Glossary of Semi-Trailer Terms

A list of common semi-trailer terms and definitions commonly used in the freight, transportation, and trucking industries in the United States.

Air Control System, Suspension: An arrangement of valves and tubing connected to the trailer air system, which is designed to regulate the ride height (or mounting height) of an air suspension.

Air Leveling Valve: (See Valve, Air Leveling)

Antilock: (See Brake, Antilock)

Auto Transporter: (See Trailer, Auto Transporter)

Axle: A beam with a spindle or spindles about which wheels rotate.

Axle, Air Lift: A single air operated axle which when lowered will convert a vehicle into a multi-axle unit and provide the vehicle with a greater load carrying capacity.

Axle, Camber: The controlled convexity of a trailer axle designed to compensate for deflection of the axle under load, to compensate for the curvature or crown of certain roads and to minimize tire wear thereby.

Axle, Drop: An axle in which the centerline of the beam portion is lower than the centerline of the spindles.

Axle, Flip or Removable: A frame and axle which can be rigidly pinned or pivoted at the rear of a trailer or an axle assembly that can be flipped up and onto the rear of the trailer to which it is attached.

Axle, Multi: A grouping of axle along the length of a trailer designed to distribute the load over as large an area of road surface as possible.

Axle, Single: The machine steel journal or shaft (usually heat treated and hardened) at each end of an axle, which is contoured to accept wheel bearings and seals, and around which the wheels rotate.

Axle, Spindle: One axle mounted independently of any other axle.

Axle, Spread Tandem: A two-axle assembly in which the axles are separated beyond the spacing of a normal tandem assembly in order to qualify for maximum axle loads allowed by regulations.

Axle, Tandem: A two-axle assembly having a means of distributing or transferring weight between the two axles.

Axle, Tri: A three axle configuration consisting of tandem or spread tandem axles at the rear and a single, liftable. Third axle separated ahead or behind the tandem axles. There are no means of distributing or transferring load from the third axle to the tandem axles other than through the trailer structure.

Axle, Tridem: A three-axle assembly having a means of equalizing weight between the three axles, which are equally spaced.

Axle, Trunnion: A short axle pivoted at or near its mid-point about a horizontal axis transverse to its own centerline. Normally used in pairs in conjunction with a walking beam in order to achieve two axis of oscillation.

B-Train: (See Trailer, B-Train)

Bar Spreader: A transverse rigid member attached at intervals between the side walls of a trailer, commonly employed on open top van trailers to stabilize the upper rails.

Beam, Main: A main structural member of a lowbed, platform or chassis trailer, usually one of a pair.

Beam, Meat Rail Support: A transverse load-carrying member installed between the upper side walls of refrigerated van trailers and containers to which meat rails are attached.

Beaver Tail: The rearmost section of a trailer sloped downward to facilitate loading and unloading from the rear.

Binder, Load: A mechanical device, consisting of a lever-operated toggle and lock, generally used to tighten a cargo securing system. Winches are sometimes used as load binders.

Bogie: (See Undercarriage).

Bolster, Container Chassis: Transverse structure with locks on each end for securing a container to the chassis. Also provides vertical load support for a container.

Booster: A mechanism, which attaches additional axles to the rear of a trailer. The trailer to booster connection may rely on mechanical, air (or other gas), or hydraulic means to provide load transfer.

Box, Connection: Contains fittings for trailer emergency and service brake connections and electrical connector to which the lines from the towing vehicle may be connected.

Brake, Antilock: Device which, when incorporated into a conventional service brake system, automatically controls the degree of rotational wheel slip at one or more wheels of an undercarriage during braking.

• 2S1M – Two sensors, one modulator

• 2S2M – Two sensors, two modulators

• 4S2M – Four sensors, two modulators

Brake Caliper Assembly, Disc: The non-rotational components of a disc brake, including its actuating mechanism and pads, which generate friction forces on the disc.

Brake Chamber: A mechanical device mounted near either end of an axle, adjacent to the wheel, which receives pressurized air from a valve and exerts force through the slack adjuster and camshaft to the foundation brake.

Brake Control, Load Proportioning: A system or device, which regulates the input force to the brakes on an axle in proportion to the load on that axle.

Brake, Disc: A type of wheel brake in which the brake linings or pads are squeezed against either side of a flat, circular disc by means of a caliper-type clamp assembly.

Brake, Drum: The cylindrical rotational member of a drum-brake assembly attached to the spoke wheel or hub and acted upon by the brake shoe and lining assemblies.

Brake Dust Shield: (See Dust, Shield, Brake).

Brake, Foundation: The non-rotational components of a brake, including the actuating mechanism for development of retarding forces.

Brake Hop: (See Wheel Hop)

Brake Hose: A flexible tube (usually rubber or coiled synthetic) used to conduct pressurized air in an air brake system. (May pertain also to conduit for other fluids).

Brake Hose Coupler: (See Gladhand).

Brake Lines, Trailer Air: The system of conduit or tubes carrying compressed air from the gladhands to the trailer brake system. There are two separate lines: supply (emergency) and control (service).

Brake Line, Trailer Control (Service): The conduit or tube, which transmits the brake actuating signal from the tractor to the trailer air brake valve. (This line is normally not pressurized).

Brake Line, Trailer Supply (Emergency): The conduit or tube which carries air from the tractor compressor through the trailer brake valve to the reservoir(s). (This line is normally pressurized).

Brake, Parking: A mechanism designed to prevent the movement of a stationary motor vehicle.

Brake Rotor, Disc: The flat, circular rotational member of a disc brake assembly which is attached to the spoke wheel or hub and is acted upon by the caliper and friction pads assembly.

Brake, Service: The primary system or mechanism designed to retard or stop a motor vehicle.

Brake Slack Adjuster: (See Slack Adjuster).

Brake, Spring: A mechanism (usually integral with or attached directly to a service brake chamber) which employs the stored energy of a coil spring to actuate the foundation brake.

Brake Spider: That portion of the foundation brake, which is anchored to either end of the axle bar, variously bolted or welded.

Brake System, Air: A type of brake system which uses compressed air to transmit operator signals and to provide the motive force required for actuation.

Brake System, Emergency: A wheel brake system used for stopping a vehicle in the event of a malfunction in the operation and/or control of the service brake system.

Brake System, FMVSS 121: A trailer air brake system composed of, but not limited to, drums or disks, foundation brakes brake lining, slack adjusters, brake chambers, air tanks, air lines, gladhands, appropriate pneumatic fittings and parking brake system, an air supply control valve, protective devices and the mechanically actuated part of the brake chamber.

