Wheel hardness is measured by a durometer, an instrument used to measure the hardness of rubber or plastics. The hardness is then assigned a numerical rating along with a durometer rating in the following types: A, B, C, D, DO, E, O, OO, and OOO. Skateboards traditionally have harder wheels (between 99a-101a), while longboards are a bit softer (75a-80a). Longboard wheels provide a smoother, softer ride and offer more grip. Harder wheels on skateboards provide a rougher ride but are better for skateparks.