Bottom Bracket/Crankset sizing and Chainline
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  1. #1
    RoadBikeReview Member
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    Bottom Bracket/Crankset sizing and Chainline

    I looked and I looked and didnt see any threads discussing this specific question, but if its covered somewhere else let me know

    I got a new Cannondale Capo recently and want to upgrade the bottom bracket and cranks -- they seem really junky. It came stock with a Shimano BB-UN26 68mm which measures 110mm wide at the tips after taking the cranks off. http://bike.shimano.com/catalog/cycle/products/component.jsp?JSESSIONID=G27wT1J1HZgyJLqN1Qcy2ZJQx w5QNSZPYyJvjZVvvTccyMNtTshJ!758611947&PRODUCT%3C%3Eprd_id=845524441771271&FOLDER%3C%3Efolder_id=2534374302050437&ASSORTMENT%3C%3East_id=1408474395181674&bmUID=1186872177127
    The cranks are 130BCD and 172.5mm in length. From what I understand about chainline I will want to get a 110mm spacing bottom bracket and cranks that are designed to fit 110mm as well, although the BCD wont really matter if they come with a chainring.

    I was wondering the following:

    -Do I absolutely have to use a 110mm bottom bracket and crankset? Or can I get by with like 109, 110.5, or 111mm? How bad will that mess up the chainline?

    -How close in size do the cranks and bottom bracket have to be in relation to each other? I noticed that some cranks are advertised as being compatible with 103-107. If I had a 110mm bottom bracket would a 110.5 or 109mm crankset work?

  2. #2
    Hoopy Frood
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    Assuming that bike has standard 120mm rear spacing and track hubs, you want a chainline of 42mm. That matches up with a road double crankset with a 109.5mm bottom bracket spindle.

    Generally, you can go about 0.5 - 2mm off and not notice a major problem with noise or throw a chain. So I think the 109, 109.5, or 110mm spindle would be fine.

    Here's a good resource on chainline:

    http://www.sheldonbrown.com/chainline.html

  3. #3
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    Phil

    I am considering the Phil Wood BB but it only comes in 108 and 110.5 sizes that are near mine... do you suppose the 110.5 would be ok?

  4. #4
    Growing Older, Not Up
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    Quote Originally Posted by ProsperityRed
    I am considering the Phil Wood BB but it only comes in 108 and 110.5 sizes that are near mine... do you suppose the 110.5 would be ok?
    Phil Wood BBs are supposed to offer some adjustability for chainline so it shouldn't be a problem.

    I am a relative newb so maybe a fixie veteran can confirm this for us.

  5. #5
    RoadBikeReview Member
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    Quote Originally Posted by crumjack
    Phil Wood BBs are supposed to offer some adjustability for chainline so it shouldn't be a problem.

    I am a relative newb so maybe a fixie veteran can confirm this for us.
    This is true, the lockrings adjust +/- 5mm, so you can dial the chainline in perfectly as long as the BB is close to the right size....this capability is great to have in any case as hubs, cogs and cranksets, etc. can vary so the cog or chainring may be slightly off the 42mm standard even using good quality track parts, the PW BB can be adjusted to compensate. FWIW Sheldon Brown says the chainline should be within +/- 3mm on a fixie....

  6. #6
    RoadBikeReview Member
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    So I measured my current (stock) chainline front and rear:

    Front = 47.5mm
    Rear = 42mm

    Does this mean it came stock from the factory with a terrible chainline? It does use a Shimano 9 speed chain but doesnt that seem a little odd Cannondale would ship it with such a wide margin? 4.5mm seems like bad planning, or no planning to me. Or was it just a cost saving measure?

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