Brake Systems

1. Braking Theory
Friction of objects
When an object is placed on any surface and the object is pulled, a force is needed for the object to start
moving.
This force that is required for an object to move while sliding, is called friction force. Friction force is a constant
determined between the 2 objects, multiplied by the load applied to the object. It is expressed with the
following formula.

F = μW F: Friction force (kg)

μ: Coefficient of friction
W: Weight (kg)

The constant determined between the 2 objects is called the coefficient of friction and is expressed with μ (mu).
Typical values are shown below.
2. Tire Slip Ratio

TIRE SLIP RATIO

When a vehicle is driven with the brakes applied, the friction between the lining and the drum and the friction
between the tires and the road surface must be considered.
When the vehicle is traveling while the brakes are applied, the condition between the lining, drum, tires and road
surface may be as follows.
1. The tires rotate without slipping.
2. The tires rotate while slipping on the road surface.
3. The tires fully slip on the road surface without rotating.
The tire slip ratio when a vehicle is driven while the brakes are applied is defined as in the picture below.

In other words, when the tires rotate without slipping at all, the value in the brackets in the formula above is equal
to the vehicle speed, so the slip ratio is 0%. When the tire rotation stops and the vehicle slips, the value in the
brackets is 0, so the slip ratio is 100%.
3. Friction between Tires and Road Surface

FRICTION BETWEEN TIRES and ROAD SURFACE

When tires rotate while slipping, the friction force acts in the opposite direction to the driving direction.
Under constant road surface conditions, the coefficient of friction between the tires and road surface changes
greatly according to the slip ratio. The picture below shows an example of test results. It shows that the coefficient
of friction increases dramatically until the slip ratio reaches approximately 20%, the value is highest between 20
and 30%, and then it starts to decrease again.
When the slip ratio is 100%, in other words when the tires are locked, the coefficient of friction is usually
significantly below its maximum value.
It follows that to brake the vehicle most effectively, the ideal point to use is when the slip ratio is 20 to 30%.

Test data (μ b-s curve, concrete road surface)

4. Categories of Brakes

Items printed in red, are specific for trucks

Control Method of a Main Brake

The control method of a main brake has been categorized three types.
Hydraulic brake … Most passenger cars are equipped with this because of its good response and simple structure.

Air Over Hydraulic Brake … This system has both merits of air brake and hydraulic brake.
Pedal side: Reduce operational force by using air
Brake side: Good response by using brake fluid

Full Air Brake … Previously compressed air of high pressure is kept in an air tank and used at the time of braking.
Great braking force is obtained with less applying force to the pedal.
5. Drum Brakes
Drum Brakes
A drum brake has a structure where a shoe is pushed against a rotating drum. Because it has an action called a "self
servo action", it can provide a stronger braking force. For this reason, it is widely used in trucks and buses. In terms
of cost as well, it is less expensive to manufacture than a disc brake. Further, because it can be made even more
compact and lightweight than a disc brake, it is often used as the rear wheel brake in passenger vehicles.
(1) Characteristics of drum brake
- When the brake shoe is pushed against the rotating drum, pulling (dragging force) is generated in the drum and it
produces a strong braking force. This refers to the self servo action.
- Because the drum brake has a sealed structure, it retains more heat than a disc brake, and the brake performance
reduces when used frequently.
The brake shoe works as a "Leading shoe or Trailing shoe". The "Leading shoe or Trailing shoe", switches function
depending on the direction of the wheel rotation (Forward or Reverse).
Leading shoe
The Leading shoe provides a stronger brake force than the trailing shoe. Because, the leading shoe is for forward
drum rotation and tends to become wedged into the brake drum when brakes are applied.
Trailing shoe
The Trailing shoe provides a weaker braking force compare to the leading shoe. Because, the trailing shoe works
against the drum rotation, it tends to push into the wheel cylinder by the drum rotation.
5.1. Brake Components

(2) Drum brake components

1) Brake shoe
The brake shoe is fixed to the inner side of the brake drum and is pushed against the rotating drum to generate
braking force.
The brake shoe usually has a T-shaped cross section and is made of materials such as rigid rolled steel, cast steel,
cast iron or welded steel sheets. It must be curved so that the centers of the shoe and the drum are fully aligned.
In general, a cast shoe or plate shoe is used, as shown in the below picture. A fixed type or floating type is used as
the shoe support method. The floating type is often used in passenger vehicles.
The characteristic of the floating type is that the end surface of the shoe is curved and makes contact with the
anchor, and because it can slide at the anchor, the drum contact is even. However, because the shoe position is
difficult to hold, drag occurs easily. A return spring is used as a counterbalance to prevent the self servo force acting
on the brake linings.
5.2. Brake Drum and Linings
2) Brake drum
Because the braking force is generated by pushing the shoe against the inner surface of the brake drum, cast iron
with a high coefficient of friction and high wear resistance is commonly used for the brake drum. Because the brake
drum holds heat during braking, a strong drum with good cooling performance is required for stable brake use.
Various types are available for achieving this.

