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Shimano GRX 1x - 810 or 812 Rear Mech?

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Shimano GRX 1x - 810 or 812 Rear Mech?

Old 12-15-20, 11:46 AM
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996
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Shimano GRX 1x - 810 or 812 Rear Mech?

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!
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Old 12-15-20, 07:39 PM
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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.
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Old 12-16-20, 10:57 AM
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Originally Posted by 996 View Post
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!
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.
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Old 12-16-20, 11:10 AM
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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.
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Old 12-16-20, 12:05 PM
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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.
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Old 12-16-20, 12:37 PM
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Originally Posted by shoota View Post
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.
Got it - appreciate the reply. Did you end up going with a GRX 812 rear mech?
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Old 12-17-20, 08:54 AM
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Originally Posted by 996 View Post
Got it - appreciate the reply. Did you end up going with a GRX 812 rear mech?
It's Di2 so it's the RX817: https://bike.shimano.com/en-EU/produ.../RD-RX817.html
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Old 12-21-20, 06:46 AM
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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.


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Old 12-21-20, 01:17 PM
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Originally Posted by MAGAIVER View Post
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.

https://www.youtube.com/watch?v=71PgvwolgEE
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.
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Old 12-21-20, 01:24 PM
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Originally Posted by 996 View Post
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.
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.
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Old 12-21-20, 02:52 PM
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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 $$$$$!
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