rear deraileur question

[02GF74]

... need to get a new rear derailleur at some point as the spring has gone. Looking at XT and XTR they seem to come in 3 cage lengths:

short, standard(?) and long.

anyone have any info. about the different lengths - distance between jockey wheels would identify the one I have.

Presumably the short length ones are limited over the gear selection?
Also whilst on the same subject, what is the point of a rapid rise? Is it any better then the standard shifter?

Don't plan to get one as it will get confusing unless I change all bikes to be the same but just wondering.....

 

[Twenty Inch]

They're specific to the amount of "move" in your rear cassette. Closely-spaced road cassette - small cage. Big old MTB with granny gear - large cage. Get the same one that you've got on now.

 

[mondobongo]

This articlegives a good explanation about tooth capacity. I ran a medium cage Sram rear mech with no problems could not run small small and a couple of other small gears on the cassette but never used such a combi on a positive it did give snappier changes.

 

[mickle]

The longer the cage the longer the chain. Cage length is determined by total tooth difference. Easy to calculate and its printed on the deraillieur box.

(Biggest sprocket minus smallest sprocket) plus (biggest chainring minus smallest chainring) equals total tooth difference.

eg (28-12=16) + (52-42=10)=26
eg (32-12=20) + (48-28=20)=40

 

[piedwagtail91]

i have a medium cage xt on a touring bike. the cassette is something like 14-22 and the front 48,36,26. it seems to cope ok with those ratios though i set the chain length to still be under some tension on the 26 ring and 16 sprocket simply because i never use the 14 and 15 when i'm on the 26 ring.