Just switched to a 22/36 double and bash from a triple ring. Currently running an X0 long cage. Next time I mash the rear mech on a rock do I replace with a short or medium cage, and if the right answer is short cage, what on earth is a medium cage for?

Rule is long for triple chainring, medium for double, and short for single is it not?

That would be completely logical, but have seen people say that short cage is OK for a double set up

I may be wrong but I was under the impression that road mechs (ie proper short) tend to have a parallelogram angle matched to road cassettes and would thus have trouble with your average 11-32 cassette.

Me, I use mediums on triples (22/32/44 with 11-32) and shorts on doubles with road blocks (39/53 with 12-27 on a road bike and 22/36 with 11-25 on an MTB). Mediums would work on doubles with wide blocks, longs would give you more to play with on a full-sus whose effective chainstay length varies.

FWIW I've always found you can exceed Shimano's quoted rear mech tooth range by 50%. Rig it up as tight as it'll go in big-big and you're away.

my x0 long cage works perfctly on 22/32 but I would have smaller one if they were not made of carbon. It has never dropped the chain either.

Also worth considering the cassette size too as its the total capacity thats important. The total capacity is calculated by subtracting the size of the smallest chainring from the largest chainring and adding it to the difference between the largest and smallest cog in the cassette. For example, if the front chainrings were 22-36 and the rear cluster were 11-34t then, the front difference of 14 (36 minus 22 = 14) would be added to the rear difference of 23 (34 minus 11 = 23) to achieve a Total Capacity of 37 teeth. If the rear cluster were 11-28 the total capacity would be 31t.

For Shimano

Long cage 45t (SLX)
Medium cage 35t (SLX)
Short cage 23t (Saint)

Look at the mech that catches your eye.

Check the spec on the box or manufacturer's website like Sram or Shimano etc that state which ratios it will work with and total chain length.

Job done.

OK, thanks all. Think I will try an X9 medium cage and see how it goes. Might even RTFM before buying as you suggest zasker

Edit – just looked at the SRAM website. Unless I am missing something, it doesn't even mention different cage versions let alone how to work out compatability