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  1. #1
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    new chain, rear derailer question

    currently the small idler pulley on my real derailer rolls against the cassette (the chain keeps them apart), I've noticed this for a while (don't know if this is proper or not). According to my chain measure tool, the old chain is at a .75 stretch and I am replacing it.

    Once the new chain is on (I will make it the same number of links as the old one), if the pulley still rolls against the cassette, should i take out one more link? Would this create any problems?

    thanks in advance.
    Charlie

  2. #2
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    Sheldon Brown replies..........


    Angle adjustment ("B-tension")
    Modern derailers have two spring-loaded pivots. The lower pivot, sometimes called the "a pivot" winds the cage up to take up slack as you go to smaller sprockets. The upper "b pivot" adds additional slack take-up ability by pushing the derailer's parallelogram backwards.
    The tension of the two springs needs to be balanced for best shifting.

    Most derailers have an angle adjustment screw (Shimano calls it "B-tension adjustment"). This adjusts the tension of the upper ("b") spring of the parallelogram, and thus the height of the jockey pulley. The looser this screw is, the closer the jockey pulley will be to the cluster.
    The angle adjustment will need to be set according to the size of the largest rear sprocket. If you change to a cluster with a larger or smaller low-gear sprocket, you will need to re-adjust this setting. You will also need to adjust this if you change the length of your chain.

    If the angle adjuster is set too loose, however, the jockey pulley will bump into the largest sprocket when the bicycle is in the lowest gear (large rear, small front). This is the gear you should check the adjustment in.

    Since a derailer shift is caused by forcing the chain to run at an angle, the greater the angle, the sooner it will shift. The closer the jockey pulley is to the cluster, the sharper the angle will be for a given amount of sideways motion of the derailer. Thus, the looser the angle adjuster screw is, the better the shifting will be.

    http://sheldonbrown.com/derailer-adj....html#btension
    It's only pain......

  3. #3
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    If your upper pulley is "touching" the cassette, or very close to it as you describe it, you need to adjust the "B" screw on it. Here is a link on how to do it:

    http://www.parktool.com/repair/readhowto.asp?id=64

    When installing the new chain I'd recommend you verify proper length not just by using the same number of links, but with some other method. I've had good results with the Big-Big plus two method:

    -Wrap the chain around the Big Ring and Big cog, bypassing the Rear derailleur. To this length add two links (one inch) of chain. And that should be your length.

  4. #4
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    this is drifting a bit but I like to use a different method for checking chain length (not wear). With both wheels on the bike and the bike on the ground, I put the chain on the big ring and the small cog and make sure the der cage is vertical-to-slightly-pointing-toward-the-front-wheel.

    I tend to run my chain short since I wouldn't ever pedal in the "Big-Big plus 2" position.

  5. #5
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    Quote Originally Posted by dir-T
    this is drifting a bit but I like to use a different method for checking chain length (not wear). With both wheels on the bike and the bike on the ground, I put the chain on the big ring and the small cog and make sure the der cage is vertical-to-slightly-pointing-toward-the-front-wheel.

    I tend to run my chain short since I wouldn't ever pedal in the "Big-Big plus 2" position.

    Just two points of advice. Be carefull running you chain short. If you ever were to shift into the big big by acident you could snap off your rear derailer.

    Second the drivetrain (chain, cassette and chainrings) wear together as a set. When you replace a chain because it is worn out you need to replace the whole drivetrain (not the derailers, crankset, shifers, etc...). I know it's expensive but if you just throw a new chain on your old chainrings and cassette and it's going to peddel funny. You may notice that the chain suddenly jumps when you are pushing hard or something like that.

  6. #6
    The devil is an angel too
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    Quote Originally Posted by Maida7
    Just two points of advice. Be carefull running you chain short. If you ever were to shift into the big big by acident you could snap off your rear derailer.

    Second the drivetrain (chain, cassette and chainrings) wear together as a set. When you replace a chain because it is worn out you need to replace the whole drivetrain (not the derailers, crankset, shifers, etc...). I know it's expensive but if you just throw a new chain on your old chainrings and cassette and it's going to peddel funny. You may notice that the chain suddenly jumps when you are pushing hard or something like that.
    No, you don't.

    If you wait too long -i.e. chain past the 1% mark- you may have to, you will probably have to actually. But if you replace your chains at the .75% mark you don't have to replace the cassette and chainrings every time you change your chain. Chainrings even less than the cassette, actually.

    Do you sell cassettes or something?

  7. #7
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    this is going on a different tangent but I also don't change cassette and front rings every time I change a chain. Fine adjustment of rear der. and clean and lubed chain works great. If I was sponsered or had a trust fund then I might change the entire drive. Heck , I'll get a new bike when its time for a new chain.

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