Glossary

ABS (Antilock Braking System).
System that electronically controls wheel slip, thereby preventing sustained wheel locking when braking and stopping on surfaces with low friction.

Adaptive Cruise Control.
Radar based system relying on radar sensing and on on-board computer that adjusts a vehicle’s cruise speed automatically to accommodate changing traffic conditions.

Aftercooler.
Often referred to as an intercooler, this device is a heat exchanger which cools the induction air before it enters the cylinders of an engine.  It is most often used in diesels downstream of the turbo charger.  If used properly, it can lead to longer engine life.

Air Application Gauge.
A gauge designed to read the amount of pressure applied during air brake operation.

Air Ride Suspension.
Suspension which supports the load using air-filled rubber bags rather than steel springs.  Compressed air is supplied by the same engine-driven air compressor and reservoir tanks that provide air to the air brake system.

Air Brake.
A brake in which compressed air actuates the braking mechanism by pushing on a diaphragm within a brake chamber.

Air Compressor.
Device designed to build-up and maintain the necessary air pressure in the brake system reservoir.

Air Deflector.
Contoured panel typically positioned on the roof of a commercial vehicle cab that minimizes air flow between the truck cab and body or trailer.

Air Dryer.
Device designed to remove moisture from the compressed air of the braking system.  It is normally installed between the compressor and service reservoir.

Air Filter Restriction Gauge.
Indicates how dirty the air filter is and when maintenance is needed.

Air Gauges.
Dashboard mounted gauges (reservoir pressure, application, manifold pressure) which indicate the air pressure in pounds per square inch.

Air Line.
Pressure tubing used to circulate the compressed air of the air brake system.

Air Slide Fifth Wheel.
A fifth wheel design that utilizes a switch on the dashboard to activate an air cylinder mounted to the fifth wheel slide plate which moves and locks the position of the fifth wheel.

Articulated Vehicle.
A vehicle consisting of two or more separable wheeled units connected so as to allow flexibility of movement, such as a vehicle towing a trailer or tractor and semi-trailer combination.

Aspect Ratio.
The relationship between the tire sidewall height and the section width.

ATA.
American Trucking Association.

ATC (Automatic Traction Control).
Optional feature based on ABS which prevents spinning of the drive wheels under power on slippery surfaces by braking individual wheels and/or reducing engine throttle.

Automatic Slack Adjuster.
Device designed to automatically control the proper stroke lengths of the push rod and adjustment of the foundation brake.

Auxiliary (lift) Axles.
Axles which are lowered to the ground when heavy loads are carried, such as on dump trucks.

AVI (Automatic Vehicle Identification).
System combining an on-board transponder and roadside receivers to automate identification of vehicles.  Uses include electronic toll collection and stolen vehicle detection.  See IVHS.

AVL (Automated Vehicle Location).
Class of technologies designed to locate vehicles for fleet management purposes and for stolen vehicle recovery.  Infrastructure can be land-based radio towers or satellites.  See IVHS.

Axle.
Horizontally aligned structural member to which the wheels, brakes and suspension are attached.

Axle-Lift.
Device designed to raise and lower the liftable axle of a vehicle.

B Train.
Tractor and semi-trailer with 5^th wheel.  The semi-trailer has seven axles.

Baffle.
A plate or wall for restricting movement of fluids.

Ball Joint.
A mechanical joint in which a spherical end moves freely in a recessed socket.  It is also called a ball and socket.

Barn Doors.
Rear doors on a trailer that swings open.

BBC.
Distance from a truck’s front bumper to the back of its cab.

Bias-Ply.
Tire construction where the cords run diagonally to the tire’s bead or directional rotation.

Bill of Lading.
Itemized list of goods contained in a shipment.

Binder.
Chain length adjustment device designed for securing cargos to the trailer.

Blind Spot.
Areas around a commercial vehicle that are not visible to the driver either through the windshield, side windows or mirrors.

Bobtail.
Tractor operating without a trailer.  A straight truck can also be called a bobtail.

Bogie or Bogey.
Assembly of two or more axles, usually a pair in tandem mechanically linked and suspended as one unit.  Normally added to carry more weight.

Box Trailer.
Enclosed trailer for transporting freight.

Brake Actuator.
See Brake Chamber.

Brake Backing Plate.
A circular plate in a hydraulic braking system to which the brake shoes, brake adjuster and wheel cylinder are mounted.

Brake Blocks.
Friction material or lining attached to a brake shoe.

Brake Chamber.
Chamber in which air pressure is converted to mechanical force for actuation of the air brakes.

Brake Chamber Diaphragm.
Diaphragm within the brake chamber designed to convert air pressure into mechanical force via a push rod.

Brake Compounding.
The result of stepping on the treadle valve with the spring brakes applied.  If too much pressure is applied, the brake components can be damaged.

