GRX812 Mullet
 

I run a 40t 1x w/an 11-42t cassette on my gravel bike. I am fat and old. I've read that a 42t cassette is max w/the GRX812 rear der. - but some run an 11-46 with no issues? Internet stories.

Any experience w/a 1x 40 by 46 in the rear w/a GRX812 der?

I was thinking a 46 or 50 in the rear; do I need to go w/a Road link DM?

I've run 11-46 with the 812. Works great, no modifications.

I've heard, but have not been able to test myself, that the GRX822 (12 speed) rear derailleur works with 11 speed shifters, and would let you run an 11-51 11 speed cassette.

There is a YouTube channel that tested it and it seems to be a perfect match. The same channel put a sgs XT cage on the 812 and ran a 11/51 on it without any issues. It really is a good time for people who want wide range 1x gearing.

Why not a smaller ring? Top end speed?

The maximum RD wrap depends on the chain stay length, the largest spocket pnd the chain ring size chosen. A 40/46 uses 21.5 inches of chain, while a 38/46 uses 21 inches. A 16 inch, or 406.4 mm chain stay uses 32 inches of chain. The next even number is 16.5 inch or 419 mm.

The point is that some combinations may leave the chain slack in the smallest combination, in order to wrap the big/big, while others can make use of every inch. Chain length can only be changed in 1 inch increments.

RDs always have some excess wrap capacity to account for less than ideal chain ring and sprocket combinations.

My 46/44 big/big uses 22.5 inches of chain and the 415 mm chain stays use an odd 32.68 inches of chain. Adding 1inch for the 2x RD, the total is 56.18 inches, so a 57 inch chain is needed. It doesn't make full use of the 57 inch length.

Looking at an XT 11-s cassette, the final jump goes from 37 to 46! On the 11-51, the final jump is only 45-51 (m5100 cassette).

Might try the true mullet and go big!

I was considering the same 1x set up, didn't like the jump of the last big cogs of Shimano either. But there's this that might work:
https://www.ebay.com/itm/125446611428
Microshift makes an H-version of this cassette for less $, but a little heavier.
For me the 1x- 40/46 should be low enuf for when gravel turns into more gnarly MTB terrain, they often do here.

That is a much better jump than the XT. I will check out SRAM also. I think 40/46 will get me up anything local. I can grind middle green gulch in my 40/42 but it would be so much more 'pleasant' if I could spin the first ramp of the climb.

Note to self; never spec / build a gravel bike / gearing when you are in great shape. Think ahead!

My thought as well. If you could get something with a 34 ring it would lighten everything up as well since you wouldn't need a pie-plate cassette and a massive rear der. cage. And hopefully smaller jumps as well.

It strikes me as a really weird ring size for a 1X. It's not tall enough for the pointy end of a race, but it's not low enough for steep dirt or gravel. Just kind of stuck in the middle of no-man's land.