Originally Posted by
deermouse
If the cage is longer than what you currently have you may need a longer chain.
Nope. Cage length on a rear derailleur has no effect on required chain length. It does effect how much chain it will "wrap up" and, therefore, how wide a gearing range it will tolerate before going slack in small-small. Notice a rear derailleur at full stretch (i.e. in the big chainring-big cog combination) is fully in a straight line with the chain and it's length has no bearing on the situation.
Remember, the safest way to size a chain is to wrap it around the largest chainring and the largest cog
without going through the rear derailleur and then add 1". The rd cage length is not a factor.