Truck Terms

Body

The part of the vehicle designed to carry items related to the use of the vehicle rather than the operation of the unit. This does not normally include the cab except when the cab is an integral part of the body as in a school bus.

Cab

The part of the vehicle that encloses the driver and vehicle operating controls. The term "cab" may also include the front end, sheet metal housing, the engine, front fenders, etc.

Axle, Full-Floating

The shafts of these axles have nothing to do but drive the wheels. The housing supports the entire rear wheel weight through double opposed wheel bearings, which absorb all load and wheel stresses. Should the axle shaft break, the truck can be towed since the wheel is supported by the wheel hub and bearings.

Geared Speed

The theoretical vehicle speed based on engine rpm, transmission gear ratio, rear axle ratio, and tire size.

Chassis

(1) Entire vehicle as produced by the factory when no body is included (cab, frame, powerplant, drive line, suspensions, axles, wheels, and tires); (2) Same as (1) except excluding cab and other sheet metal; or (3) Frame only with brackets, bumper, and other miscellaneous parts directly attached to the frame.

Differential

(A) Standard — gear assembly on the drive axle that permits the wheels to turn at different speeds. (B) No-Slip or Limited-Slip — gear assembly on the drive axle that will not permit one wheel to spin while the other is motionless, such as when a truck is stuck in snow or mud. Torque is transmitted to both wheels of the driving axle.

Resisting Bending Moment (RBM)

(Frame section modulus) x (Frame yield strength). The resulting number is used when comparing the strength of two frames made of different materials.

Maximum Rolling Grade

(Gradability) Greatest grade a vehicle is able to climb while under motion, or the number of foot rise the vehicle can attain continuously for each 100 feet of horizontal movement. Maximum rolling grade is calculated with the vehicle in motion with rated load and with gearshift settings to obtain greatest gear reduction.

Maximum Starting Grade

(Gradability) Greatest grade a vehicle is able to start on from complete stop. Approximately 10% grade loss from the rolling gradability. (Starting Gradability (%) = Rolling Gradability (%) -10%).

Disc Brakes

A brake assembly containing a disc, which rotates as the wheel turns. A caliper device grabs the disc to stop the wheel from rotating.

Drum Brakes

A brake assembly with brake shoes, which are pressed against a brake drum to stop the wheels from rotating.

Power Takeoff

A device usually mounted on the side of the transmission or transfer case, or off the front of the crankshaft, used to transmit engine power to auxiliary equipment such as pumps, winches, etc.

Trailer, Full

A full trailer is a truck trailer constructed so that all of its own weight and that of its load rests upon its own wheels (see Full Trailer).

Power Curve

A graphic illustration of maximum output of power and torque at all operating speeds. These curves are established from data obtained by running a sample engine on an engine dynamometer. Net power figures are used in the vehicle.

Horsepower

A measure of the amount of work that can be done by an engine in a certain amount of time. One horsepower is equal to 33,000 ft.-lb. of work per minute. The horsepower of an engine depends upon the torque and speed of the engine.

Air Resistance

A measure of the drag produced by a vehicle in motion. Because it varies theoretically as the square of the speed, it effects the ability of the vehicle to reach top speed as well as the gradability at fast speeds.

Road Rolling Resistance

A measure of the retarding effect of the road surface to forward movement of the vehicle and varies with the type and condition of the road.

Section Modulus

A measure of the strength of frame side rails determined by the cross-section area and shape of the side rails.

PR (Ply Rating)

A measure of the strength of tires based on the strength of a single ply of designated construction. An 8-ply rating does not necessarily mean that 8 plies are used in building the tire, but simply that the tire has the strength of 8 standard plies.

Powertrain

A name applied to the group of components used to transmit engine power to the wheels. The powertrain includes clutch, transmission, universal joints, driveshafts, and rear-axle gears.

Trailer, Semi

A trailer having axle (or axles) only at the rear; the front of the semi-trailer is supported by a tractor fifth wheel. A semi-trailer may be operated as a full trailer by using a converter dolly to support the front of the trailer.

