A shopping cart in a supermarket, or a luggage cart at an airport, has 4 wheels underneath. Careful observation shows that the wheels and wheels of the supermarket shopping trolley are not fixed, but can rotate freely around a vertical axis. If the axle is fixed on the car body, the swivel wheels can only go straight ahead, and the car cannot turn.
You may also find that when you push the cart forward, the wheels always follow the rear of the axis of rotation. If changing from front to rear, the wheel will suddenly rotate 180 degrees near the vertical axis, or at the position of the subsequent axis of rotation. Generally speaking, the rolling direction of the universal wheel must be consistent with the forward direction, so that the shopping cart can be synchronized with the local movement. Once the rolling direction of the wheel is inconsistent with the forward direction of the vehicle body, the friction force on the ground will generate torque on the vertical rotation axis in front and push the wheel to the same position as the walking direction.
Similar to the casters of a supermarket shopping cart, bicycles have a similar phenomenon. If the axis of rotation of the front fork of the bicycle extends towards the ground, the intersection point between the fork and the ground is always in front of the contact point between the front wheel and the ground. When you push the pad forward, the front wheels follow the direction smoothly. If the stroller is rolled backwards, the opposite position of the front wheel and front fork is exactly the opposite. The sideways friction forces are in the opposite direction, and the front wheels oscillate.
The relative position of the front fork pivot and the contact point of the front wheel with the ground is very important to ensure the stability of the bicycle. To see this effect, build a special bicycle such that the axis of rotation of the bicycle's fork intersects the ground on the back side of the point of contact between the front wheel and the ground. On a particular bike like this, no matter how clever your cycling skills are, your bike is going to be unstable.
Although the bicycle has been invented for more than 200 years, it is difficult to explain why the fixed wheels of the cart are pushed down, and the mechanical principle of riding is stable. The position of the front wheel axle is an important factor affecting the stability of the bicycle, even more important than the wheel rotational inertia effect.
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