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					                           Heavy Vehicle Braking

                                                   What You’ll Learn
                              After reading this chapter you will be able to:

                                       explain why heavy vehicles take longer to stop than smaller
                                       passenger vehicles

                                       explain why air brakes are used on heavy vehicles

                                       describe the basic scientific principles of braking
                                       identify the elements of stopping distance
Heat is a form of
energy. In an internal                 explain the effects of weight and speed on stopping distance
combustion engine, the
heat energy produced                   explain how to brake safely in different situations
by engine combustion is
converted to the energy
of Motion through the
moving parts of the
                            Starting and Stopping
engine and drive train,     To most people, stopping seems simple. When you drive a car and you
turning the wheels.         want to stop, you press on the brake pedal and the car comes to a stop.
                            And, when you want to go somewhere in a car, you start it, press on the
Even though the
energy of motion turns      gas pedal, and the car begins to move.
the vehicle’s wheels,       But what actually happens to cause the vehicle to stop? And what causes
the vehicle won’t go
                            a vehicle to move? To answer these questions, we need to know some
anywhere unless there
                            basic scientific principles.
is Traction between
the tires and the road      Your car’s engine, like the diesel engine of a truck, is a heat conversion
surface.                    machine, taking the energy of heat from the exploding mixture of fuel
In stopping a vehicle,      and air in the combustion chamber, and converting it to motion through
the energy of Motion is     the engine crankshaft and drivetrain to the wheels.
converted to the energy
of Heat through Friction    A brake — whether a brake for a car or a commercial vehicle — is also
between the lining/pad      a heat conversion machine, but works exactly opposite to an engine.
and drum/rotor surfaces     Brakes convert the energy of motion back into the energy of heat through
of the vehicle’s brakes.    the friction between the brake drums or rotors and the brake linings
                            or pads.

Chapter 2

   Fast Fact                         A simple kind of brake is that used by a skater on rollerblades — the
                                     skater tips the rollerblade to the rear, and a pad rubs against the
   The final factor in                pavement to slow and stop the skater. If you were to feel the pad after
   stopping is in the                the skater has stopped, it would be hot, as would the pavement. This
   contact between the               is because the energy of motion has been converted to the energy of
   vehicle and the road              heat through the friction between the pad and the pavement. Both the
   through the tires.                pad and the pavement need to be able to absorb the heat created while
   Bald or defective tires           stopping.
   may degrade braking
                                     A car’s brakes work on these same principles, as do the brakes on a
                                     truck or bus. Attached to each wheel is either a drum brake or a disc
                                     brake. The drum or disc rotates with the wheel. To stop the vehicle,
                                     brake linings rub against the brake drum, or in the case of disc brakes,
                                     brake pads rub against the brake rotor. This creates friction, converting
                                     the vehicle’s energy of motion into the energy of heat, which stops
                                     the vehicle. The heat is absorbed and dissipated into the atmosphere,
                                     primarily through the brake drums/rotors.

                                     You might say the energy has gone full circle:

                                       Heat Energy     ➝      Energy of Motion        ➝     Heat Energy
The basic principle behind braking
systems: friction converts the       (from the engine) ➝    (through the drivetrain) ➝ (through the brakes)
energy of motion to heat energy.

                                     Drum brake                              Disc brake

                                                                                     Heavy Vehicle Braking

                                     Force Multipliers
                                     The force generated at the wheel to stop is a lot more than the force you
                                     apply when pushing down on the brake pedal.

The driver is using a tool to gain
leverage to measure the brake
chamber pushrod travel.

                                     In the diagram above, the driver is pulling on an air brake slack adjuster
                                     to measure if the brake is within adjustment tolerance.

                                     The slack adjuster, in addition to its use to adjust for brake wear, acts as a
                                     lever. Leverage is a form of force multiplication.

                                     Trucks and buses are much heavier than cars, so they need even more
                                     mechanical advantage.

