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).

shimano-grx-drivetrain-1f0d0b3.jpg 
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GSPChilliwack
I like shopping for clearance and used stuff, but this is one place where I think I'd just spend the money and do it right. I've found that what works on one bike, doesn't necessarily work on every bike when maxing the capacity of a rear derailleur. 
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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.
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pete kumble
SO I won't make any comments about how seamlessly SRAM Rival works?
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EBG18T

Shimano is conservative sometimes... But I recently setup a GRX 812 on a 11-46 per a request. The GRX 812 is spec’d By Shimano for 11-42 max, but some have stretched it to 11-46. The issue we ran into is the B screw is so far ‘in’ to get it to shift the 46, it shifts poorly in the lower 2 cogs. He was willing to accept 2 sloppy gears to get a 46T bailout. Personally I’m doing just the 42T (Shimano recommend max) as I can’t deal with sloppy shifting.


Please post up your GRX400 findings after. 

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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.
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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.

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 Where did you see the RX400 cable pull is the same as 11 speed? That's not reflected in the Shimano compatibility chart (unless I'm reading them wrong).. 
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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-GRX-RD-RX400-Rear-Derailleur
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