The wheels on a skateboard are called wheels. They are typically made of polyurethane, which is often referred to as urethane. Skateboard wheels are measured by their size and hardness. Skateboard wheels range in size from 50 mm to 70 mm+. They have a hardness scale (durometer) that goes from 78A to 100A+. Smaller and harder wheels are better for skate parks and smooth surfaces. Larger and softer wheels are more suitable for cruising, rough terrain, and street skating.