                                     What is compressed air?
                                     Air can be compressed (squeezed) into a much smaller space than it
                                     would normally occupy. The tires on a car are filled with compressed air
                                     to support the weight of a vehicle.

The compressed air in the tire
supports the load.

   Fast Fact

   Air brakes were invented
   by George Westinghouse
   in 1869 to improve the
   stopping power of trains.         The smaller the space into which air is squeezed, the greater the air’s
   They were first applied            resistance will be to being squeezed. This resistance creates pressure,
   to trucks in the 1920s.           which can be converted into mechanical force to apply the brakes. Air
                                     brakes generate more braking force than hydraulic brakes.

Chapter 2

                                    If a constant supply of compressed air were directed through a pipe that
                                    is one-inch square (see diagram below), and if a one-inch square plug
                                    were placed in the pipe, the compressed air would push against the plug.
                                    Holding a scale against the plug would register how many pounds of
                                    force the air was exerting against the plug.

   Fast Fact                        If the scale registered 10 pounds, for example, then it could be said that
                                    the force was 10 pounds on the one square inch surface of the plug. This
   Air pressure is measured         would be 10 p.s.i. or 68.9 kPa.
   in pounds per square
   inch (p.s.i.). The metric        The more the air is compressed, (that is, the greater the air pressure), the
   equivalent measurement           greater the force that would be exerted on the face of the plug.
   is kilopascals (kPa),
   named after Blaise               Leverage and air pressure
   Pascal (1623-1662), a            Air chambers are very powerful. A typical type 30 chamber, if applied
   French philosopher and           with air pressure at 100 p.s.i. (690 kPa), develops a pushrod force of
   mathematician.                   3,000 pounds. This force is then applied to move the lever (the slack
   10 p.s.i. = 68.9 kPa             adjuster) to apply the brakes.

Through force multiplication, 100
p.s.i. (690 kPa) of air pressure
produces a pushrod force of 3,000

                                                                                      Heavy Vehicle Braking

                                       Power to Move and Power to Stop
                                       A typical compact car weighs about 1,000 kilograms and has about 120
                                       horsepower. It can accelerate to 100 km/h within about 200 metres and
                                       in less than 10 seconds.

A heavy tractor-trailer combination
will take a lot longer to accelerate
than a car – and a lot longer to
stop, too.

                                       A heavy tractor-trailer combination can weigh up to 63,500 kilograms
                                       when loaded. Even though it may have a diesel engine producing over
                                       400 horsepower, because of the weight of the combination and its load,
                                       it might take over one kilometre to accelerate to 100 km/h and take over
                                       one minute to do so.

                                       Now think about stopping a tractor-trailer combination that is going 100
                                       km/h. How much energy is needed to stop it? You certainly would want
                                       to stop it in a much shorter distance than one kilometre and in much less
                                       than one minute. In an emergency, the combination might have to be
                                       braked to a stop in as little as seven seconds — roughly 1/9 of the time it
                                       took to reach 100 km/h.

                                       To stop the vehicle this quickly would require a stopping force of nine
                                       times the acceleration force — the equivalent of approximately 4,000

                                       With a tractor-trailer combination equipped with eight equally-loaded
                                       axles and 16 brakes, each brake would have to provide approximately
                                        /16 of the braking force. If one or two of these brakes were not correctly
                                       adjusted, the others would have to do more than their share of the
                                       braking, resulting in a longer stopping distance. If the axles were not
                                       equally loaded, wheels with lighter loads may skid, also resulting in a
                                       longer stopping distance.

                                       As well, the other brakes would have to work harder and that might
                                       be more work than they were manufactured to do. Excessive use of the
                                       brakes would then result in a buildup of heat greater than the brakes
                                       could absorb and dissipate. Too much heat would result in brake damage
                                       and possible failure.