Brake Drum.
A bowl-shaped component of the braking system that is attached to the hub.  When the brake shoes, with a friction material lining, expand against the inner surface of the drum, braking force is produced.

Brake Fade.
Decline of braking effectiveness which results from overheating of the brake shoes.

Brake Horsepower (bhp).
See Horsepower.  Engine horsepower rating as determined by brake dynamometer testing.  Gives actual or useful horsepower.

Brake Lining.
See Brake Blocks.

Bridge Formula.
A bridge protection formula used by federal and state governments to regulate the amount of weight that can be put on each of a vehicle’s axles, and how far apart the axles (or groups of axles) must be to legally carry a given weight.

Budd System.
A bolt and nut combination for holding disc wheels on to the hub.  The design was originated by the Budd Corporation.

Budd Wheel.
See Disc Wheel.  A single or dual mounted ten-hole disc wheel.

Build Sheet.
See Line Sheet.

Bulker.
A large trailer used for hauling low density bulk products which are pumped on and off.

Bulkhead.
A panel attached to the front of a trailer used for storing chains and binders and to protect the driver in a cab from heavy cargo shifting forward.

Bus.
Any motor vehicle designed for transporting passengers, including taxi cabs.

CA.
The dimension from the center line of the back of the cab to the center line of the rear axle which determines how long the trailer can be.

Cab.
The driver’s compartment of a commercial vehicle.

Cab Air Suspension.
Pneumatic suspension system of the driver’s compartment of the commercial vehicle.

Cab Cards.
Official identification cards to which permit stamps and state operating permits are attached.  They are referred to as “bingo cards” by drivers.

Cabover (Cab-Over-Engine, COE).
Truck or tractor design in which the cab sits over the engine on the chassis.

Cab Protector.
See Headache Rack.

Caging.
The process in which a person compresses the parking brake spring manually to release the spring brakes without the use of air pressure.

Caliper.
A hydraulic unit which, when pressurized by a brake application, squeezes the friction pads against a rotating disc.

Cam Brakes.
A drum brake in which the brake shoes are actuated by the rotation of a camshaft.

Camshaft (Brake).
Located on the axle, the cam rod creates a rotating force.  One end of the rod is an “s” shaped cam which expands the brake shoes against the rotating drum.

Cargo Weight.
Combined weight of all loads, gear and supplies on a vehicle.

Cartage Company.
A Company that provides local pick-up and delivery, within a town, city or municipality.

Cast Spoke Wheel.
Wheel with five or six spokes origination from a center hub.  The spoked portion, usually made of cast steel, is bolted to a multiple-piece steel rim.

Cast Steel Top Plate.
One type of manufactured fifth wheel top plate.  See Fifth Wheel Plate.

CDL (Commercial Driver’s License).
License which authorizes an individual to operate commercial motor vehicles and buses over 26,000 pound gross vehicle weight.  For operators of any trucks, the maximum size which may be driven without a CDL is Class 6 (maximum 26,000 pounds gross vehicle weight).

CE (CF, LP).
Distance from back of a truck’s cab to the end of its frame.

CFR.
A legislative acronym, Code of Federal Regulations.

CG (Center of Gravity).
The point within the length and width of a vehicle around which its weight is evenly distributed or balanced.

Chassis.
The section of a commercial vehicle which includes the frame, suspension, wheels and brakes, steering mechanism, etc., but not the vehicle body or engine.

Chassis Weight (Curb Weight, Tare Weight).
Weight of the empty truck including fluids, but minus occupants or load.

Check Valve.
Valve designed to prevent a loss of air pressure.

Chocks.
Blocking equipment used to restrain movement of the wheels of a tractor or trailer.

Class 8.
Trucks in 60,000 to 80,000 pound range.

Clearance Lights.
Also referred to as marker or running lights, these lights outline a vehicle’s length and breadth.

Clevis.
A “U” shaped yoke that screws onto the pushrod and is pinned to the slack adjuster.

Clutch.
Device used for engaging or disengaging the engine’s power to the drive line.

Clutch Brake.
Mechanism that slows the rotating clutch during disengagement which allows the driver to engage a gear while the motor is at idle.

Clutchless Shifting.
Shifting a manual transmission without depressing the clutch.  Also called slip shifting.

Coach.
See Bus.  Single deck multi-passenger vehicle used for long distance operation.

COE.
See Cabover.

Coefficient of Friction. 
The interaction of an object (e.g. a tire) and a particular surface (e.g. roadway) calculated by dividing the force required to overcome friction on the object by the object’s weight.   A higher number means greater friction, and greater stopping ability in the case of a vehicle on a roadway.

Combination Vehicle.
A power unit, typically a tractor, pulling one or more cargo units.

