Rear Derailleur Cage Lengths

[chriscross1966]

Don't think a hanger extension will help on a bike that wants to run a wide-range cluster, it would be for running a close but low ratio cluster, for instance on an hill-climber or cyclocross-style machine when the only mech in your spares box was a short-cage...

 

[antnee]

Ah so this basic item would just extend the derailleur hanger so allowing the larger sprocket cassette to be fitted I Thought that some one would have thought of this as on reading an acticle by Sheldon Brown sometime ago and even thought about making one Typical I never even though to look on eBay
Just looking at the post above The cage wasn't really as short as the original one that was one when I got the bike as the cassette it was fitted with a 12-25 one

 

[Gary E]

I recently bought the parts to convert my short cage to a medium cage. A careful look at the exploded parts diagram for both versions (which give the part numbers for each component) showed that all of the parts were identical except for the Inner and outer cage plates (obviously) but also the plate axle assembly. This is the pin that the cage rotates around and also includes the spring (and a couple of small spacers).
I've not had chance to actually do the conversion yet so I've not been able to compare the parts directly but I would guess that the different versions either have a different range of movement or need a different spring tension to operate properly.
Not saying this is the case for every derailleur but it'll be worth having a good look at the parts diagrams for both versions of the derailleur you're looking to convert.