Join Date: Feb 2007
Location: Tariffville, CT
Bikes: Tsunami Bikes
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Here's the issue with testing the gear combination - it may be something else that caused the chain to hop off, like a bump in the road created the perfect harmonic wave in the chain so at the peak of your power stroke the chain ended up derailling itself. Or something like that.
I've had the chain come off more than I like while sprinting in a top gear. I've eliminated shifting (I was already in the gear) but a host of other problems could cause it.
Some of my findings:
- link on chain was twisted. It may have been the result of a chain slipping off. Chain was...3 months old? Clean, properly installed, etc.
- chainring warped a bit. Changing crankarms (because the crankarm holds the ring) worked the best. Using thick chainrings also helps.
- worn components, especially chainring and/or chain.
- chain on the long side, so it bounces too much. Tighten up the b-screw or equivalent to increase tension in chain.
- chainline - with my 39 cm stays my chainline is a bit more aggressive than others. On what appears to be an otherwise perfectly good bike I've had the chain come off while in the 53 or 55x11, and the chain comes off to the outside of the ring. I even adjust my derailleur so it's super tight on the outside, for situations like this.
You can check by rolling the pedals forward slowly while the bike is on a stand/trainer/etc. Check for any link that doesn't seem "fluid". Pedal backward; use the small-small because that accentuates the link rotation in the rear derailleur. Check your chainrings. Cranks. Look down when you pedal - does the chainring move left-right more than a few mm?
I hope you were on the drops when you were doing your sprints. I've had various incidents over the years where something happened to disconnect my feet from the rear wheel - BB spindle broke, cleat broke, chain derailed (most common), pedal broke, probably others - and each time I managed to recover without falling. Most of those were at speed, 38-45 mph, at high effort. Scary, yes. Pain, sometimes. Fall, no. I attribute this to the firm grip I had to the drops and the lucky drop onto the bike.
For solutions, other than making sure your chainring/cranks are okay, is get a cassette with an 11 and don't use it much. Running the big ring with the second smallest cog will give you a better chain line than the big-little.
Hope this helps, good luck with your bike.