BRAKE SYSTEM

Automotive Brakes
• Provide a means of using friction to
either slow, stop, or hold the
wheels of a vehicle
• When a car is moving, it has
kinetic energy (inertia)
• To stop the vehicle, the brakes
convert mechanical (moving)
energy into heat
 Brake Operation
• When the driver pushes on the
brake pedal, lever action pushes a
rod into the brake booster and
master cylinder
• The pressure developed in the
master cylinder forces fluid
through the brake lines to the
wheel brake assemblies
• The brake assemblies use this
pressure to cause friction for
braking
 Drum and Disc Brakes
• Two common
types of brake
assemblies:
• Disc brakes
– often used on
the front
wheels
• Drum brakes
– often used on
the rear wheels
 HYDRAULIC BRAKES
• Automotive brakes use a hydraulic system
• Hydraulic brakes use confined brake fluid
to transfer brake pedal motion and
pressure to each of the wheel brake
assemblies
• The master cylinder acts as the pumping
piston that supplies system pressure
• The wheel cylinder acts as the power
piston, moving the friction linings into
contact with the rotating drums or discs
Hydraulic Brake Action
 Components of Brake
system

A lever to increase the force applied

to the master cylinder piston
 Master Cylinder

• Foot-operated pump that forces

fluid to the brake lines and wheel
cylinders
• Develops pressure to apply the
brakes
• Equalizes pressure required for
braking
• Keeps the system full of fluid as
the linings wear
• May maintain a slight pressure to
keep contaminants from entering
 Master Cylinder

Brakes applied Brakes released

Master Cylinder
Components
 Master Cylinder
Components
• piston
– used to pressurize the system
– when they are pushed forward, they
trap fluid, building pressure
• Intake port
– allows fluid to enter the rear of the
cylinder as the piston slides forward
– fluid flows from the reservoir, into the
area behind the piston and cup
 Master Cylinder
Components
• Compensating port
– releases pressure when the piston
returns to the released position
– fluid can flow back into the reservoir
through the compensating port
• Residual pressure valves
– maintain residual fluid pressure of
approximately 10 psi (69 kPa) to help
keep contaminants out of the system
 Master Cylinder
Components
• Rubber boot
– prevents dust, dirt, and moisture from
entering the back of the master
cylinder
• Reservoir
– stores an extra supply of brake fluid
– cast as part of the housing or added
as a removable plastic part
 Dual Master Cylinder
• Uses two separate hydraulic
pistons and two fluid reservoirs
• Each piston operates a hydraulic
circuit that controls two wheel
brake assemblies
• If there is a leak in one of the
hydraulic circuits, the other circuit
can still provide braking action on
two wheels
Dual Master Cylinder
 Dual Master Cylinder
(Normal Operation)

Both pistons produce pressure to all

four wheel brake assemblies
 Dual Master Cylinder
(Rear Brake Circuit Leak)

Primary piston pushes on the

secondary piston, two wheel brake
assemblies still work
 Dual Master Cylinder
(Front Brake Circuit Leak)

Secondary piston slides forward, primary

piston operates two wheel brakes normally
 Power Brakes
• Use a booster integrated with
either vacuum or hydraulic
pressure to assist brake pedal
application
• The booster is located between the
brake pedal linkage and the
master cylinder
• When the driver presses on the
brake pedal, the brake booster
helps push on the master cylinder
pistons
 Vacuum Booster
• Uses vacuum produced in the
engine intake manifold or by a
separate pump to apply the
hydraulic brake system
• Consists of a housing that encloses
a diaphragm
• When vacuum is applied to one
side of the booster, the diaphragm
moves toward the low-pressure
area
Vacuum Booster Operation
Vacuum Brake Booster
 Hydraulic Booster
• Uses power steering pump
pressure to help the driver apply
the brake pedal
• Known as hydro-boost or hydra-
booster
• Commonly used on vehicles with
diesel engines
– diesel engines do not produce a
usable amount of intake manifold
vacuum
Hydraulic Booster
 Hydraulic Booster

When the pedal pushes on the unit,

the spool valve allows more oil to
enter the pressure chamber,
pushing on the power piston
 Brake Fluid

• Specially blended hydraulic fluid

that transfers pressure to the
wheel brake assemblies
• Rated by the SAE and DOT
– SAE (Society of Automotive
Engineers)
– DOT (Department of Transportation)
 Brake Fluid
• Brake fluid must have the following
characteristics:
– correct viscosity at all temperatures
– high boiling point
– noncorrosive
– water tolerant
– lubricates components
– low freezing point
 Brake Lines and Hoses
• Transfer fluid pressure from the
master cylinder to the wheel brake
assemblies
• Lines
– made of double wall steel tubing
– tubing ends usually have double-lap
flares
• Hoses
– made of reinforced rubber
– used where flexing action is
Brake Lines and Hoses
 Disc Brake Pads

• Steel plates to which linings are

riveted
• Linings
– made of heat-resistant organic or
semimetallic friction material
– semimetallic linings withstand higher
temperatures without losing their
frictional properties
 Brake Disc (Rotor)
• Uses friction from the brake pads
to slow or stop wheel rotation
• Normally made of cast iron
• Constructed as part of the hub, or
a separate unit
• May be solid, or a ventilated rib
construction
 Drum Brake Assembly

A large drum surrounds the brake

shoes and the hydraulic wheel
 Backing Plate
• Holds the springs, wheel cylinder,
and other parts inside the drum
• Helps keep road dirt and water off
the brakes
• Bolts to the axle housing or the
spindle support
 Wheel Cylinder Assembly

