Why is it difficult to balance a stationary two-wheeler?

Stationary objects experience only the gravitational force. Such objects can be balanced only when the line of action — the line connecting the centre of gravity of the object towards the earth — touches the ground at a point within the base of the object.

If the base area is large, slight deviations due to external forces such as the wind will not push this point out of the base. So it is easy to balance such objects.

In the case of a two wheeler, the base is very narrow, limited to the thin strip connecting the two points where the wheels touch the ground. Hence any slight disturbance will push the line of action outside the base area, leading to the vehicle becoming imbalance.

When the two-wheeler is moving, the situation is entirely different. In addition to gravity there is another force exerted by the engine, acting in the forward direction.

The resultant of these two forces is still in the forward direction and thus the vehicle moves forward independent of where the line of action touches the ground. This is why it’s easier to balance a moving vehicle.