Chain tension varies....- Mtbr.com
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  1. #1
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    Chain tension varies....

    Sorry if this is a stupid question or has been discussed before....

    I ride an EBB SS (Niner SIR 9) with a Race Face chain ring and Niner cog. I notice that when adjusting chain tension, the chain is tighter/looser at certain spots when rotating the crank. It's not a real problem since the chain tension doesn't vary that much...it neither snaps nor comes off the ring at the tightest/loosest spots. Since observing this phenomenon, I make sure I adjust the tension at a loose spot to allow for the subsequent tightening when the crank rotates.

    I'm assuming this has to be caused by the chain ring not being perfectly round? Or perhaps the rear cog? I've checked them both for roundness and they seem to be OK. For a while there I thought Race Face had re-introduced Bio-Pace; every fad in cycling comes back sooner or later.

    Could it be the crank spider arms (Shimano XT)? The bolt pattern on the RF chain ring not machined correctly? The Niner cog too loose on the freewheel body? I'm going nuts trying to figure this out.

  2. #2
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    totally normal.

    t's because of the 4 bolt chainring and crank.

    loosen the chainring bolts slightly, turn the crank until it's at its tightest and give the chain a good squeeze halfway between the ring and cog. tighten the crank bolts up again and see how it feels for tension throughout the crank's rotation.

    may take a few tries to get it right....

  3. #3
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    I have a Middleburn splined chainring and i have the same problem so it may not be the 4 bolt ring that is the issue. My fixed gear track bike does it and i even visited the local bike shop and the owner showed me how all the bmx's do it too.

    i agree that it is annoying and i don't really understand where the innacuracies lie in my drivetrain but if it doesn't cause any issues then don't worry too much.
    'Bicycles themselves are the perfect blend between artistic craftsmanship and technical application' - Carl Schlemowitz

  4. #4
    velocipede technician
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    looking for 20-21" P team

  5. #5
    sop
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    Thanks for the link and feedback. I though it was just my setup, good to know it's common. I've not had to adjust chain tension on a SS since my Schwinn Stingray days. As noted, it's not really a problem, more of a frustration not being able to pinpoint the cause. Thanks again for the help.

  6. #6
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    I think this is pretty universal - I tried the methods above but had to admit failure in the end!

  7. #7
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    dunno on my fixie (real nice old sugino superbee cranks) it doesn't happen.

    on my rainy day commuter (cheap old trvativ crank) it is bordering on ridiculous but since it really doesn't affect my riding i never f-ed with it.

    on my mtn ss (lx crank) it is there but barely.

    in my experience it comes mostly from an unevenly drilled crank spider. i have even heard of people sending back new cranks because of this.

    it is cureable but it might just not be worth it.

    some cranks are definitely drilled to improper specs and (like in the case of my commuter) no matter how much you adjust, you'll never get rid of it. new cranks would be the answer but i really don't spend money on that bike unless i HAVE to.

  8. #8
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    That was a realy good qusetion. I have been dealing with the same thing on my fixie. So now I can forget about it.......
    "Roll your own......." Smokebikes.com

  9. #9
    Bend, OR
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    I had this problem on my trials bike, bad enough that when my good foot was forward, the chain was far too slack, broke it a few times. I've concluded that it is actually the cranks however, since I've rotated the chainring, cog and chain without any different results. It was only a problem because I was trying to SS without a tensioner with vertical dropouts. Don't think it bothers so much now with a tensioner. We'll see.

  10. #10
    nachos rule!
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    cheap freewheels are another big source of this problem. there's a world of difference between say an acs and an eno, acs will have major differences in tension, the eno will have none (assuming your spider & ring are cool.)
    plus a change, plus c'est la m'me chose - alphonse karr

  11. #11
    Poorly Disguised Poser
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    I agree with almost all that has been said..here is a solution.

    I recently installed an eno crank, with eno ring on my bike with a Phil BB and an eno FW. I did not expect to see that this varying chain tension would be gone...at least gone enough that I can not really discern a tight spot.

    I can not say if it is mostly the BB, the Crank, the Chain ring, or the free wheel. But I can tell you that the freewheel by itself did not change a thing.

    You can try and adjust it out with your chainring bolts as mentioned by others, but I have always noticed that this worked only for about half a ride. Not sure if it is the torque induced shift of the ring, or the occasional bash of the ring against a tree or rock. Either way, the chain ring bolts are simply not strong enough to eliminate movement.

    I am sold on the one piece chain ring idea. There are many sound arguments against them, but none matter to me and my riding.
    My ego is bigger and better looking than yours.

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