Types of Brake Drums

3) Brake lining
The brake lining is one of the most important parts affecting brake performance. Materials with a friction
performance suitable for the braking conditions must be used.
Required performance for the brake lining:
•An appropriate coefficient of friction
•Little change in friction coefficient with respect to temperature
•High mechanical strength and low wear
•Fast recovery from water or oil infiltration
•No unpleasant odors or sounds during braking
5.3. Lining Types
Molded lining
Currently, this is the most commonly used lining, where the short fibers of different types of friction materials, are
pressured formed using a binder. By varying the components, a wide range of characteristics can be generated.
Linings using natural rubber or synthetic rubber as the binder are called rubber molds, and those using synthetic
resin are called resin molds. Due to issues regarding the heat resistance and wear resistance of rubber molds, resin
molds that address these issues are now most commonly used.
Also, some resin molds contain metal powder or fine metal wire. These are called semi metallic linings and because
they have good heat conduction, they have the advantage of not being prone to the brake fade phenomenon.

Metallic lining
Metallic linings are made from a centered alloy of copper and iron. They have good resistance to fade and wear, but
because they generate brake noise easily and are expensive, they are not usually used in vehicles.
5.4. Drum Brake Types
Drum Brake Types
The following types of internal expanding drum brakes are available, which differ according to how the brake shoe
and wheel cylinder are installed.

Leading-trailing type

Advantages:
A simple structure and reliable operation. The same braking force is generated whether traveling forward or in
reverse.

Disadvantages:
The braking force is slightly less than other brake types with the same brake drum diameter and same lining area.

← Forward
2 leading type

Advantages:
Reliable operation. A large braking force is generated when traveling forward.

Disadvantages:
When reversing it has 2 trailing shoes, which reduces the braking force to approximately 1/3 of the force when
traveling forward. Also, because 2 wheel cylinders are used in the target area, it is relatively expensive.

← Forward
Dual 2 leading type

Advantages:
A large braking force is generated when traveling both forward and in reverse.

Disadvantages:
Because 2 double acting wheel cylinders are used, it has cost disadvantages. Also, if one of the piston caps of
the wheel cylinder wears and leaks oil, the operation of the other piston will also become defective.
Uni servo type

Advantages:
A characteristic of servo brakes is that because the expansion force assisted by the primary shoe is used for the
expansion of the secondary shoe, the braking force of the secondary shoe increases. An extremely high braking
force can be generated even with a limited brake drum diameter and lining area.

Disadvantages:
Although a characteristic of servo brakes is their high braking force, this also tends to make locking more likely.
Because the braking effect is sudden and strong, a slight time difference between the locking of the left and right
wheels can cause spinning or tail swing. Also, the braking force when reversing is significantly lower, dropping to
1/5 to 1/7 of the force when traveling forward.
Duo servo type

Advantages:
This type incorporates the advantages of the uni servo while also generating approximately the same braking
effect when reversing as when traveling forward.

Disadvantages:
The same as the uni servo, except for the improved braking force when reversing.
Z cam type mechanical wheel brake
The brake lining is pushed against the brake drum by the rotational motion of the brake cam. The brake shoe rises
from the brake shoe support plate. As such, the brake lining is installed around the same center as the brake drum,
to prevent it from wearing.
The brake mechanism is completely sealed and coated with grease before being set on the cam housing. All
components are completely protected from dust and moisture, which minimizes part damage and helps reduce
maintenance costs.
S cam type mechanical wheel brake

This is a leading-trailing shoe type of wheel brake that is fixed with anchor pins.
When the cam positioned in the center top rotates, the brake shoe opens, generating braking force through friction
with the drum.

