I'm building a 18' revolve and expect a 18' X 14' flat constructed set on it with a little furniture and 4/5 people on it at any given time. My question is in caster land...do I want rubber or polyurethane casters, 4" or 3" (why) and how should I estimate load. Final question...I see than van and others place the casters in concentric circles leaving casters further apart as they move away from the hub....is there a reason for so many so close to the hub? I know that there has been a reasonable amount of discussion on the revolve/turntable but I did not see a definitive weigh in on these.