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triple rear derailleur question???????(3 posts)

triple rear derailleur question???????slow-ron
Jan 12, 2002 4:36 PM
I understand why a new front derailleur is needed when going from a double to a triple (larger range). And I also understand why a longer cage is needed for the rear derailleur when going to a bigger cassette (take up chain slack). My question is, why do you need a new rear derailleur when going from a double to a triple crank? What is different in the design of a double rear derailleur vs. a triple rear derailleur? I'm working with Campy here so I'm not sure if this question applies to Shimano owners.

thanks

ron
re: triple rear derailleur question???????Rusty Coggs
Jan 12, 2002 5:31 PM
With a triple chain ring you may need the long cage rear der to wrap more chain,due to the granny ring in the system.The long cage is not an absoulte requirement,but If you use a short cage rear with a triple,there will likely be several of the smaller cogs you cannot use when in the granny due to insufficient wrap capacity. Often not an issue on road bikes, as the granny is most often used as a bailout with the largest cogs. Long cage is more critical on MTB since a greater range of rear cogs may be need when in the granny.The are two rear der specs, one is wrap capacity,a function of cage length and the other is max rear cog capacity, a function of parallelogram design. In the case of the ultegra double and triple derailers, both have a max cog cap of 27 teeth,but the long cage wraps more chain..
I have a double with a triple derailleur....Bruno S
Jan 12, 2002 5:31 PM
The triple has the arm (that holds the two very small gears) longer. So far it has worked fine. I guess the arm has to be longer to pull the chain more when in the small gear of the triple. Thats the basic difference but I guess someone else can get into the fine details of the matter.