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A tractor-trailer has several pieces, much like any other kind of vehicle. Although each of these components serves a specific purpose, collectively, they drive the tractor-trailer effectively and efficiently.


As a professional driver, you must comprehend car design and operation fundamentals .This will assist you in maintaining a secure car and enable you to identify issues before they become severe and result in a breakdown or accident.

Remember that not all cars will have the components covered as you read this chapter.



The framework around which the car is put together is called the frame. The frame comprises cross members that join the frame rails on the left and right sides. The frame is strengthened and supported by this. All truck components are directly or indirectly related to the frame. Engine mounts hold the engine to the frame. Through the frame, wheels and axles are attached to the suspension system. Many cargo tanks and van trailers are among the vehicle kinds that lack a structure. The weight of the automobile is supported by its body.


Suspension System


The suspension system holds up the truck’s body and frame. Torque arms, airbags, springs, and mounting hangers make up its construction. The suspension system is connected to the front and back axles, and the frame is supported. In reaction to changes in the terrain, the suspension system enables the axles to shimmy up and down without significantly altering the load. It is possible to disperse the stress of road shocks uniformly throughout the frame by securing the suspension system at several locations. The system’s strength and longevity depend on the size and weight of the vehicle itself and the size and weight of the payload being transported. The qualities required for a decent suspension system are as follows:


  • The ability to carry the burden;


  • Capability of fully transferring braking force to the chassis structure


  • A comfortable ride for the driver and the cargo, whether it is loaded or not


  • Axles must be secured to ensure proper driveline alignment.


  • Lightweight components and simple maintenance.


Airbag and leaf spring suspension systems are the two primary varieties.


Leaf spring suspension — Metal strips of various lengths are joined and affixed to the spring mounts or hangers in this suspension system.


Air bag suspension — In the case of this kind of suspension system, airbags are positioned between the axle and the frame. The majority of the time, trailers and truck tractors employ this technique. Regardless of whether the car is loaded or empty, this technology offers as one of greatest features of both systems may sometimes be merged in these systems.



Shock absorbers — Shock absorbers lessen the motion of the car’s body while its wheels travel over uneven terrain. They are used in airbag and leaf spring suspension systems.




Generally speaking, the axles sustain the weight of the automobile and its load and link the wheels to the rest of the car. Different axles serve different purposes.


Front tractor axle — The steering axle is this one. It links the brakes and steering system.


Rear tractor axles — These are the driving, or power, axles. They act as the hub where the brakes link and transmit power from the engine and drivetrain to the wheels.


Axles may be divided into two groups or types:

  • Dead axles — Dead axles do not transmit power. They serve to sustain the weight.

  • Live axles — Live axles use a differential gear housing to transfer power from the driving shaft to the wheels. A hollow live axle is used. As a result, power may be transferred to the wheels via this region through the gears and axles.




The power for the vehicle comes from the engine. Either diesel or gasoline is used to power it. You could also discover the usage of propane or natural gas as fuel in today’s motors. An internal combustion engine powers a tractor-trailer. This indicates that the engine’s closed chambers are where fuel is burned. Cylinders are the name given to closed chambers. The engine’s cylinders are where the vehicle’s power is generated. The engine has five basic parts:


  • Engine block;

  • Cylinders;

  • Connecting rods

  • Crank shaft.

  • Pistons.


The engine is powered by diesel or gasoline, as already indicated. There are certain parallels between the two internal combustion engines, such as the usage of air and fuel for ignition.

Today’s gas engines may be equipped with a carburetor, spark plugs, or fuel injectors that resemble those in a diesel engine. Several fuel injectors are included in the diesel engine. During compression, fuel and air are combined. Diesel engines have substantially greater compression ratios, and the fuel/air combination is ignited due to the compression pressure.


Fuel System


Fuel is supplied through the fuel system to the engine. It decides when and how much gasoline is injected into the engine’s cylinders. It also regulates the quantity of fuel given to the engine.


