I’ve just changed my BB, cranks, rings, chain and cassette (all new except cranks/chainrings) and the chain seems to be jumping in the 3rd and 4th cassette sprockets only.

Shifting seems ok otherwise. I thought this was due to a worn cassette which was why I bought a new one.

What’s next to look at?

chain tension?

Check chain length. A chain that’s too long will jump.
Also double check the cassette is fitted correctly, and all sprockets are evenly spaced.

Make sure your mech hanger is straight.

Mech hanger loose?

I’ll check the mech hangar but it looked to be straight from glancing at it, and I’m pretty sure I’d know if I’d bashed it hard enough to bend it anywhere.

odd issues like this very tricky to diagnose remotely.

Could you post up a video?

Just to further this, over the weekend I removed and refitted the mech hanger, cleaned the gear cables, shortened the chain, removed and refitted the cassette and tightened a rear hub cone in the process.

I had a test ride and all seemed fine on the road but as soon as I got to some singletrack it was skipping in the same sprockets as before.

The mech hanger looks fine, but I think it’d be worthwhile picking one up to try it out.

Anyone got any other ideas?

Try it with a different cassette. Just to see if the cassette is the culprit.

Try new inner and outer gear cable(make sure it’s long enough not to snag on moving suspension parts and cause ghost shifts too), you’ve tried everything else and cables are often the culprit with dodgy shifts.

Okay I’m still having problems with this. Now I’ve also fitted new inner & outer cables, fitted a new mech hangar and changed the rear mech. I’ve also tested it with the old cassette again.

It skips in the 3rd sprocket only. Any more ideas?

The only thing I haven’t changed now is the shifter… and surely nothing could be wrong with that?

You checked the teeth on the cassette itself properly?

Yeah they all look fine to me, although I wouldn’t say I was that experienced with what notches cassettes should and shouldn’t have.

It’s a SRAM cassette so I even took it apart and put it back together to make sure all was lined up properly.

Even the old cassette doesn’t look too worn (although it is 2 years old)

I`m with coatesy on this.
I assume it has rear suspension ? if so then the cable outer is too short where it comes from frame to arm.

so its fine on the road? even up climbs when there is a lot of force on? just trying to confirm above diagnosis…

does your rear mech outer go under the bb ? if so I would say very likely ghost shifting when suspension moves, as suggested by others – I had this a few yrs ago on a Spesh Enduro and took me ages to work out what it was and about 5 mins and a new bit of outer to fix it ! -)

It’s a hardtail!

And it was fine on the road that one time, but I think that was just coincidence because I was riding smoothly. All tests since have been on the road and it still skips, mostly when I put it under a lot of pressure.

New outers were cut to within a few mm of the old ones.

seems weird it does it in 3rd and 4th… i have it previously but normally it shows in the smallest cogs as the torque is highest there…
is it jumping into other gears or just slipping but staying in gear?

Could be a chance its slipping on the old chainrings.

Could be a chance its slipping on the old chainrings.

wouldn’t it do it in every gear then? or in the smaller gears where theres more torque?

if you turn bike upside down and put it in that gear does it turn properly? and if you check each point for the chain engagement, so press the chain onto the cogs of the cassette, the rear mech do they all sit properly?

It’s slipping down, from 3rd into 4th briefly then quickly back up again.

It sounded exactly like a slight bent mech hangar to me, but I’ve changed that and it’s still the same.

Aha! Thats not slipping, its skipping gears. Slipping is where the chain rides up and slides over the top of the teeth.

Could be a twisted/stiff link in the new chain (possibly where you joined it) or something stuck in the cassette (like a twig) thats lifting the chain.

Some new chains need to be lubed

I’ll take a careful look at the chain, but I haven’t joined it anywhere as it has a powerlink; just removed some links.

If it was skipping UP (meaning to a larger sprocket) I think it would make more sense to me, I just can’t think of anything which would make it jump down.

It needs exorcising, no really. I had a similar problem with my gears last year could not find the problem I kept tinkering and tinkering. I ended up stripping the whole thing down and washing in holy water, put everything back relubed and worked perfect.

I had the same problem just recently. It turned out to be the bearings in my freehub that were knackered, leading to the cassette wobbling. It seemed odd that it only affected the 3rd and 4th gears but there ya go. Might be worth checking?