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  1. #1
    CTB
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    BB Spindle Length, Chain Line, etc - Tutorial?

    I've been trying to learn about cranksets and BB's, and I haven't really found anything that outlines *why* there are various spindle lengths, if they'll work with each other, etc.

    For example: I have an old Shimano XT Octalink crankset (FC-M751) that was installed on a BB-ES70 (73 x 113). From what I've measured, the chainline on this crankset is 47.5mm.

    If I wanted to update this crankset to something newer but using the same BB, would it simply not work if the crankset says it should have a 121 spindle? I'm confused as to what the effects are of chainline, spindle length, etc., and it is somewhat paralyzing if you want to buy components separately to save money. By this I mean a crankset separate from a BB, etc. Certainly buying the whole deal at once should result in a compatible setup regardless of the bike, right?

    I haven't found a good tutorial on this. Anyone? Thanks.

  2. #2
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    The XT crankset you have was designed for the 113mm or 118mm spindles. Some frames (full suspension was common) needed a little more room so the 50mm chainline was an option. Current external bearing cranksets are almost always using a 50mm chainline now.

    The 121mm and 126mm Octalink BB's were originally used for the LX FC M572 crankset (47.5mm and 50mm chainlines). This BB should not be used with the XT FC M751 crankset, it will put your chainrings too far outboard and your front derailleur will not function as needed.

  3. #3
    CTB
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    So would it also be completely wrong to try and bolt, say, a Shimano FC-M443 crankset (Octa) onto the 113 BB? For example, looking at this Shimano tech doc:

    http://techdocs.shimano.com/media/te...9830657937.pdf

    Does this mean a 113 BB is only feasible for the FC-M443 and 521, but none of the others on the page? As I read it, this document indicates that the chain line would be all wrong for the M442 crankset with a 113 BB, but that the 443 could use it.

    Thanks for the help in understanding all this. I'm AMAZED at the massive lack of standardization in the bike world.

  4. #4
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    Yes...you need to use the recommended spindle lengths.

    I'm AMAZED at the massive lack of standardization in the bike world.
    It didn't used to be as bad, but many larger companies (and a few small ones) are using proprietary standards now days. In addition, Shimano is always changing their standards every few years. Its a pain...

  5. #5
    CTB
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    Ok, I think I'm starting to finally get my head around this (sorry for being so slow today). Here's what I think I learned:
    * Chain line is god. The design of the crank arm interface determines how long of a spindle you need to make either a 47.5 or 50mm chain line.
    * The frame doesn't matter much. This is the part I was struggling with. It seems the frame only matters if you have a large diameter tube for the front D to attach to; wide tubes make a 47.5 chain line hard to achieve, so 50 can be used, provided you get the right BB to move the arms to the right place.

    I have one last question (I hope): Are there 47.5 and 50 front derailleurs, or do all derailleurs have the capacity to work with either one?

    mtnbiker72, thanks for the help. You've been understanding of my stupid newbie questions, and I appreciate it.

  6. #6
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    I have one last question (I hope): Are there 47.5 and 50 front derailleurs, or do all derailleurs have the capacity to work with either one?
    Most will handle both...though I did have an issue with a SRAM X-7 when I went to a 51mm chainline (what Truvativ uses for their external bearing) from a 48.5mm chainline (what Truvativ uses for their ISIS). I replaced it with a Deore and it works fine.

  7. #7
    CTB
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    After re-reading the document in the link I posted, I realize that I had it wrong in my post above. The M442-8 (Octa) will work only with a BB-ES25(-E); 113 for a 47.5 chain line, 118 for 50. And so on for the other cranks listed.

    Thank goodness for these tech docs, or I'd never get this right.

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