A A

A-arm A lateral suspension locating link in theshape of the letter A. The two legs attach to thechassis by pivots and rubber bushings that allowthe outboard top of the A (connected to thewheel assembly) to pivot up and down.Sometimes the A-arm is made up of two sepa-rate links. The system is widely considered togive excellent wheel-locating geometry, and it isused for the rear suspension of most racing carsand many high-performance road cars. Alsocalled wishbone. Also see unequal-length A-arms.

ABS See antilock braking system.

acceleration The rate of change of velocityin relation to time; measured in ft/sec/sec orft/sec2.

accelerator A foot pedal linked to the throt-tle valve, controlling the flow of air and there-fore, by means of the carburetor or fuel injectionsystem, also the amount of fuel admitted to theengine. Also called the throttle.

A-arm

accelerator pump A carburetor device thatenriches the fuel mixture for acceleration.

accelerometer A mechanical or electricalinstrument for measuring and recording accel-eration.

access panel See hood.

accumulator In a hydraulically assisted brakesystem, a reservoir filled with hydraulic fluidand kept under pressure during engine opera-tion; in case of engine failure, this pressure pro-vides reserve braking power that can stop the carseveral times with something approaching thedegree of assistance available with the enginerunning.

Ackermann steering Steering geometry thatallows the outside wheel to turn fewer degreesthan the inside wheel to compensate for thelarger circle tracked by the outside wheel. Therelationship between the angles of the inner andouter wheels is not fixed; the tighter the turn, thegreater the disparity between the wheels’ desiredturning angles.

2

accelerator pump

20o

30o

accumulator

Ackermann steering

3

active suspension

Steering geometry that does not give the idealrelationship in all types of turns results in tirescrub. Ackermann steering remains perfectlyaccurate only at very low speeds, before the non-linear characteristics of the tires become a fac-tor. However, Ackermann steering minimizesscrub in the vast majority of conditions encoun-tered by production cars, though it is a designcompromise that corrects for normal drivingconditions rather than for all possible turns.

active-ride suspension See active suspension.

active safety Aspects of a car’s engineeringthat affect its accident-avoidance capabilities,especially those under driver control. Includessuch factors as steering, brakes, handling, andtires — in sum, a car’s total roadholding and eva-sive capability. Also see passive safety.

active suspension A suspension system witha double-acting hydraulic actuator at each wheel,driven by a variable-rate hydraulic pump andcontrolled by an electronic control unit (ECU).

The ECU monitors forward speed, body atti-tude, lateral-versus-longitudinal velocity whencornering, wheel/hub velocity, angular dis-placement, and load. It also controls an onboardhydraulic power source that supplies the inter-nal pressure necessary to counteract the externalforces and optimize the vehicle’s body position.The main advantage of active suspension is supe-rior body and suspension control under all con-ditions, including near roll-free cornering behav-ior and a virtual absence of bounce and pitch.An active suspension is truly an intelligent sus-pension that senses outside forces and generatesits own internal forces to counter them, therebycontrolling body position; conventional sus-pensions can only react to the forces influenc-ing them, and are thus passive systems.Generally, an active suspension will still haveconventional springs to support the car’s staticweight when the car is at rest and the engine isoff. Also known as active-ride suspension or full-active suspension.

A

active suspension

Modern electronic engine controls with theability to “remember” major changes in envi-ronmental and operating conditions and adjustto them. An example is the adaptive feedbackidle-speed control of certain engine-manage-ment systems, which shifts its adjustment rangeduring the break-in period of a new engine andtherefore needs no further attention for the car’slife span, barring malfunctions.

adhesion The ability of a tire to remain incontact with a road surface without loss of trac-tion. Also, the ability of an oil to remain in con-tact with a metal surface.

adiabatic engine An engine in which theheat of combustion remains within the com-bustion chamber and cylinder where it can beconverted into power, making the engine moreefficient and allowing it to operate without acooling system.

adjustable shock A shock absorber whosejounce and rebound characteristics can be stiff-ened or softened, either manually or electroni-cally, to compensate for wear or to fine-tune asuspension for rough roads, heavy loads, cor-nering, or racing. Manual adjustments are madeon the shock absorber; some contemporary carshave shock absorbers that can be adjustedelectromechanically via a switch inside the car.

advance To adjust the timing of a camshaftor distributor spark or valve operation, so thata valve opens or a spark plug fires earlier in theengine’s cycle; also, the mechanism for doingthis. The opposite of retard.

aerodynamic drag The resistance of air tothe forward movement of a body such as a car.It has three sources:

Drag resistance, a function of the body’sshape. Protruding objects such as mirrors, muf-flers, and license plates can increase drag resis-tance considerably at higher speeds. Of specialimportance is the shape of the rear part of thebody, which determines the amount of turbu-lence in the vehicle’s wake.