Full Trailer

A trailing load carrying a vehicle, which is entirely supported by its own suspension systems. The powered unit merely tows this type of trailer and does not directly support any of its weight. Sometimes referred to as a "pup" when towed behind a truck with a mounted body or behind a tractor/semi-trailer combination. Tractor/semi-trailer/full-trailer combinations are often referred to as "double" or "double bottoms."

Transmission

A transmission contains a number of gears that, when a connection is made between a specific set, provide a choice of ratio. Connection is made by sliding the teeth of one gear into mesh with another, or by engaging a tooth-type clutch that has one part fastened to a gear already meshed to another, and the other part splined to a shaft. Synchromesh transmissions use gear speed synchronizers to ease engagement.

Overdrive Transmission

A transmission in which the high gear ratio is less than one to one. This permits the truck, under favorable conditions, to maintain a higher road speed with any given engine speed or a given road speed at a lower engine rpm. The primary use in trucks is for fuel economy on empty return trips.

Synchromesh Transmission

A transmission with mechanisms for synchronizing the gear speeds so that the gears can be shifted without clashing, thus eliminating the need for double clutching.

Gradability

Ability of a truck to negotiate a given grade at a specific GCW or GVW.

Maximum Speed

Ability of a vehicle to attain speeds under full load conditions. This speed is calculated using level road conditions and with best concrete road surface. When the vehicle power is great enough to exceed geared MPH, the geared MPH becomes the maximum speed. Speeds are calculated in the best gear to obtain the highest speed (using a lower gear if necessary).

Gross Vehicle Weight (GVW)

Actual weight of the entire vehicle including all equipment, fuel, body, payload, driver, etc. This is for the individual unit only, such as a truck or tractor.

Slack Adjuster

Adjustable brake lever on air brake assemblies.

Nominal Truck Rating

An arbitrary classification of truck capacity in tons, such as ½-ton, 1½-ton. Although this classification is still used, the correct rating of truck capacity is gross vehicle weight (GVW).

Engine Brake

Brake device using engine compression pressure as a retarding medium.

AF

Dimension between the center of the fifth wheel or the center of gravity of the body and rear axle. Maximum AF is the longest dimension permissible to ensure against load damage to the frame. AF dimensions are based on frame strength and do not consider adaptability of average trailer or bodies to the available space behind the cab.

BBC

Dimension from the front bumper to the back of the cab.

BA

Dimension from the front bumper to the centerline of the front axle.

Body Clearance (BC)

Distance between the back of the cab and the installed body to prevent cab-to rear body contact due to flexing of chassis frame.

Displacement

Engine displacement is the volume displaced by a piston during one stroke multiplied by the number of pistons. (bore) x (bore) x (stroke) x (no. of pistons) x (.785)

Trailer Length (TL)

Front of body to bumper.

Planetary Drive

Gear reduction system with sun gear transmitting reduction through planetary gears to main output shaft.

Helical Gears

Gears with slanted teeth, usually used in transmissions. The teeth are positioned diagonally across the face of the gear for quieter operation and more gear tooth contact.

Spiral Bevel Gears

Gears with spiral-shaped teeth used primarily to change the direction of transmitted power, such as from the propeller shaft to axle shafts.

Pintle Hook

Hook mounted on the truck or semi-trailer used to couple on a full-trailer.

Hotchkiss Drive

Hotchkiss drive is a term applied to that type of chassis design where the rear springs are mounted at the forward end in a stationary bracket (not shackled as at the rear end) and all driving and braking forces are cushioned by the springs and transferred directly to the frame side members. Open-type universal joints and propeller shafts are used in this design.

Hypoid Gears

Hypoid gears and pinions have a tooth form that permits the drive pinion to mesh with the driven gear below the center of the driven gear.

Fifth Wheel

Load supporting plate mounted to the frame of the vehicle. Pivot-mounted, it contains provision for accepting and holding the kingpin of a semi-trailer providing a flexible connection between the tractor and the trailer. Center of fifth wheel (where kingpin is held in position) should always be located ahead of the centerline of the tractor rear axle or axle group.

