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Derailleurs
NB: Originally posted elsewhere on the Global Riders Network and appears via syndication.
Just woundering what the reasoning behind different cage lengths are? About to purchase a new sram xo and the price difference depending on length varies alot.
Cheers
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Go to the SRAM website, Click at the very top where it says "SERVICE" in tiny letters.
On the new page, go to the botttom, on the right hand side where it says "Popular Manuals and Guides" click on "Technical Manual - SRAM - 2010" and download the file, (which is a pretty big PDF file). It contains all the secrets about SRAM. Don't tell anyone else otherwise everybody will know.
If you have a large difference between your highest and lowest gears, then it takes a longer chain to cover that range in the highest ratios. And if you have a longer chain, then you need a longer rear derailleur cage to handle the extra chain length (keep tension on the chain) when you are in the lower gears.
At the other end of the scale, if you have a single front chain ring and close ratio rear cassette, then the required chain length is probably not so huge and you could use a shorter rear derailleur cage.
http://alexwetmore.org/?p=414
You add the difference in teeth from the smallest chainring to the biggest chainring to the difference between the smallest cog and the biggest cog. From the resulting number you can work out what length derailleur you need.
or Tell us what chain rings and cassette size you have and we'll work it out for you.