chain slack with 2x setups with large rear cassettes

Question

I would like to upgrade the transmission of my bike and was thinking to a 2x setup (36/26) with a 9/46 rear cassette, with the latest SLX. There was some contradictory info the cassette manufacturer, saying on one page that there's no problem to use their cassette on a 2x setup as long as the rear derailleur has enough capacity, and another page saying that their cassette are not compatible with any long cage derailleur (1x or 2x).
So I asked them, and their reply was it is not recommended, as it would imply to have a very long chain, that would have too much slack in some gear configurations. I then gave them a the example of the RD-M8000-SGS, that has the needed capacity and max-sprocket size, the explanation remained the same and they push to the LBS (that doesn't sell their brand anyway).
But I have to say I don't buy the explanation:

to limit slack is not reason rear derailleurs are rated for a given capacity and clutch derailleurs have been invented?
with the last SLX and XT (M7100 and M8100 — I'm aiming for SLX), the only 2x offering is 36/26 and 10/45. 9/46 doesn't seem to be so far off. So why it should be bad for them and good for the very conservative Shimano.
going to a 1x would require to have an even longer chain (if the chainring is bigger than 32 for a 50T cassette), that should be handled by a derailleur with an even smaller - but there would be more clearance between the chain and chainstay, at least in the front.

Do they have a point? Or am I missing something?

leftaroundabout · Accepted Answer

So I asked them, and their reply was it is not recommended, as it would imply to have a very long chain

Well, it doesn't really require a very long chain. It would require a long chain if you were to use all possible gear combinations, as new 2× bikes typically allow – but the large-large and small-small combinations, i.e. cross-chaining, aren't really supposed to be used anyway. If you make sure never to use large-large, then you can make do with a reasonably short chain, and by avoiding small-small you also avoid situations with excessive slack.
As you say, a 1× system like everyone is riding nowadays actually requires the derailleur to take up more slack (with same, or smaller, gear range). The benefit is 1× allows for a narrow-wide chainring, which means you hardly need to worry about the chain coming off the chainring anymore even in gears with a lot of slack. But from my experience, a modern 2× system isn't much worse in that regard either – just make sure you're in a middle gear before going into really rough stuff, and after it load the drivedrive a bit carefully in case the chain has come off (which seldom happens, but if it does it's usually enough to once shift down and up again with the front derailleur under ghost-pedalling).
Another question is of course whether you really need such a huge gear range. 26:46 is a ratio of 0.57. Now, I do like a <1 gear for long steep fire roads, but that low does seem excessive. On most climbs where this sort of gear could be desirable, riding in the saddle just gets too unstable and it's more efficient to get off and push or even carry the bike instead. Whereas for sprints out of the saddle, you need a gear of at least 1.2 or so.On the other end, 36:9 can certainly be useful, if only on flat tarmac. Even then, you have to work quite hard to make use of it when running knobby tyres, but – why not.

juhist · Answer

It is true that if riding on bumpy ground in the gear combination with the most slack, a clutch derailleur has a smaller probability of dropping the chain from chainrings or having the chain hit the chainstay. For example, I have a 26"x4.8" tired fatbike with a ridiculous 34T narrow/wide chainring, meaning of the 11-46T cassette the only useful gear is the 11T sprocket. Unfortunately this is the sprocket that has the most chain slack. Initially when purchasing the bike, I was not aware of the need to activate the clutch mechanism (it was not activated from the factory), so I dropped the chain riding on bumpy ground and had to get my fingers stained with chain oil to raise the chain to the narrow/wide chainring, not having a front derailleur to do the same. Now I have bought a 42T chainring and have to install this someday. It would somewhat reduce the chain slack because there would be other useful sprockets than the 11T sprocket if using a 42T chainring.
Most likely if a derailleur is advertised by the manufacturer (Shimano) to work with a 10-45T, you can use with 9-46T provided you won't exceed the capacity and the cassette fits to your hub (most hubs only accept 11T small sprockets). Usually Shimano is very careful in listing the compatibility, only listing combinations that work with perfect shifting. However, long time ago chainrings had no ramps and had no profiled teeth, sprockets were not side-cut and had no profiled teeth, and nobody complained about bad shifting. With today's technology perfect shifting is well ... just perfect, so slightly less than the best possible shifting performance is still acceptable.
If you have 36T chainring and 46T big sprocket, then >32T chainring and 50T big sprocket requires a longer chain, yes. But the derailleur still sees less slack. The capacity of a derailleur is listed as big_ring - little_ring + big_sprocket - little_sprocket. In 1X systems it's only big_sprocket - little_sprocket. So 36/26T and 9-46T requires 47T capacity but 1X with 9-50T would require only 41T capacity.

chain slack with 2x setups with large rear cassettes

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