# Glossary of automotive terms

This glossary of automotive terms is a list of definitions of terms and concepts related to automobiles, including their parts, operation, and manufacture, as well as automotive engineering, auto repair, and the automotive industry in general. For more specific terminology regarding the design and classification of various automobile styles, see Glossary of automotive design; for terms related to transportation by road, see Glossary of road transport terms; for competitive auto racing, see Glossary of motorsport terms.
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## A
### accelerator

    A throttle in the form of a foot-operated pedal, or sometimes a hand-operated lever or paddle, by which the flow of fuel to the engine (and thereby the engine speed) is controlled, with depression of the pedal causing the vehicle to accelerate.

### admission stroke

    See induction stroke.

### air brake

    1. A type of brake in which the force that actuates the brake mechanism is provided by compressed air acting on a diaphragm within a brake chamber or servo. Such systems are used primarily in commercial vehicles.
    2. An aerodynamic spoiler designed to retard high-speed vehicles.

### airless injection

    A type of fuel injection which relies on mechanical pressure only; the normal method of injection in a diesel engine.

### alternator

    A small electrical generator that converts mechanical energy into electrical energy in the form of alternating current, typically by using the engine to drive the rotation of a magnet which induces a current in a stationary armature. This electrical energy is then stored in the battery.

### antifreeze

    Any chemical, commonly ethylene glycol, that is added to the cooling water of an engine in order to decrease its freezing point for winter operation, when ambient temperatures may be below the normal freezing point of water.

### articulated vehicle

    Any vehicle consisting of two or more usually separable wheeled units, such as a towing vehicle (e.g. a tractor unit) combined with a towed vehicle (e.g. a trailer or semi-trailer) that is often unpowered and carrying a load. Articulation is primarily in the steering mode, though some degree of horizontal axis articulation is usually also necessary to enable the combined tractor-trailer to negotiate road surface irregularities.

### axle

    A horizontal transverse shaft or beam with spindles on which road wheels are mounted. There are many different types of axles, typically distinguished by their placement relative to the vehicle's center of mass and their intended use. Vehicles are often classified by the number of axles they possess.

### axle ratio

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## B
### back-fire

    An explosion of unburned or partially burned fuel that occurs in an exhaust or inlet system, rather than in the combustion chamber as intended.

### ball and socket

    A type of mechanical joint in which a spherical end (the ball) moves freely within a recessed cavity of the same shape (the socket), commonly used in suspension and steering linkages.

### brake

    Any device designed to apply friction to a moving or moveable surface in order to slow it, stop it, or prevent it from moving, especially one that slows, stops, or prevents the rotation of one or more of the wheels of an automobile. Most wheeled vehicles are built with at least one brake for each wheel, which can be applied either independently of each other or, in four-wheeled vehicles, all at the same time, by depressing one or more foot-operated brake pedals or hand-operated levers. If the vehicle is already in motion, the act of applying the wheel brakes, known as braking, will ultimately cause the entire vehicle to slow or stop as the friction generated by the contact between the tires and the ground overcomes the vehicle's inertia. If the vehicle is stationary, the same brakes or a separate brake may also be applied to prevent the wheels from rotating while the vehicle is parked. There are many types of brakes which operate on a wide variety of physical principles. Modern automotive braking systems rely on a large and complex network of precisely controlled mechanisms in order to deliver optimal braking pressure for specific road conditions, to prevent brakes from locking and overheating, to reduce the amount of input required from the driver, and ultimately to maximize safety, efficiency, and durability.

### brake dive

    See dive.

### brake puck

    See brake pad.

### breeches pipe

    See Y pipe.

### bumper

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## C
### caliper

    In a disc brake system, the mechanism that brings the brake pads to bear on disc by a clamping or pinching action.

### camber angle

    The angle between the plane defined by the circumference of the wheels and the vertical axis of the body or chassis of a vehicle, as viewed from the front or rear of the vehicle. If the top of the wheel is tilted outward, away from the body, the wheel is said to have positive camber; if the top is tilted inward, towards the body, it is said to have negative camber. Angled wheels are sometimes an intentional part of a vehicle's design, but most ordinary road vehicles are intended to have minimal or neutral camber (i.e. the wheels are completely parallel to the body and perpendicular to the road surface).

