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Automotive Dictionary

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Air Bag: The air bag, also known as a Supplemental Inflatable Restraint System, is a passive safety device, supplemental to safety belts, that inflates to provide a cushion to absorb impact forces during moderate to severe frontal collisions. This system can help to lessen the chance of contact with the steering wheel, instrument panel and windshield. The air bag is actuated automatically by sensors located in the front of the vehicle. To maximize effectiveness, seat and shoulder belts must always be used in conjunction with this system.

Airfoil: An aerodynamic device designed to improve traction by increasing the downforce on the car. The use of airfoils (also called spoliers or wings) increases the cornering capability and improves stability at speed, but often at the expense of additional aerodynamic drag.

Air Injection: A system that injects air into the exhaust ports of the engine for combustion of unburned hydrocarbons in the exhaust gases, thus producing "cleaner" exhaust emissions.

Alignment: Generally refers to wheel alignment, which is the proper adjustment of the car's front and rear suspension for camber, toe, caster and ride height.

A-Pillar: In the side view, the foremost roof support of a vehicle, located in most instances between the outer edge of the windshield and the leading edge of the front door upper. Also known as an A-Post.

Alloy Wheels: A generic term used to describe any non-steel road wheel. The most common alloy wheels are cast aluminum. Technically, an alloy is a mixture of two or more metals. These wheels are known for their light weight and strength.

All-Wheel Drive: Often confused with Four-Wheel Drive (4WD), this drive system features four, full-time active drive wheels to reduce wheel slippage and provide greater driver control over the vehicle. All-Wheel Drive automatically splits engine torque between the front and rear wheels as needed, improving on-road traction in unfavorable road conditions. Unlike Four-Wheel Drive, All-Wheel Drive is an on-road system and is not designed for off-road use. AWD does not require the driver to actively engage the system. It is operational at all times, and requires no switches, lights or visor instructions for system operation.

Anti-Lock Brake System (ABS): On a vehicle equipped with Anti-Lock Brakes, the wheels are equipped with speed sensors. When a sensor determines that a wheel is decelerating so rapidly that lockup may occur, the electro-Hydraulic Control Unit (EHCU) is activated. The EHCU then modulates the brake pressure in the appropriate brake lines by means of the solenoid-operated valves. This is intended to prevent wheel lockup and help the vehicle maintain directional stability during potentially hazardous braking situations. (See also: Rear-Wheel Anti-Lock and Four-Wheel Anti-Lock.)

Automatic Locking Front Hubs: Found in some four-wheel drive vehicles, this allows the driver to engage, or "lock," the front axle hubs without leaving the vehicle.

Axle Ratio: The ratio between the rotational speed (RPM) of the drive shaft and that of the driven wheel. Gear reduction in final drive is determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear.

B-Pillar: The roof support between a vehicle's front door window and rear side window, if there is one.

Balance Shaft: A shaft designed so that, as it turns, it counter rotates the rotational direction of the engine crankshaft in a manner that reduces or cancels out some of the vibration produced by the engine.

Ball Joint: A flexible joint consisting of a ball within a socket. Ball joints act as pivots which allow turning of the front wheels and compensate for changes in the wheel and steering geometries that occur while driving.

Base Coat/Clear Coat: A paint system that adds a final clear-coat paint layer over primer and color coats to provide a deep, "wet-look" shine that resists fading.

Belted Radial Tires: A reinforcing bank, normally textile, fiberglass or steel, running around the circumference of a tire and strengthening the tread area. The most common type of tire available today.

Bias-Ply Tires: A type of tire in which the plies or layers of cord in the tire casing are laid diagonally, criss-crossing one another at an angle of 30 to 40 degrees. Often the original type of tires found on muscle cars.

Body-On-Frame Construction: A type of automobile construction in which the body structure is attached to a separate frame. Common on cars until the 1970's when car companies switched to uni-body construction on most cars (but not on trucks and some SUVs).

Boost: The amount of extra air (compressed air) pumped into the intake manifold by a supercharger or turbocharger. Usually measured in psi (pounds per square inch), inches of mercury, or bar.

Bore: The diameter of an engine cylinder or bearing. Used in determining engine displacement.

Brake Fade: A condition brought about by repeated brake applications, resulting in build-up of heat that causes a temporary reduction or fading of braking effectiveness.

Brake Horsepower (bhp): The actual horsepower of an engine, measured by a brake attached to the driving shaft and recorded by a dynamometer.

Brake Linings: The replaceable friction material which contacts the brake drum in a drum brake system to slow or stop the car.

Brake Master Cylinder: A cylinder containing a movable piston activated by pressure on the brake pedal. The piston produces hydraulic pressure that pushes fluid through the lines and wheel cylinders. This forces the brake lining or pad against the drum or disc to slow or stop the car.

Brake Pads: In a disc system, they are the replaceable flat segments consisting of a rigid backing plate plus frictional lining that takes the place of the shoe and lining in a drum brake. Brake pads are sometimes referred to as brake pucks.

Brake Shoe: The arc-shaped carrier to which the brake linings are mounted in a drum brake. They also force the lining against the rotating drum during braking.

Brakes, Disc: A type of braking system in which brake shoes, in a vise-like caliper, grip a revolving disk mounted on a wheel to slow or stop disc and wheel rotation for braking. Properly called "Caliper Disc Brakes," they are used on most modern cars.

Brakes, Drum: A type of braking system that utilizes a metal drum mounted on a wheel to form the outer shell of a brake. The brake shoes press against the drum to slow or stop drum and wheel rotation for braking. Common on vehicles in the '60s and '70s, Drum brakes are only used on some vehicles rear wheels today.

Burned Piston: When a cylinder runs lean (too much air in the air-to-fuel mixture) and excessive heat burns or melts the piston.