Hi,

Im having an issue with my chain jumping off the highest gear at the rear into the frame. This only happens when I’m not pedalling and going hell for leather down rocky terrain. I’ve spent a lot of time with the bike on the stand attempting to make the necessary adjustments (i am no mechanic) I’ve adjusted the H screw to a point where the dérailleur looks perfectly lined up at the rear but it happened again on a recent ride (nothing is bent)
I’ve also played with B screw adjustment but I’m finding it difficult to find a balance between the two where everything runs quietly and smoothly. I also have an chain guide and a narrow wide front ring (1×10).

Chain too slack perhaps. When it’s in highest gear / smallest sprocket, is the mech cage swung way back almost or actually touching the upper part of the mech? If so that would suggest too much slack in the chain.

Is it a clutch mech? If it is, if SRAM test it’s tough / plenty of resistance to swing the cage forward, if Shimano make sure the clutch switch is turned on. If not a clutch mech, maybe consider one as they do take a lot of slack out of chains. Though if the chain is not sized right in the first place then you’ll have issues.

Could be a number of other things however, just a couple of suggestions.

Chain was my first thought, if your end stop was out on the mech it would throw the chain as you shifted up and while you were pedalling.

It’s a sram XO type 2. I have actually debated chain length. I’ve gone from from two upfront to a 34 and 40 tooth expander at the rear (lbs job) They fitted a new chain but not sure what adjustment were made in that respect.

Too much b-tension?