### caster angle

    The angular displacement of the steering axis from the vertical axis of a steered wheel, as seen from the side of the vehicle.

### chassis

    The structural lower part of a vehicle to which the running gear and body are attached, or more generally the main load-bearing framework which supports all of a vehicle's mechanical parts and other components and on which the body is mounted. Compare rolling chassis.

### choke

    A valve that restricts the amount of air entering an engine on the induction stroke, thereby enriching the fuel-to-air ratio for ease of starting and running, especially when the engine is cold.

### clearance volume

    The volume remaining in a cylinder above the piston when it reaches top dead center. See also swept volume.

### clocking

    Illegally tampering with a vehicle's odometer reading.

### combustion chamber

    The part of an internal combustion engine in which combustion occurs; in particular the space within a cylinder bounded by the top of the piston when at top dead center and the cylinder head in a reciprocating engine. Since most of the fuel/air mixture's combustion takes place in this space, its volume, shape, and design greatly affect the power, fuel efficiency, and emissions of the engine.

### compression ratio

    The ratio between the maximum and minimum combined volume of an internal combustion engine's cylinder and combustion chamber.

### coolant

    Any substance, usually a liquid, that is circulated around a vehicle's engine in order to reduce and regulate temperature by absorbing excess heat and then transferring it to another medium (e.g. the surrounding air) where it is safely and efficiently dissipated. An ideal automotive coolant has high thermal capacity, low viscosity, and is chemically inert and non-toxic; though specific properties can vary widely by intended usage, most ordinary personal vehicles use water as the base coolant with additives such as ethylene glycol or methanol to lower its freezing point, raise its boiling point, and prevent corrosion.

### crankcase

    The housing that surrounds the crankshaft.

### crankshaft

    A linear shaft that translates reciprocating motion created by the pistons of an engine into rotational motion via a series of "crank throws" or "crankpins" – additional bearing surfaces which are slightly offset from that of the shaft and which are attached to the connecting rods of the pistons. The crankshaft is often attached to a flywheel and vibrational dampers to reduce the pulsation characteristic of four-stroke cycles and other undesirable movement.

### crankshaft journal

    The part of the crankshaft that rotates in a main bearing, with connecting rods attached to the engine's pistons.

### crankshaft throw

    The lateral distance between the centerline of the crankshaft and the furthest offset of a big end journal, equivalent to half the length of a piston stroke; i.e. the distance by which the journals are offset from the main shaft.

### curb weight

    See kerb weight.
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## D
### differential

    A system of gears capable of dividing the input torque of one shaft between two output shafts where the two shafts are likely to rotate at different speeds, as in cornering. The differential is used as the final drive of vehicles with two or more driven wheels.

### dipstick

    See oil level indicator.

### dispersant

    A motor oil additive which separates and holds solid or liquid contaminants in suspension, preventing their settling or clumping and thereby reducing sludge deposition in the engine.

### displacement

    The total volume swept by all of the pistons in all of the cylinders of an internal combustion engine, excluding the volume of the combustion chambers. Engine displacement is commonly used as a measure of an engine's size, and by extension as an indicator of the power the engine is hypothetically capable of producing and the amount of fuel it can be expected to consume.

### displacement factor

    An index of vehicle performance usually expressed as the product of the engine displacement and axle ratio divided by the product of the drive wheel rolling ratio and the vehicle's gross weight.

### dive

    The dipping of a vehicle's front end that occurs when the brakes are applied. Dive is caused by a load transfer from the rear to the front suspension, as the inertial force of forward motion passes through the vehicle's center of gravity, which is higher than the points where the braking forces are exerted on the ground.

### donut

    See spare tire.

### drive axle

    The axle or axles used to transmit torque from the drivetrain, imparted via a differential, to one or more drive wheels; a driven axle.

### drive shaft

    The shaft or shafts by which mechanical power and torque is transmitted from the rest of the drivetrain to a differential (and hence to a driven axle and driven wheels).

### drive wheel

    The wheel or wheels used to transform torque from the powertrain into tractive force, and to transmit that force from the tires to the road, overcoming stationary forces and causing the vehicle to move forwards or backwards; i.e. the wheels that are driven, as opposed to merely rolling under the momentum imparted to them. Vehicles may have any combination of driven wheels; power applied to the two front wheels or two back wheels is two-wheel drive (2WD), and power applied to all four wheels is four-wheel drive (4WD). Which wheels are driven is variable in all-wheel drive (AWD).

