CHAPTER 30 Principles of Braking Introduction The braking

CHAPTER 30 Principles of Braking

Introduction • The braking system is critical. – Gradual braking is ideal. – Rapid braking may be needed to avoid an accident.

The History of Brakes (1 of 5) • Early automobiles used scrub brakes. – Simple mechanical system – Leverage forced friction block against wheels. – Outdated with rubber tires • Needed alternative that did not apply friction material directly to tires

The History of Brakes (2 of 5) • Band brakes – Metal band lined with friction materials – Clamped around a small-diameter wheel – Or drum-mounted to the axle or wheel – Unwound in reverse; soon abandoned

The History of Brakes (3 of 5) • Drum brakes – Similar to today’s drum brakes – Two shoes push against the inside of the brake drum. – Difficult to maintain equal braking forces at each wheel

The History of Brakes (4 of 5) • Modern hydraulic drum brake system replaced mechanical drum brake system. • Automatically equalizes braking forces at each wheel

The History of Brakes (5 of 5) • Disc brakes – Developed early 1900 s; common use 1960 s – Advantage in dissipating heat led to greater use of disc brakes on most vehicles.

Electronic Brake Control (1 of 3) • Risk of too much brake force – Loss of traction, control – Led to electronic brake control (EBC) • Anti-lock brake systems (ABS) – Reduce accidents – Computer monitors wheel speeds, maintains maximum braking power

Electronic Brake Control (2 of 3) • Demand for more safety led to: – Traction control systems (TCS) – Electronic stability control (ESC) • TCS – Reduces engine torque • Helps prevent tires slipping during acceleration – Applies brake pressure to slipping wheels

Electronic Brake Control (3 of 3) • ESC – Adds functionality to ABS and TCS – Helps prevent loss of traction with aggressive steering, evasive maneuvers • Both TCS and ESC – Can apply individual brake units even though driver is not stepping on the brake pedal

Brake Assist and Brake-by-Wire Systems (1 of 4) • EBC systems have additional features. – E. g. , brake assist (BA) • Greater computer control • Better reaction time • Example of BA: brake-by-wire system

Brake Assist and Brake-by-Wire Systems (2 of 4) • Full brake-by-wire system – Hydraulics replaced with: • Sensors and wires • Electronic control unit (ECU) • Electrically actuated motors – Individual brake units at each wheel

Brake Assist and Brake-by-Wire Systems (3 of 4) • Brake-by-wire process – Driver presses a brake pedal emulator. – Emulator communicates with computer. – Control unit sends signals to brake actuators. – Actuators generate clamping force, slow vehicle. – Sensors report data to the control unit.

Brake Assist and Brake-by-Wire Systems (4 of 4) • Greater computer control increases safety. – E. g. , brake-by-wire reduces braking time, stopping distance – Computer detects quick accelerator release. – Control system lightly applies the brakes. • Dries moisture, takes up clearance in the system

Regenerative Brake Systems (1 of 2) • Improved fuel economy in hybrid vehicles • Brake-by-wire system turns electric motor into a generator. – Converts vehicle’s energy into electrical energy – Slows vehicle

Regenerative Brake Systems (2 of 2) • Amount of electricity generated controls the amount of stopping power. – More stopping power, more electrical output from the generator • Electricity stored in a high-voltage battery – Used later by electric motor to drive the vehicle

Brake Fundamentals (1 of 5) • Factors that influence braking – Road surface – Road conditions • Water, ice, gravel reduce traction and increase stopping distances. • Hot asphalt can soften, be slippery.

Brake Fundamentals (2 of 5) • Factors that influence braking (cont’d) – Vehicle weight • Heavier vehicles have: – More braking force – Greater stopping distance – Larger wheel brake units

Brake Fundamentals (3 of 5) • Factors that influence braking (cont’d) – Load on the wheel during stopping • Heavier loads increase downward force, traction.

Brake Fundamentals (4 of 5) • Factors that influence braking (cont’d) – Vehicle height • Taller vehicles: greater leverage on contact point – Increases, decreases the load on tires – Much more difficult to control the vehicle – How the vehicle is driven • Aggressive driving, increased speed

Brake Fundamentals (5 of 5) • Factors that influence braking (cont’d) – Tires • Tire composition, tread style, and condition affect traction. • Using the wrong tire will affect stopping power.

