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Drivetrain Issues

Old 09-05-08, 12:44 PM
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natonium
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Drivetrain Issues

Recently I've noticed that my chain will tighten up and loosen up during a rotation. I never saw this on my previous setup. Any ideas? I have recently swapped out the BB and crank arms.

Thanks,
_nate_
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Old 09-05-08, 12:48 PM
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Recently I've noticed that my chain will tighten up and loosen up during a rotation.

Sounds like the chainline is no longer straight.
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Old 09-05-08, 12:50 PM
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Chainrings and cogs are not perfectly round, and as a result, the chain tension changes depending on the orientation of each. If your chain is very tight, just loosen it until it no longer binds.

If that makes it too loose, you need to center your chainring. Sheldon Brown explains how to do that.
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Old 09-05-08, 01:05 PM
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Originally Posted by iDrake View Post
Sounds like the chainline is no longer straight.
How would that lead to varying tension during rotation of the drivetrain?
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Old 09-05-08, 01:29 PM
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Originally Posted by zacked View Post
Chainrings and cogs are not perfectly round, and as a result, the chain tension changes depending on the orientation of each. If your chain is very tight, just loosen it until it no longer binds.

If that makes it too loose, you need to center your chainring. Sheldon Brown explains how to do that.
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Old 09-05-08, 01:37 PM
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Originally Posted by kyselad View Post
How would that lead to varying tension during rotation of the drivetrain?
Just thought it would be putting unnecessary pressure on the chain during rotation. I'm no engineer.
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Old 09-08-08, 07:15 AM
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Thanks for the feedback. I'll try out Sheldon's method to see if that fixes anything. I'll let you know if that works. If not could it be a bumb BB? Like one end could be bent or angled wrong?
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Old 09-08-08, 07:39 AM
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Originally Posted by natonium View Post
If not could it be a bumb BB? Like one end could be bent or angled wrong?
Not necessarily bent, but cheaper components may not have been manufactured as accurately. Square tapers need to be cut very carefully, especially in the crank. Look for any side to side movement in your chainring during a revolution. clamp a ruler or some other thin indicator to the seatpost and see how it interacts with the chainring during a revolution. If you can, see if there is any odd wobble in the crank spider itself as you turn the crank.

Normally, Sheldon's alignment procedure will work to eliminate tight spots, but an improperly manufactured component may be possible.
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Old 09-08-08, 07:58 AM
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Its a brand new Sugino BB. But I will try out Sheldon's tips to see if that fixes it.

Thanks.
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