By: Noah Thurm Most successful major leaguers drive the ball in the air consistently with a slight upward swing, but at the extremes this approach may bring sacrifices. Players are often discouraged from a fly-ball oriented approach in the name of contact and whiff rates, but I think plate coverage is most strongly hurt by a steeper swing. I’m most interested in how much of the zone a player can effectively control, not just throw the bathead to, so in this article I’ll use the 2D area captured by all of a player’s barreled balls as they reach the plate, or “Barrel Area,” to break this down. The next step is estimating each player’s Attack Angle, or the angle of the bat path relative to the ground through the zone (large positive values indicate steep upward swings, negative values indicate the bat impacts the ball on the way down). Attack Angle isn’t directly measured by pitch-tracking technology, but with a little intuition and some physics we can get pretty close. The bat-ball collision brings us three angles of interest, the (usually) downward angle of the pitch (Vertical Approach Angle), the (usually) upward angle of the bat (Attack Angle), and the resulting angle of the batted ball (Launch Angle) (The centerline angle will contribute to batted ball spin, but isn’t of immediate importance here). Balls that are hit the most “square” or “flush” leave the bat with launch angles closely matching the attack angle of the swing, limiting launch angle “deflection”, as I refer to it (for a more in-depth explanation and implications for pitching strategy, check out my last article). This forms the basis for our estimation of attack angle, because we can be fairly confident that a player's hardest hit balls leave the bat at about their swing’s attack angle. To come up with the final estimation, I took the peak of the launch angle distribution (i.e. the most common launch angle) for the top 20% of each Major Leaguer’s batted balls by exit velocity. As a check, we can compare the estimations of attack angle to average launch angle and see that they track together quite well, with more barrels at higher angles as we’d expect: Charting those calculated attack angles against Barrel Area seems to answer the initial question pretty convincingly: A 13° attack angle maximizes the area covered on the regression line, with attack angle extremes at each end fairly quickly compromising plate coverage. Given that most pitches enter the zone at downward angles between ~3° and ~12°, that swing path “matches plane,” giving the hitter the greatest opportunity to get the barrel to the ball. Unsurprisingly, Barrel Area is fairly strongly correlated with a player’s barrel rate (r = 0.55) and xwOBA (r = 0.52), seen below. Estimating attack angle can also give us a glimpse into a player’s barrel control and barrel accuracy, as highly accurate players will minimize their average launch angle deflection, preventing mis-hits. Like barrel area, we see that an attack angle in the low teens seems to produce the most barrel accuracy: That being said, I’d caution the use of average launch angle deflection as a measure of swing control and discipline because it fails to accurately represent players who avoid mishits by just not hitting the ball at all in unpreferred locations. High whiff rates are disguised by the appearance of consistency in launch angle where a player tends to miss balls outside of their Barrel Area. To recap so far, we can be fairly confident that:
So is plate coverage a production panacea, and should hitters accordingly seek to maximize it by swinging with an attack angle in the low teens? As is so often the case, the answer isn’t quite that simple, and we can see it by looking at two seemingly opposite players, C.J. Cron and Freddy Galvis. Given what we’ve just established, aren’t these guys getting it all wrong? Galvis is covering the plate but not producing (4.43% Barrel rate), and Cron dominates a much smaller wheelhouse (18.5% Barrel rate). With that coverage disparity, you’d expect Cron to have a much steeper attack: Well, Cron is a power hitter, so probably produces more barrels by hitting the ball harder: Huh. If a Barrel is driven in the air, Cron definitely hits fly balls more frequently, right? Cron and Galvis swing nearly identically, hit the ball with equal authority on average, and drive the ball in the air at roughly the same rate. So what gives? To answer, we need to go back to one of the first pieces of intuition laid out: a player's hardest hit balls leave the bat at about their swing’s attack angle The flip side of this is that any ball not hit at around the attack angle loses some exit velocity on account of that “deflection” and the reduction in contact quality. Taking a random sample of 5% of the batted balls in the Statcast era (~30,000 events) makes this point quite clearly. The more a ball is deflected away from a player’s attack angle, the more velocity they will lose, and maximal exit velocities are clustered around a deflection of 0°. Taking that into account, the gap in barrel rate between Cron and Galvis begins to make sense, particularly once we add Max EV to the equation. Neither Cron nor Galvis has a very steep swing, so both players will need to go through quite a bit of deflection to produce a Barrel (defined by Statcast here). The problem for Galvis, then, is that he hits his fly balls at an average of 22° away from his attack angle! Using our modeled relationship between deflection and exit velocity, that produces a loss of more than 9 MPH off his maximum exit velocity. That effectively limits him to a maximum possible Fly Ball exit velocity of just 102 MPH, good for 280th in the MLB last year. It’s no surprise, then, that his average hovers around 89 MPH on fly balls, producing a hit probability of just .027! Average exit velocity is not terribly predictive, but in this case tells us the quality of contact Galvis can expect more regularly when deflecting the ball into the air. Starting with a lower Max EV, he simply cannot afford the decrease in exit velocity required to put the ball in the air swinging as he does. Cron, on the other hand, hits the ball harder, which means he has more exit velocity to “lose” as his launch angle increases, producing way more Barrels. He has more behind his fly balls, and with the swing discipline to stay in his (albeit smaller) wheelhouse, he is a more efficient and effective power producer with a nearly identical swing and batted ball profile. Being able to control the zone is helpful, but if expanding the “wheelhouse” compromises contact quality and creates more mishits, that coverage will not translate to production. Galvis has an above average barrel spread, but the vertical and horizontal deflection that more swing-happy approach generates hurts his ability to punish those pitches. If you cannot hit a pitch hard consistently, swinging at it is unlikely to do you much good. Ranking the variables we’ve discussed so far by their correlation with Barrel rate shows us this: We know we must hit Barrels to produce at a high level, so hitters need to prioritize how hard they’re hitting their fly balls. If you can get the barrel to any pitch chalk to chalk but can’t drive them in the air with much authority, you’re facing an uphill battle to be successful. This is an important realization that could nuance the way we think about swing path. In my experience, we tend to want to reserve steep swings for powerful hitters who can “afford” to hit the ball at higher angles because they have the exit velocity to turn those fly balls into homers and extra base hits. What Galvis’ production screams to me, however, is that it's guys without superhuman strength who could benefit the most from trying to induce a little more loft with a steeper swing. It is nonnegotiable that balls must be hit hard and in the air to maximize production, but a flat swinger cannot do that without compromising exit velocity. (Exit velocity to spare, Exhibit A. Attack Angle: 12°)
Hitters without exit velocity to spare would be best served by training attack angles that preserve exit velocity on fly balls by matching the desired launch angle. Put simply, if you want your hardest hit balls launched between 18° and 30° (where the most valuable batted balls are hit), you should think about swinging nearer to that range -- plate coverage be damned.
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