Brake, Wedge: A type of wheel brake in which the brake shoe and lining assemblies are spread apart by forcing a wedge between opposing ends of the assemblies pushing them outward against the brake drum.

Bridge: On a double drop lowbed or drop frame trailer, that portion of the structure beginning with the rear drop and extending to the rear end.

Bridge Weight Formula, Federal: The Federal standard specifying the relationship between axle (or groups of axles spacing and the gross weight those axles may carry, expressed by the formula):

W = 500 (LNN/N-1 + 12N + 36)

W = Overall gross weight on any group of two or more consecutive axles.

L = Distance in feet between the extreme of any group of two or more consecutive axles.

N = Number of axles in group under consideration.

Bulkhead: A fixed structure at the lower end of a front wall to protect against shifting cargo or fork tine impact. An alternative structure (fixed or removable) installed across a trailer’s width to compartmentalize a trailer and/or to protect against damage caused by shifting cargo.

Bull Boards (Grain Boards): Wooden boards installed across the width of a van trailer or container generally stacked one upon another in slots located on either side of the rear door opening used to protect doors from rearward shifting of cargo, or to contain bulk commodities such as grains.

Bull Nose: A type of trailer front end construction in which the front corners are beveled (usually 45°) in order to provide additional swing clearance on trailers with longer than normal kingpin locations. (Example 56-inch swing radius with a 48-inch kingpin location).

Bumper: A device mounted to the trailer structure for absorbing shock or preventing damage. (also see rear impact guard).

Caging, Spring Brake: To compress the parking/emergency brake spring by trapping it between its pressure plate and the spring brake housing using air pressure or manually by tightening the release bolt until heavy resistance occurs.

Capacity, Cubic: Usable internal load-carrying space as related to van trailers, bulk trailer, tank trailers, containers, etc. (Usually expressed in cubic feet and sometimes referred to as “cube” or volumetric capacity).

Capacity, Rated Trailer: The rated trailer capacity does not indicate trailer payload capability, but rather the total of the payload and the vehicle weight. The assignment of the rated capacity value is the responsibility of the trailer manufacturer taking into account all factors relating to the intended usage including (but not limited to) payload and its location and distribution, method of loading and unloading, safety factor, operating environment, handling characteristics, center of gravity, etc.

Capacity, Towing: (See Towing Capacity).

Cargo Control Post: (See Post, Cargo Control).

Certification Label: (See Label, Certification)

Chain, Safety (Safety Cable): A back-up device consisting of chains or cables between the towing and towed vehicles used to prevent separation of the vehicles in the event of malfunction or loss of the primary towing connection.

Chain, Spreader: A chain connected at intervals between the side walls of a trailer to prevent their being pushed out by the cargo.

Chassis: The structural framework comprising the load carrying elements on all trailers other than those of monocoque construction.

Chock: (See Wheel Chock).

Clearance, Road: Distance between a trailer component and the road surface.

Clearance, Support: (See Support Clearance).

Clearance, Swing: The minimum distance between a trailer and its power unit or tractor while turning.

Connection Box: (See Box, Connection).

Connector, Plug Electrical: That component part of a tractor or converter dolly electrical system attached to an electrical jumper cable and designed to plug into a mating receptacle connector on a trailer.

Connector, Receptacle, Electrical: That component part of a trailer electrical system usually attached to the front of a trailer and designed to accept the plug connector on the tractor jumper cable. Receptacle connectors are also located at the rear of trailers used in doubles and triples operations and at various locations on oilfield and lowbed trailers.

Conspicuity Treatment: Retro-reflective tape or special reflex reflectors used to make trailers and tractors more conspicuous, especially at night.

Construction, Beam: A frame-type trailer consisting of two main beams, with short crossmembers between, designed to transport mobile equipment of various types by supporting its undercarriage at points between the wheel or track assemblies.

Construction, Full Frame: (See Chassis).

Construction, Monocoque: That type of structural design employed in frameless van trailers, tanks and other trailers such in which the trailer body or shell is the principal load carrying element.

Construction, Perimeter Frame: A type of chassis design in which the two main load carrying members are spread apart so that they constitute the side frames of the trailer.

Controller Logic, Antilock: That part of an electro-mechanical antilock system, which receives and processes electrical signals from the wheel sensors and in turn transmits electrical controlling signals to the antilock air valve.

Converter Dolly: (See Dolly, Converter).

Coupler, Air Hose Dummy: Metal Fittings, usually attached to the rear of a tractor or to a converter dolly, and used to store the loose ends of air brake hoses when not in use. They also provide an effective seal preventing the entry of contaminates into the gladhand openings.

Coupler Assembly, Rocking Upper: An upper coupler plate and pin structure pivoted about an axis parallel to the longitudinal axis of the trailer.

Coupler, Assembly, Upper: The structural element at the front of a trailer, including the kingpin, which receives and transfers the load from the forward portion of the trailer’s load carrying elements to the tractor fifth wheel.

Coupler Height: The nominal design dimension from the ground, or road surface, to the bottom of the upper coupler plate with the trailer unladen.

Coupler Pickup Plate, Upper: (See Plate, Pickup).

Coupler Plate, Upper (Upper Fifth Wheel Plate): The flat plate on the underside of the upper coupler through which the kingpin protrudes and which rests directly on the tractor fifth wheel.

Coupler TOFC Plate, Upper: (See TOFC Plate).

Crossmember: A transverse member in a trailer chassis or underframe.

Crossmember, Rear: The transverse member at the extreme rear of a trailer to which the bumper is normally mounted and on which the stop, tail and turn lights are often installed.

Crossmember Spacing: Distance between crossmember centers.

Cubic Capacity (CUBE): (See Capacity, Cubic).

Curbside: That side of a trailer nearest the curb when the trailer is traveling in a normal forward direction. Opposite to “roadside” or “driver side”.

Deck: The load carrying area on a platform, lowbed or chassis-type trailer.

Deck, Drop Side: That portion of the loading deck on a lowbed or drop deck trailer, which falls outside the main frames and which is several inches lower than the top of the main frames.

Decking, Double: A removable secondary floor with supporting members, located between the permanent flooring and the roof to permit separate loading compartments when full height stacking is not desired.

Dolly, Converter: An auxiliary undercarriage assembly consisting of a chassis, fifth wheel, and towbar used to convert a semitrailer to a full trailer.

