A-pillar: the roof support on either side of a car's 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: the 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.

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