996

Hi - looking to build up my gravel bike with a 1x system. I have Shimano 105 11sp hydraulic shifters... looking for a rear mech. I wanted to clarify a few things with GRX 812 and GRX 810.

GRX 812 specs:

• Top sprocket: Max. 11T/Min. 11T
• Low sprocket: Max. 42T/Min. 40T
• Total capacity: 31T

GRX 810 specs:

• Maximum front difference: 17T
• Top sprocket: Max. 11T/Min. 11T
• Low sprocket: Max. 34T/Min. 30T
• Total capacity: 40T

So it seems as though the GRX 810 is for a 2x setup and the GRX 812 is for a 1x setup. My question is the following: Can I run the GRX 810 with a 1x setup? The 40T total capacity of the 810 should in theory let someone run a 10-50 cassette in the rear with a single up front? Has anyone had any experience with this?

Any input would be greatly appreciated. My goal is to run a 38 or 40 1x up front with a 10-46 or 10-50 cassette in the rear with my Shimano 105 R7020 shifter(s).

Thank you!

dsaul

The 810 has a Max large cog size of 34t, so you could go to a 36t without any problems, but a 10-50 cassette is out of the question. The total capacity of 40t means that it can take up the slack from a difference of 40 teeth, but the geometry of the derailleur will not allow the upper pulley to clear anywhere near a 50t cog.

If you want a 1x, use the GRX 812, but a 10-50 cassette would still be too big. If you want low gearing, go with the 2x and the 46/30 GRX crankset. 1x just doesn't make much sense for a gravel bike, unless you live in a relatively flat area.

shoota 

I just looked into this as well and settled on 1X 40 - 10/42. I've read more than one success report of using a 10T (technically not allowed) and the ability to still shift up to the 42. I think that will be about the best you can do with this setup.

msu2001la

GRX 1x max cassette size is 11-42. Some internet forums indicate 11-46 can work, so this might be the biggest option for you.

You'll need a MTB rear derailleur to work on a 10-50, and there isn't one that would be compatible with 105 road shifters. If you want to dive into SRAM eTap AXS, you can run a "mullet build" with MTB rear mech, and road shifters.

2x is the obvious solution, but that's answering a question you didn't ask.

Elvo

Sram Type 3 road/gravel long cage rear deraillers will also fit 10-46T no problem. For 10-50+ you will need to hack a SRAM eagle mountain bike derailer.

996

Got it - appreciate the reply. Did you end up going with a GRX 812 rear mech?

shoota 

It's Di2 so it's the RX817: https://bike.shimano.com/en-EU/produ.../RD-RX817.html

MAGAIVER

The 812 is the 1x specific rear derailleur, Shimano claims a 42t maximum rear cog but I run an XT 11-46 on my bike with the 812 and have 0 problems. I have a second wheelset with an SLX 11-42 cassete and I swap back forth witthout any need to readjust anything. I even filmed the shifting with the 11-46 cassete the day I got the bike built.
If you need the 10t cog you may find a 10-46 cassete from brands like Sunrace and Zitto but you will have to upgrade your rear hub to an Xd Driver. And if you want to go all out, get the Di2 stuff and use a GRX Di2 shifter with an XT or XTR Di2 rear derailleur, the XT can handle an 11-50 just fine,I run an 11-50 with an XT M8000 derailleur on my XC bike.

996

Wow thank you for this! So you have no shifting problems even on higher gears? (11T cog) - no sloppiness? Very good to hear. I was thinking of doing a 36T up front with an 11-42 in back... but this opens me up to maybe a 40T 1x in the front and an 11-46 in the rear.

MAGAIVER

I haven't been able to ride it as much as I'd like to but I had no issues with it so far, shifts perfect, the 812 looks a lot like an XT 11 speed RD and Shimano is usually very conservative on the derailleur capacity quotes. I am running a 40t ring on that bike.

rosefarts

Are you loaded up on the bike?

Are you using it on very technical rocky and rooty steep climbing?

I ask this because those situations are where ultra low gearing really shines.

For gravel riding, in Colorado, with occasional mediocre mountain biking thrown in, you're about as low as you need.

40x42 is almost enough for anything a one day unloaded bike might see. If you go 38x42 or 42x46, you're not going to need more.

Down the hills, especially on washboards and switchbacks, I rarely find myself pedaling much past 20mph. If I find myself on pavement, sure I run out of high gears but it's fairly inconsequential with a good tuck.

I currently run 40x11-36 most times and occasionally switch to 11-42. On a 22 lb bike. That's where I base most of this on.

My new bike, a Swiss Cross, will be running 11-46 and a 44 for 90% of my rides. I'm giving myself the option to switch to a 38t chainring for oddball days.

I do think you're thinking in the right direction, gearing for all major groups is reflective of fast riders on fast courses and there's not a single complete group on the market that fits so many peoples needs. Ekar is the closest thing but $$$$$!

sean.hwy

if you convert a 2x bike that came with grx 810 to 1x can you use a road link so the 11-46T would work?

or do you need the grx 812 to make the 11-46T work?

I know grx 812 works with the 11-46T because it came that way from the factory on my grx equiped Ibis MX.



https://www.ibiscycles.com/bikes/hakka-mx

HTupolev

Somewhat, although it's unlikely to shift as well as the 812 in that case. I actually know a guy who was using an 11-46 cassette with the 810 on a 2x drivetrain, but it was really straining the boundaries of good behavior.

Rooney 

Old thread but I watched this video earlier this week and it seems at least somewhat applicable to the conversion.

chembers

Hi, I'm hoping someone can advise here.

I have a 2x11 set up on a gravel bike with Shimano GRX810 rear derailleur. It's damaged but a friend has an 812 from his Cannondale Topstone (written off) he said I can have.

I'm a noob when it comes to the technical side of things. Can I just straight swap this on to mine. My cassette is 11-34 and the front is 46/30

I've read the stuff at the start of this thread but don't really understand.

Many thanks.

sean.hwy

If you are getting it for free just try it and report back. I think you "might not" have enough chain wrap. My Ibis hakka grx 812 had an 11-46T cassette = 35 spread. That's already outside
Shimano recommendation.
https://bike.shimano.com/en-US/produ.../RD-RX812.html
Total capacity: 31T




You have 39 spread.
46-30 = 16 in the front
11-34T = 23 in the back

16 + 23 = 39

Since it's free just try it. If it has too much slack in the small and small gear you can then decide on buy a grx 810 rear mech or buying a smaller rear cassette.