Ride, Wrench, Swap, Race
Join Date: Jan 2010
Location: Northern California
Bikes: Cheltenham-Pederson racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
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First look for a bent tooth on the big ring.
If the chain falls to the outside, it would help to slightly bevel the left side of the chainring's teeth.
To do this, hold a file against the tips of the ring at a ~45-degree angle while rotating the cranks backward.
I've solved many a chain-throw problem on unusual setups using this method, even using a rock (as a file) when I needed to fix the problem while out riding. It's quick and easy to adjust the tooth profile enough to encourage the chain to go to one side of the teeth or the other.
I am assuming that normal adjustments have been performed already. One thing often overlooked is the foreward-pointing tab on the outer cage plate of the front derailer. Especially on older models, this should be bent in as far as clearance allows in order to control any tendency to overshift to the outside of the big ring. The derailer must first be set to the right height, typicallt about 2mm above the tips of the teeth when in the big ring.