Ceramic (Si3N4) with Teflon. These jockey wheels are perfect when you want to save as much energy as you can on long distance rides. Decreasing friction was key when developing this product. The bearings are made out of Si3N4, a kind of ceramic known for its low friction character, and lubricated with grease to minimize maintenance requirements. Teflon is added to the wheel, which reduces friction with the chain. Finally, a low-friction seal is used to reduce friction with the bearings to a minimum. All these aspects together make this a one of a kind jockey wheel with which you can save energy on a long ride, but also with regards to maintenance.