A gear change mechanism for a bicycle may involve a tensioner that has a wheel engaged with a chain in a bicycle drivetrain. This tensioner may be configured to rotate or otherwise change orientation in response to slack in the chain to maintain tension in the chain. A sensor may be used to measure the orientation of a tensioner of a chain in a bicycle drivetrain, and/or the physical orientation of components indicative thereof. Actions may be triggered by signals generated by the sensor.