Common Carrier.
A term for a transportation company which serves the general public.  It may be regular route service over designated highways on a regular basis or an irregular route between various points on an unscheduled basis.

Compressor.
A device designed to create and maintain the necessary air in the brake system reserve and air ride suspension.  It may be belt or gear driven.

Condensation Type Air Dryer.
A type of air dryer in an air brake system that accepts hot air from the compressor and cools it.  The water and other impurities condense and are expelled out the purge valve.

Constant Mesh Gearbox.
All forward gear pairs remain in mesh for the transmission with the driving gear pair being engaged by a clutch.

Container (Shipping Container).
Standard-sized rectangular box, flat or tank used to transport product by ship, rail and highway.  International shipping containers generally are 20 or 40 feet long, conform to International Standards Organization (ISO) standards and are designed to fit in ships’ holds.  Containers are transported on public roads atop a container chassis towed by a tractor.  Domestic containers, up to 53 feet long and of lighter construction, are designed for rail and highway use only.

Container Chassis.
Single-purpose semi-trailer designed to carry a shipping container.

Contract Carrier.
Company that transports freight under contract with one or a limited number of shippers.

Conventional.
A truck or tractor style consisting of a long hood with the driver sitting behind the engine.

Converter Dolly.
Coupling device equipped with a fifth wheel and one or two axles.  It allows a trailer to be coupled to a tractor or trailer combination.

Coupling.
The procedure for connecting a tractor to a trailer, creating what is referred to as an articulated vehicle or tractor-trailer combination.

Cube (Cubic Capacity).
Interior volume of a truck body, semi-trailer or trailer, measured in cubic feet.

Curb Weight.
See Chassis Weight.  The weight of a commercial vehicle with fuel, oil, lubricants and coolant and all standard equipment, but minus the driver and cargo.

Curtain Sider.
Commercial vehicle trailer with removable sides made of a flexible fabric which can be pulled back for side access to the cargo.

CVSA.
Commercial Vehicle Safety Alliance.

Dayton Wheel.
See Cast Spoke Wheel.

Dead Axle.
An axle that is not powered by the engine.  Its purpose is to help carry the load.

Dead Heading.
Operating a truck without a cargo.

Department of Transportation.
The federal agency responsible for enforcing the Federal Motor Carrier Safety Regulations.

Desiccant Air Dryer.
A type of air dryer in an air brake system in which the water is stripped from the air by running the air through a bed of desiccant beads.

Diaphragm.
See Brake Chamber Diaphragm.

Diesel Engine.

A compression ignition, internal combustion engine.

Differential.
A gear assembly which permits one axle shaft and wheel to turn at a different speed than the other.

Direct Drive.
A match between the transmission output shaft speed and the transmission input speed.

Disc Brakes.
A braking system in which friction pads are brought into contact with the face of the disc brake with the assistance of a caliper.

Disc Wheel.
Single-piece rim/wheel assembly of pressed steel or forged aluminum, anchored by 8 or 10 nuts to a hub.

Double.
A tractor plus semi-trailer plus full trailer having 5 or 6 axles with typically two 26-29 foot trailers.

Double Check Valve.
A valve positioned between the two reservoir air tanks in an air brake system.  In the event of a failure, it will seal off the damaged tank, thereby preserving the air in the second tank.

Double Clutching.
Shifting the gears of a non-synchronized truck transmission to keep maximum control of the vehicle by depressing and releasing the clutch pedal twice; the driver must downshift in order to gain enough RPM to reach the next lower gear.

Double Drop Deck Trailer.
A trailer bed that is not only lower than the fifth-wheel plate, but is even lower than the trailer axles.  The floor of the trailer comes within a few inches of pavement.

Drag Link.
The link in a steering system that connects the Pitman arm to the steering arm.

Drawbar.
A horizontally hinged rigid bar or A-frame by which a full trailer is towed and steered.

Drive Axle.
By way of a differential and transmission arrangement, the drive axle transmits power from the engine to the wheels.

Driveline.
All the components which together transmit power from the transmission to the drive axle(s).  These consist of at least one drive shaft (propeller shaft) with a universal joint at each end.

Driver’s Qualification File.
Includes hiring information, documents concerning the driver’s qualifications to drive, including employment testing, motor vehicle records, citizenship, notice to driver and certificate of compliance, current driver’s license, notices of violations to employer, drug testing, medical statements.

Drivetrain (Powertrain).
See Powertrain.  All the components, excluding engine, which transmit the engine’s power to the rear wheel: clutch, transmission, driveline and drive axle(s).

Drop Deck Flatbed Trailer.
A trailer with a flatbed which is lower than the fifth wheel on the tractor.

Dry Tank.
The air reservoir from which the brake system draws air.

Dual Circuit Brake System.
A redundant air brake system designed so the second system will take over in the event of a failure by the first.

