The Shimano GRX rear derailleur, with its integrated chain stabilizer, ensures quiet, safe, and precise shifting on mixed terrain. For riding on smooth surfaces, the chain stabilizer can be deactivated using the ON/OFF switch on the derailleur. The Di2 version of the RD-RX817 offers the digital precision of Shimano's electronic shifting combined with Synchronized Shifting and is compatible with road cassettes with a maximum 42T on the largest cog.