Skateboard wheel hardness is measured on a durometer scale. The level of hardness refers to how hard the material used to make the wheel is. The durometer is measured on a scale to 100. There are 2 different scales, the A scale and the B scale. The B scales measures hardness at 20 points lower than the A scale meaning an 80B wheel will be the same hardness as a 100A wheel. Particularly hard wheels, like that professional skateboarders will ride will be 80B+.
Generally the softer the wheel, the slower and smoother the ride. Soft wheels are generally found on longboards and are often preferred by street skaters who need more grip and forgiveness when rolling over rough surfaces, cracks and pebbles.
Hard wheels on the other hand are fast and offer less grip. These are suited to smooth surfaces and vert ramps where there is no loose debris to contend with.
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