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Clunk noise from front cogs
#1
Hello All,

Hope you can help me with this problem:

I have a stock Trek 3700 bike with 3x7 speeds Shimano Acera rear derailleur and Shimano C051 front derailleur. Crankset model is Shimano M151 42/34/24. Model year around 2009-2010.

I've had it since new and wasn't ridden a lot nor abused. Nothing seems to be bent. All seems well, shifting and all except an annoying clunk when pedaling on gears 5 , 6 and 7 while on the middle front cog. Upon close inspection I located the clunk source: The chain is touching the inside of the biggest front cog where a notch is located on the cog. These notches or indentations protrudes towards the inner side of the cog and that's where the protrusion touches the chain.

I took photos to show you exactly how it happens. I would like to hear your expert opinions before tinkering.

Thank you kindly in advance.

PS: The chain is not worn, it's practically new, neither is the chain ring. The touching point on the cog is happening in the spot where there are some kind of indentations or notches on the cog, there are 4 evenly distributed on the cog, seems like a feature cos it has an arrow just below and the writing "Special Teeth Profiles Shifting" as you can see in the photos. Maybe they are the spots that facilitate the chain to jump to the next gear, I'm not sure but this is how it always was since new. And the clunk sound is not new , I've been hearing it since forever.

   
   
   
   
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#2
chain rings look worn, replace the crank.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#3
(06-03-2020, 08:15 AM)Painkiller Wrote:  chain rings look worn, replace the crank.

Could it be worn already? The bike wasn't ridden much, not more than 2000 Kms... What's the life expectancy of it?
Suppose I file down the notch just a little bit, around 0.5-1 mm, would it compromise the structure or functioning in any way?
Thank you for your reply.
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#4
not really out anything by filing down the spot, but if it has been like this from day one I would upgrade to an Alivio crank set. It can only be two things, the chain or the rings. You could always start with a different brand chain to see how it plays, Like Sram. Then replace the crank if need be, making sure the bottom bracket is the proper length for the new crank too.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#5
Thank you for your input, much appreciated!
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#6
Reasons why the chain might be touching the chain rings:
  • botttom bracket axle too short for crank set
  • chain rings bent
  • bb axle bent

I have seen the first one, the bike was set up so the chain line was optimised for the smallest chain ring. To some extent I have witnessed the bent chain rings - not so much the axle...
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#7
Could this have something to do with the centering of the rear wheel in the frame? Maybe if the rear wheel could be moved about 1 mm to the left within the frame then the chain angle would change and miss the notch on the biggest front chain ring... Or could it have something to do with the kickstand making the rear wheel slightly off-centered within the frame? I have never removed the rear wheel btw.
I don't know I'm just guessing, it just doesn't make any sense that nothing is bent and this is happening. My friend has the exact same bike, we bought them together and his doesn't have that problem... Gotta take a closer look at his.
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#8
(06-04-2020, 06:00 PM)Joe_W Wrote:  Reasons why the chain might be touching the chain rings:
  • botttom bracket axle too short for crank set
  • chain rings bent
  • bb axle bent

I have seen the first one, the bike was set up so the chain line was optimised for the smallest chain ring. To some extent I have witnessed the bent chain rings - not so much the axle...

Hi Joe! Sorry I must've missed your reply somehow.
About the bb axle being a bit short, I think you might be on to something there: I have a couple of friends who bought the exact same bike at the same time, theirs do not have this problem.
could the chain ring bolt be overtightened somehow? can it be spaced out just by 1 mm? Cos I think it couldn't be short as the bike was bought new and came as is from Trek... Or maybe it could..
Thank you for your input!
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#9
your crank does not have chainring bolts as it is rivited, If you meant the center crank bolt, no, you could not over tighten that to change anything, you would break the bolt or crank arm first.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
  Reply
#10
(06-06-2020, 10:13 AM)Painkiller Wrote:  your crank does not have chainring bolts as it is rivited, If you meant the center crank bolt, no, you could not over tighten that to change anything, you would break the bolt or crank arm first.

Yes I meant the center bolt. I was thinking maybe it is overtightened now that's why the chain is hitting the largest ring.
Could it be possible to loosen it a bit to maybe move the chain ring outward just by 1 mm? That was my question, sorry if I expressed it wrong. English is not my first language.
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#11
No, no way. You can put a spacer on the bottom bracket threads (where it is screwed into the frame). That would move it out. Had to do this, as I ordered one with a too short axle (the smallest chain ring was a bit too close to the frame for my taste) - put that one together from scratch, and messed up measuring...

Don't mess with the wheel axle spacers, it'll just make things worse (unless you want to open the bearings and grease them, many come pretty dry from the factory, and assuming it's a cup and cone bearing).
  Reply
#12
(06-06-2020, 10:51 AM)TheCrow Wrote:  
(06-06-2020, 10:13 AM)Painkiller Wrote:  your crank does not have chainring bolts as it is rivited, If you meant the center crank bolt, no, you could not over tighten that to change anything, you would break the bolt or crank arm first.

Yes I meant the center bolt. I was thinking maybe it is overtightened now that's why the chain is hitting the largest ring.
Could it be possible to loosen it a bit to maybe move the chain ring outward just by 1 mm? That was my question, sorry if I expressed it wrong. English is not my first language.
Yes I figured what you meant, Of course it would be nice to have your bike in my stand but I think I got the picture of whats going on. you to seat the crank arms so loosening of those. I will revert back to my first two responses of your post and stick with that as a fix for you. Nothing else I can help with on this matter. However Like Joe has mentioned it may not hurt to double check proper spindle length to your crank set, It could be wrong from the factory but probably not. With that said, I have seen where changing it to a slightly shorter or longer felt better for my purpose and best chainline for radical gearing mods. but you describe a basic stock bicycle
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
  Reply
#13
Thank you so much Joe and Painkiller for your replies.
I would love to upgrade the chain and chain rings, but things are very tough right now and I cannot afford to buy anything especially that where I live everything is 4x the prices I see online.
Ordering online is out of the question due to exorbitant fees and bad postal service, so I guess I'm stuck for now.
I put some thick tape over the notches on the largest chain ring and this has reduced the noise greatly for now.
Maybe I will try to put a 1mm spacer on the BB threads if I ever manage to find a crank puller...
Thanks again!
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