The fuel injectors receive fuel from the fuel tanks through the fuel lines, fuel filters, and fuel pumps. An injector pump drives the fuel injectors, which spray a fine gasoline mist into the engine cylinders.


When it comes to the fuel system, cleanliness is crucial. The engine may be harmed by even the tiniest quantity of dirt or other debris. A well-functioning fuel system will remove many contaminants, but you must also fill your automobile with gas carefully.


Fuel tank — The gasoline is kept in the fuel tank. The size and nature of the vehicle will determine whether there are one or two fuel tanks. Added tanks may contain fuel for heaters and refrigerators.


Clear the area around the fuel cap. Avoid letting dirt or other debris into the gasoline tank.


Fuel filters — The gasoline is cleaned throughout the fuel system by several filters. These filters clean the fuel system of debris, moisture, and corrosion.


Fuel system heaters — In particular, fuel system warmers are crucial for automobiles driven in cold climates. There are three main categories of heaters:


  • Tank-mounted heaters that heat gasoline within the tank;

  • In-line heaters that warm up gasoline as it travels from the tank to the injector system;

  • Filter heaters that assist in heating the fuel as it travels through the injection system.


There are several diesel trucks without fuel warmers. Some of the gasoline that passes through massive engines is not burnt; instead, it is returned to the fuel tanks through a fuel return line. Some businesses rely on this technique as their “fuel heating” system since it warms the fuel.


Air Intake and Exhaust System


The diesel engine’s ability to operate correctly depends on fresh air. The engine’s ability to work effectively depends on how the air intake and exhaust systems process fresh air.


Air intake system — The air intake system sends a lot of clean air to the cylinders while filtering water, dust, and other impurities.


Exhaust system — Following combustion, the used gases are collected by the exhaust system, sent via the turbocharger, and finally released through a tailpipe or exhaust stack.


Turbo charger — The turbocharger transforms the energy it receives from the exhaust fumes into energy the engine can utilize.


After cooler — The turbocharger’s air intake is cooled by the after cooler, which raises the temperature to a safe level.


Pyrometer — The pyrometer is a device used to gauge exhaust gas temperatures. The average temperature is between 600° and 1,000° F, depending on the vehicle. The engine and turbo might be damaged if the exhaust temperature is too high.


Lubrication System


There are three uses for the lubricating system. The engine is lubricated, cooled, and cleaned.

  • Clean oil is delivered to various engine components via the lubrication system. This prevents the engine components from rubbing against one another as they move (friction). This improves engine performance and protects against harm.

  • The engine generates a lot of heat due to its moving components. Oil flowing through the machine keeps the engine temperature at a safe level, which absorbs part of this heat and eliminates it.

  • Oil maintains the engine’s cleanliness. It gathers dirt and debris that might harm the machine as it passes through the engine. The engine filters these particles away. By covering stationary engine surfaces with oil, rust and corrosion are also prevented.


Maintaining the lubrication system — Making ensuring your car’s engine runs smoothly and efficiently may be achieved with proper preventative maintenance. According to the manufacturer’s guidelines, replacing the oil regularly is essential.


The condition of the oil filter should be evaluated as part of the oil change. The dirt and debris that the oil removes from the engine are dumped in the oil filter. Clean oil may flow through the machine thanks to a clean filter.


Additionally crucial is regular checking of the engine’s oil level. The dipstick may be used for this. The engine’s side is where you’ll find the dipstick. To gauge the oil level in the engine:



  • Turn off the engine.

  • Put on gloves (the engine may be hot and you need to protect your hands).

  • Take out the dipstick and clean it with the cloth.

  • Place the clean dipstick back in.

  • Take out the dipstick once again.


The amount of oil in the engine, which is expressed in gallons, should now be determined. If the reading is unclear, clean the dipstick, insert it again, take it out, and read it once more.


Before starting the car again, add oil if the dipstick says “low” or “add.”


Oil filter system — The oil filter may help the engine last longer by eliminating impurities as oil runs through. Three different styles of oil filter systems exist:

  • Systems with the entire flow;

  • Bypassing systems;

  • Systems that combine bypass and total flow.



