Shimano GRX Compatibility Question?
 

Hello,

Does anyone know or can point to documentation as to whether or not you can use an RX-812 (long cage) rear derailleur on their normal 2X setup. I believe that the difference in cog sizes are 34 max on the 2X and 42 max on the 1X. The difference is only 8 teeth which I would hope would be made up with the long cage rear der.

I am working on speccing a new build with a 2X GRX drive train and would like the option to provide lower gearing if needed in the future without replacving the rear der. I know that this trick would work on older Shimano drive trains but don't know about this one.

Yes. It's will work. Don't disturb dude

RD-R812 is not "long cage." Its cage is actually shorter than the one on RD-RX810, since due to being a "1x" derailleur, it does not need to be able to wrap as much chain.

Personally I'd probably err toward the RD-RX810 for what you're trying to do. It can tolerate a larger cassette than spec: I know multiple people running the RX400 or RX810 on their gravel bikes with an 11-40 or 11-42 cassette and a 16-tooth front difference, with pretty reasonable results.

Thanks for the clarification. I wasn't considering the 1X factor.

I think that the RX-810 comes with the 2X kit that will be going on the new bike. Wonder why Shimano doesn't mention the max cog size of 40-42 in the spec for the RX-810? Their "consumer" literature only mentions the 11-34/36 cassette with the 2X group.

From what I understand, the difference is in the path the derailleur takes. 812 takes a path more suitable to larger cogs. As many have experienced, it does work. Modern medium cage is yesterday’s looong cage.

Shimano generally only specs derailleurs to handle what they intend to sell them alongside, which in this case is an 11-34 (RD-RX810).

They're also conservative in only spec'ing things that they test and guarantee to work dead-on first-try if you follow the setup procedure. When I say that friends have gotten "pretty reasonable results", I don't mean that they just bolted the parts to their bicycles and it instantly shifted as crisp as a 53-39 11-25 Dura Ace racing drivetrain. There's often some messing around with b-screw adjustment and/or hanger extenders, and sometimes the shifting ends up slightly balky in the high gears. When exceeding wrap, you also need to pay close attention to chain sizing and be careful about drivetrain behavior in the cross-chained gear combos: using Shimano's big-big-plus-1" sizing method, this means that the drivetrain may get very slack in the small-small region, the high gears on the small chainring.

Sort of. Like most "1x" derailleurs, it uses a large offset between the jockey wheel and the cage pivot, which causes the jockey wheel to move downward and away from the cogs as the derailleur wraps more chain. This causes it to take a path more suitable to larger cogs when a larger cassette is used, and more suitable to smaller cogs when a smaller cassette is used. This allows a single derailleur design to behave well over a wide variety of cassettes, and can help a 1x drivetrain perform better if the user is occasionally swapping in a smaller cassette than it was originally set up with.

The challenge is, the offset between jockey wheel and cage pivot means that the position of the jockey wheel is also affected by wrap from front shifting. This is why the RD-RX810 puts its jockey wheel concentric with the cage pivot: the position of the jockey wheel doesn't get affected by front shifting at all, instead the path taken by the jockey wheel is determined entirely by the slanted parallelogram.

The drawback here is that the RD-RX810's parallelogram isn't slanted quite as much as you'd want for super-wide cassettes. So if you set the b-screw (and/or use hanger extenders) so that it clears a very large cassette, the jockey wheel will end up pretty far from the cassette when you're in the small cogs, which can result in balkier shifting in the high gears.

Both derailleurs have their drawbacks when used in a drivetrain with both a super-wide cassette and wide front shifting.

I have giant Revolt Advanced 2 with Rear Derailleur Shimano GRX RX-810 with 2x up front.
https://www.giant-bicycles.com/us/revolt-advanced-2


I installed 11-40 Shimano Deore XT M8000 11-speed Cassette. It works good enough for me. It does not
shift as nice as the 11-34 that came with the bike. I prefer the 11-40 for the mnt bike trails that are over 15%. I can't imagine how the 11-42 would fit. The 11-40 is pretty tight.

The biggest draw back is when I put my road wheel set on ( 11-30 ) I have to make adjustments to the b screw. Before I had to do nothing switching between my gravel wheel set when it was 11-34 and my road wheel set of 11-30.

Thanks for the data point!