A bearing is the component that sits inside the wheel hub and allows it to roll. Bearings are measured with ABEC Rating and Skate Rating. To learn more about each rating system, check out our glossary. Bearings come in two sizes to fit on different truck axels. 8mm bearings are the standard size and the most common. 10mm bearings are used on trucks that are built for high speeds. Our professional staff is happy to answer any questions and help find the best bearings for you.