Glossary of Mechanics Terms

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.

balance shalt -  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.

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 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 turbo charged cars because it helps overcome turbo lag.

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.

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.

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.

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.

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.

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.

differential -  A special gearbox designed so that the torque fed into it is split and delivered to two outputs that can turn at different speeds. Differentials within axles are designed to split torque evenly; however, when used between the front and rear axles in four-wheel-drive systems (a center differential), they can be designed to apportion torque unevenly.

disc brakes -  Properly called caliper disc brakes: a type of brake that consists of a disc that rotates at wheel speed, straddled by a caliper that can squeeze the surfaces of the disc near its periphery. Disc brakes provide a more linear response and operate more efficiently at high temperatures and wet conditions than drum brakes.

drivability -  The general qualitative evaluation of a powertrain's operating qualities, including idle smoothness, cold and hot starting, throttle response, power delivery, and tolerance for altitude changes.

driveline -  Everything in the drivetrain, less the engine and the transmission.

driveshaft -  The shaft that transmits power from the transmission to the differential.

drum brakes -  A type of brake that has an iron casting shaped like a shallow drum that rotates with the wheel. Curved brake shoes are forced into contact with the inner periphery of this drum to provide braking.

EPA fuel economy -  Laboratory fuel-economy tests administered by the Environmental Protection Agency using simulated weight and drag to re-create real driving conditions. The city fuel-economy test, also used to test emissions compliance, is based on a drive through typical Los Angeles urban traffic of about twenty years ago. Of course, such nostalgic conditions are purely nostalgic these days. The highway test uses a higher, steadier speed, averaging 49.4 mph.

Exhaust Manifold -  The network of passages that gathers the exhaust gases from the various exhaust ports and routes them toward the catalysts and mufflers of the exhaust system. A manifold with free-flowing passages of a carefully designed configuration, called a ""header,"" can improve breathing.

Exhaust Port -  The passageway in the cylinder head leading from the exhaust valves to the exhaust manifold.

Fiberglass -  A composite material that relies on small glass fibers for its strength.

Floorpan -  The largest and most important stamped metal part in a car's body. Usually assembled from several smaller stampings, the floorpan forms the floor and fixes the dimensions for most of the car's external and structural panels. It is also the foundation for many of the car's mechanical parts.

Fluid Coupling -  Any device that transfers power through a fluid between its inputs and outputs. A fluid coupling basically consists of two fans in a sealed, oil-filled housing. The input fan churns the oil, and the churning oil in turn twirls the output fan. Such a coupling allows some speed difference between its input and output shafts.

Four-Wheel Drift -  A somewhat imprecise term that describes a cornering situation in which all four tires are operating at large slip angles.

Four-Wheel Steering -  A steering system that actively steers the rear wheels as well as the fronts to improve handling and maneuverability.

Fuel Injection -  Any system that meters fuel to an engine by measuring its needs and then regulating the fuel flow, by electronic or mechanical means, through a pump and injectors. Throttle-body injection locates the injector(s) centrally in the throttle-body housing, while port injection allocates at least one injector for each cylinder near its intake port.

Gearset -  A group of two or more gears used to transmit power.

Ground Effect -  The phenomenon that occurs when the airflow between a moving object and the ground creates downforce.

Handling -  A general term covering all the aspects of a car's behavior that are related to its directional control.

Heim Joint -  An extremely rigid articulating joint, commonly known as a ""spherical rod-end,"" used in any precision linkage. Heim joints are often used in the suspension links of race cars because they locate wheels very precisely.

Hemi -  A term used to describe any engine that has hemispherical combustion chambers in its cylinder head. Although a four-valve design is more efficient, a hemi head provides room for a pair of large valves and offers good breathing characteristics.

Horsepower -  The common unit of measurement of an engine's power. One horsepower equals 550 foot-pounds per second, the power needed to lift 550 pounds one foot off the ground in one second: or one pound 550 feet up in the same time.

Independent Suspension -  Any suspension in which the camber of a wheel is not directly affected by the vertical motion of the opposite wheel.

Intake Manifold -  The network of passages that direct air or air-fuel mixture from the throttle body to the intake ports in the cylinder head. The flow typically proceeds from the throttle body into a chamber called the plenum, which in turn feeds individual tubes, called runners, leading to each intake port. Engine breathing is enhanced if the intake manifold is configured to optimize the pressure pulses in the intake system.

Jounce -  The motion of a wheel that compresses its suspension.

Jounce Bumper -  an elastic cushion used to stiffen the suspension gradually as it approaches the end of its jounce travel.

Kickdown -  A downshift in an automatic transmission caused by depressing the throttle.

Knock Sensor -  A sensor mounted on the engine that is designed to detect the high-frequency vibrations caused by detonation. By employing a knock sensor, a computerized engine-control system allows an engine to operate very near its detonation limit: thereby improving power and efficiency.