### drivebox

    See gearbox and transmission.

### drivetrain

    All of the components of a motor vehicle that are involved in delivering power to the drive wheels, excluding the engine or motor that generates the power. The drivetrain typically includes the clutch (if present), transmission, drive shaft, differential, and one or more axles. Contrast powertrain.

### drop arm

    See steering arm.

### dynamometer
## E
### econometer

    An instrument that measures and displays instantaneous fuel consumption or fuel efficiency.

### economizer

    Any device or system designed to reduce fuel consumption by preventing unnecessary energy expenditure or by repurposing energy that would otherwise be wasted to perform useful functions. The term commonly refers specifically to a system that automatically modifies the ratio of the air/fuel mixture supplied to the engine in order to save fuel when cruising at constant speed.

### engine block

    The structure, typically cast in one piece, which contains and houses the cylinders, pistons, and other components of an internal combustion engine. Modern engine blocks usually also have an integrated crankcase, as well as coolant passages and oil galleries.

### exhaust system

    The assembly of parts that conveys the gaseous products of the engine's combustion reactions (collectively known as exhaust) out of the combustion chambers, away from the engine, and ultimately into the atmosphere. In the most basic design, exhaust gases flow from each of the cylinder heads into an exhaust manifold and then through one or more exhaust pipes, which expel their contents behind, above, or to the side of the vehicle. In modern systems, the exhaust may also flow through a turbocharger, various emissions control devices (e.g. a catalytic converter), and/or a muffler and resonator (to reduce noise and vibration) before exiting through one or more tailpipes.
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## F
### fan belt

    The endless belt that transmits rotational motion from the engine to the cooling fan.

### fender

    1. The fixed part of a vehicle body exterior that frames the wheel well, with the primary purpose of blocking sand, mud, rocks, liquids, and other road spray that is thrown into the air by the rotating tire; or any deflector plate or structure mounted at the front or rear of a vehicle near ground level.
    2. A fixed side-panel that partially shrouds a tire from view and similarly serves to deflect road spray.

### firing order

    The numbered sequence in which the cylinders of a multi-cylinder internal combustion engine are ignited.

### footprint

    The shape of the contact interface of a loaded tire with the ground. See also contact length.

### footwell

    The lower interior part of the scuttle, which accommodates the feet of the driver and the front passenger, as well as the foot pedals and often other control features on the driver's side.

### four-wheel drive (4WD)

    A drivetrain system in which torque generated by the engine is delivered simultaneously to all four wheels of a four-wheeled vehicle. Thus all four wheels are drive wheels, as opposed to two-wheel drive, in which only one pair of wheels receives power at a time (either the front or rear wheels). Four-wheel drive can provide greater traction when road conditions are suboptimal, but typically has negative effects on fuel efficiency and handling at speed. It may be either permanent or selectable on-demand; in the latter case it is often referred to as all-wheel drive.

### friction clutch

    A type of clutch in which torque is transmitted by pressure of the clutch faces on each other.

### front-wheel drive (FWD)

    A two-wheel drive drivetrain system in which torque generated by the engine is delivered only to the two wheels at the forward end of the vehicle, as opposed to rear-wheel drive and distinct from four-wheel drive.
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## G
### gas pedal

    See accelerator.

### glove compartment

    A compartment built into the dashboard of an automobile above the front-seat passenger's footwell, often used to store miscellaneous items such as small tools and paperwork and, traditionally, driving gloves. The compartment usually closes with a latch and in some vehicles can be locked with a key.

### governor

    A device which limits the maximum rotational speed of an internal combustion engine, usually by controlling the fuel supply, in order to prevent mechanical damage, overheating, or unacceptable levels of exhaust emissions. They are used primarily in diesel engines.

### gradeability

    A measure of the ability of a vehicle to ascend an incline of a specified grade.

### grille

    A grid or grating covering an opening in the body of a vehicle and allowing air to pass through it, especially an ornamental one at the front end of a vehicle designed to ventilate the radiator and engine compartment.

