rbrides

After a 4-day trip climbing some too steep and too long gravel roads & trails that required some hike-a-bike, I want to upgrade my current gravel bike's drive train from the original equipment 11 speed Shimano R8000 11-34 cassette and Shimano GRX derailleur to the Shimano XT M8000 11-40 and Shimano RD RX812, which has a stated max sprocket of 42 teeth. I will stay with 2x chain rings and of course need new longer chain.

Is there a mechanic or technical reason this would not work on my 2020 Specialized Diverge Comp? Since my wheels are 11 speed and I intend to stay with 11 speed, and want to keep all else the same, I don't know any reason this larger cassette & derailleur would not work.

Am I missing something?

alcjphil

You would have to verify if that new derailleur has enough chain wrap capacity for your new setup. You didn't mention what size chainrings you have. The RX812 derailleur has a chain wrap capacity of 31 teeth and is designed for 1X drivetrains. You would need more wrap capacity than that for a 2 ring crankset

Rolla

Be sure the pull ratio of your shifter is compatible with an XT cassette.

rbrides

thanks. That’s the kind of info I am looking for.

rbrides

thanks. I didn’t think of that and appreciate you contribution.

pennpaul

I was running an 11-speed 11-32 with 46/36 in the front. I went to an 11-40 with 50/36. To make the change, I just needed the bigger cassette, a longer chain, and a Roadlink from Wolftooth.

Bmach

If you do a search there are success stories of people running up to 42 with the GRX RD. I will see if I can find it again.

try this

Upgrading cassette on GRX-600 Gravel Bike?

HTupolev

The RD-RX812 is rated for large cogs because it's targeted at 1x applications, not because it's intended for wider-range drivetrains than the RD-RX810.

I know a number of people who use RD-RX400 or RD-RX810 on 2x drivetrains with 11-40 or larger cassettes, with reasonable overall drivetrain behavior. On some frames, you might need a hanger extender (like a Wolf Tooth Roadlink DM) to make it comfortably clear the big cogs.

This generally isn't a problem. Shimano's 11-speed road and MTB cassettes have very similar cassette pitch. The big difference in their dimensions is that, on the MTB cassettes, the largest cog "overhangs the spokes." This allows the 11-speed MTB cassettes to fit on 8/9/10-speed freehubs, and means that you'll need to put an appropriate spacer behind the big cog when mounting them to a wider 11-speed road freehub body. (Shimano's 11-34 11-speed cassettes follow this convention. They're basically an "MTB cassette", and require a 1.85mm spacer when used on an 11-speed road freehub body, but are compatible with Shimano's 11-speed road drivetrains.)

rbrides

Thanks for sharing that video and your input.

rbrides

Does this logic work?
Although I have the 2x chain rings installed, I can stay in the small ring at all times, effectively operating it as a 1x system so that the derailleur capacity equation is simplified and going with the 11-40 or 11-42 is more plausible? (Or, maybe big ring ONLY when rear cassette is in highest 3 gears?) Perhaps I could set up in small ring and disconnect the front derailleur so I never go to big, while on these multi-day bikepacking trips.

rbrides

Does this logic work?
Although I have the 2x chain rings installed, I can stay in the small ring at all times, effectively operating it as a 1x system so that the derailleur capacity equation is simplified and going with the 11-40 or 11-42 is more plausible? (Or, maybe big ring ONLY when rear cassette is in highest 3 gears?) Perhaps I could set up in small ring and disconnect the front derailleur so I never go to big, while on these multi-day bikepacking trips.

alcjphil

I suppose that it might. In my earlier answer I asked about your chainrings. What size are they? Going to smaller chainrings might solve your problem unless they are already as small as you can go

Kapusta

If you are beyond the chain wrap capacity of the RD, and you size the chain to only work with the small ring, you really need to remove the large ring or at least make sure there is absolutely no way to be able to shift into it, because if you do shift into it and your large cog, you could do some serious damage to your drivetrain in a single pedal stroke.

Have you considered a sub-compact crankset (if you don’t already have one)? That is another option that will retain your current gear spacing, but at the expense of losing you highest gear combo.

rbrides

I have 48-32 and also 50-34. I run the 48-32 for bikepacking, to help accomodate for the extra weight I'm carrying.

rbrides

I use 48-32 chain rings when bikepacking (that originally came with the bike) and also have 50-34 that I use on more tame gravel day-rides.

Rolla

I would measure the chainline for the small ring; I'm not sure it's in the optimal position for a 1 x 11 drivetrain.

tyrion

One possibility/hack is to make the chain long enough to accommodate big/big rings. The chain might be floppy in small/small rings, but not catastrophic.

jp911

An 11-speed XT cassette works fine with the GRX 812 (I've been using an 11-42 in a 1x setup for over 3k miles with zero issues). I haven't tried it with a front derailleur, so can't really help there.