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  1. #1
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    General crank questions

    A few general crank questions:

    1) What is the purpose for going with a 170 or 172 vs a 175mm crank arm?
    2) Purpose of a 4 bolt vs a 5 bolt pattern?

    thanks in advance!

  2. #2
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    Good questions. Longer crank lengths originated based on the concept that cranks should be somewhat proportuned to leg length. While cranks are still proportuned when making kids bikes, the subtle (3%) difference between 170, 172 and 175 is more a matter of preference. Longer cranks make sense for tall riders and power pedallers who benefit from the slightly higher leverage, while shorter cranks are preferred by spinners.

    4 bolts vs. 5 is strictly a marketing difference. Years ago cranks had 3 bolts, then 6, then 5, now 4. I'm pretty sure that sometime soon some marketing whiz will bring back 3 bolts taking us around full circle.

    Technically there are subtle differences in possible stiffness achieved with differing bolt patterns, with more arms and larger spiders being stiffer at the expense of weight and minimum chainring size. I'm convinced that half the reason for all the bolt patterns is to raise costs for after market chainring makers, and capture more replacement business for the OEM producer.
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  3. #3
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    Try reading this article from the ol' master Sheldon Brown http://www.sheldonbrown.com/cranks.html for some background on crank length.

    As far as 4 arm vs 5 arm cranks it's more about marketing than anything else IMHO; the bcd involved has a relationship with the size chainrings that can be utilized but becomes a moot point for choice and availability for most current mountain cranks as they tend to be 4 arm 104/64 bcd (at least now).
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  4. #4
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    Thanks for the responses/info!

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