Diagnosing and fixing a wobbly crankset

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Diagnosing and fixing a wobbly crankset
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flag tmrozewski
Posted: Oct 1, 2012 at 12:30 Quote
I've noticed a wobble in my front chainring. I'm trying to figure out the cause of the problem.

My chain snapped when I was riding across down and had to dump it at a shop to have it replaced. The mechanic showed me the wobble and said it probably caused the break. He said it was a problem with the chainring, I believed him, and swapped it. Problem not solved.

I'm guessing the issue is with the bearings, or maybe with the spider arms. Does anyone have advice for diagnosing the problem?

I'm pretty new to bike repair - teaching myself to fix things rather than pay someone else.

Thanks!
T
flag Reignx08
Posted: Oct 1, 2012 at 15:51 Quote
i think the first step is to determine if its A) your crank arm, B) bottom bracket, or C) possibly and prob not likely maybe your chain ring (s).
Some info would help... do you know what set-up you have? Brand or model? there are a few possibilities all designed and assembled differently.
\in any case, grab your crank arm firmly and give it a "wobble check". check your crank bolts are tight. check your chain ring bolts are all tight and might not hurt to at least visually inspect your bottom bracket and see if you see any signs of it being loose. If the crank bolts are loose you may get away with just tightening them, but if you rode far on loose crank arm the spline is prob not in the best of shape. \if the bearings in the BB are loose youll neeed a new BB but may get away with using the same crank arms if you buy the same interface.
\i\ve never had a chain ring come loose but its possible.
hope that helps a bit,maybe give you a place to start
flag tmrozewski
Posted: Oct 1, 2012 at 16:24 Quote
Thanks for that. I'll run through the checks you suggest. The bike is a Fuji Track with no fancy gear. Having just replaced the chain ring, I can verify that the crank arms and chainring are tight, so my guess is it's the bracket.

Jumping ahead a bit, let's assume the issue is the bracket and/or bearings. Generally, is it possible to replace just the bearings or would I have to replace the entire bracket?

PS, I think I was mis-using the term "crankset" for "bracket" in my original post.
flag Reignx08
Posted: Oct 1, 2012 at 17:23 Quote
Generally you would replace the whole bottom bracket. Mtn bike bottom brackets run anywhere from $30 ish and up, your fuji is a road bike so \i'm not 100% sure what set up it has (square taper crank, isis ect) but its pretty safe to say that you would be best to swap out the whole BB.
If you do need to replace it it usually requires a couple speciality tools to complete the job. You'll need to figure out what size & interface it is so it matches your bikes BB shell (width) and cranks interface (how the cranks attach to the bottom bracket)
this link may help:
http://www.bikeforums.net/archive/index.php/t-776979.html

they list the bb as 68 mm (thats the width of the BB) and 108 (length of the spindle).
flag bigburd
Posted: Oct 2, 2012 at 4:01 Quote
Grab both crank arms and give them a wobble , if both sides move then it's usually the Bb bearings , if only one side moves you have crank interface problems.

If one arm is moving , as reignx08 said , chances are you have allready done too much damage to the interface , whip the crank off and check the contact points for wear and burring.
flag tmrozewski
Posted: Oct 2, 2012 at 11:14 Quote
Thanks for that. However, nothing appears to be loose.

It seems that the spindle is not rotating cleanly on its axis. That means the chainring isn't parallel to the frame as it turns. Does this sounds like a spindle issue or a bearing issue?
flag bigburd
Posted: Oct 2, 2012 at 11:47 Quote
tmrozewski wrote:
Thanks for that. However, nothing appears to be loose.

It seems that the spindle is not rotating cleanly on its axis. That means the chainring isn't parallel to the frame as it turns. Does this sounds like a spindle issue or a bearing issue?



It sounds to me like a bent crank arm or bent spider/chain ring tabs if it is moving sideways.

Do the bearings run free and smooth ?
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