Cooling System


Even while heat is necessary for an engine to function, too much of it may harm an engine. A machine that is running generates fewer heat thanks to the cooling system.


The water pump moves coolant, a mixture of water and antifreeze, through the engine’s water jackets after being stored in the radiator. The coolant absorbs part of the engine’s heat before returning to the radiator. The hot coolant in the radiator circulates via tubes, where it is cooled. The process is then repeated with the cooled coolant.


Always choose coolant under the recommendations of the engine manufacturer. Ensure that the appropriate quantity of coolant is added to the radiator. Both too much and not enough might harm the engine. Also, consider that various operating circumstances (weather, etc.) need varying concentrations of the water-antifreeze combination.



Electrical System


There are several uses for the electrical system. It is used to start the engine and control the electrical features of the car, such as the lights and instruments.


Four sections make up the electrical system:

  • Charging circuit;

  • Cranking circuit;

  • Ignition circuit; and

  • Lighting and accessory circuits.


Charging circuit — It generates power in the charging circuit. This powers the electrical circuits and keeps the battery charged. There are six main components in the charging circuit.


Battery — An electrochemical device called a battery provides and stores electrical energy. The battery’s energy turns on the starter.


Alternator or generator — Although most modern automobiles have alternators, some still operate on generators. These gadgets are in charge of charging the battery. Electricity is produced by the alternator when the engine is operating. The alternator is primarily responsible for supplying the vehicle’s energy demands.


Ammeter — A gauge on the car’s instrument panel called an ammeter displays how much current is coming from the alternator. It indicates whether or not the battery is charging or draining.


Voltmeter — Another gauge on the instrument panel of the car is the voltmeter. You can see how fully charged the battery is.


Voltage regulator — The component that controls the voltage output of the alternator or generator is the voltage regulator. The voltage regulator prevents an excessive increase in battery voltage. The battery may overload if the voltage is too high.


Cranking circuit — Electricity from the battery is transferred to a starting motor via the cranking circuit. This operation is initiated by depressing the starting switch inside the car.


Ignition circuit — In a gasoline engine, the fuel and air combination is ignited by sparks produced by the ignition circuit, which is an electrical circuit. A diesel engine does not need an ignition circuit.


Lighting and accessory circuits — These circuits provide the vehicle’s lights, horn, instrument lights, windshield wipers, etc., with power.


Drive Train


A drive train is a group of mechanical components that transfer engine power to the tractor’s driving wheels.


The following are the primary components of the drive train:


  • Clutch;


  • Transmission;


  • Drive shaft;


  • Universal joints; and


  • Differential.


These components change the engine’s torque/twist and speed, converting them into power to propel the car forward.


Clutch — The component of the drive train that enables the driver to connect or detach the engine from the drive train is the clutch. The driver can change gears thanks to the clutch. The clutch consists of six main components:


  1. Clutch housing;
  2. Flywheel;
  3. Clutch disc(s);
  4. Pressure plate;
  5. Release assembly;
  6. Controls.


The clutch on the majority of vehicles has three plates, two of which are securely pressed together. The driving member (clutch disc), the plate in the center, is attached to the shaft going into the gearbox. The driving elements on the other two plates are connected to the engine. The two driving parts are forced together by a powerful spring or spring. As a result, their hold on the central plate becomes firmer, and they eventually turn together.

The initial driving component is made of the engine flywheel. It has a smooth surface where the driven plate presses up against it.


The pressure plate is the other driving component. It is a large, smooth-surfaced cast iron ring. For them to all rotate simultaneously, it is secured to the cover and bolted to the flywheel. It is attached to allow for back-and-forth movement.


The driven plate is a steel disc with friction facing both sides and flat. The plate is attached to a shaft heading to the gearbox via splines. The plate may move back and forth on the shaft by sliding into grooves, allowing them to revolve together. Compared to the other plates, this disc is softer. By doing this, additional components of the drive train won’t be harmed as the disc ages.


