50 to 34 keeps over shifting.

[Globalti]

About one time in five when I shift from my 50 to my 34 for a hill the chain over-shifts and drops off. Sometimes I can pick it up again with the derailleur and "re-rail" it onto the 50 and start again. There seems to be no pattern or consistency in this; it happens wherever the chain may be on the cassette and whether I'm applying pedal pressure or not. I've adjusted the limiter screw on the derailleur so that it's not moving any further inwards than it needs to and this hasn't improved matters.

Any suggestions? Is it something to do with my technique?

 

[jimboalee]

1/ Chain is too long.
2/ Rear mech cage spring is weak.

Adjust the front mech LOW limit so that when the chain is on 34T and largest, the chain is JUST MISSING the front mech cage. It doesn't matter if the ring 'out of true' causes the chain to hit the front mech cage one or two rivets per rev because you're not on this gear very often, and when you are, you're grovelling up a steep hill at 40 rpm. You should NEVER change rings when the chain is on largest sprocket.

Try not to change down rings with the bike rocked over to the left. Keep an 'even keel' while changing.
Try not to change down rings so that when the chain drops on, the revs go up over 95ish rpm. Try to keep a fluid motion so revs increase from 75ish to 85ish. Too high revs when changing down rings will have the chain wagging around like an angry snake.

If possible, use both changers simultaneously so when the chainring reduces, so does the sprocket, and the revs won't fly UP so much.

 

[Globalti]

Thanks, very helpful info. I'll check that chain length tonight; I guess an over-long chain would be more inclined to fly off.

 

[Tynan]

the chain wagging around like an angry snake.

that's bad?

interesting read, I din't know most of that