Skateboard wheels, usually made of polyurethane, come in many different sizes and shapes to suit different types of skating. Larger sizes like 65-90 mm roll faster, and also move more easily over small cracks in pavement. Smaller sizes like 48-54 mm keep the board closer to the ground, require less force to accelerate and produce a lower center of gravity, but also make for a slower top speed.
Wheels also are available in a variety of hardnesses usually measured on the durometer 'A' scale. Skateboard wheels range from the very soft (about 75a) to the very hard (about 99a). As the scale stops at 100a, any wheels labelled 101a or higher are harder, but do not use the appropriate durometer scale. Some wheel manufacturers now use the 'B' or 'D' scale, which has a larger and more accurate range of hardnesses.
Modern street skaters prefer smaller wheels (usually 48-55mm), as small wheels make tricks like kickflips,and ollies easier. Street wheels also need to be quite hard, as small soft wheels absorb too much energy.
Vert skating requires larger wheels (usually 55-65mm) as vert skating involves high speeds that smaller wheels are unable to sustain. Vert wheels are usually very hard, so they can roll faster. As they are only used on ramps and parks that are smooth they are usually on the harder end of the durometer scale.
Even larger wheels are used in longboarding and downhill skateboarding. Sizes range from 65mm right up to 100mm. These extreme sizes of wheels almost always have cores of hard plastic that can be made thinner and lighter than a solid polyurethane wheel.
