Fitted this to a new HT frame. New chain (pc071) and very newish PG990 cassette. Middle chain ring – when jumping or riding over big roots/bumps chain jumps off ring onto granny – doesn't seem to occur on the outer chainring and less on the granny.

The chain is correct length, rear mech is long cage XT non-shadow (newist version).

The only think I can think is the chain tension is poor – poss the return spring in the mech or perhaps screw in the B screw may give me a bit more tension?

Anyone else had a problem with FSA K-Force cranksets? My old LX set on an older frame never did this?

I've never had any bother. I'd look at the spacing of the BB, it may be different to your old crank. You may just need to adjust your front mech.

Yeh – was going to wind out the front mech cage to keep the chain on if it jumps but also the chain jumps vertically and then slips (both are new!!) when putting torque on after going over bumps – so Hmm? My BB is 68mm so 2 spacers drive side and 1 non-driveside should should be OK no?

Did you replace your middle ring with a new one? I have to change mine once a years along with the chain and cassette, if not the chain jumps. The outer and inner rings last much longer as they get less use.

But, if it is jumping off just over bumps and roots and not under power then maybe look at trimming your front mech?

All new except the rear cassette which is about 3 month old 🙂

My BB is 68mm so 2 spacers drive side and 1 non-driveside should should be OK no?

I think mine has the spacers the other way around

LBS fitted mine – hmm

sticky freewheel?

Don't think so as serviced the rear end recently? Hope Pro II – makes a nice rackets as bearings all changed last Xmas 🙂

Think I might just increase the chain tension a bit – maybe shorten teh chain – I would guess a longer chain would make it worse?

How high is the front mech? I tend to run mine very close to the top ring, maybe if the gap is too big???

Nah – 2mm off the big ring 🙁