Shimano derailleur length?

Phideaux

I can't seem to find any technical literature that says when a short, medium, or long derailleur is needed.

I've got a 53/39 chainset and a 9 speed 11-23 cassette, but might want to go up to a 25 or 27.

Which length derailleur do I need?

Why isn't here any literature about this sort of thing on the Shimano site?

Help most appreciated.

John.T

You can go up to a 27 with a short cage mech. Shimano only do 2 road mech sizes. Short and long. Both have a max rear cog size of 27 teeth but the long one is for use with a triple chainring.
All the Shimano tech info is on their site. Go to the site for the mech you are using (ie Tiagra,105 etc). There are links to the info sheets and exploded diagrams.

Phideaux

Yup, I've got the exploded diagrams, but I can't actually find a document that explains the action of the dérailleur and the difference in use between short and long. From the sounds of it the long dérailleur should work for both double and triples, which begs the question "why have a short cage version".

whyamihere

The short cage is lighter. That's basically it.

Sometimes, but not always, the short cage will have stiffer springs which gives better shifting, as they're used more for racing bikes where long cage are more on touring bikes.

John.T

Short cage (SS) looks better and is good for a max tooth range of 31 teeth. The long cage (GS) will take 37 teeth range but as you say can be used on a double as well.