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Discussion Starter · #1 ·
Hi. I have a Scott scale 940. I had about 3 dropped chains after I upgraded my front chain rings from a 42t to a single Race Face Narrow/wide 36T chain ring. My bike came with a Shimano XT RD-M781 SGS, Shadow Type, 30-Speed. I was thinking of getting a rear derailleur with a built in clutch, such as the Shimano XTR M986. My question is, is it better to go with a clutch or get a shorter rear derailleur? Not sure if that makes sense. When I say shorter I mean that the cog wheels are not so far spaced apart. I know some use a chain guide but I prefer not to use that. Thanks
 

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Was this 42T the big ring of a triple? If so, your stock drivetrain inlcuded a long cage rear derailleur to provide the needed capacity, where:

Capacity = (largest ring in front - smallest ring in front) + (largest cog in back - smallest cog in back),

C = F[SUB]L[/SUB] - F[SUB]S[/SUB] + R[SUB]L[/SUB] - R[SUB]S[/SUB]

Measured in teeth (T).

With any single ring setup F[SUB]L[/SUB]-F[SUB]S[/SUB]=0, and so capacity is just R[SUB]L[/SUB]-R[SUB]S[/SUB]. On a 11-36, that's 25T.

A modern mountain short-cage rear derailleur has, unsurprisingly, a capacity of 25T, to go with the defacto standard cassette of 11-36. If you run a 10-42, or an aftermarket cog to go 11-40/42, the capacity is over 25T, and that's why those setups call for a medium-cage derailleur.

The reason to not keep your long-cage, and to avoid a medium cage, is that the shortest cage you can get away with (has sufficient capacity) will handle chain slack better than the next cage length up.

But perhaps not so much better that getting a medium-cage wouldn't be a bad idea, as a hedge against your trying out something like the Hope or General Lee adapters, or the wide range cassettes coming out after Sea Otter.

Clutch has nothing to do with this so far. I ran a Raceface N/W on a 9-speed clutchless system for months with zero drops. Recently went to 10-speed clutch and continue to have no chain drops. So I would say the clutch can't hurt and very likely helps. They are basically standard on 10-speed stuff now, so why not add one. But don't think the clutch will make up for poor choices in cage length and chain length. Those should be your first concern.

Good luck.
 

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Discussion Starter · #5 ·
Yes I had my LBS shorten the chain as much as possible. 42T was the biggest ring in the front. I'm thinking about going with a Medium rear derailleur. It's a Shimano XTR M986 Rear Derailleur with clutch.
 

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Cage length doesn't matter with a 1 x setup.

The clutch is what keeps the chain on, it limits the forward movement of the derailer.

If you shorten the cage you will not be able to use that mech if you decide to go back to the original set up.
 

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I run a 1x10 with a 11/36 in the back using a short-cage x9 clutch derailleur. I only have about 300 miles on the bike but I have dropped the chain a couple of times now going over very rough stuff. Now I have gotten into the habit of switching into the larger cogs during those descents to have more tension on the chain. Since I started doing that no more dropped chains but it's too early to say for sure. I would definitely recommend a type 2 rear derailleur and a short cage if you plan on sticking with 1x10 and 11/36.

I am not sure why you are asking clutch or short. You can get a short cage with a clutch -- at least from SRAM you can but probably Shimano as well.
 
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