derailleur ?
 

what is the difference between a short cage and a long cage derailleur ( why would you use one over the other )?
Thanks,
John

The long cage will take up more slack in a chain. This is useful on a bike where there is a large difference between the lowest and highest gear. On road bikes you will most often see a long cage on a triple. You need enough chain to get around a 52t chainring which leaves a lot of slack chain when you use the 30t ring.

I think the shimano mega range cassette and freewheel requires a long cage.