Setting chain length with a mid-length rear derailleur cage

[hookooekoo]

Those who have one will know that the Shimano RD-M900 has a mid-length cage. Its cage length is about halfway between a road dérailleur cage (double chain ring), and a full length MTB dérailleur cage (triple chain ring).

My chain is now as short as possible, but it is still slightly too long to use the smallest two sprockets with the granny ring. If I remove another complete link (2 inches), it will be a mechanical disaster if I forget that the big-sprocket big-chainring combo is forbidden.

Possibly I could slightly improve the situation by buying a half link, but is it really worth it? Mostly I find that if I need the granny ring then I probably need the largest sprockets too.

My setup is 28-12 and 46-36-24. I'm guessing that's slightly too much for the 'Total 33T' maximum difference that's stamped on the dérailleur cage, although I can't remember how to calculate the maximum difference on my setup.

If you have one of these mid-length rear mechs, how did you set your chain length? Did you bother with a half link?

 

[d8mok]

Re:

Why would you use the granny ring with smallest cogs and why on earth would you be anywhere near using the big/big ?

Cross chaining and the fact that ratios will be doubled up by using better combos

I think you are trying to solve a problem that doesn’t (shouldn’t) exist

 

[FluffyChicken]

Re:

That's perfect, as lo g as big-big doesn't snap or jam, it fine. For those accidental days.

Small. Small and a few at he back missing doesn't matter, the chain just jumps around.
Given I use a wider range than you, don't worry.
It's a race groupset compromise, or grab an SGS cage.

 

[legrandefromage]

dont use the smallest sprockets on the granny ring

'mid cage'

Pffftttt!!!

 

[Woz]

^ Agree.

While you are over the rated capacity limit for a GS due to the triple, in practice with sensible shifting practices it's a non-issue.

(46 - 24) + (28 - 12) = 38T

If you do not do big-big: (46 - 24) + (24 - 12) = 34T

If you do not do small-small: (46 - 24) + (28 - 13) = 37T

You are best not bothering with silly half-links. Set the chain length for small-small where the cage tension just starts
without the upper jockey wheel hitting the lower chain run and you will be fine. The mech will cope with the rest
with only a potential issue of chain-slap on the small-small combos if you are riding over bumpy ground at speed.

I routinely push Shimano mechs over the rated limits, not only capacity but also max sprocket size.

 

[hamster]

Re: Re:

Why would you use the granny ring with smallest cogs and why on earth would you be anywhere near using the big/big ?

Big-big only ever happens during emergencies - for example when the big black Range Rover decides to make a close pass just when you are about to do a double downshift and you have no time; or 250km into an Audax and you feel shattered etc.

Big-big prevents a calamity from becoming a disaster. If you want to ride with gears set up with a deliberate self-destruct option (to save the weight of 2 chain links) then up to you.

 

[jimo746]

Re:

Sounds like you've set it up correctly, you definitely don't want a disaster when in Big/Big. If the chain is slightly too long in small/small then I dont think you can do much more with your setup. The time spent in small/small is probably tiny, and easy to avoid.

Short answer... get a longer cage rear mech or live with the relatively small inconvenience .

 

[d8mok]

Re: Re:

Why would you use the granny ring with smallest cogs and why on earth would you be anywhere near using the big/big ?

Big-big only ever happens during emergencies - for example when the big black Range Rover decides to make a close pass just when you are about to do a double downshift and you have no time; or 250km into an Audax and you feel shattered etc.

Big-big prevents a calamity from becoming a disaster. If you want to ride with gears set up with a deliberate self-destruct option (to save the weight of 2 chain links) then up to you.

Nothing to do with weight. Just don’t want a longer than necessary chain flapping about especially on a retro bike where there isn’t a clutch mech.

I still can’t think of a scenario where big/big would happen. As you shouldn’t be in big (rear) and about to double downshift as you should of already dropped it to middle ring by the middle of the cassette. Move the chain across the cassette in line with the front cogs. That’s way I was taught years ago and have never had a issue.

 

[legrandefromage]

1x drivetrains are always using 'big big'

 

[d8mok]

1x drivetrains are always using 'big big'

Didn’t realise 1x had a “big”on the front. Since there is only one chainring surely it can’t be classed as the big , the small or the middle since there isn’t any others.