Air friction on the body’s external surfaces(skin). For the more or less standardized surfacefinish of modern passenger cars, this amounts toabout 10 percent of total air resistance.

Airflow through the car for cooling or venti-lation. This can be resistance-increasing or -

4

adhesion

adjustable shock

advance

5

airbag

decreasing, depending on the function, loca-tion, and aerodynamic design of the air channelsor orifices.

Aerodynamic drag, or air resistance, increasesas the square of vehicle velocity; the powerneeded to overcome it varies as the cube of vehi-cle velocity. Also see coefficient of aerodynamicdrag and coefficient of aerodynamic lift.

aggressive The French now describe a car asaggressive, or refer to its aggressivity, if its struc-tural passive safety characteristics are such thatit imposes excessive injury to a pedestrian, to anoccupant of another vehicle, or to another vehi-cle in an accident. For example, the shape of thefront end and the height of the bumper of anaggressive car would tend to throw a pedestrianunder the car rather than up and onto its hood.Similarly, the front end structure would be capa-ble of penetrating another vehicle upon impactrather than crushing progressively. Also see crushzone and passenger cell.

aggressivity See aggressive.

airbag A large deflated pillow or balloon con-cealed in the steering-wheel hub, instrumentpanel, or dash, designed for rapid inflation in afrontal impact to cushion and restrain front-seat occupants.

One or more deceleration sensors triggerinflation within milliseconds, typically byburning a solid chemical that converts to a gas

A

aerodynamic drag

airbag

that inflates the bag; the bag then deflates, alsovery quickly.

Airbags are an example of a passive restraintmandated for fixed percentages of cars sold inthe United States by all manufacturers beginningwith model year 1987. Also see SupplementalRestraint System.

air bleed passage See compensating jet.

air capacity See breathing capacity.

air cleaner A device mounted on an engine’sintake system that contains a wire mesh or paperfilter to trap dust and dirt, preventing themfrom being drawn into the engine. Also calledair filter.

air-cooled engine An engine cooled by thepassage of air around external cylinder fins ratherthan by passage of a liquid coolant throughinternal water jackets.

air dam, front and rear An aerodynamicdevice that reduces the amount of air flowingunder, and directs airflow around, a car, thusreducing its aerodynamic drag and lift

Originally devised for racing, air dams havebecome a popular accessory and de rigeur fornewer high-performance production models.Air dams are typically made of aluminum, fiber-glass, or flexible plastic, the last being most prac-tical for road use given their proximity to theground and consequent vulnerability to scrap-

ing and bending. They’re generally distinguishedfrom front spoilers by being somewhat deeper.A recent development is the rear air dam, some-times referred to as a skirt and generally seen withextensions of a car’s rocker panels. Also seeground effect.

air filter See air cleaner.

airflow sensor The device in a fuel injectionsystem that measures the flow of air through theintake manifold to determine the amount of fuelto be delivered; also called mass airflow sensor.

Bosch’s mechanical K-Jetronic fuel injectionsystem employs a mechanical sensor consistingof a plate (“flap”) in the airstream, attached to alever arm that in turn moves a plunger in the fueldistributor. The force of air on the plate movesthe lever up and down, causing the plunger toopen and close holes in the fuel distributor. TheL-Jetronic system also uses a flap in theairstream; its movement creates voltage changesin a potentiometer (variable resistor) attached toit. These changes are relayed to the system’s elec-tronic control unit and, in turn, to the fuel dis-tributor. Both these systems measure the volumeof airflow.