Dolly, Load Divider: A short frame-type trailer complete with upper coupler, fifth wheel and undercarriage assembly and designed in such a manner that when coupled to a semitrailer and tractor it carries a portion of the trailer kingpin load while transferring the remainder to the tractor fifth wheel.

Dolly, Permanent: An undercarriage assembly with a permanently attached turntable and a towbar and which, in aggregate, comprises the forward axle(s) or a full trailer.

Door(s), (Rear or Side): Panels provided on the sides or at the rear of a trailer or container and articulated in any of several ways to allow access to the cargo area when opened.

Door(s), Overhead (Sometimes Referred to as Roll Up): An assembly of counterbalanced, hinged panels mounted in tracks on either side of the door opening designed to allow the door to be raised or lowered to control access to the loading area.

Door Header: The transverse structural member comprising the top of a door opening.

Door Hinge: The movable joint or device by which a door is articulated.

Door Hold Back: A device used to temporarily hold doors in the open position along the sides of a trailer.

Door Lock: A device used to secure doors in their closed position by means of mating parts on the door and the trailer door frame. Most door locks are designed to accept a seal or padlock for security purposes.

Door Sill: A transverse frame member at the bottom of a door opening.

Door, Swing: A door panel articulated by means of hinges along one vertical side.

Doors, Vent: Small doors used to provide and/or control air circulation.

Doubles: A trailer combination consisting of a truck tractor, semitrailer, a converter dolly and a semitrailer coupled together. (Also called doubles trailer or pups).

Doubles, Rocky Mountain: A trailer combination consisting of a truck tractor, a standard-length semitrailer, a converter dolly and a shorter semitrailer coupled together.

Doubles, Turnpike: A trailer combination consisting of a truck tractor, a standard-length semitrailer, a converter dolly and a standard-length semitrailer coupled together.

Drain, Kazoo: A self-opening and closing ruby bladder used for the collection and drainage of liquids from drain gutters or troughs usually in insulated trailers.

Drain Gutter: A trough across the front and/or rear of an insulated trailer floor used to accumulate and drain liquids from the interior of the trailer.

Ducts: Passageways installed within a trailer or incorporated into the linings, floor or insulated envelope for the purpose of transmitting air or other gases.

Duct-Six Finders: An assembly consisting of six flexible passages, usually of different length and suspended from the ceiling; which is used to distribute air from a trailer refrigeration system.

Duct-Straight Chute: A single passage suspended from the ceiling and used for the distribution of air from a trailer refrigeration system.

Ductboards: Loose over-the-floor sections which permit air circulation or drainage beneath the cargo.

Dust Shield Brake: A non-rotating plastic or metal cover bolted to the brake spider on either end of an axle, to restrict the entry of foreign material into the brake drum.

Fenders: Plastic or metal shields or deflectors fixed to the trailer above the wheel assemblies principally to control spray thrown from the rotating tires.

Fifth Wheel: A device used to connect a trailer to a tractor, converter dolly or jeep dolly in order to permit articulation between the units. It is composed of a plate and latching mechanism mounted on a trunnion which most commonly allows freedom of movement about the transverse axis.

Fifth Wheel, Full Oscillating: A fifth wheel assembly whose trunnion mechanism allows freedom of movement about both the transverse and longitudinal axis.

Fifth Wheel Pickup Ramp: (See Ramp, Fifth Wheel Pickup).

Fifth Wheel, Sliding: An assembly designed to incorporate a manual adjusting and locking feature, which allows the fifth wheel to be moved or relocated forward or backward. Most commonly used on truck tractors to affect changes in weight distribution and/or overall combination length.

Fifth Wheel Spacer: A type of adapter or spacer designed to incorporate a kingpin and plate on the bottom and a rigid fifth wheel on top. Used to lower the effective upper coupler height of a gooseneck in some lowbed applications designed to operate normally in connection with a load divided dolly or “jeep”.

Floor: The cargo-carrying surface. (also called the “deck”).

Floor, Composite: Flooring consisting of alternate narrow metal sections (generally “hat-shaped”) and wood floorboards.

Floor, Laminated: A floor consisting of trailer-length boards manufactured by bonding smaller pieces of wood together.

Floor, Shiplap: A floor consisting of shiplap boards, the term “shiplap” referring to the practice of alternately offsetting or rabbeting opposing edges of a board so that, when joined to others of its kind, adjacent boards “lap” one another.

Floor, Tongue & Groove: A floor consisting of tongue and groove boards, the term “tongue & groove” referring to the practice of alternately placing a tongue and a groove on opposing edges of a board so that, when joined to others of its kind, the tongue on one board engages the groove on the adjacent board.

Floor Loading Height: The unladen height of a trailer floor above the ground measured at the rear of the trailer.

Floor System: A floor system consists of four principal structural elements: (1) the loading deck material; (2) the crossmembers to which the deck material is connected; (3) the fastening system at either end of the crossmembers; and (4) the lower sidewall structure.

Floor System Load Rating: (See Rating, Floor System Load).

Frame Beam: (See Construction, Beam).

Frame Trailer, extendable (Extendable Flatbed): A frame or flatbed whose length may be readily increased or decreased within prescribed limits and with prescribed variations in load carrying capability.

FRP: An acronym taken from the words “Fiberglass Reinforced Plastics.” The term as used to describe the composite panels utilized in truck trailer construction should be written FRP/Plywood, FRP/Honeycomb, FRP/Foam, etc. to properly describe the core construction.

Gate, Chain: A movable barrier composed of horizontal and vertical straight link steel chains, usually installed across the rear door openings of van trailers with or without conventional doors.

Gate and Curtain: A chain gate with a canvas or plastic curtain attached.

Gate, Lift: A powered platform, usually installed at the rear of a trailer, used to load and unload cargo.

Gate, Scissors: A scissor-action folding metal gate usually used across the rear door openings of van trailers with or without conventional doors.

Gladhand: A paired mechanical joining device used to connect flexible air brakes hoses to one another or to portions of trailers or towing vehicles.

Gladhands, Polarized: A variation in gladhand design incorporating a feature, which prevents the crossing, or improper connection of supply or control lines.

Gooseneck: On a drop frame trailer, that portion of the trailer which extends upward and forward from the front of the loading deck to, and including, the upper coupler and front crossmember. On a container chassis the front portion of the structural frame that extends upward and forward.

Gooseneck, Folding: A gooseneck which is articulated in such a manner that it may be extended flat upon the ground upon being disconnected from the tractor. The motive force required to operate such goosenecks is usually obtained through the use of the tractor winch line or hydraulic cylinders.

