Originally Posted by elmo_4_vt
I guess I can't think of a real (physics) type explanation, but I would try taking some 1/4" flat stock and cutting/drilling holes into it, three for each side instead of the washers. That way, everything still has full contact between the hitch and frame, and there is a much smaller chance of things moving around.
Exactly waht I was thinking,
More contact area, much stronger, less chance of sheering off a bolt.