dkyser

I have switched over to mostly gravel & paved trails due to the drivers in the St Louis area.
I am looking for lower gears from my current setup. Ultegra 8000 50/34 & 11/32 cassette with GS derailleur on my Gunnar Hpyer-X disc frame.
I have a 11/34 Cassette on its way but thinking I may be better off going 11/42 and adding the GRX RX812 derailleur. Can I use my Ultegra compact crank and FD and GRX RD?
Thoughts?

Crankycrank

Check this chart here. Bicycle Cassettes & Drivetrains Chart - Google Sheets GRX specs start at column 70. Gives you pull ratios (same as 8000, row 85), max size cassette cogs, etc. Looks like it should work. FD and Crank should also work.

Badger6

Thoughts? Are you getting ready to switch this to a 1x setup? That RD is spec'd with a 31T total capacity, and a maximum low cog of 42t. An 11-42 cassette is 31t, leaving no specified room for a FD...the RD will be unable to wrap the chain on that chains with an additional 16t between the 50 and 34 chainrings.

You could probably get away with a SRAM 11-36 cassette with your current RD, with some b-screw adjustment. You'd be outside the spec'd capacity, but plenty of folks claim they can make it work. But, when you get into the territory of matching an 11-42 cassette with a 50-34 chainset, you're gonna have issues getting it to work, if you can ever get it to work.

So, I used to go to St Louis for work a couple of times a year, and I always took a bike, and I rode a bunch of the bike paths (and up and down the Missouri and Mississipi rivers), and I gotta be honest, I can't figure out why you think you need a gear lower than 34/32 or 34/34. I'm not saying this to be a jerk, but the best off road bike riding on that side of the river (and the levee paths on the Illinois side) is flat.

dkyser

I probably will switch to 1X, I seldom use the 50T sprocket as I am not one to pedal much on descents. My weight and gravity take care of that LOL.
I have a paved path that leads from my subdivision to Quail Ridge Park which has some steep grades and two-track paths I will want to use.

I may try the 11/34 first and see how that works.

Badger6

I am definitely biased toward my big ring, and frequently climb with it if the grade is consistent and below 7-8%. I need to do a better job of remembering that not everyone else does the same...I never rode in that spot, but I see what you mean by pulling up a terrain overlay. The upside is that those climbs and descents all look to be between 10-20m total, which is quite short (this is good news!).

To your original question, and as I stated above, the GRX RD will not work with a 2x chain set. There is an older Shimano MTB RD, the RD-M8000 with an SGS cage, that has a 47 tooth capacity (for an 11-42 cassette, that's 31T, plus the 16T of the 50-34 chain set) that works with up to a 42T big cog in a 2x set up. But, it's a MTB RD, and the pull ratio is different than your shift levers. Wolf Tooth makes something called a Tanpan that will adapt the pull ratio from your lever to properly index that RD, so you could run it....but that said, it'd be faster and easier to get a SRAM 11-36 (11 spd) cassette and try that first, with the only adjustment necessary to make it work is to adjust the b-screw...yes, the 36T low cog is outside the Shimano spec, but it's only 2T outside (both in terms of max cog and capacity) and Shimano is known to conservatively spec their equipment, real world testimonials indicate that the 11-36 will work fine.

jp911

I have a very similar setup to what you're describing: Ultegra shifters, GRX812 rear derailleur, and an 11-42 cassette (crankset is 1x Easton EC90 with a 40 tooth chainring). Works perfectly!

As has been pointed out, it's really not going to work with a 2x crankset and front derailleur.

shelbyfv

This GRX 46-30T crank with your 34T cassette would give you a good range. Use your same bottom bracket. You may or may not need a GRX front derailleur, reports vary. The swap would likely be less expensive than a 1X conversion.

Badger6

It won't work. Period. The RD can't wrap the amount of chain required for a 50/42 combo when it's set at 34/11. (Yes, I know those are not really usable, but even if the chain is set to properly go over 50/28, it most certainly will be incredibly slack on 34/12.) That RD is specifically for 1x setups, like you are using. The OPs only option for a cassette that wide with a 2x setup—I assume that is the goal, since there was no mention of 1x—is to use a "mullet" setup with an RD-M8000-SGS and then use an inline "reduction" adapter to make the different cable pulls of the shifter (1.1) and RD (1.4) compatible.

dkyser

My GRX 812, 11/42 and 1X 40T arives today