We were recently asked: "How do I tell if I need a medium or long rear derailleur?"
Excellent question! And while there are a few factors that impact your choice of cage length (Long, Medium or Short), you can work it out using a simple formula.
Why does it matter?
The derailleur cage is a swinging link, with two pulleys. As the chain moves between different size sprockets (front and rear), the amount of chain required changes. The derailleur cage picks up this slack.
A cage that's too short has the potential to rip your derailleur off your bike, when the cage reaches its capacity....
A cage that's too long adds unnecessary weight to your bike, runs closer to the ground (when it's not necessary) and there's also the potential for more dropped chains (a longer cage generally has less leverage, and therefore runs less spring tension).
Working it out:
[ Largest Front Ring - Smallest Front Ring ] + [ Largest Rear Ring - Smallest Rear Ring ] = Required Capacity
e.g. if you are running a 22-32-42T front crankset, and a 11-34T rear cassette.
[ 42 - 22 ] + [ 34 - 11 ] = 43T
Choosing a cage:
Then you can choose which cage length you need. General guidelines for maximum capacity for each brand are (though it does vary between models, so best to check the model-specific capacity):
Shimano
- Long = 45T
- Medium = 33T
- Short = 28T
SRAM
- Long = 45T
- Medium = 37T
- Short = 30T
So in the example above, you would opt for a Long Cage derailleur in either brand. (Be sure to check the rating for the exact model you are looking at).
The other factor to consider is if you are riding a long travel dual suspension bike - if the calculation is on the edge, you may wish to go for the longer option cage (as the rear suspension goes through travel, it may pull on the chain if the cage is too short). There is no hard and fast rule on this, however there is an easy way to test - once you have your new derailleur set up, shift to the largest ring(s) on your bike, release all the pressure out of your rear shock (or remove it, if you have a coil sprung bike), and then check your chain does not limit your travel.
If you're having any trouble choosing the right derailleur, then feel free to contact us and we'll sort it out!
To view our range of derailleurs,Click Here!