jwill911

Hi, I have a Trek Checkpoint 6 Ultegra 50-34 11s with 11-34 cassette. I want to go lower for some of my gravel off road adventures. Initially I thought a Wolftooth RD extender would be the way to go, but came aware of the new GRX RD-RX812 and figured the addition of the clutch to reduce chain slap and other advantages would be better for going lower, plan a 11-40 cassette. Most of the dealers selling the GRX state 1x. But reading the Shimano documentation it does not state 1x or 2x only 11 speed.
My questions: does it work, has anyone done it successfully?
I understand the GRX group 1x FD and crank are shifted outboard 2.5 mm for additional wheel clearance.
I'm tempted to get the GRX RD-RX812 and try it, but was hoping for a little confirmation for those that have been there done that.
Thanks,
John

dsbrantjr

Well, according to this document https://bike.shimano.com/en-AU/produ.../RD-RX812.html the RD has a total capacity of 31T, so a 11-40 cassette would only leave 2T difference for your front crankset. So, it doesn't STATE 1X but in practical terms you are stuck with 1X.

melikebikey35

If you feel that you need something lower than 34/34 I'd recommend switching your crankset and front mech (2x has the 2.5mm offset, as well) to GRX. A 48/31 (or 46/30) crank and a 11-34 should give you plenty of gears, and won't have the large jumps between them that you'd get with an 11-40.

tomtomtom123

11-40T and 50/34T is a total of 45T chain wrap capacity.

You can easily find the wrap capacity on the Shimano specs site. You're going to want an RD with a SGS long cage, like RD-M8000-SGS or RD-M9000-SGS. But they use flat bar trigger shifters.

https://productinfo.shimano.com/#/sp...r%20Derailleur

20T tooth jump between front double is doable, but you'll have to find another crankset with a smaller BCD.

Eggman84

The RD-RX810 is designed for the 2 x 11 gravel drivetrain, with a total capacity of 40T but a maximum large cog of 34T. Shimano also specifies for this RD a maximum front difference: of 17T, which implies the 2x front drivetrain. The RD-RX812 does not include a similar specification for a front drivetrain, thus implying 1x. It seems the "oversize" cogs (40+ T) are spec'ed for the 1x options by both Shimano and SRAM.

jeffwhitfield 

Actually, with an RX810 rear derailleur, you can run it with a 40T cassette. Gravel Cyclist wrote about this and I posted up commentary on it as well on my blog (https://velonut.com/blog/gravel-cycl...1-40t-cassette). I tried it and it works great. Basically, while Shimano says that the highest is 34T, it can actually go a little higher. However, beware of cross chaining! You absolutely can not use the biggest gears on both. Highest you can go while using the higher front chainring is the second gear in the rear. Otherwise, you will run into problems. Watch the video and you'll see what I mean.

tomtomtom123

The RD-RX810 won't work for what the OP wants to do, since it only has a chain wrap capacity of 40T. If the OP is intending to run 45T chainwrap and tries this, then possibly the outermost 4 or 5 cogs will have the pulley dragging the chain when on the small chainring, if the chain is cut long enough for the big-big to clear. The OP needs SGS length cage for 45T chain wrap capacity.

You can find the specs here:
https://productinfo.shimano.com/#/sp...r%20Derailleur

If you do use a MTB RD and keep using your existing road shifter, then supposedly Jtek says their Shiftmate 8 or 8a can convert the cable pull.
https://www.jtekengineering.com/shift...harts-choices/

jwill911

Well based on the replies I got I think my plan going forward will be to swap out the front chain for the GRX 48/31 or the 46/30 (haven't decided yet) and the GRX FD. I'll likely keep the 11/34 rear cassette initially and my Ultegra mid-cage RD. Sounds like that should work.
If I want to try to go lower later can get a 11-40 and swap out the RD for the RX810.
This should get me where I want to go and save some money.
BTW in the area I live we have lot of hills, and I'm planning to do the Eroica gravel/dirt event in April and it has some really long steep climbs, which I did in November but we split it up and did one climb each for three days. The Cyprus Mt climb which comes at the end has really steep sections approaching 20% for extended periods.
Thanks for the info provided.
John

jeffwhitfield 

I went with the 48/31 crankset with a 11/34 rear cassette initially for my wheel sets. Ends up being really good for me since I switch between a road-only wheelset and a few gravel-specific ones. Once I found out that I could use an 11-40 cassette, I switch out one of my gravel wheel sets with an 11-40. Ends up being a very, very nice compromise. A 1x groupset just seems too limiting to me. Great for a gravel-only bike with specific rides in mind (dropper post). But not great if you plan on riding on some road rides too.

