Skateboard bearings are made with steel ball bearing and are the round metal ring-shaped pieces that fit inside the wheels and mount the wheels to the axle. Regardless of wheel size, all skateboard bearings are the same size and will fit any skateboard wheel. The universal measurements for bearings are 8mm (core), 22mm (outer diameter), and 7mm (width). Skateboard bearings are sold in sets of eight, as each wheel requires two bearings. Most skateboard bearings on the market today are measured by an 'ABEC' rating system. This rates the quality of materials and precision used in manufacturing the skateboard bearing. A higher ABEC rating means higher quality and higher precision. This ABEC rating system includes grades 1, 3, 5, 7, and 9. A higher ABEC rating means higher quality and high precision, and ultimately more speed, longer durability, and less friction while you ride.