Front derailleur randomly not changing from small to big ring

[Fifelad]

On a 70 mile ride today everything was working sweet, but on a couple of occasions the chain wouldn’t budge from the small ring to the big ring, the derailleur moved but not the chain. It’s a new cable. Slick as anything on the next attempt. Baffling and infuriating. It’s a new chain and chain rings are in good nick. The only thing I can think off is cable tension, I’ve tweaked the inline barrel adjuster to tighten the cable while on the ride. Could the cable have done with being a bit tighter in the first place when I put the cable in and first clamped it in ? I did it hand tight and there is only so much purchase I can’t get by hand, I don’t want to pull it with pliers if I can help it Suggestions please ?

 

[Ming the Merciless]

What’s the chain set? Have you got any lateral movement in the RH crank such that the chain rings are not a fixed distance from the frame? Thus sometimes the big ring sits outside the range of the FD as set.

 

[Alex321]

Which model of derailleur is it?

With some models, what appear to be the High stop is in fact not a stop, but rather is what actually controls the lever that pushes the mech.

 

[Cycleops]

Insufficient tension I'd guess. Put the chain on the small ring , depress the lever or brifter to high, tighten the cable and nip it up and try again.
Before you do, with the cable off check the Ho-Lo adjustment for travel.

 

[Lee_M]

A new cable stretches, like a guitar string, so I'd expect to have to tighten it up until it settles in

 

[figbat]

A cable shouldn’t stretch enough to cause no shift effort - there could be some settling of the ferrules in their stops. Or it could be that the shifter has a two-step shift - for trimming.

 

[Lee_M]

A cable shouldn’t stretch enough to cause no shift effort - there could be some settling of the ferrules in their stops. Or it could be that the shifter has a two-step shift - for trimming.

"shouldn't" isn't the same as "won't"

If it's not the correct tension in the first place a minor stretch can cause all sorts of issues.
Occams razor applies in my view, look for the simple solution first

 

[Kajjal]

Try lubing the pivots. I had one once that stopped changing cleanly and that was the issue.

 

[Ajax Bay]

My first remedial action would be to unscrew the top limit screw one turn and test ride. Then reduce till it still works every time to minimise chance of unshipping the chain outwards. Only then address cable tension.
Cables (new or old) don't "stretch" (beyond Young's Modulus elastic tiny %ages which will not affect shifting), btw (not "shouldn't" or "won't"). Ferrule/stop settling is not 'stretching', but doubt this is the issue here.

 

[HLaB]

On a 70 mile ride today everything was working sweet, but on a couple of occasions the chain wouldn’t budge from the small ring to the big ring, the derailleur moved but not the chain. It’s a new cable. Slick as anything on the next attempt. Baffling and infuriating. It’s a new chain and chain rings are in good nick. The only thing I can think off is cable tension, I’ve tweaked the inline barrel adjuster to tighten the cable while on the ride. Could the cable have done with being a bit tighter in the first place when I put the cable in and first clamped it in ? I did it hand tight and there is only so much purchase I can’t get by hand, I don’t want to pull it with pliers if I can help it Suggestions please ?

Does indeed sound like cable tension. Sometimes when you are in a smaller sprocket at the back it'll still shift up at the front still but on larger sprockets there isn't quite enough chain tension to get it to shift up to the big ring. What I do when fitting a new cable to the front mech is: shift to the wee ring but manually push the derailleur to the big ring put something that wont damage the frame in the gap temporarily to hold the derailleur in position then pull the cable tight by hand and tighten with a hex. Remove the object (that pull the cable nice and tight, certainly tighter than I could by hand) and shift up/down and if it needed tweak with the barrel adjuster.

 

[Paulus]

As above, or shift to the smaller chain ring, loosen the cable clamp and pull the cable tight. If you have a cable puller all the better, and then do up the clamp. The tension should now be good, adjust the barrel adjusters as needed.

 

[Dimples 6890]

I had same problem when I upgraded chainring. It turned out to be the derailleur had worn inside. If not, then maybe your shifter has worn so not giving the same pull.

 

[iluvmybike]

It could be sticking pivots on the derailleur - give it a shove with your hand and see if you can move it inwards. If you can then you need to clean and lube the pivots