Here Is Your Fly-Ball Revolution

My sense is that the fly-ball revolution isn’t something I even need to explain. You know the argument, you know the theory, and you know about some of the most successful cases. Ground balls are mostly unhelpful, right? So why not…not…hit ground balls? Francisco Lindor has attempted to stop hitting ground balls. He’s doubled his home-run total, from 15 to 30. Good luck finding someone who scouted him in the minors who thought he’d hit for that kind of pop.

Yet, largely, the fly-ball revolution’s very existence feels anecdotal. One can’t help but notice the league-wide numbers, and how little they’ve shifted. Compared to last year, this year’s average ground-ball rate is down half a percentage point. The league has hit 44% ground balls. A decade ago, the league hit 44% ground balls. Who cares? Is anything actually going on?

It is. It’s just a little bit hidden. Thanks to the wonders of Statcast, we can see where the league has been actively changing.

As is always the case, Statcast information has been pulled from Baseball Savant. Information exists for the better part of three years, and although three years isn’t very much, we can still put the data set to use. Let’s begin with simplicities. Here are the league-average launch angles, by season.

• 2015: 10.1 degrees
• 2016: 10.8
• 2017: 11.1

Slightest hint of a trend there. The fact of the matter is that launch angle has increased by one degree from a couple years ago. But this isn’t the kind of thing you’d notice if you didn’t go looking for it. It’s subtle. And to go with the above, here are the league-average exit velocities, by season.

• 2015: 87.3 miles per hour
• 2016: 87.7
• 2017: 86.7

Pretty weird, right? Between 2015 and 2016, league-wide isolated power increased, and league-wide exit velocity increased. Between 2016 and 2017, league-wide isolated power has increased again, yet league-wide exit velocity has decreased, by a full tick. A full tick feels substantial. If the numbers are to be believed, this year’s hitters are hitting the ball more weakly than they used to. Suffice to say I found that surprising.

Yet, that trend can be analyzed further. Let’s take it as granted that exit velocity is down, overall. Exactly where is it down? Is it down by a similar amount across all launch angles? In the following plot, I’ve gone with the bucketing method. I’ve combined 2015 and 2016, and compared those years against 2017. Here is exit velocity by launch angle.

At the lowest angles, EV is down more than two ticks. To be precise, it’s down 2.4. Between the angles of -30 and -20 degrees, EV is down 2.0. Then, going left to right, down 1.5, then down 0.3. Then on and on, until eventually it’s down just 0.1, then up 0.3, then up 0.5. At the very highest angles, batters have hit the ball slightly harder. It’s very slight, but the trend is consistent. Moving left to right in the plot, the differences get decreasingly negative, and eventually positive.

It’s worth taking a look at frequencies, as well. How have batted balls been distributed, by launch angle? This plot does the same as above.

At the lowest angles, you see an increase in frequency in 2017. But then there’s a pretty significant decrease between -20 and 0 degrees. At all positive angles, there’s been some kind of increase. The way I interpret this is that hitters are indeed trying to hit more balls in the air. I think the increases at the lowest angles reflect mis-hits. If you try to hit a ball in the air but get your timing wrong, you’re likely to hit a weak grounder, and a weak grounder frequently goes almost straight down. This is a theory of mine, and it’s far from being anything proven, but we’re seeing more air-ball contact and lousy ground-ball contact, while also seeing less decent-grounder contact. I doubt that’s a coincidence.

In case you’re curious, here’s how wOBA on batted balls has changed, again by launch angle.

I don’t know how to explain the increase between -20 and -10 degrees. I’m open to your interpretations. Otherwise, everything looks stable, until you get to 20 degrees or so. This is true even if you compare 2017 against just 2016, when the home-run spike truly blossomed. Production is up significantly between 20 and 30 degrees, and it’s also up significantly between 30 and 40 degrees. These are the home-run angles, the extra-base-hit angles. Therefore, there’s more damage to chase at higher angles, and this is why batters want to hit more balls in the air in the first place. In part, I think this reflects intentionality — batters are doing better at those high angles because they’re trying to do better at those high angles. And, in part, I think this reflects why the high angles are even increasingly desirable. The ball’s taking off like never before. Anyone who used to have even middling power is now far more of a home-run threat. Recall, again, the 30 home runs by Francisco Lindor. He wouldn’t even be trying to do this if there weren’t something substantial for him to gain.

The fly-ball revolution is happening. It’s just in the earlier stages, and you have to work in order to find it. It’s not like the league can ever just completely stop hitting balls on the ground; pitchers have a say, too, and they want the hitters to not be productive. There are opposing forces at play. But, remember how the league-wide average launch angle is up 0.3 degrees, compared to last year? Looking only at balls hit at least 90 miles per hour, the league-wide average launch angle, compared to last year, is up 1.5 degrees. The good contact is finding more air, which is the whole point. Bad contact can stay on the ground. If it’s bad enough, it can even go all the way back around to good. Batters, increasingly, are chasing liners and flies. It’s happening, and it’s probably far from finding its own equilibrium.