In the Bosch LH-Jetronic system, the sensoris an electrically heated platinum wire. Air pass-ing over the wire, which is part of a bridge cir-

6

air bleed passage

air dam (front)

airflow sensor

7

air pollution

cuit, tends to cool it, thus altering its electricalresistance. An electronic amplifier detects theseminute changes in resistance and regulates cur-rent to keep the wire at its original temperature.The current required is thus a measure of airmass, an even more fundamental indicator ofhow much fuel to supply than the volume mea-sured by K- and L-Jetronic systems. Thus theheated wire is called an air-mass sensor.

air foil A device used to improve traction byincreasing the aerodynamic downforce on eitherend of a car. In cross section, an airfoil is basi-cally an inverted wing. Thus, instead of provid-ing lift, as on an airplane, it causes air to pushthe car closer to the ground. The use of airfoilsincreases cornering capability and improves sta-bility at speed, but at the expense of additionalaerodynamic drag. Also called wings. Also seedownforce and ground effect.

air-fuel mixture The blend of air and fuelsupplied to an engine by the carburetor or fuelinjection system.

air-fuel ratio The ratio of the mass of air tothe mass of fuel supplied to an engine. The sto-ichiometric, or chemically correct, air-fuel ratiois the exact ratio necessary to burn all carbon andhydrogen in the fuel and produce carbon diox-ide and water with no oxygen remaining.

air injection system An emission controlsystem that injects fresh air into the exhaustports or a thermal reactor for conversion of car-bon monoxide into carbon dioxide and for com-bustion of unburned hydrocarbons in theexhaust gases. Also called pulsed air injection.

air lock See air pocket.

air-mass sensor See airflow sensor.

air pocket A quantity of air that prevents thenormal flow of a liquid. Air pockets sometimeoccur in brake and clutch hydraulic lines, oillines, or in the cooling system. Also see bleed.

air pollution Unwanted particles, mist, orgases put into the atmosphere primarily as aresult of motor vehicle exhaust or the operation

A

air injection system

of industrial facilities. Also see exhaust emissioncontrols and exhaust emissions.

air pump The device that supplies the freshair needed by an air injection system.

air resistance See aerodynamic drag.

air scoop An opening in a body panel usedto duct outside air for purposes of ventilation orcooling, generally to the engine, brakes, radia-tor, or an oil cooler. On the hood (or the roofor rear deck of a rear- or mid-engine car) an airscoop can be used to force ambient air into theintake system. Also called scoop.

air suspension A suspension system using atype of airbag rather than metal springs to sup-port a car and control its ride motions. Air sus-pension can result in excellent riding comfortover a wide range of vehicle loading because anair spring’s natural frequency of vibration doesnot vary with load as does that of a metal spring.Air springs can be made very soft for a lightlyloaded condition and their pressure automati-cally increased to match any increase in load,thus maintaining constant spring vibration char-acteristics for any load. Also called air springing.

air-to-air intercooler See intercooler.

air-to-water intercooler See intercooler.

air-valve carburetor A carburetor in whicha spring- or weight-closed air valve opens inresponse to engine demand. Through suitablelinkage, it varies the fuel opening to give thedesired mixture ratio throughout the range ofoperation. SU, Stromberg CD, and Keihin CVcarburetors operate on this principle. Alsoreferred to as constant-depression, constant-vac-uum, and variable-venturi carburetor.

alignment Generally refers to wheel align-ment, the proper adjustment of a car’s front orrear suspension for camber, toe, caster, and rideheight.

all-indirect gearbox A manual transmissionin which none of the forward gears has a direct(1:1) ratio.

alloy A metal composed of two or more ele-ments; one or more elements is added to a puremetal to alter properties such as strength andelongation.

alloy wheel A generic term used to describeany lightweight road wheel. The usual alloys arealuminum or magnesium, the latter materialhaving led to the common usage of the term magwheel, often referring to any nonsteel wheel.Alloy wheels can be cast (formed by pouring liq-uid metal into a mold) or forged (formed by

8

air pump

air pump

air scoop

9

alternator

heating the metal and then hammering or press-ing it into a specific shape). Forged wheels aregenerally stronger than cast alloy wheels.

all-wheel drive (AWD) A drive system inwhich the transmission is connected by the dri-ving axle(s) to both the front and the rear wheels.For off-road vehicles it is usually termed four-wheel drive (4WD). Also see four-wheel drive.

all-wheel steering (AWS) See four-wheelsteering.

