As for friction, if F=uN, it means that friction has nothing to do with contact area, so why should automobile tires be widened?

Here are a few misunderstandings, to help you clarify; And answer your questions.

1.F=uN？

In fact, this formula is only one of the forms of friction. F=uN, the application premise of this formula is "Coulomb friction law", which is the simplest description form of friction.

It is impossible to teach too complicated knowledge in middle school (just as students in grade one of primary school can only understand what a few plus a few equals, and it is impossible to understand what sine and cosine are and what functions are). After going to college, if you are a mechanical major, you can have a new understanding of friction by consulting and studying professional literature. If you continue to study as a graduate student after graduating from college and choose tribology as your research direction, you will learn more friction formulas.

So, what's wrong with this simplest formula F=uN?

Yes, there is a problem. In fact, friction may be related to pressure, speed and temperature. However, usually we don't care about these complicated factors.

Since there is a problem, is this formula wrong?

You can't say that either. Simplifying complex problems in real life is not only science, but also art, which is also one of the essences of mathematics and physics. F=uN Put aside complicated factors, grasp the most fundamental factors and simplify the problem. And those complicated problems are left to graduate students, doctors, professors and scientists to study.

2. There is a great resistance when driving with a flat tire, because the friction increases?

This understanding is actually wrong. From a professional point of view, friction is only a very minor factor leading to tire running resistance.

Simply put, the driving resistance of tires is mainly caused by the deformation of tire rubber, not the friction of the ground.

Complexly speaking, the tire is deformed due to the self-weight of the vehicle, and local materials (especially rubber materials) of the tire are bent, compressed and sheared; This deformation occurs alternately every time the tire rolls. Due to the viscosity of rubber, these deformations will inevitably consume energy (refer to the relevant knowledge of viscoelastic mechanics), and at the same time, part of the energy will be converted into heat energy, which will raise the tire temperature. This energy loss process is the principle of tire rolling resistance.

This can explain why riding a bicycle is particularly tiring when the tire leaks-because the air pressure is small and the rigidity of the pneumatic tire is small at this time. Under the same load, the deformation will be larger (the difference of tire deformation in the contact area can be seen by careful observation), and the greater the energy required for deformation, the greater the rolling resistance.

3. What is the pressure between the tire and the ground?

This problem seems simple, but in fact it is very complicated.

Simply put, the pressure between the tire and the ground mainly depends on the inflation pressure of the tire. The greater the inflation pressure, the smaller the grounding area and the greater the pressure between the tire and the ground.

Complexly speaking, the pressure between the tire and the ground depends not only on the inflation pressure, but also on the size, shape and internal structure of the tire itself. Moreover, the distribution of this pressure is uneven (the pressure at each point in the ground plane is different), and the characteristics of the distribution also depend on many factors.

4. When the tire becomes wider, the ground contact area will increase? Or, if the tire gets wider, the pressure will decrease?

(On the premise of constant vehicle weight, the pressure is inversely proportional to the contact area, so these two problems are completely equivalent. )

The answer is, not necessarily.

If the air pressure is constant, according to the above explanation, the pressure mainly depends on the inflation pressure of the tire. Therefore, the ground contact of wide tires does widen, but it will shorten in the longitudinal direction. Therefore, even if the tire width is doubled, the area of the whole contact area (envelope area) will not change much, usually not more than 20%, and sometimes it will even decrease.

However, as mentioned earlier, "the pressure between the tire and the ground depends not only on the inflation pressure, but also on the size, shape and internal structure of the tire itself;" In addition, the distribution of this pressure is uneven. "As a tire developer, when developing new tires, we will try to increase the contact area (even if it is only 5% or 10%) and make the pressure distribution as uniform as possible.

What is the purpose of increasing the grounding area?

Make the tire more wear-resistant; Improve driving stability; Improve grip. Every article is complicated to explain. Limited to space, I won't explain it in detail.

5. Why should car tires be widened?

Different cars have different performance of concentration. Wide tires are not all good, and the purpose of widening is usually to pursue handling performance. Generally, wide tires have greater driving force (faster speed), greater braking force (faster braking), and better control accuracy and stability (for example, when turning at high speed, they will not roll over and turn accurately); But the comfort performance is poor. This is why sports cars and racing cars usually have flat and wide tires, while many luxury cars don't.

6. The same model has the same quality. Why are the tire sizes different?

Cars of the same brand, series and version usually have the same tire size when they leave the factory. However, because automobile manufacturers usually choose two or three tire suppliers, although the tyre size produced by different manufacturers is the same, the specific size will be slightly different (not too different). The reason is that different manufacturers have different design concepts.

Some cars have different specifications of front wheels and rear wheels, which are also due to different performance considerations-for example, the front wheels have to make complicated turns and the rear wheels are responsible for driving; Large front wheel load, small rear wheel load, and so on.

In addition, when the car itself is modified or upgraded, the tire size may also change.

Finally, the driver modifies or upgrades the tires himself.

7. Other explanations

If you keep seeing this place, it shows that you have a strong thirst for knowledge.

I am a professional tire developer, so I need to know this problem thoroughly, but you don't need to know that much. If you are still interested in explaining these problems more clearly, you can consider buying some professional books to study, such as tribology, automobile tires, and so on.