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  1. #1
    Uncle
    Reputation: Entrenador's Avatar
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    Measuring CS / frame alignment question

    I have a low end 29er steel frame that thus far has served me well. Rides reasonably, but while setting it up SS last night, I noticed that the chainline looked a bit odd, which made me wonder about the frame alignment / build. With the bare eye, the wheel seems to sit straight and centered overall -- tire line center lines up with the seat tube, tires clear the chain and seat stays equally, etc. Here are the numbers I got after a few measurements:
    RH CS length: 443.5mm
    LH CS length: 440.5mm


    Measurement process: I left the bike assembled, and measured from the centerpoint on the crank spindle to the center of the QR ends (hole on the drive side, center of clamp on the non-drive side). The cranks are XT 760 cranks, and are hollow on the drive side. On the non-drive side, I measured to the center of the crank arm tensioner cap. I took these measurements twice and got the same results.

    Question: Does this sound like an accurate and useful measurement? Should I be able to conclude from this measurement that the dropouts are not quite in the proper place, or was my measurement flawed -- i.e. is there something about this way of measuring from the cranks, or from the QR ends, that might make for improper measurements?

    Lastly, and probably most complex: Ideally, there would be no variance in this number I'd expect, but how much variance is commonly understood as insignificant? This is a loaded question I'm sure, so maybe I should reframe the question: How much variance would you tolerate on your own personal frame?

  2. #2
    mtbr member
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    the short answer is no. The bottom bracket spindle is narrower than the axle and usually offset to one side. The QR skewer is not in the center of the axle and usually not straight. It would be a bit like attempting an automotive wheel alignment using your wheel covers as points of reference.

  3. #3
    Moderator Moderator
    Reputation: Walt's Avatar
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    If it ain't broke

    ...don't fix it.

    You've said it rides well, and it doesn't sound like you have the tools to do a proper check of alignment (beyond seeing if the wheel fits straight) so forget about it and ride it.

    -Walt

  4. #4
    Uncle
    Reputation: Entrenador's Avatar
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    I asked mostly because I was having trouble finding the right chainline, which made me wonder if the vertical dropouts were even, and leaving the rear cog out of whack. At this point, I'm going to take your advice.

    Thanks for chiming in.
    -Josh

  5. #5
    mtbr member
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    A quick and dirty way of checking lateral frame alignment is to tie a piece of string from one dropout, up and around the headtube and back to the other dropout. the perpindicular distance from the seat tube to the string should be the same on either side, as should the distance from the dropout to the head tube. It's crude, and you should treat the results with a bit of caution, but it may give you a bit of piece of mind.

  6. #6
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    one thing you haven't said is whether the wheel you're checking with has been checked, have the wheel checked, or simply flip it in the frame to check if the tyre is still central either way. If it is then the wheel is going to be running in basically the right place so if you haven't had any problems with handling the chainline issue is something else - bb length/spacers on hub etc

    Matt

  7. #7
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    If you have the means, try removing the crank and BB, and the rear wheel. Then measure the frame directly (I suggest trailing edge of the BB shell to the leading edge of the axle opening. If all is well this will be spot-on side-for-side, removing any inaccuracies from crank offset and QR.

  8. #8
    Who turned out the lights
    Reputation: Francis Buxton's Avatar
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    Drop the chain off to the inside of the ring/cog, and put a big carpenter's square (or other straight edge) on the chainring and aim it toward the rear wheel. That should tell you where the chainline "wants" to be.

  9. #9
    Old school BMXer
    Reputation: Blaster1200's Avatar
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    Quote Originally Posted by Francis Buxton
    Drop the chain off to the inside of the ring/cog, and put a big carpenter's square (or other straight edge) on the chainring and aim it toward the rear wheel. That should tell you where the chainline "wants" to be.
    That's where it wants to be relative to the front chainring. Ideally, you'd also want to do the same for the cog, but the square just won't fit in there well. If the rear end is misaligned (or the wheel is not straight in the dropouts), the rear cog may want the chain to be somewhere different up front.

    To the OP, if there is a single mm of difference in chainstay length, the wheel will be very crooked in the frame - it won't be inline with the centerline (or top tube) of the bike.

    The string method described above is a quick and dirty way of identifying some alignment issues.
    May the air be filled with tires!

  10. #10
    Uncle
    Reputation: Entrenador's Avatar
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    Right on, Y'all. Much appreciated.

    I had a trouble free ride on it today -- fairly quiet the whole way, so like Walt suggested, I'm ready to roll it the way it is for now. In the event that I remove the X-BB cups, I'll take some measurements. At this point, ignorance is blissful.

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