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

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).

https://cimg3.ibsrv.net/gimg/bikefor...beb90886be.jpg

I don't see any reason why it should not work. Example, people have used stock M2000 to M4000 series (with b-link) with 11-42.

I am using Alivio MC20 which is a much older generation (read: shorter cage) and it performs perfect with 11-42.

https://cimg4.ibsrv.net/gimg/bikefor...c1b0fee43e.png

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.

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.

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.

Yeah I’m actually a bit confused as to why an 810 RD wasn’t used instead. I’ve seen a build with 2x 11-40 using that RD.

Hi All - jumping in here... so will the GRX 400 derailleur work 1x10 with an 11-42 in the rear? That is probably what I am going to use for my build. Will I need a wolf-tooth extender perhaps?

I would have to use a GRX 1x crank for the chainline correct? or that would be preferred?

Thanks!

it would work 10 sp 11-42 with GRX400 shifters or the latest Tiagra 10 speed shifters. But not 105 or Ultegra 10 speed shifters.

the point is to not need a Wolf Tooth Road Link, because a standard road 5701 or 6800 rear derailleur will shift 11-42.

My setup is 1x so A RD-RX812 would have worked perfectly because it is rated for 11-42 out of the box, but it is $120. The RD-RX400 is $67. So, I thought it was worth a test. Now I know the 812 is worth it.

Understood - thanks. Amazing how this can work 11-42 10 speed. Running it 1x won't be a problem?

Edit - let's say i'd like to run a 1x with a 30T chainring up front... is that too much range?

Thanks again.

Yes a 30t 1x front chainring works.

The GRX 400 has these specs:
LOW SPROCKET MAX 36T (42T works)
LOW SPROCKET MIN 30T
TOP SPROCKET MAX 11T
TOP SPROCKET MIN 11T
TOTAL CAPACITY 41

Drive train capacity can be calculated with the following formula: (maximum difference chainrings)+(maximum difference largest and smallest sprocket). E.g. The FC-RX600-2 crank is 46|30, the maximum difference is 16t. Assume an 11-36 cassette, so the maximum difference is 25. Then the capacity of the drivetrain is 16+25=41t.

But if you only have a 1x with an 11-42, your capacity is just the cassette =31.

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

https://cimg3.ibsrv.net/gimg/bikefor...6c8138b035.jpg
https://cimg4.ibsrv.net/gimg/bikefor...9ae7726b00.jpg
https://cimg5.ibsrv.net/gimg/bikefor...4695946974.jpg

Thank you! This was very helpful Erik_A

Did you mean that the RD RX 812 isn't worth it because the RD RX400 works with an 11-42?

Also been looking around on how to run a 1x with a 30T or 32T up front. How can I stick with GRX cranks and achieve this? or would you recommend something like Easton or an MTB crank?

Thanks!

I converted a friends Jamis Renegade gravel bike (Tiagra 4700 10sp shifters) to a 1x10 setup using a GRX RX-400 10sp derailleur, with a 38t x 11-42t setup and works perfectly. The derailleur is rated to that capacity (as per the post above) but the b-tension is at maximum.

On my personal Renegade I'm running 1x11, with a 40t and 11-46 cassette. That is however with a Shimano XT M8000 11sp GS (medium cage) MTB derailleur + Wolftooth Tanpan. Clears is with a a few mm's of B-tension to spare. Compared the cage length and it appears the MTB 11sp derailleurs are slightly longer in the cage vs. the GRX stuff (makes sense, needs to compensate more for chain grown on full-suspension bikes etc).

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

My wife has a stock Liv Devote 1 with 32 - 48 Chainrings 11 - 34 cassette grx 400 10 speed.

Using the capacity formula, the biggest cassette is 11 - 36 to get 41t, is that correct? So I can't go more than the official spec and stay 2x?

Thanks

Yes.

The official spec also has a max cog size limit of 36T, so it's not like you dodge the issue even if you go 1x.

That said, plenty of people violate these specs without that much trouble. I know a few people running 11-42 cassettes on GRX400 or GRX810 derailleurs with 16-tooth front differences. If you exceed the max cog rating, you'll potentially want to get a Wolf Tooth RoadLink DM. You'll also need to use a longer chain, or else shifting into the big-big region of the drivetrain may damage the bike (note that because you're exceeding chain wrap, this will likely make the chain go slack in the small-small region, so you'll have extra reason to avoid small-small cross-chaining).

Grx 400 probs
 

Hi.
I've acted on some of the info here and decided to buy a GRX400 rear mech to use with my current system which is 11 speed 34 cassette with r7000 front mech, absolute black chainrings 30/46 and 5800 105 11 speed shifters. So all 11 speed except for the new rear mech.
This system was running perfectly with the 105 rear mech but although I can get it running on all cogs with the GRX 400 it is very, very rough.
I Was going to hopefully put a 36 or 40 cassette on but surely that will only make things worse.
What could be wrong here?
I've tried various b screw positions. It doesn't need to be fully in with the 34 cassette.
This system would seem to be very similar to others here that seem to work perfectly so I don't know what The problem could be.

Try turning off the clutch.

with apologies to Sir William

To work or to work perfectly is the question,
whether tis better to the shifting to suffer
The slings and arrows of out of spec gear
Or to match specs against a sea of troubles

I know we all want as cheap as possible and sometimes just want to try things for the heck of it, be IME with newer gear you need to think of it more and more as a system than components. It is the system that has to work. YMMV but I have found pushing specs and mix and match often does not give me the desired performance (shifting especially)

Hi Eric.
Yep. Tried it both ways.
With a little more tweeking i have got it running very slightly better. But not much. I'm wondering if the chain is a bit short. I will insert a few links tomorrow with the quick links and see if it works better but i would have though that even if the chain was a little too short it should still run fine in the small ring but it's still noisy. I wouldn't ride it like it is at the mo for fear of breaking something.

Just to say. I've added (temporarily) a couple of extra links with quick link connection and that does seem to have solved the problem. I just wanted to make sure before I go ahead with a larger cassette.
With that in mind, do you think that my system would be okay with an 11/40 cassette? Bearing in mind that I have oval 30/46 on the front. Or might you expect chain wrap.
Grx400 rear mech, 105 front mech, oval 30/46 chainrings.
Cheers.

Shimano Deore cassette CS-M4100 10x 42-11 works great
 

Some posts were confusing and I was not very sure will it work or not. Now I can confirm, it works great!

I have installed the Shimano Deore CS-M4100 10x to a Cannondale Gravel bike Topstone 2 with the original full GRX 400 equipment.
The B-screw must be adjusted to the maximum distance. Then shifting is smooth as before.

Only one point, that has to be mentioned. My chain is too short with 112 links, i.e. the largest 46 front chain ring does not work with the largest 42t ring on the cassette. This combination shouldn't be chosen, of course, but when it is done accidentally the wheel can block.

Maybe I will add 2 or 4 more links, sometimes.