43.6%) 51 88.5 (7) 17.7 (14) 11.0 (7) 2150 | 99% | 0 (44) 56.9% (93)
Cutter (14.1%) 50 85.4 (13) 24.2 (27) 2.4 (40) 2101 | 73% | 30 (27) 47.5% (34)
Changeup (20.3%) 134 79.5 (65) 33.6 (56) 16.4 (86) 2179 | 97% | 0 (95) 50.4% (60)
Curveball (15.9%) 91 74.9 (12) 62.0 (82) 6.6 (32) 2285 | 80% | 15 (49) 48.6% (89)
2021 stats; percentile ranks in parenthesis

Marco’s changeup grades out really well via this updated methodology but the rest of his arsenal is lacking. It really emphasizes the critical role his command plays in his success. If he can’t locate his pitches — like he was struggling to do early this season — his whole approach falls apart.

July update: Gonzales has started throwing his four-seam fastball a lot more often in the past month. By Stuff+, it’s a better fit for his pitch mix than his mediocre sinker and he’s done an excellent job of locating it in good spots in the zone.

August update: That uptick in four-seam usage was short lived as Gonzales really leaned into using his sinker in July and his first two starts in August. That’s pretty uncharacteristic for him. In the past, he’s used most of his repertoire equally, but his usage pattern is heavily skewed towards his sinker now.

September update: Gonzales continued to lean on his sinker in August and it’s produced some surprisingly good results. He’s upped the whiff rate on the pitch to 18.6%, well above league average for that pitch type. His run of really good starts since the All-Star break coincides with his increased usage of his sinker and far better control of his entire repertoire.


LHP Yusei Kikuchi

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Four-seam (35.5%) 142 95.2 (78) 13.7 (71) 9.9 (80) 2222 | 95% | 30 (55) 51.4% (53)
Cutter (34.7%) 111 91.3 (89) 23.5 (21) 0.2 (6) 2305 | 55% | 45 (29) 50.6% (56)
Changeup (10.4%) 88 86.3 (69) 30.8 (35) 10.6 (19) 1458 | 87% | 45 (15) 41.2% (11)
Slider (19.3%) 84 82.7 (25) 42.6 (87) 2.5 (19) 2406 | 30% | 45 (29) 49.5% (79)
2021 stats; percentile ranks in parenthesis

Yusei Kikuchi has the best raw stuff on the Mariners pitching staff with a four-seamer and cutter that grade out particularly well. He’s also able to locate both of his secondary pitches extremely well. His changeup isn’t liked by Stuff+ but opposing batters have chased and whiffed against it at a healthy rate anyway.

July update: Kikuchi’s stuff and pitch mix stayed stable in June. Why fix what isn’t broken?

August update: Kikuchi’s stuff has seen a slight decrease in effectiveness after the sticky substance crackdown was implemented in mid-July. He’s also started to use his changeup a bit more often as a weapon to keep right-handed batters at bay. That pitch is running an excellent 41.3% whiff rate.

September update: Kikuchi’s struggles in August seem to stem from a drop in effectiveness from his fastball. The average velocity on the pitch dipped a bit last month and it’s shape looked a little different with some additional horizonal break. After his fantastic start against Houston during that last homestand in August, he mentioned that he was working through some mechanical issues all month long which suddenly clicked in that outing.


RHP Chris Flexen

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Four-seam (37.7%) 83 92.7 (34) 13.6 (72) 2.0 (7) 2098 | 92% | 15 (23) 50.5% (47)
Cutter (30.8%) 117 89.0 (67) 27.1 (44) 5.0 (85) 2292 | 54% | 45 (28) 49.9% (50)
Changeup (16.0%) 107 82.8 (86) 30.3 (32) 12.3 (33) 1322 | 98% | 45 (15) 44.0% (23)
Curveball (15.5%) 97 77.2 (27) 59.1 (74) 7.8 (39) 2702 | 72% | 15 (54) 46.5% (73)
2021 stats; percentile ranks in parenthesis

Flexen’s success in Korea was built upon his fastball and his curveball but those two pitches grade out pretty poorly by this metric. Instead, it’s his cutter and changeup that look like they’re carrying his arsenal. One thing to note are his struggles to locate his any of pitches in beneficial regions.

July update: Flexen improved the location of his pitches in June and his Stuff+ scores have all seen a slight bump. He’s also started to throw his curveball a little more often and has started seeing more strikeouts as a result.

August update: Flexen has continued to improve his control of his entire repertoire.

September update: Flexen just keeps on baffling hitters with his four-pitch mix. It’s hard to pinpoint exactly what makes his pitches so difficult to square up. His four-seamer, cutter, and curveball all have a hard hit rate over 40%, yet each of those pitches has a xwOBA under the league average for their respective pitch types. He might want to think about throwing his changeup a bit more often too. It has a pretty good whiff rate and opposing batters have a .209 xwOBA against the pitch.


