Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ 07458 Braking System Principles.

Automotive Brake SystemsJames D. Halderman

© 2014 Pearson Higher Education, Inc.Pearson Prentice Hall - Upper Saddle River, NJ 07458

Braking System Principles

chapter4

Chapter 4 Braking System Principles



ObjectivesObjectives• Discuss the energy principles that apply to

brakes.• Discuss the mechanical principles that apply

to brakes.• Discuss the friction principles that apply to

brakes.

Bret

Jessy/Author:LOs have been replaced. Please verify changes.




ObjectivesObjectives• Describe how brakes can fade due to

excessive heat.• Describe how deceleration rate are

measured.

This chapter will help you prepare for the Brakes (A5) ASE certification test.

Bret

Jessy/Author:Change "rate" to "rates", or change "are" to "is"?




FIGURE 4–1 Energy ,which is the ability to perform work , exists in many forms.




Energy Principles: Kinetic EnergyEnergy Principles: Kinetic Energy• Kinetic energy is a fundamental form of

mechanical energy. – It is the energy of mass in motion.

• The job of the brake system is to dispose of that energy in a safe and controlled manner.




Energy Principles: Kinetic EnergyEnergy Principles: Kinetic Energy• Every moving object possesses kinetic

energy, and the amount of that energy is determined by the object’s mass and speed. – The greater the mass of an object and the faster

it moves, the more kinetic energy it possesses. – Even at low speeds, a moving vehicle has enough

kinetic energy to cause serious injury and damage.




FIGURE 4–2 Kinetic energy increases in direct proportion to the weight of the vehicle.

FIGURE 4–3 Kinetic energy increases as the square of the increase in vehicle speed.




Energy Principles: Kinetic Energy Energy Principles: Kinetic Energy and Brake Designand Brake Design• The relationships between weight, speed,

and kinetic energy have significant practical consequences for the brake system engineer. – If vehicle A weighs twice as much as vehicle B, it

needs a brake system that is twice as powerful. – But if vehicle C has twice the speed potential of

vehicle D, it needs brakes that are, not twice, but four times more powerful.




FIGURE 4–4 Inertia creates weight transfer that requires the front brakes to provide most of the braking force.




FIGURE 4–5 Front-wheel -drive vehicles have most of their weight over the front wheels.




FIGURE 4–6 A first-class lever increases force and changes the direction of the force.




FIGURE 4–7 A second-class lever increases the force in the same direction as the applied force.




FIGURE 4–8 A third-class lever reduces force but increases the speed and travel of the resulting work.




FIGURE 4–9 A brake pedal assembly is a second-class lever design that provides a 5 to 1 mechanical advantage.




Mechanical Principles: Mechanical Principles: Mechanical AdvantageMechanical Advantage• Leverage creates a mechanical advantage

that, at the brake pedal, is called the pedal ratio.– For example, a pedal ratio of 5 to 1 is common

for manual brakes,• Which means that a force of 10 lb at the brake pedal

will result in a force of 50 lb at the pedal pushrod.




Mechanical Principles: Mechanical Principles: Mechanical AdvantageMechanical Advantage• In practice, leverage is used at many points in

both the service and parking brake systems to increase braking force – While making it easier for the driver to control

the amount of force applied.




Friction Principles: Coefficient of FrictionFriction Principles: Coefficient of Friction• The amount of friction between two objects

or surfaces is commonly expressed as a value called the coefficient of friction. – It is represented by the Greek letter mu (μ).




Friction Principles: Coefficient of FrictionFriction Principles: Coefficient of Friction• The coefficient of friction, also referred to as

the friction coefficient, is determined by dividing tensile force by weight force.

• The tensile force is the pulling force required to slide one of the surfaces across the other.

• The weight force is the force pushing down on the object being pulled.




FIGURE 4–10 The coefficient of friction in this example is 0.5.




FIGURE 4–11 The type of friction material affects the coefficient of friction, which is just 0.05 in this example.




Friction Principles: Friction Contact AreaFriction Principles: Friction Contact Area• For sliding surfaces, such as those in wheel

friction assemblies,– The amount of contact area has no effect on the

amount of friction generated.

• This fact is related to the statement that brake friction materials always have a friction coefficient of less than 1.0.




Friction Principles: Friction Contact AreaFriction Principles: Friction Contact Area• To have a friction coefficient of 1.0 or more,

material must be transferred between the two friction surfaces.

• The amount of contact area does not affect the coefficient of friction.– It does, however, have significant effects on

lining life and the dissipation of heat that can lead to brake fade.




Friction Principles: Static and Friction Principles: Static and Kinetic FrictionKinetic Friction• There are actually two measurements of the

coefficient of friction: the static friction coefficient and the kinetic friction coefficient.

• The static value is the coefficient of friction with the two friction surfaces at rest.

• The kinetic value is the coefficient of friction while the two surfaces are sliding against one another.




FIGURE 4–12 The static coefficient of friction of an object at rest is higher than the kinetic (dynamic) friction coefficient once in motion.




Friction and HeatFriction and Heat• The function of the brake system is to convert

kinetic energy into heat energy through friction.




Friction and HeatFriction and Heat• It is the change in kinetic energy that

determines the amount of temperature increase – And kinetic energy increases proportionately

with increases in weight, and as the square of any increase in speed.




Friction and HeatFriction and Heat• If the weight of the vehicle is doubled to

6,000 Ib, the change in kinetic energy required to bring it to a full stop will be 180,602 ft-Ib.

• The temperature increase computed with this equation is the average of all the friction-generating components.




Brake FadeBrake Fade• The temperature of a brake drum or rotor

may rise more than 100°F (55°C) in only seconds during a hard stop, – But it could take 30 seconds or more for the rotor

to cool to the temperature it was before the stop.

Bret

Ed.:Can't fix space after degrees symbols.




Brake FadeBrake Fade• If repeated hard stops are performed, the

brake system components can overheat and lose effectiveness, or possibly fail altogether.

• This loss of braking power is called brake fade.




Brake FadeBrake Fade• The point at which brakes overheat and fade

is determined by a number of factors – Including the brake design, its cooling ability, and

the type of friction material being used.




Brake FadeBrake Fade• There are four primary types of brake fade:– Mechanical fade– Lining fade affects– Gas fade– Water fade




FIGURE 4–13 Mechanical fade occurs when the brake drums become so hot that they expand away from the brake lining.




FIGURE 4–14 Some heat increases the coefficient of friction, but too much heat can cause it to drop off sharply.




FIGURE 4–15 One cause of GAS brake fade occurs when the phenolic resin, a part of the friction material, gets so hot that it vaporizes. The vaporized gas from the disc brake pads gets between the rotor (disc) and the friction pad. Because the friction pad is no longer in contact with the rotor, no additional braking force is possible.




SummarySummary• Energy is the ability to do work. – A vehicle in motion represents kinetic energy,

which must be absorbed by the braking system during a stop.

• The front brakes must provide a higher percentage of the braking force due to weight bias and weight transfer.




SummarySummary• The brake pedal uses mechanical advantage

to increase the force applied by the driver to the master cylinder.

• Coefficient of friction represents the amount of friction between two surfaces.




SummarySummary• Friction creates heat during a stop, and the

braking system must be able to absorb this heat.

• Deceleration rates are expressed in feet per second per second, or ft/sec2.




SummarySummary• Brake fade results when the heat generated

by the brakes causes changes in the friction materials that reduce the braking force – Or when water gets between the brake drum and

the linings.

Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ 07458 Braking System Principles.

Documents