Question if anyone has tried this combination. I have just purcahsed a new bike with GRX 820 1x setup with a 10-51 Cassette.
I’d like the option to run a road 11-36 Cassette, has anyone tried that?
Thanks,
If the bike runs a 10-51 cassette, it has a mountain bike rear derailleur. Shimano MTB rear derailleurs are only compatible with 12-speed cassettes and only 10-45 and 10-51. A 11-36 cassette is too small and might have too few cogs.
Thanks for the reply, I should have been more clear.
The bike is running the 1x specific GRX 12 speed rear derailleur RD-RX822-SGS
I’m talking about the 12 speed 11-36 road cassette like the Shimano 105 CS-HG710
I understand they “Technically” Aren’t meant to work together. I was wondering if anyone had any real world experiance running a 12 speed road cassette like the above with the new 12 speed 1x specific derailleur from the new 820 GRX groupset.
They are not compatible as the cassette you intend to run is too far out of spec. According to Shimano’s own specs even the 10–45 cassette is not officially supported. (Although that should work as the rear derailleur should be a rebadged MTB rear derailleur.)
Usually, you can push the specs a little and e. g. run a 11–36 cassette when officially only an 11–32 cassette is supported (as was the case with 11-speed Shimano drivetrains).
So you will need to get a new rear derailleur if you want to run that cassette.
Let’s reverse the question: are you sure this is the right gear range? Assuming you have a 40-tooth chain ring, the gear range that a 11–36 cassette offers might be too narrow.
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Sure, again I get that it’s not in spec. I’m really asking if anyone has tried it and has any experience.
A long cage 105 mech can run anywhere from 12-23 to 11-34 no problem. The idea that a mech is designed for only one cassette and will work with no other cassette doesn’t really make all that much sense to me.
For your other question, yes 40 with 11-36 cassette would be great.
I’m planning on using the bike offroad in the summer, for that the 40x10-51 is great.
I’d like to use the bike for Winter training on the road also, for that 40x11-36 should also be great.
40x11 is 47kph @ 100 RPM more than enough for a winter training bike with big tyres and mudguards.
40x36 is equvilant to 36x32 which is what I currently have on my winter road bike.
Another issue you might have is the freehub body. The 10-51t cassette is for 12spd microspline, and the road cassette is not. If it could work, you would need to swap out freehub body, or use a different wheel with the cassette you want.
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Thanks, yes my plan was to get a road wheel set for the winter training as well. Then i can have a set of wheels with 45c knobby tyres and a 10-51 cassette on and a pair of wheels with 32c slicks and a 11-36 on.
across manufacturers and their respective mechs/cassettes, many combinations are possible.
the mfg recommendations are based on different mechs being optimized for certain cassettes, which is completely reasonable and based on their actual mechanical design.
rear derailleur linkages (parallelogram) move in different ways, ideally they closely follow the steps (# of teeth) of the cassette through the shifting range.
also there is the issue of cage length and how much chain slack can be taken up by a certain rear mech in different drivetrain configurations.
there is generally some wiggle room, and you can do things like max out B screw adjustments or go for aftermarket parts etc, but shifting IMO/IME is never as good in all respects as a perfectly configured setup.
I understand you are all trying to be helpful. But my original post is asking if anyone has tried it. Real world experience and what it’s like.
Again, I realise it’s out of spec and won’t be perfect but I’m not looking for theoretical possibles I do understand them. But the real world could be anywhere from completely unusable to slightly slow shifting otherwise fine.
you literally wrote this earlier today.
I haven’t heard anyone trying it, because the specs are so widely different that it is very unlikely it will work.
The same goes for SRAM’s mountain bike rear derailleurs and the 10/11–44 cassettes, to my knowledge nobody has tried it for the same reason.
Of course, if anyone has heard of someone trying it, please post a link here. 
I’ll see if I can borrow a wheel from a friend a give it a go. I’ll post back here if I do.
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Local shop just did it and it worked fine. 
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The thing I don’t understand about it being “wildly out of spec” is that all the cogs in a 11-36 are contained within a 10-51. The mech has no problem moving over them within a 10-51 cassette so why would it struggle when you put a 11-36 on?
Why can my current mech work fine with a 11-23 and a 11-34. But the GRX mech ONLY work with a 10-51 and no other cassette?
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The main issue in your situation is whether the rear mech will keep the chain taut and whether it will keep the chain securely on the sprockets, especially towards the climby end of the cassette.
That is because the RD design presumes a certain cassette geometry so a certain side-to-side movement will result in a corresponding up-down movement.
The relative and absolute size differences are a lot larger than in your example or even with the XPLR RD, where the difference is 6-8 teeth as opposed to 9-15.
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the chain tension is not an issue imo, as @hollandgdavid said, the derailleur is capable of taking up the slack in the smaller cogs of a 10-51t for example.
it is the rd design/movement path that can cause wonky shifting as you mentioned, along with possible b-tension screw/derailleur hanger tab interface issues.
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I’m not 100 % convinced. At the very least he needs to use a shorter chain, no?
If you try it, try it out on a repair stand first. Check that the bike can shift the whole cassette, up and down. Pay attention to the chain length and pulley cage angle.
So many people setup their drivetrain without doing this, then the first time they shift into the their easiest gears they rip off the rear mech. Or they have way too long of a chain and it pops off when they hit some bumps.
Don’t worry it’s not my first rodeo with weird gears.
I was running 58 single ring with a 11-36 aftermarket cassette on a Ultegra 6800 groupset for a couple of months last year.
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Any update on this I am Curious as well.