That's another good point, Bob. We found a cart at the dog shows that had 4 pneumatic tires on it, little itty bitty ones. They are 50 X 200 mm (wheel diameter right at 4" overall diameter 7.5") with a 35 psi max pressure rating. I think the cart is rated to 300 lbs., but it sure is easy rolling from parking lots to show rings across the grass and dirt. Maybe an eight inch wheel would work by keeping the CG down low. The only thing is, to make it work like casters would be a trick. Just a thought.