Friction PowerPoint

1. In this presentation you will:
Friction
 examine friction and kinetic energy in automotive brake systems
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 identify factors that influence brake design

2. Introduction
When a vehicle moves, it has a kinetic energy that depends on its mass
and speed.
To stop a vehicle, the brake system uses friction to transform kinetic
energy to heat energy.
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3. Speed
Kinetic Energy
Kinetic energy is the energy processed by a vehicle due to its motion.
At a set speed, doubling the mass of a vehicle doubles its kinetic energy.
It depends on the mass and speed of the vehicle.
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At a set mass, doubling the speed of a vehicle quadruples its kinetic energy.
2,000 kg
Kinetic energy
4,000 kg

4. Kinetic Energy
The relationship between
mass, speed, and kinetic
energy can be written as
an equation:
KE = ½ mv2
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where:
KE = kinetic energy in joules
m = mass in kg
v = speed in m/s
m
v KE

5. Moving vehicles are
slowed or stopped
using ‘friction’ brakes.
Friction brakes function
by converting kinetic
energy to heat energy.
Friction and Kinetic Energy
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Two surfaces in contact,
such as a brake disc and
brake pad, are not
perfectly smooth.
When they slide over
each other, there is a
resistance to movement,
called friction.
Brake Rotor
with Pads
Brake Drum
with Shoes

6. In some automotive
systems, friction must
be minimal.
The gear box uses a
fluid to reduce friction
between components.
Friction in Vehicles
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Other systems rely on
friction to work, such
as the tyres.
The brake system
makes positive use of
friction, to help stop or
slow a vehicle.

7. How do standard brakes dissipate the kinetic energy of a vehicle?
Question 1
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B) They use friction to transform it to heat
C) They transfer it to the road surface
D) They supply it to the engine
A) They store it

8. How do standard brakes dissipate the kinetic energy of a vehicle?
Question 1
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B) They use friction to transform it to heat
C) They transfer it to the road surface
D) They supply it to the engine
A) They store it
Brakes use the friction between a disc or drum and a pad or shoe to
dissipate a vehicle’s kinetic energy as heat.

9. The coefficient of friction
is a measure of how
much friction there is
between two surfaces.
There are two types;
static (used when
surfaces are stationary)
and kinetic (used when
surfaces are moving).
Coefficient of Friction (C.O.F.)
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Static friction holds an
object in place, while
kinetic friction slows an
object down.

10. Increasing the pressure
on surfaces, which are in
contact, increases the
amount of friction
between them.
As the friction increases,
the surfaces will get
hotter. This affects the
C.O.F. and the braking
ability of a vehicle.
Pressure and Temperature
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To counter this, brake
components are
designed to maintain a
near constant C.O.F.
The larger the surface of the brake area,
the easier it is for the material to absorb
the heat produced by friction
150 °C
50 °C
50 °C
50 °C
Coefficientoffriction
Material temperature
Good material
Poor material

11. The smoothness of a
friction material affects its
C.O.F. and its wear rate.
Rough surfaces have a
high C.O.F. They wear
quickly, lose stopping
power, and should be
avoided in brake design.
Friction and Surface Texture
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Smooth surfaces have a
lower C.O.F. so wear more
slowly. More pressure
must be applied to achieve
the same braking force as
a higher C.O.F. material.
Modern brake pads are made
with low C.O.F. material and
designed to be worn away. Smooth
Rough

12. Different materials have
different coefficients of
friction. Brake materials
are designed to have and
maintain the correct C.O.F.
The pressure applied,
contact area, material
finish and material type
must be taken into
account.
Type of Friction Material
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If the C.O.F. is too low,
then excessive brake
pressure must be used to
stop the vehicle.
If it is too high, the
brakes will perform too
well, causing the wheels
to lock up.
Correct C.O.F.
Low C.O.F.
High C.O.F.

13. The heat generated by braking must be dissipated as quickly as possible to
prevent brake components overheating.
Friction and Heat
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Repeated hard braking can produce excessive heat that damages
the brakes, increasing the stopping distance.
Pedal presses:
Stopping
distance

14. Brake fade is the term given to the situation where brake efficiency is
reduced due to overheating.
Brake fade is very dangerous as the heat generated can wear brake linings
and warp discs.
Brake Fade
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It can cause premature failure of the hydraulic system, wheel bearings,
and seals.

15. To prevent this, brake
pads are designed to fade
at a set temperature.
Once this temperature is
passed, the frictional
material will melt and
the brakes will no longer
stop the vehicle.
Brake Fade
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Excessive heat
produced by hard
braking, can damage
brake components.
The fade point of the
material is designed to
allow for most braking
circumstances.
Without a set fade temperature, tyres
could be heated enough to catch fire.
ConnorLuddy

16. Heat needs to be
removed from the
brake components as
quickly as possible.
The size of the friction surface
area on modern brake
systems is designed to allow
maximum heat dissipation.
Heat Dissipation
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Brake discs and drums
have almost all of their
surface area exposed
to the air, giving good
heat dissipation.
Some brake discs
have fins and holes to
increase the heat
dissipation capacity.
Brake drums use the complete outer surface
of the drum to dissipate heat.

17. Several factors must be taken into account when designing brake systems.
Brake Design Considerations
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 Front brakes do more work than the rear due to the vehicle’s weight
distribution when braking
 Traction between the road and the tyres must be maintained to
prevent skidding
A moving vehicle will skid if braking stops a wheel from rotating

18. The size of the engine will have an effect on the braking ability of a vehicle.
Brake Design Considerations
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When the accelerator pedal is released, an engine will still try to draw in air.
This creates drag on the drive train, which helps to slow the vehicle.

19. Brake Design Considerations
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If water reaches the brake linings, it will act as a lubricant.
Disc brakes are more efficient at removing water than drum brakes, as drum
brakes can trap water.

20. Materials used on brake
shoes and pads should
have the following qualities:
Brake Lining Characteristics
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 A consistent coefficient
when hot or cold
 Resistance to fade at
high temperatures
 Fade at specific point
to avoid damage to
other components
 Provide braking action
when wet
 Stop the vehicle
smoothly and quietly
 Last for tens of thousands
of miles

21. Lining materials can be classified as three different types:
Brake Lining Material
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 Non-metallic
 Semi-metallic
 Metallic
Metallic brake linings are the
hardest of all those available and
cause wear on the drums or discs.
Semi-metallic are the most
common and are harder than
non-metallic so last longer.
Non-metallic linings are quieter than
other types. They provide the lowest
coefficient of friction and the least
braking power.

22. Brake pads and shoes are marked on their edges to show their C.O.F. at
low and high temperatures.
Good linings will have a coefficient that is the same for both temperatures.
Temperature Markings
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A larger range indicates a poorer quality of lining.
Friction Material Code Coefficient of Friction
C <0.15
D 0.15-0.25
E 0.25-0.35
F 0.35-0.45
G 0.45-0.55
H >0.55

23. Question 2
Which graph shows the ideal relationship between coefficient of friction and
temperature of a brake?
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B)
C) D)
A)
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature

24. Question 2
Which graph shows the ideal relationship between coefficient of friction and
temperature of a brake?
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B)
C) D)
A)
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature
Coefficientoffriction
Material temperature
The coefficient of friction should ideally remain constant throughout the
brake temperature range.

25. Summary
 how brakes use friction to slow vehicles
In this presentation you have seen:
 the factors affecting brake design
End