Uses master cylinder pressure to

force the brake shoes out against
the drum
Wheel Cylinder Components

• Boots
– keep road dirt and water out of the
cylinder
• Pistons
– metal or plastic plungers that transfer
force out of the cylinder to the brake
shoes
• Cups
– rubber seals that keep fluid from
leaking past the pistons
Wheel Cylinder Components
• Springs
– hold the rubber cups against the
pistons when the wheel cylinder
assembly is not pressurized
– metal expanders may be used on the
ends
• Bleeder screw
– provides a means of removing air
from the brake system
 Brake Shoes
• Rub against the revolving brake
drum to produce braking action
• Made by fastening organic friction
material onto a metal shoe
– rivets or bonding agents may be used
 Brake Shoes
• Retracting springs
– pull the brake shoes away from the
brake drums when the brake pedal is
released
• Hold-down springs
– hold the brake shoes against the
backing plate when the brakes are in
the released position
Drum Brake Assembly
Traction and Stability
Control Systems
 Traction Control Systems
• Designed to prevent the vehicle’s
wheels from spinning and losing
traction under hard acceleration
• Most systems work with the anti-lock
brake system to cycle hydraulic
pressure to the wheel spinning the
fastest
 Traction Control Systems

• The control module is capable of

applying only one wheel brake at a
time
• Some systems also reduce the
engine’s power output to reduce
slipping
• The indicator light is illuminated
anytime the traction control
system is activated
– warns the driver that the tires are
 Stability Control Systems
• Advanced system that reduces tire
spin upon acceleration and
prevents tire skid when cornering
too quickly
• Uses more input signals from
various sensors to provide greater
control under severe cornering,
braking, and acceleration
conditions
 Understeer

• If understeer is detected, the

control module will apply braking
force to the rear wheel on the
opposite side of the vehicle
– this brings the front of the vehicle
back under control for making the
turn
Understeer
 Oversteer

• If oversteer is detected, the control

module will apply braking force to
the outside front wheel
– this prevents the rear of the vehicle
from sliding sideways in a turn
Oversteer
 Stability Control System
Inputs
• Wheel speed sensors
– detect individual wheel speeds
• Steering angle sensor
– measures how sharply the steering
wheel is rotated
• Lateral acceleration sensor
– measures how much side force is
generated by a turn
 Stability Control System
Inputs
• Yaw sensor
– measures the direction of the thrust
generated by vehicle movement
• Throttle position sensor
– measures the demand for power
• Brake pressure sensor
– measures the amount of brake
system hydraulic pressure generated
by the driver
Parking Brakes
 Parking Brake Operation

• When the hand lever is activated,

it pulls a steel cable that runs
through a housing
• The cable pulls on a lever inside
the drum or disc brake assembly,
forcing the brake linings against
the rear drums or discs
 Parking Brake
• The parking brake system uses
cables or rods to mechanically
apply the rear brakes
• Provides a system for holding the
wheels when the vehicle is parked,
or stopping the vehicle during
complete hydraulic brake system
failure
Air brake systems
 Braking distance
• It refers to the distance a vehicle will
travel from the point where its brakes are
fully applied to when it comes to a
complete stop.
• It is affected by the original speed of the
vehicle, the type of brake system in use,
the reaction time of the driver/rider and
the coefficient of friction between its tires
and the road surface.
• Note that these theoretical formulas do
not take account of the driver's reaction
time
• Braking distance is calculated by:
dMT = 0.039 V2⁄a
dMT = braking distance, m
V = design speed, km/h
a = deceleration rate, m/s^2
• Actual braking distances are affected
by the vehicle type and condition,
the incline of the road, the available
traction, and numerous other factors.
 Stopping sight distance
• It is the distance a vehicle driver
needs to be able to see in order have
room to stop before colliding with
something in the roadway, such as a
pedestrian in a crosswalk, a stopped
vehicle, or road debris.
• Insufficient sight distance can
adversely affect the safety or
operations of a roadway or
intersection.
 Sight distance needed
• Stopping sight distance is the distance traveled
during the two phases of stopping a vehicle:
perception-reaction time (PRT), and maneuver time
(MT).
• Perception-reaction time is the time it takes for a
road user to realize that a reaction is needed due
to a road condition, decided what maneuver is
appropriate (in this case, stopping the vehicle),
and start the maneuver (taking the foot off the
accelerator and depressing the brake pedal).
• Maneuver time is the time it takes to complete the
maneuver (decelerating and coming to a stop).
• The distance driven during perception-reaction
time and maneuver time is the sight distance
needed.
 … .
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• The values of stopping sight distance used
in design represent a near worst-case
situation.
• For design, a conservative distance is
needed to allow a vehicle traveling at
design speed to stop before reaching a
stationary object in its path.
• A generous amount of time is given for the
perception-reaction process, and a fairly
low rate of deceleration is used.
• The design sight distance allows a below-
average driver to stop in time to avoid a
collision in most cases.
Driver perception/reaction distance is
calculated by:
dPRT = 0.278 V×t (Metric)
Where:
dPRT = driver perception-reaction distance, m
V = design speed, km/h
t = brake reaction time, in seconds
• Based on the results of many studies, 2.5 seconds
has been chosen for a perception-reaction time.
• This time will accommodate approximately 90
percent of all drivers when confronted with simple
to moderately complex highway situations.
• Greater reaction time should be allowed in
situations that are more complex.
• Stopping Sight Distance (SSD) is
the sum of reaction distance and
braking distance
SSD = dPRT + dMT
SSD = {0.278 V×t }+ {0.039 V2⁄a}
in m.