UKH

New chain skipping under load on old cassette at the back but much less so on big chainring at front.

Shifting OK.

Chain looked a bit long so took a link out of chain. B screw looks pretty fully in.

No improvement, still skips when any load applied unless big chainring & the bigger sprockets.

New cassette?

Yup, probably a new chain ring too. 

I think chainrings last forever as they don't seem to cause problems.

Chainring unlikely, but cassette for sure

New cassette ordered, rear mech reasonably clear of 34T sprocket so got 11-36 rather than 11-34.

Mine did after 40,000 miles

Check your chainring - if that's worn it'll cause premature wear on the new cassette and chain.

At 40,000 miles it's touch and go which has greater lifespan, me or the chainring.

Sorry, I miss read that. I thought you meant the new chain was skipping on big chainring as well.

I realised it was badly written. As usual for me

play it safe and get a new bike 😀

Check that the top and bottom alignment screws set correctly.  If they're out, it can shift fine but skip under load.  You might well ask how I know that a new chain and cassette (both needed) don't fix this basic problem.  You could also check the alignment of the rear mech, but that needs a proper tool and not just a Mark 1 eyeball

Skipping at the back on a new  chain and new cassette is my other bike

Not having much luck (or competence) at the moment.

Try a new gear change cable, I had a similar problem last year and that helped, after the expense of a new chain and cassette didn't work!!

How long had the original chain been on the bike?  How many miles had you put on it?  What led you to replace the chain in the first place?

i.e. had you been keeping an eye on it with a chain wear tool and changed it out as soon as it reached 0.75%?  Or the bike was 10 years old, the chain had been on the bike since day #1 and you've done a heap of miles on it?

How did you measure the new chain vs the old chain?  Did you lie them both side by side and line up the links to make sure you were replacing, for example, 60 links with 60 links?  Or you eye balled it?

Here is deal:  Chains are consumables .. just like tyres and lube.  A new chain on a new chain-ring/cassette will sit perfectly in inbetween the teeth loading each side of the tooth equally.  When you ride the bike, the chain slowly stretches - you can monitor this with a chain wear tool [1] and you want to replace the chain when the stretch = 0.75%.  If you fail to replace the chain when it reaches 0.75% and instead pedal on then the chain will continue to stretch.  As it stretches it will start to unevenly load the teeth on the chain-ring/cassette and create an wear pattern i.e. 1x side of the teeth will start to wear out while the other side is likely experiencing no wear/no load.  Now .. this isn't a problem, per say, when you leave your old stretched chain on the bike ... because that old stretched chain fits the now asymmetric teeth on cassette & chain ring. The problem occurs .. when you change out your old chain and put on a new one.  Now it won't fit.  On the chain ring you'll typically see it picking up the new chain and dropping it on the down side and on the cassette you may experience what you've described. 

Either way ... not good.  If this is what's happened - you'll need to replace both the chain ring and the cassette before you'll get it back to normal again.  

Separately .. if the cassette is worn out .. then pretty sure that the chain ring will be too.  It's usually chain ring first and then (if ever) the cassette 2nd.  There are more cogs on the cassette than the chain ring meaning that each cog on the cassette shares a smaller % of the wear - hence wears out slower.  If you're on a 1-by drive chain then ever rotation of the pedals is 100% on the same single chain ring.  

[1] https://www.wiggle.com/p/park-tool-chain-checker-cc-3-2?utm_source=google&a...

For the bike I posted about, I don't think it it needs new chainrings as although it's much abused off-road, the chainrings are steel and fairly new.

However you were spot on for my other bike fixed (commuter rather than off-road but much greater milage and aluminium chainrings) a new small chainring largely fixed the skipping. 

That bike is now 10 years old so I think it's overdue some TLC & replacement parts. 

What do people do with old cassettes, chains, chainrings etc? Seems a waste of scrap metal to put them in the bin. In the recycling bin? Something else?

You could make something artistic with the parts.

https://www.etsy.com/uk/listing/511672898/cycling-gift-cycling-art-wall-pla...

Let there be joy!

After thinking I'd fixed it a couple of times by adjusting the indexing, I got it up on a work stand and saw it was skipping about very two turns of the cranks so I thought stiff link as chain length a bit more than twice number of teeth on big chainring.

Sure enough, one link extremely stiff. Tried waggling* it but no difference. Pushed pin one eighth* of a turn with chain tool - FIXED!

*see URL below

https://www.bikeradar.com/features/how-to-fix-a-stiff-chain-link/ which doesn't tell me to push it an eighth of a turn the other way back

I should have thought of that earlier as I've previously had a stiff link on a new chain.

Hopefully not a lesson I will have to learn for a third time

Now to my other bike....

Similar position to you - new chain caused exactly what you describe. Bit surprising, as the existing chain and cassette were relatively low mileage, although had been used for touring in some quite gritty conditions, so may have worn prematurely. I do also swap wheels on that bike with another set with an identical cassette - hopefully, that one is not as worn as this one!

Anyway, new cassette and BB sat in garage until I have a few minutes to swap them.