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Another high gear crank/chain vibration thread

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Another high gear crank/chain vibration thread

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Old 07-09-18, 09:27 AM
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zammykoo 
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Another high gear crank/chain vibration thread

I'm experiencing crank/chain vibration on a recent used bike purchase and wanted to see if anyone can point me in the right direction. It's a 9-speed Specialized Tricross and I only feel it in the last 2 cogs. The highest cog does it the most.

I've swapped out the chain for a new Shimano one but that didn't make a difference. Then I swapped out the cassette for one in my spare parts bin from the original HG50 to HG70, and that helped a bit. The vibration is still there but probably half as noticeable. Some say that this is just how it is and that I should be riding at the correct speed to be in those gears, but I wonder if there is anything else that I could look at.

I have not taken a look at the bottom bracket bearings because I don't have the lock ring tool (FSA mega exo), but the cranks spin smoothly on lower gears. I will probably replace the bearings soon anyway once I can.

Any other thoughts? Thanks!
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Old 07-10-18, 02:47 PM
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That's most likely how it is, and yes you should be riding the proper gears for your speed. If you want to know more about the vibration, and some possible way to reduce it, just Google chordal action.
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Old 07-10-18, 04:45 PM
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Check your chainline

Chordal action can be increased/decreased by chainline. Putting a spacer in the BB, bringing the chainline a bit outwards helped to reduce the vibration in my bike, though not completely eliminate it.
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Old 07-10-18, 08:41 PM
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Originally Posted by Abu Mahendra View Post
Chordal action can be increased/decreased by chainline. Putting a spacer in the BB, bringing the chainline a bit outwards helped to reduce the vibration in my bike, though not completely eliminate it.
Not sure this is true since chordal action is an issue of tooth count and power pulses. How the chain feeds off the cog isn't an aspect of chordal action. But really poor chain angle will induce some amount of roughness rhythmic with each tooth/link release. Andy
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Old 07-10-18, 11:43 PM
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2.2.1 Chordal Action
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Old 07-11-18, 02:53 PM
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Interesting. I haven't checked my chain line since everything on the bike was original. I assumed that it was set up how the manufacturer intended.

It didn't seem like anything has been modified before I purchased it, but I guess we can never be too sure. I'll take a closer look and take measurements.
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Old 07-11-18, 05:16 PM
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Does it only do it while pedaling?
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Old 07-11-18, 06:30 PM
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Originally Posted by Troul View Post
Does it only do it while pedaling?
Yes, only while I put some muscle on the pedals in higher gears. On lower gears it seems perfect.
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Old 07-11-18, 06:40 PM
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Originally Posted by zammykoo View Post
Interesting. I haven't checked my chain line since everything on the bike was original. I assumed that it was set up how the manufacturer intended.

It didn't seem like anything has been modified before I purchased it, but I guess we can never be too sure. I'll take a closer look and take measurements.
it may not be chordal action at all, but rather poor chainline.
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Old 07-12-18, 01:38 AM
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Originally Posted by zammykoo View Post
Yes, only while I put some muscle on the pedals in higher gears. On lower gears it seems perfect.
more than likely the chain alignment being felt.
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