Attempting To Explain The Jesse Chavez Home Run Issue

Without the Rays scouting department handing over their report on Jesse Chavez, there is only so much we can tell from the numbers. However, there is some data that might be worth noting. It could be something, but it could just as easily be nothing.

While sifting threw Jesse Chavez's 2009 pitch f/x data, I noticed a little hiccup along the way. A few disclaimers: 1) Small sample size rules apply. It's one season of data and we have been breaking it down into even smaller sizes. 2) Home run rates are volatile from year to year, and even more so for a relief pitcher. 3) Pitch F/X classification aren't always 100% exact so keep that in mind.

But back to the hiccup...

Looking at the overall numbers, it seems Chavez struggled with the long ball this past season. He allowed 11 home runs in 67.1 innings (1.47 HR/9), and a carried relatively large HR/FB of 12.7. That HR/FB is high in its own right, but even more for a neutral pitcher. For comparison, J.P. Howell had the exact same HR/FB rate, but since he gets more ground balls, fewer balls hopped the fence. HR/FB rates fluctuate a lot year-to-year, but alas we only have one full year of data to work with.

After skimming the data, I noticed that seven of the 11 home runs came within a small window. From 7/20-9/21, Chavez made 26 appearances and allowed seven home runs in 25.1 innings. This means he allowed just four homers over the remaining 42 innings.  Seven home runs in 25.1 innings gives you a HR/9 of 2.50, while four in 42 innings leaves us with a much more manageable 0.85.

Pitch selection/usage is usually a good place to start when we see fluctuations in a pitcher's numbers. Sure enough, there was a bit of a change in selection between the two data groups. During the home run happy summer, Chavez threw his fastball 69% of the time. This was pretty normal as he threw it 70% of the time during the rest of the year. The real change is in secondary pitches.

From people familiar with Chavez, we've been told his slider is his real plus pitch. According to Fangraphs pitch values, that observation is confirmed. With a wSL of 4.1, Chavez's slide piece was arguably his best pitch (sss). When things were going well (42 innings), Chavez was throwing the slider between 11-12% of the time. During the same stretch, he was throwing his change up around 19%. However, when the home runs starting flying his slider usage dropped to 6% while his change up rose to 25%.

While his home runs decreased dramatically not everything was perfect. When more sliders were thrown, his BB/9 increased and his K/9 decreased slightly. Normally, this is a bad thing, but I rather give up an extra walk here in order to save a few home runs.

With just this data alone we cannot say that because he threw less sliders and more change ups this magically explains the increase in home runs allowed, but it is something to keep in mind. Also keep in mind that none of the 11 home runs came off of a slider, and in the small sample size of the season he got 15% swinging strikes on the slider; the best of his trio of pitches.

Both teams (Pirates and Rays) have more and better data available and I'm assuming both sides are familiar with the the fluctuation in homers and the change in selection. Maybe there was something physical that prevented him from throwing the pitch during those two months and he corrected it at the end. Maybe that is what Rays saw in Chavez. Regardless of the actual reason, the Rays saw something in him that they liked. I hope we get to see the same.