Chapter 2

 Fast Fact                   Stopping distance and stopping time
                             To stop a vehicle, you need to See-Think-Do. Total stopping distance is
 Reaction time is the
                             the distance your vehicle will travel from the moment you:
 time it takes between
 recognizing a hazard            •   See — a hazard
 ahead and applying the
 brakes.                         •   Think — decide to stop
 An average driver’s             •   Do — place your foot on the brake pedal until you stop
 reaction time is about
  /4 of a second.            This distance can also be expressed as the time it takes to stop.

                             When you see a problem ahead while driving, it will take you about
                              /4 of a second of Perception Time (See-Think) and another 3/4 of a
                             second of Reaction Time (Do). Only then will your vehicle begin to slow.
 Driver’s Perception Time    A car’s brakes begin to work almost instantaneously when you press
                             on the brake pedal. This is not the case with air brakes because there
     + Reaction Time         is a Brake Lag Time of approximately 4/ of a second between when
     + Brake Lag Time        you apply the brake and when the brakes begin to function. Add to this
     + Braking Time          the Braking Time, which is the time — or distance — the vehicle travels
                             before it stops.
     = Total Stopping Time
                             This is why it is so important to allow enough distance when following
                             other vehicles

                                 •   passenger cars and light truck drivers should use the two-
                                     second rule. Watch the vehicle ahead pass some checkpoint on
                                     the roadway, such as an overpass or sign post. Start to count:
 Driving Tip
                                     “One thousand and one, one thousand and two.” That’s two
 Total Stopping Time                 seconds.
 will be even longer if          •   drivers of buses, trucks and other large heavy vehicles should
 brakes are incorrectly              never be less than five seconds behind the vehicle ahead at
 adjusted or vehicle is on           highway speeds.
 a downgrade.
                                     Refer to Chapter 3 — Basic Driving Skills for more information
 The most common                     on following distance.
 air brake system
 defect found during             •   stay even further back when conditions are less than ideal.
 a commercial vehicle
 inspection is brakes that
 are out of adjustment.

                                                                                                                     Heavy Vehicle Braking

   Fast Fact

   Total stopping distance
   or time depends on
   the ability of the brake
   linings or pads to
   produce friction, the
   brake drums to dissipate
   heat and the tires to
   grip the road.

Total stopping distance includes
perception time, reaction time,
brake lag time and braking time.
Always allow enough following
distance or time and choose a
                                    Speed and weight facts
speed that will allow you to stop   Speed and weight affect the stopping power required to stop any vehicle,
safely.                             and how far it will travel before it stops. You need more stopping power
                                    whenever the speed you are travelling and/or the weight of your vehicle

   Fast Fact                             •      2 times vehicle speed requires 4 times the stopping power
                                         •      2 times vehicle weight requires 2 times the stopping power
   Driving without due
   care and attention and                •      2 times vehicle speed and 2 times vehicle weight requires 8
   unsafe speed are the top                     times the stopping power
   two factors assigned
   to commercial drivers
   involved in police-
                                                        Stopping Power Required to Stop a Vehicle
   reported casualty                   Speed unchanged — weight doubled                           Weight unchanged — speed doubled
   Traffic Collision
                                          10,000kg 30 km/h                                  10,000kg 30 km/h
   Statistics: British
   Columbia (2002)

                                          20,000kg 30 km/h                                  10,000kg 60 km/h

                                     2 X vehicle weight requires 2 X the stopping power     2 X vehicle speed requires 4 X the stopping power

                                                                    Both weight and speed doubled

                                                    20,000kg 60 km/h

                                                             2 X vehicle weight + 2 X vehicle speed requires 8 X the stopping power

                                    A typical compact car weighs about 1,000 kilograms. The truck in the
                                    illustration above may weigh 25,000 kilograms when fully loaded — over
                                    25 times heavier than a car. And heavy tractor-trailer combinations can
                                    weigh up to 63,500 kilograms.

