Automotive Basics (3) What is the role of ABS?

In the last issue of "Automobile Basics", we divided the chassis into four major systems according to their functions: driving system, transmission system, brake system and steering system (click here for the specific introduction of the four systems: "Automobile Basic Knowledge (2) The Structure of Automobile Chassis"). So in this issue, we will focus on the braking system of the car.

● How to evaluate the braking system

Before introducing the braking system, let's clarify a question: how to evaluate the braking system of a car? In other words, what makes a good braking system?

Many people will say, "It must be the braking distance", while some old drivers will say, "The braking distance is short, but the braking is stable", and the cardholders who often walk on mountain roads will say, "The heat dissipation performance is also very important." ”

Therefore, to truly evaluate the braking performance of a car, it is necessary to design indicators in three dimensions:

1) Braking efficiency.
That is, braking force and braking distance;

2) Stability of braking efficiency
Brake heating and water will cause performance degradation, so it is necessary to evaluate the thermal decay resistance and water decay resistance of the brake;

3) Stability of the direction of the car when braking
In other words, braking not only stops the car, but also allows the car to stop in the place specified by the driver. This does not allow the vehicle to slip sideways (rear wheel lockup), lose steering ability (front wheel lock), etc. - how many accidents are caused by the loss of control of the vehicle during braking.

● Disc brakes aren't all advantages

In recent years, with the development of high-end trucks, "disc brake standard" has become a slogan that many people are shouting. But are disc brakes really that good?

To paraphrase the evaluation criteria of the braking system we mentioned earlier, the advantage of disc brakes is that they have good drainage and heat dissipation performance, which means that they have better "braking efficiency and stability" than drum brakes.

But from the structure, we can see that the friction area of the disc brake is not as large as that of the drum brake, so its braking force is not as good as the drum brake. That's why trucks have been using drum brakes for a long time.

Increasing the braking power of the disc brakes requires increasing the size of the brake discs and calipers (such as the large calipers that stand out on sports cars), but this will undoubtedly increase the cost of the braking system significantly.

Therefore, for trucks that often walk on flat roads, the stability of braking efficiency is not so important, but the braking force is more critical, so drum brakes are obviously a better choice; For trucks running in mountainous areas, it is very dangerous to overheat the brakes caused by long downhills, so the thermal decay resistance of the disc brakes is more advantageous.

● The role of ABS is not to reduce the braking distance

It is often seen that everyone is discussing whether the braking distance will increase or decrease after installing ABS. In fact, cardholders who discuss this issue may not understand the purpose of ABS design.

Let's go back to the question raised at the beginning: evaluating the braking ability of a car can be divided into three dimensions. The simplest and most effective way to reduce the braking distance is to increase the braking power of the vehicle – larger brake drums, wider tires. The emergence of ABS is to solve the third problem: to improve the directional stability of the vehicle when braking - its name translated into Chinese is actually "anti-lock braking system".

Many veteran drivers know that the front wheels of a car are locked, which will make the vehicle lose its ability to steer; And if the rear wheels are locked, there will be a tail flick (the brother with a high level will drift according to this principle). The best braking state is to let the wheels in a state of "rolling and slipping", at this time the braking distance is short, and the car will not lose control, so before there is no ABS, the old driver will use the point brake to let the vehicle enter this state.

To sum up: if the brakes are lightly pressed and the wheels are not locked at all, the braking force is not strong enough; If the brakes are pressed too hard and the wheels are completely locked, the direction will easily lose control, so it is necessary to grasp the degree in the middle. As we all know, the response speed of the machine must be much faster than that of people, so we use sensors and small motors to help people operate the brakes, and find the one in the middle that can take into account the braking distance and braking stability.

The specific working principle of ABS is relatively boring, and some calculation formulas are designed, and it is recommended that interested cardholders Baidu the concept of "slip rate".

All in all, ABS is born to ensure braking stability: when the ground is slippery, ABS can help your vehicle not to skidding to a certain extent, but the braking distance will not be significantly improved, and the slippery road should still slow down and brake in advance. And: Preventing side slippage only works to a certain extent.

● ABS can derive a lot of functions

As mentioned earlier, ABS is to keep the wheels in a state of "rolling and slipping". So if you upgrade this system, it can actually achieve more functions:

ASR drive anti-skid function: ABS sensors can know the state of the wheels, and then the engine ECU can limit the engine speed after knowing this state to prevent the wheels from slipping when starting and accelerating

ESP body stability system: ABS controls the braking force and ASR controls the driving force, which can be combined to allow the vehicle to control the body attitude when steering and prevent the wheels from slipping.

In addition, there are TCS traction control system, EBA electronic control brake assist system, etc., which are all vehicle safety systems derived from the basic principles and basic hardware of ABS. The overall goal is to keep the wheels in reasonable condition and the driver to have better control over the vehicle.