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Panhard rod: a long lateral link that provides lateral location of a rigid axle. It usually sits roughly parallel to the axle, with one end attached to the body and the other attached to the axle. pent-roof: a combustion chamber whose upper surface resembles a shallow peaked roof. Usually used with four valves per cylinder. 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. planetary gears: a gearset in which all of the gears are in one plane, grouped around each other like the planets around the sun. The central gear is called the "sun gear." In mesh with it is a circular grouping of gears, called "planet gears," mounted on a rotating carrier. The planet gears also engage teeth on the inner periphery of the "ring gear." By holding any one of the three gear elements motionless, different ratios can be produced between the other two. Planetary gearsets are common in automatic transmissions. 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. polar moment of inertia: the resistance of an object to rotational acceleration. When the mass of an object is distributed far from its axis of rotation, the object is said to have a high polar moment of inertia. When the mass distribution is close to the axis of rotation, it has a low polar moment of inertia. A mid-engined car has most of its mass within its wheelbase, contributing to a low polar moment of inertia, which, in turn, improves cornering turn-in. 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. power band: the subjectively defined rpm range over which an engine delivers a substantial fraction of its peak power. The power band usually extends from slightly below the engine's torque peak to slightly above its power peak. powertrain: an engine and transmission combination. profile: the aspect ratio of a tire. 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. push: a slang term for understeer. pushrod: A general term for any rod that transfers force in compression. In a valvetrain, pushrods are used to transfer reciprocating motion from the cam followers to a more distant part of a valvetrain, typically the rocker arms. |
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