Chain dropping off big ring

[derrick]

Have an Ultegra crank, 53/39. I can be cycling along when i change into one of the larger rings on cassette the chain drops of the large front ring. it'sDi2 the front mech does not move when this happens. Any idea's The chain is not worn.


I have an idea what it is, just want comfirmation, What you got?

 

[cyberknight]

is the rear mech altering its position vertically to adjust chain tension as well as moving sideways ?

 

[Globalti]

What do you mean by "one of the larger rings"? What size?

 

[derrick]

is the rear mech altering its position vertically to adjust chain tension as well as moving sideways ?

Rear mech working as it should do.

 

[derrick]

What do you mean by "one of the larger rings"? What size?

between 25 to 30. tooth. on the cassette.

 

[Globalti]

That's crossing the chain too far. I don't know how Di2 deals with a badly crossed chain but are you sure it's not knocking the chain onto the small ring to save it from wear? Or alternatively the chain is coming from so far inboard that it's just derailing off the big ring?

 

[derrick]

That's crossing the chain too far. I don't know how Di2 deals with a badly crossed chain but are you sure it's not knocking the chain onto the small ring to save it from wear? Or alternatively the chain is coming from so far inboard that it's just derailing off the big ring?

I know it cross chaining but it's not normally a problem.

 

[cyberknight]

Is the chain too short ?

 

[derrick]

Is the chain too short ?

Only if it's shrunk in the rain, been fine for all the time the bikes been used, this started happening 2 days ago.

 

[bikingdad90]

Your Di2 has been infected by malware and is trying to extract bitcoins from you to send to an African prince who will make you very rich. Ha ha ha

 

[Ajax Bay]

Di2 deliberately does this because it's selecting a better combo of ring/sprocket. You can disable this function in Di2.

 

[derrick]

Di2 deliberately does this because it's selecting a better combo of ring/sprocket. You can disable this function in Di2.

 

[Ajax Bay]

the chain drops of (sic) the large front ring [onto the small ring].

Have you checked hanger alignment?

Shift modes / Synchronized shifting

All road Di2 builds that use the modern BT-DN110 battery support different shift modes and synchro-shifting.
This means the system will switch the rear derailleur when you change chainring, or shifts the front derailleur automatically when you shift the cassette to a certain sprocket.
Synchroshift: how does it work and How to customize Synchroshift settings

 

[derrick]

Have you checked hanger alignment?

Shift modes / Synchronized shifting

All road Di2 builds that use the modern BT-DN110 battery support different shift modes and synchro-shifting.
This means the system will switch the rear derailleur when you change chainring, or shifts the front derailleur automatically when you shift the cassette to a certain sprocket.
Synchroshift: how does it work and How to customize Synchroshift settings

It's nothing to do with synchronized shifting, as that has never been set up, And also that would shift at the same place every time.

 

[the snail]

Di2 deliberately does this because it's selecting a better combo of ring/sprocket. You can disable this function in Di2.

Must be depressing when your derailleur is cleverer than you