A shopping cart in a supermarket or a trolley in the airport is usually equipped with 4 industrial casters. When installing, if the axle and the car body are fixed together, the industrial casters can only go straight ahead and cannot rotate freely. The axles and body of the industrial casters are not fixed, but are free to rotate about the vertical axis.
In addition, industrial casters always follow the rear of the rotating shaft when you run the trolley. If you change from forward to backward, the caster will suddenly turn 180 degrees around the vertical axis and still be in the position following the rotary axis. Under normal circumstances, when the trolley moves along the ground, the rolling direction of the casters must be consistent with the direction of advancement. Once the rolling direction of the caster does not coincide with the forward direction of the car body, the friction of the ground will generate a moment to the vertical axis of rotation of the front, pushing the caster to the position where the walking square is consistent.
Similar to the casters of the shopping cart, the reason for the free rotation of the bicycle is similar. If the axis of rotation of the bicycle front fork is extended to the ground, the intersection with the ground will always be in front of the front wheel and ground contact points. When you hold the seat cushion forward, the front wheel smoothly follows the forward direction. If the cart is reversed backwards, the relative positions of the front and front forks are just opposite, and the lateral friction is pushed in the opposite direction, and the front wheels will shake side to side.
Therefore, the fixed and relative distance of the axles of the stroller industrial casters is very important to ensure the stability of the trolley.