Shimano Di2 shifting chain prob.

[Vantage]

It falling of the big front ring. Quite simple.

It really isn't.
The very fact that the chain hooks over the teeth on the ring prevents this. The only way it'll fall off is either through being bumped off while going over rough ground if it's under spring tension via a derailleur or chain tensioner or being forced off via front derailleur.
It cannot just fall off.

 

[Gary E]

But he did mention nothing done apart from cleaning, But thats interesting what you say. i have never come across a firmware update that could do that, i will look out for that as i look after a lot of Di2 bikes,

To be fair, this isn't something I've come across myself. I did a quick Google search and found several accounts of this issue all saying that this started to happen (along with other Di2 syncro setting changes) immediately after a firmware update.. As with most things on the interweb, that's no guarantee that its a real thing! I did however think it was worth mentioning

 

[derrick]

It really isn't.
The very fact that the chain hooks over the teeth on the ring prevents this. The only way it'll fall off is either through being bumped off while going over rough ground if it's under spring tension via a derailleur or chain tensioner or being forced off via front derailleur.
It cannot just fall off.

5. When a chainring is worn out, it will produce intermittent chain suck when shifting to the inner chainring, and I suppose it could also cause occasional inward chain drop from the big ring. Look at the teeth to see if they are hook-shaped and check to see whether you can pull the chain straight forward off of the front of the chainring and create a gap between it and the ring through which you can see light. If you can, replace the ring (and, of course, replace the chain if it’s worn out, too).
― Lennard
https://www.velonews.com/2016/08/bikes-and-tech/technical-faq-leaky-forks-di2-dropped-chains_419285

 

[derrick]

5. When a chainring is worn out, it will produce intermittent chain suck when shifting to the inner chainring, and I suppose it could also cause occasional inward chain drop from the big ring. Look at the teeth to see if they are hook-shaped and check to see whether you can pull the chain straight forward off of the front of the chainring and create a gap between it and the ring through which you can see light. If you can, replace the ring (and, of course, replace the chain if it’s worn out, too).
― Lennard
https://www.velonews.com/2016/08/bikes-and-tech/technical-faq-leaky-forks-di2-dropped-chains_419285

I have had it happen a few times with Rotor cranks and once with a Shimano crank. putting a new chain and chain ring cured the problem.

 

[Ajax Bay]

Had from new, done 2000km dry miles

Worn chain and chainring.

OT regarding involuntary shifting:
After 2000 miles, the chain has elongated - and one can measure it. But critically the %age elongation does not matter: what matter's is whether the associated cassette wear has gone beyond accepting a 'new chain'. It'd be worth fitting a new chain asap, without throwing away the old one, and seeing if the cassette will accept it (ie go for a quick test ride and give a good hard push up a gentle hill). If it will: happy days (and make a note to change in another 2000 miles). If it won't: refit the old chain and that chain and the cassette can grow old(er) together, in harmony. (Of course the 'new' chain can be kept to use with a new cassette when the time comes.)
How would the chainring be worn after 2000 miles riding? And which chainring are you talking about @derrick ?

 

[derrick]

OT regarding involuntary shifting:
After 2000 miles, the chain has elongated - and one can measure it. But critically the %age elongation does not matter: what matter's is whether the associated cassette wear has gone beyond accepting a 'new chain'. It'd be worth fitting a new chain asap, without throwing away the old one, and seeing if the cassette will accept it (ie go for a quick test ride and give a good hard push up a gentle hill). If it will: happy days (and make a note to change in another 2000 miles). If it won't: refit the old chain and that chain and the cassette can grow old(er) together, in harmony. (Of course the 'new' chain can be kept to use with a new cassette when the time comes.)
How would the chainring be worn after 2000 miles riding? And which chainring are you talking about @derrick ?

If you are constantly cross chaining it can wear the chainrings out a lot quicker.