Gooseneck, Full Width: A gooseneck the same width as the trailer deck.

Gooseneck, Removable: A gooseneck, which can be separated from the trailer and reconnected, usually through the use of large hooks or removable pins. The motive force required to remove such goosenecks is usually obtained through the use of the tractor winch line or hydraulic cylinders.

Gooseneck, Stinger: A variation of the conventional gooseneck, not normally used on trailers designed to be pulled by truck tractors.

Gross Axle Weight Rating: (See Weight Rating, Gross Axle).

Gross Combination Weight Rating: (See Weight Rating, Gross Combination).

Gross Vehicle Weight Rating: (See Weight Rating, Gross Vehicle).

Gross Weight: (See Weight, Gross)

Guide, Container Chassis: Structure at the nose of a container chassis, which guides the container into place and may retain the container from forward movement. (Also called “horn”).

Gutter, Drain: (See Drain Gutter).

Handling, Vehicle: The response or controllability of the vehicle to driver steering and braking inputs experienced in normal highway maneuvers.

Hat Box: (See Fifth Wheel Spacer).

Headerboards: Wooden boards installed across the width of a trailer, generally stacked one on top of the other in slots located in either front corner of a van trailer or container.

Height, Floor Loading: (See Floor Loading Height).

Height, Inside: The unobstructed inside loading height measured from the floor to the lowest point of roof construction.

Height, Overall: The nominal design dimension from the ground to the highest point on the trailer, based on a specified coupler height, with the trailer unladen.

Hitch: A connecting device at the rear of a vehicle used to pull a full trailer with provision for easy coupling and uncoupling. (See Tow Bar, Pintle Hood).

Holder, Certificate: A moisture proof container provided for storage and easy access to trailer related literature such as licenses, registration, defect and maintenance information. (Also called Registration Holder and Defect-Card Holder).

Hook, Pintle: A kind of hitch, normally employing a vertical horn or hook, sized to accept a lunette eye, and equipped with a safety latch. (See Hitch).

Hose, Brake: (See Brake Hose).

Ice Hatch and Ice Hatch Door: An opening and its cover located in or near the front corners of an insulated van or truck body through which ice is loaded.

Ice Pan: That portion of the floor designed to accept and store ice in the front of an insulated van.

Insulation: Any of several materials, or classes of materials, used in the construction of trailers and containers whose function it is to control the transfer of heat in or out of the vehicle.

Jack, Stabilizing: A device, variously designed, to provide temporary support to the front or rear of a parked vehicle and used to prevent nose-diving and taildiving when loading and unloading.

Jackknife: A condition existing in a truck or truck tractor trailer(s) combination when the longitudinal axis of the combination moves from a relatively straight line to one having one or more exaggerated angles on the order of 90° at kingpin or pintle connections. Usually occurs in connection with loss of traction while operating over-the road or when maneuvering or backing in close quarters.

Jeep: (See Dolly, Load Divider).

Kingpin: An especially machined stub shaft which extends vertically from the lower surface of an upper coupler assembly and which locks into a fifth wheel.

Kingpin Plate: (See Coupler Plate, Upper).

Kingpin, Removable: A special adaptation of the standard kingpin shape which allows it to be readily removed and relocated to other positions or settings in the upper coupler assembly. Screw-in and drop-in types are sometimes used.

Label, Certification – (National Highway Traffic Safety Administration): A label or plate permanently affixed to the forward left side of a trailer providing specific information required by law relating to that vehicle.

Lamp: A lamp is a light emitting device permanently attached to the vehicle and energized by the tractor electrical system.

Lamp, ABS: Lamp mounted on the roadside rear of a trailer to indicate malfunction of the anti-lock brake system.

Lamps, Clearance: Lamps mounted on the front and rear of a vehicle which show to the front or to the rear of a vehicle permanently mounted as near as practically possible to the left and right extreme edges to indicate the overall width. (Amber to the front, Red to the rear).

Lamp, Combinational Top & Tail: A single red lamp having two separately energized filaments, which fulfills the requirements of both a stop lamp and tail lamps.

Lamp, Identification: Red lamps deployed horizontally in a group of three, spaced within specified limits, and used on the rear of trailers usually located as high as possible on the longitudinal centerline.

Lamp, License Plate: A clear lamp or lamps used to illuminate the license plate on the rear of a vehicle.

Lamp, Side Maker: Lamps which show to the side of a vehicle, permanently mounted as near as practicable possible to the front and rear edges to indicate the overall length, and also midway along the side on vehicles 30 feet long and over. (Amber at front, amber midway along the side when required, red at the rear).

Lamp, Stop: Red lamps spaced as far apart as practicable on the rear of a vehicle and designed to operate upon application of the service brakes with greater brightness than tail lamps.

Lamp, Tail: Red lamps spaced as far apart as practicable on the rear of a vehicle and designed to operate when tractor headlamps are illuminated.

Lamp, Turn Signal: Lamps spaced as far apart as practicable on the rear of a vehicle and designed to operate intermittently upon activation of a hand control mounted on the tractor.

Landing Gear (See Supports): Devices, generally adjustable in height, used in a trailer support assembly.

Landing Gear Cross Shaft: The transverse shaft connecting the gear mechanism of the landing gear legs.

Landing Gear Extended Length: The maximum length, which each leg is designed to extend to provide maximum upper coupler elevation.

Landing Gear Foot Member: A device attached either directly or indirectly to each lower leg to serve as ground contact support for the trailer.

Landing Gear Location: (See Support Location).

Landing Gear Retracted Length: The minimum collapsed length, which each leg is designed to retract to provide the maximum clearance between the bottom of the foot member and the road surface.

Landing Gear Sand Shoe: A pivoted flat plate attached to the bottom of each leg, which affords increased bearing surface on the ground (a type of foot member).

Landing Gear Stiff Leg: A variation in leg design employing non-powered telescoping tubes adjusted by means of mating holes and through shear pin.

Landing Gear Travel: The distance, which the lower leg is designed to travel from a fully retracted position to a fully extended position.

Landing Gear Wheels: Dual Wheels attached to the bottom of each lower leg providing ground contact for the supports (a type of foot member).

Ledge Loading: A projection or slot on the back or sides of a lowbed trailer, which is used to temporarily attach loading ramps.

Length, Inside: The unobstructed inside loading length measured at the floor of a van trailer or container.

Length, Outside Trailer: The length measured from the front vertical plane of the foremost transverse load carrying structure to the rear vertical plane of the rearmost transverse load carrying structure, including door hardware.

