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Shimano GRX Di2 Setup using RD-RX817 in 2x Configuration
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Hoping this helps someone who might be considering the same thing, that is to try to get a much better gear range than that which is standard using Shimano's 1x or 2x GRX. I mainly want to do this so that the bike can be more suitable for bikepacking.

I generally work in Gear Inches when looking at Gear Ratios. And the GRX 2x using 11-34 Cassette and 48:31 Chainring gives a range of 117.8 to 24.6 Gear Inches (using Sheldon-Brown's page). Ideally I'd want something in the range of 100 to 20 for Bikepacking.
With a 1x GRX setup. 11-42 Cassette and 40T chainring the range is 98.2-25.7 Gear Inches. Even with pushing the cassette up to 11-46, you still only get down to 23.5 Gear Inches.

Theoretically if you could pair an 11-42 Cassette (I'm not proposing to push as far as 11-46 at this stage) with the 2x GRX chainring of 48-31 you get a range of 117.8 to 19.9. I think this would be just about ideal. I think it is a gearing range of 592%

This short video is just to show the setup and that it works.
It uses the following key components:
Shimano GRX FC-RX810-2 48-31T 2x11 Chainring.
Shimano XT CS-M8000 11Sp Cassette 11-42T.
Shimano GRX Di2 Rear Derailleur RD-RX817.
Shimano Ultegra Di2 R8050 11 Speed Front Derailleur.

The RD-RX817 is only supposed to be used in a 1x arrangement as its capacity is 31T. However after viewing a Comment to an online article (https://opencycle.com/updates/campagnolo-ekar-1x13-new-drivetrain), I decided to give it a try on a Salsa Warbird.
I started off using a 2x Compatible Di2 Rear Derailleur (Shimano Ultegra Di2 R8050 11 Speed Rear Derailleur) and an 11-34 Cassette. This was important as you can't programme the Di2 shifting using the Etube project if the components are incompatible (The RX817 is incompatible).
With this configuration I then programmed the shifting so that the front derailleur would not go Big:Big or Small:Small (I actually programmed it to shift to the small chainring when on the third largest cassette sprocket, and to shift to the large chainring when the third smallest cassette sprocket was reached).
Once the programming was done, and I'd configured the shifters using the Etube project (because afterwards I wouldn't be able to). I swapped in the larger cassette and the RX817 rear derailleur.
It all worked well and as you can see the chain is never unduly tight or loose.

If I want to use Etube in the future with this, I will need to unplug the RX817 and plug in (though not necessarily install) the original 2x compatible rear derailleur. I am considering removing all direct access to the front derailleur in full synchro mode, and instead programming the left shifter to switch data screens on my bike computer (Hammerhead Karoo).

Obviously there are risks with this. If the system resets and you end up using the Di2 in manual mode, then you could rip off the derailleur and damage the frame. So please decide if you want to take this risk, and proceed accordingly as I am disclaiming all liability.
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