1. F=uN？
in fact, this formula is only one of the forms to represent 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 sine and cosine are and what function is). After going to college, if you majored in machinery, you can have a new understanding of friction by consulting and studying professional literature. If you continue to be 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 complex influencing 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 the complicated factors, grasped the most fundamental factors, and made the problem simple. And those complicated problems are left to graduate students, doctors, professors and scientists to study.
2. The running resistance of a flat tire is great, because the friction is increased?
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 on the ground.
In a complicated way, the tire is deformed by bearing the weight of the vehicle, and the local materials (especially rubber materials) of the tire are bent, compressed and sheared; This deformation alternates every time the tire rolls. Because of the viscosity of rubber, these deformations must consume energy (refer to the relevant knowledge of viscoelastic mechanics), and at the same time, some 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 is flat-because at this time, the air pressure is low, the rigidity of the pneumatic tire is small, and when bearing the same load, the deformation amount will be greater (you can see the difference of tire deformation in the grounding area by careful observation), and the more energy needed for deformation, the greater the rolling resistance.
3. what is the pressure between the tire and the ground?
this problem seems simple, but it is actually 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.
In a complicated way, 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 is different in the ground plane), and the characteristics of the distribution also depend on many factors.
4. when the tire is wider, the ground contact area will increase? Or, if the tire gets wider, the pressure will decrease?
(The pressure is inversely proportional to the contact area when the weight of the car is constant, so these two questions 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. As a result, the ground contact of the wide tire does widen, but it will shorten in the longitudinal direction. Therefore, even if the tire width is doubled, the area (envelope area) of the whole contact area will not change much, usually not more than 2%, and even sometimes it will 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; Moreover, the distribution of this pressure is uneven. " As a tire developer, we will try our best to increase the contact area (even if it is only 5% or 1%) and make the pressure distribution as uniform as possible when developing new tires.
what is the purpose of increasing the grounding area?
make tires more wear-resistant; Improve driving stability; Improve grip. Each article is very complicated to explain. Due to the limitation of 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, they will not roll over when turning at high speed, and they can turn accurately); But the comfort performance is poor. This is why sports cars and racing cars usually have flat and wide tires, but many luxury cars don't.
6. the quality of the same model is the same. why are the tire sizes different?
cars of the same brand, series and version are usually the same in tyre size when they leave the factory. However, because car manufacturers usually choose two or three tire suppliers, although the specifications of the tires produced by different manufacturers are the same, the specific sizes 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 revised or upgraded, tyre size may also change.
Finally, the driver modified or upgraded the tires himself.
7. Other notes
If you have insisted on seeing this place, you have a strong desire for knowledge.
I am a professional tire developer, so I need to understand this problem thoroughly, but you don't need to know so much. If you are still interested in making these problems more clear, you can consider buying some professional books to study, such as Tribology, Automobile Tire, and so on.