Lifting Equipment: The term “Lifting Equipment” refers to straddle carriers, traveling and fixed gantry cranes, counter-balanced lift trucks, derricks, and similar types of equipment used in transporting trailers from one place to another by lifting.

Lift Gate: (See Gate, Lift).

Lift Pads: (See Pads, Lift).

Lift Shoe: A lift shoe is the device used by lifting equipment, which contacts and transmits the lifting forces to the trailer through the trailer lift pads, in intermodal operations.

Lights, Running: Marker, clearance, tail, and identification lights as required by regulations.

Load Binder: (See Binder, Load).

Lunette Eye: A steel eye mounted on the drawbar of a trailer or converter dolly designed to couple with a pulling vehicle having a pintle hook.

Nosediving: The tipping down at the front end of an uncoupled trailer resting on its supports while being loaded or unloaded.

OEM: An acronym taken from the words “Original Equipment Manufacturer”.

Outrigger: Structural load-carrying members attached to and extending outward from the main longitudinal frame members of a trailer. (See side bracket).

Over-The-Road Service: The transportation of goods by vehicles designed and manufactured for use over improved highways and city streets, which are maintained in good condition, and in terminal yard or parking areas, which are maintained in relatively hard and smooth condition.

Pads, Lift: A lift pad is a structural reinforcement attached to the lower rails of a van trailer or container and intended to prevent localized mechanical damage to the vehicle when engaged by lifting equipment.

Panel: Material used to cover the interior and exterior surfaces of a trailer.

Payload: The net weight of the cargo carried by a vehicle.

Pintle Hook: (See Hook, Pintle).

Planks, Widening: Planks used in connection with side brackets or extensions to temporarily widen the deck on a drop frame or lowbed trailer.

Plate, Pick Up: That portion of the upper coupler plate forward of the kingpin which initially encounters the fifth wheel and guides it toward the kingpin.

Plate, TOFC: (See TOFC Plate).

PLC: Power line carrier.

Pockets, Stake: Rectangular openings or enclosures installed at intervals around the perimeter of platform trailers to accept removable stakes and/or stake sides.

Pogo Stick: A flexible, vertical springwound tube installed on a truck tractor frame behind the cab providing an elevated support for longer than normal tractor air hoses and electrical cable.

Pole or Rope Ridge: A rigid member or rope installed down the centerline of an open top van trailer to lend support to the tarpaulin and provide good drainage.

Post: A vertical structural member utilized in the construction of van trailers and containers to which panels, side sheets and, in some instances, doors are attached.

Post, Cargo Control: A specialized variation wherein a steel post is provided with one or two rows of punched slots along the length of the post designed to accept various cargo control fittings.

Racks: Removable wood or metal wall sections (usually slatted) held in place on flatbeds normally by stake pockets.

Rail, Side Protector: An exterior protector bar or strip extending horizontally along the sides (on some types of trailers across the front and rear) and intended to provide scuff protection for the trailer. On some van trailers there may be more than one row extending horizontally along either side.

Rails, Meat: Supports suspended longitudinally beneath the ceiling of an insulated van or container on which meat is hung for transport by means of hooks or other devices.

Rails, Upper Side (Roof Rails): The main longitudinal frame members on vans and containers used to connect the roof and the side wall structures. Upper rails comprise the principal compressive structural elements used in frameless (monocoque) construction.

Rails Lower Side (Chassis Rails): The main longitudinal frame members on vans and containers used to connect the sidewall and floor structures. Lower rails comprise the principal tensile structural elements used in frameless (monocoque) construction.

Ramp(s): Articulated or removable bridge-type structures to load or unload lowbed or drop frame trailers.

Ramp, Fifth Wheel Pickup: A sloping structure permanently attached behind the fifth wheel on a truck tractor and intended to lift and guide the upper coupler assembly of a trailer being coupled.

Ramp, Flip: A ramp hinged in such a manner that it may be folded aboard the trailer when not in use (usually installed on the rear of lowbed or drop frame trailers).

Ramp, Motor Grader: A ramp attached to either side of a lowbed gooseneck designed to support the front wheels of a motor grader being carried.

Rating, Floor System Load: A load rating for trailer or container floors expressed in terms of the maximum gross load which may be imposed by a fork truck front axle having specific wheel “prints” or load areas.

Rating, Gross Axle Weight: (See Weight Rating, Gross Axle).

Rear Impact Guard (RIG): A device installed on or near the rear of a vehicle so that when the vehicle is struck from the rear, the device limits the distance that the striking vehicle’s front end slides under the rear end of the impacted vehicle.

Reefer: Jargon for “Refrigerated Trailer”. (See Trailer, Refrigerated).

Reflector, Reflex: Device used on vehicles to give a warning indication to the driver of an approaching vehicle by means of incident light reflected from the headlights of the approaching vehicle.

Refrigeration Unit: A mechanical Device Designed to be mounted on a refrigerated trailer or container for the purpose of cooling the cargo area.

“Remak”: The remanufacture of a trailer utilizing certain components salvaged from existing vehicles as provided for in Title 49 Code of Federal Regulations Part 571.7(f).

Reservoir, Air: A container or tank used to store compressed air.

Ridge Pole: (See Pole, Ridge).

Ridge Strap: (See Strap, Ridge).

Ring, Bull (“D” Ring): A heavy articulated fitting installed at or near floor level and used as a cargo tie-off. On lowbeds commonly installed at intervals along the side frames.

Ring, Lashing: A device permanently attached to a trailer underframe and designed to accept tiedown equipment.

Ring, Tie: A particular kind of cargo tie-off consisting of an articulated ring, usually installed in a recessed pocket in the sides or floor.

Rim: A metal ring uniquely designed and manufactured to receive a pneumatic tire (or tire-tube assembly) and transfer its operational loads to a wheel assembly or disc.

Riser: Vertical mounting brackets or struts employed in some types of running gears to transfer suspension loads to the trailer sub-frame.

Roadside: The side of the trailer farthest from the curb when traveling in a normal forward direction. Opposite to “curbside”. (Sometimes called “driver side”).

Rolling Tail Pipe: A full width cylindrical member located at the rear of the bed of a platform or drop frame trailer and level with or projecting slightly above the floor. Its purpose is to facilitate the loading of cargo such as skid mounted equipment through use of a cable passing over the rolling tail pipe so that the tail pipe may rotate and facilitate loading by reducing friction and scuffing by either the cable or the cargo. (See TTMA RP no. 64).

Roof Bow: A transverse member in the roof of a van trailer or container which is connected at either end to the roof rails (upper rails) and which supports the roof sheet.

