Saturday, July 02, 2022
Statcast Lab: When Angles Attack
As we are continuing our look into the Bat Swing data we've been receiving, a centrepiece article that you should read or otherwise keep in the back of your mind is from Alan Nathan from 2015, who naturally has been doing this for even longer and that article is a terrific summary of everything he had learned, put in a way that a baseball fan can appreciate. We're just standing on the shoulders of giants here.
One of the key pieces is the Attack Angle, which put simply is the trajectory of the bat at the collision point. To put it a bit less simply, it is the tangent line to the path of the bat. To put it even less simply, it is based on the velocity vector, normalized ("unitized"). While we are interested in the angle at the collision point, we can actually calculate this angle along any part of the trajectory of the bat. All we need to do is calculate its velocity vector.
The vector can be helpfully be broken down into a vertical component and horizontal component. The vertical component is a contributing factor to the resulting launch angle. The horizontal component is a contributing factor to the resulting spray direction.
It's always helpful to use a perfect launch swing, and those are easy enough to find: look for long homeruns, like this video. And the Attack Angle for the swing, from the start to the end, can be seen in this chart (click to embiggen), where 0 is at or very close to the impact point.
So what are we seeing exactly here? It's probably easiest to start with the bottom chart, even though that will be the less interesting one. The swing goes full circle: it starts at one horizontal angle, and eventually comes full circle back to the same angle. If you can remember that plus 180 degrees and minus 180 degrees is the same point, then you can see how the swing indeed comes full circle. At the impact point, it's at minus 10 degrees. In other words, the bat path is 10 degrees to the pull side, which is pretty normal for a HR that was pulled. Had the contact happened 4 milliseconds earlier, the bat path was at close to 0 degrees (straightaway). When we talk about a game of inches and milliseconds, we mean it quite literally.
We do see a dip right after the collision point. That, I believe, is an artifact of the way the data is generated. In order to create the bat path, the swing is broken down into a pre-impact and post-impact trajectory, and then "connected". When you connect two disjointed lines like that, things like this will happen. Eventually we should create a process to smooth this out, but we're still in the development phase here, and we're sharing as we're learning.
Now, the more interesting part is the top part, the vertical component of the Attack Angle. This is because the launch angle is far more important for the batter than the spray direction. And we see it follows a golf type path where the vertical angle dips heavy into the negative at minus 50 degrees, at a point 50 msec from impact. In other words, had the collision point happened at that point, that bat would be hitting the ball at a heavy downward angle. In the actual case, he hit it perfectly, at an Attack Angle of +17 degrees (in the vertical portion). If you check out Alan's article, you will see "Table 2: Optimum swing parameters for maximum distance", with a value of around +18 degrees. Score another one for creating a model to match eventual reality. And like above, being early by 4 milliseconds would have made a big difference: + 9 degrees instead. (And that blip exists here as well. Again, just something we have to work our way through.)
While a swing speed is critical, and being able to make contact at the sweetspot is important, the most important part is in fact making contact in the first place. It's obvious to say, but MLB players are so incredibly gifted, this assumption of fact at this level is nowhere close to a given at the lower levels.
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