Brake Systems (1 of 10) • Two brake systems on all vehicles • Service brake – Slows, stops vehicle – Foot pedal – Drum and/or disc brakes

Brake Systems (2 of 10) • Service brake (cont’d) – Two options • Disc brakes on front wheels, drum brakes on rear • Disc brakes on all four wheels

Brake Systems (3 of 10) • Parking brake – Holds stationary vehicle in place – Usually operated by hand – Some use a footactivated pedal.

Brake Systems (4 of 10) • Modern braking systems – Hydraulically operated – Two main sections • Brake assemblies at the wheels • Hydraulic system that applies them

Brake Systems (5 of 10) • Driver pushes brake pedal – Mechanical force to pistons in master cylinder – Pistons apply hydraulic pressure to fluid in cylinders.

Brake Systems (6 of 10) • Lines transfer pressure equally to hydraulic cylinders. – Hydraulic cylinders at the wheel assemblies apply brakes.

Brake Systems (7 of 10) • Force transmitted hydraulically through fluid – Cylinders of the same size: • Forces transmitted, applied are the same value.

Brake Systems (8 of 10) • Cylinders force friction linings into contact with braking surfaces. – Friction generates heat energy. – Slows the vehicle

Brake Systems (9 of 10) • Drum and disc braking mechanisms

Brake Systems (10 of 10) • Both drum and disc systems – Components must withstand high temperatures and high pressures. • Modern brake systems have refinements such as EBC systems.

Kinetic Energy (1 of 2) • Energy of an object in motion – Affects all moving objects – Heavier objects have more than lighter objects moving at the same speed. • If weight doubles, kinetic energy doubles.

Kinetic Energy (2 of 2) • Increases by the square of the speed – If speed doubles, kinetic energy increases by four times. • Function of braking system – Stop vehicle by converting all kinetic energy to another form • In most cases heat

Acceleration and Deceleration (1 of 4) • Newton’s first law of motion – “An object will stay at rest or uniform speed unless it is acted upon by an outside force. ” • Acceleration: increase in an object’s speed – Driver steps on throttle pedal. – Engine’s power output increases. – Vehicle accelerates.

Acceleration and Deceleration (2 of 4) • Acceleration requires energy. – Heavier vehicles need more energy to accelerate to a given speed. • Deceleration: decrease in an object’s speed – Outside force is needed. • Comes from Earth’s mass

Acceleration and Deceleration (3 of 4) • Brakes – Connect the vehicle to the road • Via rolling wheel and tire assembly – Apply varying amount of force from the ground to the vehicle • Causes vehicle to decelerate

Acceleration and Deceleration (4 of 4) • The force of the brakes absorbs the vehicle’s kinetic energy. • Heavier, faster vehicles – More kinetic energy must be dissipated. – Brakes have to work harder.

Energy Transformation (1 of 5) • Law of conservation of energy: energy cannot be created or destroyed. – Energy used to accelerate, decelerate must be transformed from one form to another. • Gasoline, diesel fuels – Potential energy in chemical form

Energy Transformation (2 of 5) • Some of fuel’s chemical energy is transformed within the engine. – First into heat energy – Then into mechanical energy • Only 25% to 35% – The rest is wasted as heat energy.

Energy Transformation (3 of 5) • Mechanical energy accelerates the vehicle. – Converted to kinetic energy • More energy needed to accelerate than to maintain speed – Affects fuel efficiency • Usually better at steady speeds vs. stop-and-go

Energy Transformation (4 of 5) • Deceleration requires energy transformation. – Kinetic energy is removed. – Transformed into another form of energy – Heat energy in a standard vehicle

Energy Transformation (5 of 5) • It takes the same amount of energy to slow a vehicle down as it does to accelerate it. – Stopping is expected to be faster than acceleration. – Braking system transforms energy faster than the engine.

