Bicycle Mechanics - Knocking noise and vibration only on the 3rd smallest ring in cassette

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fstshrk
02-12-12, 05:56 PM
The bicycle is a Jamis Aurora. It uses a mountain bike rear cassette 12-32. The crankset is a Shimano Ultegra triple.

I have this weird knocking noise on the rear cassette only when it is on the third smallest chainring. Any other chainring and any other front chainring selection is OK as long as I don't crosschain of course. The chain is not hitting the FD. The knocking happens on both the largest and the middle chainring in the front.

There is no visible freeplay on the hub or the cassette. The RD shifts perfectly with no RD noise.

Any thoughts?

Thanks for all advice.

HillRider
02-12-12, 06:04 PM
I'd look at that particular cog to see if it's damaged or improperly made. There may be a bent or otherwise irregular tooth.
ThermionicScott
02-12-12, 06:07 PM
Is that sprocket (chainrings are in the front) excessively worn compared to the others?

dsbrantjr
02-12-12, 06:09 PM
Is an older (as in not new) cassettte? If so, I'll bet that you use that cog more often than the rest and it has worn more than the others. This is especially likely to cause problems if you have recently replaced the chain.
fstshrk
02-12-12, 06:12 PM
Is an older (as in not new) cassettte? If so, I'll bet that you use that cog more often than the rest and it has worn more than the others. This is especially likely to cause problems if you have recently replaced the chain.

Hi

Thank you all for the helpful suggestions. The cassette has two thousand miles on it, the chain about a thousand. The teeth of that particular cog look OK to me (but I am no mechanic). However, I think it is a good suggestion to check to see if it is bent or not. I will take a look.
Kimmo
02-12-12, 07:26 PM
If you can't find what's causing it, I have a possibility for you: it could be a worn or faulty cassette body.

Sometimes, when loaded from a particular direction (by the chain being on a certain cog), a single pawl can disengage while the other/s remains engaged; this can make for something about halfway between a tick and a clunk. Occasionally it may let go and jump, feeling similar but noticeably different to the chain skipping a tooth.

This can sometimes be fixed be removing a shim or two from inside if it has some play.