friction

In this presentation you will:

Friction

examine friction and kinetic energy in automotive brake systems

Next > identify factors that influence brake design

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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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

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 joulesm = mass in kgv = speed in m/s

m

v KE

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

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.

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

How do standard brakes dissipate the kinetic energy of a vehicle?

Question 1

Next >

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.

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.

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

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Material temperature

Good material

Poor material

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

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.

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:

Stoppingdistance

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.

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

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.

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

The size of the engine will have an effect on the braking ability of a vehicle.


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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.


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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.

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

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.

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

Next >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

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)

Coe

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Material temperatureC

oeffi

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t of f

rictio

nMaterial temperature

Coe

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Coe

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Question 2

Which graph shows the ideal relationship between coefficient of friction and temperature of a brake?

Next >

B)

C) D)

A)

Coe

ffici

ent o

f fric

tion

Material temperatureC

oeffi

cien

t of f

rictio

nMaterial temperature

Coe

ffici

ent o

f fric

tion


Coe

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ent o

f fric

tion


The coefficient of friction should ideally remain constant throughout the brake temperature range.

Summary

how brakes use friction to slow vehicles

In this presentation you have seen:

the factors affecting brake design

End

friction

Engineering