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 shifting quality could suffer.
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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).
Indexing, trimming, or fine-tuning Di2 rear derailleurs is done through micro-adjustment. Read how to set up your Di2 rear derailleur perfectly and ride without those grinding noises.
Read how to install the RD-RX805 / RD-RX815 / RD-RX817 Di2 rear derailleurs.
