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
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Does your chain drop during shifts, or while just running on the inner? Each has a different cause.
If it happens during shifts, it's possible you're keeping too much tension on the chain during the shift, especially if it happens when you've already shifted to the inner cassette sprockets. The chain is coming to the outer chainring from an inside angle. If you disengage it under tension, it'll look to jump inward because of the tension and overshoot the sprocket. If you relax and lower the chain load during the shift, gravity will drop the chain straight onto the sprocket.
If it happens randomly when using the inner chainring, especially off the innermost rear sprocket, it's a chainline/tooth profile issue.
Imagine a pulley for a moment, when feeding straight everything is fine, but if you feed the rope from too much of an angle it'll run up and over the flange and fall off. Same thing with chains and chainrings, except the pulley guides run inside the chain. The teeth are pointed, and pick up and guide the inner links of the chain (outer links don't count here). As the chain angle increases it reaches a point where the upcoming teeth touch the inner plate instead of sliding in smoothly. If they bump the plate enough they'll pick it up and let it run over the side.
There are a few fixes.
1- some chains have more bellmouth or chamfer in the inside of the inner plates. These act as wide mouth funnels to pick up teeth and guide them to the center. Next time around look at various chains and buy the one with the most inner bellmouth.
2- many modern chains have plates that are about the width of the rollers, others use plates that are wider than the rollers, forming sort of a "V" pulley, which helps keep the chain on track. This design has gone out of fashion, so save weight, and to reduce tooth damage from hard shifting under load. If you can find a chain with the older big plate design, it'll be less likely to derail.
3- you can re-profile the inner sprockets teeth so the points are a bit more inboard. This will make them more able to meed the gap between the inner plates coming from the inside. Set a medium file at an angle against the outer taper of the inside sprockets teeth. Spin the crank to file all the teeth down a bit as if you had a lathe. Stop and check progress often because if you get carried away, you'll cause problems when the chain comes from the outside.
Or, you may simply buy one of a number of chain keeper devices which catch errant chains and guide them back home.
An ounce of diagnosis is worth a pound of cure.
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