Front derailleur headache

[gbb]

If it were mine I'd do the following...mostly to satisfy myself the basics were right.

Check the cable run. Is it sitting correctly in ferrules, guides, particually under the BB (if yours is routed that way).
Release the cable and pull the mech by hand, does it travel correctly in and out (or up and down if you prefer). It almost certainly will because you stated it does on the stand.

Then operate the shifter while holding the cable under tension (still diaconnected from the FD)...does it pull the cable consistently, does it release the cable as you shift up and down. It seems it should as it works ok on the stand.
This of course requires you to re-attach the cable and tension everything up when youve finished which you may or may not be comfortable with.

So is something pulling the cable tight while you're sitting on the bike, something loose, something too tight...is the cable outer free and not resticted of movement between the shifter and FD. (It needs some freedom to prevent the swing of the handlebars pulling the inner /outer inadvertently)

None of these things are likely...just simple checks to go through to

Right at the very extreme end of possiblities, is the frame cracked and causing frame flex that could pull the cable tight...extraordinarily unlikely of course...

 

[Tim Hall]

Right at the very extreme end of possiblities, is the frame cracked and causing frame flex that could pull the cable tight...extraordinarily unlikely of course...

I was thinking that too. @atlantis , have you noticed any new creaking noises from the bike?

 

[Yellow Saddle]

But he says its under load that the problem occures...so check the chain anyway..simple enough

Don't confuse the issue. This has nothing to do with chain wear. Chain wear does not present in this way. A shotgun approach to "ideas" is not helpful.

 

[Ajax Bay]

Try unscrewing the T limit screw one full turn. With enough cable tension you should be able to shift to the large chainring on the road, and it should not be over-stiff. Then screw it back in a quarter turn at a time till it doesn't quite rub when in smallest sprocket. If it then won't shift down, you have too much tension in the cable. It's an iterative process which needs to be tested on the road outside, once you've got it OK on the stand.

 

[raleighnut]

Try unscrewing the T limit screw one full turn. With enough cable tension you should be able to shift to the large chainring on the road, and it should not be over-stiff. Then screw it back in a quarter turn at a time till it doesn't quite rub when in smallest sprocket. If it then won't shift down, you have too much tension in the cable. It's an iterative process which needs to be tested on the road outside, once you've got it OK on the stand.

The OP has a triple, I still think it's corrosion related, summats sticking somewhere, either cable or pivot, cos it used to work. It could be cable tension but I'd have thought that would show up in the middle ring (chain rubbing the mech)

 

[gbb]

I'm racking my brains, I had this on a Campag setup, extreme difficulty getting onto the big ring, very very tight cable.
Nothing had changed, nothing had been played with, no corrosion, it seemed a gradually worsening problem that quickly got worse. I never found out why...after I released the cable from the mech, did some checks etc, re-attached it all...everything was fine.
Nothing worse than solving a problem...and not knowing what the cause was.

 

[Levo-Lon]

Don't confuse the issue. This has nothing to do with chain wear. Chain wear does not present in this way. A shotgun approach to "ideas" is not helpful.

Ok i wont then...

 

[atlantis]

Yes the chain is fine , pretty regular with checkups, I will certainly go the cable route, will dismantle it completely and get back to you, thanks once again

 

[atlantis]

Hi ! Thank you , thank you to all, problem has been sorted. I started checking the cables, all was well ! until I saw the cable stop, it's a plastic part and its bolted under the down tube, this was somewhat flexing when pressure was applied, and would return when pressure was relieved on the cable. I just didn't notice it. So firstly I changed the outer, inner and ferrules. Then glued the cable stop and bolted it down hard. Shifting is a breeze the entire assembly is nice and stiff, the shifter has softened as well. Will post an image of this cable stop