Misaligned Crank?

Discussion in 'Cycling Equipment' started by Palisades, Oct 11, 2006.

  1. Palisades

    Palisades New Member

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    Hi,

    Ok I bought a Motobecane Immortal Force Carbon fiber bike off of Bikesdirect.com after reading a bunch of positive stuff about it and riding the one my friend bought. But this is not a thread about bikesdirect, so please dont gun me down as a bikedirect shill. Ok the bike looks great, everything seems ok when I put it together. Took a test ride in the small ring, everything is ok, a little FD rub when the chain was crossed, not a big deal, maybe needs a bit of limit screw adjustments. The big ring is a completely different story. I get chain/FD rub in all rear cogs on the big ring. I fiddled with the limit screws and no help. I am concerned that either crank and chain rings are crooked or bent, or that the crank was not screwed into the bottom bracket correctly. In the big ring, the chain will rub one side of the derraileur and then if I rotate the crank/pedals 180 degrees, it is rubbing on the other side of the derraileur. If I look directly from behind the bike, and spin the crank, you can see the chain-ring moving horizontally left to right. It is not aligned properly with the bike. In the small ring the effect is not as pronounced and I rarely get rub unless I am crossing the chain pretty badly in a small rear cog. I have adjusted the front derraileur's limit screws multiple times to try and find a sweet spot that the chain wouldn't rub and it was impossible. No matter where it is adjusted the front derraileur rubs in all rear gears when on the big ring.

    Although it isn't so much of a problem in the small ring, you can still see the lateral movement of the chain. This makes me think it is not the chain ring, but the entire crank being out of alignment with the bike. I emailed Bikesdirect, and they said that I should get in contact with FSA who makes my compact crank. They said they have heard of this issue before and that it was simply that the crankset needed to be tightened down properly on the bb spindle. And that I need to use chromoly/steel bolts to do this and not the aluminum ones that are currently installed. Can anyone explain this to me? I have an email in to FSA, but no response yet. What can I do to fix this problem? Any help would be greatly appreciated. Otherwise the bike was an excellent purchase, smooth and stiff, excellent components, just need to get this issue worked out. Thanks for the help guys.
     
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  2. John M

    John M New Member

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    It may pain you to do so, but maybe a quick trip to the local shop is warranted. The can check BB/crankset installation, frame alighnment, FD adjustment, chainring straightness, etc...

    If it is a crank arm that is loose on the BB, you should NOT ride the bike further until it is tightened or you could permanently ruin the crank arm.

    It could be that the crank was not tightened down all the way (is this in ISIS BB?). What they mean by not using aluminum for tightening is that the lightweight bolts are not sufficiently strong to tighten the crank down but are strong enough to hold on an already tight crank. The proper procedure is to use chromoly (steel) bolts to tighten down the crankset and then remove the steel bolts and put in the light alloy ones to hold it on. You can get steel bolts at any local shop.
     
  3. carbonguru

    carbonguru New Member

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    This is an easy fix. Take the FD and loosen the clamp screw and the Shift Cable Screw. Realign the FD so that it is about 1 dime thickness above the Big Ring. Manipulate the FD so that it is parallell with the rear cogs. Tighten the clamp, then pull the Shift cable with a plier and tighten the bolt. Reset the Limit Screw by turning it all the way down and then back it up about 3 1/2 turns. This should fix it. :rolleyes: If all else fails the LBS would love to take a look at it. Good luck!

     
  4. Palisades

    Palisades New Member

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    I took it to my local bike shop tonight, and they took a quick look at it. They said it looked like the crank was bent. Because when they spun the crank they could see the chain ring as well as the spider moving side to side. Could this still be caused by a crank that is not tightened enough? How can I tell if the crank is crooked, vs it's not tight enough. The shop wants to start replacing things and see if it fixes it. I have a hard time believing the crank is bent already, or that it came bent. I have a much easier time believing the bike wasn't assembled properly. Could I just buy the tools necessary to remove the crank, take a look at the bb, and then if it seems fine, use the steel bolts to tighten the crank all the way?
     
  5. ScienceIsCool

    ScienceIsCool New Member

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    Find a new shop... Bent crank?! I'm sure you can imagine what kind of impact it would take to bend a new crank. Does it look like it fell from an airplane? No?

    Likely explanations include:

    - incorrect installation of the crank onto the spindle causing it to not be fully engaged with the taper on the splines. I.e., not tight enough or grit in the interface?

    - BB shell was not properly "faced". That is, the two sides of the bottom bracket shell on the frame are not parallel to one another.

    - Other explanations?

    Maybe I'm totally wrong, but to start off with "bent crank arm" without checking the obvious stuff seems... amateur?

    John Swanson
    www.bikephysics.com
     
  6. DMF

    DMF New Member

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    No, I agree with the LBS.

    What the OP is describing is a classic sign of a bent shaft. If the shaft is straight the rings may go out of perpendicular, but no misalignment of the bearings can cause the wobble described.

    I too find it hard to imagine what could have bent the axle, but that doesn't matter; it's bent. The only to prove it is to disassemble the BB.

    Let the LBS do their thing, not least because you want the opinion of professional mechanics when getting bikesdirect to make good on it.
     
  7. John M

    John M New Member

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    The shop did an inadequate examination of the problem. Go to a different shop. If you have an ISIS BB, the crank should be bottomed out onto the BB--the metal ring or flange of the crank should be flat against the shoulders of the BB and parallel.

    You can buy the tools and remove the crankset/BB.

    I would still start with a new shop. Go to one that has lots of $5000+ bikes hanging in there.
     
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