redlude97

Huh, all my gears work with an rx800 which has a total wrap capacity of 39t with a 50/34 11-40 setup

jwill911

I pulled the trigger on the GRX 48/31 chain set and GRX front derailleur my local bike shop ordered it yesterday. I am for the time being keeping the 11/34 cassette and Ultrgra mid-cage RD. I'll check back once it's done and I've had a chance to get out in the dirt/gravel.
John

Grimnur

I just accidently picked up the GRX RD-RX810 for my Warbird, I had wanted to get the RD-RX812 to run a 42t cassette but the 1x GRX rear derailleur that REI is selling is the 810 (which I should have spotted since it said max 34t rear, but I didn't notice until I received the box). I had an RD-M8000 SGS inner and outer plate assembly on hand and the RD-RX810 lower cogs appear to be the same length as a SGS. I had read about people putting RD-M8000 plates on the RD-RX812 to increase the take up in the free chain to run it as a 2x rear setup with a 42 tooth rear. It appears from photos of the two that the RX812 is running a GS plate on the lower cogs, does anyone have a RX812 to compare with some GS lengths?

I'd like to use the same rear derailleur for both a 2x and a 1x setup, the RX810 will do this, but I'll need a fairly small front crank cog (maybe 38?) if I am stuck at 36t or 40t on the rear. I'm torn on whether to modify the plates on the RX812 to get chain takeup and the 42-46t cassette granny gear or to just go with a Roadlink on the RX810. The two GRX RDs cost the same and the Roadlink costs the same as the RD-M8000 inner and outer plates. So I guess it just comes down to which would work best (or at all for my setup) and have the least chance of failing midride.

I tried to post a few pictures of the RX810 next to the RD-M8000 SGS plates, but I don't have enough posts to include pictures it appears. The cog mount holes lined up and there was only a little more protective cage area on the RD-M8000 than the RX810.

LetsGoBike

HI All,

I'm getting new bike in next 2 weeks Cube Cross Race SL 2020 - I cant include link as I'm a new forum user.

This bike is using REAR DERAILLEUR: Shimano GRX RD-RX812, Direct Mount, 11-Speed with CASSETTE: Shimano CS-M7000, 11-42T and CRANKSET: Shimano GRX FC-RX600, 40T. Then it looks like RD-RX812 can handle 42T cassette.

I'm just wondering if decide to go and change the Crankset to this: FC-RX600-11, 2x11, 46-30T and I can still use that RD-RX812 on the back with new Crankset?

Do you think it is gonna work or not?

Thanks

Matt

hermanchauw

I am using a much older generation rd (read: shorter cage) with 2x and 11-40 rear. Can.

The thing with 1x, 2x or 3x is the chain slack take up capacity, which is dependent on the cage length. Older medium cage rds can do 3x. I don't see any reason why a newer longer cage rd cannot.

hermanchauw

I just did some calculations.

34-50 front, 11-40 rear gives 45 tooth chain capacity.

An old school 22-32-44 front, 11-32 rear gives 43 tooth chain capacity. And a medium cage range of motion is nowhere near its maximum.

Another old school set up of 22-36-48 front, 11-32 rear gives 47 tooth chain capacity.

Your set up can work

tomtomtom123

No. Rx812 is 31t capacity. 46/30 11-42 is 47t wrap.

LetsGoBike

It looks like this setup will work but with long cage rear derailleur (RD-RX810) I found video showing it working but I'm new on this forum and I cant include one here!

Grimnur

I have the 46-30t 2x GRX 600 cranks with the GRX 810 and 11-42t rear on a Salsa Warbird. I ended up using the longer Wolfstooth extra long b screw to make the adjustments, but that might not be absolutely necessary. Sets up and adjusts easily. Slightly rough shifting on the 2nd and 3rd largest rear cassette gears, but nothing serious. Shifts fine into largest.

EDITED to correctly identify this as a mod to allow the GRX 810 rear derailleur to work with a 11-42t rear cassette (up from the 34t max Shimano states). Which if I had read closer wasn't the topic of the post I was responding to, my apologies!

tomtomtom123

Hm... Maybe Shimano put 31t wrap capacity because of 42-11=31, and didn't bother giving the real number if they didn't want people using it with double chainrings. If you say it works on your setup then that's at least 47t wrap.

Grimnur

My mistake, apologies for not reading the post closely enough. My post was my setup for the GRX 810 rear (even though I said 812 in the post). I'll go edit my initial post so others don't get the incorrect information on the 812.

