Rear Mechs.Cage size.What's it all about ?

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the great roberto

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Medium,small ,long..........

Are there any rules as to what cage should go with which cassette or is it to do with the number of chain rings on the crank?

In the past ( for that read always) I have used SGS(long) caged Shimano rear mechs with cassettes ranging from 28-11 upto 34-11 teeth.Always with 42/32/22 chainrings on the front.

Now I'm giving SRAM rear mechs a go, but their medium cage is only (between pulley centres) 6mm smaller than shimano SGS (long).

SRAM long is way longer than Shimano SGS.

Given my cassettes and chain rings described above is a SRAM medium what I need or a long.
Or, should I not worry as both are fine.............
 
It is to do with chainring/cog spread - the wider the spread the longer the cage needed - shimano's website used to have a guide and i think most new derailleurs have the limits of their range on the box.
 
The mech will be given a capacity in it's specification, it should be in the technical info. Just work out what you have and keep in that spec.

To be honest you can go outside it with some adjustment. Or with the realisation you either cannot use large-large or your chain will snap or get stuck there.
Or you cannot use small-small else the chain is floppy and skips around.
 
Simply put the arm length determines how much extra chain you have to play with. A swing from the longer arm has more chain in it's length (from from bottom of the front ring to the tension pulley.)

i.e. the ability to take up the slack.

So if you run 48/36/24 at the front and 11/28 at the back you have a (48-24)+(28-11) capacity needed.
So 41T capacity needed.
As Shimano SGS (long cage) with capacity of 45T will all ow you to use all gear with the slack taken up if chain length is set correctly as per shimano.

A Shimano GS (medium cage) with capacity 33T will work but you have to compromise decrease your gear range and risk the slack chain or snapped chain, your choice.
 
the great roberto":2fzt4tma said:
Accordington to sram long is 45t and medium is 37t. So what does that mean?
Click to expand...

So to answer your question
So go long ;) if you like to use your wider range at the back and have no problems

11-28 at the back gives your setup 37T,

but the 11-34 is 43T
so if you setup the chain problem, then the lowest get usable in low front would be a 17T at the rear, all below that will give slack chain
 
Any use?
whatmeansthecapacitymaizl5.jpg
 
Got it ,thanks.

The bit that confused me was that a shimano medium(gs) 33T is not the same as a sram medium 37T.........

Given the above capacities for medium, SRAM/Shimano can't sell many :?
 
FluffyChicken":30mpy543 said:
Or with the realisation you either cannot use large-large or your chain will snap or get stuck there.
Or you cannot use small-small else the chain is floppy and skips around.
Click to expand...

I've always ridden with that ethos :) so I'm quite pleased I've always been getting it 'right'!

What a fantastic post, thanks everyone, it's been extremely helpful ;)
Steve
 
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