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Here’s the reason why Madison Bumgarner has leaned on the curveball this season

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It’s not great news.

San Francisco Giants v Colorado Rockies Photo by Justin Edmonds/Getty Images

You’ve probably heard all about how Madison Bumgarner is no longer the pitcher he once was. Indeed, you could probably draw a line from the dirt bike accident to today and recall several instances and sets of numbers that indicate nothing but a steady decline for the Giants’ #1 starter.

That happens, though. It’s a part of the game, and if the accident was the direct cause, that’s a real bummer, but ultimately, a decline of some kind was going to happen at some point because that’s what happens to every baseball player. And with pitchers, that decline can come suddenly and without the aid of a dirt bike accident.

No matter the reason, he’s clearly trying to adjust to the situation. As Kenny noted in his review of Madison Bumgarner’s new approach:

The thing that jumps out is the increase in curveballs. He’s throwing a greater percentage of curves and they’re mostly coming in two strike counts or when he’s ahead. While it’s not getting as many whiffs as before, it’s still an above-average swing-and-miss pitch for him. By pitch value, his curve has been the most effective it’s ever been. It’s been worth 2.42 runs per 100 bendy boys that he’s thrown. The most it had ever been was 1.72.

That analysis was in direct response to the obvious drop in Bumgarner’s fastball velocity. A quick review of the Statcast data suggests that it’s not just the fastball that’s suffering, however — his velocity is seemingly down across the board. Perhaps most alarmingly, his pitch spin rate is also down across the board.

What the heck do I mean by “spin rate”? That’s just the rate a ball spins after a pitcher releases it. We didn’t have the technology to measure it until recently, and those numbers weren’t publicly available until even more recently.

Going back to 2015 when MLB.com published its results from the first year of Statcast data, we learned the following about specific pitch types:

4-seam FB: high spin good for swinging strikes and flyballs, low spin good for groundballs.

2-seam FB: can also be a sinker. Have less spin and less velocity; hence, more groundballs.

Cutter: faster than a slider but more spin and late movement than a 4-seam FB.

Slider: faster than a curveball but with less spin and less movement, but also not as fast as a fastball (FB) but more movement than a FB.

Curveball: velocity between 72-85 mph with lots of spin. High spin curveballs dive out of the strike zone (for comparison’s sake: a high spin FB seems to “rise” up through the zone).

Changeup: low spin and low velocity, about 6-10 mph slower than a FB.

Statcast’s scouting report says of Bumgarner:

Bumgarner relies on 5 pitches. Sinker (35%), Cutter (34.5%), Curve (22.3%), Changeup (8.2%), Four Seamer (0.1%)

Now, these are just the Statcast computer’s distinctions and we know that pitchers have their own designations. As Eno Sarris wrote in 2014:

[Bumgarner] knows that some people call it a slider, or think he has two different pitches. “I call it a cutter but I feel like it’s in between the two — I think people call it different things because I change speeds with it,” he said. “But I throw it the same.” Sliders and cutters traditionally have the same grip, so no big deal really.

Over the past two seasons, Statcast has stopped labeling any of Bumgarner’s pitches as a slider and instead switched over to cutter, so that’s what I’ll be using to examine the decline in Bumgarner’s spin rate across the board.

Every pitch except the curveball has gone down in velocity and the spin rate has dropped off on every pitch he throws. Even with that increase in curveball usage, we’re not seeing as much spin as we’ve seen before with it. A lower spin rate means it doesn’t “drop” out of the zone as sharply.

Some of that could be the result of Bumgarner messing around with an eephus pitch or simply varying speeds on the curve to create a bit more deception. He might be changing speeds on all his pitches to compensate for his lower velo ceiling, and that’s all well and good, but the simple fact is that he’s getting less spin on his pitches.

Going back to that Statcast primer article again, it used data from 2015 and noted that the year-to-year spin rate numbers might fluctuate a bit, but here are those averages (measured in rpm: revolutions per minute) posted at the time:

4-seam FB: 2,226 rpm / 92.9 mph

Cutter: 2,185 rpm / 88.0 mph

Slider: 2,090 rpm / 84.6 mph

Curveball: 2,308 rpm / 78.2 mph

Changeup: 1,746 rpm / 83.9 mph

The diminished velocity of the fastball is one thing, but the diminished velocity and spin on every other pitch could be the indication of either physical degradation or looming injury. Bumgarner has leaned on the curveball more than ever before because it has been his best pitch — maybe even his only good pitch. Everything else in the arsenal seems to have been effective-ish because of cunning, guile, and sequencing.

No matter what the case, Bumgarner’s decline warrants a bit more scrutiny going forward.