Leaf Spring -  A long, flat, thin, flexible piece of spring steel or various composite materials that deflects by bending when forces act upon it. Leaf springs are used primarily in suspensions.

lift -  A vertical force directed upward, produced by the airflow around a moving object: such as a car body.

Limited-Slip Differential -  A differential fitted with a mechanism that limits the speed and torque differences between its two outputs. Limited slip ensures that some torque is always distributed to both wheels, even when one is on very slippery pavement.

Live Axle -  A rigid axle incorporating a differential and axle shafts to power the two wheels it is supporting.

Lockup -  The juncture at which a tire starts to skid during braking. A tire's maximum braking force is developed when it is on the verge of lockup, so a car's shortest stopping distances are produced when its front and rear tires approach lockup simultaneously. This is very hard to achieve under varying conditions of load and traction, so one end typically locks up before the other. Front-wheel lockup is inherently more stable than rear-wheel lockup.

Main Bearings -  The bearings in an engine block that support the crankshaft.

Multileaf Spring -  A leaf spring with several leaves bundled together by steel bands.

Multilink Suspension -  A rear suspension consisting of at least four links, or "arms," and no struts. Because multilink suspensions assign specific wheel-locating duties to each element, they provide great flexibility for optimizing both ride and handling.

Neutral Steer -  A cornering condition in which the front and rear slip angles are roughly the same. Although seemingly an ideal state of balance, perfect neutral steer is not as stable as slight understeer.

Overdrive -  Any gearset in which the output shaft turns faster than the input shaft. Overdrive gears are used in most modern transmissions because they reduce engine rpm and improve fuel economy. Occasionally, a separate gearbox with an overdrive gearset is coupled to a conventional transmission.

Overhead Cam -  The type of valvetrain arrangement in which the engine's camshaft(s) is in its cylinder head(s). When the camshaft(s) is placed close to the valves, the valvetrain components can be stiffer and lighter, allowing the valves to open and close more rapidly and the engine to run at higher rpm. In a single-overhead-cam (SOHC) layout, one camshaft actuates all of the valves in a cylinder head. In a double-overhead-camshaft (DOHC) layout, one camshaft actuates the intake valves, and one camshaft operates the exhaust valves.

Pitch -  The rotation of a car about a horizontal axis, which causes its nose or tail to bob up and down. Dive and squat are pitching motions.

Plenum chamber -  A chamber, located between the throttle body and the runners of an intake manifold, used to distribute the intake charge evenly and to enhance engine breathing.

Port fuel injection -  A type of fuel injection with at least one injector mounted in the intake port(s) of each cylinder. Usually the injector is mounted on the air intake manifold close to the port. Port fuel injection improves fuel distribution and allows greater flexibility in intake-manifold design, which can contribute to improved engine breathing.

Pound-feet -  The unit of measurement for torque. One pound-foot is equal to the twisting force produced when a one-pound force is applied to the end of a one-foot-long lever.

Power -  The rate at which work is performed. Power is proportional to torque and rpm and is measured in horsepower.

Powertrain -  An engine and transmission combination.

Progressive-rate spring -  A spring with an increasing spring constant. For example, if the first inch of spring motion requires 100 pounds of force, the second inch would require more than an additional 100 pounds, and the third inch would require still more. Progressive-rate springs become stiffer as they are compressed, unlike single-rate springs, which have a fixed spring rate.

Psi -  Pounds per square inch, the common unit of measurement for pressure. Normal atmospheric pressure at sea level is 14.7 psi.

Rack-and-pinion -  A steering mechanism that consists of a gear in mesh with a toothed bar, called a ""rack."" The ends of the rack are linked to the steered wheels with tie rods. When the gear is rotated by the steering shaft, it moves the rack from side to side: turning the wheels.

Rebound -  The motion of a wheel that extends the suspension. The opposite of jounce.

Recirculating-ball -  A steering mechanism in which the steering shaft turns a worm gear that, in turn, causes a toothed metal block to move back and forth. Ball bearings in a recirculating track reduce friction between the worm gear and the block. As the block moves, its teeth rotate a gear connected to a steering arm, which then moves the steering linkage.

Redline -  The maximum recommended revolutions per minute for an engine. In cars equipped with a tachometer: an instrument that measures engine rpm: the redline is usually indicated by, surprisingly enough, a red line. Some tachometers mark the redline with a colored sector. Others have two lines: the lower one marking the maximum allowable sustained engine rpm, the higher line indicating the absolute maximum rpm.

Rigid axle -  A simple non-independent suspension, consisting of a rigid transverse member with wheel hubs solidly bolted to it. The axle can be attached to the body by leaf springs, or by a combination of suspension arms and links.

Ring-and-pinion gear -  Any gearset consisting of a small gear (the pinion gear) which turns a large-diameter annular gear (the ring gear).