Torque Converter

Multiplies engine torque. Made up of a pump, a turbine, and a stator. When torque multiplication nears a one-to-one ratio, the converter acts as a fluid coupling between the engine and the transmission. At all other pump-turbine ratios, torque is automatically multiplied according to the load imposed on the vehicle, within the limits of the converter.

BW

Outer track — measures the distance between the dual rear wheels from the outside of the outer wheels.

OH

Overall height of chassis measured from the ground to the topmost point of the cab.

OAL

Overall length of chassis measured from the front bumper to the end of the frame.

Center of Gravity

Point where the weight of the truck and/or body and payload appears to be concentrated and, if suspended at that point, would balance front and rear.

Ratio

Proportion input revolutions to output revolutions of a unit (axle, transmission, steering gear, etc.). A two-to-one ratio (2:1) means that two complete revolutions must be made on the input shaft of the item to obtain one complete revolution of the output shaft. This is used primarily to multiply torque (turning force), which is the opposite of speed. To interpret a ratio in terms of torque, the expression becomes the proportion of the output to the input. Thus, a 2-to-1 ratio means that 2 units of force are available at output shaft for each unit of force applied to input shaft.

SRW

Single Rear Wheel.

Springs, Semi-Elliptical

Springs basically consisting of one main leaf with eyes at each end for connection to spring shackles and brackets and a number of shorter leaves of uniformly decreasing length shaped in the form of an arc.

Springs, Progressive Type

Springs that automatically adjust to load or road conditions, ensuring a smooth, comfortable ride.

Springs, Auxiliary Type

Springs that do not come into operation until a predetermined load is placed on the chassis. They are designed to provide riding comfort whether the truck is empty or under partial load.

Frame Cut-Off

Standard frame on most models extends behind the rear axle, far enough to support a body mounted on the vehicle. For special purpose bodies that may be unusually short for the wheelbase of the vehicle on which it is mounted, or in most tractor operations, this frame extension behind the rear axle may be shortened. The shortest allowable extension for each vehicle is referred to as "maximum frame cut-off."

Crossmember

Structural unit that connects side rails of the frame.

Taxable Horsepower

The N.A.C.C. (National Automobile Chamber of Commerce) adopted an arbitrary formula for estimating horsepower to enable comparison of engines on a uniform basis. It assumes that engines deliver their rated power at a piston speed of 10 feet per minute and that mechanical efficiency will average 75% Taxable Horsepower = (Diameter of Bore) 2 x Number of Cylinders/2.5 = D2N /2.5. Advancement in engine design since this formula was developed has rendered the formula obsolete as a basis of estimating true engine output. The formula is still used in some states for licensing purposes, however.

Brake Horsepower

The actual horsepower delivered by the crankshaft and is measured by means of an electric dynamometer.

Gross Brake Horsepower

The actual horsepower delivered by the crankshaft and is measured by means of an electric dynamometer.

Gross Combination Weight (GCW)

The actual weight of a vehicle at the ground with a trailer or trailers including vehicle, equipment, driver, passengers, fuel, and payload (everything that moves with the vehicle).

Chassis Weight

The actual wight of the fully equipped vehicle including all fluids, but not including body or driver.

Spring Capacity At Pad

The amount of sprung weight that will bend a leaf spring its maximum amount.

Engine Torque

The amount of twisting effort exerted at the crankshaft by an engine. The unit of measure is a pound-foot, which represents a force of one pound acting at right angles at the end of an arm one foot long.

SAE, Net Brake Horsepower

The brake horsepower remaining at the flywheel of the engine to do useful work after the power required by the engine accessories (fan, water pump, generator, etc.) has been provided as measured in accordance with SAE standards.

Deflection Rate

The deflection rate of a spring is the force required to compress or deflect the spring a distance of one inch. For torsion springs, this distance is measured at the end of the control arm attached to the springs.

CA

The dimension from the back of the cab to the centerline of the rear axle. This dimension is important when determining the body application or fifth wheel mounting and weight distribution.

