CHAPTER 30 Principles of 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 Outdated with rubber tires Needed alternative that did not apply friction material directly to tires

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

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

Modern hydraulic drum brake system replaced mechanical drum brake system. Automatically equalizes braking forces at each wheel

Disc brakes Developed early 1900s; common use 1960s 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

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

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

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-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.

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.

Factors that influence braking (cont’d) Vehicle weight Heavier vehicles have: More braking force Greater stopping distance Larger wheel brake units

Factors that influence braking (cont’d) Load on the wheel during stopping Heavier loads increase downward force, traction.

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

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 foot- activated 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 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

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

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

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.

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

Deceleration requires energy transformation. Kinetic energy is removed. Transformed into another form of energy Heat energy in a standard vehicle

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

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

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

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 Disc 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 Rotational force
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

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

Effort distance From fulcrum to the point effort is applied Load distance From fulcrum to the point the load is applied

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

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

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

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.

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

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.

Compression stroke absorbs kinetic energy. Slows the vehicle Lower gear slows it more quickly. Engine is turned over more rapidly by wheel movement.

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.

Rotor attached to wheel hub Rotates with the tire Stationary pads clamp against the rotor. Creates friction, slows the vehicle

On light vehicles, both kinds are hydraulic. Hydraulic fluid transfers force from the driver. Brake pedal operates a master cylinder. Hydraulic lines, hoses connect master cylinder to wheel brake units.

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.

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

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
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 Transmission-mounted parking brake Small drum brake prevents drive shaft from turning. Sometimes called a transmission 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 Electric/hydraulic

Brake-by-Wire (2 of 9) Electric Electric/hydraulic 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, drum- style, 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.

CHAPTER 30 Principles of Braking.

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CHAPTER 30 Principles of Braking.

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