I just got back from a ride where other people had odometers on their bike. They read 26 km. My iPhone's HealthKit said 7.8 km of walking and running.
Is HealthKit just not useful for biking, or is there some way to get it to recognize biking?
Cycling doesn't trigger the 'step' recognition the way actual steps do.
The subsystem responsible for the step sensor in fact does a great deal of work to filter out anything that isn't an actual step.
So the fact that some cycle moves make it through could actually be considered a flaw.
What actual cycling apps do instead is:
This is why you can possibly find cycling data in someone's Health database – apps can use it to store their own sensor data or metrics. But so far (early 2016) iPhones don't directly feature built-in cycling sensors.