Maybe the radial slicks having more give on the sidewall helps when not screwed to the rims? But us Drag Radial guys, I have a few buddies that hit the tire hard enough at the hit that the tire was moving 1" to 1-1/2" on the rim. I was marking the rims/tires myself before each hit. Maybe because a drag radial does not wrinkle up as much as a slick?

I still screw my drag radials to the rims however. Beadlocks are coming for me this winter anyway.