Duals.
Two wheels mounted together.

Dump Trailer.
Framed or frameless trailer, used for transporting granular construction materials, coal, etc.  Unloaded by use of a hydraulic cylinder that raises the front end of the trailer and dumps the cargo out the rear of the trailer.

Eastern Turnpike Double.
Vehicle with nine axles consisting of a tractor and two 45- 48 foot trailers.

EDI (Electronic Data Interchange).
The business-to-business interconnection of computers for the rapid exchange of a wide variety of documents, from bills of lading to build tickets at auto plants.

Emergency Brake System.
Secondary braking system used to stop a vehicle if the main braking system fails.

Exempt Carrier.
Company which transports commodities exempted from Interstate Commerce Commission (ICC) economic regulation, i.e. farm products, some fish products.

Exhaust Brake.
Designed to restrict the flow of engine exhaust gases, increasing the retarding effect of the engine and resulting in slowing of the vehicle.

Extended Hood.
A conventional tractor, only with a longer hood.

FCC.
Federal Communications Commission.

FHWA.
Federal Highway Administration.

Fifth Wheel.
Coupling device designed to connect the semi-trailer to the tractor.  The fifth wheel is located on the rear frame of the tractor and consists of a flat rounded plate with a locking bar mounted on the tractor and a kingpin attached to the semi-trailer.  The fifth wheel bears the weight of the front half of the semi-trailer and provides freedom for articulation.

Fifth Wheel Plate.
Plate located on the front underside of the trailer, fastened to the bottom of the trailer floor and resting on the fifth-wheel.  It supports the weight of the nose of the semi-trailer and the cargo inside.  See also Cast Steel Top Plate, Stamped Steel Top Plate, Fixed Fifth Wheel, Manual Sliding Fifth Wheel, Air Slide Fifth Wheel.

Fixed Fifth Wheel.
A fifth wheel whose position is not adjustable since it is bolted directly to the truck frame.

Fixed Tandem.
Assembly of two axles and suspension that is attached to the chassis in one place, and cannot be moved fore and aft.

Flatbed Trailer.
A semi-trailer with no sides, top or end above the platform of the trailer.

Flexible Brake Hose.
High-pressure line which connects the brake line from the master cylinder to the wheel cylinder.

Float Shifting.
Slang for gear changing without use of the clutch.

Flotation Tire. 
A wide –based tire that is used at extremely low pressures.

FMCSR.
Federal Motor Carrier Safety Regulations.

Foot Valve.
A valve that is mounted either on the floor or fire wall of the cab and operates both the tractor and trailer air brakes when depressed.

For-Hire Carrier.
A company in the business of transporting product belonging to others.

Frame Dump Trailer.
A type of dump truck in which there is a frame between the kingpin and axles that carries the load and provides strength and stability.

Frameless Dump Trailer.
A type of dump truck that has no frame but uses a platform to support its weight and cargo.

Front Brake Limiting Valve.
Sometimes controlled by a two-way switch mounted on the dashboard. Designed to limit the amount of braking force applied to the front axle brakes to avoid locking up the brakes on slippery surfaces.  Found only on tractors with front axle air brakes.

Full Trailer.
Trailer that supports its entire weight and cargo on its own tires.

GAW (Gross Axle Weight).
The total load a particular axle is supporting.

GAWR (Gross Axle Weight Rating).
Maximum weight an axel is rated to carry by the manufacturer, measured at tire-road interface.  Measurement includes the axle weight and the portion of the vehicle carried by the axle.

GCW (Gross Combination Weight).
Total weight of the loaded combination vehicle including fuel, driver, trailer and cargo.

GCWR (Gross Combined Weight Rating).
Maximum combined safe weight of the vehicle as well as any towed or attached vehicles.

Gear Fast/Run Slow.
Term used in the industry to describe spec’ing a truck that will be geared to run at a high road speed but limited by the ECM to a certain speed to increase fuel economy.

Gear Ratio.
The ratio of teeth or revolutions of one gear to the teeth or revolutions of the gear with which it is engaged.

Geared Speed.
The calculated speed of the vehicle based upon the engine’s rpms in each gear or top gear; the overall gear ratio to the driven wheels and the diameter of the driven wheels.

Glad Hands.
Mechanical couplings mounted on the front of the trailer to connect the air lines from the tractor to the trailer.

Gooseneck Trailer.
Drop deck trailer with a long narrow neck whose geometry permits the trailer to pivot without striking the truck.

Governor.
Device designed to limit the maximum speed of the engine by controlling the fuel which limits the rpms; they are found mostly on diesel engines.

GPS (Global Positioning System).
Satellite system used to pinpoint a geographic position of the truck, tractor or trailer.