RO/RO: An acronym taken from the words “Roll-On/Roll-Off” and used to describe a particular type of trailer-on-ship operation.

Running Gear: (See Undercarriage).

Sand Shoe: (See Landing Gear Sandshoe).

Seal, Oil: A device used to retain liquid lubricant in the bearing area of a wheel or hub assembly. The seal normally rotates with the wheel and “wipes” a mating stationary part of the spindle inboard of the inside bearing.

Semitrailer: (See Trailer, Semi).

Sensor, Wheel: That part of an electro-mechanical antilock system installed in the hub of a wheel, which generates and transmits to the logic controller an electric current proportional in strength to the rotational speed of the wheel.

Shut-Off Cock: A type of valve used to control or stop the flow of air in a line.

Sideboards (see swinging side brackets): Removable boards used in conjunction with swinging or pivoted side brackets to increase the effective width of the loading deck on lowbed or drop deck trailers.

Skylight: A panel of clear or translucent material installed in the roof, nose or sides of van-type trailers to admit light to the cargo area.

Slack Adjuster: An adjustable mechanical lever fixed to the brake camshaft on one end, to the brake chamber push rod on the other, and designed to transmit brake chamber energy to the camshaft.

Slider: (See Undercarriage, Sliding).

Sockets, Pipe: Short sections of pipe installed at intervals usually along the top of trailer or sides to receive tarp bows or spreaders. Sometimes used on flatbeds in lieu of stake pockets.

Spacer, Rim: A device used in connection with spoke wheel assemblies to produce the desired dual tire spacing with demountable rims.

Span: (See Wheelbase).

Spider, Brake: (See Brake Spider).

Splash Guard (Mudflaps): A flexible plastic or rubber deflecting shield installed behind running gear tires to control spray.

Spring, Air (Air bag): A flexible pneumatic cushion pressurized by trailer system air and regulated by leveling valves to provide the desired performance in an air suspension.

Spring Deflection: The reduction in effective suspension spring height brought about by imposed load.

Spring Seat (Axle Seat): A suspension component affixed to a trailer axle and used to position and support a suspension spring.

Stake Pockets: (See Pockets, Stake).

Stanchion, Free Standing: A trailer support, which accommodates the trailer’s kingpin, but is not locked or rigidly attached to the transporter.

Stanchion, Locking: A trailer support that locks the trailer kingpin and is rigidly attached to the transporter’s deck.

Stanchion Plate: On trailers intended for TOFC Service, a structural extension of approximately 24 inches on the rear of the upper coupler assembly.

Stinger Steered Vehicle Combination: A truck tractor or truck towbar semitrailer combination where the towbar connection is located just to the rear and below the top of the rear tires of the towing vehicle.

Strap, Ridge: (See Ridge Rope).

Stretch, Over-The-Road Van: The lengthening of a van trailer from its original manufactured overall length to a longer specified overall length.

Stretch, TOFC Van: The lengthening of a previously certified TOF van trailer, which is in structurally sound operating condition.

Strip, Scuff: Material installed inside van trailers and containers to minimize mechanical damage from within to lower side walls and nose.

Structure, Permanent Front End: (See Bulkhead).

Supports (See Landing Gear): Structure generally adjustable in height used to support the front end of a semitrailer in an approximately level attitude when disconnected from towing vehicle. Includes landing gear legs, foot members, mounting brackets, braces and related fasteners.

Support Clearance: The distance between the rearmost interference point of a truck tractor or dolly and the trailer landing gear or its supporting structure during any degree of turning or maneuvering of the tractor.

Support, Location: The distance from front of the trailer to the vertical centerline of the landing gear legs.

Suspension: Any one of several types of articulated mechanical assemblies used to position and secure axle and wheel assemblies to a trailer frame or sub-frame.

Suspension, Air Ride: A suspension employing flexible pneumatic cushioning devices, or air springs, which are pressurized by tractor air and regulated by height control valves or variable pressure regulators.

Suspension, Four Spring (3-Point Suspension): A two-axle suspension equipped with elliptical (or plate-type) springs wherein the springs on one axle are caused to interact with the springs on the other by pivotal rockers or equalizer beams.

• Front Hanger:

  The forward most frame mounting bracket which normally incorporates a progressive spring seat and an anchor point for one torque arm.

• Rear Hanger:

  The rearmost frame mounting bracket, which normally incorporates a progressive spring seat.

• Rocker (Equalizer Beam):

  A suspension part containing a progressive spring seat on either end and a transverse pivot connection in between, allowing vertical translation of the wheel and axle assemblies it connects.

• Rocker Hanger (Equalizer Hanger):

  An intermediate frame mounting bracket, which incorporates a pivoted rocker, or equalizer beam, and an anchor point for one torque arm.

• Torque Arm, Adjustable (Adjustable Radius Rod):

  An articulated, adjustable member, which is attached to, an axle bar at one end and anchored (usually through another suspension part) to the trailer frame or sub-frame at the other end. Its function is to absorb certain dynamic axle loads and to provide the means for axle alignment.

• Torque Arm, Non-Adjustable (Fixed Radius Rod):

  Same as (d) except it is of a fixed length.

Suspension, Overslung: A suspension employing flat plate or tapered springs wherein the springs are attached above the top of the axle bar.

Suspension, Single Axle: A suspension usually employing two springs and designed to accommodate one axle.

Suspension, Single Point (Sometimes referred to as a 2-spring tandem): A tandem two-axle suspension consisting of two springs deployed much in the same manner as walking beams between the axles and pivoted half way between the axles on a trunnion shaft at the connection to the trailer frame.

Suspension, Spring Beam: A type of two-axle suspension, which employs a spring beneath each walking beam to enhance ride when the trailer is traveling in the unladen condition.

Suspension, Steel Spring: Any one of several types of suspensions, which employ either flat plate or tapered steel springs to reduce shock loads from the axle and wheel assemblies.

Suspension, Underslung: A suspension wherein the springs are air beams are attached below the bottom of the axle bar.

Suspension, Walking Beam: A type of unsprung suspension comprised of parallel beams pivoted on a common transverse axle or trunnion between the axles and attached at the ends to axle and wheel assemblies. Most commonly used in two-axle assemblies but also to lesser degree in three and four-axle variations.

Suspension Mounting Height: The vertical distance from, and perpendicular to, the horizontal centerline of the axle(s) in a suspension to the horizontal line representing the lower surface of the suspension sub-frame. Normally taken with the suspension in the unladen condition.

