hey Lee, i run a long cage SRAM X.O rear derailleur on both my Giant Reign and Giant DH bikes. i was thinking about picking up a spare XO rear, to have on hand if i bash either one of those to death.

but short, mid or long? i could use this opportunity to go short, and put the new short cage on the DH, saving the long cage for the spare, but now i am confused by chain wrap, slack pickup and more.

thanks! robyn

Hey Robyn,

Always optimizing, eh?

TDC
Every SRAM derailleur has a TDC —Total Drivetrain Capacity. That is the sum of the tooth differentials for the chainrings and cogs. For example:

11-34 cassette = 23 tooth differential

22/34/44 rings = 22 tooth differential

Total differential = 45 teeth

The long and the short of it
Short cage = 30t capacity

Medium cage = 37t capacity

Long cage = 45t capacity

To make it simple:
One ring (whatever x 11-34) – Short cage

Two rings (22/36×11-34) – Medium cage

Three rings (22/34/44×11-34) – Long cage

If you run a tighter cassette, you can squeeze in more rings. Just find your total differential, then rock the derailleur with the right TDC.

Cons of too short – You either have to run the chain too loose, or you can’t get the big/big combos.

Cons of too long – You have too much chain; lots of slapping and noise.

Get it as short as you can while honoring the TDC.

And have fun!

—Lee

Also read: Long or medium cage rear derailleur? This article talks about Shimano XT.