Wedge type wheel brake

This is a leading-trailing shoe type of wheel brake that is fixed with anchor pins.
When the expander positioned in the center opens, the brake shoe opens, generating braking force through friction
with the drum.
6. Disc Brakes
Disc Brakes
In addition to having high braking force, brakes must always deliver the braking force in a stable
condition. Because drum brakes have a self servo action, although the braking force is high it can also
fluctuate easily, and the inside of the drum is prone to heating. This means that the fade
phenomenon occurs easily.
These drum brake disadvantages can be resolved by using a disc brake. A disc brake is a circular disc
that rotates together with the tire, and which is pressed strongly from both sides to stop the disc
rotation.
Characteristics of disc brake
•Stable braking force is generated even when braking at high speed or when braking repeatedly.
•Because the rotating disc is exposed to the air, heat dissipation from the friction surface is good and
the fade phenomenon does not occur easily.
•Because there is almost no self servo action, there is no braking force fluctuation and almost no
difference in braking between left and right, resulting in excellent directional stability.
•Because the rotating disc is exposed to the air, it recovers quickly even if it becomes wet.
•The pads can be easily replaced.
Disc brake components

1) Disc
The disc is installed onto the hub and rotates together with the tire. Usually, cast iron with a high coefficient of
friction and high wear resistance is used for the disc. Usually the discs are hollow and cooling fins are used to
improve the fade
6.1. Disc Brakes

Caliper
Because the caliper is subjected to the braking reaction force and a strong force when the pads are pushed against
the disc, it is manufactured to be strong using cast iron. The shape of the caliper is a double bridge with an open
center. This has advantages such as enabling a large pad to be used, making pad replacement easy and ensuring
high rigidity. The fixed caliper type is installed directly onto the knuckle. The floating caliper type is installed onto
the axle via a support bracket and enables the caliper to slide during braking.

Pad
In a disc brake, the pads are equivalent to the lining in a drum brake. Because there is no self servo action in a disc
brake, the disc must be pressed with greater force, which means the pads are used under severe conditions. For
this reason, semi metallic pads are used that have excellent heat, wear and fade resistance.
7. Hydraulic Brakes

Principle of hydraulic brakes

Currently, hydraulic brakes are widely used. These brakes are based on Pascal's principle. The pedal force on the
brake pedal is converted into hydraulic pressure by the master cylinder and then transmitted equally to each wheel
cylinder by components such as the brake pipe. Hydraulic brakes generates stable braking force with this hydraulic
pressure.
7.1. Pascal's Principle

Pascal's principle is defined as "Pressure added to any point in an enclosed fluid is transmitted with an equal
strength to all points in the fluid. "
If the applied force is constant, the output is in proportion to the area of the piston on the output side and in
inverse proportion to the area of the piston on the input side. This means when 20 kg is applied to piston A that has
an area of 10 cm2 as shown in the below picture, the pressure is 20 kg/10 cm2 = 2kg/cm2, and a force of 10 cm2 ×
2 kg/cm2 = 20 kg is generated on piston B that has an area of 10 cm2. In other words, when the piston areas of A
and B are equal, the input and output are the same.
When the area of piston A is 5 cm2 (A'), pressure of 20 kg/5 cm2 = 4 kg/cm2 is generated, and a force of 10 cm2 × 4
kg/cm2 = 40 kg is generated on piston B'.
7.2. Leverage Principle

The brake pedal transmits the pedal force to the master cylinder. Using the principle of leverage, it can transmit a
large force to the master cylinder even from a small pedal force.
A single bar is used with a fulcrum supporting the bar at a certain point. When force is added to one end, force acts
on the other end in proportion to the distance from the fulcrum. The same is true for the movement amount of the
power point and the application point.
In the Figure, when force F1 is applied to power point A and it is moved by distance X1, force F2 that acts on
application point B and movement amount X2 are expressed with the following formulas.
8. Master Cylinder

Structure
The picture shows a typical master cylinder structure, consisting of a reservoir that stores the brake fluid and a
cylinder section. The cylinder has a feed port for injecting the brake fluid and a return port for returning the brake
fluid to the reservoir. A check valve is used at the brake fluid outlet. This valve acts to always maintain an
appropriate residual pressure within the pipe system.
8.1. Master Cylinder
Operation
A secondary cup and a piston cup are installed on the piston, and they are pushed to the left by the return spring as
shown in the picture below. When the brake pedal is applied, the piston is pushed by the push rod and moves to
the right. After the primary cup blocks the return port, the brake fluid inside the cylinder is pressurized, the check
valve opens and hydraulic pressure is applied to the wheel cylinder.
Next, when the brake pedal is released, the piston returns quickly to the left due to the spring force of the return
spring. The brake fluid in the pipe pushes up the check valve (the valve is closed) and returns to the cylinder. When
the piston fully returns to its original position, the brake fluid returns from the return port to the reservoir and the
hydraulic pressure inside the pipe decreases.
Regarding the passages that return the brake fluid from inside the pipe to the tank, the piston head has several
small holes which are closed by the spacer and primary cup when the piston moves to the right, and are opened
when the piston moves to the left.
If the brake pedal is applied and released quickly, the piston returns quickly to the left due to the spring force of the
return spring. At this time, because the hydraulic pressure inside the cylinder on the right side is lower than that on
the left side, the brake fluid flows into the right side via the holes on the piston head. When the piston moves
further to the left and returns to its original position, the return port opens and the excess brake fluid returned
from the wheel cylinder and returns to the reservoir via the return port.
8.2. Check Valve

