The mechanical Shimano GRX rear derailleurs, RD-RX810 and RD-RX812, are characterized by quiet, smooth, precise and reliable shifting even in the most demanding terrain. This is mainly due to the Shadow RD+ chain stabilisation system which prevents the chain from slapping and dropping as well as from unnecessary movements of the derailleur's cage, thus ensuring uninterrupted shifting performance. That is why an On/Off switch can be found next to the upper pulley, much like Shimano’s mountain bike derailleurs. When activated the rear derailleur pivot takes a firm hold to reduce excessive movement, noise and general "chain chatter".The RD-RX810 rear derailleur will accommodate cassettes with 11-30 teeth up to cassettes with 11-34 teeth. Instead, the RD-RX812 rear derailleur is suitable for 11-40 and 11-42 teeth cassettes.