[QuiQuaeQuod]

Ok, a strange drivetrain noise puzzle. I got it after 3 tries.

My wife said her drivetrain was noisy. It sounded like the rear D was out of whack, and the chain was not setting right on the cogs, like the links were hanging on the teeth and then sliding into place. No problem, thought I.

So I set the cable tension and stops. I added just a bit of tension, and adjusted the lower stop just a smidge. Noise gone on the stand.

Not on the road.

Ok, second try. I rechecked the adjustment. Spot on. Lubed chain and checked for a stiff link (nope) and dug some gunk out of the cogs that MIGHT have been causing the chain not to seat properly on the teeth. Looked like sticks, turned out to be flimsy plant material.

Nope, noise on the road, still. Now it get's interesting. We rode, and I listened. The noise was bad in the 3rd easiest cog. It was totally silent in all other combinations, at least in the middle ring. But the noise was definitely from the cogs and not the rings. Also, the noise was under pressure of acceleration, pedaling at constant cadence on the flats meant no noise, but speed up or climb and the noise was back.

I fixed it very quickly on getting home.

Campy Mirage, 9 speed drive train.

What was wrong? Please explain WHY the problem only occured in the third cog. In other words, show your work!

 

[bikeboy389]

Hoo,

I feel like there's something I'm missing here. Are you sure all the information I need to solve this (minus expertise, of course) is here?

Had you had the cassette apart?

Otherwise, I'm stumped.

And impatient.

 

[AIE]

That 40/21 combo is a favorite.

I'm surprised you had a spare 21T cog just laying around.

 

[mtbbmet]

A bit of dirt between the cassette and FH body?

 

[konaken]

Worn Chain?

My guess would be the chain was worn and the 3rd cog down was not getting as much wear as the rest? Complete opposite guess from above, but hey, this is supposed to be a puzzler!

 

[Mr. Versatile]

My wife has a bike with a triple front. In one particular gear combo, and only one, the chain just barely contacts the chain ring. When under power, apparently the bike, BB, crank, chainring, (take your pick), flexes just enough to make the noise. I went nutz checking to see if either der. was rubbing or needed trimming, but found nothing. It took me some time to see that it was thr chain ring.

 

[QuiQuaeQuod]

Had you had the cassette apart?

Nope, never touched it.

Around 1000 miles on the drive train, new bike, new(ish) noise.

 

[QuiQuaeQuod]

I'm surprised you had a spare 21T cog just laying around.

Nah, nothing wrong with the cog. I did look for a problem when adjusting the derailleur, no bent/broken teeth.

 

[QuiQuaeQuod]

My guess would be the chain was worn and the 3rd cog down was not getting as much wear as the rest? Complete opposite guess from above, but hey, this is supposed to be a puzzler!

Good guess, but not enough miles on the drive train to be a wear problem... unless my wife has been riding centuries without me

 

[QuiQuaeQuod]

My wife has a bike with a triple front. In one particular gear combo, and only one, the chain just barely contacts the chain ring. When under power, apparently the bike, BB, crank, chainring, (take your pick), flexes just enough to make the noise. I went nutz checking to see if either der. was rubbing or needed trimming, but found nothing. It took me some time to see that it was thr chain ring.

I was AFRAID it would be something like that. Some complex interaction of many factors making it a bear to deal with. But no, it was a simple thing with only one component causing the problem. It was just that the symptom was so strange that makes it tough to figure out.

If no one gets it, I'll post an answer tomorrow sometime.

 

[QuiQuaeQuod]

A bit of dirt between the cassette and FH body?

I'm not sure how that would produce the symptom. Given the tight fit of the cogs on the splines of the body, I am not sure how it would happen. I can see how dirt might stop the cogs from seating all the way on, but that would seem to put everything out of whack... and either make the problem happen in all the cogs or be adjusted out by reseting the stops and cable tension.

 

[QuiQuaeQuod]

Did the BB loosen slightly, moving outward enough to cause a rub?

Nope. Plus if it did, it should have caused problems in all cogs, no? Same for a loose crank, or anything else that altered the chainline as a whole.