Grade.
Steepness of a slope, expressed as a percentage.  Example: a vehicle climbing a 5% grade rises 5 feet for every 100 feet horizontally.

Gradeability.
The measure of a vehicle’s ability to climb an incline while maintaining a given speed.

Gross Horsepower.
The brake horsepower with the engine void of its accessories.

GVW (Gross Vehicle Weight).
Total weight of a vehicle and everything aboard, including its load.

GVWR (Gross Vehicle Weight Rating).
Maximum allowable weight of a commercial vehicle including the trailer and cargo; this figure is used in licensing the truck or trailer.

Hand Valve.
A valve located on the steering column that is designed to apply only the trailer brakes.

Hazmat.
Hazardous materials, as classified by the U.S. Environmental Protection Agency (EPA).  Transport of hazardous materials is strictly regulated by the U.S. Department of Transportation.

Headache Rack.
Heavy protective barrier mounted behind the tractor’s cab.  It is designed to prevent injuries caused by load shifting forward from the trailer and crushing the cab.  Useful place for hanging chains and equipment.

High Cube Van.
Van whose body, wheel and tire design allows more space to give maximum loading area.

High Hitch.
This occurs when the trailer height is too high.  As a result, the fifth wheel latches cannot properly grasp the kingpin.

Hopper Trailer.
Trailer designed with an open top for easy filling of grains and other bulk products.  It is unloaded through doors in the bottom of the trailer.

Horsepower (hp).
Refers to the amount of work that can be done over a given amount of time; the measure of power defined by a rate of working of 33,000 foot-pounds per minute.

Horsepower, SAE Net.
Horsepower capability of an engine with full accessories and exhaust system.  Test procedures per standards of Society of Automotive Engineers (SAE).

Hot Loads.
Rush shipment of cargo.

Hours-of-Service.
U.S. Department of Transportation safety regulations which govern the hours of service of commercial vehicle drivers engaged in interstate trucking operations.

Hub-Piloted Wheels.
A type of disc wheel that is centered by the close fit of the wheel center hole to the wheel hub.

ICC.
Interstate Commerce Commission.

Independent Trucker.
One who owns a tractor and trailer and hauls products for self or others on either a regular or as needed basis.

Inter-Axle Differential.
A device designed to link tandem axles and to prevent slippage during icy or wet road conditions.  It is also referred to as the power divider.

Jack-knife.
A result of loss of control of the articulated vehicle; the trailer ends up at a sharp angle to the tractor.

Jacobs Engine Brake.
See Jake Brake.

Jake Brake.
An engine compressor brake which builds up back pressure by preventing exhaust from escaping thus the engine slows as the cycle at the top of the power stroke is interrupted, which provides a retarding force.

Jeep Wheel.
Gooseneck dollies used with very heavy or extra long loads.  See Joe Dog.

Jiff Lock.
Two fifth wheels on a three axle tractor; a gear set designed to fit into the frame of a two axle tractor converting it to a three axle tractor.

Joe Dog.
A device with a dead axle that converts a single-axle tractor to a tandem-axle tractor.  It hooks over the tractor’s fifth wheel, replacing it with another for the semi to be hooked onto.  Also referred to as Jeep Wheel.

Kingpin.
Anchor pin at the center of a semi-trailer’s fifth wheel plate which is captured by the locking jaws of a tractor’s fifth wheel to attach the tractor to the semi-trailer.

Landing Gear.
Retractable legs which support the front end of a semi-trailer when it is not coupled to a tractor.

LCV (Long Combination Vehicle).
Vehicles longer than a standard doubles rig (tractor and two 28-foot semi-trailers).  Some examples are twin 48-foot trailers and  triple 28-foot trailers.

Leaf Spring.
Suspension system consisting of superimposed, flat, narrow steel plates.

Lessee.
The company or individual which leases vehicles.

Lessor. 
The company which owns the vehicle and leases it to customers.

Lift Axle.
An un-powered axle used only when the vehicle is loaded in order to meet federal and state standards.  It is attached to the undercarriage with an air spring suspension which allows the axle to be raised when it is not in use.

Line Sheet.
A production list of all components that go into the manufacturing of a truck, tractor or trailer.

Livestock Trailer.
Trailer designed for livestock transport.

Load Lock.
A bar type device designed to hold the cargo in the trailer.

Locking Main Differential.
A differential that locks both shafts together so equal torque is applied to left and right wheels.

Logbook.
Book carried by truck drivers in which they must record their hours-of-service and duty status.

Lowboy.
Trailer designed with open flat-bed with the deck very low to the ground, most commonly used for hauling heavy machinery for construction, bulky or tall loads.

Low Profile Tires.
Tires with a profile ratio of 80 or less.

LTL Carrier.
A trucking company which consolidates less-than-truckload cargo for multiple destinations on one vehicle.

Lumping.
Loading and unloading cargo.

