SergeyKrog

Hi,

I'm building Salsa Fargo TI (dropbars) light bike packing off-road touring based on the gears I have on my MTB.
The chosen group set is Shimano GRX except front crankset (cause the frameset has BSA73 bottom bracket) and the brake calipers (Here I'm planning to use Hope rx4 Postmount)

So my question is will I have issues with the rear derailleur (RX812) paired with 11-42 MT8000 cassette and 26/36 front chainrings?

Thanks in advance!

dsbrantjr

The specs on that derailleur call out a max capacity of 31 teeth, you are trying to exceed that by 10 teeth so I would say you will have problems. I am not familiar with that particlular part but I by the specs I suspect that it is intended for a single front chain ring only.
https://bike.shimano.com/en-EU/produ.../RD-RX812.html

SergeyKrog

Great, then I should be well with RX-810 (40T capacity) with the following setup: 36-26 / 11-40 XT M8000

HillRider

I think that set up may work only if you limit the use of the 26T chainring to a cog no smaller than 21T. That will stay within the wrap capacity of the derailleur. Any smaller cogs will let the chain go slack. Be aware it is essential the chain be made long enough to allow big-big (36x41).

SergeyKrog

Checked with bike gears calculator, and you are definitely right.
I have the same setup on my MTB: 36-26 / 11-42 with XT rear derailleur without any issues in rides.

dsbrantjr

Um, no, that one is specified for a 34T max big cog, it is mentioned twice on this sheet: https://bike.shimano.com/en-EU/produ.../RD-RX810.html

SergeyKrog

I have several possible compatibility issues with GRX and MTB FD, as well as this rear compatibility. So decided to move 1x11 at the stage 1. (RD-RX812).
Thanks for you help!

Bill Kapaun

That should work since you still have 1T wrap left.
You can fudge a couple teeth if you are willing to have some chain "sag" in Small:Small. A gear (or 2) that you are unlikely to use anyway due to severe cross chaining.

tomtomtom123

https://productinfo.shimano.com/#/sp...r%20Derailleur
The rx810 Rd says up to 34t max cog.

Instead of 40/11 36/26, if you have a 64bcd crank, then using 34/11 36/22 would give you a similar gear range with closer stepping.
36/11 36/22 would give you a little more range than 42/11 36/26.
You can even find 20t for 64bcd if you are willing to shave a little bit of the spider. You could then try 32/11 36/20.
You could try for maxed out 36/11 36/20 for 589% range. You'd be 2t over the max cog and 1t over max chain wrap but it's probably still ok
You could even go for 36/12 36/20 to get closer stepping.
One problem with 11 speed cassette is that usually the big cogs are riveted to a spider so you can't mix and match them, and road 11 cassette is limited to 34t. The MTB 11 cassette also don't have many options for 36t. But Miche 11 speed has an option to build your own cassette. There are a few dealers on the internet offering the service. If I had the choice of 11 speed Miche, I might go with 12,13,14,15,16,18,20,22,25,29,34, And see if I can get 38/20 chainrings. If you had 25.5 inch tires, that would give you 80.75-15.00 gear inch range.
But I like having a lower gear closer to 14 gear inch for very steep climbs, but it's difficult to customize that 36t cog for 11 speed cassettes.

Oh I see that you have 29 inch tires. Then I would prefer 40/14 and 38/20. You could mix a road 14-28 with MTB 11-40. The 3 big + 2 big cogs on the MTB can't be disassembled so you'd get
14,15,16,17,19,21,24,27,31,35,40. But then you can't use the rx810 since it's max 34t, and you'd have to get a MTB Rd and shifter
But some people use something like a wolftooth road link to drop the road Rd so that you can go up to 40t. (oh, also 40/14 and 38/20 is 44t chain wrap, which is higher than the 40t capacity of the rx810, but you might be able to get away with dragging the chain on the outer 3 cogs when on the small chainring)
https://www.wolftoothcomponents.com/products/roadlink

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

hermanchauw

My set up is very similar to yours. 26-38 front, 11-40 rear, with a much older generation rd.

Definitely no issues.

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!

www@instagram@com/p/B1U0AxMHtct/?utm_source=ig_web_copy_link <--------------------- Please REPLACE "@" with "."

Bill Kapaun

Sad when you have to quote yourself with bogus info.
I suggest you learn-
1. Max cog SIZE
2. Chain Wrap Capacity.

LetsGoBike

And I suggest you learn why people use forums like this -
1. Exchange knowledge and information
2. Check if someone else already did it.
3. Thinking outside the box (I suggest you should try this some times)
bikepacking.com/gear/wide-range-grx-2x11/
instagram.com/p/B1U0AxMHtct/?utm_source=ig_web_copy_link

Oso Polar

I humbly suggest to our new mighty lord to check first if he'll be able to install a front derailleur on the bike in question before changing 1x crankset to 2x...

LetsGoBike

Hi,

Thanks for your advice but "New mighty lord" did his homework and cross bikes have much bigger clearance plus the GRX front derailleurs have an additional 2.5mm outboard clearance to provide space for wider tyres, up to 42mm. For this reason, the front mech must be used with the matching GRX chainset which is similarly pushed outward from the frame by 2.5mm.

It looks like rear derailleur 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 in two 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 maybe 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.

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 the correct tire size to calculate everything.

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