Quote Originally Posted by dachshund View Post
The IRD 11-34 is also supposed to work with a Campy compact 34-50 and a medium cage derailleur.
That would have to be a medium cage mountain derailleur. If you had a triple up front you'd need a long cage mountain derailleur with that cassette.

A 'medium cage' road derailleur is for a triple up front and a road (i.e., no more than 27) cassette.

The maximum cog refers to the maximum number of teeth a RD can handle. For road, it's 27/28 and for mountain it's 34.

Capacity refers to the number of teeth between the hardest and easiest gears (biggest chainring minus smallest chainring) - (biggest cog minus smallest cog).

I don't know about the designations for Campy, but Shimano uses SS for road RDs meant for a double crankset. GS for R