Rear derrailleur positioning ??- Mtbr.com
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  1. #1
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    Rear derrailleur positioning ??

    So I've noticed different ways mechanics (people) place the rear derailleur in the hanger. Some seem to be placed quite forward under the cassette and the chain wraps around the cassette almost completely. Others (like mine) further back, or higher up and the chain wraps around the back and top of the cassette. What is the difference, if any, in positioning with regards to performance?

  2. #2
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    B-Screw Adjustment

    After setting the L-screw, check the "B-screw" for an adequate setting. The B-screw controls the derailleur body angle, hence the name, B-screw. Shift to the innermost rear cog, which is the largest cog. View the upper pulley relative to the largest cog. If the pulley is rubbing against this cog, tighten the B-screw to increase upper pivot spring tension, which pulls the pulley back and away from the cog. If there is a large gap between the upper pulley and cog, loosen the screw. To find a good setting, loosen the B-screw until the upper pulley begins to rub, then tighten the screw to get clearance

  3. #3
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    There is no real choice in the rear derailleur mounting. There is some adjustment in the B screw but the main difference is chain length and cage size. This will be driven by the range of gears.
    A road bike has the derailleur nicely tucked up under the cassette - but the cassette might only be from 11 to 23 - 12 teeth and the front difference will be 10 teeth.
    Compare that to a MTB with 11 to 36 cassette - 25 teeth and the front difference of 20 teeth.

  4. #4
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    Quote Originally Posted by keen View Post
    B-Screw Adjustment

    After setting the L-screw, check the "B-screw" for an adequate setting. The B-screw controls the derailleur body angle, hence the name, B-screw. Shift to the innermost rear cog, which is the largest cog. View the upper pulley relative to the largest cog. If the pulley is rubbing against this cog, tighten the B-screw to increase upper pivot spring tension, which pulls the pulley back and away from the cog. If there is a large gap between the upper pulley and cog, loosen the screw. To find a good setting, loosen the B-screw until the upper pulley begins to rub, then tighten the screw to get clearance
    Shimano recommend 5-6mm between the guide pulley and largest/lowest gear.

  5. #5
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    Longer d hangers can be purchased. I also have machined hangers as needed.

    There are in fact options.

    When running larger diameter cogs I go to the longer hanger.

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