Even if you've made it several seasons without smashing your rear derailleur, the pulley wheels will still wear out. If your pulley teeth have rounded out, or the bearings no longer spin freely, it's in your best interest to replace them to get the best shifting performance out of your drivetrain. These are genuine SRAM replacement pulleys for X-Sync 11-speed rear derailleurs, with stainless steel sealed cartridge bearings included. Each kit comes with two pulley wheels, enough for one derailleur.