### gross axle weight

    The specified maximum carrying capacity of an axle, as measured at the tire–road interface.

### gross power

    The total measured power output, typically expressed in watts or horsepower, of an intact, unmodified engine operating in optimal conditions without the burden of any power-absorbing auxiliaries such as electric generators, pumps, or silencers.

### gross train weight

    The total combined weight of the tractor and trailer of a commercial vehicle.

### gross vehicle weight (GVW)

    The maximum total weight at which a vehicle can be legally operated, including the kerb weight plus the weight of any payload.
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## H
### handbrake

    Any brake operated manually by a hand lever, but especially a parking brake.

### hidden headlamp

    A style of headlamp on some vehicles which allows the headlamps to be retracted and concealed when they are not in use.

### hood

    The hinged cover over the engine compartment of a motor vehicle. Hoods can be opened to allow access to the engine and most other power and accessory components for maintenance and repair.

### hub

    The central portion of a wheel, by which the wheel is fastened to the vehicle's chassis.

### hubcap

    A round plastic or metal disk which covers all or part of the exterior face of a wheel, particularly the central hub, in order to protect the fasteners which attach the wheel to the vehicle's chassis from dirt and moisture. Hubcaps are often decorative.

### hybrid vehicle

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## I
### idle port

    An opening in the throttle body through which the carburetor dispenses fuel while the engine is idling, i.e. while the throttle is otherwise fully closed.

### idle speed

    The rotational speed of an unloaded engine at the minimum throttle setting.

### idling

    Running a vehicle's engine while the vehicle is not in motion, without any loads except the engine accessories, e.g. while waiting at a traffic light.

### inline engine

    An internal combustion engine layout in which all cylinders are aligned in one row without any offset, i.e. in a single plane of the crankshaft axis. Inline layouts tend to be simpler in design and easier to build than flat or V-shaped layouts, smaller in physical dimension, and more broadly compatible with various engine designs because they can be mounted in any direction.

### internal combustion engine

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## J
### jump lead

    One of a pair of heavy-duty electrical leads by which a vehicle with a discharged battery may be connected to an external source of power, such as a charged battery in another vehicle, in order to recharge the dead battery, a process known as a jump start.

### jump start

    A procedure of starting the engine of a vehicle with a discharged or depleted battery by using jump leads from a charged battery or other external power source.

## K
### kerb weight

    The total mass of an unladen vehicle including standard equipment and all consumables necessary for operation, including water, motor oil, transmission fluid, brake fluid, coolant, etc., and (sometimes) a full tank of fuel, but excluding passengers and cargo. The precise definition varies by location. Compare gross vehicle weight.

### kickdown

    A system that enables a driver to select a lower gear than the one automatically engaged by an automatic transmission, e.g. when accelerating rapidly, usually by depressing the accelerator pedal fully.

### kingpin

    1. The vertical or inclined shaft about which a steered wheel assembly pivots.
    2. The main fastening member between a fifth-wheel and semi-trailer.

### knock rating

    See octane rating.

### knockout axle

    A type of detachable axle, usually on a low-loader, the removal of which allows further lowering of the loading bed.

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### main drive belt

    See serpentine belt.

### make

    The manufacturer of a particular vehicle.

### model

    The specific style, size, and/or type of a particular vehicle produced by a particular manufacturer; the set of features and design elements which unify a particular class of vehicles produced by a manufacturer. See also marque.

### motor oil

    Any of various viscous liquids used to lubricate the mechanical components of an internal combustion engine, with the primary purposes of reducing friction and wear on moving parts, cleaning and preventing the accumulation of sludge and varnish, improving the static seal created by gaskets and piston rings, and cooling the engine by carrying heat away from the moving parts. Modern commercial motor oils typically consist of a mixture of base oils produced from crude petroleum and enhanced with various additives, including detergents, dispersants, and other chemical protectants, which is then circulated under pressure through oil galleries in the engine block.

### multivalve engine

    An engine having more than the conventional one intake valve and one exhaust valve in each cylinder, a design intended to increase the flow of fuel-air mixture into the cylinders and of exhaust gases out of them.
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## N
### nave

    1. The hollow or dished center part of a road wheel on which the rim is mounted.
    2. The face of a road wheel that is bolted to the hub.

