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Shimano writes in the description of the Shimano GRX Shadow Plus RD-RX812 11-speed rear derailleur that it is compatible with shifters GRX ST-RX810 and ST-RX600 shift/brake levers.
Should I understand that it is "only" with these shifters because of a pulling ratio different from their road but also from their MTB standard? An information that I can't find anywhere. Or is the derailleur a standard road one compatible with their road shifters?
The rear pull ratios used by Shimano are:
Essentially with ever more closely spaced sprockets, it emerged that while pulling less cable means a shift requires more force, it makes shifting more accurate/precise/easier to setup. Hence after the introduction of 10-speed MTB which pulls more cable per shift, and the introduction of hidden cabling for second-generation 10-speed road, which adds more right-angles/friction, Shimano came up with an intermediate shift ratio between MTB and the previous standard. Since the old ratio didn't work that well with hidden cabling & 10-speeds or more, they essentially have one standard that is used for all new 10 & 11-speed, and probably will be kept for 12-speeds as well.
So the RX812 is not '11-speed', since the RD is not indexed. It will work with shifters that use Shimano's new road pull ratio, which is anything GRX (10 or 11-speed), anything 11-speed, and Tiagra 4700 10-speed.
There are some older 10-speed road shifters using the standard-for-everything-for-a-quarter-of-a-century-pull ratio, but they are no longer in the Shimano range.
The thing specifically with RD-RX812 is it's marked for a 1x drivetrain, and Shimano don't have 1x levers for road (only for gravel), so they only show it compatible with the GRX levers, since these have brake-only levers. There's however nothing stopping you using it with, say, SL-4700-R, which is a flat bar 10-speed lever, and a Deore 10-speed 11-42 cassette.
The Shimano compatibility documentation gives the official answers
The RD-RX800 is compatible with all the 11 speed road shifters
The RD-RX812 is compatible with GRX levers only, but that is because the RX-RX812 is for 1x setups, only the GRX range has 1x levers, with the left unit being a brake lever only.
Note that although both the 800 and 812 derailleurs expect the cable pull length from an 11 speed shifter the 812 actually has a slightly different actuation ratio than the 800, because it is designed to be used with mountain bike wide range cassettes, which have different sprocket spacing that road cassettes.
Shimano does not publish any information for compatibility between road (including gravel) and MTB shifters and derailleurs. It's generally understood that mixing road and MTB group components does not work.
The reason the rx 812 is listed as 1x only is because it's maximum range is taken up by the cassette and adding to that range will decrease chain wrap to smaller cogs potentially to a point where they will not interface properly and skip gears. There is nothing intrinsically different about a 1x derailleur that stops it working with a front derailleur.
GRX lever pull ratios can be converted with a WolfTooth Tanpan to MTB derailleur pull ratios. And you can extend the capacity with either/or a derailleur hanger extender such as WolfTooth Goat-Link 11 speed and an oversized jockey wheel cage with larger wheels. The extra teeth take more of the spare chain up.
As far as I can tell, the pull ratios on all 2xs are essentially the same. If it does need converting...longer throw moves less for more leverage than MTB..a Tanpan should work but I don't think it's needed on front.
I have a fairly decent light aluminium hybrid frame with all the mounts but shitty gearing. I do 100 mile rides heavily laden up mountains. I've been grappling with initial setup but i wanted a 3x for the range. an 11-51 cassette paired with GRX 810 chainrings would give me pretty much the same range and solve that.
So my final setup idea, Deore 11-51, with an M8000 rear derailleur, kcnc SXT oversized hanger, a WolfTooth Goat-Link, and a GRX 810 crank, with 810 front derailleur.
I have seen this setup working 51t cassette...grx crankset. I didn't believe a 51t would work until I saw it which would have killed my ideas of a 2x but I could live with those ratios. And have an flat bar setup with normal 2 X 11 shifters, and a dropbar cockpit I can swap out. With a tanpan to convert the ratios.
I've done a lot of research and know in principle the raw parts work in terms of sizes and pull ratios. The slight elephant in the room may be that the deore 5100 cassette seems to be 12 speed spacing and married to the 5100 rear derailleur..which I can't use, direct mount. But I've seen it working with m8000 derailleur and shifter, so by extension it should shift with tanpan and grx levers.
When I'm finished I should have the gearing for fast paced road rides and everyday commutes and also be able to handle a bike packing trip up a mountain. All with indexed gears. The rest of the bike will be specced between a cross country and trail bike, trail wheels for more weight and durability with 4 pot brakes, potentially 2 pot on rear. And the ability to mount whatever I want and swap between flat bars. The geometry will be slightly strange kinda touring geometry with slightly bigger suspension (100 mm travel). Itl be a weird mix of gravel/touring bike but specced to handle the stresses of loaded touring and mountain biking. It will go up and down any mountain or trail, in much the same capacity as a gravel bike could, but more built up to cope with the stresses/gear ratios and braking ability of carrying a week's worth of stuff.
