Knowledge points of bicycle structure and physical principles

Knowledge points of structure and physical principles of bicycles 1. Friction on a bicycle

(1). Application of Friction Force —— Why can a bicycle move forward?

Bicycles, like other means of transportation, move forward by friction between wheels and the ground. Due to its own quality and self-weight, the wheel is uneven with the ground, and static friction will be generated when it is pressed on the road surface. When a person rides a bicycle, the bicycle begins to move with force, and the static friction force between the rear wheel and the ground is the same as the forward direction of the bicycle, thus pushing the bicycle forward.

(2) The greater the pressure, the greater the friction-why can the bicycle stop?

Braking devices (brakes) play a very important role in bicycles. Traffic accidents caused by poor braking are not uncommon. How to brake a bicycle?

Bicycle braking uses friction to slow down and stop the bicycle. When we use the brake, the friction between the brake shoe and the wheel stops or slows down, and the friction between the wheel and the ground changes from rolling friction to sliding friction. Strong sliding friction is opposite to the forward direction of the bicycle, which makes the bicycle slow down (or stop moving quickly).

(3) Increase roughness and friction-Why are there uneven patterns on bicycle tires?

There are uneven patterns on the bicycle tires, which can increase the roughness between the bicycle and the ground, increase friction and prevent the bicycle from slipping.

When braking suddenly, sliding friction will cause great wear on the outer tire of the vehicle. If the wheels always roll instead of sliding, the wheel wear can be avoided. Therefore, it is conceivable to install another spare brake (metal or rubber ...) on the bicycle. During normal movement, the brake disc is lifted off the ground. When braking, the brake disc is pressed to contact the ground, causing the brake disc to slide on the ground, resulting in sliding friction, which makes the vehicle slow down quickly until it stops, and prevents the bicycle tire from being worn.

(4) Rolling friction is smaller than sliding friction-where are the steel balls installed on the bicycle? Why Zhu Gang?

Bicycles should be as easy and flexible as possible, and the more labor-saving. So there are steel balls everywhere the bicycle rotates, such as the central axle, the rear axle, the handlebar rotation, the pedal rotation and the flywheel. The purpose of installing steel balls at the rotating position is to reduce friction, protect parts and save power. Because rolling friction is much smaller than sliding friction, rolling instead of sliding can greatly reduce friction, and lubricating oil is often added to separate the contact surfaces, which makes the friction smaller and more labor-saving.

Second, the lever, axle and wheel on the bicycle.

Lever to control the steering of the front wheel: The handlebar of a bicycle is a labor-saving lever. People can turn the front wheel of a bicycle with little force to control the direction and balance of the bicycle.

Lever to control the brake: the brake handle on the handlebar is a labor-saving lever, so people can press the brake on the rim with relatively small force, and the pressure is relatively high.

Pedal and big gear on the central shaft: form a labor-saving axle, and the pedal radius is greater than the big gear radius.

Bicycle handlebar and front axle: form a labor-saving axle, and the outer radius of handlebar is larger than the front axle.

Gears on the rear wheel shaft and rear wheel: constitute a laborious shaft, and the radius of the gears is smaller than that of the rear wheel.

Common bicycle wheel diameters are 559mm(22 inches), 6 10mm(24 inches), 660mm(26 inches) and 710/mm (28 inches). Riding a 28-inch car is more laborious than riding a 24-inch car, but it is faster. Because the radius of the 28-inch wheel is large, the distance traveled by the wheel per revolution is longer, so it is faster. Axle radius is large and laborious. In the early days, all kinds of wheels were wooden wheels and iron wheels with potholes. Modern bicycles use inflatable inner tubes mainly by using the principle that the volume of a certain mass of gas decreases and the pressure increases. When the uneven road surface brings impact to the tire, the compression and recovery of the inflatable inner tube has a good buffering effect, reducing bumps, protecting the bicycle and reducing the forward resistance, which makes people feel comfortable.

3. The principle of bicycle speed change

Bicycles can move forward because people step on the pedals to provide power and drive the rear wheels to rotate to generate friction. The sprocket on the central shaft and the rear wheel shaft are connected by a chain to drive the rear wheel to rotate. Each tooth of the two wheels corresponds to the hole between the chains. When a big gear turns a tooth, the gear must also turn a tooth, and it is absolutely impossible to slip.

In the new type of variable-speed bicycle, there are several toothed discs with different diameters and teeth numbers on the middle axle sprocket and several toothed discs with different diameters and teeth numbers on the rear axle flywheel. If gears with different numbers of teeth are selected and driven by chains, the speed of the rear wheels will change, which is called a variable-speed bicycle.