### nave cap

    See hub cap.

### neutral (N)

    A gear selector position in which no gear is engaged, such that there is no transmission of torque between the input and output shafts of the gearbox. The neutral position effectively disconnects the driveline between the engine and the wheels. Power may still be generated by the engine, but it cannot be delivered to the wheels; unless braking prevents it, the wheels of a vehicle in neutral are still capable of rotating in either direction under the momentum imparted by any external force.

### north-south

    Describing an engine layout in which the cylinders are arranged along the vehicle's longitudinal axis, from front to rear, as opposed to an east-west layout.

### noseweight
## O
### off-road vehicle

    Any vehicle capable of or specifically designed for traveling on unpaved, uneven, and/or low-traction surfaces such as gravel, sand, mud, snow, rocks, or other natural terrain, especially a vehicle having four-wheel drive, long suspension, high ground clearance, and specialized tires such that it performs well on these surface types, with lesser risk of being damaged or immobilized than an ordinary street vehicle.

### oil filter

    A filter designed to trap and remove particulate contaminants from engine oil, transmission oil, hydraulic oil, or any other lubrication system.

### oil gallery

    Any space or passage within an engine block, crankcase, or cylinder head through which oil is intended to flow under pressure.

### oil gauge

    An instrument that measures and displays the instantaneous pressure and/or temperature of the oil circulating within a lubrication system, especially engine oil.

### oil level indicator

    A long, flexible metal rod with graduated markings that is manually inserted into and then removed from the motor oil reservoir in order to give an approximate measure of the amount of oil in the reservoir.

### oil pump

    An electrically powered device which pressurizes and distributes oil to all parts of a lubrication system. When not otherwise qualified the term usually refers to the engine oil pump.

### overspeed governor

    See governor.

### oversteer

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## P
### parking brake

    A mechanism used to keep a vehicle securely motionless when parked, generally consisting of a cable connecting two conventional wheel brakes (usually on the rear wheels) to a pulling mechanism in the vehicle's cab, typically either a hand-operated lever or handle located near the steering column or a foot-operated pedal near the other pedals.

### platform

    A set of common design, engineering, and production efforts, as well as major automotive components, shared between a number of outwardly distinct vehicle models and even different vehicle classes which are often manufactured by different though usually related marques. Auto manufacturers commonly introduce new models based on existing platforms in order to reduce the costs associated with the development of entirely new products.

### play

    Free movement or looseness within a mechanical system, especially when not intended by the mechanism's design, e.g. any motion of a steering wheel before the steered wheels respond by turning.

### power stroke

    The stroke of a reciprocating engine during which the piston is forced outward by the immense pressure generated by the combustion of fuel in the combustion chamber, typically considered to include that part of the cycle during which the piston moves from top dead center to bottom dead center.

### powertrain control module (PCM)

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## R
### rack and pinion

    A type of linear actuator comprising a circular gear (the pinion) engaging a linear gear (the rack), which operate together to translate rotational motion into linear motion or vice versa: rotating the pinion causes the rack to be driven linearly, and driving the rack linearly causes the pinion to rotate. Rack-and-pinion mechanisms have been employed in many different automotive systems and components, as in rack-and-pinion steering.

### radiator

    A heat exchanger that serves to cool the engine block by pumping spent coolant (previously heated by the engine) through a system of tubes which are then exposed to relatively cool air from the vehicle's exterior, often by way of a fan blowing air over them, thereby transferring excess thermal energy from the engine to the air.

### radiator fan

    One or more sets of fan blades situated behind the grille which serve to move air from the exterior around the tubes of the radiator, thereby dissipating heat from the engine block.

### rear-wheel drive (RWD)

    A two-wheel drive drivetrain system in which torque generated by the engine is delivered only to the two wheels at the rear end of the vehicle, as opposed to front-wheel drive and distinct from four-wheel drive.

### redline

    1. The maximum rotational speed, in revolutions per minute (rpm), at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or to other parts of the engine assembly. The redline of a particular engine depends on various factors such as stroke, displacement, and the mass, composition, and balance of the engine components. The term is derived from the red-colored line or colored sector of a tachometer display which indicates the maximum allowable sustained engine speed.
    2. To operate a vehicle above its engine's redline. Straying above the redline usually does not mean instant engine failure, but may increase the chances of damaging the engine.

