While turning the cranks backwards, look from the top down to inspect for:

  • Bent crank arm spider (generally the chainrings will appear to move side to side as you spin)
  • Bent chainring (make sure each chainring appears straight)
  • Bent teeth (make sure there are no bent or broken teeth)

Continue spinning the cranks backwards, now check the rear freewheel/cassette cogs looking for any bent cogs or teeth.

Buy a chainwear tool¬†and check the chain wear on any used bicycle you’re inspecting. If the chain is worn out, assume that at minimum the cassette/freewheel needs to be replaced. Look at the chain rings to check for shark fin shaped teeth which indicate worn chainrings. A worn out drivetrain can significantly devalue a used bicycle.

Lastly, make sure that the space between the chain stay (part of the frame) and the inside of the crank arm is the same on both sides (add picture), if not this may indicate a bent crank arm, bent rear triangle or incorrect bottom bracket spindle length. Ensure that the crank arms match.