Originally Posted by
urbanknight
I have been told that some guy has an 11-34 9 speed cassette on his road derailleur and it works just fine. The only catch is you need to avoid cross shifting if it's a short cage. If you try this, I recommend doing it early and getting some rides in to make sure it works before you had to Death Valley without a backup plan.
He doesn't have a triple with a 22 small ring (22/36/46). A major function of the rear derailleur is to maintain chain tension. The rings play an important role in determining the need for a long-cage mountain bike RD.
Cross shifting is no additional problem (not a good idea in general) as the chain should be sized big enough so it fits the big ring and the big cog, otherwise you'll brake the chain if your forget. Small ring (22) to small cog (11 or 12) will cause the chain to rub on itself at the jockey wheels even with a long cage RD. That makes a noise which tells you that you are crossed, but it causes no damage or problems.
Al