My drivetrain is eccentric :(- Mtbr.com
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  1. #1
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    My drivetrain is eccentric :(

    I have a ISAR chainring up front, and a White freewheel on a White Eno Eccentric hub in the back, and my chain tension is still varying as I pedal. It's nothing dangerous, but I'm not sure where the variation is coming from. I've tried "adjusting" the chainring, but to no avail. Currently I suspect the chainring bolt holes in my m960 crankset were drilled slightly off. Any ideas?

  2. #2
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    How long have you had the setup?
    Loosen it, until the chain goes slack. Tighten the nondrive side first, then the drive side. There will be a degree of slack no matter what.

  3. #3
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    It's not that I have slack in the chain, it's that the chain tension varies as the crank turns.

  4. #4
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    Quote Originally Posted by bad mechanic
    It's not that I have slack in the chain, it's that the chain tension varies as the crank turns.
    Not all chainrings are perfectly round. My ENO's bikes all are like that to some degree. Tight spot on the chain, then a loose spot. Even my bikes with horizontal dropouts - same thing.
    From Sheldon Brown:

    Set the rear axle so that the chain pulls taut at the tightest part of the cranks' rotation. One at a time, loosen up each of the stack bolts, and tighten it back just finger tight. Spin the crank slowly and watch for the chain to get to its tightest point. Strike the taut chain lightly with a convenient tool to make the chain ring move a bit on its spider. Then rotate the crank some more, finding the new tightest spot, and repeat as necessary.

    This takes a little bit of your hands learning how hard to hit the chain, and how loose to set the stack bolts, but it is really quite easy to learn.

    Tighten up the stack bolts a bit and re-check. Tighten the stack bolts in a regular pattern, like the lug nuts on a car wheel. My standard pattern is to start by tightening the bolt opposite the crank, then move clockwise 2 bolts (144 degrees), tighten that one, clockwise 2 more, and so on. Never tighten two neighboring bolts in a row. You may prefer to go counterclockwise, but try to get in the habit of always starting at the same place and always going the same way. This reduces the chances of accidentally missing a bolt.

    Once you have the chainrings centered and secured, adjust the position of the rear axle to make the chain as nearly tight as possible without binding. Notice how freely the drive train turns when the chain is too loose. That is how freely it should turn when you are done, but with as little chain droop as possible.

  5. #5
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    I'm using an ISAR chainring, and those are supposed to be perfectly round.

    I'm already tried Sheldon Brown's method and it only helped a little.

  6. #6
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    My stock set-up was like that on my XXIX - I found one way to check whether it was the crank or the chainring, process of elimination:
    Pedal around with the bike in a rack or stand. Note whether the tight and loose spots are consistent with crankarm position. If so, then it is somewhere in the crank/ring part of the system. If not, it must be in the rear of the system (pretty rare).
    Note where the arm is when the chain is tight - note it on a diagram showing where the arm is at (for example, 10:00 position). Remove the chainring, turn it two chainring bolt positions around the bolt circle, and put it back on. Now check for the same tight/loose chain condition. If it was the ring, the arm should now be around 4:00 where the chain is tight (opposite 10:00 in my previous example). If it is the crank, it will still be tight at around 10:00. Of course, there is a slight chance that the ring bolts were pushing the ring in one direction or another (ernesto's scenario above). I put an ISAR ring on my bike and the eccentric issue was gone.
    R.I.P. Corky 10/97-4/09
    Disclaimer: I sell and repair bikes for a living


  7. #7
    Nightriding rules SuperModerator
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    AFAIK, that is "normal" up to a certain degree....

  8. #8
    AZ
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    Even with a "perfectly round" chainring the way it is bolted on to the crankset can affect its concentricisity . Try loosening the CR bolts and recentering the chainring. Your freewheel can also contribute to this , not anything you can do to affect that though . Its very common , but can usually be brought into an acceptable condition .


    Edit : some creative file or Dremel work is sometimes needed .
    Last edited by AZ; 03-11-2010 at 09:50 AM.

  9. #9
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    Every ss setup i have ever had has that same problem but it's not really a problem, just tension the chain at the tightest spot and ride it.
    Perfectly normal


    Quote Originally Posted by bad mechanic
    I have a ISAR chainring up front, and a White freewheel on a White Eno Eccentric hub in the back, and my chain tension is still varying as I pedal. It's nothing dangerous, but I'm not sure where the variation is coming from. I've tried "adjusting" the chainring, but to no avail. Currently I suspect the chainring bolt holes in my m960 crankset were drilled slightly off. Any ideas?

  10. #10
    Schipperkes are cool.
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    again,
    Quote Originally Posted by banks
    It can also be related to the crank arm, the bottom bracket, the rear cog, the freehub body, the freewheel, rear hub axle.
    Quote Originally Posted by mikesee
    Better suited to non-aggressive 125# gals named Russell.
    I ride so slow, your Garmin will shut off.

  11. #11
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    Quote Originally Posted by banks
    again,
    I have a feeling a goofy gnome sneaks past you and sends weird jubu to yer bike - happens now and then. The only way to prevent such disasters is to show them a picture of a giant crab. They are so afraid of crabs that they don't shower for a reason.

  12. #12
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    consider yourself lucky.. my drivetrain is abstrusely apopleptic!
    If steel is real then aluminium is supercallafragiliniun!

  13. #13
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    Quote Originally Posted by byknuts
    consider yourself lucky.. my drivetrain is abstrusely apopleptic!
    apopleptic?!?! NO! that's the worst - especially when you have to follow it around with a bag and a stick.

  14. #14
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    my bag and stick are just fine thank you.
    I keep them away from anything with a chain, that involves mashing or stomping, or teeth that're out of round...
    If steel is real then aluminium is supercallafragiliniun!

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