The bike stays upright because of the gyroscopic effect of the wheels turning. The faster the wheels are rotating, the greater the gyroscopic effect. That's why it doesn't fall over when you're moving, but it does want to fall over when you stop. You can keep it upright when stationary using balance, but you only need to use balance when there is little or no gyroscopic effect.
I don't think that's quite right. There is
a gyroscopic component, and it does
increase with speed, but it's not the most important factor.
A cyclist in motion is constantly falling to one side or the other, and steering to correct his fall. It's this steering that keeps the bike upright.
From the cyclist's point of view, steering in a curve of radius r
generates a centrifugal force* of magnitude mv²/r
That means that the steering input required to correct a given lean decreases rapidly as v
On the other hand, as v
drops towards zero, the steering input required to correct that same lean increases rapidly until the steering angle required becomes too great, or r
is less than the wheelbase of the bike.
It's as that point approaches that we have to come up with new strategies to balance, either shifting our weight around on the bike, or shifting the bike around underneath us.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.