setting up front derailleur

formerlyknownasbonj

in re setting up my gears on my road bike my front mech has now got a problem, whereby it sometimes throws the chain right over the big ring. It seems that it is shifting it such that to start with, the very edge of the chain is resting on the very edge of the chainring, and then if it falls 'inward' it goes on ok and if it falls outward then it's come off.
I'm sure it isn't the limiter screw, as i've got it so that the mech can only move just far enough for it not to rub when it's on the big ring. Thought it might be a bad chain line or the chain being too loose, but don't think it's either of these as albeit possibly slightly less, it still does it when the rear mech is on the larger sprockets. I've tried it at different tensions so many times that i've knackered a cable so need a new one tomorrow.
any ideas of avenues of investigation greatly appreciated.

Pullman1937

What make is the chainset and front mech? .

Need above information before solution can be offered.

peanut

If the chain links are riding up onto the outer chainring teeth and then comes off then the front mech must have been set so that it is traveling too far .
Have you checked that the cage is just 1-2mm above the teeth of the chainringl and perfectly in line with it ?
Also might be worth checking that the chainring is not buckled at all.
I would suggest taking the chain off the chainring and spinning the chainset and check the chainring is not buckled..
Then adjust the front mech so that the front face of the cage is directly over the outer chainring and check there is 1-3mm gap max and that the cage is in line with the chainring.

Suggest you reassemble and adjust the F Mech travel to the minimum needed to move the chain onto the outer chainring.

hope that helps

formerlyknownasbonj

I had an 8 speed mech, with a 9 speed chain (and cassette). the bike shops tells me that it will shift better with a 9 speed front mech, to be honest I didn't know they were chain-width specific. people on cyclechat tell me 8 sp vs. 9sp shouldn't make a difference, but i'm inclined to take it with a pinch of salt, as i can see how having the limit screw adjusted loose enough to enable the inner plate to move the chain far enough, was causing the outer plate to be too far out to prevent it throwing it off.

So I've ordered a 9 sp front mech and completely re-set it up now i know how to better and see if it works.