Transmission — A box or case of gears serves as the transmission. It is attached to the clutch housing and is placed behind the clutch.


The engine’s output of power is adjusted via the gearbox. This delivers the necessary speed and torque. To move the vehicle forward, it transfers this power from the engine to the drive or powered axle(s).

The gears in the gearbox regulate the car’s speed. The vehicle may travel at various speeds despite the engine having just one rate.


Torque is another attribute of the gearbox. This causes things to spin. The quantity of power a vehicle generates is correlated with the torque of the gears of a gearbox. The driver may adjust the speed and torque (force) the engine sends to the wheels using the kits. When choosing bags, remember that the ratio of torque to speed decreases with increasing speed and vice versa.


You may alter the ratio between the engine and the rear wheels to generate additional speed or torque by adjusting the gearbox gears.


Drive shaft and universal joints — A propeller that goes to the back of the vehicle is located behind the gearbox. The driving shaft is what it is called. The drive shaft, which links the gearbox to the vehicle’s back, may be solid or hollow steel. Universal joints are located at the front and back of the shaft. Most of the time, they consist of two U-shaped parts at right angles to one another and are joined by an equal-length cross.


The drive shaft transmits the engine’s revolving action to the rear axle. The post is joined to the gearbox and the rear axle via universal joints.


The cross’s arms serve as the pivot for the U-shaped sections. The two shafts may be at an angle to one another and yet revolve around and transfer power since there are two pivots. They don’t have to run parallel to each other.  This is crucial because they start to move out of alignment with each bump in the road. While the gearbox moves relatively little since it is attached to the frame, the rear axle swings up and down with the wheels. The driving shaft may rotate smoothly even when its two ends aren’t constantly parallel, thanks to the universal joints.


Differential — The drive train component known as the differential divides the driving axle in half, enabling one wheel to spin at a faster or slower speed than the other. This facilitates turning for a vehicle. Your right wheel moves at a different pace than your left wheel because the outer wheel must revolve more quickly than the inner wheel while making a turn.


Brake System


The car slows down and comes to a halt thanks to its braking system. There are three main components to it:


  • Service brakes;

  • Emergency brakes; and

  • Parking brakes.


Service brakes — Service brakes reduce speed and bring the car to a halt. The foot and trolley valves on the driver’s vehicle are used to control the service brakes.


Service brakes come in two varieties: drum and disc.


Drum brakes — This drum-like metal is fastened to both ends of an axle via bolts. The drum’s inside contains the braking mechanism. The brake shoes and linings must be pressed or shoved against the interior of the drum to stop. As a result of the friction, the vehicle slows down and generates heat. Too much heat production may cause the brakes to fail, making stopping more difficult or even impossible in certain circumstances.


Disc brakes — In disc braking systems, the wheel’s inside is linked to a fixed disc that spins together with the wheel.


Emergency brakes — When there is a problem with the air system, emergency brakes slow down and stop the vehicle.


Parking brakes — Parking brakes stop a car from moving while it is destroyed.


Air brakes — Air brakes use compressed air to increase braking power. The following is a list of the components that make up an air brake system.


Compressor — This device aims to increase and maintain the necessary air pressure in the braking system reservoir. The vehicle’s engine powers the compressor. When the engine starts, it starts. The engine’s lubrication system lubricates any belts, pulleys, shafts, and gears that the compressor may use. It typically operates between 90 and 125 pounds per square inch (psi). The compressor continues to work until 125 psi of air reservoir pressure is reached. After then, it comes to a standstill.


Governor — The governor controls the compressor. The compressor can maintain the proper pressure by opening and shutting intake valves.


Air reservoirs — The compressed air is kept in air reservoirs. The three tanks used to store air are the wet tank, dry tank, and trailer reservoir. The damp tank removes most of the moisture and supplies air to the dry tank. For usage in both routine and emergency scenarios, the trailer reservoir stores compressed air adjacent to the trailer brake chambers. Depending on how many tanks there are and how big the braking sections are, the tanks come in different sizes


One-way check valve — This valve stops air from the air reservoirs from entering the compressor again. The brakes won’t operate if air flows backward.


