Every Di2 system needs a brain, and Shimano built it into the RD-RX825 rear derailleur. The derailleur handles wireless communication with the ST-RX825 shifters, hosts the charging port for the internal battery, and runs the multi-function button that lets you check battery level, pair devices via Bluetooth, and enter adjustment mode for both derailleurs. That consolidation eliminates the separate junction box that older Di2 systems required, which means fewer components to install and fewer potential failure points when you're fifty miles from pavement.

The GRX RD-RX825 uses Shadow RD+ technology with a gravel-tuned clutch that adds chain tension for rough terrain. The ON/OFF switch makes wheel removal straightforward when you flip it off, and when it's engaged, chain slap disappears and shifting stays precise over washboard roads and chunky gravel. The clutch tension is calibrated specifically for gravel riding rather than borrowed from the mountain bike side, so you get the chain retention you need without the stiff pedaling feel that comes from an overly aggressive clutch setting.

Cassette compatibility covers the 11-34T and 11-36T 12-speed options, with a 40-tooth total capacity that handles the 48/31T and 46/30T crankset pairings in the GRX lineup. The derailleur also works with road Di2 systems running Dura-Ace, Ultegra, or 105 12-speed shifters if you're mixing components or converting a road bike to gravel duty. Full and Semi-Synchronized shift modes let you configure front and rear shifts to work together automatically or keep them independent depending on how you prefer to manage your gearing.... Read More