Upper and lower low-profile polyurethane roller spin on sealed bearings and keep your chain securely wrapped around the chainring delivering consistent and reliable performance. The polyurethane compound found in the S4 rollers was extensively tested to deliver the best blend of quiet operation and long term-durability. The polycarbonate bash guard utilized in the S4 guide protects the chain, rollers, and backplate from contact with rocks, roots, logs, or anything else for that matter and directs impact away from the frame.