alphanumeric rating system For tires; dat-ing from 1968, this system is based on the load-carrying capacity of a tire rather than on a directmeasurement of the section width. The capac-ity and size of the tire are indicated by lettersranging from A through N, with N represent-ing the largest tire with the highest load-carry-ing capacity. A typical alphanumeric tire size isBR78-13, where B is the load/size relationshipand R represents radial construction. If the R ismissing, the tire is of bias construction. Thenumber 78 is the aspect ratio and 13 is thewheel diameter in inches.

alternating current (AC) Electrical currentthat reverses its flow in a circuit at regular inter-vals. The reversal typically occurs between 60and 120 times per second, expressed as cycles persecond (CPS). Compare with direct current. Alsosee alternator.

alternator The part of a car’s electrical systemthat converts mechanical energy from a drive

A

alloy wheels

all wheel drive

belt into electrical energy to operate the ignitionand electrical accessories and charge the battery.An alternator generates alternating current (AC)and then transforms it into the direct current(DC) used by automotive electrical systems.The alternator has replaced earlier DC genera-tors, which are less efficient.

aluminum A relatively soft silvery metal usedwhere lightness is required. In cars, aluminumis always found as an alloy because in its pureform it lacks strength and rigidity and is diffi-cult to machine. Aluminum alloys can be cast,

formed into sheets, or forged. Uses include bodypanels, wheels, engine blocks, radiators, trans-mission and differential housings, suspensionmembers, and chassis.

ammeter An instrument that measures theamount, or number of amperes, of current flow-ing in an electrical circuit.

ampere A unit of electric current. The num-ber of amperes is equal to the voltage divided bythe resistance (in ohms) of the electrical circuit.

analog instrumentation Gauges that use asymbolic representation, typically a pointermoving along or around a scale, to display infor-mation such as engine speed, miles per hour, oilpressure, voltage, fuel supply, or time (the“hands-and-face” clock); contrasts with digitalinstrumentation, in which information isexpressed by numerals (digits). Traditionally,analog-gauge pointers operated by purelymechanical means such as a cable drive, butthese have increasingly given way to electronicoperation. Even newer are electronic graphicdisplays, where small liquid crystal or vacuum-fluorescent elements form a band that performsthe same function as a pointer.

annular gear A ring gear. See ring and pinion.

10

aluminum

analog instrumentation

alternator

11

anti-lift

anode The positively charged electrode in anelectrolytic cell. An automotive battery consistsof electrolytic cells. Also see electrolyte.

anodize To coat or plate a metal (typicallyaluminum) with a protective material by elec-trolytic action.

anti-backfire valve A part of the air injectionsystem that diverts air from the air pump awayfrom the exhaust ports for a brief time when theaccelerator is first released during deceleration.This prevents fuel-rich unburned exhaust gasesfrom mixing explosively with fresh air and caus-ing a backfire.

anti-dive The characteristics of a car’s sus-pension intended to resist unwanted motionduring braking. To achieve anti-dive reactions ina front suspension featuring unequal-length A-arms, the upper arm is angled upward towardthe front and the lower arm is angled downward.The angling produces lift components in reac-tion to brake torque that “hold up” the frontend. Also see dive, lift, and anti-lift.

antifreeze Any of several substances (com-monly liquids and typically ethylene glycol)mixed with water and added to a car’s coolingsystem to lower the freezing point of the coolantand to inhibit formation of rust and otherdeposits that could clog the radiator and coolantpassages and reduce cooling efficiency.

anti-friction bearing Any bearing in whichmoving parts are in rolling contact. Also seeball bearing, roller bearing, and needle bearing.

anti-knock agents Substances (such astetraethyl lead or ethanol) added to gasoline toraise the octane number and reduce the gaso-line’s tendency to knock or ping. Also seedetonation.

anti-lift The characteristics of a car’s suspen-sion intended to resist unwanted motion duringhard acceleration. Trailing arms and semi-trail-ing arms are often used in rear suspension sys-tems because they provide strong resistance tolift. Also see lift, dive, and anti-dive.

A

anode

anti-friction bearing

antilock braking system (ABS) A systemthat provides rapid, automatic cadence brakingin response to signs of incipient wheel lockup byalternately increasing and decreasing hydraulicpressure in the brake line(s) of the affectedwheel(s). This action prevents wheel lockup,thereby preserving steering control and reduc-ing stopping distances on some road surfaces.

