Shimano GRX RD-RX400 RD w/ 11-42 cassette

[Erik_A]

Has anyone tried the Shimano GRX RD-RX400 rear derailleur with an 11-42 cassette? I am currently running a 1x11 using Ultegra 6800 shifters and rear derailleur with a WolfTooth Road Link and an 11 speed 11-42 MTB cassette (w/ 42T front chainring). It has trouble dropping into the 11 tooth ring sometimes while riding, so I am looking for a setup that doesn't rely on the Road link. I know the GRX RD-RX400 is only rated to 36T max, and the GRX RD-RX812 is for 42; but shimano is very conservative with capacity, so I am guessing it would work, and be a less expensive option. I also know that the RD-RX400 is sold as a 2x10, but it uses the same cable pull as 11 speed (like the older Tiagra group before it).

[fogrider]

not sure, but since the 6800 rear mech is designed for the road, it doesn't have a clutch and the spring strength for the 11/42 range. the simple solution is to use a GRX rear mech or a XT.

[doomridesout]

This never occurred to me but I'm now watching this space for my commuter bike.

[jwin]

The Ultegra RX 800 can use an 11-40 is what the interwebs says. Also the GRX 600 and above can take an 11-42, but not with a 2x (supposedly). But all say that it is basically the same as an Ultegra RX 800, so.

[Erik_A]

I went ahead and bought the Shimano GRX RD-RX400 rear derailleur. I have heard that the RD-RX810, which is rated for 34 max works well with an 11-40 cassette. I am hoping that the GRX RD-RX400 (which is rated for a 36 max) will clear the 11-42. Will report back once it arrives and I install.

[Pinned]

The RX-RX400 is 10sp and 6800 shifters are 11sp - I think you're going to run into issue. The RX600 and RX800 are the 11sp versions.

[Erik_A]

I installed the Shimano RD-RX400 today with the ST-6800 shifters and the 11 speed 11-42t cassette (1x setup w/ 42t front chainring). It shifts through the whole cassette with b-tension screw almost all the way to its max. It easily clears the 42t cog on the cassette.

So far it shifts a bit rough at either the low end or the high end depending on adjustment. Time will tell if it is better than the RD-6800 derailleur and Wolf Tooth road link, which had the same issue with the 11-42 cassette.

It definitely shifts worse with the the clutch on than with it off.

So, I don't recommend it for the 11-42 cassette, but it would be a decent budget solution for either a 11-36 or 11-40 cassette with existing 11 speed shifters.

[Erik_A]

The GRX 400 shifters (just like the latest versions of Tiagra) use 11 speed technology in the rear derailleurs, but just pull more cable to shift to ten speed (than Shimano's 11 speed shifters do).* Therefore the 10 speed Tiagra and GRX 400 can use 11 speed rear derailleurs and vice versa.* But it's not officially supported.

Originally Posted by Pinned

The RX-RX400 is 10sp and 6800 shifters are 11sp - I think you're going to run into issue. The RX600 and RX800 are the 11sp versions.

[doomridesout]

Are there different part numbers at the 400 series too? Keep in mind GRX has a different 1x and 2x RD.

[Erik_A]

Yes GRX has 3 rear derailleurs (they are all likely compatible with the 800, 600, and 400 shifters)

RD-RX812 - 1x (42 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

RD-RX810 - 2x (34 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

RD-RX400 - 2x (36 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

Originally Posted by doomridesout

Are there different part numbers at the 400 series too? Keep in mind GRX has a different 1x and 2x RD.

[IFRider]

Originally Posted by Erik_A

Yes GRX has 3 rear derailleurs (they are all likely compatible with the 800, 600, and 400 shifters)

RD-RX812 - 1x (42 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

RD-RX810 - 2x (34 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

RD-RX400 - 2x (36 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur

Works great with the B screw adjusted accordingly.

Having researched the viability of the 40 tooth rear I gleened the RD differences, the 810 is double capable and the 812 is only single up front. Not sure the geometry difference. the 400 is the 10 speed and I am watch here closely to see how that works with a mtn cassette.

I am told GRX and road are interchangeable so might be a good option on a Tiagra Checkpoint my son has.

[Erik_A]

My long term review of the Shimano GRX RD-RX400 rear derailleur with an 11 sp 11-42 cassette using Ultegra 6800 11 speed shifters:* It works great!* After a few long* gravel/ MTB and road rides, it is shifting great!* Much better than the 11 sp Ultegra RD-6800 with Wolf Tooth Road Link.* I would say that it is definitely worth the $63.00*https://www.jensonusa.com/Shimano-GR...ear-Derailleur

[Moneywatch]

Thanks for the summary. Do you think the RX400 RD will work with a Dura Ace 7800 group/shifters to allow for lower gears since the cranks are 53/39?

[Erik_A]

I think that yes, they will work with any Shimano 11 speed shifters. But if all you want is a 34 or 36, than a Wolf Tooth Road Link may be equally good. This works well for 42t cassettes.

Originally Posted by Moneywatch

Thanks for the summary. Do you think the RX400 RD will work with a Dura Ace 7800 group/shifters to allow for lower gears since the cranks are 53/39?

[Dired]

I have a similar setup using an xt rd and roadlink to 105 shifters, shifts perfectly.