Designed with your favorite classic Spitfire Classic wheel designs and manufactured with the quality high performance, Formula Four urethane. Formula Four Urethane lasts longer, withstands more abuse and performs superior to all other wheels. Designed to combine all the desired properties of a performance wheel without sacrificing any one attribute at the expense of another. Manufactured using a 100% pure urethane compound, Formula Four wheels out-performed and resisted flat spots longer with less wear than all other wheels. Higher rebound for faster speeds over rough and smooth surfaces with a hard durometer for a smoother, faster ride that won't mush out over time. High Rebound Compound Formula Four Urethane – Formula Four urethane is unlike any other. Both in the streets and under high-speed stress tests in Spitfire's lab, Formula Four wheels retained more material, coned less and kept their original shapes and sizes longer than any other wheel.