Q: Why is it easy to balance on a moving bike, but almost impossible to stay upright when it stops?
A: First, let's dispel the myth that is usually put forward at this point, which suggests gyroscopic effects have something to do with this. I built a bike with a reverse-running rotor attached to the front axle, which cancelled out all gyroscopic effects, and it was no more difficult to ride than a normal bike. Some simple sums will confirm that this is the case.
The way we stay upright on a moving bike is by active control through steering. This is why we have to learn to ride a bike. If, as learners, we find ourselves falling over to the left then we learn to steer the bike to the left, which generates forces that tilt us back upright again, thereby putting the wheels back under our centre of gravity. Beginners are very wobbly, but as we become expert the corrections become smaller and we can ride in a straight line.
“We stay upright by active control through steering. This is why we have to learn to ride a bike”The faster we ride, the smaller the steering adjustment needs to be, simply because the bike moves much further in a given time. When riding very slowly the steering adjustments required are very large. When completely at rest, active steering can do nothing for us.
A good analogy is to ask, "Why is it easier to hop (or pogo-stick) along a straight path than it is to stand still on the ball of one foot?" The reason is that we use each hop to generate correcting forces and also to put our foot down in a new place that is closer to where we need it to be in order to maintain our balance.
Hugh Hunt, Dynamics and Vibration Research Group, Department of Engineering, University of Cambridge, UK
(there were several other contributions - see the full New Scientist article at 9 Dec 2006)