The document summarizes the key components and functions of braking systems for cars. It discusses two main types of brakes - drum brakes and disc brakes. Drum brakes work by using brake shoes that squeeze against a drum attached to the wheel to slow the car down. Disc brakes use brake pads that squeeze against a disc attached to the wheel. The document also explains how anti-lock braking systems (ABS) work to prevent wheels from locking up during braking on slippery surfaces. ABS uses sensors to monitor wheel speed and release/reapply brake pressure as needed to maintain traction and steering control during hard braking.

1. 1
The Brakes
The brakes function by absorbing in friction the energy possessed by the moving car. In so
doing they convert the energy into hear. There are tow types of brakes, the drum brake and the
disc brake. Either or both types may be fitted. But where both types are used it is usual for the disc
brakes to be fitted to the front wheels. In both drum and disc brakes, a hydraulic system applies
the brakes. The hydraulic system connects the brake pedal to the brake parts at each wheel.
1 、 Drum Brakes
The drum brake consists of a pair of semicircular brake shoes mounted on a fixed back plate
and situated inside a drum. This drum is fixed to the road wheel and rotates with it .One end of
each shoe is on a pivot and a spring holds the other end in contact with the piston of a hydraulic
cylinder.(In front brakes it is usual to use two hydraulic cylinders in order to equalize the pressures
exerted by the shoes, as shown in Fig.7.1)Each shoe is faced with material ,known as brake
lining ,which produces high frictional resistance.

2. 2
The hydraulic system comprises a master cylinder and the slave cylinders which are the
cylinders on the road wheels. The slave cylinders are connected to the master cylinder by tubing
and the whole system is filled with hydraulic fluid .A piston in the master cylinder is connected to
the brake pedal, so that when the driver depresses the pedal the fluid is forced out to each slave
cylinder and operates their pistons. The fluid pushes the pistons out of their cylinders. They ，in
turn, push against the inner ends of the brake shoes and force them against the drums in each
wheel. We say that the brakes are on. This friction of the shoes against the drums, which are fixed
to the road wheels, slows down or stops the car.
As the brake pedal is allowed to come up, the hydraulic fluid returns to its original position,
the pistons retract, and a spring attached to each brake shoe returns it also to its original position.
Free of the brake drum. Now we say that the brakes are off.(Fig.7.2)
The brakes may also be operated by mechanical linkages from the foot pedal and hand brake
lever. Common practice is to operate both front and rear brakes hydraulically with a secondary
mechanical system operating the rear brakes only from the hand lever. One of the great advantages
of hydraulic operation is that the system is self-balancing, which means that the same pressure is
automatically produced at all four brakes, whereas mechanical linkages have to be very carefully
adjusted for balance. Of course, if more pressure is put on one of the brakes than on the others
there is a danger that the car will skid.
The mechanical linkage on the rear brakes is a system of rods or cables connecting the
handbrake lever to the brake-shoe mechanism, which work entirely independently of the hydraulic
system. Drum brakes are prone to a reduction in the braking effort, known as “fade”, caused by
the overheating of the brake linings and the drum. Fade can affect all or only some of the brakes at
a time, but it is not permanent, and full efficiency returns as soon as the brakes have cooled down.
However, fading is unlikely to occur except after the brakes have been used repeatedly in slowing
the car from a high speed or after braking continuously down a steep hill. Descending such a hill,
it would have been preferable to use engine braking by changing down into a lower gear. Drum
brakes can be made less prone to fade by improving the cooling arrangements, by arranging for
more air to be deflected over them, for example.

3. 3
2 Disc Brakes
The disc brake consists of a steel disc with friction pads operated by slave hydraulic cylinders.
The steel disc is attached to the road wheel and rotates with it. Part of this steel disc is enclosed in
a caliper. This caliper contains two friction pads，one on each side of the disc, and two hydraulic
cylinders, one outside each pad. The pads are normally held apart by a spring, but when the driver
depresses the brake pedal, pistons from the hydraulic cylinders force the pads against the sides of
the disc. Because the disc is not enclosed all the way round, the heat generated when the brakes
are applied is dissipated very much more quickly than it is from brake shoes which are entirely
enclosed inside a drum. This means that disc brakes are less prone to fade than drum
brakes.(Fig.7.3)
3 、Anti-lock Brake System (ABS)
The function of an anti-lock, or anti-skid, braking system is to prevent the wheels from locking
under hard braking. Maximum braking force is obtained just before the wheels lock and skid.
Such anti-skid system ate useful on slippery surfaces, such as ice and snow, where the wheels may
lock easily. Locked wheels are dangerous because the car needs a much longer distance to stop.
Locked wheels also can cause the driver to lose control.

