Corona AC 2500 Flat Free Knobby Tire Flat-free micro-cellular polyurethane foam consists of trillions of air cells The polyurethane foam is self-skinning foam which creates a tough outer skin that is two times stronger than rubber Includes ball bearings for years of smooth performance and grease fittings for easy lubricating Knobby tires provide added traction and longer wear The non-marking skin is abrasion and chemical resistant