### roadworthy

    Describing a vehicle which is in suitable operating condition to be safely used on public roads, in which all parts are mechanically sound such that no part contravenes accepted standards for safe driving or transport of people or cargo or any other road transport regulation. The term often implies that the vehicle is street-legal.

### rolling ratio

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## S
### solid injection

    See airless injection.

### spark ignition

    A system that uses a high-voltage electrical arc or "spark" to ignite a fuel/air mixture.

### spark plug

    An insulated plug that supports the electrodes between which the electrical arc or "spark" passes to initiate ignition of a spark-ignition engine.

### speedometer

    A gauge that measures and displays the instantaneous speed of a vehicle. Universal in modern automobiles and typically mounted on the dashboard behind the steering wheel.

### stick shift

    See manual transmission.

### straight engine

    See inline engine.

### strangler

    See choke.

### suspension

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## T
### tachometer

    An instrument which measures the rotational speed of a shaft or disk, as of an engine, and/or the analogue dial or digital device which displays this measured speed, usually in revolutions per minute (RPM).

### tailpipe

    The rearmost pipe of an exhaust system, downstream of the rear muffler, by which most or all of the vehicle's exhaust gases exit the system. The term is also sometimes used to refer to intermediate pipes. Large vehicles may have more than one tailpipe.

### telematics

    The range of electronic signals, displays, controls, and information provided to drivers so as to make driving easier and safer, whether inside the vehicle, on other vehicles, or on the roadside; or the interdisciplinary field that studies the implementation of these technologies. Examples include turn signals, brake lights, hazard lights, digital instrument panels, and any of the wide variety of features offered by onboard computer systems, such as turn-by-turn navigation and proximity sensors.

### tire rotation

    The practice of periodically moving the wheels and tires of an automobile from one position to another on the same vehicle in order to ensure that all of the vehicle's tires receive approximately the same amount of wear over time; i.e. that no particular tire wears out more quickly than any other. This is a consequence of the fact that a tire's lifespan is often strongly biased by its particular position on the vehicle (e.g. front or rear, driver's side or passenger side) because the masses supported by and the forces applied to different parts of the vehicle are often uneven; for example, in front-engine vehicles, the front axle typically supports a much greater load than the rear axle, meaning the tires on its wheels experience different forces that may cause their tread to wear out faster than the tread on the rear tires.

### transverse engine

    An engine mounted inside a vehicle so that the axis of the crankshaft is perpendicular to the direction of travel. This configuration is common in front-wheel drive passenger vehicles. Contrast longitudinal engine.

### trunk

    The primary storage or cargo compartment inside an automobile, especially a large interior space at the rear of the vehicle that is accessible from the exterior by one or more dedicated doors or hatches.

### two-wheel drive (2WD)

    A drivetrain system in which torque generated by the engine is delivered to only one pair of wheels at a time, either the two front or two rear wheels. Thus there are exactly two driven wheels, while the other two wheels merely roll under the momentum imparted to them and do not create tractive force of their own. This is in contrast to four-wheel drive (4WD), in which all four wheels receive power simultaneously. Some vehicles are only capable of two-wheel drive such that it is permanent, while in others it is a selectable mode, with the vehicle capable of switching between two-wheel drive and four-wheel drive on-demand.
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## U
### undercarriage

    The wheels, axles, and suspension system of a vehicle, particularly a commercial vehicle; more generally, all of the parts of an automobile accessible from the underside of the vehicle.

### unsprung mass
## V
### vehicle identification number (VIN)
## W
### wheel well

    See fender.

### wheelbase

    The longitudinal distance between the front and rear wheel axes of a vehicle.

### wheelfight

    A rotary disturbance occurring in the steering wheel that is produced by forces acting on the steered wheels.

### windshield

    The front window of an automobile, providing visibility in the forward direction of travel while protecting the occupants from the elements. Modern windshields are usually made of laminated safety glass, consisting of two curved sheets of glass with a plastic layer laminated between them, which is bonded to the window frame.

### wing

    British English for fender.

### wiring harness

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## Y
### Y pipe

    A two-branch exhaust manifold connecting the exhausts of a V engine to form a single exhaust.
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