Rear Derailleur Question
 

When using a double or a triple chain ring set-up are there specific rear derailleurs?
Thanks!

It depends on how much chain needs to be wrapped by the derailleur. Triples generally require a lot of chain wrap, which would call for a long-arm rear derailleur, but compact doubles can require just as much. Racing bikes with relatively close gearing don't need to wrap as much chain, and thus can use a shorter arm, and thus lighter, rear derailleur.

Determine how much chain wrap is going to be needed. Difference between tooth count of largest and smallest cog in the rear PLUS the difference between the tooth count on the chain rings equals the chain wrap capacity required.

Brad

When you calculate the chain wrap you need you might want to consider allowing for a possible future change to a larger rear cog. A lot of folks ask about lower gearing on this forum and a larger cog is often suggested. The weight and performance penalties for a longer-than-needed derailleur arm are small if even perceptible.

Thanks for your help!