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OK... so cap req'd (T) = (big ring - small ring) + (big cog - small cog).

So for me that's (38-24) + (36-12) = 38T

SRAM say long = 43T; medium = 37T; short = 30T

Now me, I think with a need for 38T that I would go for the closest... so a medium cage BUT the fecking SRAM thing below says I should be a long cage.

Keep with me... With a 37T Medium Cage I potentially can't use my BIG-BIG combo (or there may be issues) but with the long 43T cage I could get serious chain slap...

OR do I need the 43T long cage to take up the rear suspension?

Bike nerds... please help!

I tend to go for the theory of:

single ring up front - short cage
double - medium
triple - long cage

Not sure how technical it is (safe to say 'not very' ), you might get 0.1% more efficiency and a bit less chain slap working it out properly, but I've been staring at that chart for a while now and it's just given me a headache.

That's probably not all that helpful, but it's worked out o.k for me so far?

Chris

dito, what set up are you wanting to run?

I'm just thinking about the rear derailleur - then I saw they are short, medium and long...

I assumed the medium was the right one but SRAM say long - just wasn't sure...

Right, I think i've just about managed to understand the equations and the chart (it's been a long day ) . The 37 vs 38T thing is probably no cause for concert, the springs in the mech will be able to cater for a bit of leeway.

Chris

It is because you have the wide ratio 36T Cassette that SRAM recommend the Long Cage, but it is based on the assumption that you will use all the ratios. If you choose the medium it may mean that 36/38 combo will not be available (or less likely, that, if you are on a full sus, depending on your suspension set-up the chain growth may exceed the rear mech spring and snap the chain). If you are happy to forgo the 36/38 combo (you'll have the same ratio in the middle of the cassette with the 24t ring) the medium will be OK.

Chris321 and phinw that's where I was - as I don't want to lose gears or chance snapping the chain I'll opt for the long mech...