Can i change grx 2x 12 speed to mountain bike 2x 12 speed

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Re: Can i change grx 2x 12 speed to mountain bike 2x 12 speed

Postby Duck! » Sat Dec 23, 2023 10:19 pm

nezumi wrote:
Fri Dec 22, 2023 2:23 pm
Duck! wrote:
Fri Dec 22, 2023 12:05 am

The OP specifically asked about changing the crankset. Only two replies out of about 20 have remotely brushed on answering the question; the remainder discussing derailleurs & cassettes, and all being 11-sp. are totally irrelevant to the question posed!
With respect, I'd say that there's a difference between what the OP asked, and what they are trying to achieve. The desired outcome is clear: have lower gearing than the default that the bike ships with. The means by which this can be achieved may be smaller chainrings, but that's not the only solution, and definitely not the automatic option.
In the context of Shimano 12-sp, changing the crank is actually the least complicated way of achieving the OP's goal, although it's not without potential traps, as noted previously.

To change to a larger cassette, one needs to jump over to the MTB family, where the next size on offer from the OP's stock 11-36 is a 10-45. There may something third-party that I haven't heard of, but to the best of my knowledge there is nothing in the 38-42 big sprocket range in 12-sp. from anywhere.

Where the 10-45 (and next step 10-51) makes things messy is that due to the 10T top gear it needs a specific freewheel body, which may not be available to suit certain hubs. It is shorter to allow the smallest sprockets to sit over the end (interlocking with the adjacent sprockets to drive them), with a unique spline pattern (MicroSpline). Road-derived cassettes (up to 36T) also use a new, fundamentally similar but different finer spline, which is cleverly designed so the cassettes can also be fitted to road 11-sp. hubs. The matching 12-sp.-specific hubs will only fit these cassettes.

Then there's the derailleur, and I can only go on what I've read, because when I left the industry, 12-sp. GRX wasn't even rumoured as A Thing, and road 12 had only just appeared in Di2-only form. From what I can tell, there are two quite different GRX derailleurs, one a derivative of the related road models, and as such an evolution from the 11-sp. family, the other more closely related to MTB derailleurs, and this is the one that is needed for the big cassettes. The road-derived model is oriented to 2x drivetrains, and although officially limited to 36T rear sprocket, if its geometry is as close to the 11-sp. family as it looks, it will fit up to a 42T rear sprocket, and should be able to also handle a 2x crank with a fairly narrow chainring difference. But as noted above, actually finding a 12-sp. cassette in that size range will be the biggest problem.

The 1x-oriented model is more closely related to the MTB family, and from what I can tell has a longer upper body, and a considerable offset of the top pulley from the cage pivot, and even moreso on the 51T-compatible SGS variant to gain the necessary sprocket clearance (there are two versions of this derailleur, but only the cage is different, and can be swapped). The cages are not compatible with the road-based 2x model. The unknown thing for me, simply because I'm not able to experiment with them, is how or if they'll behave when front shifting is thrown into the mix. But crunching numbers, although it lacks the really big sprocket clearance, the 2x-oriented version has the greatest overall gear range capacity.
I had a thought, but it got run over as it crossed my mind.

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