Mr. Tuffy is a remarkable tire liner made of durable, lightweight polyurethane. When inserted between the bike tire and the tube it provides a protective layer between road hazards and the inner tube Thorns or glass that may penetrate are blocked from the inner tube. Proven Tough-For over two decades Mr. Tuffy has been the world's leading flat-tire preventive with millions upon millions of satisfied cyclists. Mr. Tuffy tire liners are lightweight, weighing less than liquid fills and thorn-resistant tubes. The liners will not cause tire or tube damage nor loosen spokes and they give the bicycle a smooth, stable ride and add to bike safety.