Why does a new chain jump

[Davidc]

The answer to this is of no practical use - I'm just interested in knowing.

I just changed the chain on one of my bikes, and it's jumping in the highest gear. The old chain never did. The cassette is a bit worn, especially the small sprocket.

having the new chain jump when the old one didn't is quite common. So why doesn't the old one start jumping as the cassette wears?

(The cassette has lasted 2 chains. It would probably have lasted a 3rd if I'd been a bit sharper at changing the chain this time, but that's not the question.)

 

[Hydra]

The chain wears the cassette. The old, stretching chain would have worn the cassette with it. So basically the cassette is moulded to fit the ever-stretching chain.

Putting a new chain on an old cassette will skip because the teeth on the cassette will be wider apart (thanks to Mr. Stretchy-Chain).

 

[Davidc]

So if I didn't change the cassette how long before the new chain adapts by 'stretching' and works on the old cassette?

 

[Hydra]

It's unlikely to, I'd imagine, as it can't get a footing on the cassette to get worn/stretched. You'll need to replace your cassette as well.

 

[Davidc]

It's unlikely to, I'd imagine, as it can't get a footing on the cassette to get worn/stretched. You'll need to replace your cassette as well.

Thanks for the thoughts. They prompted me to getting the bike up on the stand and having a close look with a magnifier.

BTW I already have replaced the cassette, and ordered the next one!

The teeth on the two smallest sprockets (old cassette) are quite well ground away so it's not surprising the chain jumped. I had a good look and the reason for the new chain jumping is clearly that apart from the one at the higher end of the sprocket the rollers don't meet the chain. The oddity is that when the old chain is on the sprocket its rollers don't quite meet the chain either. The only difference is that on the elongated (old) chain they just about connect before lifting off so the margin by which it is still functioning may be dangerously small.

With the new chain and cassette the rolers are a tight fit to the teeth of the sprocket, so there's very little chance of lift-off happening.

I suspect that the margin by which the old chain stays in the sprocket is not large, but I'm still not convinced this is a complete answer.

I won't worry much about it, once every 2 or 3 chains the cassette needs replacing and knowing the detailed mechanics of why the worn ones don't work won't change that at all. I just keep a spare cassette and chain in stock.