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Glossary

A

A-pillar the roof support on either side of a cars windshield.

Active Suspension An extremely sophisticated, computer-controlled suspension system that uses powered actuators instead of conventional springs and shock absorbers. The actuators position a car's wheels in the best possible manner to deal with road disturbances and handling loads.

Aerodynamic drag Drag produced by a moving object as it displaces the air in its path. Aerodynamic drag is a force usually measured in pounds; it increases in proportion to the object's frontal area, its drag coefficient, and the square of its speed

. Air Dam A front spoiler mounted beneath the bumper and shaped to reduce the airflow under the car. Air dams can increase the airflow to radiators, reduce aerodynamic drag, and/or reduce lift.

Anti-Dive A tuned-in front suspension characteristic that converts braking-induced forces in the suspension links into a vertical force that tends to lift the body, thereby reducing dive under braking.
Anti-Lock Braking System A braking system that senses when any of the wheels have locked up—or are about to—and automatically reduces the braking forces to keep the wheels rolling. Commonly called ABS, such a system can control all four wheels or only two.
Anti-Roll Bar A suspension element (used at the front, the rear, or both ends of a car) that reduces body roll by resisting any unequal vertical motion between the pair of wheels to which it is connected. An anti-roll bar does not affect suspension stiffness when both wheels are deflected equally in the same direction. Often incorrectly called a sway bar.
Anti-Squat Similar to anti-dive, this suspension characteristic uses acceleration-induced forces in the rear suspension to reduce squat.
Apex The point(s) or region on the line through a corner that touches the corner's inner radius.
Aspect Ratio Generally the ratio between two dimensions of an object. In tire terminology it applies to the unloaded sidewall height of the tire divided by its overall width. A lower aspect ratio implies a shorter, wider tire. When used to describe a wing it is the span of the airfoil (the long dimension perpendicular to the airflow) divided by its chord (the dimension parallel to the airflow).
Axle Tramp A form of wheel hop that occurs on cars with live axles, caused by the axle repeatedly rotating slightly with the wheels and then springing back.


B B-pillar The roof support between a car's front door window and rear side window, if there is one.
Balance Shaft A shaft designed so that, as it rotates, it vibrates in a way that reduces or cancels some of the vibration produced by an engine. Not essential to an engine's operation, balance shafts are nonetheless becoming increasingly common as a means of engine refinement. Balance-shafted four-cylinder engines use two shafts turning in opposite directions on either side of the engine's crankshaft. A single balance shaft is used when fitted to three-cylinder and V-6 engines.
Ball Joint A flexible joint consisting of a ball in a socket, used primarily in front suspensions because it can accommodate a wide range of angular motion.
Beam Axle A rigid axle supporting the non-driven wheels. Also called a dead axle.
Beltline The line running around a car's body formed by the bottom edges of its glass panels
Bevel Gears A gearset employing gears shaped like slices of a cone, which allows the axes of the gears to be nonparallel. Bevel gears are used to transmit motion through an angle.
Boost Pressure The increase above atmospheric pressure produced inside the intake manifold by any supercharger. It is commonly measured in psi, inches of mercury, or bar.
Brake Bias The front/rear distribution of a car's braking power. For the shortest stopping distance, brake bias should match the car's traction at each end during hard braking brake modulation: the process of varying pedal pressure to hold a car's brakes on the verge of lockup. Ideally, the brakes will unlock with only a slight reduction in the pressure needed to lock them. Typically, however, a considerable pressure reduction is required.
Brake Modulation The process of varying pedal pressure to hold a car's brakes on the verge of lockup. Ideally, the brakes will unlock with only a slight reduction in the pressure needed to lock them. Typically, however, a considerable pressure reduction is required.
Brake Torquing A procedure generally used in performance tests to improve the off-the-line acceleration of a car equipped with an automatic transmission. It is executed by firmly depressing the brake with the left foot, applying the throttle with the car in gear to increase engine rpm, then releasing the brakes. Brake torquing is particularly effective with turbocharged cars because it helps overcome turbo lag.
Breathing (engine) A term used to describe an engine's ability to fill its cylinders with air-fuel mixture and then discharge the burnt exhaust gases. In general, the more air-fuel mixture an engine burns the more power it produces.
Bushing A simple suspension bearing that accommodates limited rotary motion, typically made of two coaxial steel tubes bonded to a sleeve of rubber between them. The compliance of the bushing in different directions has a great effect on ride harshness and handling.



