# Bicycle Mechanics - Meaning of cage length in rear derailluer

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Fat Boy Biker
08-08-05, 05:08 PM
What is the meaning in cage lenght for the rear derailluer (short versus long, maybe even a medium). It is my understanding that a long cage for the front derailluer is for a 3 chain rings, and a shorter cage would be for 2 (over simplified I'm sure). But how does length come in to play in the rear?

Thanks,

Steve
-confused, and trying to build a bike

sydney
08-08-05, 05:40 PM
Chain wrap capacity.Longer wraps more chain. Generally regarded as needed for a triple and sometimes used with a compact double and wide range cassette. Formula for the wrap spec you need: big chainring teeth-smallest chainring teeth + large cog teeth- small cog teeth = required wrap capacity. The 30 granny on the typicall 30/42/52 triple adds an addtitional 12 teeth of required wrap, relative to a 42/52 double.

Retro Grouch
08-08-05, 05:44 PM
The "cage" is tha arm that extends downward from the derailleur. It's function is to take up the chain slack as you shift into smaller sprocket combinations. While saying that a long cage is for 3 front chainrings and a short cage is for 2 chainrings is somewhat oversimplifying, it's close enough for most applications.

Fat Boy Biker
08-08-05, 05:45 PM
Chain wrap capacity.Longer wraps more chain. Generally regarded as needed for a triple and sometimes used with a compact double and wide range cassette. Formula for the wrap spec you need: big chainring teeth-smallest chainring teeth + large cog teeth- small cog teeth = required wrap capacity. The 30 granny on the typicall 30/42/52 triple adds an addtitional 12 teeth of required wrap, relative to a 42/52 double.

Great explanation. Thanks Sydney.

Steve
-a little less confused.