[Closed] tow ball mounted racks-how do they stay on and not slip about?

you do up a cup that grips the ball.

I've stood on my Thule towball mounted rack and it's not slipped.

do they grip around the circular bit of metal that the tow ball is cast as part of or do they just grip the ball part? if they only grip the ball,i dont see how they cannot move?

if you look closely, a towball looks like a D not a sphere, the racks count on that flat bit

any low down racks that the bikes sit "on" that mount via the two bolt holes for the tow ball?

? as in lower than the towball? Probably ground clearance issues there. Level with or above the ball mounted on the backplate, Thule, Pendle, Avenir, etc etc the manufacturers all do something