UKC

I got a new bike last year with 11 speed Ultegra fitted. For the last few rides I've noticed that sometimes, when I shift quickly to the smaller cogs at the rear (higher gear), the chain will be thrown off the big cog at the front. This is a bit of a pain as momentum is lost while it sorts itself out.

Anyone else had this sort of issue? I had 10 speed Ultegra on my previous bike and that worked pretty much perfectly for five years or so.

Thrown off the crank side? Like right off?

Sorry should have been clearer. Actually thrown down to the smaller cog at the front (opposite to what I would expect)

Springs in the rear mech? Sounds like the slack in the chain from going down in size on the rear, is not being taken up quick enough and it's causing the front to jump. 

Or maybe the chain is too long? with a similar effect?

My Ultegra 11 spd is faultless in operation.

You can change spring tension I think if that is the problem, but requires a dismantle of the top pivot.

Either. There must be a little ripple of chain coming forwards that is lifting or flicking it off.

Given the other thread i am assuming this is mechanical and not Di2 but as mechanical always works its a weird one ;-]

Does the front mech stay where it is ? If its adjusted properly it shouldn't really be able to get back to the inside, sounds like a front mech adjustment issue to me with a little bit of chainline funkyness thrown in

Some sort of ripple effect seems the only explanation, just not sure how to fix it. Will try the tension.

When in the bigger cogs at the back you adjust the front mech to stop chain rub. I suspect this give the chain the leeway to jump but it still shouldn't.

I believe I found the cause. The chain-ring bolts were working loose (they may have used and unsuitable thread-lock material - not oil resistant) The bolts got so loose I could here the rings rattling. Since I tightened them it seems fine.