Aaron Bummer, strike thrower?

Aaron Bummer has never been able to throw strikes. He entered the 2024 season with a career-average BB/9 of 4.30; by comparison, only five qualified relievers have posted a  higher BB/9 across the six-year span since Bummer’s MLB debut. Bummer bottomed out in 2023 with a BB% in the 4th percentile, which contributed to his career-worst ERA (6.79) and FIP (3.58). 

But then something funny happened. After seven years on the south side of Chicago, Bummer joined the Braves and promptly stopped walking people. He improved his BB% to the 59th percentile, the second-highest single-season improvement in baseball this year, behind only Kansas City’s Lucas Erceg. It’s probably not a coincidence that Bummer managed to cut his ERA in half (3.58) and establish his best FIP (2.23) since the COVID-shortened 2020 season. 

When a pitcher improves his walk rate so drastically, the first instinct is to assume that they started throwing more strikes. If we look at the pitchers with the top-five decreases in BB% from 2023 to 2024, that’s largely what happens — with one exception. 

	Change in BB%	Change in In-Zone%
Lucas Erceg	-8%	+1.9%
Aaron Bummer	-6.1%	+0%
Bryan Woo	-5.6%	+5.9%
Nick Martinez	-5.5%	+5.2%
Carlos Estevez	-5.3%	+3.6%

Woo and Estevez transformed into two of baseball’s preeminent strike-throwers. Martinez is more or less in the middle of the pack, with a lean towards the plus side. Erceg still throws strikes at a similar rate to Bummer — which is to say not often — yet he nonetheless benefited from a bump in In-Zone%. But Bummer, for the third season in a row, threw pitches in the zone just 47.8% of the time, in the 36th percentile among pitchers who faced at least 200 batters. He generated called strikes at a lower rate in 2024 than he did in 2023. 

Well, okay. We’ve reached our first dead end. If Bummer is walking fewer people without throwing more pitches in the zone, then maybe he’s simply getting hitters to chase more often. That would be a reasonable assumption: More chase means more strikes. 

Erceg, for instance, increased his Chase% by 8.7% and saw his Chase Contact% drop by 1.4%, which helps explain why he slashed his walk rate without throwing more pitches in the zone. He simply induced a lot more swing-and-miss out of the zone. Yet that’s not what’s happening with Bummer, either. FanGraphs and Baseball Savant differ on the particulars of Bummer’s Chase Rate. But that ambiguity alone gives us enough pause to rule out Chase% as a defining factor, in contrast to Erceg, where the evidence is blatant.

Lucky for us, there’s an answer that sheds more light on what’s happening with Bummer: In 2024, Bummer’s Chase Contact% soared to a career-high 58.9%. He experienced the third-highest Out of Zone Contact increase in baseball. 

The quality of contact matters here, too. Bummer, always proficient at limiting barrels, curtailed hard contact all together in 2024, ranking in the 11th percentile in Hard Hit%. These traits hold true on pitches outside of the strike zone, too. So while Bummer may or may not be generating more chase, he is certainly generating more contact on these swings. That seems to have helped Bummer, and somewhat significantly. 

Let’s look at the group of pitchers that, like Bummer, saw a massive increase in Chase Contact%, and how they fared on these swings:

	wOBA	Hard Hit%
Aaron Bummer	.152	14.7%
Brennan Bernardino	.249	17.6%
Phil Maton	.222	18.4%
Emmanuel Clase	.091	15.2%
LEAGUE AVERAGE	.158	18.3%

This has been a skill for Bummer in bad years, too. Take 2023, for example: When hitters chased, they posted a .099 wOBA and 17.9% Hard Hit%, both marks better than league average. 

The logic makes sense, too: Because of his propensity to lose the zone, Bummer should want to maximize contact on pitches outside of the zone. The more weak contact he can induce to end an at-bat earlier — and he reached a 3-ball count a career-best 7.6% of the time this year — the better. You can’t walk a batter if you don’t get to three balls. 

So, we’ve established that this is a unique skill set that Bummer somehow maximized as a proxy for reducing his walk rate, a longtime Achilles Heel. That segues right into our next question: How did he maximize this strength? Why are hitters suddenly making much more contact when chasing Bummer’s pitches out of the zone? 

The best — if not the most satisfying — answer appears to come down to location. We’ll start specifically with Bummer’s slider (or, more accurately, sweeper), the only one of his five pitches to see both a decrease in O-Swing% and an increase in O-Contact%, according to FanGraphs. Using visuals from Baseball Savant, we can reach four main conclusions: 

• First, if we look at the sliders that Bummer threw outside the zone first in 2023 and then in 2024, we see a clear difference in location. In 2024, he concentrated his sliders closer to the bottom of the zone, rather than clustering them low and away. In totality, they’re more competitive. As we’re about to find out, this is a calculated decision. 

• Looking at a side-by-side comparison of the O-Swings induced by Bummer’s slider in 2023 and 2024, we see that the chases come against pitches in a similar cluster, beneath the outside corner of the strike zone. That's mostly where Bummer concentrated his sliders in 2023.

• When hitters chase AND make contact against Bummer’s slider, the pitches are not low and away. Rather, they’re simply low, dipping beneath the zone -- right where Bummer clustered the pitch in 2024.

• When hitters chase AND whiff against Bummer’s slider, these swings occur against pitches in a vastly different spot, centered low and away from the hitter. Of particular note, Bummer stopped throwing his slider in this location this past season. 

Where does that leave us? Well, Bummer is proficient at throwing his slider out of the zone in a spot where a) batters are prone to chase and b) after chasing, batters are likely to make contact. He placed a particular emphasis on the latter this season. That helps slash his walk rate, because it puts balls into play and eliminates deeper counts, where Bummer’s inability to find the zone is more likely to hurt him. Plus, since Bummer is exceptional at inducing weak contact and minimizing barrels, the contact is conducive to success. And all of a sudden, it doesn't matter as much that Bummer hardly comes into the zone.

It’s unorthodox, yet undeniably successful. The next question, of course, is sustainability. Can Bummer keep his walk rate down without throwing strikes? The Braves certainly think so: They agreed to a revamped two-year, $13 million deal with Bummer, buying out a pair of his options and offering more guaranteed salary. And it’s not like Bummer hasn’t had success before; he’s just a completely different pitcher now, less of a hard-baller geared towards swing-and-miss, instead more oriented towards finesse and soft contact. Perhaps it’s in part the natural evolution of an eight-year pro relishing a new organization for the first time. Or maybe the roots for this strategy were laid a while ago: His slider’s O-Contact% has increased in each of the last five seasons. 

Nonetheless we have, in part, solved the mystery of Aaron Bummer, who has maximized an innocuous skill set to do the improbable: slash his walk rate without throwing more strikes.