Chapter 2

                                     There is only so much stopping power available, and the faster the
                                     vehicle is travelling, the more power it needs to stop. This is why it is so
                                     important to drive at a safe speed, at a safe following distance, and within
                                     the vehicle’s stopping capability.

                                     Compared to cars, trucks and buses have larger brake components,
                                     more wheels and more brake linings/shoes or rotors/pads. Despite these
                                     features, trucks and buses have longer stopping distances.

The truck is going to take much
longer to stop than the car — even
more so if going downhill.

                                     Another factor involved in stopping distances is the slope or grade of the
                                     road. A vehicle travelling down a hill will need a longer stopping distance
                                     than a vehicle travelling at the same speed on a level surface because of
                                     the effect of gravity. A vehicle travelling up a hill will stop in a shorter
                                     distance than a vehicle travelling the same speed on a level surface, again
                                     because of the effect of the grade.

                                     How much heat can brakes handle?
Most brake components operate
best at around 120° C and should
not exceed 250° C.

                                                                                   Heavy Vehicle Braking

   Definition                      In normal stopping, the brake components heat up to about 120° C
                                  (almost 250° F). The brake components are constructed to withstand
   Brake fade occurs when         more heat, and can handle up to about 250° C under hard braking.
   your brakes stop working
   properly because they          If excessive hard braking is done, heat will continue to build up to a point
   are overheated. Your           where it exceeds the capability of the brake components to absorb and
   vehicle takes longer to        dissipate the heat. As brake drums heat up, they expand away from the
   stop, or may not stop          brake linings. Too much heat can result in brake fade, brake damage
   at all if you have not         and/or brake failure. Brake components or tires may actually catch fire.
   properly controlled the

                                  Retarders are designed to provide auxiliary slowing of the vehicle, such as
   Definition                      for controlling the speed on long downgrades without the use of the main
                                  braking system. Engine retarders (also known as engine brakes) help to
   Retarders provide an
                                  save the main braking system for emergency stopping requirements.
   auxiliary slowing effect
   and assist the main            There are four main types of retarders in use:
   brake system to slow the
   vehicle.                           •   Engine retarders
                                      •   Exhaust retarders
                                      •   Hydraulic driveline retarders
                                      •   Electric driveline retarders

                                  Many retarders can absorb as much or more horsepower as the engine
                                  can develop.


An engine brake control switch.

Chapter 2

                             Using retarders
                             An engine retarder is more efficient at a higher engine r.p.m. and a lower
 Driving Tip                 vehicle speed, so it is important to select the right gear. Select your gear
                             before you start down a hill. You are more likely to miss a shift if you
 Make sure you are           wait until you are on the downgrade.
 familiar with the
 manufacturer’s operating    Retarders/engine brakes should be used with caution on slippery road
 procedures before           surfaces. Some can actually cause the drive wheels to lock up, stalling
 operating any retarding     the engine, and in some cases result in loss of control or even cause a
 device.                     tractor/trailer to jackknife.

                             All modern trucks are manufactured to meet safety
                             standards, including noise levels. A well-engineered
                             truck with an engine retarder and a properly
                             maintained muffler system should not be noisy.
  The term r.p.m. stands
  for revolutions per        Many municipalities post signs restricting drivers
  minute. This refers to     from using engine retarders/engine brakes. These
  the number of complete     signs must be obeyed.
  turns the crankshaft
  makes in one minute.
  Most trucks have a         Braking
  tachometer in the          Bringing your vehicle to a stop on a level roadway usually means
  instrument panel to        squeezing gradually and firmly on the brake pedal with your foot.
  indicate the engine’s      However, as the speed or weight of your vehicle or the degree of
  r.p.m.                     emergency increases, you may have to react more quickly.

