Do I need a new chainring?

Question

Here's my setup:

It runs great. But the chain is a little rusty. I measure the chain wear and it looks like about 1/32 of an inch over 12 inches. So not much, right? But chains are cheap so I try a new chain. The new chain "clicks" at the chainring, especially under high torque. I can feel it in the pedals.

I would think I am perhaps due for a new chainring and rear sprocket, but neither appear that worn:

But the new chain "clicks" and that does not inspire confidence under high torque, and is annoying, so I put the rusty chain back on and all is well. What gives?

Possibly relevant: The new chain sat in the its package for at least 12 months. I rode maybe a mile or two with the new chain and the clicking before switching back to the old chain. The old and new chain are both KMC 8 speed chains.

Answer 1

Chainring wear and noises on fixed/singlespeed drivetrains can be a little nebulous and I think by experimenting with switching back to the old chain you've probably diagnosed the issue as well as possible. I wouldn't expect that ring or cog to be exactly worn out either, but if you look at it you can see the contact side of the teeth is worn down visually from the back side.

If your chainline isn't great that could conceivably create a situation where noise is exacerbated, but if the old one isn't doing it then I doubt it's much of a factor.

Given the choice, I tend to put 1/8" singlespeed chains on every kind of singlespeed drivetrain because I think they add a little bit of smoothness and tend to run quieter and last longer, all else equal. There's no issue with putting them on 3/32" parts like yours. But if wear is causing this then it likely wouldn't make a big difference.

If wear is causing the noise, it will subside when the wear state of the chain catches up to the rest of the parts. Personally I hate playing that game and would rather enjoy a quiet bike.

Answer 2

The rear cog's teeth definitely look worn to me. You can tell that by comparing the "working" sides of its teeth to the non-working sides of same teeth. It does not have to be visually apparent for the worn teeth to cause clicking with a new chain. After all, a chain is considered terminally worn out at 1% of relative elongation, a value that a human eye cannot see without a gauge.

See if you can turn the rear cog around and reinstall it on the hub, so that unused sides of the teeth become those to drive the chain. If the hub/cog allows it, you have just gotten the second life out of your rear cog.

The same could have been said about your front chainring, but both the crank's spider and the ring itself look asymmetric. It does not look likely to me that you'll be able to "reflect" the chainring's position.

However, the front ring is not as worn as the rear cog. You might be able to get a bit of more life out of it. This is expected as the front ring simply has more teeth to distribute the wear than the rear cog.

The old front chainring may still cause clicking with a fresh new chain. I usually wait until the new chain gets "broken in" after a short while. The clicking goes away after that.