What makes a new chain jump other than a worn cassette?

Question

My bike recently passed 5,000 miles. A chain gauge says the original chain isn't worn out and when I measure 24 links I get exactly 12-1/16", so the gauge isn't wrong.

However, I used wax on that chain and twice I've had it half break (one side of one link breaks). Both times it was likely due to water washing the wax off. Once from slush and the second time from thick fog. It's a 1x11 drivetrain and I never use the lowest 2-3 gears so after the first break I just took out two links and stopped using the first gear. After the 2nd break I decided to replace the chain.

I couldn't find a replacement chain that was exactly the same so I bought a chain from the same manufacturer that's one model up (higher quality I suppose). It's an 11-speed, just like the original, and it's the exact same width when measured with calipers.

That chain only came with 116 links and my original had 118, so I took two links off my old chain and added them to the new chain.

I put synthetic lube on the new chain and all the links seem to turn smoothly.

For some reason when the chain has a lot of tension it jumps. The lower part of the derailleur jerks forwards, the pedals jump and there's a clanking noise.

I thought the derailleur indexing was off but I messed with the cable tension and the high limit screw and couldn't fix the problem. It happens frequently enough on the smallest cog that I can't use it. It also happens on the second smallest cog but far less frequently and correlates directly to high chain tension, like when I'm standing on the pedals.

• The chain is the right length.
• The chain is lubed and all links seem to turn smoothly.
• The old chain isn't worn out so the cassette shouldn't be either.
• The high limit screw is set properly.
• The shifter cable tension is set properly.

One tooth on the smallest cog has it's leading corner chipped off (the one the chain doesn't touch under load). I thought that was the problem but then realized it wouldn't explain why it still happens in the 2nd smallest cog.

Shifting is smooth both up and down, and I can use first gear again. I don't see any problems at all other than the damn clunking at high tension. What am I missing?

Package from the new chain:

Other than being two links shorter and $20 cheaper, I don't see any difference between the chain I bought and the manufacturer's replacement chain: https://www.statebicycle.com/products/ybn-11-speed-chain

Answer 1

What you are describing is sprocket wear. Even though your old chain wasn't "worn" by the metrics you have used, the 11 and 12 tooth sprockets have been used hard enough to have worn down and lost their ability to retain the chain under high load.

For some cassettes (eg some new Shimano and Sunrace) it is possible to buy the 3 smallest sprockets as a replacement part so I think your problem is far from unique, even in the eyes of the manufacturers.

Anecdotally, I see this alot with undergeared mountain bikes (too small chainring encourages overuse of smallest sprockets) and also (slightly differently, as takes longer) with road riders where the middle of the cassette gets alot of use and will start jumping after several chain changes.

You will need to change the jumping sprockets.

I can see even in your not-very-hi-res photo that the small 11t sprocket is completely hammered.

Answer 2

I agree with Noise that your cassette is worn. Substantially worn. It's most pronounced in the smaller cogs as expected, but there's a large burr surrounding the contact surface of most of the teeth, and the tooth profile has been worn to be visibly asymmetric. YBN chains are very good--it's possible that the cassette is weaker/softer than the chain and was being worn away even though the chain looks good. Get a new cassette (maybe even one with smaller large cogs since you don't use the current ones anyways) and you should be good. Worst case, you are now the proud owner of a spare cassette for when you'd (inevitably) have to change it at some point.