Every Di2 12-speed drivetrain needs a central hub for wireless communication and charging, and Shimano put all of it into the rear derailleur housing on the GRX RD-RX825. This is the 2x12 gravel-specific Di2 rear derailleur, and it handles the wireless connection to 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 via Bluetooth, and enter adjustment mode for both derailleurs. If you've been running an older Di2 build with a separate junction box or bar-end charging port, this consolidation is the biggest architectural change to get used to.

The Shadow RD+ clutch is the other piece that defines this derailleur, and Shimano tuned it specifically for gravel rather than borrowing the mountain bike calibration. Chain slap disappears with the switch flipped on, and shifting stays precise over washboard and chunky gravel because the cage holds tension against the chain instead of letting it bounce around. Flip the switch off when you're pulling the rear wheel and the cage swings freely, which makes tire changes and travel disassembly a lot less of a fight.

Cassette compatibility runs 11-34T and 11-36T on 12-speed spacing, with a total capacity of 40T and a maximum front difference of 17T. That covers the standard GRX 48-31T crankset plus the wider-range cassettes gravel riders actually reach for. The derailleur pulley cage uses 11T upper and 13T lower pulleys, and mounting is direct attachment with direct-mount hanger compatibility for frames that support it. Full and Semi-Synchronized shift modes are both configurable through the E-Tube Project Cyclist app.... Read More