C C-pillar The roof support between a car's rearmost side window and its rear window. On a vehicle with four side pillars, the rearmost roof support may be called a D-pillar.
Cam Profile The shape of each lobe on a camshaft. The profile determines the amount, or "duration," of time the valve is open; it also largely determines the valve's maximum opening, or "lift."
Camber The angle between the plane of a wheel's circumference and a vertical line, measured in degrees and minutes. The tops of a car's wheels tilt inward when the camber is negative, outward when it is positive.
Camshaft A shaft fitted with several cams, whose lobes push on valve lifters to convert rotary motion into linear motion. The opening and closing of the valves in all piston engines is regulated by one or more camshafts.
Carbon Fiber Threadlike strands of pure carbon that are extremely strong in tension (that is, when pulled) and are reasonably flexible. Carbon fiber can be bound in a matrix of plastic resin by heat, vacuum, or pressure to form a composite that is strong and light—and very expensive.
Caster The angle between a vertical line and the car's steering axis when viewed from the side, measured in degrees and minutes.
Catalytic Converter Often simply called a "catalyst", this is a stainless-steel canister fitted to a car's exhaust system that contains a thin layer of catalytic material spread over a large area of inert supports. The material used is some combination of platinum, rhodium, and palladium; it induces chemical reactions that convert an engine's exhaust emissions into less harmful products. So-called three-way catalysts are particularly efficient; their operation, however, demands very precise combustion control, which can be produced only by a feedback fuel-air-ratio control system.
Center Differential A differential used in four-wheel-drive systems to distribute power to the front and rear differentials.
Chassis A general term that refers to all of the mechanical parts of a car attached to a structural frame. In cars with unitized construction, the chassis comprises everything but the body of the car.
Coil Spring A bar of resilient metal wound into a spiral that may be compressed or extended without permanent deformation. Coil springs have many automotive applications but are particularly important as suspension springs.
Combustion Chamber The space within the cylinder when the piston is at the top of its travel. It is formed by the top of the piston and a cavity in the cylinder head. Since most of the air-fuel mixture's combustion takes place in this space, its design and shape can greatly affect the power, fuel efficiency, and emissions of the engine.
Compliance A slight resiliency, or "give," designed into suspension bushings to help absorb bumps. Good compliance allows the wheels to move rearward a bit as they hit bumps but doesn't allow them to move laterally during cornering.
Composite Any material that consists of two or more components, typically one or more of high strength and one an adhesive binder. The most common composite is fiberglass, which consists of thin glass fibers bonded together in a plastic matrix. The structural properties of composites can be altered by controlling the orientation and configuration of the high-strength components.
Compression Ratio The ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.
Connecting Rod The metal rod that connects a piston to a throw on a crankshaft.
Constant-Velocity Joint A particular kind of universal joint designed so that there is no cyclic fluctuation between the speeds of its input and output shafts.
Control Arm A suspension element that has one joint at one end and two joints at the other end, typically the chassis side. Also known as a wishbone or an A-arm.
Cornering Limit The maximum speed at which a car can negotiate a given curve.
Coupe A closed car with two side doors and less than 33 cubic feet of rear interior volume, according to measurements based on SAE standard J1100. A two-door car is therefore not necessarily a coupe.
Crankshaft A shaft with one or more cranks, or "throws," that are coupled by connecting rods to the engine's pistons. Together, the crankshaft and the con rods transform the pistons' reciprocating motion into rotary motion.

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