Chain dropping off big ring

[Jimmy Welch]

To add a bit more speculation to this...

If the cable's too tight the front mech cage will be sitting a bit closer to the centreline of the bike than it should be when in the big ring. If the chain is on the low gear (inboard) sprockets at the rear, this may not be a problem. But when changing up to the higher gears at the back, the chain will move across a bit, come in contact with the front mech cage, and drop down into the little ring. That's my theory.

View: https://youtu.be/LAwhlHnd3Vw?si=3RsPBmw2YSRLcE38

 

[tom73]

Just ask Kelvin

View: https://www.youtube.com/watch?v=ZNG7g83lI-s

 

[Ajax Bay]

Have you changed or refitted your chainset? Was there a spacer between the right hand crank and the bottom bracket shell? Should there be?

 

[Jimmy Welch]

Have you changed or refitted your chainset? Was there a spacer between the right hand crank and the bottom bracket shell? Should there be?

No I did put a 105 on but the was in the summer so that's been sound since then I'm going to start from scratch new cable new housing lazer the mech dead straight and height , follow kelvin at Park tools video to the letter , when it's disconnected and I adjust the derailleur by hand it stays on no bother so it's got to be an alignment problem

 

[Jimmy Welch]

If re done everything, still not quite right this is the chain position at the top is this correct

 

[Jimmy Welch]

If re done everything, still not quite right this is the chain position at the top is this correct

 

[Ajax Bay]

Think maybe wrong image shared just now. That is a chain(!)
One other thing, after a couple of runs through the video upthread.
I wonder whether your FD (cage) is too high wrt the top of the large ring teeth: 1mm gap is optimal.

 

[Jimmy Welch]

Yes I realigned it this morning got it dead straight but your right it does look high doesn't it

 

[Jimmy Welch]

Ok I've dropped the front derailleur and still it jumps please see attached video which shows the point where it leaves the chairing

View: https://youtu.be/RLGwFcbOZ-w?si=cbW2AM62ZnpmH9G7

 

[Ajax Bay]

It cannot be the FD cage outer plate that's pushing the chain off the big ring: as you change up the cassette the chain will move AWAY from that.
I suggest this is a chain line issue and your chainset is too far out (hence my earlier Q).
So try this: roughly how close is the cassette lock ring from the drop out? >2mm?

 

[Jimmy Welch]

It's less than 2mm .....picture is a 1mm hex just fitting

 

[Jimmy Welch]

View: https://youtube.com/shorts/ppSf56Yv1Nw?si=C_NtyFb2dIk9jz1C


But if I press the derailleur arm so it comes out a little bit further the problem stops

 

[boydj]

View: https://youtube.com/shorts/ppSf56Yv1Nw?si=C_NtyFb2dIk9jz1C


But if I press the derailleur arm so it comes out a little bit further the problem stops

Slacken the cable a touch to allow the mech to move out further.

 

[Jimmy Welch]

Slacken the cable a touch to allow the mech to move out further.

Cheers , I'll give that a go , I'd have thought a tighter cable would pull it further, is that not the case

 

[boydj]

Cheers , I'll give that a go , I'd have thought a tighter cable would pull it further, is that not the case

The tight cable will not allow the mech to get its full travel, and there will be a bit of a pull on the mech when the chain is on the big ring. I've done lots of front mechs, including triples, and the cable only needs to be tight enough to get the mech onto the big ring, which means there is a bit of slack when it's on the small ring.