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.

 

[PJC01]

There are several factors at play here in the physics of bicycle riding. The first is the concept of angular momentum, which is the tendency of a rotating object to maintain its rotational motion. In the case of a bicycle, the wheels are constantly rotating, creating a significant amount of angular momentum. This helps to stabilize the bike and keep it upright, even at low speeds.

When you are moving forward at 10 cm/sec, the wheels are still rotating and creating this stabilizing angular momentum. However, when you are completely stopped, there is no rotational motion and the bike becomes much more unstable. This is why it is easier to maintain balance at a slow speed rather than when completely stopped.

Another factor to consider is the center of mass of the bicycle and the rider. When the bike is in motion, the rider's body and the bike are in a dynamic equilibrium, with the center of mass constantly adjusting to maintain balance. When the bike is stopped, the center of mass is no longer in motion and becomes more difficult to maintain balance.

Lastly, the gyroscopic properties of the wheels do play a role in maintaining balance, even at slow speeds. While they may not be spinning rapidly, they still have some gyroscopic effect that contributes to the overall stability of the bike.

In conclusion, the little bit of velocity that you have when moving at 10 cm/sec is enough to maintain the necessary angular momentum and dynamic equilibrium to keep the bike stable. Without this velocity, the bike becomes much more difficult to balance. So next time you're riding your bike, remember the physics at play and appreciate the small but important role that velocity plays in maintaining balance.