I keep confusing the two derailleurs because I had initially ordered the 812 but received an 810 instead (same box for all the GRX derailleurs it seems, just a pen mark next to the one that should be in the box). The 810 ended up working fine for my needs when I found out I could swap the b screw and have it work with the 11-42 rear cassette. But since I had ordered the 812 I had been planning to mod it to work with 2x front cranks based on the below information. I have NOT done the mods to the 812 below, but my experience with the 810 makes it seem very reasonable if you plan to keep the 11-42 rear cassette.

When I initially purchased the 812 I had planned to swap out the stock inner and outer plates on the 812 cage assembly to the SGS plates for the Shimano RD-M8000-SGS to get the additional chain wrap for the 2x (approximately +8t). I purchased the M8000-SGS plates based on a proof of concept video on youtube. I can't post URLs yet, so if you go to Youtube and search " Modified Shimano GRX RD-RX812 1x11 spd rear derailleur to supports 2x11 spd w/ 11-46T cassette"
The creator is "Service Customer". Also Youtuber "Moz on Bikes" has a video titled "HACK-Tiagra 10 speed with GRX 11 speed clutch derailleur" where he demonstrates the 812 working in a 2x setup with Tiagra 4700 shifters (so yeah clearly some undocumented STI shifter compatibility beyond what shimano put in the spec sheets).

In comparing the SGS plates to the cage on the 810 that I actually received, they are almost exactly the same dimensions (this is how I first figured out that I was shipped the wrong derailleur). Which lead me to believe that the chain capacity would have been very similar between the stock 810 and a cage swapped 812 (so around 40t)

The plate replacements for the longer SGS cage only cost about $30 at the time. Since then Garbuk has put out a purpose built rear cage replacement with a 50t top end for the GRX 812 that should have a pretty similar cage length, but a better angle for the first cog. So I would spend the extra $20-30 and just go with the Garbuk if you want the extra chain capacity (+8t).

Garbuk rear cage can be found by searching for Garbuk Rear Derailleur Cage for Shimano GRX 11-speed, they are $65 and come in a bunch of colors.

Bikepacking.com has a few articles on mullet builds and they found the stock GRX 812 can handle a 11-46t cassette (4t more than Shimano's stated) with no additions, just adjustments. They took the 812 up to a 50t rear with the addition of the Wolfstooth GoatLink (+4t). The Bikepacking article can be found by searching Bikepacking.com GUIDE TO MULLET DRIVETRAINS: PAIRING ROAD SHIFTERS WITH MTB DERAILLEURS/CASSETTES, it was published April 21, 2020 by Logan Watts (who has a bunch of great articles on this kind of equipment modding).

The swap to get the 812 working with the 2x setup with an 11-42t cassette was a bit more than expanding the range of the the 810 in the rear (so I just stuck with the 810 that I was sent). But if you are keeping the 11-42t rear cassette and the 812 is stock with a 31t capacity, then you already have +4 capacity stock as Bikepacking found, and you gain at least 8t from the cage swap giving you at least +12 with just the change to the cage and some adjustments. Given the fact that the swapped out SGS cage is the same length as the 810 stock cage, and the 810 works stock with the 2x GRX600 46-30 cranks, there should be plenty of capacity if I were doing this to my Salsa Warbird. That said, it is $112 for the GRX 810 which will work in 1x and 2x with any crank between 30 and 50t and any cassette below 42t.

LetsGoBike

Hi,

First of all, Thanks for your time explaining everything in detail and like you I do keep confusing two derailleurs 812(this should be used with x1 crankset) and 810(should be used with x2 crankset) saying this if you check the video on youtube with title "Modified Shimano GRX RD-RX812 1x11 spd rear derailleur to supports 2x11 spd w/ 11-46T cassette." channel "Service Customer" you will see that he is using Modded 812(inner and outer plate are changed to the XT RD-M8000-SGS super long cage rear derailleur to support the front double-speed crank) with crankset FC-6800 50-34T.

But I personly on the end decided to take a completely different approach (cheaper and I can do it into stages) I will stay with crankset x1 but I will change my chainring on GRX600 from 40T to 42 or 44T(make more sense to go with 44T) from WolfTooth and in the future try to see if I can change my wheel freehub to use Sram XDR to use SRAM Cassette with 11 speed but 10-42 and that means that I'm only adding +2T or +4T to the setup and leave it to chain length do extra work.

I'm just trying to match my old bike setup range what I mean highest and lowest gear. The old setup was road bike 34-50T and cassette 11-32. I just use it last 5 years.

There is a nice website that you can put all details to check the gear ratio look for "home earthlink net/~mike sherman" please replace "space" with "." or use google - I can't post links yet!
Make sure that you use correct tyre size to calculate everything.

BTW I'm collecting my new bike tomorrow. I will order and change the chainring in the next 2 weeks.