Diagnosing drivetrain issues

We all love our bikes, but you can take care of the bike until the cows come home and it will still have issues once in a while. Some things are easy to diagnose and easy to fix (flat tires), some are easy to diagnose and hard to fix (seized forks) and others take more thought, but can be fixed quickly if not let on for too long.

Recently in a race I suddenly had a chain skipping predictably every pedal stroke, it felt like a bent chain, but I hadn’t mis-shifted I thought. Getting off the bike while I was holding 3rd place in a race is the last thing that I want to do, so the first trouble shooting I did was to check through my entire gear range. Oddly enough I could use my easiest 3 gears without a problem, but past those 3 I would drop my chain right off. For context, my drive train is a 1×10 with a SRAM XX 11-36 cassette, 34T direct mount front ring, and XO type 2 rear derailleur. Eventually, after my chain got jammed between my chainstay and chain ring a few times, the chain wouldn’t stay on at all. Not typical bent chain symptoms.

What had happened was that one of my chainring teeth was bent inward, which affected the chain by picking it off when the chain was crossed to the outside of the cassette. The thing that made it worse was that “chain suck” pulled the chainring in, and warped it so that I couldn’t even do one revolution anymore. Bent teeth are easy to overlook on a casual drivetrain inspection, and definintely not as obvious as a bent chainlink but are easily fixed by bending the tooth back. If the chainring itself gets warped, you will likely break it by trying to true it, as they are typically made of an alloy and not steel. Keeping an eye on the condition of your chainring and staying on top of bent or notched teeth can save a ride/race, as opposed to allowing it to cause permanent damage by warping the whole ring.

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