Batter HR/FB Rate and Statcast Metrics — A History

With three seasons in the Statcast Leaderboard books, it’s time to dive in to its short digital history. While the various metrics published on the site are sure to provide a better understanding of what drives a handful of statistics, I’m most interested in what fuels home runs. The leaderboard shares a number of interesting metrics, several of which would intuitively correlate with HR/FB rates. Given the leaguewide home run surge — for the second straight year, HR/FB rate hit a new high since 2002, the first season FanGraphs has batted ball data — we might find some answers on the virtual pages of Statcast’s records.

First, let’s discuss the league averages by season for the most relevant Statcast metrics.

In order to get rid of pitchers on the leaderboards, I set the minimum batted ball events to 60. That left me with populations of 444, 450, and 455 players in 2017, 2016, and 2015, respectively.

We’ll begin with everyone’s favorite new term, exit velocity. However, this isn’t just average exit velocity for all batted balls, but rather exit velocity for only fly balls and line drives. We don’t care how hard a player hits his ground balls, only when he lifts the ball in the air. A year ago, I found the correlation between FB/LD EV and HR/FB to be 0.769, which is quite strong. Though, there is actually a metric that correlated even better with HR/FB rate, which I’ll get to shortly.

From the table, we see the EV rise from 2015 to 2016 and remain constant this past season. But that jump is essentially meaningless. In 2017, hitters ranged from a low in the mid-80 MPH range, like Billy Hamilton at 84.1 MPH, all the way up to leader Aaron Judge at 100.2 MPH. So as tempting as it is to point to the increase in EV as the explanation for a higher HR/FB rate (“hey, batters are hitting the ball harder, how obvious!”), this really isn’t true.

Next, we slide on over to the newest addition to the leaderboard, 95+ MPH %. This is simply the percentage of batted balls hit at least 95 MPH. It’s seemingly more of a “cool” and “interesting” stat than a predictive one, but it’s fun to look at. The most interesting observation is perhaps that Yandy Diaz sits fifth on the leaderboard with a 50.8% 95+ MPH %. After a relatively quick look, it’s clear why — he actually sits atop the leaderboard in ground ball EV with a 90.8 MPH mark, a whopping 2.2 MPH greater than the second ranked player!

Diaz’s high ranking in 95+ MPH % is precisely why the metric is really not a good indicator of home run power. Well, at least not in a descriptive sense. It’s surely possible that Diaz translates that power from the ground to the air and enjoys a power surge, but for now, the metric’s inclusion of all batted ball types poses a problem. That said, it’s still a bit surprising to find that 95+ MPH % jumped in 2016, but then in 2017, dropped back down to below the 2015 mark.

Last is my favorite stat of all, Brls/BBE, which has a 0.824 correlation with HR/FB, even better than FB/LD EV (of course, because it includes EV, plus an additional component). And would you look at that, there was a meaningful spike in 2016 that stuck in 2017! It’s a surprise that there wasn’t another jump in 2017, but the move from 5.5% to 6.4% is far more significant than the FB/LD EV rise. As you could have guessed, Aaron Judge easily led baseball with a 25.7% mark.

So that solves it then, right? Batters are hitting more “barreled balls”, and since barreled balls are the most ideally struck balls, they go for homers at the highest rates. Wellll, uhhhh, yeah, but it’s not that simple, because there are two components of a barreled ball. Before we finish the Brls/BBE discussion, let’s bring back the table above, but this time with an addition.

I brought along fly ball percentage to the party. As HR/FB rate has been rising, so has FB%. Aha! That’s all that launch angle stuff and how hitters are increasing their average launch angle to participate in that exciting “fly ball revolution”, right?! Wrong.

First of all, HR/FB rate already controls for fly balls. It’s in the denominator, so FB% has no influence on the metric. Batters could hit every single ball into the air in the future, and that still wouldn’t provide the explanation for a rising HR/FB rate. Now, you could argue that the change in their approach has helped optimize their swing to hit more of those flies over the wall, but that’s a different explanation than simply hitting more fly balls.

So we have now ruled out fly balls as an explanation, as by the definition of HR/FB rate, it literally cannot be. And we have ruled out FB/LD EV as an explanation, because it stalled in 2017, and barely budged off of 2015. So what’s left?

Well, remember I said that “barrels” is made up of two components? Those components are exit velocity (which appears to not have increased across the league) and launch angle (there’s that term again!). I’m going to quote Mike Petriello from his article linked to in the first table as he defines what a barreled ball is:

A “barrel” is defined as a well-struck ball where the combination of exit velocity and launch angle generally leads to a minimum .500 batting average and 1.500 slugging percentage

As you can see, the “Barrel Zone” is an area that begins at 98 mph between 26 degrees and 30 degrees, and expands outward from there. The higher the speed of the ball, the wider the range of launch angle exists for a ball to be considered a barrel. At 99 mph and up, for example, between 25 degrees and 31 degrees is “barrelled.” At 100 mph and above, batted balls between 24 and 33 degrees will always be considered a barrel, and so on, expanding as balls get hit harder. Those aren’t arbitrary definitions; that’s based on a review of all of those batted-ball types and outlining the area where you get your minimum of “.500 batting average and 1.500 slugging percentage.

The key part of the definition is the ranges of the ideal launch angles. The leaguewide HR/FB rate hasn’t been exploding because batters are increasing their average launch angles, as that will generally just lead to a higher FB%. Instead, batters must be hitting their well-struck balls at increasingly ideal launch angles.

So, if a batter hit 250 balls at 95+ MPH, like Manny Machado did this past season, each of those 250 balls will qualify as a “barrel” if it was also hit in the ideal launch angle range associated with that exit velocity. Let’s say 50 of those balls qualified as barrels(with “ideal” launch angles), while 10 of them were hit at launch angles too high to qualify. The next year, this player still hit 250 balls at 95+ MPH and kept his FB/LD EV constant, but five additional balls that were hit too high to qualify last year now fell into the ideal launch angle range, qualifying them for barreled ball status. This player’s FB% would be unchanged as he’s simply rebucketing some of the well-struck balls from a less-ideal launch angle to an ideal one, both of which classify them under the fly ball umbrella.

So now the question is, if we have figured out that it’s the increased rate of ideal launch angles leading to the home run binge, how do we continue on with the currently available data? I’m not aware of any sort of historical “ideal launch angle percentage when ball is well-struck” data. Is this even a repeatable skill? Considering the league power run, it may very well be. This could be one of the missing ingredients for a backwards-looking HR/FB rate estimator (along with horizontal angle).

Last, what’s the deal with the HR/FB rate leap in 2017 that was not accompanied by a jump in Brls/BBE? Essentially, a barreled ball in 2017 flew over the fence more frequently than in 2016. Why? Is this more proof of a change in the ball?