The difference between a cart caster and a bicycle wheel

The difference between a cart caster and a bicycle wheel

Shopping carts at the supermarket, or carts carrying luggage at the airport, have four casters below. If you look closely, you can see that the wheel shaft and body are not fixed, but can rotate freely around the vertical axis. Because if the axle is fixed with the body, the caster can only go straight ahead, and the car cannot turn.

In addition, it can be found that when you push the cart forward, the caster always follows the rear of the axis of rotation. If once into to retreat, castor will suddenly around the vertical axis around 180 °, still follow the location of the axis of rotation. In general, the wheel's rolling direction must be consistent with the forward direction to make the shopping cart move smoothly. Once the wheel's rolling direction is inconsistent with the direction of the body, the friction on the ground will generate torque on the vertical axis of rotation, pushing the caster to the same position as the moving direction.

Similar to shopping cart casters, bicycles have similar phenomena. If the rotation axis of the front fork of the bicycle is extended to the ground, the intersection with the ground is always in front of the front wheel and ground contact point. As you push the pad forward, the front wheels follow in a compliant direction. If you push back, the front wheel and the front fork are opposite, and the lateral friction is pushed in the opposite direction, and the front wheel wobbles.

The relative position between the front fork rotation axis and the front wheel and the ground contact point is very important to ensure the stability of the bicycle. To understand this effect, a special bicycle can be specially made to make the point of the rotation axis of the bicycle's front fork and the ground intersect at the rear of the front wheel and ground contact point. Ride on this special bicycle, and let your bike ride skill, bicycle can't be stable.

Although bicycles have been around for more than 200 years since their invention, it is not easy to explain the mechanics of why stationary bikes are pushed back, but they are stable. The position of the front wheel rotation axis mentioned above is an important factor that affects the stability of the bicycle, and is even more important than the inertia effect when the caster rotates.