RHP Logan Gilbert

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Four-seam (61.6%) 131 95.1 (77) 12.2 (87) 5.5 (26) 2190 | 95% | 15 (48) 48.7% (27)
Changeup (7.8%) 151 79.3 (100) 38.8 (94) 14.0 (56) 1549 | 98% | 30 (49) 34.4% (0)
Curveball (6.8%) 51 74.1 (11) 58.2 (72) 4.9 (21) 2144 | 71% | 15 (27) 33.0% (4)
Slider (23.8%) 68 82.9 (27) 36.1 (46) 6.9 (75) 2303 | 35% | 30 (22) 39.0% (18)
2021 stats; percentile ranks in parenthesis

Unsurprisingly, 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. Surprisingly, his two breaking balls do not grade out very well. They both have an interesting shape but are held back by a lack of velocity and some really poor location.

July update: Gilbert has started mixing in his changeup more often instead of his curveball. It’s only 34 pitches but the data is fascinating so far. It has the largest velocity differential of any changeup-fastball combo in baseball and it’s vertical movement sits in the 96th percentile. If he can harness his command of the pitch, it has the potential to be a plus pitch for him.

August update: Gilbert has continued to increase the usage of his changeup, though he often struggles with his feel for the pitch. It can be erratic, but it’s incredibly effective when he can locate it where he wants to against left-handed batters.

September update: Gilbert has really struggled with the feel for his secondary pitches, but he’s continued to tinker with them to find something that works. He’s added two ticks of velocity to his slider and is throwing it out of a different arm slot. Those adjustments haven’t translated into positive results yet, but he’s making progress towards getting a better feel for the pitch. His curveball made a triumphant return to his pitch mix in his start on September 1, earning a big strikeout against Yordan Alvarez. His changeup also looked pretty different against the Astros in that start; the velocity differential on that pitch dropped from 15.5 to 12.1, still elite but not among the best in baseball anymore.


LHP Tyler Anderson

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Four-seam (39.7%) 77 90.4 (11) 15.5 (42) 9.4 (73) 2307 | 98% | 0 (82) 52.7% (67)
Sinker (7.7%) 100 88.9 (9) 28.4 (88) 18.5 (98) 2253 | 97% | 30 (53) 54.9% (82)
Cutter (26.9%) 68 85.0 (8) 27.8 (54) 2.2 (38) 2500 | 48% | 60 (50) 50.2% (52)
Changeup (25.0%) 118 80.8 (80) 28.3 (22) 12.7 (39) 1847 | 97% | 15 (75) 48.2% (48)
2021 stats; percentile ranks in parenthesis

The newest member of the Mariners rotation utilizes a fastball-heavy approach, with three different types of heaters making up the bulk of his pitch mix. The addition of his cutter to his repertoire this year gives him a third look to keep batters off-balance, not unlike the success Lance Lynn has enjoyed with his fastball-heavy approach. Without the elite velocity that Lynn has, Anderson has to rely mostly on deception and sequencing, which he does well when he pairs his fastballs with his great changeup.

September update: Anderson has essentially swapped the usage rate of his cutter and changeup since joining the Mariners. The former is a fine pitch, but it doesn’t get that many whiffs and simply serves as a third look to keep hitters off-balance. His changeup is really good however, so it’s a nice sign that he’s started to emphasize it more.


LHP Justus Sheffield

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Sinker (44.0%) 95 92.3 (41) 24.1 (64) 15.0 (46) 2050 | 81% | 60 (38) 50.4% (43)
Changeup (22.9%) 66 85.7 (27) 33.7 (53) 12.6 (32) 1766 | 72% | 75 (15) 46.2% (32)
Slider (32.5%) 75 81.8 (19) 43.3 (91) 7.2 (77) 2573 | 44% | 15 (24) 39.8% (22)
2021 stats; percentile ranks in parenthesis

Despite the low Stuff+ score, Justus Sheffield’s slider is the only pitch in his repertoire he can use to get whiffs, and the pitch’s whiff rate is rather average. His other two pitches are used to generate weak contact, but that’s a fine line to walk for a pitcher with a shallow arsenal.

July update: Nothing has really changed with Sheffield’s repertoire and that’s not an encouraging thing.

September update: The table above hasn’t changed because Sheffield hasn’t made enough appearances since July to update the data. But he did make a relief appearance on September 1. His sinker averaged 92.6 mph, just a touch above his season average for the pitch.


RHP Justin Dunn

Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Pitch Type Stuff+ Velocity Vertical Movement Horizontal Movement Spin Rate Control
Four-seam (51.6%) 124 93.7 (49) 12.4 (86) 9.8 (79) 2366 | 95% | 0 (80) 50.0% (40)
Curveball (33.1%) 106 80.2 (56) 42.4 (11) 16.7 (97) 2514 | 56% | 45 (22) 55.5% (100)
Slider (14.6%) 109 84.1 (40) 39.6 (65) 7.1 (77) 2511 | 39% | 60 (54) 45.5% (55)
2021 stats; percentile ranks in parenthesis

Justin Dunn is the only starting pitcher on the Mariners with a full arsenal of above average pitches. Looking at the raw characteristics of his two breaking balls shows them to be strong pitches on their own, but when taken together, you can see why he needs to work on differentiating them more.

July update: Nothing new to report since Dunn made just two abbreviated starts in June.