I'm thinking about putting a compact crank on my road bike. As I understand it (or mis-understand it), the only difference between rear derailleurs for doubles and triples is that the triple der has a longer cage to handle more variation on chain length. If so, should I use a triple rear for a compact crank?

I'm thinking about putting a compact crank on my road bike. As I understand it (or mis-understand it), the only difference between rear derailleurs for doubles and triples is that the triple der has a longer cage to handle more variation on chain length. If so, should I use a triple rear for a compact crank?
You understand correctly. The only reason to change would be if you like the looks of the shorter RD. - TF

I changed my DA double crankset to an FSA compact a couple of days ago. I left the rear cassette (12-23 9 speed) and DA rear derailleur alone. The only adjustments were to lower the front derailleur and take one link out of the chain. The rear shifts as well as it always did but the front takes a little longer with the bigger jump. The DA rear is supposed to handle a 27 tooth difference so with the 16 tooth front plus the 11 tooth rear it is within spec. If you have a larger rear difference, you may have to change to a tripple RD.

Jim

Kalukis, you have to state what cassette size you're running. Like Jim mentioned, 11-23 or 12-23 or 12-25 should be fine with a DA rear. A 12-27 cassette may be tight.
Shimano underates their RD capacity.
We run DA on our mtb, and those have upto 35t diff. That is certainly the outside of the capacity, and leaves the chain dragging on the little RD tab when in small-small, but why would you be in that gear...

I'm thinking about putting a compact crank on my road bike. As I understand it (or mis-understand it), the only difference between rear derailleurs for doubles and triples is that the triple der has a longer cage to handle more variation on chain length. If so, should I use a triple rear for a compact crank?

You could use either a short cage or a long cage rear der with a compact 50/34...even with a 12-27. If I had to pick I would go with the short cage because it shifts slightly quicker, but you can just use whatever you already have.

I agree with LC. Use the one you have as they both will work.

I have used short cages with both a 12/26 SRAM and 12/27 Shimano cassette. Also used both Campy and Shimano short cage rear derailleurs without problems.

Both Shimano and Campy indicate you are exceeding the maximum chain wrap (27), but this is likely their lawyers talking because it works fine.

I would suggest you think about changing your front derailleur to help with shifting. The IRD Compact FD is about $45 (braze on only) for Shimano, or if you want Campy specific they are now making excellent FDs for compact cranks. I have also used the standard FDs from my groupsets, and they just shift slower. The compact specific FDs shift much better in the front.