4. 4
The system uses a sensor that knows when one wheel (or a pair of wheel) is skidding.
(Fig.7.4) The sensor sends a signal to a computer, which signals a modulator valve. The modulator
connects into the hydraulic system and can momentarily release the brake pressure and prevent the
wheels from locking.(The pressure release is so fast that a driver is seldom aware of it.) pressure is
then reapplied until the sensor again senses that the wheel is about to lock up. Thus, this system
keeps the wheels as close to lock up as possible, without actually allowing the wheels to lock up
and skid. This is called incipient lock up. Maximum braking occurs at that point. If any part of the
system should fail to work, the system goes into a “fail-safe” mode. The brakes operate normally,
as they would on a car that is not equipped with ABS. Today , ABS is an optional or standard
feature that typically is found on expensive luxury cars and sports cars. In the future, ABS may be
available for all cars.
3.1、ABS overview
Anti-lock braking system is using the body of a rubber balloon, while hitting the brakes, will
give brake oil pressure, feeding through to the ABS body, using of the air in the middle of the air
layer to return the pressure, make wheels evade the locked points. When the wheel will arrive next
locked point, brake oil pressure makes balloon in a repeat function, so can function eight to thirty
times in one second equivalent constantly brake, relax, namely, similar to the "mechanical braking
". Therefore, ABS anti-lock braking system, can avoid the orientation losing control , the wheel’s
lateral sliding coming up and the wheels rubbing on one point with the ground without being
locked in the emergency brake，
so it can make the brake friction efficiency achieves ninety percent.
It also can reduce the braking consumption and prolong the brake wheel drum, disc and tire twice
with the service life of the vehicle in the ABS. On the dry tarmac road or on snowy or on rainy
days, the slippery performance reached 80% - 90%, 30% -10%, 15% - 20%. Ordinary braking
system on a wet road surface brake, or in the emergency brake, it’s easy for wheels to be locked
owing to the braking force exceeds the friction force of the tires and ground. In recent years, the
consumers of the vehicle emphasis on the safety , so most of the cars have ABS listed as standard.
Without ABS, emergency brake usually cause tire locked, then, the rolling friction becomes
sliding friction, as a result, braking force dramatically decreased. And if front wheels are locked
first, the vehicle will lose the steering ability; if the rear tires are first locked, the vehicle is easy to
slide laterally , so the direction becomes impossible to control. Through electronic or mechanical
control, the ABS system controls the braking fluid pressure at a fast speed to avoid the wheels

5. 5
locked. Insure the tire have the biggest braking force and the turning ability when braking, and
make the vehicle have the ability to evade the obstacles in emergency braking. With the rapid
development of the automotive industry, safety increasingly become the important basis of that
people choose and buy cars. At present the widespread adoption of holding brake system (ABS)
that people can fully meet the safety of requirements. Automobile brake prevent embrace system,
referred to as the ABS, is to improve the car an important device passive safety. Someone said
brake prevent embrace system is auto safety measures relay belts after another major progress.
Automobile braking system is the bus passengers safety is the most important relationship to one
of the second systems. With the rapid development of the automotive industry, automobile safety
for people appeals more and more attention. Automobile brake prevent embrace system, is another
major progress to improve the safety.
ABS braking system is controlled by automobile microcomputer, when braking, it can keep
the wheel rotating to help driver control vehicle parking safely. The anti-locked braking system
detects wheel speed by speed sensor, and then send the wheel speed signals to the microcomputer.
The microcomputer controls wheel slipping rate by increasing or decreasing the brake pressure
repeatedly according to the input wheel speed to keep wheel rotate. In braking process, keeping
wheel rotate not only ensures the ability of controlling driving direction, but also provides higher
brake force than the locked wheel in most circumstances, .
3.2、The working principle of ABS
It includes control devices and ABS warning lights, in different ABS system, the structure of
brake pressure adjusting device and working principle of electronic control devices are often
different, the internal structure and control logic of ABS system usually includes the wheel speed
sensors, brake pressure adjusting device, electronic identical and so on. In common ABS system,
each wheel is installed a rotational speed sensor on the wheel speed, input the signal to electronic
control device. The electronic control unit states monitoring and determination according to each
wheel speed sensors’ signal about each wheel movement, and has formed the corresponding
control instruction. Brake pressure adjusting device is mainly composed by pressure regulating
solenoid valves, electric pump composition and liquid container components compose an
independent whole, braking main cylinder and the cylinder brake wheel connected by the brake
pipe. Brake pressure adjusting device is controlled by the electronic unit to control all brake wheel
cylinders’ brake pressure. The working process of ABS can be divided into general braking, brake
pressure kept brake pressure decrease and brake pressure increase stage. In general braking phase,
ABS doesn't intervene brake pressure control, pressure regulating electromagnetic valve assembly
in various into liquid solenoid valves are no electricity and is open, each produced liquid are no
electricity and electromagnetic valve is in the closed position, electric pump also operates without
electricity, and brake main cylinder to each brake wheel cylinder brake lines are in communication
condition, and the brake wheel cylinder to liquid brake lines are in close condition, the brake
wheel cylinder brake pressure change with the output pressure brake master cylinder, the braking

