Just to be clear, you're talking about a 42T on the back, not on the front?!

Are you limited to Sram components? If you're starting from scratch then Shimano have excellent cross-compatibility between road and MTB groupsets. It looks as though a new XT rear derailleur could handle a 42t sprocket in 2x setup, although with a max capacity of 39T and using an 11-42T cassette that would only leave you with a max difference of 8T up front, so it's a bit of swings and roundabouts.

The issue you have is you are aiming for a super small gear which is better accomplished by changing the front rings. Here is SRAMs official compatibility chart:

https://www.sram.com/..._road_components.pdf

The non-clutch 2x rears are designed to work with a super compact 50/34 setup. The 34-42 combo is really extreme especially when you have the option of just running a smaller front ring. You can get the 1x clutched system up to a 42 with the long cage and you could run the clutch system as a 2x with something like a 11-36 cassette and a 46-36 front combo. But you really haven't gained any lower end range in this setup and the shifting will likely be compromised.

If I were in your position I would run a 46-30 front combo with the standard 11-32 cassette. (The 30 might be a bit difficult to find but they are standard for triple crank sets so someone must still make a compatible option). The 46-30 is the same jump as the 50-34 but pushes everything towards the lower end. I would need to run the numbers by I think the 30-32 combo is close to a 34-36.

If you go with Shimano the R8000 Rear derailleur is rated to 34 teeth but youtube videos show people using it with a 40

https://www.youtube.com/watch?v=roUXWvAN-9k

One option I think would be Ultegra r8000 with a subcompact 46/30 crank and 11-36 cassette. 30/36 low gear is just slightly higher than 34/42. Another option would be a Rival 1 setup. You can put on one of the e*thirteen 9-46 cassettes and probably a 38 chain ring and that gets you even more range than the 46/30 w/ 11-36.

Does the Fargo take road cranks? I thought most people generally build them up with mtb parts.

I agree with the others about going lower up front, but to directly answer your question....


The SRAM 10 speed mountain bike derailleurs have the same cable pull as 11 speed road and are a very common swap in the cyclocross world. Since the shifter dictates how many 'gears' a derailleur has, the 10 speeds are inherently 11 speed when connected to an 11 speed shifter. The 10 speed flavor of a GX rear derailleur will easily clear a 36 tooth on any bike and a 40 tooth on most. I have no experience trying to cram a 42 into one. Here's one for $60 at the very first hit on the Google. https://www.jensonusa.com/...-SPD-Rear-Derailleur

This is probably your best option. There's now an R8000 GS rear derailleur that is clutched that's probably ideal for your friend's application.

Or maybe an older XTR triple?

The Fargo takes MTB cranks with a listed max of 42/28 in a 2x setup.

Personally, I would prefer to use Shimano, but I was under the impression that the pull ratios were different.

Thanks for the info so far!

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Steve

"If you ain't first, you're last." Reese Bobby Talladega Nights

If you are talking about the rear cassette - i recently used a road link from wolftooth components to extend my Shimano 8000 rear der to shift with an 11 - 40 rear cassette - shifted flawlessly on Di2.

https://www.wolftoothcomponents.com/collections/derailleur-optimization/products/roadlink-dm

I can confirm that a 42 cog works just fine with that derailleur. That's exactly the derailleur I use on my Fuji Jari (SunRace 11-42 cassette)...but, in my case I'm using 10sp Red shifters. But, as you point out, that should work just fine with 11sp SRAM road shifters as well.

BTW, for those saying it may not work well with larger rings in the front...I actually have the above combined with a road standard 53-39 chainset (I wanted to run an old Quarq CinQo PM on there). Everything shifts perfectly.

So...to the OP, just get the GX 10sp rear derailleur and run it with the Rival22 HRD controls as you planned. Problem solved :-)

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http://bikeblather.blogspot.com/

Thanks for confirming Tom!

I agree that it absolutely would work with larger chainrings, it's the chain capacity and not the size of the front rings that matters. Big rings are just not necessary for the event that OP is talking about. Even your 39 would probably be large enough!

After reading a few replies, I'm not so sure that OP and their friend even has the SRAM shifters already.

This is the trick, with Rival22 Hydro brifters. That derailleur with an 11-36T cassette would pair nicely with the 42T/28T SRAM S1000 2x10 crankset (as used on the Giant ToughRoad SLR1)... if you can find one.

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"They're made of latex, not nitroglycerin"

Thanks (again) for all the info, guys.

I just wanted to add that this is a "from scratch" build. None of the drivetrain parts have been purchased, yet. Also, the Salsa Fargo takes a MTB crankset and is BOOST.

Right now, I'm leaning toward the following combo...

Controls: Sram Rival 22 HRD
RD: Sram GX Type 2.1 10sp (Long Cage)
Cassette: Shimano XT CS-M8000 11sp 11-40
Crankset: Sram GX 1400 2x11 BOOST 36/24
FD: Sram GX 2x11
Chain: Shimano CN-HG701 11sp

Jenson has the Long Cage version of the GX RD listed as 47T total capacity. Based on that and Tom's success using even a 42T big cog, I think this should work just fine. Does anyone see any other compatibility issues with this combo?

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Steve

"If you ain't first, you're last." Reese Bobby Talladega Nights