Friction and Friction Brakes (1 of 4) • Brakes transform kinetic energy. – Standard brakes use friction. • Friction: resistance of surfaces in contact – Static: between nonmoving surfaces • Parking brakes – Kinetic: between moving surfaces • Standard brakes

Friction and Friction Brakes (2 of 4) • Operating the brakes – Moving friction components in contact • Generates heat – Kinetic energy converted to heat energy • Slows the vehicle • Example: scrub brake on a go-kart

Friction and Friction Brakes (3 of 4) • Coefficient of friction – Friction between two moving surfaces in contact – Expressed as a ratio of two forces: • Amount of force pushing the surfaces together • Resistive force between the surfaces sliding against each other

Friction and Friction Brakes (4 of 4) • Example – Stationary steel surface – Pushed against a moving steel surface with 100 lb of force – Might generate 20 lb of resistive force – Coefficient of friction is 20/100, or 0. 20.

Heat Transfer (1 of 2) • Heat transfers from hot to cool areas. • Braking converts kinetic energy to heat. – Must be dissipated – Heat transfer is critical. – Most radiates into the atmosphere. • Process depends on type of braking system.

Heat Transfer (2 of 2) • Drum brakes – Heat is created inside the drum. – Transfers to outside surface, then atmosphere • Disc brakes – Heat is created on outer rotor surfaces. • In contact with the atmosphere – May have internal ventilation to speed transfer

Brake Fade (1 of 4) • Reduction in stopping power caused by a change in the brake system • Three factors – Most common is heat fade. • Heat buildup in the braking surfaces • So hot they cannot create any additional heat

Brake Fade (2 of 4) • Coefficient of friction drops. – Kinetic energy cannot be reduced. – Brakes cannot generate stopping power until heat dissipates. • Cause: overuse of brakes – E. g. , going down a long, steep hill

Brake Fade (3 of 4) • Water fade – Caused by water-soaked brake linings • Water lubricates • Lessens coefficient of friction – Hard brake pedal, very little braking power – When water evaporates, friction is restored.

Brake Fade (4 of 4) • Hydraulic fade – Brake fluid overheats, boils. • Partially converts to vapor • Can be compressed • Can’t transfer force effectively to wheel brake units – Brake pedal becomes soft.

Rotational Force (1 of 6) • Generated by brakes on a moving vehicle – Friction tends to twist brake support in the direction of wheel rotation. – Body tends to rotate in the same direction.

Rotational Force (2 of 6) • Example of rotational force – Motorcycle rear wheel lifting off the ground with hard front braking

Rotational Force (3 of 6) • Suspension components – Usually control rotational forces – Can become worn, allow movement – Clunk or pop during braking • Rotational force – Tends to push nose down, lift rear – Weight transfer to front wheels

Rotational Force (4 of 6) • Rotational force, weight transfer – Also happens because vehicle’s centerline is higher than that of axles – Center of gravity tends to move forward when brakes are applied firmly.

Rotational Force (5 of 6) • Weight transfer – Increased traction in front wheels – Front wheels: more stopping load – Rear wheels: less traction • Reduces the load they can bear • Factored into brake design – Avoids lock-up from excess stopping power

Rotational Force (6 of 6) • Lock-up is avoided with: – Properly sized master and wheel cylinders – Valving that modifies hydraulic pressure to rear wheels under hard braking

Levers and Mechanical Advantage (1 of 7) • Used to apply service and parking brakes – A bar is a lever. • Fulcrum – Point around which a lever rotates – Supports lever and load

Levers and Mechanical Advantage (2 of 7) • A lever helps you lift heavy things. – Small force at one end • Applied over a certain distance – Large load at the other end • Moved a smaller distance

Levers and Mechanical Advantage (3 of 7) • Effort distance – From fulcrum to the point effort is applied • Load distance – From fulcrum to the point the load is applied

Levers and Mechanical Advantage (4 of 7) • If effort distance is greater than load distance – Effort required is less than load – Mechanical advantage • If load distance is greater than effort distance – Effort required is greater than load – Mechanical disadvantage

Levers and Mechanical Advantage (5 of 7) • Three lever types • Lever of the first order – Fulcrum in the middle, between load and effort – Example: seesaw – Force applied in the opposite direction of the load

Levers and Mechanical Advantage (6 of 7) • Lever of the second order – Load between effort and fulcrum – E. g. , wheelbarrow – Force applied in direction of the load – Mechanical advantage built into pedal

Levers and Mechanical Advantage (7 of 7) • Lever of the third order – Effort in the middle, between load and fulcrum – Example: an oar when paddling a canoe – Force is in the direction of the load.

