Formula Four Wheels retained more material, coned less and kept their original shapes and sizes longer than any other wheel. Formula Four Urethane is made using a 100% pure urethane compound. On the hills, in the streets and parks and in forced flat spot testing in our lab, Formula Four wheels out-performed and resisted flat spots longer with less wear than all other wheels. Formula Four's exclusive combination of High Rebound and Hard Duro produces a faster wheel everywhere. Higher rebound for faster speeds over rough and smooth surfaces. Hard Duro for a harder, faster ride that won't mush out over time Formula Four's exclusive anti-stick slide gives a fast/consistent, smooth slide all the way through.Slide with confidence start to finish knowing your wheels won't skip or catch mid-slide.