Manifest.
Bill of lading describing the cargo of the vehicle.

Manual Sliding Fifth Wheel.
A type of fifth wheel that utilizes a pull lever mounted on the sliding plate that allows repositioning of the fifth wheel or the tractor.

Maxi’s.
See Spring Brakes.

MGM.
See Spring Brakes.

Michigan Special.
Vehicle with 11 axles.

Moving Floor Trailer.
Trailer with a moving floor designed for easy unloading of cargo.

NHTSA.
National Highway Traffic Safety Administration.

NPGA.
National Propane Gas Association.

NSC.
National Safety Council.

NTDA.
National Trailer Dealers Association.

NTDRA.
National Tire Dealers and Retreaders Association.

NTEA.
National Truck Equipment Association.

NTSB.
National Transportation Safety Board.

NTTC.
National Tank Truck Carriers.

NUTA.
National Used Truck Association.

OEM’s.
Original Equipment Manufacturers.

Off-Tracking.
A term referring to the difference in path taken by the rear of a vehicle relative to the front when turning.  At low speeds the radius of the rear wheel path for a vehicle is smaller than the radius of the front wheel path.  Off-tracking increases with increased vehicle wheel-base.  An articulated vehicle will exhibit less off-tracking than a non-articulated vehicle of the same wheel-base.

Ontario Special.
Tractor semi-trailer combination with 8 axles.

Open Top.
Van type trailer with an open roof.

Overdrive.
A transmission gear ratio where the engine crankshaft turns at a slower rate than the vehicle drive shaft thus the engine speed is reduced for more economical high-speed driving.

Overload Spring.
An additional spring on a leaf spring suspension system that is used in trucks to increase the suspension stiffness at high load.  Also called helper springs.

Owner-Operator.
A trucker who owns and operates his own tractor (sometimes trailer) and is leased to a motor carrier or private carrier.  Also called an independent contractor.

Parking Brake Valve.
The valve that directly controls the application of spring brakes on the tractor.  It appears as the yellow knob on the dashboard.

Payload.
The cargo that a commercial vehicle hauls, or the weight of that cargo, subject to local definition.

Perception.
The conscious understanding that an event has taken place.

Piggyback.
Semi-trailer built with reinforcements to withstand transport by a railroad flatcar.

It is also another name for a spring brake chamber.

Pigtail.
Electrical cable used to transmit power from the tractor to the trailer; it resembles the coiled tail of a pig.

Pintle Hook.
Coupling device used in double trailer, triple trailer and truck-trailer combinations.  It has a curved, fixed towing horn and an upper latch that opens to accept the draw bar eye of a trailer or dolly.

Pitman Arm.
Lever that converts rotary output from steering box to linear movement of a drag link.

Pop-off Valve.
Pressure relief valve fitted in the air tanks to avoid over-pressurization.

Pot Belly Trailers.
Name for double-deck or triple-deck livestock trailer.  Also called Possum Belly Trailer.

Power Divider.
A part of the dual axle drive system designed to transfer power between two or more axles of a tandem axle arrangement.

Powertrain.
The series of components that make up the power transmission and transfer engine power to wheels; consists of engine, clutch, transmission, drive shafts and drive axles.

Private Carrier.
Business which operates trucks primarily for the purpose of transporting it s own products and raw materials.

PSI (Pounds per Square Inch).
The unit of measurement for tire air pressure, air brake system pressure and turbocharger boost.

PTO (Power Takeoff).
A device designed to transmit engine power to auxiliary equipment such as concrete mixer or refuse packer.

Pull Trailer.
Full trailer supported by axles front and rear with a variety of tongue lengths.

Pup Trailer.
Short semi-trailer, usually between 26 and 32 feet long, with a single axle.

Push Rod.
A rod designed to transmit linear displacement to an actuating mechanism when in compression.

Pusher Axle.
Powered rear axle of a tandem arrangement, where the forward axles are non-powered or “dead.”

Pyrometer.
Instrument installed to monitor the temperature of the exhaust gases of a diesel engine.

Quick Release Valves.
Valves designed to accelerate the release of air pressure from the brake chambers of the air brake system.

Radius Rods/Arm.
Suspension coupling designed to provide fore and aft location of an axle.

Reaction.
The reflex or voluntary motion that occurs in response to a person perceiving an event.

Rear Axle Ration.
Ratio of the speed of the propeller shaft to the speed of the axle shaft.

Reefer Trailer.
A trailer designed for transporting food. Consists of insulated walls and self-powered refrigeration unit.

Relay (Relay Driving).
A practice in which one driver takes a truck for 8 to 10 hours, then turns the truck over to another driver.

Relay Valve.
Valve designed to decrease the response time of the air brakes particularly on long vehicles.  It is fitted near the local reservoir and brake chamber of the air brake system.