Safety valves — Safety valves, also known as pressure relief valves, prevent harmful air pressure from building up. When the system’s pressure hits 150 psi, a safety valve typically opens, releasing air into the atmosphere and reducing system pressure. The air governor may need maintenance if the safety valve activates often or constantly. A trained mechanic should be the only one to do this. Never carry out these modifications by yourself. Everybody has undoubtedly heard an air discharge from a large vehicle while nearby. It is usual for the safety valve to release in this manner.


Drain cocks — Reservoirs for the air brake system’s moisture are discharged using drain cocks. If this moisture is not eliminated, the braking system might be harmed, particularly during the winter when the water can freeze. It would help if you drained the humidity every day to avoid harm. The car should be adequately choked, and this should be done on a flat surface. To ensure that all the moisture is discharged, let the air pressure escape.


Air pressure gauge — On the instrument panel is where you’ll find the air pressure gauge. It informs you of the system’s pressure level. 90 to 120 psi is the standard working range. Always include this gauge in your visual inspection of the dashboard gauges. This will enable you to find problems before they become emergencies. It’s essential to remember that this might be one gauge with two needles or two different gauges, primary and secondary.


Low-pressure warning signal — You are alerted by this indicator that the air pressure has decreased to a potentially dangerous level (below 60 psi). Either a buzzer will ring, or red warning light will illuminate. If this occurs, cease right away. You risk losing additional pressure, and your brakes lock up without quick stopping. Before you may go on your journey, something has to be fixed.


Air application pressure gauge — This indicator shows the amount of air pressure being applied to the brakes. The gauge will indicate 0 psi if the service brakes are not under pressure. (Not on every truck.)


Treadle valve — The treadle valve, commonly known as the foot brake or brake pedal, regulates the airflow necessary for the brakes to function. This valve is managed by applying pressure to (depressing) the brake pedal. The air the system receives increases the farther down the brake pedal is depressed until it reaches maximum capacity. By allowing air to escape from the brakes, the brakes are released when the brake pedal is released.


Independent trailer brake — The independent trailer brake is a hand-operated device that applies the trailer brakes by controlling the airflow to the trailer unit (also known as the trolley valve or hand valve). Never use the brake to reduce speed or to stop. (Not on every trucks.)


Tractor parking valve — The tractor parking brake may be released by pushing in on this hand-operated knob, and it can be reset by pulling it out. In addition, it controls the spring brakes.


Glad hands — Glad hands are connectors that are attached to the trailer’s front and are used to connect air lines from the tractor. The connection is solid once the happy hands lock. An O-ring seals the bond, limiting air leakage and the entry of dirt and debris. Before connecting the airlines, you should inspect the O-ring for damage or dirt and debris. These O-rings have distinct colors. Typically, the emergency line is red, and the service line is blue. To keep out dirt and debris, the airlines should be sealed with protection plates after being unhooked. Brake failure is less likely if the airlines are kept clean.


Quick release valve — This valve, situated close to the braking chambers, enables the brakes to release quickly. The compressed air in the sections immediately escapes into the atmosphere as you take your foot off the valve. As a result, the brake shoes might swiftly disengage.


Relay valve — The application and release of the rear wheel brakes are accelerated using this valve. In other words, the relay valve is in charge of ensuring that all brakes engage simultaneously.


Emergency relay valve — Either the trailer reservoir or the trailer frame near the braking chambers is where the emergency relay valve is mounted. This valve regulates the brakes’ application and offers automated trailer brake application in the event the trailer separates from the tractor. The trailer brakes may also be immediately released using the emergency relay valve.


Brake chamber — The air-filled braking chambers are positioned near each tire. A pushrod with a slack adjuster (cam brake) or a pushrod alone connects them to the brakes (wedge brake). The braking lines are where air enters the chamber. The force is mechanically applied from the brake shoe and drum to the section.

Air brake system operation — What does the primary function of an air brake system?


