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  1. #1
    Senior Curmudgeon
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    EXPERT advice needed on triple to double

    I did a "search the forums" for this topic, and found many threads, none of which answered my question exactly. I'm sure this has been asked before, but since I couldn't readily find it, please (again) be tolerant with this potential repeat question.

    My situation is thus: I have a hybrid bike with 135mm rear spacing. The bike came originally with a square-axle Truvativ triple. The original axle is 113mm wide. The chainline was perfect with the chain on the middle chainwheel and the middle rear cog.

    I plan to replace the triple with a Shimano 105 double with an octalink axle. I further (and this is important!), plan to use the 39t chain ring ONLY, mounting it (the 39t) on the outside of the spider, where the larger ring normally goes. This bike will be run without a front derailleur and without a "large" front chainring.

    My question is this: What axle length do I need to exactly match the chain line that I had with the middle chain of the triple? The answer must take into account:

    1. The change from square axle to octalink (if it matters)
    2. The change from triple to double (I don't know how this affects the spacing, but it must)
    3. The fact that only the outer ring of the double will be used
    4. The fact (if it matters) that the 135mm rear spacing will have wider chainstays than a road bike

    If you don't KNOW the answer here, please don't reply. I'd really like to be sure before I go spending money on a bottom bracket. Thanks to all for your patience and your help!

  2. #2
    Licensed Bike Geek Davet's Avatar
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    Unfortunately, when you use the OctaLink bottom bracket you only have two choices of BB spindle width; 109.5 and 118.5, neither of which will give you the exact same chainline as you had. The 109.5 BB will come the closest and not be too far from your ideal.

  3. #3
    Senior Curmudgeon
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    Quote Originally Posted by Davet
    ..when you use the OctaLink bottom bracket you only have two choices of BB spindle width; 109.5 and 118.5
    Well, that certainly narrows the choices! Thanks for the prompt reply.

  4. #4
    Senior Member sydney's Avatar
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    Quote Originally Posted by Davet
    Unfortunately, when you use the OctaLink bottom bracket you only have two choices of BB spindle width; 109.5 and 118.5, neither of which will give you the exact same chainline as you had. The 109.5 BB will come the closest and not be too far from your ideal.
    Well actually, the xtr octalink is compatible and has 112.5 and 116 spindles.

  5. #5
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    sydney! I knew you would have the scoop. Is the 109.5 still my best choice?

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