XTR Rear Der - long or med cage?
I'm after a little advice on rear derailleur. I'm going with the XTR low-normal (rapid rise) RD-M970. There appears to be two offerings on most bike parts websites - Long or Medium cage. Going by the Shimano website (Australia) they list the following:
RD-M970-GS: 34T max, 11T min, 33T capacity, 22T max front difference
RD-M970-SGS: 34T max, 11T min, 43T capacity, 22T max front difference
I'm guessing the max and min are the rear cassette max & min, and it seems the only difference is the capacity, which makes me think the SGS is the long cage? But if someone can explain the 'Capacity' part, as well as the font difference, that would be great.
Basically I want to also know: if intending to run a 11-34 rear cassette, along with a 22-32 (or potentially 22-34) double front, can I run the medium cage, and would the shifting be slicker/quicker as a result?
P.S. I know the trend these days is to use the 'shadow' variant, esp. since I'm running rapidfire shifters - not DCLs. But that's what I like, and I've looked closely at the clearance differences between shadow and normal and can't see how a few mm will make that much a difference for the cost increase.
Derailleur cpacity explained here
When to use a Long Cage vs Short Cage Derailleur?
Manufacturer stated derailleur capacities are as follows:
Shimano long = 45T; medium = 33T
SRAM long = 43T; medium = 37T; short = 30T
The easy capacity formula is to add your big ring & cog sizes, then subtract your small ring and cog sizes. It looks like this:
cap req'd (T) = (BIG ring - small ring) + (BIG cog - small cog)
...so for a typical 44-32-22 mountain crank & 11-34 cassette...
T = (44T - 22T) + (34T - 11T)
.. = (22T) + (23T)
.. = 45T
Thanks Rocky. So it looks like a 32-22 up front and a 11-34 out back will work, but if I ever want to increase the front ring to 34, I'll have to go back to 11-32 out the back. Unless the 'conservative' part Speedhub.Nate talks about comes into play, in which case i'll be fine.
The next question is whether running a 2-ring specific chain guide (like a Blackspire stinger) will affect any of this?
Seeeriously easy Livin
If your only running 2 chain rings up front a medium cage will work fine, even if you wanted to step up to a 36 t ring, the only limitations of shorter cage rear derailleurs is if you shift into the granny ring and smaller cogs in the rear with too wide a range it won't take up all the slack, but really you should never be in those gear any way.
I run 2x9 with an 11-34 cassette and a short cage rear derailleur, and it works just fine because I only use the granny for steep climbs, never leaving the top 4 cogs. The short cage does a much better job of keeping chain tension, so I get much less chain slap.
If you shift correctly, then you can probably do a mid cage derailleur. Noone is going to be running 22-11 or 44-34 because that would be two extreme cases of bad chainline. If you are mindful of your shifts keeping the chain in the first 4 cogs for 22T, mid 4 for mid ring and the last 4 for big ring, then you can probably get away with the mid cage.