Shimano's GRX gravel components lean towards road in performance and capabilities, but durability and special features also come from the mountain bike side of the design table. The RD-RX817 Di2 version offers the digital precision of Shimano electronic shifting and synchronized shifting. and is compatible with mtb cassettes with a 42T low sprocket max.The increased strength is built in to handle the rigors of constant abuse on rough gravel roads, or worse, yet Shimano has found a way to keep the components within close range of Ultegra R8000 road models. The 11-speed GRX RD-RX817 Di2 adds more from MTB technology into the rear derailleur to optimize chain tension for a more efficient, stable, and reliable 1x drivetrain over rough roads and terrain. With an integrated chain stabilizer mechanism, the GRX derailleur provides a quieter riding experience and shifting security with Shimano drivetrain precision on mixed terrain. For rides on smooth surfaces, the derailleur features an On/Off switch for the chain tensioner. The RD-RX817 Di2 rear derailleur is compatible with cassettes with a 40 or 42T low sprocket max for a 1:1 or better ratio with a 40 or 42T single chainring. Many aspects of this design were first released in mountain bike derailleurs for double chainring set-ups several years ago, and it was updated and improved for MTB electronic and mechanical 1x11 drivetrains - which greatly informs the RD-RX817 Di2 .

The now familiar Shadow cage and parallelogram design sits much closer to the frame and under the cassette, protecting the cage from ground contact in case of accident or from bike to bike interaction when transporting multiple bikes. The pathway the derailleur action travels when shifted is also more linear, and smooth. A shorter throw now results in quicker, more precise shifts using the RX817 with less effort, even over the wider spread of the 11-40 or 11-42 cassette. Adding that greater range opens more cyclists up to 1x all purpose drivetrains, for gravel, endurance, adventure, etc. One important difference between the mountain and road application of the Shadow design is the very welcome presence of a compact barrel adjuster for manual cable tension adjustments. The B-link standard hanger mount can be uncoupled from the direct mount pivot if your frame accepts the direct mount system.

Ultegra-level quality materials and finish means you get a robust derailleur body, links, and axles which are strong and light. Flouric coated link bushings keep the movement smooth and the linkage torsionally stiff and aligned. The stouter cage plates are made from machined aluminum, so you gain a few grams but gain max durability to go along with excellent cage stiffness. The cage design is torsionally and laterally stiff, resulting in reliable shifts and chain performance under load, no matter what ring and cog combination you find yourself in- all the way from 11-42. The upper and lower pulley wheels of the cage have specific profiles, and ride on Stainless Steel bolts that ensure longevity and resist corrosion.