                             Just before your vehicle comes to a full stop, reduce the pressure you
                             are placing on the brake pedal. This will prevent your vehicle from
 Fast Fact                   jerking back. Practise stopping until you become familiar with how much
                             pressure you need to ease off the brake to smoothly stop where you want.
 You may be fined as
 much as $2,000 if you       Watch the driving technique of the driver ahead. If the driver in front
 are operating a vehicle     of you is a tailgater, expect frequent panic stops. Professional drivers
 with brakes that are not    maintain a safe following distance and plan for gradual stops. By making
 adjusted correctly or not   your stops slowly you give the driver behind you plenty of notice that you
 performing adequately.      are reducing your speed.

                             Heavy commercial vehicles have longer stopping times and distances
                             than most other vehicles. To stop safely, you must ensure that your brakes
 Definition                   are correctly adjusted, your load is balanced and your tires are in good
                             condition. You should also be certain that your tires are properly inflated.
 Tailgaters are drivers
 who follow other            To ensure a smooth, controlled stop every time:
 vehicles too closely.
 These drivers are               •   travel at a speed that allows you to see a safe distance ahead
 allowing the vehicle            •   maintain a safe following time or distance
 ahead to be in their own
 vehicle’s Danger Zone.          •   use good braking skills

                                                                                  Heavy Vehicle Braking

                                     Icy roads
                                     To stop on icy roads, use extremely light pressure on the brake to control
   Driving Tip
                                     your vehicle’s speed. This will help prevent the wheels from locking up.
   Don’t wait until you spin         A slowly revolving wheel on an icy surface will be more effective than
   out on a hill to put on           a locked wheel skidding on an icy surface. ABS equipped vehicles will
   the tire chains.                  require a different technique for braking on an icy road surface, as shown
                                     later in this chapter.
   It is extremely difficult
   to mount chains                   Using snow tires and/or chains on icy roads is
   when you are on a
   hill. Instead, use a
                                     a good idea. Many highways have designated             CHAIN UP
                                     chain-up areas to allow you to mount chains
   designated chain-up               before getting onto steep hills. Make sure you           AREA
                                     are familiar with how to mount chains on tires
   Use your “chain sense”                                                                         E X IT
                                     — practise mounting them before you find
   to stay out of danger in          yourself in conditions where you need to put
   adverse road conditions.          them on your vehicle.

                                     Make certain tire chains are properly sized for your tires and tightly
                                     mounted (do not deflate tires to install tire chains). You should stop and
   Fast Fact                         check tire chain tightness at the first safe opportunity. Reduce speed
                                     when driving with tire chains.
   Frequent use of the
   service brakes will cause         In extremely bad conditions, it may be safer to park your vehicle than
   the service brakes to             to continue driving. Check weather and road conditions before you
   heat up, reducing the             encounter icy roads.
   stopping ability of the
   vehicle.                          Downgrades
   The result can be a               There is a practical limit to the amount of heat that brakes can absorb
   dangerous condition               and dissipate. The highest brake temperatures occur when braking from
   called brake fade.                highway speeds while descending long downgrades, or from repeated use
                                     of the brakes without enough time between applications for the brakes to
                                     cool. Almost all brake failures and downhill runaway crashes are caused
                                     by overdriving the ability of the brakes to deal with heat. In other words
                                     – poor speed control.

                                                          Whether in town or on a highway, in most cases,
Some steep hill warning signs
                                                          you will need to descend a hill at a speed below the
show the steepness of the grade.                          speed of other traffic in order to avoid overdriving
The higher the percentage, the                            your brakes. With today’s diesel engines, you
steeper the hill. This hill has an                        should be in a lower gear to go down the hill than
18 per cent gradient, which is                            used to climb it. Never shift to a higher gear on
very steep.                                               a downgrade unless the speed on the grade can
                                                          be controlled with a retarding device or engine

