Skateboard wheels are designed in many different sizes and hardness. Skateboard wheels are made of a rubber-like material called urethane. The size and hardness of the wheels affect the smoothness of the ride on a skateboard, how well it handles, and its speed. Skateboard wheels are normally 52 mm to 60 mm and have their own ball bearings built in. Street skateboard wheels are usually very hard, which allows them to slow down quickly and store energy as you move along. Park wheels are usually made of a softer urethane for better bite and handling. Modern skateboard wheels are bidirectional, meaning that they can be attached in either direction unlike the wheels on a car or the back wheel of a bicycle. This means that if the edges start to wear, you can flip the wheels and use them on the other side. Skateboard wheels are attached to the trucks by hex head axle bolts and ABEC (or another company’s) ball bearings to provide the free-spinning movement of the wheels.