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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Chains break for reasons completely unrelated to how they wear. Wear occurs as the pins move in the journals when the chain winds onto and off sprockets. This allows the links to move slightly farther apart changing the pitch by a few thousandths of an inch, and causing the effect called stretch.
Breakage usually occurs from sloppy or aggressive shifting, usually under load, which puts twisting and bending stress on the chain. Eventually some plates get spread to or beyond the ends of the pins, so that when the chain is next under tension a plate falls clear, and the chain snaps. Note that, depending on conditions, lots of time can elapse between what sets up the failure and the actual breakage.
In your case, odds are that your RD went out of trim, possibly from a hanger bending, possibly just a cable adjustment. That caused the auto shifting, which included some under load and eventually the chain breakage.
Replace the chain, and lesson for the future, when the RD starts acting up deal with it before it causes more damage.
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