The Juiced Ball is Helping Flyball Pitchers, Not Sure Why.

I was wrong a few weeks back when I wrote:

With the new juiced baseball, home runs rates are up, and they are the main driver for the scoring increase. If a pitcher can limit home runs, they will be affected less and the easiest way to do this is to generate ground balls.

By using OPS for and against, the production flyball pitchers have gotten worse over the past 2.5 seasons. By examining the problem using the difference between both FIP and xFIP to ERA, I found the opposite answer and can’t explain why.

I expected this article to go a different route than it did. When I wrote that owners should focus on groundball pitchers, I knew I wanted to eventually determine how much the difference between ERA and both FIP and xFIP shrunk for extreme flyball pitchers.

Historically, extreme flyball and groundball pitchers see their ERA suppressed compared to their xFIP and FIP. Extreme groundball pitchers achieve this by inducing more double plays and hitters have limited extra base hits making them station-to-station. The key for extreme flyball pitchers is the sky high popups and fly balls they allow. These high-hit flyballs become easy outs, not home runs. For pitchers, they want their pitches to fall into the extremes and not in the Danger Zone.

With the new juiced ball, I figured the curve moved with extreme flyball pitchers having less of an advantage. I was wrong.

For the study, I divided up all pitchers into a 2002 to 2014 group and a 2015 to 2017 group. With the juiced ball showing up in the middle of 2015, I included that season with the second group, especially with its limited sample size.

For the first test, I found the weighted average difference between ERA and both FIP and xFIP by single GB% increments. Then I used Excel to plot a simple polynomial equation for the normal and juiced ball time frames. Here are the results:

It looks like a mess but concentrate just on the lines. In the juiced ball era, the Danger Zone shifted towards groundball pitchers by about 5% points. Also, heavy fly ball pitchers see better results while the extreme groundball pitchers still see some ERA suppression.

With the scattered nature of the data and limited samples at each percentage, I decided to group the groundball rates into 5% groups. Here are those results:

The curve differences aren’t as extreme as in the first graph. Overall, the curves haven’t changed but a couple of trends continue from the first graph but to a lesser extent. First, extreme flyball pitchers are suppressing their ERA more now in the juiced ball era than in previously. Next, the juiced era peak for ERA-xFIP moved from 45% to 55%.

For a final study, I will go to just three groups, less than 40%, greater than 55%, and in-between.

Extreme flyball pitchers are suppressing their ERAs more than previously with the average pitcher’s results are exactly the same. The extreme groundball pitchers are the ones seeing their advantage diminish some. My overall conclusion is owners don’t need to shy away from flyball pitchers in this juiced ball era. Instead, they may actually want to target high flyball pitchers as a tiebreaker.

So why the advantage to flyball pitchers? Truthfully, I am having a hard time believing my conclusions. They seem to be counter intuitive. For now, I can’t even come up with some narratives to test. Please suggest any in the comments.

The only one I have for now is that with the juiced ball, a pitcher needs to stay out of the ideal launch angle as much possible and heavy flyball pitchers meet this criterion. It can’t be tested since no pre-2016 batted ball data exists to test the theory.

For now, owners shouldn’t shy away from flyball pitchers just because more home runs are being hit. Even though I’m not sure why, they seem to be suppressing their ERA at a similar or better rate than previously measured.