Suspension Setting: The distance from the rear extreme of the trailer (or dolly) to the vertical centerline of the axle, if single axle, or to the vertical centerline of the axle arrangement when two or more axles are employed in the suspension.

Suspension Subframe: A structural assembly, usually employed in frameless (monocoque) construction, to which the suspension and axle assemblies are attached.

Tandem Axle: (See Axle, Tandem).

Tarp (Tarpaulin): A water resistant fabric cover used to protect the cargo on an open trailer.

Tarp Basket: An open framework rack or storage compartment attached across the front of a trailer and used to store a tarpaulin when not in use.

Tarp Bow: A transverse support installed at intervals along the top rails of open top trailers, or removable side panels, and designed to elevate and support a tarpaulin or tarp.

Tarp Hook: Any one of several types of fittings attached at intervals around the outside of open top trailers, or trailers with removable side panels, and used as anchor points for securing a tarpaulin or tarp.

Tarp Tie Rod: Rods or tubes attached horizontally around the outside of open top trailers, or trailers with removable sides, and used in lieu of tarp hooks.

Tie-Down Assembly: A system of chains, cables, or straps along with the accompanying attachment hardware used to secure cargo in or on trailers for support.

Tire: A device made of rubber, fabric, steel or other materials which, when mounted on the rim at wheel and filled with air, cushions and sustains the load.

Tire Carrier: A rack or other device for carrying a spare tire.

Tire Clearance: The distance or dimension between a tire or tires and the nearest point of interference or contact on the trailer. Normally taken as the vertical distance above the periphery of a tire, but may also be construed to mean lateral clearance.

TOFC Intermodal (Piggyback): An acronym taken from the words “Trailer-on-Flat-Car” and used to describe the transport of highway trailers on the rail system.

Tow Bar: A strut or column-like device temporarily attached between the rear of a towing vehicle and the front of the vehicle being towed. Its purpose is to maintain the interval or distance between the two. (Not commonly used with truck trailers).

Towing Capacity: The total weight of the trailer or trailers towed.

Track, Wheel Tread: The distance or dimension between the vertical centerlines of single tires or dual tier combinations as measured transverse to the longitudinal centerline of the trailer.

Track, Axle: The projected nominal wheel track for a given axle bar and spindle combination as used in specifying trailer axles.

Track, Cargo Control: A vertical or horizontal structural member normally attached to the inside of sidewall posts, having pierced slots for use with cargo-tie-off straps or double decking components.

Trailer: A vehicle without motive power designed for carrying property and for being drawn by a motor vehicle. The term “trailer” includes full trailers and semi-trailers as sub-groups.

Trailer, Agricultural Commodity: A trailer that is designed to transport bulk agricultural commodities in off-road harvesting sites and to processing plants or storage locations.

Trailer, Auto Transporter: The semi-trailer portion of an auto transporter truck and trailer combination designed and manufactured expressly for the loading, transportation and unloading of automobiles.

Trailer, B-Train: A type of doubles semi-trailer incorporating a uniquely designed suspension sub frame-fifth wheel combination, which obviates the need for a converter dolly when towing a full trailer.

Trailer, Carryall: (See Lowbed).

Trailer, Container Chassis: A semi-trailer chassis designed especially to transport one or two containers over the highway.

Trailer, Converted Semi: A semi-trailer and converter dolly combination.

Trailer, Desert Float: (See Trailer, Oilfield Float).

Trailer, Dolly: (See Dolly, Converter)

Trailer, Doubles: (See Doubles)

Trailer, Drop Center (Double Prop): A trailer design employing an offset or drop in the cargo deck immediately behind the supports and another immediately in front of the suspension, the purpose of which is to lower the cargo deck and/or to provide the greatest height possible for cargo in the dropped area.

Trailer, Drop Frame: A trailer design employing one offset or drop in the cargo deck immediately behind the supports.

Trailer, Extendable Platform: (See Frame Trailer, Extendable).

Trailer, Flat Bed: (See Trailer, Platform).

Trailer, Full: A load carrying vehicle equipped with two or more axles and constructed with a front tow bar coupled to a swiveling or steerable undercarriage assembly at the front of the trailer.

Trailer, Heavy Hauler: (See Trailer, Lowbed).

Trailer, Industrial: A general purpose trailer sometimes equipped with a tilt bed or ramps and towed by means other than a truck tractor.

Trailer, Insulated Van: A van trailer constructed with (or without) insulation in the floor, and with insulation in the walls, nose and roof.

Trailer, Piggyback: (See Trailer, TOFC)

Trailer, Platform (Flatbed): A trailer chassis consisting of load carrying main frames and a flat loading deck without permanent sides or roof.

Trailer, Platform, Extendable: (See Frame Trailer, Extendable).

Trailer, Pole: A trailer consisting of an upper coupler assembly connected by a tube (or pole) to a running gear assembly and designed to carry long loads capable of supporting their own length.

Trailer, Pulpwood: Any of several types of chassis trailers designed exclusively for transporting pulpwood.

Trailer, up (Pull): A short trailer typically less than 30 feet long and typically pulled behind a trailer or truck. (See Trailer, full).

Trailer, Refrigerated: An insulated van trailer designed to possess specific thermal properties and intended for use with self-contained refrigeration systems.

Trailer, Semi: A load carrying vehicle equipped with one or more axles and constructed so that its front end is connected to and supported by a vehicle, which pulls it.

Trailer, Straddle: A trailer that is driven or backed over the cargo it is to carry in order that lifting devices integral with the trailer can position and secure the cargo for transport.

Trailer, Tag-A-Long: (See Trailer, Industrial).

Trailer, Tank: (See Trailer, Refrigerated)

Trailer, TOFC: A trailer designed for carriage on a railcar and which, if built after January 1, 2973 for straight floor van trailers or after June 1, 1975 for container chassis, contains an Association of American Railroads certification plaque described in AAR Specification M-931 or M-943. Visually a TOFC trailer may be identified by the AAR certification plaque ear the DOT certification label, lift pads located along the bottom side of the trailer (except for container chassis and tank trailers) and a plate extending about two feet to the rear of the upper coupler assembly.

Trailer, Van: A trailer designed to have a totally enclosed cargo space comprised of fixed sides, nose, roof and with articulated panels across the rear and sometimes at selected places in the sides.

Trailer, Wedge Van: A van trailer having a straight floor in which the inside clear height at the rear is greater than at the front.

Trailer Body: A trailer body is that portion of the assembly whose function it is to contain or accommodate the cargo.

