Here's the thing about the RX820 rear derailleur — it's the mechanical counterpart to the Di2 version, and if you're building a 2x12 or 1x12 GRX bike without going electronic, this is the one derailleur that covers both configurations. Shimano built the RX820 around 11-34T and 11-36T 12-speed cassettes, which puts it in the same gear-range territory as a road drivetrain but with the chain security and clutch behavior gravel actually needs. If you want the wider 10-45T or 10-51T mountain cassettes, that's the RX822 with a Micro Spline freehub — a different part number entirely.

Shadow RD+ is the piece doing the real work here. The friction-based stabilizer slows the cage from swinging fore-and-aft when the rear wheel bounces through washboard or braking bumps, which is where dropped chains and dead-slot shifts usually happen. Unlike a roller-type chain guide, it doesn't add drag to the drivetrain when the trail smooths out, and the ON/OFF switch at the cage pivot lets you dial the clutch off entirely for pavement sections or wheel removal. The 42-tooth total capacity handles a 2x setup with a 48/31 crankset paired to an 11-34, or a 1x build with the 11-36 cassette.

Cable pull is Shimano's road 12-speed ratio, so this pairs with ST-RX820 GRX levers or Ultegra R8100 / 105 R7100 mechanical shifters — not Di2, and not the older 11-speed GRX RX810. Chain choice should be Shimano 12-speed HG+ (CN-M8100 or the road CN-M8100-equivalent), and the recommended cassettes are the Ultegra CS-R8101 11-34 or the HG710-12 11-36. The direct-mount hanger interface is standard, and Shimano includes an OT-RS900 240mm outer housing section with a long nose cap in the box for a clean cable run into the derailleur body.... Read More