A friend recently chatted with the editor and asked: "You always talk about anti-lock brakes and anti-lock brakes. Why do you need anti-lock brakes? When I ride a bicycle, I have to lock the brakes to stop. Come on down!” After listening to this, the editor was thinking that we still need to learn more about why cars need anti-lock brakes and why all cars are equipped with ABS now.
Why anti-lock brakes are needed
I believe many friends may see obvious brake marks when driving past the scene of a car accident. Just like shown in the picture below.
In fact, these skid marks are the marks left by the tires on the road. So we carefully observed the braking process of the car and found that the marks left by the tread on the ground were a gradual process from wheel rolling to locking and sliding. The impressions can basically be divided into three segments: the shape of the impressions in the first segment is basically the same as the tire tread pattern, and the wheel is close to pure rolling, that is, pure rolling state; the impressions of the tire tread pattern in the second segment can be distinguished, However, the pattern gradually becomes blurred, and the tire not only simply rolls, but also has a certain degree of relative sliding between the tread and the ground, that is, the wheel is in a state of rolling and sliding. As the braking intensity increases, the proportion of the sliding component becomes larger and larger; The three sections directly form a thick black mark, and no trace of pattern can be seen. The wheel is held by the brake and completely slides on the road, that is, it locks and slides.
So we see that as the braking intensity increases, the wheel rolling component becomes less and less, and the sliding component becomes more and more. Therefore, the concept of slip rate (s) is specifically used to describe the amount of tire slip during braking. That is to say, a percentage is used to express the proportion of tire motion components during braking. The greater the slip rate, the more sliding components. Obviously, if it is pure scrolling, s=0, when rolling and sliding, s is between 0 and 100%, and if it is a locked and dragging state, s=100%.
Then the relationship between the braking force coefficient, lateral force coefficient and slip rate is obtained based on the test.
So as you can see, the braking force coefficient generally reaches the maximum value at about 15%-20% of the sliding rate. As the sliding rate increases, the braking force coefficient decreases, while the lateral force coefficient is It's been going down! Therefore, if the wheels are locked and slipping (slip rate s = 100%), the braking force coefficient is not the largest and the lateral force coefficient is the smallest. At this time, the tire is easily affected by the lateral force and causes sideslip or sideslip! Therefore, cars now need an anti-lock braking system (ABS) to control the slip rate during braking to about 15%-20% to achieve a larger braking force coefficient and a higher lateral force coefficient. This provides the best braking performance and good lateral stability.
Obviously, as mentioned before, when braking on a bicycle, the wheels must be locked, and the slip rate is 100%. That's because the bicycle is slow, light, easy to brake, and can be controlled without turning sideways by human power. Cars are fast and weigh many times more than bicycles. If there is no ABS, the braking distance will be long, and it is also very easy to swerve or slide to the side.
ABS system
ABS anti-lock braking system is a car safety control system with the advantages of anti-skid and anti-lock, and has been widely used in cars. ABS is mainly composed of ECU control unit, wheel speed sensor, brake pressure adjustment device and brake control circuit.
During the braking process, the ABS control unit continuously obtains the wheel speed signal from the wheel speed sensor and processes it to determine whether the wheel is about to be locked. The characteristic of ABS brakes is that when the wheel approaches the locking critical point, the brake cylinder pressure does not increase with the increase in the brake main cylinder pressure, but the pressure changes near the locking critical point. If it is judged that the wheel is not locked, the brake pressure regulating device will not participate in the work, and the braking force will continue to increase; if it is judged that a wheel is about to lock, the ECU will issue an instruction to the brake pressure regulating device to close the brake cylinder and brake wheel. The passage of the cylinder prevents the pressure on the brake wheel from increasing; if it is judged that the wheel is locked and slipping, an instruction is sent to the brake pressure regulating device to reduce the oil pressure of the brake wheel cylinder and reduce the braking force.
Electronic body stability system ESP
Some friends have also heard of the ESP electronic body stability system. What kind of system is this? In fact, the body electronic stability system is a patent of Bosch.
Other companies have also developed similar systems, such as BMW's DSC, Toyota's VSC, etc. Their functions are similar. Don't think that if they don't have this name, they won't have this function.
When a car drives or turns quickly, the lateral force generated will make the car unstable and prone to accidents, and the ESP system can prevent this situation before it happens. When an obstacle suddenly appears in front of the vehicle, the driver must turn left quickly. At this time, the steering sensor transmits this signal to the ESP control assembly. The sideslip sensor and lateral acceleration sensor send out a signal that the car is understeer, which means that the car is understeer. Will rush directly to the obstacle. At this time, the ESP system will instantly brake the rear wheels in an emergency, so that the reaction force required for steering can be generated, allowing the car to drive according to the steering intention. If you reverse-steer in the left lane after the car has turned, the car is in danger of oversteering, with too much torque to the right, causing the rear of the car to swing to the left. At this time, the ESP system will brake the left front wheel and the torque will be reduced, allowing the car to turn smoothly.
So ABS is basically standard equipment on cars at present. If it can be equipped with an electronic body stability system ESP (or a similar system), your car should drive better on the road. Safety. But it should be noted that ESP and EPS are two completely different things. If you are interested, let’s talk about the electric power steering system EPS next time.
This article comes from the author of Autohome Chejiahao and does not represent the views and positions of Autohome.