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« Home Road Stewart | Main | Defining the Strike Zone »

Wandy Rodriguez and the Curve

Wandy Rodriguez is regarded as one of the better starting pitchers in baseball. He was rewarded as such this off-season when the Houston Astros signed him to a three-year, $34 million extension.

However, the left-hander struggled mightily during the first half of the 2010 season. In his first 14 starts, he posted a 6.09 ERA with 52 strikeouts and 34 walks in 75 and one-third innings. In his next 18 starts, he posted a 2.03 ERA with 126 strikeouts and 34 walks in 119 and two-thirds innings. What happened?

At Baseball Prospectus, Christina Kahrl suggested that Rodriguez refined his curve. Using data from Baseball Analytics, we shall find out exactly how Rodriguez changed.

We will start out with some of the results, first looking at the batted ball splits. In the first half, Rodriguez induced 46 ground balls out of 82 total batted balls (56 percent); in the second half, he induced 89 grounders out of 137 total (65 percent). As we can see by the league trends on batted balls (and intuitively), it is significantly harder to hit for power when the ball hits the ground.

Did he change his general location? During the first half, he was hitting the middle of the plate more often overall. His performance against lefties stayed about the same (.276 wOBA to .273) but improved drastically against right-handers (.341 to .290). The heat maps showed that he hit the corners better and even expanded outside of the strike zone.

Wandy Rodriguez, first half vs. RHWandy Rodriguez, second half vs. RH

In both halves, Rodriguez opted not to use the curve in hitters' counts, choosing instead to use them in pitcher-favored and even counts. His performance in pitcher-favored counts did not vary much at all (.257 wOBA to .226) but in even counts, his wOBA allowed dropped from .355 to .283. The heat maps show you the severe change in location as well.

Wandy Rodriguez, first half, even counts vs. RHWandy Rodriguez, second half, even counts vs. RH

Many have tried to explain Rodriguez's second-half transformation, but the answer may simply be that he located his curve better. After a bit of Googling, I could not find out if pitching coach Brad Arnsberg had Rodriguez change his grip -- or arm slot, or anything else for that matter -- but it would not surprise me if that were the case.

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