Adjustable Brake Pedal System • Driver can raise or lower brake and throttle pedal assembly for comfort. • Usually adjusted by electrically driven motors – Switch on steering column or dash

Principles of Engine Braking (1 of 5) • Engine crankshaft, wheels – Mechanically connected when vehicle is in gear • Engine applies turning force to the crankshaft. • Transmission applies that force to the wheels.

Principles of Engine Braking (2 of 5) • If force is applied to the wheels, the transmission applies that force to the engine through the crankshaft. – Principle behind push-starting a vehicle with a manual transmission – Can start without a starter motor

Principles of Engine Braking (3 of 5) • Engine braking uses same principle. – Foot off the accelerator – Engine stops applying force to the wheels. • Begins to act as a brake – Wheel momentum keeps engine turning over.

Principles of Engine Braking (4 of 5) • Compression stroke absorbs kinetic energy. – Slows the vehicle – Lower gear slows it more quickly. • Engine is turned over more rapidly by wheel movement.

Principles of Engine Braking (5 of 5) • Steep or long decline – Risk of brake fade with wheel brakes alone. – Use brakes to slow the vehicle initially, then shift into lower gears. • Assist from engine inertia – Safer; reduces brake wear and tear

Brake Repair Legal Standards and Technician Liability • Technicians can be: – Legally liable, sued for improper brake repair – Found criminally negligent if they are determined to have acted maliciously • Always follow manufacturer’s procedures. • Never take shortcuts; safety first!

Types of Brakes • Brake systems vary. • Must be appropriate to vehicle application – E. g. , trailers, trucks • Hydraulic brakes are difficult. • Some heavy trucks use air-operated brakes.

Drum and Disc Brakes (1 of 6) • Friction brakes use two kinds of wheel brake units. • Drum brakes – Drum attached to wheel hub. – Rotates with the tire – Stationary brake shoes expand, create friction.

Drum and Disc Brakes (2 of 6) • Disc brakes – Rotor attached to wheel hub – Rotates with the tire – Stationary pads clamp against the rotor. – Creates friction, slows the vehicle

Drum and Disc Brakes (3 of 6) • On light vehicles, both • Hydraulic lines, hoses kinds are hydraulic. connect master cylinder to wheel – Hydraulic fluid brake units. transfers force from the driver. – Brake pedal operates a master cylinder.

Drum and Disc Brakes (4 of 6) • Most modern light vehicles have either: – Disc brakes on the front wheels and drum brakes on the rear, or – Disc brakes on all four wheels – Disc brakes require greater forces than drum brakes. • Disc brakes usually include power brake booster.

Drum and Disc Brakes (5 of 6) • Modern systems often fitted with ABS. – Prevents wheel lock-up or skidding • No matter how hard brakes are applied • Or how slippery the road surface • Driver has better control. • Generally reduces stopping distances

Drum and Disc Brakes (6 of 6) • ABS system components – Brake pedal – Power booster – Master cylinder – Wheel speed sensors – ECU – Hydraulic control unit (HCU) • Hydraulic modulator

Air Brakes (1 of 5) • Air-operated braking systems – Commonly called air brakes – Compressed air provides large forces at the brake assembly. – Drum or disc style

Air Brakes (2 of 5) • Air-operated braking systems are used on heavy vehicles.

Air Brakes (3 of 5) • Some brake canisters have a separate air chamber and large spring assembly. – Referred to as spring brakes – Spring applies brakes when air pressure is released. • Air pressure applied to chamber compresses spring, releases brakes.

Air Brakes (4 of 5) • System functions – Parking brake – Safety measure • Applies brakes if system loses air pressure • Minimum amount of air pressure is required to compress the spring, release brakes.

Air Brakes (5 of 5) • Engine-driven compressor creates air pressure. – Pumps pressurized air into storage tanks – Driver-controlled valves direct air to wheel units. • Valve releases air when driver releases brakes. • Brakes retract.

Exhaust Brakes (1 of 2) • Exhaust brakes provide increased braking for heavy-duty vehicles.

Exhaust Brakes (2 of 2) • System closes butterfly valve in exhaust manifold. – Restricts flow of exhaust gases through engine • Results in high pressure in exhaust manifold, engine cylinders • Engine slows down; slows wheels through transmission or power train.

