The wider chainline that makes GRX cranksets accommodate bigger tires also means the front derailleur has to sit further outboard than a road unit—which is exactly why dropping a Dura-Ace or Ultegra Di2 front derailleur onto a GRX crankset doesn't work cleanly. Shimano's GRX FD-RX815 is the Di2 front derailleur built specifically for that 2.5mm outboard chainline, letting you run the wider tires GRX drivetrains are designed around without fighting chain rub or mis-shifts. It's the mechanical answer to running a 2x gravel setup with electronic shifting.

The derailleur handles a 17-tooth capacity across 46-50T top gears, which covers the 48-31T combination on the FC-RX810-2 crankset as well as smaller sub-compact gravel gearing. Cage geometry is tuned for the wider chain line, and programmed trim positions adjust automatically as you move through the cassette—so you're not manually trimming the front to eliminate rub when you drop to the lower gears on the cassette in the big ring.

Because it's Di2, you also get access to Synchronized and Semi-Synchronized shift modes through E-Tube. Full Synchro treats the two derailleurs as one system, throwing the front shift automatically when the rear reaches the programmed sprocket. Semi-Synchro keeps front shifts manual but compensates the rear by a couple of cogs to smooth out the ratio jump when you change chainrings—useful on rolling gravel where you're changing rings more often than on the road.... Read More