Friday, June 17, 2022
Statcast Lab: What is Swing Speed?
This seems like this should have an obvious answer, but it does not. What is Swing Speed? Let me count the ways. There are at least FIVE reasonable answers to this question.
- You can decide to measure the linear speed at the head (top) of the bat. That seems to give you a constant number. It won't. This is especially true when the rotational point is something other than the knob of the bat, which happens when you lunge for a ball for example.
- You can decide to measure the linear speed at the presumed "sweetspot" (about 6 to 7 inches from the top of the bat). This seems even better, since that's (mostly) where the collision happens. It's still not constant though. The rotational point from above applies here.
- You can decide to measure the linear speed at the collision (impact) point. This is where the energy transfer actually happens, and so, where the calculations are centered. Which makes good enough sense. Except what do you do for non-collisions, like swing and miss? Even if you decide to use a "default" sweetspot point for swing-and-miss, you won't have an actual collision time, and so, WHEN do you take the speed of the bat? And imagine when you just "barely" touch a ball nowhere near the sweetspot: now you have different places for measurement based on outcome, which is NOT something you want to do.
- You can take the angular speed. This would at least seem to be a reasonable baseline, bypassing the need to know the impact point. Think of a record player, where it doesn't matter what kind of disc you put on the turntable, it'll spin at the same (angular) speed, even though the outside of the disc is spinning at a faster (linear) speed (the outside of the disc covers more linear distance than the inside, but both travel the same amount of time). The unit here is radians per second. So, 50 radians per second means... what exactly? It means 8(*) rotations per second (if you try to use the turntable analogy), but what does THAT mean either? Maybe a Bugs Bunny cartoon would like to represent a swing as 8 rotations per second. The swing-and-miss timing issue still applies.
- Finally, we can take advantage of the angular speed by establishing a constant point, like 27 inches from the rotational point (I'll call this R27). 27 inches conforms roughly to the sweet spot. A bat is 33.5 to 34.0 inches, and so that puts the sweet spot at 6.5 to 7 inches from the head (assuming the rotational point is the knob). An angular speed of 50 radians/sec is 76.7 mph (**). This at least gives us the units to tie swing speed to launch speed, and at a point that is roughly where the energy transfer is targetted to happen.
(*) A circle is 2 x PI, or about 6.28 radians. 8 rotations is therefore ~50 radians.
(**) 27in / 12in/ft x 3600s/hr / 5280ft/mi = 1.534 mph x sec, and that's the multiplier for angular speed (1/sec) to R27 (in mph)
What is Swing Speed? It really depends on the question. And you can have five possible answers.
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