Why Can You Balance a Bicycle in Motion But Not When Stationary?

[SkepticJ]

When I'm riding my bicycle, I can be barely moving forward--say 10 cm/sec.--and I'm able to maintain my balance easily. But if I'm completely stopped, I cannot do it.

Of course some people can maintain their balance on a unicycle when they're not moving, but I don't have balancing skills which are too unusual.

So, what's going on here? What difference does that little bit of velocity make?

I don't think it can be the gyroscopic properties of the turning wheels, because they're barely spinning.

 

[mgb_phys]

Castoring - as you lean one way the pivoting front wheel moves to counter it.
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

 

[rcgldr]

When you're moving, even at slow speed, you can generate more side force (which produces a rolling torque) and side displacement at the front tire contact patch, than you can while stopped. Because the contact patch is behind the pivot axis, it is possible to balanace while standing still, something that veledrome bicycle racers are good at, but it's difficult do the very small range of relative movment of the contact patch with respect ot the bicycle when stopped. When the bicycle is moving, the contact patch moves in the direction the tire is steered, which allows for much more movement and side force.

 

[AJ Bentley]

Of course some people can maintain their balance on a unicycle when they're not moving,

No they can't. They have to keep moving backwards and forwards, turning slightly each time to adjust their balance.