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: thread-like 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, 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": 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.

cylinder: the round, straight-sided cavity in which the pistons move up and down. Typically made of cast iron and formed as a part of the block.

cylinder head: the aluminum or iron casting that houses the combustion chambers, the intake and exhaust ports, and much or all of the valvetrain. The head (or heads, if an engine has more than one bank of cylinders) is always directly above the cylinders.

cylinder liner: the circular housing that the piston moves in when the cylinder is not an integral part of the block. Also known as a "sleeve."