Any experience purchasing these would be appreciated. Trying to manage two projects in a too small shop, I need a set of these to help maneuver vehicles. Summit has two different models, then some more expensive units and of course some uber expensive ones I will pass on.The summit lines will run about 130 to 160 for 4 dollies. The next step up is about 245 for a set of four.One last question: Are these designed so that the car can remain up on them or are you supposed to remove them from underneath and get the car back on the floor? Seems like leaving the car sitting up on them could cause some issues, like flattening the casters or overloading the bearings.Thanks, any knowledge will be appreciated.Neal ZImmerman, Eugene Oregon