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EspañolShopping carts in supermarkets, or luggage trolleys in airports, have 4 casters underneath. If you look closely, you can find that the axle and body of the caster are not fixed, but can rotate freely around the vertical axis. Because the axle is fixed with the car body, the casters can only go straight forward, and the car cannot turn.
In addition, it can be found that when you push the cart forward, the casters always follow the rear of the rotating shaft. If you change from forward to backward, the caster will suddenly turn 180° around the vertical axis, still in the position of following the rotation axis. In general, the rolling direction of the casters must be consistent with the forward direction so that the shopping cart can move smoothly. Once the rolling direction of the casters is inconsistent with the forward direction of the vehicle body, the frictional force of the ground will generate a moment on the vertical rotation axis in front, and push the casters to a position consistent with the walking direction.
Similar to the casters of shopping carts, bicycles have a similar phenomenon. If the rotation axis of the bicycle front fork is extended to the ground, its intersection with the ground is always in front of the contact point of the front wheel with the ground. When you push the cart forward with your hands on the seat cushion, the front wheels follow the direction of travel very obediently. If you push the cart backward, the relative positions of the front wheel and the fork are opposite, and the side friction force is pushing in the opposite direction, and the front wheel will sway from side to side.
The relative position between the rotation axis of the front fork of the bicycle and the contact point between the front wheel and the ground plays a very important role in ensuring the driving stability of the bicycle. To understand this effect, a special bicycle can be made so that the intersection of the extension of the rotation axis of the bicycle fork with the ground is behind the front wheel and the ground contact point. On this particular bike, no matter how good your riding skills are, the bike will not be stable.
Although the bicycle has a history of more than 200 years since its invention, it is not easy to explain the mechanics of why a stationary bicycle falls over when pushed but is stable while riding. The position of the front wheel rotation axis mentioned above is an important factor affecting the stability of the bicycle, even more, important than the inertial effect of the wheel rotation.