This really is a puzzler, because the symptom is so screwy. Also, please note that I said I FIXED it quickly. I did not figure it out from the symptom and go to fix it. I was going process of elimination at that point and stumbled across it. But once I knew the problem I did manage to figure out WHY the symptom was so strange and limited.

Process of elimination should mean someone hits the cause soon... but can they figure out how the cause produced the symptom?

 

[handsomerob]

The rear derailer pulley is slightly too "high" due to the increased tension in the chain when you are actually riding the bike and doesn't give enough clearance for the chain to engage and stay in the third cog.

or possibly

if you have deep dropouts, maybe the axle was not seated correctly leaving an angle that only created trouble in the third cog

 

[JP]

She had a really funny bend in the little bit of derailleur cable past the anchor bolt, and in that combination it hit the chain.

OK, I'm fishing.

 

[mtbbmet]

The cable, where it attaches to the RD, was on the wrong side of the bolt.

 

[mohair_chair]

If the cassette lockring is loose, you get a strange rattling that tends to come and go, until they get really loose and it rattles all the time. This happened to me one time, so this is my guess.

 

[QuiQuaeQuod]

If the cassette lockring is loose, you get a strange rattling that tends to come and go, until they get really loose and it rattles all the time. This happened to me one time, so this is my guess.

WE HAVE A WINNER! And I won't even ask you to explain the symptom.

The lockring was undertorqued. Not rattling loose or anything, but no where near tight enough. Keep in mind that campy cogs are all individual, not riveted together.

The 3rd cog is pretty much in line with the middle ring. Bike flex under power seemed to be enough to rock that cog back and forth very slightly, wiggling it under the chain and causing the noise. The other cogs were off line enough that, while they must have been angled slightly under chain pressure, they were held at that constant pressure. If it had been a cogstack riveted in one piece I think there would have been no noise at all.

The lockring must have loosened a bit over time, and since it is relatively new I had never had the cogs off. The looseness was JUST right to produce this weird symptom, any looser and it would have caused more problems I think, as mohair said.

As soon as I put the tool to the lockring I knew it was the cause. I was going to take it all apart and clean every darn thing there, but it was not necessary.

And I enjoyed the whole search to destroy the noise, sick puppy that I am. But then I like puzzles.

 

[bikeboy389]

WE HAVE A WINNER! And I won't even ask you to explain the symptom.

The lockring was undertorqued. Not rattling loose or anything, but no where near tight enough. Keep in mind that campy cogs are all individual, not riveted together.

The 3rd cog is pretty much in line with the middle ring. Bike flex under power seemed to be enough to rock that cog back and forth very slightly, wiggling it under the chain and causing the noise. The other cogs were off line enough that, while they must have been angled slightly under chain pressure, they were held at that constant pressure. If it had been a cogstack riveted in one piece I think there would have been no noise at all.

The lockring must have loosened a bit over time, and since it is relatively new I had never had the cogs off. The looseness was JUST right to produce this weird symptom, any looser and it would have caused more problems I think, as mohair said.

As soon as I put the tool to the lockring I knew it was the cause. I was going to take it all apart and clean every darn thing there, but it was not necessary.

And I enjoyed the whole search to destroy the noise, sick puppy that I am. But then I like puzzles.

No Fair! I rejected this explanation because on my Campy-equipped bike this exact same problem caused creaking/clattering on several cogs, not just one.

 

[QuiQuaeQuod]

No Fair! I rejected this explanation because on my Campy-equipped bike this exact same problem caused creaking/clattering on several cogs, not just one.

Who said life is fair? Besides, isn't that when we get in the most trouble when wrenching, when we think "It can't be THAT, so..."? But seriously, it was a freak thing that it was just that loose. A couple hundred more miles and it would probably have been as you describe.

 

[mohair_chair]

What do I win?

I won't theorize the mechanics, but I actually had this problem before, and it took me a lot longer to find than it did you. Even several bike shops were no help. So now it's always the first thing I check whenever I hear a rattle on my bike or someone else's.