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Mini Blog 5: Validating Stalker Pro IIs

I ran a little study with a few friends this past winter and am just getting around to finalizing the results. Most of what we looked at will be presented in a much longer post that is nearly complete. But we ran another small test within the larger study, attempting to validate the Stalker Pro IIs. (TLDR: It works.)


There has been a ton written on different radar guns, the most comprehensive of which I know of being by Zac Morain (Part 1, Part 2, Part 3). Rapsodo 1.0 and 2.0 were both validated against Trackman, the gold standard of somewhat portable movement tracking systems, by Driveline. They also ran a Trackman validation study. I believe the current system used in the big leagues is validated with a fancy slow motion camera system, but unfortunately I can’t find the link.

I have yet to see any validation of the newish, spin-rate-reading Stalker Pro IIs radar gun.

Anyway, I wanted to test it out, and the wonderful folks at Haverford College, my alma mater and former stomping grounds, let me use their new Stalker Pro IIs and OG Rapsodo 1.0 for a few days.

We measured 130 fastballs from 1253 to 2565 RPM, taking measurements with both devices. We threw out any readings that were bonkers. Then we ran a simple correlation. The R-sqaured came out to .99.

As you’ll see in the graph below, there is not much data in the middle. We had 3 former players throwing in the mid 60’s to mid 70’s and one throwing in the low to mid 80’s, all with different spin profiles and none of whom consistently spun the baseball in the 2000-2200 RPM range. While it would be great if someone else wanted to give this test a go, I’m pretty confident that the Stalker IIs accurately measures spin rate.


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