Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 9 Post(s)
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Let's take a close look at exactly what's happening in slo-mo. It could be a simple FD adjustment or chainline/chainring tooth profile issue.
Use a repair stand, or lean the bike on a wall with about a foot of room to roll forward. Shift to the problem combination, and with the chain tensioned, adjust the trim of the FD so it clears the chain at both the inside back, and outside front at the same time. If you can't do this, it might help to rotate the FD very slightly to bring the back inward, but understand that this might cause trim and shift issues for the outside/outside combinations.
If the FD can be trimmed to totally clear the chain on both sides of the cage, and so can't be blamed for causing the shift, it's time to look at how the chain engages and chainring.
Set the bike back up in the problem combination with the FD clear (or removed). Use your left hand to hold the rear wheel back while you load the pedal with your right. Slowly relax the left slightly so the crank can move a bit while looking through the chain from the top as the chainring teeth come up between the inner plates of the chain. The point of each tooth should slide nicely past the outer plate, then move the chain over as the wider part of the tooth engages. If the point of the tooth touches the edge of the outer plate, it'll tend to lift the chain and start a derailment. If that's the case, you have a few options, including of course, not using the problem combination.
1- try a more effective chain lube to try to help the tooth glide by more smoothly, but this will only help in a borderline case.
2- change to a chain with more inside bevel or bellmouth, to widen the target for the entering teeth.
3- trim the FD to push the chain over, but doesn't work under high load, and eventually saws through the cage.
4- if there's room, bring the crank inboard more to improve chainline.
5-and if none of the above work, and you still want to ride this combination, consider re-profiling the teeth of the outer chainring to bring their points more inboard and improve pickup from that angle. The easiest way is by holding a file against the outer bevel and spinning the crank. Do this only as a last resort and by degrees only enough to cure the problem. BTW- though modification is frowned upon these days, filing chainring teeth was SOP in the dark ages of derailleurs, when chains falling off was more common than staying on.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions”
- Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance
Last edited by FBinNY; 05-02-10 at 09:06 AM.