6. 6
process at this time is completely the same with conventional braking system braking process.
In braking process, when the electronic control detects that the wheels primarily tend to
embrace dies according to the wheel speed sensors of the wheel speed signal input, ABS came into
the braking anti-lock process. For example, when the electronic control unit judges the front-right
wheel tends to embrace, the electronic control unit will make control scrape the dynamic pressure
front-right wheel failure into liquid solenoid valves electricity, make the fluid electromagnetic
valve closed, brake main cylinder output brake fluid no longer enter into brake wheel cylinder,
right now, right at the end of a fluid electromagnetic valve still energized and closed, the right
brake wheel cylinder brake fluid also won't outflow, the right brake wheel cylinder scraping
dynamic pressure stays certain, and other arms tend to be dead wheel with still brake pressure
braking main cylinder and the increase of output pressure increases; If the right brake wheel
cylinder brake pressure keep certain, the electronic control unit front-right wheel failure still tend
to judge lock, the electronic control unit and move out liquid solenoid valves also electrify into a
state of opening, the part right brake wheel cylinder brake wave will pass is open from fluid
electromagnetic valve flow back to liquid container, make right brake wheel cylinder brake
pressure holding diminishes quickly front-right wheel failure will start to eliminate death trend,
with the right brake wheel cylinder brake pressure decreases, the front-right wheel under the
action of inertia force will speed up gradually; accelerate gradually When the electronic control
device determines the lock front-right wheel failure to completely eliminate according to the
wheel speed sensors input signal, the electronic control unit makes right into the fluid
electromagnetic valve and a liquid solenoid valves are without electricity, make into fluid
electromagnetic valve to open, use liquid into closed electromagnetic valve, also make electric
pump operation, energized to brake wheel cylinder pump brake fluid, output by brake main
cylinder brake fluid electromagnetic valve into the right brake wheel cylinders, make right brake
wheel cylinder brake pressure increased rapidly, opening up a front-right wheel failure and slow
rotation. ABS control the sliding rate of wheels which tend to be locked through holding the brake
force of wheels that tends to be locked repeatedly ,adhesion coefficient is in peak within the scope
of the sliding rate, until the bus speed reduced to very low or brake main cylinder pressure no
longer tend to be locked. Brake pressure to adjust cycle of frequency can reach 3 ~ 20HZ. In the
ABS each has on the fluid and the fluid electromagnetic valve in corresponding at each brake
wheel cylinder, but by the electronic control device to control, therefore, respectively, the brake
wheel cylinder brake pressure can be independently adjustment so that four wheels are not occur
braking lock phenomenon. Although various ABS structure form and working process is not
exactly the same, are based on the brake holding pressure adaptive cycle adjustment of wheel
which tends to be locked, to prevent the controlled wheels holding in death occurred when brake.

7. 7
3.3、 ABS function
The braking performance is one of main auto performance, it is also related to the security of
driving. Evaluate the braking performance of a car , the basic index is brake acceleration ,braking
distance and direction of braking time and braking stability.
Braking stability points to that the vehicle can still be specified in the direction of auto brake when
driving in the direction of track. If cars proceed high-speed brake (especially when emergency
brake) and make the wheel fully embrace die, it is very dangerous. If front wheel is locked, it will
make cars lose steering ability. If the rear wheel is locked, it will appear to swing tail or switching
(running deviation, sideslip) especially in the road is all wet slippery case, it will cause the traffic
safety great harm. Automobile braking force depends on the brake friction, but the braking force
which can make the car brake to slow, and also restricted by ground adhesion coefficient. When
brake produces braking force increases to certain value, the tires will appear slipping on the
ground. Its sliding rate
δ = (Vt - Va) / Vt x 100%
δ: delta - sliding rate;
Vt - theoretical speed of the car;
Va - the actual speed of the car.
According to the experiments confirm, when wheel sliding rate delta range from 15% to 20% , the
ground adhesion coefficient reach maximum, therefore, in order to get the best braking effect, we
must control the slip rate in 15% ~ 20% range.
ABS function will namely decrease the brake force when the wheels will embrace dies, , and
when the wheel will not hold died and increase braking force, so repeated action, braking effect is
the best.
3.4、The problems needing attention when ABS is used
(1) After replace brake or replace hydraulic brake system components, it should exhaust the air
in the brake pipe, lest affect braking system work normally.
(2)The car equipped with ABS should be replaced every year. Otherwise, brake fluid
hygroscopic is very strong, water will not only reduce the boiling point to make it easy to produce
corrosion, and still can cause braking performance recession.
(3) To examine ABS braking system should pull power firstly.
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