Under braking, an electronic control unit(ECU) receives signals from electronic sensorsmonitoring wheel rotation. If a wheel’s rate ofrotation suddenly decreases, the ECU orders ahydraulic control unit to reduce line pressure tothat wheel’s brake. Once the wheel resumesnormal rotation, the controls restore pressure toits brake. Depending on the system, this cycleof “pumping” or cadence braking can occur upto fifteen times per second. Also see StopControl System.

anti-percolation valve A carburetor ventused to prevent vapor lock by releasing hot,expanding fuel vapors. In emission-controlledengines, vapors are not released to the atmos-phere but are routed into an evaporative-emis-sion-control canister, where they are stored andthen metered to the engine the next time it isstarted. Also see evaporative emission control andvapor lock.

anti-roll bar A transverse bar linking bothsides of a suspension system; generally, a torsionbar with rubber bushings mounted on the chas-sis that allow it to turn freely. The bar’s ends areconnected to or shaped as lever arms, withattachments to the suspension linkages at eachside via ball-joint links, rubber-bushed pivotlinks, or, on race cars, spherical rod ends calledHeim joints. When both wheels take a bumpequally, the wheels move the same amount with-out twisting the anti-roll bar. Individual wheelmovement or body roll will force the bar totwist as the lever arms are variously moved,thereby adding the bar’s own spring rate to that

12

antilock braking system

antilock braking system (ABS)

anti-roll bar

of the car’s springs. Although an anti-roll bar’smain function is to reduce body roll in corner-ing, it also influences overall handling. Installingor enlarging a front anti-roll bar tends to increaseundersteer; fitting or enlarging a rear barincreases oversteer tendencies. Also called sta-bilizer bar or, erroneously, sway bar and anti-sway bar.

anti-skid See antilock braking system.

anti-slip See anti-wheelspin.

anti-spin See anti-wheelspin.

anti-squat The characteristics of a car’s sus-pension intended to resist unwanted motionduring hard acceleration. Trailing arms andsemi-trailing arms are often used in rear sus-pension systems because they provide strongresistance to squat. Also see squat, anti-dive, andanti-lift.

anti-sway bar See anti-roll bar.

anti-wheelspin Electronic controls thatmaintain vehicle traction by automaticallyapplying the brakes and/or reducing enginepower during acceleration when the systemdetects one or more wheels spinning. As in elec-tronic anti-lock braking, sensors monitor wheelrotation; in this case, though, a sudden increasein a wheel’s rate of rotation signals the anti-spincontrols to effect gradual application of the

brakes, reduce the throttle opening or tur-bocharger boost, or perform a phased shutdownof engine cylinders until power to the wheel issufficiently reduced that it resumes normal rota-tion. Also called anti-spin, anti-slip.

apex seal In the Wankel or rotary engine, theequivalent of a reciprocating engine’s pistonring. Fitted to each of the three apexes on arotor, the seals serve to prevent compressed-gasleakage and blowby of combustion gases, andalso to release part of the heat captured by therotor into the wall of the housing. Also called tipseal. Also see Wankel engine.

A-pillar A car’s foremost roof pillars, sup-porting the windshield and front portion of theroof. Also called A-post.

A-post See A-pillar.

aquaplaning A tire’s tendency to ride on alayer of water instead of maintaining direct con-tact with the road surface. Generally more pro-nounced in low-profile tires with relatively largecontact patches, aquaplaning can lead to loss ofcontrol. For this reason, virtually all road tireshave grooves designed to channel water awayfrom the tread. Also known as hydroplaning.

ARI See Automatic Radio Information.

armature A wire-wrapped iron or steel coreforming a movable coil within the starter motor.