Chapter 2

   Fast Fact                           On highways
                                       Truck advisory speed limits are often posted. Follow these speed limits
   The braking effect from             where posted. Use your four-way flashers to alert other drivers that you
   engine compression is               are driving slowly down the hill. Stay in the far right lane where possible.
   greatest near governed              You must control your speed all the way down every hill so you can
   maximum engine r.p.m.               respond to any emergency.
   and with the transmission
   in a low gear. Save your                •   select a safe speed that is not too fast for the weight of your vehicle,
   brakes so you will be                       length, and steepness of the grade, weather, and road conditions
   able to stop if road and
   traffic conditions require               •   select an appropriate low gear to hold that speed
   their use.                              •   use the vehicle’s retarding device

                                       If you have selected an appropriate safe speed, are in the right low
   Fast Fact                           gear and are using your vehicle’s retarding device, you should be able
                                       to go down the hill without using the service brakes.
   Always obey the
   regulatory stop and                 If this does not control your speed, and speed is increasing above your
   check brakes signs.

These road signs may be placed
before a downhill grade. The one
in the top left corner shows the
speed suggested for descending the
grade. The other signs direct you to
check your brakes.

                                       chosen speed:
                                           •   apply the brakes hard enough to reduce speed by 10 – 15 km/h
                                               — the brakes are cold at this point
                                           •   downshift to a lower main gear (do not use the splitter valve for
                                               shifting on a downhill)
                                       Continue down the grade, using engine compression, transmission
                                       gearing and your vehicle’s retarding device to control your speed. If the
                                       speed again increases to above your chosen speed, repeat this process. Be
                                       careful using this procedure on icy roads.

                                       Keep your vehicle in gear all the way down the hill.

                                       In town
                                           •   select a safe speed that is not too fast for the weight of your vehicle,
                                               length, and steepness of the grade, weather and road conditions
                                           •   select an appropriate low gear to hold that speed
                                           •   use the vehicle’s retarding device unless signs prohibit their use

                                       You must control the speed of your vehicle so it can be stopped at any
                                       time for any emergency while descending a hill.

                                                                                    Heavy Vehicle Braking

                                      Water on roadways
                                      Water entering the brake drums will reduce their braking efficiency.
                                      Avoid driving through large amounts of water whenever possible. If it is
                                      necessary to drive through water on the roadway, cover the brake pedal
                                      with your foot as you approach the water. Place a slight drag (that is,
                                      apply a slight constant pressure) on the brakes while you drive through
                                      the water. The slight drag placed on the brakes will reduce the amount of
                                      water entering the brake drums and shoes. Always reduce your vehicle’s
                                      speed before driving through large pools of water on the roadway.

                                      During extremely wet conditions, or after driving through water, test
                                      your brakes for safe operation. Do this by applying a slight pressure with
                                      your foot on the brake pedal. Keep this pressure on for a short distance
                                      to dry out the brakes.

                                      Runaway lanes
                                      Runaway lanes are located beside the road on some downhill grades.
These two road signs alert you to
runaway lanes. The sign on the left
is posted in advance of a runaway
lane; the one on the right is found
at the entrance to a runaway lane.

                                      These lanes are there to help slow and stop vehicles if their brakes fail
                                      as they are going downhill. Use a runaway lane only if you are unable to
                                      control your speed or stop.

                                      These lanes are there for safety. Do not use them for any other purpose.

                                      Combination unit braking
                                      When you step on the brake pedal of an air brake equipped vehicle, all of
                                      the brakes are applied at the same time and at the same pressure.

                                      Trailers equipped with electric or vacuum brakes work in a similar way.
                                      Applying brake pressure too forcefully may cause the wheels to lock up
                                      which can cause a vehicle to jackknife or skid. Use extra caution when
                                      applying brakes on or before a curve, on wet or icy surfaces or when
                                      vehicle load is light.

                                      Remember that the brakes, tires and suspension of a combination vehicle
                                      work best when the vehicle is legally loaded and the load is properly
                                      distributed among axles. When the cargo area is empty and there is
                                      no ABS, the vehicle’s wheels may bounce and lock up. This can make
                                      braking more difficult and may mean you will need a longer distance to
                                      stop, especially when going down hills.

