Jim, Thanks for the info on the bottom hinged doors. I never would have spotted that. I imagine when they're open they're suspended in a horizontal position and don't actually touch the ground. For this to work, the door support needs to be strong enough to kneel on and to take the weight of whatever you happen to be sliding across it. Hinging on a vertical edge certainly makes more sense, but for whatever reason I guess it isn't (or wasn't) feasible.