Just set up my bike with slx cranks and fire eye narrow wide chainring along with XT cassette and absolute black expander cog.

I replaced the 17t cog from the cassette and fitted the expander cog as per instructions. All gears work perfectly except the expander cog. It shifts up and down fine but pedaling under even modest power causes the chain to slip. What can be the cause of this?

Surely can’t be chain line as it’s the same as if it was an unaltered cassette running a chain to a middle ring. The chain length is long enough and all of the components are new so I’m a bit stumped. Can anyone suggest what’s wrong? It’s probably something obvious 🙄

Is everything brand new? If not, there’s your problem.

Otherwise, play with the screws on the mech, plus do you need a longer B-Screw? Easy solution if you haven’t one long enough is to turn it around.

Everything is brand new. So I should be screwing the b screw all the way in then I take it?

So I should be screwing the b screw all the way in then I take it?

Yep. So your derailleur clears the cog. Some expander cogs come with an extra long b-tension screw, or you can get one online. Still might be cable tension/limit adjustment though.

Can you see if the top jockey wheel is contacting the expander cog?

If so, you do need a longer b screw.

I did on both bikes I went 1×10.

I had exactly the same issue with my 1×10 set-up on the BFe. I re-measured the chain length and it was too long by a couple of links, took the links out, haven’t suffered chain slippage since.

If so, you do need a longer b screw.

As said, you can short-cut this by taking it out and screwing it in from the back.

My chain length was too short originally so I’ve bought a new chain but not taken any links out yet. What would be the best way of getting the chain length right?

Thanks for the suggestions.

I’ve adjusted the B screw to fully wound in and it’s made a big difference. Just pedalled hard along the road dragging the back brake for a bit of resistance and it’s not slipping anymore but still has a slight hint that it might. I’ll swapping the screw around as suggested for that extra half turns worth before the next proper ride.

What would be the best way of getting the chain length right?

Google?

Mine was big cog, big cog + 4, but it would depend on your mech cage length.

Not all expander rings are compatible…….SRAM ones will not work properly on a Shimano cassette as my pal found out!!!! I’d suggest checking you don’t have an incorrect expander ring….