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Discussion Starter · #1 ·
Hi,

I'm new to this forum and was curious why do triple-chainring bikes need a longer rear derailleur cage?

I'm confused about the reason for this and what role the length of the cage actually has?

Thanks a lot

Michael
 

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Eocyclist
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A long cage derailleur can take up more chain slack that a short cage derailleur. Triple chain rings typically have a larger range in size (diameter and number of teeth) between the largest chain ring and the smallest chain ring, and the derailleur has to take up more slack when shifting on the triple than on the double.

The total capacity of a derailleur is calculated from (Teeth in largest chain ring -Teeth in smallest chain ring) + (Teeth in largest cassette cog-Teeth in smallest cassette cog)

For example, a Surly LHT Complete comes with 26/36/48 chain rings and and 11-34 cassette. The required capacity is (48-26) + (34-11) = 22+23 = 45t. The capacities for a Shimano XT Derailleur are 33t for short cage and 45t for long cage. That LHT configuration would need a long cage deraileur. The short cage just won't pull enough chain slack to allow for shifting over the complete range.
 

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Discussion Starter · #4 ·
Thank you so much! So the tension pulley of the rear derailleur feeds out more chain when switching to the largest chainring and picks up the slack when shifting back to the smallest chainring?
 

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Eocyclist
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Thank you so much! So the tension pulley of the rear derailleur feeds out more chain when switching to the largest chainring and picks up the slack when shifting back to the smallest chainring?
Yes. And a similar thing happens when shifting from the largest cassette cog to the smallest cassette cog.

The extreme case is when shifting from cross chained big chainring-big cog to cross chained small chainring - small cog. Running from the largest chain ring and largest cassette cog takes more chain than running from the smallest chain ring to the smallest casette cog. The derailleur has to be long enough to take up the slack between those two positions.
 

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Discussion Starter · #8 ·
Just curious, I recently took a very basic bike maintenance course and became addicted to how it all works and how to fix everything.
 
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