Jake Brakes (1 of 3) • Engine braking is less effective in heavy diesel vehicles. – More compression energy back to crankshaft after the piston reaches top dead center – Engine wants to freewheel. • Does not create much force to slow the vehicle

Jake Brakes (2 of 3) • Jake brake (compression brake) – Extra lobe on camshaft controls an auxiliary exhaust valve at the top of each cylinder. – Releases compression stroke pressure before it can be sent back to the power stroke of the piston

Jake Brakes (3 of 3) • Substantial energy used on compression stroke – Slows the crankshaft – Increases engine’s braking effectiveness • Very noisy: machine-gun sound – Usually requires additional muffling – Banned in some areas

Electric Brakes (1 of 4) • Often used by trailers if gross towing weight exceeds a certain value – Activate drum-type friction brakes – Driver uses manual control unit. • Can increase, reduce braking • Can lessen tendency to sway in some conditions

Electric Brakes (2 of 4) • Process – Towing vehicle brakes are applied. – Brake light circuit signals control unit. – Control units send current to trailer brake actuators. – Electric current sent to brake units at the wheels.

Electric Brakes (3 of 4) • Process (cont’d) – Brake assembly uses lever-operated shoes. – Lever has electromagnet with electric current. • Draws magnet toward the spinning brake drum – Magnet contacts the drum and is pulled around, applying the brake shoes.

Electric Brakes (4 of 4) • Harder braking – Stronger electric current; increasing braking force • Auxiliary battery-powered braking system – Required for trailers above a certain weight in some places – Automatically applies the brakes – Keeps them applied if the trailer breaks away

Parking Brakes (1 of 7) • All vehicles must have: – Service brake – Parking brake • Service brake – Usually hydraulically operated – Used while the vehicle is being driven – Applies brake units at all four wheels

Parking Brakes (2 of 7) • Parking brake – Mechanically operated – Holds vehicle in place when parked – Applies the brake units on two wheels only – Several types

Parking Brakes (3 of 7) • Drum-style parking brake in the center of the rear disc brake rotors – Mechanically operated – On some vehicles with disc brakes

Parking Brakes (4 of 7) • On some vehicles, a mechanical linkage directly operates a disc-style service brake.

Parking Brakes (5 of 7) • Mechanically applies brake shoes against drum • Cable pulls on an actuating lever inside the brake drum assembly.

Parking Brakes (6 of 7) • Actuating lever is connected: – To the secondary brake shoe by a pin or tang – To the primary shoe by a strut • Lever forces both shoes against the drum.

Parking Brakes (7 of 7) • Older parking brakes include: – Front wheel-mounted parking brake • Small drum brake prevents drive shaft from turning. • Sometimes called a – Transmission-mounted transmission brake parking brake

Parking Brake Cables (1 of 2) • Parking brake cables transmit force – From parking brake actuating lever – To the brake unit • Part of the cable is inside a wound steel housing.

Parking Brake Cables (2 of 2) • Other sections are exposed. – Can be susceptible to damage – Cables are steel; can rust and stick to the inside of the wound housing. – Use manufacturer-specified lubricant.

Parking Brake Apply Mechanisms (1 of 2) • Two common methods • Hand-operated lever – Fulcrum on the bottom end – Cable attached a few inches from the fulcrum • Mechanical advantage – Ratcheting mechanism in the handle • Maintains tension on cables and assembly

Parking Brake Apply Mechanisms (2 of 2) • Foot operated, works similarly – Mechanical advantage, ratcheting mechanism – Usually under dash near kick panel • Release methods – Pull release handle – Push pedal farther down – Gear moved out of park; vacuum or electric power releases brake

Parking Brake Adjustment (1 of 2) • Methods – Nut on cable under vehicle – Brake lever assembly – Rear calipers

Parking Brake Adjustment (2 of 2) • Order of adjustment – Parking brake cable • Should have slack – Service brakes • Follow manufacturer’s procedure. – Parking brake

Hybrid Vehicles (1 of 3) • Regenerative braking – Recaptures, stores part of kinetic energy – Otherwise converted to heat, wasted • Electric motor acts as a generator. – Converts kinetic energy into electrical energy

Hybrid Vehicles (2 of 3) • Electrical energy is stored as chemical energy in a high-voltage battery.

