Shifting mystery - chain getting caught when shifting up

antonyfromoz

I apologise in advance for the long post but I have an issue with my bike that has proved to be baffling. The bike is a Bianchi via nirone and it is running shimano 105 gears with an fsa omega compact crank - all original. The problem is that the chain will not shift up to the larger chainring if there is any pressure on the driveline but will if there is not pressure ie riding down hill. If there is a slight uphill or there is some resistance on the turbo and I try to change up then the chain does not ride up correctly and ends up caught between the front derailleur and the large ring. Sometimes, if i then peddle very slowly it will shift up but usually it just grinds away where it is. So far I have taken the bike to two mechanics, with the result that the bike has a new 105 front derailleur and a new chain as well as perfectly adjusted gears but both mechanics admit that there is a problem they cannot identify a fix for. The second mechanic even swapped out the large chain ring for another one to see if this was the issue but if anything this made it worse. The rear derailleur seems to shift with no issues or hesitation. There is no movement in the bottom bracket and the chain rings seem to be in good repair. I have also tried it with two different wheels, each with a separate cassette but the issue remained. Any ideas will be gratefully received - especially as I am meant to be racing on the bike on Sunday

onbike 1939

I hesitate to make a suggestion given that two mechanics have already had a go but will do so anyway. From time to time something baffling such as this shows up. I've had a few and can only suggest that the best starting point is to take nothing for granted and to work forward in a logical way checking off the basics. So, is the chain-line correct? Is the chain the correct length? Is the chain the correct one? Have you checked the throw to the big ring by pulling on the front cable rather than using the lever? Does it work when you do this and if so, why? Is the FD correctly positioned height-wise and is it correctly aligned? The fact that it works while on the stand but not while riding should be a clue. The FD works on the upper length of the chain which is under tension, unlike the rear gears which work on the bottom, untensioned section. Does it change if pressure is lessened on the pedals while changing?

It's extremely difficult to diagnose a fault like this from a distance but a logical approach which works through all possible causes must be the way to go. Hope the above may be of some help.

Twostage

I thought I'd posted earlier but it doesn't seem to have made it . My one guess is now covered above - derailleur height.

antonyfromoz

Thank you for your suggestions! The problem now appears to have been solved (pending a trial run outside tomorrow morning :? ). After visiting two more bike shops and getting some different suggestions I managed to track down a new FSA 50t chain ring and this seems to have worked where the other Shimano rings that had been tried earlier had failed. On close inspection, once the old ring was removed and cleaned, the small pins that are designed to assist the chain on to the larger ring have been rounded off in places and this seems to have been the issue. The pins still appeared to be fine but the tolerances must be pretty small. The new ring is an FSA Gossamer while the old was was an Omega and the difference in quality from stepping up the range is pretty obvious. Happy now!

Twostage

Funny how when the previous mechanic did a swap of the chain ring it made it worse. This could have been a clue to it being a problem, maybe the one he tried was worse than the one he took off ?

petejuk

Despite the problem being solved, I am curious as to why the chain didn't shift even with the grooves being ground down since all the front mech does is push the chain. With the grooves worn down as they are, I would check your chain length as previously stated- it could be wearing unnecessarily. I'm surprised the bike shops you tried didn't pinpoint the problems.

on-yer-bike

petejuk wrote:

Despite the problem being solved, I am curious as to why the chain didn't shift even with the grooves being ground down since all the front mech does is push the chain. With the grooves worn down as they are, I would check your chain length as previously stated- it could be wearing unnecessarily. I'm surprised the bike shops you tried didn't pinpoint the problems.

Sounds like the front mech was too low. But is that too obvious?

antonyfromoz

the bike worked perfectly today in the race - which is more than can be said for my legs unfortunately . In answer to some of your questions - when the problem first appeared I had a new chain fitted so i knew the chain length and possible stretch could not be the issue. As for the derailleur height - this was removed and replaced so the height was checked several times in the different bikeshops with no improvement. My conclusion is that the derailleur did the job it was designed to do and moved the chain over towards the large chain ring but the pins on the chain ring didn't pick the chain sufficiently if there was any resistance on it. Once I had taken the old large chain ring off and cleaned it up I could see that the upper edges of the pins had been rounded off and so they did not "grab" the chain sufficiently to pick it up onto the ring. As the bike has a compact chainset this seems to be more of an issue than with a standard set (according to what I have been told). There was sufficient contact to move the chain up if there was no pressure on it - for instance when the bike was on the stand. I think the reason the other chain rings didn't fix the issue was that they were Shimano as none of the other shops had any FSA rings spare. I don't know if swapping both the smaller and large rings for Shimano would have fixed the issue but both of the mechanics who swapped them earlier felt that the larger Shimano ring made no improvement on its own. Thank you once again for all your ideas!