Aerzon

Only posted here instead of in the mechanics forum because it's on a vintage bike with vintage parts and C&V tends to be the most helpful and active for these issues.

This just recently began on a 2x6 drivetrain. At the start of rides in Chainring 1, Cog 3 there is a crunching noise that appears to originate from the middle/rear of the drivetrain. It is exclusively a problem in this specific combination. After riding for about 20 minutes in other gear combinations, I can change into that combo and it rides fine. (As in there is no problematic noise in the Chainring 1, Cog 3 combination after I ride for a bit)

I have no idea as to what this could be. I'm going to try a quick chain cleaning and rear derailleur adjustment to see if that helps, but otherwise I'm stumped. My only thought is that the chain links are stiff and the way the chain sits in this combo exacerbates the issue until riding loosens it up.

noobinsf 

Could you post a pic of the drive side with the chain sitting on the rear cog and front chainring that are problematic? A chain-level shot of the drivetrain from the rear would also be helpful.

My first thought is that your front derailleur is rubbing on the chain, in which case you would just need to gently trim it (move it slightly) to get it out of the way of the chain. That's normal behavior, both for the drivetrain and for the rider.

If your rear is index shifting, then you may need to slightly adjust cable tension to ensure the derailleur cage aligns exactly with the cog, as it may be a mm or two off when in that specific combination.

Aerzon

General pics of the drivetrain while in that gear combination. I don't think it's the front derailleur as trimming it doesn't improve the noise during the early ride, and it clears up later on as I mentioned. I also don't think it could be the derailleur cable tension, also since it clears up mid ride. Bent hanger doesn't seem likely since it wouldn't improve with use? That's why I'm stumped, if it didn't go away 20 minutes into my ride, all these would be plausible. :c

I know the drivetrain is pretty dirty, I'm going to give it a good clean and make sure it's lightly lubed afterward.

tricky 

Yup, starting with cleaning up the drivetrain will get you to a good baseline then you can begin troubleshooting. I always clean before trying to diagnose sounds or poor shifting. It goes a long way in improving issues plus you know you aren't fighting the dirt when trying to nail an adjustment.

It almost looks like your derailleur is out of alignment with that middle gear, but I could be seeing some distortion in the photo.

Does this noise occur while you are pedaling or is it when you are coasting? Do the bearings in your hub feel smooth? Is the axle broken? Any weird noises coming from your freewheel?

If it goes away eventually, my wild ass guess is it's a part heating up the lubricant and smoothing out.

cudak888 

I see some hairlike material between the first and second cog. Is there any grit between each tooth on the fourth?

-Kurt

Dylansbob 

How wobbly is the rear derailleur? If there's a lot of wiggle in the cage, it can make noise that settles under consistent power as it self aligns itself.

70sSanO

I agree that cleaning, and then possibly replacing the chain, would be my approach.

I can’t determine the chainline in the offending gear, but if that gear is lined up then shifting to another cog forces the chain on a angle. Just a pure guess, but that angle might be flexing (loosening) the chain against the crud enough so it runs quieter back in the 4th position.

John

obrentharris 

Hard to tell from the photos but the upper pulley might be just lightly touching the cog in that particular combination. Tightening the "B" adjustment screw a half a turn or so would take care of that.
Brent

SurferRosa

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