Rubber covers are "hollowed out" with some "cross bars" on the bottom for more stability. They are a bit light (in weight) so I filled some holes in the bottom with silicone and they work great. They'll probably work fine without the silicone filler, I just want a little extra bottom stability.
Milwaukee 33815 Carpeted Furniture Hardwood
10 Review
Portable Aluminum Stair Climber Cart With Rope Handle: The Ultimate Heavy Lifting Solution
33 Review
Heavy Duty 265 LB Capacity Foldable Hand Truck Dolly Cart For Moving, Portable Collapsible Luggage Trolley With 4 Wheels For Office Travel Shopping Use - Black
23 Review
Effortlessly Move Heavy Appliances With DOZAWA Telescopic Furniture Dolly - Adjustable And Secure With 4 Locking Wheels - Perfect For Washing Machines, Refrigerators, And More
14 Review