New chain skipping

[esuhl]

Today I fitted my new cassette, replaced my rear mech with an old working spare, and even with the OLD chain, it worked! No skipping at all!

I put the original rear mech on, and it still worked. I compared the two rear mechs and couldn't see or feel any differences or faults.

So, it must be the cassette, right? I put the old one on, so the bike was exactly as before... but... it still worked. Perfectly. The entire problem vanished, leaving just a worn chain to replace.

So I put the new chain on... and the skipping came back.

In the last few minutes, I've put the new cassette on, and the skipping went away.

What's so odd is that the difference between it "working" and "skipping" is so big. When the chain skips, it does so really badly, with gentle force on every stroke, on several of the smaller rear gears. When it "works", I can hold the brakes and put my weight on the pedals/chain, even using the small/small gears and there's no movement at all. There's no "in-between" where it skips just a little bit on the small/small gears. That gear combo is either un-rideable or perfectly fine (apart from crossing the chain and causing wear, of course).

So my tentative analysis so far is:
1) worn chain,
2) worn cassette and
3) poltergeists

Anyway, it seems to be working now, and I'm hesitant to touch the bike again in case I invoke evil spirits. If anyone has an explanation that doesn't involve supernatural forces, I'd love to hear it!

[gregoryoftours]

The whole skipping thing is quite logical, you can run an old chain on a new cassette and it won't skip to start with because it has variable pitch due to the longitudinal wear. By that I mean that if you lay a worn chain on a flat surface there will be 'slop' along its length, you can stretch the ends out or push them towards each other and you get a different length of chain. The worn chain can therefore conform to the pitch of the new teeth but the load will be shared by less teeth so it will just wear the sprockets faster.

An old chain won't necessarily slip on a worn cassette that it has been run down with because it can also conform to the pitch of the worn teeth. Sometimes they both have to be terribly worn before the chain will skip, but you might get sloppy shifting, increased chance of snappage and your chainrings and jockey wheels will be wearing faster too so that's why you change worn chain and cassette even though they still work together.

A new chain will skip on a worn cassette though because there is little to no give in the pitch of a new chain. It can't conform to the elongated pitch of the worn sprockets.

I'm not quite sure what was causing the new chain to sag with the old cassette

[Brucey]

what you have found is exactly what I'd expect you to find; other combinations may seem to work OK but

new chain plus worn cassette = skipping

If you lean on one pedal, hold the bike on the rear brake, then ease the brake off and let the bike forwards (all whilst keeping the pressure on the pedal), you will see the chain fail to engage with the bad sprockets as it leaves the derailleur, ride up, and then slip 1/2" as the disengaged part of the chain comes near to the top of the sprocket.

No poltergeists required, no mysteries, just normal behaviour for worn sprockets with a new chain.

cheers