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  1. #1
    Cold. Blue. Steel.
    Reputation: OneGearGuy's Avatar
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    New question here. "Mystery of the Un-Round Drivetrain"

    Please help me solve the "Mystery of the Un-Round Drivetrain" on my 29" Waltworks SS with Bushnell EBB. BTW, it is the best bike I have ever ridden...

    I cannot seem to get the chain to maintain a constant tension all the way through a complete rotation of the cranks. When I spin the cranks you can see the chain rise and fall between the front chainring and rear cog. And, to prove what my bleary eyes are seeing, if I pause the crank in one position, the chain is very tight. Rotate the cranks a quarter turn in either direction, and the chain is quite slack.

    I am posting because I have tried lots of combinations which I am listing here, and none solve the problem which I have had since day one:

    Shimano XT cranks (180mm) with the new outboard bearing BB.
    I initially used the stock XT 32t chainring switched to the outer chainring position (man did that ring wear out fast!).
    I am now on my second Surly stainless 32t ring (bent the first one in a matter of weeks on a big drop. Oops...); I have run them both in the outer position, but have now switched the new Surly ring to the middle ring position. This allows me to move the rear cog closer to the center of the freehub body... I have been using stainless shorty bolts without a chainguard to secure the ring. Same bolts all along.

    I have used 2 different SRAM chains. A PC-59 and now a PC-71.

    I have used 3 cogs, all Kick Ass Cogs, 2 are 20t and the latest is a 21t.
    The cogs have been mounted on 2 different wheelsets. One with Deore disc hubs and the other utilizes the latest sweet XTR disc hubs.

    I have tried to visually center the rings on the crank spider with the chain out of the way. No luck. I have had the chain and rear wheel in place, losened the ring bolts just a tad so the ring could re-position and hopefully center itself, spun the cranks and tightened the bolts. No luck.

    So, MTBR guru's... What shall I try next?
    I didn't have this problem with my last 2 singlespeeds...

    Thanks!
    OGG
    Spinning and Grinning...

  2. #2
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    check it out

    http://sheldonbrown.com/singlespeed.html#tension

    it's in the part about chain tension

  3. #3
    paintbucket
    Reputation: wooglin's Avatar
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    I use Sheldon's method mentioned above, and get it close enough so that I don't drop the chain. If I was real picky I'd rotate the chainring on the spider until I found where it worked best, and _then_ use Sheldon's method. I suppose its also possible that a cog is not stamped out centrally, but it seems to me that would cause less of an issue than a chainring that was off-center by the same amount so I don't worry about it.
    When the going gets weird its bedtime.

  4. #4
    mtbr member
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    How slack is quite slack?

    Since the tension synchronizes with the crank position, it has to be the BB, crank, or chainring that's not perfect. It's not the cog, chain, or hub.

    It doesn't take much deviation from a perfect circle to make the chain sag noticeably, 0.005" off and the chain drops 1/4", 0.05" off and it drops 1". So the Sheldon Brown micro adjusting method might help. I've seen chainrings wear out of round because of extra stress at 3 and 9 o'clock crank positions (on climbs) but (oops) you have a new chainring.

    I don't have the XTR stuff but are the bearings servicable? Could be an internal problem.

  5. #5
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    I'm not sure if you completely answered this: If the tight spot is once every crank rotation, the problem is on the crank side. If it is once every cog rotation, it is the on the cog side. I have my doubts it is on the cog side. My guess is that it is the chainring on the cranks, there is usually a pretty sloppy fit to acount for differing tolerances. Use Seldons method to center the chainring on the crank. Or do what I did: Adjust the chain tension in the tightest position, and if you don't drop the chain, don't worry about it!

    Mark

  6. #6
    Cold. Blue. Steel.
    Reputation: OneGearGuy's Avatar
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    Quote Originally Posted by bikeny
    I'm not sure if you completely answered this: If the tight spot is once every crank rotation, the problem is on the crank side. If it is once every cog rotation, it is the on the cog side. I have my doubts it is on the cog side. My guess is that it is the chainring on the cranks, there is usually a pretty sloppy fit to acount for differing tolerances. Use Seldons method to center the chainring on the crank. Or do what I did: Adjust the chain tension in the tightest position, and if you don't drop the chain, don't worry about it!

    Mark

    Thanks for the good input. Gettin' lots of responses on the 29er board, too...
    I tried Sheldon's method (it's my usual way) and rotated the ring a quarter way around the crank. Only helped a tad. The chain doesn't drop, so I will just continue to ride it. One day I will get another crank and maybe I'll really know then...

    OGG
    Spinning and Grinning...

  7. #7
    mtbr member
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    (psst) Paging Bikeny

    I sent a PM about the Woodman QR conversion, do you have the instructions?

  8. #8
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    My first guess would be a slightly bent BB spindle. You said you hit it hard enough to ruin a steel ring right....

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