Check Valve
Regarding the operation of the check valve section, the check valve is installed at the end of the return spring inside
the master cylinder as shown in the picture. If the pressure inside the cylinder drops, the check valve as a whole is
pushed back by the pressure inside the pipe, and the brake fluid returns to the cylinder. However, if the pressure
inside the pipe continues to be overcome by the force of the return spring, the valve closes and a certain amount of
residual pressure remains inside the pipe.
8.3. Tandem Master Cylinder

Tandem Master Cylinder

The Tandem Master Cylinder has two independent hydraulic circuits. In case of fault in one circuit, the other circuit
can operate. The structure is indicated in the picture below.
Both the Primary piston and the Secondary piston is installed in the same cylinder. The Secondary piston is fixed
with a return spring from the front and rear. Each return spring has a different preset spring force. Each chamber
has an inlet port and a return port. In addition, the Primary and Secondary inlet and return port shares the same
reservoir.
In case of a static condition, the primary piston cup and secondary piston cup must be set between the return port
and inlet port. Therefore a stopper bolt is installed on the cylinder body. The operation of tandem master cylinder is
similar to single type.
8.4. Tandem Master Cylinder Continue

In case of a rear brake leak:

The "primary piston return spring" will compress, allowing the primary piston to push directly onto the secondary
piston. Only the front brakes will operate.

In case of a front brake leak:

The "secondary piston return spring" will compress and the primary piston is pushed by the brake pedal, so only
the rear brakes will operate.
9. Integral Type Brake booster (Vacuum Brake Booster)

Integral Type Brake booster (Vacuum brake booster)

Different point of separate type brake booster and integral type brake booster is position of booster function and
operation of servo unit. The servo unit operation has been operated by mechanical action in the integral type
brake booster.
Brake pedal operation flow is as per description below:

＜Route of brake pedal force＞

Valve operation rod → Valve plunger → Reaction disk →Push rod (Diaphragm)→ Master cylinder piston
10. Load Sensing Proportional Valve

Load Sensing Proportional Valve (LSPV)

The illustration shows the load sensing proportioning valve (hereinafter known as LSPV).
The LSPV is a device that prevents early locking of the rear wheels. However, it controls the braking force according
to the carry load.
As the suspension spring installed between the axle and frame is pressed and contracted according to the carry
load, the carry load can be detected by the displacement of the frame and axle.
As shown in the picture, the LSPV is mounted on the rear frame, and is connected to the load detecting spring and
load detecting link mounted on the rear axle housing.
The LSPV unit consists of the valve, piston, plunger, spring, and other components.
By changing the hydraulic pressure control starting point according to the carry load, the LSPV controls the braking
force of the rear brake according to the carry load and deceleration speed.
11. Air Over Hydraulic Brakes

Air Over Hydraulic Brakes

Air Over Hydraulic Brakes are an ideal system that combines the advantages of both hydraulic brakes and full air
brakes. The compressed air stored in the air tank is controlled with a dual brake valve and fed to the 2 air master
relay valves. Hydraulic pressure is generated according to the compressed air pressure acting on the air master
relay valve, operating the wheel cylinders and generating braking force.
11.1. Diagram of Air Over Hydraulic Brakes

Diagram of Air Over Hydraulic Brakes

This section shows the overall layout of brake control.

To increase safety, the layout of the brakes in the air and hydraulic system is fully separated into 2 systems, for the
front axle and rear axle. The air tank, air master and brake fluid tank are also separated with 2 of each installed.
This ensures safety because if either system fails, the other system will continue to operate.
12. Full Air Brakes

Full Air Brakes

The full air brake is a brake system that provides larger braking force than hydraulic brakes.
When the brake is applied, air is sent via the relay valve to the brake chamber, which directly operates the brake. A
cam and a wedge-shaped device is installed onto the rod of the chamber, and the brake shoe is pushed and opened
via these parts to generate the braking force.
12.1. Diagram of Full Air Brakes

Diagram of Full Air Brakes

The layout of the brakes in the air and hydraulic system is fully separated into 2 systems, for the front axle and rear
axle, the same as in the combined air type. This type consists of an air tank, relay valve, chamber and S cam or
expander. The signal pressure from the brake valve is received by the relay valve, and the air pressure is fed from
the air tank to the chamber via the relay valve.