On an asphalt road if you try to achieve very high speed quickly the car may get stuck sometimes this is because the tire frictional force depends on an important parameter called slip let's learn more about this interesting topic in a detailed way. Here are perfectly rolling tire is illustrated in a perfectly rolling condition the distance traveled by the wheel will be equal to the perimeter of the wheel. The contact between the road and tire will be on the line in this case. In a pure rolling case the relative movement between road and tire is 0 and obviously slip is also 0. The exact opposite to this case is a pure sliding case or 100 percent slip. In general the tires behave between these 2 extremes.
The contact region of the tire and road will always be on an area due to the weight acting on it. Here if you observe the wheel moves less than its perimeter during a complete revolution this is because the relative motion between the tire and road won't be perfectly 01 portion of it will always be sliding let's understand more about this phenomenon. We saw earlier what are pure rolling and shore citing cases in a pure sliding case the molecules of the tire and road rub each other and the frictional forces generated opposite to the tire movement in short the whole contact patch molecules will be sliding in this case.
Now the question is what will be the natures of this microscopic interaction during say 50 percent of slip. Such cases lead to generation of another interesting region called adhesion region and in that he's in region the molecules of the tire and road surfaces form surface bonds these are called vanderwal bonds and are we can bond strength you can find the same types of bonds between the graphite layers of a pencil it. Now you might be wondering how they could be a fixed bond on a moving tired the bonds in the adhesion region are temporary due to the movement of the tire of these bonds have to get broken.
During this process the force generated by the road molecules on the tire molecules will be in the same direction of sliding frictional force. There is still confusion among researchers about the location of the adhesion and sliding regions. The total frictional force produced by a tire will be the sum of it he gin and sliding friction values. Yeah he's in frictional force value is always greater than this fighting frictional force. In a perfectly rolling condition the value of frictional force is 0. However when the wheel is 100 percent slipping the sliding friction comes into play you can expect the frictional values for the other levels of slippage to very like this. However what happens practically is totally different.
Why such a strange behavior for slip versus friction variation. To understand this behavior let's take the classic example a linearly moving block. The block doesn't move until the force applied overcomes the static frictional force. After the peak force the block slides the frictional value suddenly drops let's replace the X. axis with the slip value instead of the applied force this block example either has a 0 slip or 100 percent slip.
Since a perfectly rolling tire does not have any relative motion with the road this case is equivalent to a non moving block. A 100 percent slipping tire is obviously the same as a moving block so it is logical to expect a similar graphic for rolling tires as well. Due to the flexible properties of rubber in this case the graph will be smoother. The slip versus friction graph is quite important in a B. S. technology and other vehicle dynamics analysis. The anti lock braking system is designed in such a way that it applies brakes in the slip ratio range of 8 percent to 30 percent. Now we see how the frictional force and slip can be calculated experimentally we know the formula for calculating the friction.
Brake torque sensors are used to measure the frictional force the normal force is measured using a strain gauge using these 2 values the coefficient of friction is calculated. This slip ratio was calculated using these equations. Both angular speed and translational velocity are measured using a wheel speed sensors. The translational velocity is measured at non breaking wheels however when brakes are applied at all 4 wheels a radar is used to estimate the translational velocity of the vehicle. In this video we studied frictional force variation for straight line driving when the vehicle takes a turn the contact patch deforms. The angle between the direction in which the wheel is pointing in the direction in which the wheel is traveling is known as slip angle. In the upcoming videos of this series we will see how the slip angle affects friction.
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