The wheel frames and hardware on all of our sliding barn door hardware kits are made of either carbon steel and/or stainless steel; however, the actual wheel portion of the hangers are made of Polyoxymethylene, or POM, an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability.The wheels also utilize ball bearings for an extremely smooth and quiet operation.*MJC wheels have been tested well beyond 100k consecutive rolls.