  • The compressor pumps the pressured air to the wet tank, where moisture is removed.


  • The air travels to the dry reservoir after passing through the one-way check valve.


  • Air goes via the service airlines to the tractor brake chambers when you step on the brake (both front and rear). Air moves through the service glad hand, the tractor protection valve, and the trailer brake chambers.


  • Air escapes from the tractor and trailer via the fast-release valve and foot valve when you release the brake pedal.


Antilock brake systems — National Highway Traffic Safety Administration (NHTSA) regulations mandating antilock braking systems (ABS) for large vehicles, tractors, trailers, and buses were published in March 1995. These regulations specifically demand ABS for:


  1. Every new tractor-trailer produced on or after March 1, 1997;
  2. All new single-unit trucks, buses, and air-braked trailers manufactured on or after March 1, 1998;
  3. All brand-new, single-unit trucks and buses manufactured on or after March 1, 1999, must have hydraulic brakes.

The antilock system aims to prevent wheel lock-up, which may cause a vehicle to jackknife or slide. An electrical system that detects the speed of each wheel enables this. An air release valve is opened, and the chamber’s air is let out when the wheel speed sensor senses an impending lock-up. The valve closes, and the atmosphere is reintroduced as the wheel speed rises. If a lock-up is detected when the car halts, this happens automatically.


The antilock system engages when you push down on the brake pedal firmly enough to lock the wheels. Three to five times more quickly than a person could, the computer system “pumps” the brakes when it detects the danger of lock-up. You should never pump the brakes if a car has an antilock braking system. Make use of the computer to complete the task.


On the dashboard, most automobiles with ABS feature a warning light. When you start your car, this light will come on briefly. The ABS may not function if the light remains on or abruptly comes on while driving. The antilock function could not work even if your brakes were still functioning correctly. Check this out as soon as you can.


When the brakes are engaged, the light briefly illuminates as well.


Wheels and Tires


Wheels — The two-wheel styles most often encountered on trucks are spoke and disc wheels.


  • Spoke wheels — Two parts make up spoke wheels. They are heavier and more difficult to balance and line.

  • Disc wheels — Steel or aluminum is used to make disc wheels. Because they are secured with a single locking stud and nut that passes through both rims, they are simpler to align.


Most vehicles employ either stud-piloted or hub-piloted wheel mounting methods.


  • Stud piloted — The studs on the wheel hub are used in stud-piloted mounting systems to center and guide the wheel.

  • Hub piloted — The wheel hub uses corner-piloted mounting systems to center and guide the wheel.


Either flange nuts or ball-seat nuts are used to secure the wheels.


Tires — The majority of vehicles utilize one of three tire types.


Radial tires — Perpendicular to the tread, the body ply cords wrap around the tire. Belt plies are also present in radial tires, running beneath the track circumferentially around the tire. The traction and fuel efficiency of these tires are improved.


Because radials have more traction and produce less heat and friction when their surface area makes contact with the road, their tread lasts 40 to 100% longer than conventional tires. This improves the performance and fuel efficiency of your car.


Bias ply tires — The tire’s body ply cords cross it diagonally. Under the tread, the tire may also contain breaks or thin plies.


Belted bias tires — Belted plies wrap the tire circumferentially under the tread while the bodies ply cords run diagonally across the track. This tire’s belts are made of more substantial material.

Mixing radial and bias ply tires — Radial and bias ply tires shouldn’t often be combined. Your car’s tires should all be the same size and make. If the two tire types are combined inappropriately, the traction and turning capabilities variations might lead to issues. The tires may be mixed in a certain manner. For recommendations or instructions, speak with the tire manufacturer.


Tire tubes — Some tires come with tubes that slide over the rim. The tire tube contains the air that maintains the tire’s inflation. Because the under-inflated tube rotates within the tire, flat tires are more likely to catch fire.


Tubeless tires — Onto a one-piece wheel are installed tubeless tires. The air is stored in the tire rather than a tube. It is lighter and simpler to install tires without tubes.


