Little stumped here,

Fitted a new 12-30T rear cassette, mid length rear mech (10spd shimano) to a 48T / 36T Truativ elita GP chain set. Using the 1mm spacer+ magic spacer on my mavic wheels.

Chain line measures up fine 44.5mm, chain length + condition is fine, B limit screw + rear mech set up is fine.

I suspect that the chain (only happens in small big combo) is being pulled to far over to the outside edge by the 10spd rear cassette vs the road front chain rings, however the same model bike sells with the same chain line though different cranks.

Could it be the out chain ring teeth profile?

any suggestions or solutions greatly appreciated.

Stop back pedalling

As above.

I find I go further when pedaling forwards, nothing good ever comes from back pedaling…

You should only be in 48/12 when hurtling along and pedalling hard, if you’re riding anything technical that needs a back pedal to negotiate then you need a different gear!

Comment removed, thought it was a triple.

Really? Hummm…. this has never happened on any other bike I’ve owned, hence my concern.

It’s not that I’m riding along back pedalling though. It takes only a small turn back (1/2 tops) for the chain to fall off, so when coasting down a steep hill, it’s actually very easy for the chain to slip off (outside edge only).

is it a new chain too?

if not I’d suspect worn chain/new cassette combo.

It’s not uncommon IME – back-pedalling in the biggest gear makes the chain go slack on top, esp with a stiff freehub seal.

Just don’t do it – forward pedalling is an easy habit to form instead.