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scale 06-18-11 08:21 AM

Rear Derailleur questions....
what are the rules of the Rear Derailleur?

Does it determine the number of gears you have or is that done by the shifters themselves being that is where the cable is pulled? Im refering to indexed DT shfters or brifters.

for instance, can i use a 10 speed detrail. on a 8/9 speed cassette? I realize that i probably couldnt use a derail. that came of a 6 or 7 speed on a 8 or 9 speed rear cassette being that there probably isnt enough movement and it wasnt designed for it(or can i)?

Bikewer 06-18-11 08:28 AM

The main thing to look for in a rear DR is "capacity". Most all have the ability to handle any contemporary gear cluster as far as side-to-side capacity goes.
So the thing to think about is the ability to handle larger cogs. (Lower gears)
"Mountain bike" DRs have long "cages", the part that holds the two idler sprockets. This to allow the use of the typical low gears on MTB cassettes.
"Road" DRs have typically shorter cages for the typically smaller and "closer" gear clusters found on roadsters.

RookieRoadie 06-18-11 08:48 AM

Bikewer has said it all really, but I think you would be right in saying that it is the shifters that determine the amount of cable pull etc. due to ratchet spacing of the mechanism. That said, I'm not sure if some derailleurs have some weird cable pull/amount moved going on. Basically, for example, using a ten speed shifter on an 8 speed block wouldn't work, as the amount of cable pulled would be wrong compared to the sprocket spacing. I explained that kinda badly didn't I? :p

FBinNY 06-18-11 08:53 AM

It's the lever that controls the indexing, or the number and spacing of the clicks that position the RD under the sprockets. It's a matter of having all the components matched. Each click of the lever moves the cable a set distance. The geometry of the RD determines how far the RD moves across for that amount of cable movement (known as the pull ratio). Finally the RD movement must match the cassette's number of sprocket and their spacing.

So the levers, must match the cassette in terms of number of speeds, and the RD must have the correct pull ratio to translate the lever's clicks to the correct shift action.

It's a system, where all three must be correctly matched for it to work correctly. Note that the RD doesn't have to be of X number of speeds, only to have the right pull ratio. So the same Shimano RD can be 5s 6s 7s...9s depending on the lever it's connected to.

Generally companies try to keep the same pull ratio throughout their lines (with exceptions) so you can usually use any of a company's RDs with that companies levers. There are exceptions, for example Shimano 10s is not compatible with earlier 8s and 9s systems.

I hope this clarified it for you.

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