Rear Derailleur Compatibility Question
 

I want to set up a 10 speed Campy drive train to handle a 12-25 (coastal riding) and a 13-29 (mountain riding) rear cassette. I want to use a medium cage rear derailleur which will handle both cassettes, but all I have is a 9-speed Centaur medium cage from after 2001 (after Campy changed the cable pull length specification). Will the 9-speed derailleur handle the 10 speed shifting requirements properly? I do have a long cage Chorus 10-speed rear derailleur and I was also wondering if there were any downsides to running a long cage derailleur with a 12-25 cassette? My intention is simply to change out the cassette and the chain when moving from the 12-25 to the 13-29. Thanks for any info!!!

As long as the derailleur can handle small/small with each cassette, it should all work.

The long cage *may* be slower shifting, doubt you'd notice.

I have never tried a 9s RD on a 10s setup, but am led to believe it will work just fine. I have one in the parts drawer, just in case.

Given your scenario, I would use the LC Chorus, and never think about it again. As long as the chain doesn't rub on the bottom of the cage in the 12-25, it will work without adjustment on the 13-29.

Isn't it really only the thickness of the derailleur jockey wheels that can make a difference between 9 and 10. Couldn't you just swap the jockey wheels between derailleurs? Or i could just not know what i'm talking about:)

Medium-cage derailers will handle a 29T sprocket just fine, so long as you're using a double up front. The long-cage wrap capacity is needed if you are using a triple chainring. We've used a short-cage Chorus with a 30T cassette (an old one from Campy's now-defunct tandem gruppo) and a 39/53 double.

There is no problem using a 9-speed derailer cage with a 10-speed chain. The slightly wider spacing between the cage plates has never caused any problem for me. I don't know for sure (because the bike I used it on used friction shifters) that the indexing will work properly, too, but I think it will. The reason is that the Jtek Shiftmate doesn't specify 9- vs. 10- speed derailers in its package insert for making 10-speed Campy shifters work with 9-speed Campy cassettes -- it just assumes you are using any post-2001 Campy derailer, which you are. (I'm currently using a 9-speed chain in a 10-speed derailer on our tandem, and that works fine, too.)

I would *not* put 10-speed pulleys in the 9-speed derailer cage. If you use the 9-speed bolts, they will stick out past the face of the inner cage plate and possibly hit the spokes. Also, the more times you remove those bolts, the more likely you are to strip the threads of either the bolt or the cage -- the cage plate is very thin and engages only a couple of threads. Leave 'em alone is my advice.

Long-cage derailers shift just fine on mid-range cassettes. True, shorter cages look more "manly" and scream "TOURIST!!" less loudly but honestly that is the only reason to use them. If you've already got that long-cage derailer, I'd just leave it on all the time, as Teetime said, with the chain sized for the 29T cassette, and then all you have to do is wrench on that 29T when you head for the mountains.

Thanks for all the responses. Much appreciated!!!