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Old 06-18-11, 08:53 AM
  #4  
FBinNY 
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Join Date: Apr 2009
Location: New Rochelle, NY
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Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

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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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