The Bike Sauce | It Works!! // The $127 Shimano GRX Mullet Upgrade @TheBikeSauce | Uploaded January 2024 | Updated October 2024, 1 hour ago.
No Goat Links, no Tanpans, no 3rd party mods. An all-Shimano solution to an affordable, reliable, 11 speed GRX mullet setup for up to 10-51t cassettes.

The GRX RX822 SGS 12 speed rear derailleur is compatible with GRX 11 speed shifters like GRX 600 and GRX 800. You install the RX 822 like any other derailleur, but max out the high limit screw, which limits it to 11 speeds. You will likely need a longer chain to go with your wide range cassette of choice. This video features an 11 speed Deore 11-51T MTB cassette.

Shimano GRX RD-RX822 SGS 12 Speed Rear Derailleur:
https://shorturl.at/gsMV5 (Jenson link)

Shimano CS M5100 11-51t 11 Speed Cassette:
amzn.to/47swQiT (affiliate link, thanks!)

Check out Maverick Cycles if you're nearby in Whittier, CA
maverickbicycles.com

Credit to drinksta who posted this in the r/gravelcycling subreddit a couple months ago:
reddit.com/r/gravelcycling/comments/17qsvb7/it_works_11_speed_1151t_cassette_12_speed_grx