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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If this is happening when the chain is on an inboard rear sprocket (lower gears), it's fairly normal. If it's happening even when the chain is spooling from the outer half of the cassette, is isn't, and you probably have a bent or damaged tooth on the ring, or a bent hanger. When hangers get bent inward (about the only they ever bend) the lower pulley is far inboard of the upper, so the chain in the lower loop is coming from too much of an inboard angle an climbs over the side.
Easy to check for a bent hanger. Shift to a gear which has the RD cage nearly vertical. Put a straight edge (ie. a broomstick) vertical against the right side of the rear wheel, and see ic the RD cage is parallel, or tilted bottom toward the wheel.
A slightly sticky freewheel or freehub can make this worse because it causes a slack lower loop when back pedaling.
An ounce of diagnosis is worth a pound of cure.
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