Problems shifting to large chainring

mjsmart

I've got a Specialized Roubaix Elite that I built up wit a full set of Shimano 105 with a compact on the front. I didn't have any issues with the build and got the gears setup quite nicely.

On a ride last Tuesday it suddenly started over-shifting when going up to the large chainring and chucking the chain off to the outside. I tried adjusting cale tightness and limit screws but with no luck and out of exasperation have also tried setting up the dérailleur from scratch following both the Park Tools guidance and Shimano's own instructions.

The problem I'm having is that if I adjust the high limit screw to prevent the chain being chucked off then I can't shift up into the large chainring. If I adjust it so I can change up then the chain gets chucked off. At the moment I've managed to find a compromise where I can change up if I'm in either of the two lowest (outside) gears on the cassette on the back.

Any ideas on why this might have suddenly started and why I can't adjust the gears correctly? As far as I'm aware neither front or back dérailleur has taken a knock.

Cheers,

Mike

sungod

if you can't adjust it with cable tension and limit screws, i'd check for...

a bent cage
incorrect mounting height/angle
loose mounting collar screw

mjsmart

Thanks Sungod.

The cage looks straight to my eyes. It's a braze-on style fitting so there's not a lot I can do rotation wise but the height is adjusted so that the outer plate sits a mm or so above the outer chainring.

After another 10 minutes of fettling outside in the sun I've managed to get it adjusted so that I can shift up at the front without it chucking the chain off regardless of the gear selected at the back. The shift is a bit hesitant from the higher gears as the chain only just catches one of the up-shift pins and there's a much bigger gap between the chain and the outer plate than there should be when in the lowest gear at the back. But I can shift and not lose my chain so I'm happy with that for now.

onbike 1939

My guess would be that it's the angle of the FD which is giving you the problem. There is enough rotation on a Braze-on fitting to allow you to adjust this. It's a suck-it-and-see situation.

mjsmart

I'll brave the adjustment at some point and see if I can reduce the amount of movement that's needed to perform the shift. I'm pretty sure I'll end up making it worse before I make it better though!

onbike 1939

mjsmart wrote:

I'll brave the adjustment at some point and see if I can reduce the amount of movement that's needed to perform the shift. I'm pretty sure I'll end up making it worse before I make it better though!

I've always found it better to start from the very beginning and working through. So, "L" and "H" screw settings first without cable tension and then FD position and cable tension setting. The Park guide is pretty good.It really shouldn't be a great problem with a double chain-set...now a road triple can be a pig.

drlodge

How much clearance is there between the outer plate of the FD and the largest chainring teeth? My guess is that they may be too much clearance, you want as little as possible. This will aid changing and also mean you need less adjustment of the FD when on the larger sprockets at the rear.

mjsmart

Vertically there's about 1mm above the chainring and the outer plate.

Horizontally, from the outside of the chain to the inside of the outer plate, there's about 4-5mm when the chain is on the large chainring. I know that's too much but any less and it won't shift.