CE

The dimension from the back of the cab to the rear of the standard frame. Used primarily to determine the size of the body that may be used.

Tread

The distance between the centers of tires at the points where they contact the road surface. Duals are measured from the center of dual wheels.

Axle Width (AW)

The distance between the front wheels measured from the centerline of the front tires.

Center of Gravity to Axle (CGA)

The distance measured from the center of gravity of the body and payload to the center of the rear axle (midpoint between the axles for a tandem).

Stroke

The distance traveled by a piston in a cylinder during ½ revolution of the crankshaft.

Tractive Effort

The force available at the road surface contacting the driving wheels of the truck. It is determined by engine torque, transmission ratio, axle ratio, tire size, and frictional losses in the drive train. Also known as Rim Pull.

Rim Pull

The force available at the road surface contacting the driving wheels of the truck. It is determined by engine torque, transmission ratio, axle ratio, tire size, and frictional losses in the drive train. Also known as Tractive Effort.

Cowl

The front part of an automotivecab or body, directly below the base of the windshield between the dash panel, is used to indicate the complete vehicle (less body).

Set Back or Forward Front Axle

The front steering axle is normally as close to the front of the vehicle as the design and wheel and tire size permit on conventional and set forward axle (SFA) models. When the front axle is purposely located farther toward the rear it is referred to as being set back or (SBA). The centerline of the front axle to the front bumper is normally 26 to 33-1/4 inches on Conventional and SFA models and 51-3/4 inches for set back front axle models. The purpose of moving the axle rearward is to increase loads applied to the front axle and increase maneuverability. Standard-type front axle setting generally enables more economical cab construction and meets axle spread requirements of states using the Bridge Formula.

Axle, Semi-Floating

The inner shaft of this axle is carried on an extension of the differential; the outer or wheel bearings are carried directly on the axle shaft. The axle shafts and wheel bearings not only support the total rear weight, but also transmit driving torque to the wheels and resist stresses due to skidding, cornering, and tractive forces.

Allowable Payload

The maximum load weight that can be carried without exceeding the truck manufacturer's designated maximum rating, or some component rating or legal limit (e.g. axle capacity, legal axle load limits).

Tire Load Capacity

The maximum recommended load that may be carried by the tires. Altering the size of the tires on a vehicle will have a direct bearing on the load that can be carried.

Gross Torque

The maximum torque developed by an engine without allowing for the power absorbed by the engine's accessory units such as the fan, water pump, generator and exhaust system. Used to determine gross horsepower.

Spring Deflection Rate

The number of pounds necessary to deflect the spring one inch.

Gear Ratio

The number of revolutions a driving gear requires to turn a driven gear through one complete revolution. For a pair of gears, the ratio is found by dividing the number of teeth on the driven gear by the number of teeth on the driving gear.

Net Torque

The torque available at the flywheel of the engine after the power required by the engine accessories (fan, water pump, generator, etc.) has been provided.

Landing Gear

The two small wheels at the forward end of a semi-trailer used to support the trailer when it is detached from the tractor.

CWR (Cargo Weight Rating)

The value specified by the manufacturer as the cargo-carrying capacity, in pounds, of a vehicle, exclusive of the weight of the occupants. The actual cargo weight is also called the payload.

Gross Axle Weight Rating (GAWR)

The value specified by the vehicle manufacturer as the load-carrying capacity of an axle system measured at the tire-ground interfaces.

Compression Ratio

The volume of the combustion chamber and cylinder when the piston is at the bottom of its stroke, divided by the volume of the combustion chamber when the piston is at the top of its stroke. Higher compression ratios tend to increase engine efficiency.

Shipping Weight

The weight of the basic truck including all standard equipment plus grease and oil wherever required. It does not include the weight of fuel or coolant.

Curb (Vehicle) Weight

The weight of the truck (without load or driver), including fuel, coolant, oil, body and all items of standard and optional equipment.

