Since the 1970s, skateboard wheels have been made from polyurethane, or Urethane for short, and are bought in packs of 4. Coming in a wide range of sizes and hardness, depending on the type of skateboarding you're doing. Going up to around 70 mm for cruiser and longboard wheels, and as small as 40 mm, but these days most people go for something around 51 mm - 58 mm. In addition to the size, wheels can vary in hardness, with old school soft cruiser wheels being around 78a - 80a durometer, and modern street, ramp and skatepark wheels being between 99a - 101a. Each of the 4 wheels take 2 skateboard bearings per wheel for it to spin and fix to the trucks' axel.