Short cage vs Long cage derailleurs...
Most of the rear derailleurs I'm looking at are available in short and long cage versions. What are the advantages/disadvantages to each.
And the drivertrain I'm thinking of going with is the following. Any issues with this setup?
SRAM ROCKET SHORTY GRIP SHIFT (currently have these on my bike)
SHIMANO XT FRONT DERAILLEUR (currently have this on my bike)
SRAM X.0 REAR DERAILLEUR
SRAM POWERGLIDE 9 SPEED CASSETTE
SHIMANO XT760 CRANKSET
Thanks in advance!
Here is the short answer from Jensonusa.com
What is derailleur cage length?
The cage of a rear derailleur is defined as the distance between the two pulleys. Depending on your choice of components, there may only be one choice available, or you may have a choice between a short, medium, and/or long cage version. Note the distance between the pulleys of each derailleur. (Fig. 1)
Fig. 1 - Short, Medium, and Long Cage rear derailleurs
The cage length affects capacity. Capacity is defined as the difference between the number of teeth on the largest and smallest cogs, added to the difference between the number of teeth on the big chainring and the small chainring. Long cage rear derailleurs have a high capacity, and can therefore handle a very wide range drivetrain. Short cage rear derailleurs are more limited in their capacity, working best on a drivetrain with closer gearing.
How does cage length affect shifting?
Shorter cage rear derailleurs take up less chain slack than a long cage rear derailleur. The chain is held more snugly, which can result in smoother, more precise shifting. However, longer cage rear derailleurs are capable of taking up more chain slack than a short cage rear derailleur, allowing you to run a wider range drivetrain than would be possible with a short cage rear derailleur.
For road bikes, your choice is fairly straightforward. If you plan to use a double crankset, stick to a short cage unit. For triple cranksets, you'll want to use a longer cage rear derailleur to handle the increased chain wrap necessitated by the wider range drivetrain.
On a mountain bike, your choice is slightly more difficult. Less experienced riders, or riders who ride frequently in steep terrain should probably stick to a long cage rear derailleur. This, combined with proper chain length, will allow the bike to be safely shifted into every gear on the bike. Racers might appreciate the lighter weight and slight shifting improvement a short cage rear derailleur offers. However, these benefits come with a tradeoff - because of the decreased capacity a short or medium cage rear derailleur offers, it may not be possible to safely use the small chainring in tandem with the smallest cogs. (The derailleur simply can't take up enough slack in the chain)
For mountain bikes, when in doubt, choose a long cage rear derailleur, unless you are certain you understand the limitations of a short/medium cage derailleur.
For some reason, everyone always ignores suspension bikes and the fact that most of them have expanding chainlines as the travel increases.
This means that while a short cage derailer may work while the bike is on the stand, when the suspension bottoms out, it may rip the derailer off of the mount, and possibly take some of the hanger/dropout with it.
The derailer has to take up slack on most FS bikes. Depending on the degree of chain extention, this could be a very small amount or a very large amount. A short cage derailer can not take up as much slack.
"It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth
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