In supermarkets, shopping carts or luggage trolleys in airports all have four wheels at the bottom. If you look closely, you'll notice that the wheel axles of the wheels are not fixed to the trolley body, but can freely rotate around the vertical axis. If the wheel axles were fixed to the trolley body, the wheels would only be able to move straight forward, and the trolley wouldn't be able to turn. Additionally, you'll find that when you push the trolley forward, the wheels always follow behind the rotating axis. If you switch from forward to backward motion, the wheels will suddenly rotate 180° around the vertical axis and still remain in the position of following the rotating axis. Generally, the rolling direction of the wheels must be consistent with the forward direction to enable the shopping cart to move smoothly. Once the rolling direction of the wheels is inconsistent with the forward direction of the trolley body, the ground friction will generate a torque on the vertical rotating axis in front, pushing the wheels to a position consistent with the direction of travel.
Similar to the wheels of shopping carts, bicycles also exhibit similar phenomena. If you extend the rotating axis of the bicycle front fork to the ground, the point where it intersects the ground will always be located in front of the point where the front wheel contacts the ground. When you push the bicycle forward by holding the saddle, the front wheel will obediently follow the forward direction. If you push the bicycle backward, the relative position of the front wheel and the front fork will be reversed, and the direction of the lateral friction will also be reversed, causing the front wheel to sway from side to side. The relative position between the rotating axis of the bicycle front fork and the point where the front wheel contacts the ground is crucial for ensuring the stability of the bicycle's movement. To understand this effect, you can specially make a special bicycle with the extended line of the rotating axis of the bicycle front fork intersecting the ground behind the point where the front wheel contacts the ground. Riding such a special bicycle, no matter how skilled you are at riding, the bicycle will not be stable. Although bicycles have been invented for more than 200 years, it's not easy to explain the mechanical principle of why a stationary bicycle falls over when pushed, but remains stable when ridden. The aforementioned position of the front wheel rotating axis is an important factor affecting the stability of bicycles, even more important than the inertia effect of the wheels when rotating.