Conventional Cab

This is a cab design where the powerplant is located ahead or mostly ahead of the cowl. Term may be applied to basic cab structure only or may include front fenders, hood, grille, etc.

Semi-Trailers

This is a trailering unit that is supported in the rear by its own suspension system and at the front by the towing vehicle. A separate suspension unit with towing provisions sometimes supports this type of unit, but while being used this way it becomes a full trailer. An exception is the utility-type trailer, house trailer, etc., which is towed by a ball coupling. This is referred to simply as a trailer and is not designed as a semi- or full trailer.

Design Weight

This is the maximum to which a vehicle or component may be loaded without the danger of failure and/or premature wear taking place. It is a limit imposed by the manufacturer of that vehicle or component.

Axle, Rear, Double Reduction

This rear axle has a primary reduction through a hypoid or spiral bevel pinion and ring gear and a secondary reduction through a set of herringbone or helical gears. This rear axle is designed to maintain gear strength and give a more powerful driving force to the rear wheels without sacraficing road clearance and to provide higher numerical ratios than are possible with single reduction axles.

Axle, Rear, Single Reduction

This rear axle has one driving pinion and one ring gear that turns the axle shaft. The driving torque at the rear wheels is increased or decreased according to the ratio of the teeth in the driving pinion to those in the ring gear.

Rolling Radius (Loaded Radius)

Tire-rolling radius is the distance from the center of the wheel to the road. Static radius applies when the vehicle is at rest, rolling radius for a vehicle in motion. The latter dimension is usually slightly greater than the static radius and is the figure used in determining the tire revolutions per mile.

Reduction

Used to indicate the slower output speed resulting from a ratio proportion (faster on reductions less than 1).

Auxiliary Springs

Usually rear only, used for increased load stability or capacity without affecting light ride. Mounted to act only after regular springs are partially deflected.

Truck

Vehicle designed for carrying entire load; GVW rating indicates truck capacity. GCW will also apply if a trailer is to be pulled behind the truck. GVW and GCW ratings are maximum at the ground including vehicle, payload and all equipment. A load capacity chart for each model indicates basic equipment needed for each GVW and GCW.

Tractor (Highway)

Vehicle designed for pulling loads greater than weight actually applied to the vehicle. Most heavy-duty trucks are designed for either tractor or truck service. Optional equipment is available to adapt each unit for the particular tractor or truck application for which it is to be used. GCW rating indicates total pulling capacity of a unit including its own weight when used as a tractor in a specified type of service. GVW rating also must not be exceeded.

Tilt Cab

Vehicle designed with the engine beneath the cab and having provisions for tilting the cab forward on a pivot near the front bumper to provide easy access to the engine.

Off-Road

Vehicle operation over private roads in areas with no maintained hard surface variable grades. Not subject to legal weight and dimensional limitations.

Off-Highway

Vehicle operation over private roads or asphalt or maintained crushed rock surface or similar material, variable grades. Not subject to legal weight and dimensional limitations.

On/Off-Highway

Vehicle operation over secondary roads of good concrete or asphalt construction with partial operation on well-maintained crushed rock surface or similar material, variable grades. Subject to legal weight and dimensional limitations.

On-Highway

Vehicle operation over well-maintained major highways of excellent concrete or asphalt construction, level to rolling terrain with uniform grades. Subject to legal weight and dimensional limitations.

Forward Control

Vehicle with driver controls (pedals, steering wheel instruments) located as far forward as possible. Supplied with or without body, the controls are stationarymounted as opposed to the special mountings of tilt cab models.

Model Weight

Weight of the vehicle with all items of standard equipment, 15pounds per passenger in each designated seating position, and maximum capacity of fuel, oil and coolant.

Payload

Weight or commodity being hauled. This will include the packaging, pallets, banding, etc., but does not include the truck, truck body, etc.

Allowable Body Payload

Weight rating designated by the truck manufacturer for model types that are later equipped with some type of body (e.g. stripped chassis, chassis-cowl, chassis-cab). This is the combined allowance for total weight of body and payload together.

Overall Width (OW)

Width of chassis from the widest point of the cab.