Do you really know why the tires are round? Why does the tires look backwards?

Every  day we drive, ride, on the car can be described as very familiar, but  the car there are many humble small features, parts and components is  not well known. Such as tires.
Although the tire is a common but the auto parts, but it is still full of a lot of mystery, such as why it is round. Then the car chatter on the inventory of some knowledge of the car tire cold.
Why the tires are round
First of all to exclude the difficulty of the installation, first of  all say why the round wheels to make the car running smoothly.
Figure black parts, A, B, said two points on the edge of the wheel, O said the axis of the wheel, then there are:
Length of OA = length of OB

And C to the wheel at any point on the edge, if the wheel can be smoothly rolling to meet, then, should be: OC = OA.
What if the tire is not circular?
From  the figure on the blue part of the view, OC length and OA different, so  in the process of rolling the tire axis will be up and down beating. Think about it, if riding a bike tires of the square, what would be an experience?
Why is the end of the car tire label is 5?
We  all know that the number of car tires is usually 205/45 R17 such a  format, which 205 represents the number of 205 mm tire width section. Common car tire width from small cars 165,185,195, and even  medium-sized cars and even 225,245,275 with large cars, but the question  comes, why the car tire width is the end of 5?
In the design and manufacture of tires, the need to consider a lot of  factors, which is very important point is to meet the vehicle adaptation  and follow industry standards.
This is like a computer's USB interface, is a rigid standard size. Although they can be designed into different shapes and shapes, but in  the end they are able to meet and insert a variety of  computer-compatible USB interface, and read or write data.
Tire  width is not the beginning of a good preset, each manufacturer has its  own standards, but in order to meet the requirements of different car  manufacturers, tire manufacturers in accordance with the needs of car  manufacturers to meet the minimum size in a range of grip, Maximum speed, maximum carrying capacity, cost and other conditions,  and finally gradually developed into an international standard.
In  fact, their width is not all 205mm, it may be 203mm, 204mm or 206mm,  but if these different data to mark their models, then the type of tire  will be very numerous. Because most mantissas are close to 5, the standard on the compromise took a 5. Through the following table you can find that many of the actual width of the tire is not the end of 5.
So you see, engineers sometimes we are not so hard-working!
The greater the friction surface contact, the more why the tires have patterns?
Learned physical people know that the greater the contact surface, the greater the friction. In the tread to pull out the pattern, the contact area becomes smaller, the grip will be worse, then why pull it?
In fact, this is a comprehensive problem. First, the tire and the ground is not rigid, in contact will produce  deformation, bite each other, so the car will not only rely on the  friction between the tire and the road, as well as mutual thrust, like  gears.
Second,  we bought a car can not only drive in sunny days, but also the face of  rain and snow, mud, etc., if there is no pattern, it will produce the  phenomenon of skidding. Imagine, if there is no pattern of tires in the road after the rain, it will be like
Why is the tires black?
Why is the tire on the black, car totem has published an article:  finally know why the black tires are | up posture has been interpreted.
In fact, the beginning of the automobile industry there are many other colors of the tires, such as white. This  is due to tire production additives are not fixed, resulting in a lot  of color tires, but the durability and poor resistance to aging. In order to improve the durability of the tire, scientists think of  carbon and high-temperature condensation process of hydrocarbons, making  anti-aging properties such as greatly improved, but the presence of  carbon black, leading to black tires, just like Baogong.
After  many years, many tire companies in order to show their status as a  different, but also developed a lot of other additives to make tires of  different color methods, but a long time the color will fall, will  change color, the cost will increase a lot. So the black tires have been occupying the leading position in the tire production line.
Why does the car tires look backwards?
In fact, this is not a kinematic problem, but an optical problem. Because the human eye on the image capture speed is not so fast, if  you look at the picture frame rate is higher than 10-12 FPS, when the  human eye will foolish to tell, so that the image is continuous.
Draw a simple diagram to intuitively feel:
If the tires rotate 1.2 times per second, and the human eye can only  capture images once per second, then every time you see the red line, it  will be mistaken for forward rotation, and thus feel the tire in the  forward rotation.
And if the tire and then faster, 1.8 revolutions per second, then  every time you see the red line, it will be mistaken for the tire in the  future rotation.
In  fact, at the same speed, the tire size, the number of spokes will also  affect the visual effects, in general, the higher the speed, the more  will cause this visual illusion, so sometimes we see a lot of cars on  the road The wheel seems to be spinning fast, and it is likely that it is flying fast forward.