Tire maintenance — It’s crucial to inflate tires properly. Under inflation may shorten tire life and accelerate tread degradation. Tire damage and decreased stopping power may result from overinflation. For instructions on inflation, refer to the vehicle’s owner’s handbook.


Regular tire pressure checks are advised. When the tire is cool, use an accurate gauge to do this.


Tread design — There are two different types of truck tire treads.


Rib tread — This tread’s open groove design gives you a high level of control and prevents skidding. On tractors, the front wheels should have this kind of tread.


Lug tread — Lug treads provide the greatest wear prevention and improved traction in over-the-road, high-torque applications. Drive wheels are advised to have this kind of tread. This tread often provides the car with higher fuel efficiency than rib tread.


Proper tire inflation — Increased fuel economy, a smoother ride, and a decreased risk of a blowout or tire fire are just a few advantages of properly inflated tires. Maintain regular tire inspections and use a tire gauge to check the pressure. For the right pressure, see the manufacturer’s specifications.


When the tire is cool, the pressure should be checked and corrected. The pressure measurement will be greater if the tire is heated. Do not let the air out of a hot tire if you inspect it and find that the pressure exceeds what is advised. Recheck when the tire has cooled down.


Ensure you do not exceed the rim or wheel rating while inflating a tire by keeping in mind the maximum pressure for the weight you are hauling.


The tire pressure should rise when the tires warm up while your car is in normal operation. Expect a 10-15 psi boost. Greater rises, which may produce an abnormally high heat level, might mean wrong tire size, over speeding, and/or underinflation. You need to halt, look into, and fix the issue if this occurs.


Under inflation of a tire — The following things may occur when a tire is under-inflated:


  1. The tread wears down more quickly;
  2. The temperature within the tire increases, possibly causing the separation of the tread from the body or belt ply;
  3. Deflation can occur, weakening the tire’s body cords, potentially causing a blowout.


A fire may ignite if you use duals and one of them is flat or under-inflated.


Overinflation — Overinflated tires become stiff and are more easily cut or punctured by roadside items. The ride is also less comfortable when the tires are overinflated since they cannot absorb shock.


Tire care — According to the instructions of the manufacturer, tires should be rotated frequently. This will facilitate dispersing wear evenly.


The Federal Motor Carrier Safety Regulations mandate daily tire inspections as a smart, safe practice (FMCSRs). According to Section 392.7, a driver must be certain that specific vehicle components and add-ons are in good functioning order before operating a vehicle. Tires are a part of this.


In the FMCSRs, appropriate tire wear and tread depth are also included. If a tire has any of the following flaws, you are prohibited from using your vehicle by Section 3937.5:


  1. Belt or body ply material that is visible through the tread or sidewall;
  2. Any separation of the sidewall or tread;
  3. The tire is flat or sounds like it’s leaking;
  4. The ply or belt material is exposed via a cut in the tire.


The tread groove pattern depth of each tire on the front wheels must be at least 4/32 of an inch when measured at any point on the main tread groove, according to Section 393.75 of the California Vehicle Code. When measured in the main tread groove, all other tires must have a tread groove pattern depth of at least 2/32 of an inch.


Matching and spacing of duals — If two tires of different diameters are mounted side by side, the bigger tire, which carries more of the weight on its own, will overheat and protrude from the sides.


Due to improper contact with the road, the smaller tire may wear unevenly and even detach the tread. One or both tires might blow up if the bigger tire bulges out too much and starts to contact the other tire. This will increase heat and friction between the two tires.


No matter what the tire size, keep the diameter differences to no more than 14 inches to be on the safe side. To avoid the tires contacting and building up heat, maintain the space between them at the appropriate level.