                                      You need to take extra care to safely tow two trailers. It can be difficult
                                      in an emergency to stop a multiple trailer combination in a straight line.
                                      In bad weather, slippery conditions, and mountain driving, the risk of
                                      skidding and loss of traction increases. Look far ahead, leave plenty
                                      of following distance, and slow down to help avoid having to make
                                      emergency stops.

Chapter 2

                                     Anti-lock braking systems
                                     Anti-lock braking systems (ABS) are mandatory in Canada on all trucks
                                     and buses over 4,560 kilograms manufactured since April 1, 2000 and on
                                     all commercial trailers equipped with air brakes.

                                     ABS is an addition to a normal air brake system. The purpose of ABS is
                                     to prevent wheel lockup or skids caused by over-braking on slippery road

                                     Contrary to what many people believe, anti-lock braking systems do not
                                     allow you to drive faster and they don’t always allow you to stop sooner.
                                     In fact, on some surfaces such as gravel, the braking distance needed with
                                     ABS may be longer.

                                     Anti-lock brakes can help prevent wheel lock-up on surfaces where
                                     conventional brakes usually lock up including surfaces that may be
                                     slippery because of water, ice, wet leaves, grease or spilled fluids. This
                                     means you can brake hard without skidding and losing steering control.
                                     ABS may also help you prevent your vehicle from jackknifing.

                                     Anti-lock braking systems are only as good as the driver. Learn the
                                     correct technique and practise it so that you are ready in an emergency.
                                     Read the manual from your vehicle manufacturer to learn how to use
                                     your anti-lock braking system properly.


   A full jackknife occurs
   when a tractor and the
   trailer form an angle
   of 90 degrees or less,
   relative to one another.
   Warning: this can
   happen even at very
   low speeds. Jackknifing
   is often caused by poor
   braking techniques,
   including improper use
   of retarders.

A fully jackknifed tractor/trailer

                                                                      Heavy Vehicle Braking

                        A few tips for emergency stopping with ABS:

                            •   Apply firm, hard continuous pressure to the brake pedal until
Driving Tip                     the vehicle stops.
It’s a good idea to         •   Don’t pump the brakes as this turns the system on and off.
practice using ABS so       •   Don’t be alarmed by brake noise, pedal movement or shudder.
that you are familiar           This is normal. Keep applying firm pressure.
with how it works and
feels.                      •   Be extremely cautious when attempting to steer around
                                obstacles. Remember that steering and handling characteristics
                                will be affected by the size and load of the vehicle you are
                                driving, and by the trailer you may be towing.

                            •   When operating combination vehicles, make sure you know
                                which of the units have ABS. If the tractor and trailers do
                                not all have ABS, apply the brakes as if you were operating a
                                combination without ABS.

                        Automatic traction control
                        Automatic Traction Control (ATC) is an optional addition to an ABS
                        system. It uses the pulsating signal from the speed sensors to detect loss
                        of drive wheel traction on slippery road surfaces.

                        Using the same wheel sensors used by ABS, ATC can sense if a wheel
                        has lost traction and is spinning instead of helping to accelerate the
                        vehicle. If a drive wheel begins to spin, the traction control system will
                        apply that brake, transferring power to the opposite side. In extremely
                        slippery conditions, the traction control system will reduce engine power,
                        allowing the wheels to regain traction.

                        Most vehicles equipped with ATC will have an information decal on the
                        dashboard and a dash-mounted lamp that will light if wheel spin occurs.

Chapter 2

                              Review Questions
            1. What is the final factor that will determine if the vehicle will

            2. How is brake heat dissipated?

            3. If the weight of the vehicle is doubled, how many times must
               the stopping power be increased?

            4. What causes brake fade?

            5. What can cause jackknifing?

            6. What is the difference between ABS and ATC?

            7.   What is one of the hazards of stopping a tractor/trailer

            8. When should a retarding device be used?

            9. What is brake lag time?


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