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Chain popping off front chain ring at start(3 posts)

Chain popping off front chain ring at startspoetzl
Jul 24, 2003 4:10 AM
Hello:

I have an aluminum-framed bike with Campy Daytona 9-speed components with about 3500 miles and have never had any trouble with shifting or having the chain come off.

Recently my LBS tuned it up and replaced the chain (with another C-9). Since then the chain has occassionally come off to the outside of the front chain ring when I am starting out from a stop. It happens after about two cranks. Since I'm generally out of my seat and have no momentum this is really dangerous. Once I crashed and the other times have been near misses.

We have checked the obvious: front derailleur adjustment, evidence of excessive wear on the front ring, lateral play in the bottom bracket and found nothing wrong.

The LBS mechanic said "you really shouldn't be starting out on the big ring anyway". I do downshift when stopping but if I'm on the big ring I tend to leave it there. Even if that is the conventional wisdom I would think a properly adjusted bike with good components should not throw the chain just because of a start in the big ring.

The failure happens about once per 100 miles: infrequent enough to make it hard to diagnose but often enough to be annoying and dangerous.

Any ideas on what could be wrong?

Thanks,
Brad
re: Chain popping off front chain ring at startInTheMiddle
Jul 24, 2003 6:06 AM
sorry if this borders on obvious and you already checked it.. but...

my first two guesses would be a bent something (chainring or arm on the crank).... or a not evenly seated crank on the bb.

If you spin the cranks around does it look like the rings wobble to either side during part of their rotation ?
re: Chain popping off front chain ring at startspoetzl
Jul 29, 2003 6:31 AM
thanks for the reply.

no nothing appears to be wobbly or out of plane.

since diagnosis in this forum may be really hard, let me ask a simpler question:

it is reasonable to have the bike shop state "you shouldn't be starting off on the big ring anyway" as a means of rationalizing the problem? is this conventional wisdom among riders that i just somehow missed?

thanks,
e.r.s.