Is Shimano MTB front derailleur compatible with SRAM crankset?

Question

I have 3 pieces to make this work -- brifter, FD, and the crankset -- in 2x10 setup.

From this post -- Compatibility of Tiagra 4700 shifter with MTB front derailleur -- I gather I can match Tiagra 4700 brifter with Shimano MTB FD, like Deore. Ok, so next in line...

Will Shimano Deore FD work with SRAM Boost GX-1000 2x10 crankset: https://www.sram.com/en/sram/models/fc-gx-1002-a1?

My main focus here is on the putting/shifting FD cage over given chainring correctly, so there would be no rub or other shifting problems. But I would also be grateful for comment how far I can stretch official cage capacity (in case of my current 3x9 I managed to use 20T without any problems).

I would like to focus on compatibility and not get involved in "why do you need..." discussion but those side-questions are inevitable ;-). So in short -- I need 22T (with possibility of 20T) in front with 170mm arm (or better 165mm), and I only found this particular crankset. Shimano cranksets starts from 24T and only with 175mm arm, so far from my requirements.

Answer 1

This question touches on a few different things. It is important to understand that a lot of traditional dogma about FD/crank matching that people might come up with goes completely out the window when talking about boost-era parts. There are not many boost FDs period and generally speaking, the pool of experience out there with them is limited.

• Yes, generally speaking you can match ST-4700-L with any Shimano mountain FD on a crank and chain that would normally be compatible with that FD.
• Only a handful of Shimano mountain 10 FDs are boost-compatible, and this is a restriction that can't generally be fudged. However, what's tricky about figuring out what you need here is that Shimano has presented this information in two different ways over time. In the Chainline line on their FD spec charts, sometimes boost compatibility has been delineated as "48.8/51.8" and other times as "48.8*" with an addendum of "* Compatible with the special crankset which chain line is 3 mm outboard." They are two ways of conveying the same information.
• For the most part, Shimano is not messing around with the cage capacity numbers on their 2x mountain FDs. The nominal 10t capacity ones have very vertically short cages. It's easy to get into a situation where the chain contact areas are not doing what you want when you go trying to fudge this.

Non-SRAM 10-speed FDs that can handle 36-22 with boost chainline are weird animals and the only Shimano one I can see is FD-M618-E, so that depends on whether you can run e-type. Microshift XLE could possible be a workable option; it nominally starts at 38t, but I have no idea how well it would fudge to 36-22. It might be fine. If this is really the crank you want to run, personally I would consider just trying the matching SRAM FD hooked up to the ST-4700, but be prepared to order a different thing if it doesn't work. I don't know if it's going to work, but if you can get the cable pull aspect to be acceptable, I doubt anything else will perform better, and the fact that ST-4700-L has a full compliment of trim stops makes me optimistic that it can work okay.

Answer 2

A front derailleur cage is designed around the size of the chainrings (and the chainline), more than the brand. If you want to use a Deore FD (presumably 36/26), there's a risk that the cage is too short for a 22 chainring.

Now to answer your question:

• you can have a Deore FD with a SRAM crankset (Trek sold bikes with this kind of config, for example), provided that the specs of the chainrings are close enough. There is a tolerance, but it's not possible to know without testing.
• front derailleurs (for double chainrings) are quite tolerant to the chain size, so for a 10-speed, you can also use 9-speed or 11-speed FDs. In the Acera range, there is a 2x9 spec'ed for 36/22 chainrings, available in boost (FD-M3120-M-B) and non-boost (FD-M3120-M) (pull ratios for front derailleurs are the same, no restriction for the shifter).
• You also have "1-speed" deviance for the front chainring. So you can use a 9-speed crankset on a 10-speed system. If you prefer to stay with Shimano, you can also use the Acera Crankset (FC-MT210-B2, Hollowtech II, or FC-M3000-B2 for square tapered — both being Boost).

So the answer for me is the Acera FD. For the crankset, you can choose between the Acera or the GX 1000 you linked to.