Rear Derailleur (1x)
If you've been looking at GRX Di2 components you may have seen that there are two rear deraileurs. This RD-RX817 and its lightweight cousin, the RD-RX815.
The RD-RX817 was developed by Shimano specifically for use in 1x bikes. This means that running this rear derailleur with a front derailleur is officially not supported. There are reports of users installing a front derailleur anyway, like this user on bikeforums.com. If you do, keep in mind that this is not Shimano approved and your experience (and possibly safety) could suffer.
This RD-RX817 has a total capacity of 31T, allowing you to run an 11-42T cassette. Its top sprocket max is 11T and the low sprocket max is 42T (min 40T).
Both GRX rear derailleurs feature a clutch mechanism, just like the Ultegra RD-RX805 and the mountain bike rear derailleurs. Shimano call this the chain stabiliser. It can be adjusted and prevents chain slapping (or worse, dropping your chain).
Details & Specifications
|Shimano model number:||RD-RX817|
|E-Tube Port type:||SD50|
|Component series:||GRX RX815 (Gravel)|
|Downloads:||Di2 Rear Derailleur User Manual|
GRX RX815 Dealer Manual