Reservoir Air Pressure Gauge.
A gauge designed to measure the amount of air pressure in the air tanks in psi.  If the air pressure drops below 60 psi, the vehicle is operating under unsafe conditions and a warning light or buzzer is activated.

Reservoir Tank.
See Dry Tank.

Retarder.
A device designed to assist the air brake system in slowing down the vehicle by dissipation the energy of motion by electrical or hydraulic means.  See Jake Brake.

Rim Pull Torque.
A force at the tire to road contact point.

RMA.
Rubber Manufacturers Association.

Rocky Mountain Double.
A vehicle composed of seven axles with a tractor, one 45-48 ft. trailer and one 26-28 ft. trailer.

Roll-up Door.
A style of rear door on a van trailer that is similar in operation to a garage door.

Rolling Parking Lot.
Slang for a car carrier.

Rolling Radius.
Distance measured from the center of tire/axle to the ground; the measurement is used for calculating gear speed.

Rotor.
The round disc of a disc brake assembly.

RPM (Revolutions per Minute).
The measure of speed at which an object spins.  It is most often used to describe engine crankshaft speed.

Runaway Truck Ramp.
Emergency area adjacent to a steep downgrade that a heavy truck can steer into after losing braking power.  Usually two or three lanes wide and several hundred feet long, the ramp, is a soft, gravel-filled pathway which absorbs the truck’s forward momentum, bringing it to a safe stop.  Depending on the surrounding terrain, the ramp may be level or run up or down hill.

SAE.
Society of Automotive Engineers.

Safe Haven.
A place that is approved by local, state or U.S. government officials where trucks hauling explosives can be parked and left unattended.

SCRA.
Specialized Carriers and Rigging Association.

Semi.
A term used loosely to refer to a tractor coupled with semi-trailer.  Slang for tractor-trailer.

Semi-elliptic Spring.
A leaf spring designed to operate with a pin-ended beam reacting with the loads as they are at mid-length bending.  Modern designs utilize a shallow arc.

Semi-trailer.
A trailer with only rear axles, supported and coupled to a tractor by the fifth wheel when in transit.  Length can be up to 53 feet.

Service Brakes.
Primary brakes used for normal brake application.

Set-back Axle.
Gear configuration where the front axle is set back from the front of the vehicle which creates a shorter turning radius and more of the vehicle’s weight is shifted to the front axle.

Shackle.
Pivoting link between a leaf spring and its mounting.

Shift Lever.
Lever for manually changing gear in a manual transmission.

Shifting Chart.
Displays shifting lever position for all gears in a manual transmission shifting lever.  Often found on the visor of the tractor.

Shipping Weight.
“Dry” weight of a truck including all standard equipment, but excluding fuel and coolant.

Single Reduction Drive Axle.
Drive axle in which the reduction in rotational speed between the drive shaft and the final drive is one stage.

Single-Source Leasing.
Service in which companies can lease drivers and trucks from the same source, rather than having to procure them from different companies.

Slack Adjuster.
Lever designed to be an adjustable component fitted on the brake chamber push rod which is used to compensate for clearance between the brake shoe and drum when wear to the brake lining occurs.  Some models are automatic while other are manually adjusted.

Sleeper.
Commercial vehicle cab with a sleeping compartment for the driver.

Sliding Fifth Wheel.
Fifth wheel assembly that allows it to be moved back and forth in order to adjust the distribution of the weight between the tractor and trailer axles.

Sliding Tandem (Slider).
Trailer tandems designed to be moved back and forth for the purpose of adjusting the distribution of weight between the axles and fifth wheel.

Slip Seat.
Operation where drivers change periodically, but the truck continues towards the same final destination.

Split Shifting.
Coinciding gear shifts in the main and auxiliary transmissions.

Spoke Wheel.
See Cast Spoke Wheel.

Spread Axle (Spread Tandem).
A tandem axle arrangement spaced 54 inches apart or greater, substantially greater than the spacing of a conventional commercial vehicle, which is used to circumvent weight restrictions.

Spring Brake.
Also known as dual chamber, piggy backs, MGM’s or maxi’s.  Dual air chamber which incorporates an air supplied brake chamber and air compressed spring which actuates upon sudden air loss or activation of a dash mounted brake control switch.  Brakes remain locked until the chamber is refilled with air or the spring is manually released.

Spring Capacity.
The amount of weight load that the spring in a suspension system is capable of carrying.

Spring Hanger.
A bracket on a truck chassis to which an eye of a leaf spring is attached.

Stamped Steel Top Plate.
One type of manufactured fifth wheel top plate.  See Fifth Wheel Plate.

Startabililty.
The amount of force required to begin movement.  It is a function of engine torque, transmission ration, axle ratio and tire dimension.

