Pulley wheels are considered a high-wear part and sees a lot of action in the field. Overtime the pulley wheel's teeth and bearings reach the end of usable service life and require replacement. This Chorus derailleur pulley set kit is sold with upper and lower pulley wheels and intended for factory replacement of Chorus or Athena 11 speed rear derailleur pulley wheels. The wheels are constructed from a dense rubber compound, which grips the chain while it is being passed through the derailleur. The high quality bearings roll effortlessly and offer practically zero drag and resistance. Derailleur fixing bolts are not included; these bolts are expected to be reused from existing rear derailleur.