Powell Peralta ATF 66mm 78a Black Skeleton Hands Wheels (Set Of 4) Created especially for rolling over loose gravel and other rugged surfaces, yet an amazingly fast and smooth ride everywhere else Gravel Grinders feature our Ultra High Rebound(UHR) Energy Core that absorbs the bumps from the harsh terrain and uses that energy to push you forward over it To top it off, the Energy Core is wrapped by our premium All Terrain Formula (ATF) urethane Its unreal Give your push foot a break and experience a new way to roll.