Steering System


You may move your car in any direction thanks to the steering mechanism. The steering system is made up of the following parts:


  1. Steering wheel — The direction of the car is controlled via the steering wheel. It is attached to the steering shaft and rotates it.
  2. Steering shaft — The steering wheel is connected to the steering gearbox via the steering shaft.
  3. Gear box — The steering shaft’s rotation is transmitted to the Pitman’s arm via the gearbox.
  4. Pitman arm — This part moves the drag link by being attached to the steering gearbox.
  5. Drag link — The drag link allows the left steering arm to move after being moved by the Pitman’s arm.
  6. Steering arm — The tie rod is attached to the wheels on the right side, while the drag link is on the left side.
  7. Tie rod — The tie rod regulates the operating angle of the front wheels by linking them together.


The components described on the preceding page must all line up straight. Inadequate alignment may lead to poor tire wear and problematic steering.


Wheel alignment — The manufacturer included the following alignment properties into the axle. They might be modified as required.


  1. Caster — The degree of tilt that an axle has is known as its caster. It is advised that the axle has a positive caster for this measurement, which is expressed in degrees (it tilts forward). When set up in this way, the automobile naturally wants to go straight ahead and can handle twists more easily. The positive caster makes steering easier.
  2. Camber — The degree of tilt the wheels have about the road is known as camber. It is ideal for the truck’s wheels to have positive camber to sustain the weight. In other words, the space between the top and bottom of the wheels is wider than the other. The truck’s tires line up straight with the road when a hefty load is on it.


Damage and wear are the cause of additional alignment features. Toe-in and toe-out are the two most noteworthy of them.


  1. Toe-in — The same axle’s wheels are closer in front than rear.​.
  2. Toe-out — The same axle’s wheels are closer in the rear than in the front.


Power steering — With less effort, power steering gives you better vehicle control. Power steering has various benefits, including less effort, reduced driver fatigue, improved road shock absorption, and ease of control in challenging driving conditions.


Coupling System


The tractor and trailer are joined via a coupling mechanism. The fifth wheel and kingpin are the two essential elements of the coupling system.


Fifth wheel — On the back frame of the tractor, there is a connection mechanism known as the fifth wheel. It is used to connect or couple the trailer’s front end to the tractor. The fifth wheel is a flat, circular plate with a V-shaped slot at the back. The tractor may tow the trailer by fitting the kingpin on the trailer onto the fifth wheel.


Fifth wheels come in various designs, including fixed-mount and sliding/adjustable.


  1. Fixed-mount — The baseplate, bracket subassemblies, and frame mounting elements comprise the fixed-mount fifth wheel, which is fixed in place behind the cab.

  2. Base plate — The locking mechanism is part of the base plate. Additionally, it absorbs the bulk of the coupling stress.

  3. Bracket subassemblies — The base plate is secured in place by the bracket subassemblies.
  4. Frame mounting — The fifth wheel is attached to structural steel angles that serve as the frame mounting components.
  5. Sliding/adjustable — The movable or sliding fifth wheel may move backward and forward to support various weights. A plunger that fits into a row of slotted holes or pins that fit into holes in the slider track may lock the sliding fifth wheel into position.


The slider may be adjusted manually or via a control within the cab. When the fifth wheel is moved manually, the driver shifts the fifth wheel forward or backward by physically manipulating the pins or plunger. A switch that is turned on from the cab opens the locking mechanism. Once the fifth wheel is properly positioned, the driver engages the trailer brakes and maneuvers the tractor forward and backward.


It’s critical to maintain the fifth wheel properly. This involves ensuring the fifth wheel has the appropriate quantity of lubricant to ensure optimum operation.

Kingpin — Under the front of the trailer, on the top fifth wheel plate, is where the kingpin is fastened. A two-inch steel pin is fastened to the fifth wheel to connect the tractor to the trailer.


Landing gear — When the trailer is uncoupled, the landing gears sustain the weight of the front-end load. Typically, they are turned up or down. Wheels or skid feet are connected to the bottom of a landing gear.




The fundamentals of vehicle design and operation have been reviewed in          this chapter, enabling you to maintain a secure vehicle and identify          issues before they become serious and result in a breakdown or          accident.


The major systems addressed are the chassis, suspension, axles,         engine, fuel, and exhaust, lubrication, cooling, electrical, braking,         steering, and coupling.

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