The Z Files: Average Exit Velocity and Launch Angle

Recently, RotoWire's Jeff Erickson looked at a few Statcast metrics, including batter's average exit velocity and pitcher's spin rate. Today, I'm going to delve into average exit velocity on a more granular level. Next week, it will be spin rate's turn.

The impetus for this analysis is "average". That is, like many metrics including spin rate, exit velocity is presented as the average over all batted ball types. The problem is different batted ball types have their own average exit velocity, so an individual player's mark needs to be considered in context with their hit distribution. This doesn't mean every player's ground ball, fly ball and line drive velocity is the same, just their batted ball distribution influences their average.

For the purpose of this study, batted balls will be broken into grounders, fly balls and outfield line drives. Data for each will be presented, followed by a look at some of the more intriguing batters to this point of the season.

Infield line drives and pop ups are excluded. They're obviously relevant on an individual basis and could come into play when dissecting specific batters.

GROUND BALLS

Big picture, it's fair to wonder if the decrease in BABIP this season is shift-related, or just noise. The three percent drop in frequency of grounders over the last five years is consistent with the launch angle revolution.

The consistent average exit velocity of ground balls is interesting. When many hear juiced ball, they assume it's bouncier, which should manifest in an increased average exit velocity across the board. However, the early-season research on the baseball pointed towards reduced drag once it's in flight as the actual cause. This data corroborates those findings. It's not definitive proof, but it does support those findings.

FLY BALLS

There's a lot going on here. Obviously, there have been more fly balls struck, with a greater exit velocity. The spike could be attributed to a bouncier ball; however, the previous data suggest it could be more aggressive swings.

The BABIP on fly balls is on the rise, which is probably the result of more of them going over outfielders' heads, perhaps off the bat of non-power hitters taking advantage of the reduced drag. Look at the AVG on fly balls, which unlike BABIP includes homers. While it makes sense it's surging with the record-setting pace for long balls, the delta between AVG and BABIP serves as a great reminder that BABIP needs to be used in context. Not every player's lower than historical BABIP is due to regression, especially in today's climate. If said individual has increased their homer output, many hard-hit balls that used to be doubles or triples are now clearing the fence, affecting the numerator more than the added fly ball outs influence the denominator.

It's so small, it's likely noise, but the average launch angle on fly balls is creeping downward. That's probably insignificant. The fact the average launch angle hasn't increased with the added uppercut swings is intriguing, though.

OUTFIELD LINE DRIVES

Concurrent with the growth of fly balls is a small drop in outfield liners. While the added fly balls embellish homers, the drop in outfield line drives reduces outfield hits that stay in the yard, as exhibited by the huge decrease in outfield line drive BABIP.

The stable exit velocity again supports a ball incurring less drag as opposed to a higher coefficient of restitution (bouncier ball).

Note the difference between BABIP and AVG for outfield line drives. This represents those home runs classified as line drives. There's not nearly as many as fly balls, which makes intuitive sense, but it could be relevant on an individual basis if a player has launched an inordinate number of line-drive homers.

Something to keep in mind is the electronic collection of data vastly improves batted ball classification. Still, there must be a cutoff somewhere, and those balls near the boundaries display characteristics further from the average in whatever bucket they land in. Looking at the spread within each designation is a study for another day.

Before putting some players under the microscope, he's the data germane to homers.

Again, these are averages. Most of the measured data is remarkably consistent, save for a slight increase in exit velocity the past two seasons. A plausible explanation is that hitters are stronger and hitting the ball further, but the impact is compensated for by less powerful hitters hitting more short homers.

What's obvious is a refinement in batted ball classification, which appears to have settled but more seasons are needed to know for certain.

Let's shift attention to a trio of batters who all happen to be in the National League. Sorry aficionados of the junior circuit, there just aren't any hitters as compelling as Derek Dietrich, Christian Yelich or Cody Bellinger. Before going on, please keep in mind the bulk of the presented data is descriptive, not predictive, so any conclusions are subjective in nature, but backed by tangible numbers.

Derek Dietrich, Reds

There are two questions with Dietrich: Will he keep the elevated power. and how unlucky is his .212 BABIP? First, the power.

Dietrich's fly ball and outfield line drive launch angles are in line with the league. However, his average exit velocity on both exceeds the norm, by a decent amount. Reiterating the descriptive and not predictive nature of the data, early indications are a batter owns his exit velocity faster, with launch angle being more variable. This is a great sign for Diedrich and his ability to keep pounding dingers at a career-best pace. The main concern is he'll loft fewer batted balls. Fly ball rate also takes awhile to stabilize, and Dietrich is elevating at an extreme rate.

Dietrich is a perfect example of a batter whose lower than usual BABIP is in part a function of a power surge. His career BABIP entering 2019 was .308, with a .325 mark from 2015-2017. So far this season, he's about 100 points shy of recent levels.

More fly balls along with getting no credit for hard-hit balls clearing the fence are the main culprits. However, note the lesser exit velocity on grounders as compared to the league. This explains the lower ground ball BABIP and suggests it isn't due to luck.

Dietrich's BABIP and AVG on outfield line drives is also well below league norm. Maybe he's been unfortunate, but there's a good chance he's a victim of his own success with outfielders playing him deeper than most batters, taking away long hits. In addition, the change in home venue hurts him in this regard, as there's less room to cover in the Great American Ball Park compared to Marlins Park.

It's highly probable, if not certain, Dietrich has incurred some bad luck on balls in play. However, those expecting a reversion to career levels while he also maintains his power will be disappointed. This is a true case of selling out for power, though since homers are hits too, Dietrich's average isn't suffering.

Cody Bellinger, Dodgers

It's one thing to note Bellinger sports a 36.4 percent line drive rate and thus expect regression. It's quite another to see it broken down in this manner. The man is simply crushing the ball, regardless if he's facing a righty or lefty. Without knowing exactly how many outfield line drives just missed the fly ball cutoff, speculating on the distribution the rest of the season is anyone's guess. However, there's no doubt Bellinger is living off the outfield line drive. Not only is he stroking more of them than anyone, his BABIP is well above league norm. Further, one-quarter of his homers are classified as outfield line drives, far above league average.

Something not presented in this data is a significant uptick in contact, so even if Bellinger incurs regression in the form of fewer line drives and more fly balls, his power should remain elite and his batting average won't crater.

Christian Yelich, Brewers

The most fascinating element of Yelich's game isn't so much his ability to maintain power with fewer balls in the air. At least for me, it's the ridiculous exit velocity on ground balls. Everything he hits is hard. At least Dietrich and Bellinger's ground ball exit velocities are league average. Yelich excels across the board.

As stated earlier, we're still in the embryonic stage of this research, but early indications suggest a player's average exit velocity stabilizes sooner than launch angle and most of the other leading indicators. This bodes well for Yelich. Sure, maybe he reverts to hitting more grounders, but his average will soar even more than now.

We're barely scratching the surface with this level of analysis. Over the years, I've found it beneficial to dig in early, even if the conclusions are ultimately proven wrong, as the sample increases and analytical processes are refined. Not to mention, it's fun to look at the league numbers broken down in this manner. Next week, pitching spin rates will be afforded similar treatment.