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  1. #1
    TCR
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    Riding Heavens Highway TCR's Avatar
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    New chain - how long should it be?

    Current setup:
    53 X 39 chain rings
    12 X 27 cassette

    Will occasionally switch to a 11 X 23 cassette.

    I've been told that the rear jockey wheels should be perpendicular to the floor when in the big/small combo. However, as can be seen in the pics below it's not. So, when I install my new chain do I make it the same length as the old chain that's on there or do I make it so the jockey wheels line up perpendicular?

    1st pic shows how the jockey wheels line up with a plumb-bob currently. 2nd pic shows how the current chain looks when the jockey wheels line up perendicular. Not sure how many links that would translate to but it looks like a big sag.





  2. #2
    cab horn
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    Mes compaingnons cui j'amoie et cui j'aim,... Me di, chanson.

  3. #3
    TCR
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    I guess it could have been worse. You could have just posted this:

  4. #4
    TCR
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    Hmm, interesting, when it's in the small/small the chain is almost hitting the RD, meaning the chain is almost too long (I think).



    Pic below from PT shows a chain too long. Mine isn't touching or sagging but it's really really close to touching the bottom of the RD. I'm thinking I can safely go a link or two shorter on the new chain.

  5. #5
    institutionalized PDXJeff's Avatar
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    Try using the "simple equation" for both cassettes and size it to the average.


    Park Tool
    SIMPLE EQUATION: L = 2 (C) + F/4 + R/4 + 1
    L = Chain length in inches. Round the final result to closest whole inch figure.
    C = Chain stay length in inches, measure to closest 1/8”. Use chart below to find decimal measurement.
    F= Number of teeth on largest front chainring.
    R= Number of teeth on largest rear cog.



    Fractional conversion to decimal for 1/8” measurements:
    1/8” = 0.125”
    1/4" = 0.25”
    3/8” = 0.375”
    1/2” = 0.5”
    5/8” = 0.625”
    3/4" = 0.75”
    7/8” = 0.875”

  6. #6
    Senior Member Retro Grouch's Avatar
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    It's important that the chain be long enough to safely cover the big/big combination. If it's not and you accidentally shift into that combination it can cause a lot of expensive damage to your derailleur, wheel and frame.

    I use the big/big chain sizing method as suggested in the Park Tool site. I even use it when replacing an existing chain because it only takes a minute I don't know how the old chain was sized. That way I'm sure of getting it right. If, after using the big/big method your chain goes slack in the little/little, you need to get a rear derailleur that has more slack take up capacity.

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