Roll -  The rotation of a car's body about a longitudinal axis. Also less accurately called "sway" or "lean," it occurs in corners because the car's center of gravity is almost always higher than the axis about which it rotates.

Rubber-isolated crossmember -  A laterally aligned structural member that is attached to the body or the frame via vibration-absorbing rubber isolators. By bolting suspension or driveline components to such crossmembers, automotive engineers can reduce the transmission of noise and/or ride harshness to the body.

Sedan -  As used by Car and Driver, the term "sedan" refers to a fixed-roof car with at least four doors or any fixed-roof two-door car with at least 33 cubic feet of rear interior volume, according to measurements based on SAE standard J1100.

Shock absorber -  A device that converts motion into heat, usually by forcing oil through small internal passages in a tubular housing. Used primarily to dampen suspension oscillations, shock absorbers respond to motion; their effects, therefore, are most obvious in transient maneuvers.

Single-rate spring -  A spring with a constant spring rate. For example, if a 100-pound force deflects the spring by one inch, an additional 100 pounds will deflect it one more inch, and so on until the spring either bottoms or fails.

Skidpad  -  A large area of smooth, flat pavement used for various handling tests. Roadholding is measured by defining a large-diameter circle (Car and Driver uses 300 feet) on the skidpad and measuring the fastest speed at which the car can negotiate the circle without sliding off.

Slip angle -  The angular difference between the direction in which a tire is rolling and the plane of its wheel. Slip angle is caused by deflections in the tire's sidewall and tread during cornering. A linear relationship between slip angles and cornering forces indicates an easily controllable tire.

Squat -  The opposite of dive, squat is the dipping of a car's rear end that occurs during hard acceleration. Squat is caused by a load transfer from the front to the rear suspension.

Steering axis -  The line that intersects the upper and lower steering pivots on a steered wheel. On a car with a strut suspension, the steering axis is defined by the line through the strut mount on top and the ball joint on the bottom.

Steering feel -  The general relationship between forces at the steering wheel and handling. Ideally, the steering effort should increase smoothly as the wheel is rotated away from center. In addition, the steering effort should build as the cornering forces at the steered wheels increase. Finally, the friction built into the steering mechanism should be small in comparison with the handling-related steering forces.

Straight-line tracking -  The ability of a car to resist road irregularities and run in a straight line without steering corrections.

Stroke -  The distance between the extremes of a piston's travel in a cylinder.

Strut -  A suspension element in which a reinforced shock absorber is used as one of the wheel's locating members, typically by solidly bolting the wheel hub to the bottom end of the strut.

Throttle-body -  A housing containing a valve to regulate the airflow through the intake manifold. The throttle-body is usually located between the air cleaner and the intake plenum.

Torque -  The rotational equivalent of force, measured in pound-feet.

Torque steer -  A tendency for a car to turn in a particular direction when power is applied. Torque steer is common in front-drive cars because reaction forces created in the half-shafts can generate uneven steering forces in the front tires.

Torsion bar -  A spring consisting of a long solid or tubular rod with one end fixed to the chassis and the other twisted by a lever connected to the suspension.

Transaxle -  A transmission and a differential combined in one integrated assembly.

Transmission -  A gearbox with a number of selectable ratios, used to match the engine's rpm and torque to differing vehicle requirements.

Turbocharger -  A supercharger powered by an exhaust-driven turbine. Turbochargers always use centrifugal-flow compressors, which operate efficiently at the high rotational speeds produced by the exhaust turbine.

Turbo lag -  Within a turbocharger's operating range, lag is the delay between the instant a car's accelerator is depressed and the time the turbocharged engine develops a large fraction of the power available at that point in the engine's power curve.

Understeer -  A handling condition in which the slip angle of the front tires is greater than the slip angle of the rears. An understeering car is sometimes said to push, because it resists turning and tends to go straight.

Universal joint -  A joint that transmits rotary motion between two shafts that aren't in a straight line. Depending on its design, a universal joint can accommodate a large angular variation between its inputs and outputs. The simplest kind of universal joint, called a "Hooke joint," causes the output shaft to speed up and slow down twice for every revolution of the input shaft. This speed fluctuation increases with the angular difference between the shafts.

Valve float -  A high-rpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain.

Waste gate -  A valve used to limit the boost developed in a turbocharger. A waste gate operates by allowing some of the engine's exhaust flow to bypass the turbocharger's turbine section under certain conditions.

Wheel hop -  An undesirable suspension characteristic in which a wheel (or several) moves up and down so violently that it actually leaves the ground. Wheel hop can be caused by many problems, including excessive unsprung weight, insufficient shock damping, or poor torsional axle control.

Yaw -  The rotation about a vertical axis that passes through the car's center of gravity.

Zero-offset steering -  A steering system whose geometry has a scrub radius of zero. This configuration minimizes the steering effects produced during acceleration (with front drive) or braking on varying traction surfaces.