We used those on the roundy-rounders, they're magnetic and clamp to the end of the hub. We used to run different caster on left & right, you could transfer weight to the opposite rear corner by cranking in a lot, make it bite better in the corners.
BTW, that's how they did the crash scene in Vanishing Point, cranked in all the caster it would take so it would run straight without a driver, set it straddling the road crown, and towed it with a long cable. Just to make your day, the car used for the crash was actually an early Camaro LOL