Steer Axles.
Front axles with steerable wheels.

Steering Arm.
Lever that moves the steering action from the tie rod to the steering knuckle.

Steering Gear.
Components by which a vehicle is steered.

Step Deck Trailer.
This type of trailer differs from the conventional flatbed in that the platform is typically lower than the height of the fifth wheel on the tractor.

Stop Lamp Switch.
Activates the brake lights on the rear of the tractor and connected trailer when more than 5 psi of air goes to the brake chamber.

Straight Truck.
Truck body, cab and engine all mounted to the same chassis.

Stud Piloted Wheel.
A type of disc wheel that is centered and is held onto the stud by the nuts.

Swing Doors.
See Barn Doors.

Synchronized Transmission.
Transmission with built-in mechanisms to automatically “equalize” the speed of its gears to allow smooth shifting without the need to double-clutch.

Synchronizers.
Sliding clutch mechanisms by which gear engagement is synchronized in a gearbox.

Tag Axle.
The non-driven axle of a tandem arrangement; the tag axle does not transmit power.

Tandem Axle (Tandems).
Undercarriage arrangement where two or more axles are coupled closely together, normally between 40 and 72 inches.

Tanker Trailer.
Trailer designed to carry liquids or dry bulk commodities.

Tare Weight.
Weight of a commercial vehicle exclusive of its contents including fuel, cargo, water and oil, subject to local definition.

Tie-down.
A device used to secure a load to a flatbed trailer.

Tie Rod Assembly.
The linkage that connects the left and right wheel steering arms.

Truck Load (TL) Carrier.
A trucking company which dedicates trailers to a single shipper’s cargo, as opposed to an LTL (Less than Truckload) carrier which transports the consolidated cargo of several shippers and makes multiple deliveries.

TMA.
Truck Manufacturers Association.

TMC.
The Maintenance Council.

Torque.
Twisting or turning force created by applying a torsional force.

Torque Curve.
A chart showing how the torque of an engine changes with its rpm.

Tow Bar Eye.
The ring at the end of a dolly that attaches a trailer to the pintle hook.

Tractor.
Truck designed to draw a semi-trailer with a fifth wheel coupler, referred to as a highway tractor to differentiate between farm tractor.

Tractor Protection Valve.
Valve designed to make sure air is always available to tractor brakes.  When activated (via dashboard control), it supplies air pressure to the supply system in the trailer.

Tractor Trailer.
Tractor and semi-trailer combination, an articulated vehicle.

Trailer.
A non-powered vehicle designed to be pulled by a tractor.

Trailer Air Reservoir.
The storage tanks on the trailer where pressurized air is held until needed to apply the trailer brakes.

Treadle Valve.
See Foot Valve.

Triaxle.
Three axle tractor or trailer where the three axles are located in close succession which distributes the weight.

Trip Leasing.
Leasing a company’s vehicle to another transportation provider for a single trip.

Triple.
A tractor plus semi-trailer plus two full trailers, typically having 7 axles with three 26-29 foot trailers.

Triple-deck Livestock Trailer.
Trailer with three levels for transporting animals.

Truck.
Vehicle designed for hauling cargo where the truck body is mounted to its chassis.

Twins (Twin Trailers).
See Doubles.

Two Speed Rear Axle.
A rear axle having two selective gear ratios.

Unloader Valve.
A valve designed to control the air pressure in a commercial vehicle air brake system.

Van Trailer.
Type of trailer designed with an enclosed body for transporting freight.

VIN (Vehicle Identification Number).
The number assigned by the manufacturer to each vehicle for registration and identification purposes, appears on the vehicle’s title.

Walking Beam Suspension.
A type of truck and tractor rear suspension consisting of two beams, one at each side of the chassis, which pivot in the center and connect at the front to one axle of a tandem and at the rear to the other side.  The Hendrickson walking beam suspension is one type of walking beam suspension.

Wet Tank.
First air reservoir of 3 air reservoirs where compressed air from the air compressor enters and where most of the condensation forms.

Wheel.
The disc assembly which is attached to the axle.  The tire is mounted on the wheel.

Wheelbase.
Longitudinal distance between the center line of the front (steering) axle and the center line of the rear axle, or the center of the space between the tandems.

Wheel Cylinder.
A hydraulic cylinder that forces the shoes of a drum brake apart.

Wheel Horsepower.
The horsepower of the tire on the ground that generated at the tires will be less than net horsepower.

Wheel Speed Sensor.
Device designed to record the wheel rotation speed.

WIM (Weight-in-Motion).
Scale technology for determining a vehicle’s weight without requiring it to come to a complete stop.

Yard Jockey.
Slang for a person who operates a yard tractor.

Yard Tractor (Yard Mule).
Special tractor used to move trailers around a terminal, warehouse or distribution center.

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