Trailer Chassis: (See Chassis).

Tri-Axle: (See Axle, Tri).

Tridem Axle (See Axle, Tridem)

Triple: A combination consisting of a tractor pulling a semi-trailer plus two full trailers.

Truck: A motor vehicle designed primarily to receive some sort of body or bed for the purpose of transporting cargo.

Truck Tractor: A motor vehicle designed primarily for the purpose of pulling semi-trailers.

Trunnion Axle: (See Axle, Trunnion).

Type of Vehicle: A broad separation intended to classify vehicles into groups possessing common traits, including design and purpose. Trucks comprise one group, trailers another, etc.

Type of Trailer: The first separation within the Trailer Classification. Truck Trailers comprise one type of trailer, recreational trailers another, etc.

Type of Trailer Body: The second separation within the Trailer Classification. Van Trailers comprise one body type, tanks another, etc.

Undercarriage (Running Gear, Bogie): A structural sub frame complete with suspension and axle-wheel assemblies.

Undercarriage, removable: An undercarriage capable of being separated from the trailer structure.

Undercarriage, sliding (Slider): A specialized undercarriage design, which incorporates provisions for readily moving in to new locations forward and backward to affect changes in weight distribution and/or wheelbase.

Undercarriage Locator Bar: A removable round tubular or solid bar inserted through the positioning holes of the body rail to assist in locating the sliding undercarriage at a predetermined location.

Undercarriage Stop Bar: A round tubular or solid fixed bar installed near extreme ends of a slider body rail usually through the positioning holes to serve as a safety stop when moving the sliding undercarriage from one position to another.

Upper Coupler Plate: (See Coupler Plate, Upper).

Upper Coupler, Rocking: (See Coupler Assembly, Rocking Upper).

Valve, Air Leveling: An automatic control device used on air suspensions to regulate the flow of air in and out of the air springs to maintain the predetermined ride or mounting height. There are normally two leveling valves employed in each air suspension, one on either side of the trailer.

Valve, Antilock: That part of an electro-mechanical antilock system, which receives electrical signals from the logic controller and releases air from or admits air to the brake chambers on a given axle to adjust the brake actuation force.

Valve, Booster Relay: A relay type valve used to accelerate the application and release of pressurized air to the towed vehicle(s) relay or relay emergency valve.

Valve, Check: An automatic control device designed to provide free fluid flow in one direction only.

Valve, Quick Release: A valve placed in the trailer control (service) brake line to accelerate the release of pressurized air in the line.

Valve, Relay: A secondary control unit use in an air brake system to accelerate the application and release of pressurized air to and from brake chambers.

Valve, Relay Emergency: A combination valve which incorporates the features of a relay valve along with additional provisions for the automatic application of the trailer brakes in the event pressure is lost in the trailer supply (emergency) air flow.

Valve, Tractor Protection: An automatic control device used on truck tractor air systems which: (a) Permits driver control of pressurized air passing through the air brake lines to the towed vehicle whenever the air pressure in the towing vehicle exceeds a predetermined value, (b) Closes off the flow of air to the trailer automatically when the tractor brake system pressure falls below predetermined value, and (c) Vents the trailer supply (emergency) line when flow in this line is stopped either manually or automatically.

Valve, Trailer Control (Hand Control Valve): A manually operated control device mounted in the cab of a truck tractor with which a driver can apply and release trailer brakes independent of tractor brakes.

Vehicle Identification Number (VIN): An arrangement of 17 Arabic numerals and capital letters uniquely differentiating a motor vehicle from any other so identified. (Reference: TTMA Recommended Practice No. 5 and Title 49 Code of Federal Regulations 565 and 471.115).

Weevil Pins and Sockets: Pins and sockets located on both sides of a platform trailer outboard of the rolling tail pipe. Its purpose is to contain or guide the winch cable when winching cargo such as skid mounted equipment aboard the trailer (See TTMA RP No. 64).

Weight, Axle: The weight transmitted to the ground by the trailer axles.

Weight, Gross: The combined weight of a trailer and its payload.

Weight, Payload: (See Payload).

Weight Rating, Gross Axle (GAWR): The rated load carrying capacity of an individual axle and wheel assembly.

Weight rating, Gross Combination: The maximum combined weight rating for a truck tractor and trailer(s) combination.

Weight Rating, Gross Vehicle (GVWR): The maximum rated combined weight of a vehicle and its payload based upon structural capability alone.

Weight, Sprung: That portion of the vehicle weight supported by the suspension springs.

Weight, Tare (Unloaded): The weight of an empty vehicle.

Weight, Unsprung: The weight of the axles, wheel assemblies and all other trailer parts whose weight is not borne by the suspension springs.

Wells, Coil: Recessed areas between the main beams of platform trailers designed to carry steel coils.

Wheel, Disc: A type of wheel consisting of a rim permanently affixed to a round metal plate (or disc) and designed to mate with an axle and hub assembly. The disc may be flat but is most commonly “dished” or offset in order that the wheel may be used either singly or in pairs for dual wheels.

Wheel, Spoke: A type of wheel consisting of integral hub and spokes designed to accept demountable rims either singly or in pairs with a spacer for dual tires.

Wheelbase: The distance from the kingpin to the centerline of the suspension.

Wheel Bearings: An assembly of hardened precision rollers and races designed to minimize friction at the spindle and wheel hub interface.

Wheel Boxes: Enclosures permanently attached above the tires and wheels to protect the cargo area from road spray and debris, which may be thrown upward by the tires. Generally found on furniture vans and other trailers having deep drops.

Wheel Chock: A wedge-shaped stop applied in front of or behind the wheels of parked trailers to prevent rolling.

Wheel Hop (Brake Hop): An undesirable bouncing motion of axle and wheel assemblies associated with the application of vehicle service brakes.

Wheel Rim: (See Rim).

Wheel Track: (See Track).

Width, Inside: The unobstructed inside loading width of a van-type trailer or container.

Width, Outside Trailer: The width measured across the sidemost load carrying structures, support members, and structural fasteners.

Wire Color Coding: The practice of using electrical conductors with variously colored insulating coatings to identify the different circuits in trailer electrical systems, usually in accordance with a recognized industry standard such as Society of Automotive Engineers (SAE), American Trucking Associations (ATA), etc.

Arrow Truck Sales, Inc. strives to provide accurate and reliable information on all it publishes on its website and blogs. Arrow is not responsible for technical inaccuracies and/or typographical errors. Any reliance you place on such material is therefore strictly at your own risk.