Hybrid Vehicles (3 of 3) • Amount of braking force applied by the regeneration system is computer controlled. – Quick, heavy braking requires friction brakes. – Interaction between regenerative and friction braking systems is important. • Must be carefully designed and serviced

Brake-by-Wire (1 of 9) • No mechanical or hydraulic connection between brake pedal and wheel brake units • Two types – Electric/hydraulic

Brake-by-Wire (2 of 9) • Electric – Wheel brake units – Integrated motor system • Clamping force to disc brake pads • Electric/hydraulic – Hydraulic controller • Hydraulic pressure to standard wheel brake units

Brake-by-Wire (3 of 9) • In both, brakes are computer controlled. – Brake pedal position, application speed signal driver’s intent. – Computer evaluates this and other data. • E. g. , vehicle speed

Brake-by-Wire (4 of 9) • Computer compares data to preloaded information in memory. – Commands brake controller to create specific pressure at wheel brake units • Driver can modify programmed braking with the brake pedal.

Brake-by-Wire (5 of 9) • Many vehicles use this principle to control the engine’s throttle. – Drive-by-wire – No mechanical connection between throttle pedal and throttle plate

Brake-by-Wire (6 of 9) • Easy to replicate throttle pedal feel. – Use a spring with similar characteristics. • Feel of brake pedal not as easily replicated – Spring and hydraulic pressure – Pedal has much firmer feel.

Brake-by-Wire (7 of 9) • Solution: brake pedal emulator – Sealed cylinder builds pressure similar to a master cylinder. – Sensors send brake pedal data to ECU.

Brake-by-Wire (8 of 9) • Several benefits – Reduces weight – Saves space – Drivers do not feel ABS brake pulsations. • Less likely to remove their foot from the pedal in a panic stop – Brakes applied more quickly in an emergency

Brake-by-Wire (9 of 9) • Drawbacks – Risk of system failure – Initial cost • Used by some hybrids during regeneration – Brake-by-wire system initiates braking first. • Applies a load to the vehicle from the generator – Hydraulic backup brake system can add power.

Summary (1 of 8) • Braking systems evolved from scrub brakes to band brakes to drum and disc brakes. • Electronic brake systems use computers to determine speed and stopping force. • Electronic brake systems have improved consumer safety and fuel economy. • Brake-by-wire systems replace mechanical or hydraulic connections with computers.

Summary (2 of 8) • Regenerative brake systems convert a vehicle’s energy into electrical energy and store it in the battery. • Braking systems are affected by road surface and conditions, vehicle weight and height, load on the wheels, type of tire, and driving style. • Every vehicle has a service brake and a parking brake.

Summary (3 of 8) • Hydraulic braking systems use cylinders to transfer pressure from the brake pedal to the wheel. • The braking system converts kinetic energy into an alternate form of energy. • Acceleration and deceleration use an outside force to increase or decrease the vehicle’s speed.

Summary (4 of 8) • Conservation of energy requires that energy be transformed; it cannot be created or destroyed. • Standard brakes use friction to transform kinetic energy into heat energy. • The coefficient of friction is the amount of force pushing two surfaces together compared to the amount of resistive force.

Summary (5 of 8) • Heat energy from braking dissipates into the atmosphere. • There are three kinds of brake fade: heat fade, water fade, and hydraulic fade. • Braking systems transfer weight to the front wheels by creating rotational force. • Levers can be of the first, second, or third order.

Summary (6 of 8) • With engine braking, the engine acts as a brake by ceasing to apply force to the wheels. • Technicians can be held liable for improperly repaired brakes. • Disc and drum friction brakes are used on lighter vehicles; both have anti-lock systems.

Summary (7 of 8) • Air-operated braking systems, used on heavier vehicles, apply the brakes by compressing springs with air pressure. • Exhaust brakes limit exhaust flow to create engine pressure and may supplement friction brakes in heavy vehicles. • Jake brakes use compression to slow the crankshaft and increase braking effectiveness.

Summary (8 of 8) • Trailers over a certain weight often use electric braking to give the driver control. • Parking brake styles include: top hat, drumstyle, and transmission-mounted. • Parking brakes use a ratcheting mechanism to maintain tension on the parking brake cables and assembly.

Credits • Unless otherwise indicated, all photographs and illustrations are under copyright of Jones & Bartlett Learning.