Product details Formula X Formula 97A. Durometer/Hardness 97A. Wheel Diameter = 54mm / Wheel Width 32mm. Wheel Shape V1 Standard | Perfect Balance of Radius, Width, and Speed for best All-Around Performance. The amazing new X-Formula (XF) is another industry leading innovation from the BONES WHEELS factory. X-Formula rolls over rough terrain like softer wheels do, but slides much like a harder wheel