 

[derrick]

Words of wisdom please. I have Ultegra 6800 Di2. Had from new, done 2000km dry miles, never crashed or dropped or otherwise damaged. Worked perfectly from new, not modified in ay way. Kept well maintained, cleaned regularly. Love it and has shifted like a dream since new. Lately however, when in either of the lowest gears on the cassette (nearest the hub just to be clear), it throws the chain from the big chain ring to the small ring. Have followed instructional vids and have adjusted the front mech as per the spec. Can not find an obvious cause, no point where the chain is catching or similar prob. It worked perfectly in all gears since new and I really want to get this sorted. Any help v. much appreciated.

Many thanks.

A Mc C

One thing that has not been said is you should never be in the big ring at the front and the big gear at the back, on an 11 speed set up the chain angle is just wrong.

 

[Yellow Saddle]

But he did mention nothing done apart from cleaning, But thats interesting what you say. i have never come across a firmware update that could do that, i will look out for that as i look after a lot of Di2 bikes,

You can look at the video on Shimano's online tech training university. If you are in the trade, you can get access and do the course modules. There's one on there about anti-cross chaining.

 

[Ajax Bay]

big ring at the front and the big gear at the back, on an 11 speed set up the chain angle is just 3 degrees.

FTFY. I am not suggesting that big-big or small-small is anything better than poor/lazy practice, but maybe the software can be blamed (if it didn't do what the OP has described in the OP).

when in either of the lowest gears on the cassette (nearest the hub just to be clear), it throws the chain from the big chain ring to the small ring.

Isn't this 'best practice'? A 'double change' of yore.

 

[Yellow Saddle]

If you are constantly cross chaining it can wear the chainrings out a lot quicker.

Yes, but not in the standard mode where the leading edge of the tooth is eroded.

 

[Yellow Saddle]

It falling of the big front ring. Quite simple.

OK then, let's try this: Falling off is a fuzzy answer, let's drill down. There are only four possible scenarios.

1) Good chain in good chainring
2) Elongated chain on good chainring
3) Elongated chain on worn chainring
4) New chain on worn chainring.

Please choose which scenario you've referred to in your "falling off the big chainring" answer above and then expain by what mechanism the chain just falls off.

Start by explaining how the incoming roller approaches the next available tooth and what happens from there to get the chain to fall off.

 

[Vantage]

when in either of the lowest gears on the cassette (nearest the hub just to be clear), it throws the chain from the big chain ring to the small ring.

I don't think wear, chain suck or software updates (christ it's a bike!) but simply a case of crap shifting choices.
It may not have been an issue when new as all the teeth have their full height and girth but as they've worn, the chain being used in such extreme angles is just being ripped from the teeth on the chainring.

edit: been beaten to it

 

[Yellow Saddle]

Words of wisdom please. I have Ultegra 6800 Di2. Had from new, done 2000km dry miles, never crashed or dropped or otherwise damaged. Worked perfectly from new, not modified in ay way. Kept well maintained, cleaned regularly. Love it and has shifted like a dream since new. Lately however, when in either of the lowest gears on the cassette (nearest the hub just to be clear), it throws the chain from the big chain ring to the small ring. Have followed instructional vids and have adjusted the front mech as per the spec. Can not find an obvious cause, no point where the chain is catching or similar prob. It worked perfectly in all gears since new and I really want to get this sorted. Any help v. much appreciated.

Many thanks.

A Mc C

Angus, lets start afresh with this problem.

Get the bike on a stand and your body in a position where you are eye-level with the cassette. Now pedal the bike by hand (perhaps an assistant should do this if you don't have orangutan arms) and change gear to replicate the problem. Keep on bringing the speed of things down until you can a) replicate the problem with each shift and b) see what's going on.

Specifically look to see if the FD moves the chain over or if it moves to the small chainring by itself, without FD input. Lets put an end to the speculation and solve your problem.

Report back.

 

[Dogtrousers]

This anti-cross chaining software setting is beginning to sound like Boeing's MCAS.

 

[topcat1]

Angus, also check the bottom bracket hasn't worked loose and shifted the chainset a few mm out