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I ride an SE Lager fixed. On the 30th of May this year I did a bit of an overhaul, replacing the rear cog, chain, wheels, tyres and brake pads.

I measured the chain last weekend and discovered that it was already over .75 (which I asked about here), but less than 1.0. So I decided the replace the chain with a new KMC Z410 chain. I use a Surly track cog 1/8 inch, 17t on the back and since it was only a few months old and with a single chain I decided not to replace it. The chainring is also not old and looks to be in fine condition.

Here is a photo showing the old cog (on the left) and a brand new cog (on the right). I couldn't see any appreciable difference which I took as reinforcing my decision that it didn't need replacing.

New vs Old cog

Unfortunately the next day I went for a ride the drivetrain was incredibly noisy. Clunking and grinding. I could even feel roughness through the pedals. I was thinking maybe the bottom bracket (the only drivetrain component that has not been replaced yet) might have completely fallen apart. What is causing this noise?

Fair warning: I have the answer to this question that I will post shortly. According to my research, asking and answering a question is encouraged as a source of information

Update: I should clarify that I don't have the answer to the question. I know how I fixed it, but that doesn't preclude another answer being more informative

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I appreciate the heads up. My understanding is that you post and accept your own answer if no one else provides you with the correct one. Not the case here. @Neil Fein: Correct me if I'm wrong? I don't care really one way or the other in this case, just thought it was worth asking before we set a precedent. –  zenbike Aug 2 '11 at 3:25
    
I see two issues here, actually While one wouldn't want to ask and answer one's own question terribly often (it'd be unfair to other users if done on a regular basis), I think that, as long as the question is a good one, and attracts good answers, it doesn't matter if the poster answers their own question. @freiheit might disagree with me here (correct me if I'm off-base, please), as there's a current sorta-policy that we should be asking practical questions based on real-world problems. It might be worth our while to discuss these two issues in meta before we launch next week. –  Neil Fein Aug 2 '11 at 3:54
    
@zenbike - But I think this question is fine, to answer that part of it. –  Neil Fein Aug 2 '11 at 3:57
    
@zenbike - yes, I agree that my answer is not necessarily the best one. I've updated the question with that note. I already do like your answer better :) –  Mac Aug 2 '11 at 4:00
    
@Neil Fein - Thanks for your thoughts. I agree this isn't something that you'd want to see too often. But I've also realised that even though I have the fix for the noise, I don't understand why it was noisy, which is really the core question. So there's definitely value being added by both Daniel and zenbike –  Mac Aug 2 '11 at 4:12

3 Answers 3

up vote 1 down vote accepted

Fixed gears don't have as much tolerance for mismatched wear patterns as geared bikes, because there is no "give" in the drive train system.

The small amount of wear shown on the used cog is enough to cause this kind of noise in the case of a fixie. I'd be surprised if the same amount of wear caused you any problems at all on a geared machine.

It's also possible that the chain was over tensioned. I know you said that you use a tensioner, and that you didn't change the settings, but you used a new, unstretched, and therefore shorter chain. You may have needed to relax the tension by a small amount.

As you removed the wheel to change the cog, It may be that the tension has changed as well. Hard to say for certain, as you made the noise go away.

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To clarify the tensioning - When I replaced the chain I adjusted the tension. There was a very noticeable adjustment required. But then when I subsequently adjusted the rear cog I didn't adjust the tug from it's setting with the new chain. I agree that I might have changed the tension marginally just be removing and refitting the wheel. –  Mac Aug 2 '11 at 4:03

Normally I'd suspect hooking, on a worn cog. But though there is some barely perceptible hooking on that "worn" cog, it's not nearly enough to cause a problem.

So I'm going to go with the yucky color of that chain. The bike is rejecting it! (Or perhaps the chain is just too wide.;)

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Haha. I was using the blue one for a while but my LBS stopped stocking them, so now I use yellow. It's certainly bright :) –  Mac Aug 2 '11 at 1:58
    
What is hooking? I've seen examples of the 'shark fin' wear pattern but haven't seen hooking. –  Mac Aug 2 '11 at 2:06
    
Hooking is where the teeth develop a depression. If you look real closely in the picture, looking at the two teeth at the bottom left, you can see a slight depression in the tooth, where the chain has worn the tooth away very slightly. That can get much, much worse, and will cause the chain to grab as it comes off the cog, producing a jerking motion. In this case you probably have a problem because the cog has so little wear overall that replacing the chain ate up what little tolerance there is. Plus you may have the chain tugs a little too tight. –  Daniel R Hicks Aug 2 '11 at 2:20

It turns out the rear cog was making the noise. I replaced the cog as soon as I got home and everything is running silently again.

Given how little wear there is on the cog, I'm amazed at how noisy it was.

I guess there's also an off chance that I had something badly adjusted, but I use a chain tug and didn't change the setting so I don't think the chain tension would have changed.

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So it had nothing to do with running a 1/4" chain on a 3/16" cog?? –  Daniel R Hicks Aug 2 '11 at 2:11
    
Oops -- the link you gave for the cog takes you to a page that talks about a 3/16 chain in the fine print, but I see at the top it's a 1/4 cog. –  Daniel R Hicks Aug 2 '11 at 2:13
    
@Daniel R Hicks - I think they're both 1/8". The cog has 1/8" printed on it (see pic) and the chain is also 1/8. Why do you say the chain is 1/4" and the cog is 3/16? –  Mac Aug 2 '11 at 2:13
    
If changing the cog made the noise go away, it probably was hooking and its kin, and likely the noise would have gone away on its own in a few more miles of riding. –  Daniel R Hicks Aug 2 '11 at 2:15
    
Oops squared -- divide everything by 2. I get my chains and my phone plugs confused sometimes. –  Daniel R Hicks Aug 2 '11 at 2:23

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