13

armature

A

apex seal

A-pillar

When it revolves in the magnetic field betweenthe poles, an electric current is induced.

aspect ratio The ratio of a tire’s cross-sec-tional height to its width, usually used with theexpression “-series.” The lower the number, thelower the tire profile. Thus, a 50-series tire (sec-tion height of 50 percent of width) has a lowerprofile than a 60-series tire.

aspiration The method by which an enginebreathes; used mainly to express whether theair-fuel mixture is delivered at or above atmos-pheric pressure. An engine without a tur-bocharger or a supercharger is said to be natu-rally or normally aspirated, while one so equippedmay be said to be hyperaspirated or blown. Alsosee atmospheric pressure, supercharger, and tur-bocharger.

assisted steering See power steering.

asymmetrical tread A tire tread in which theshape and size of the grooves vary across itswidth. The purpose is to provide an optimumcombination of braking, ride, handling, andwet- and dry-road characteristics.

atmosphere A unit of measure of air pressureat sea level; sometimes used for the pressure ofair forced into an engine by a turbocharger orsupercharger. One atmosphere equals 14.7lbs/in2 (psi) or, in the metric system, approxi-mately 1.01 bar.

atmospheric pressure The weight of airpressing downward per unit area. The averagepressure at sea level is 14.7 lbs/in2 (psi).

autocross See slalom.

autoignition Rapid burning of the air-fuelmixture not caused by an external ignitionsource such as a spark, flame, or hot surface. Alsosee detonation and dieseling.

automatic choke See choke.

automatic climate control A combinedheating/ventilation/air-conditioning system thatautomatically balances heating and cooling tomaintain interior temperature at a set level. Alsocalled automatic temperature control.

automatic level control See automaticleveling.

automatic leveling A suspension system thatcompensates for load variations at the front,rear, or both ends of a car, positioning it at a pre-

14

aspect ratio

armature

asymmetrical tread

15

automatic seat belt

designated level (zero pitch) regardless of load.Also called automatic level control.

Automatic Radio Information (ARI) A caraudio system that automatically interrupts radioor cassette listening with reports on local trafficconditions so a driver can take an alternate routeto avoid delays. Though national in scope, theARI network is divided into regions, each with

its own assigned VHF frequency. First developedin Germany, ARI is now available in parts of theUnited States.

automatic seat belt A passive restraintdevice, normally a front-seat shoulder belt thatfits diagonally across the wearer and is anchoredat both ends, with one end feeding out from aninertia reel. When a door is closed, the beltmoves into the correct position for restrainingthe wearer without manual fastening.

Automatic seat belts may be either mechan-ical or electric. The former typically mounts theinertia reel and outboard belt end on the door,with the inboard end anchored to a fixturebetween the seats; as the door closes, the inertiareel takes up slack in the webbing to tension thebelt. In the electric type, a small motor runs theoutboard end rearward along a track above thedoor when the door is closed (and forward whenthe door is opened); the inertia reel is combinedwith the inboard anchorage.

A

automatic climate control

automatic leveling

Automatic seat belts can be used in lieu of orin addition to airbags under the U.S. govern-ment mandate that specified some type of pas-sive restraint on a fixed percentage of all cars soldin the United States beginning with model year1987. They are simpler and therefore less expen-sive than airbags, and must be worn in con-junction with a manually fastened lap belt foroptimum restraint.

automatic temperature control See auto-matic climate control.

automatic transmission A mechanism inthe drivetrain with gearsets to vary the power andtorque delivered to the driven wheels as a func-tion of engine load and speed, usually incorpo-rating a fluid coupling or torque converter toallow stopping and reversing without a foot-

operated clutch. Also see fluid coupling, infinitelyvariable transmission, and torque converter.

autothermic piston An aluminum piston inwhich steel or alloy inserts are cast to controlexpansion of the skirt.

auxiliary lighting An additional drivinglight, fog light, or spotlight used in conjunctionwith normal high or low beams.

axle A shaft on which a wheel revolves orwhich revolves with a wheel. Also a beam, usu-ally solid but sometimes hollow, connecting thetwo wheels at one end of a car. A live axle trans-mits power, as in a front-engine/rear drive car.A beam or rigid or dead axle supports but doesnot drive the wheels, as at the rear of a front-wheel-drive car. Also see half shaft.

16

automatic temperature control

automatic transmission

17

axle windup

axle shaft See half shaft.

axle windup Phenomenon in which thetorque being transmitted by the axle shafts to thewheels produces a reaction that rotates the liveaxle about its own centerline. Obviously a greaterproblem in cars with very high torque, axlewindup can produce tramping of the rear wheelsor wheel hop as the axle winds and unwinds.Also called wheel tramp. Also see wheel hop.

A

axle windup