dBA: a unit of measure for decibels, the measure of sound intensity or pressure named after Alexander Graham Bell. It is a logarithmic measurement; every 3dB increase represents a doubling of the sound pressure. The "A" in dBA indicates that the measurement was taken with an A-weighted scale; sound pressure varies across the audible spectrum, and the A-weighted scale approximates the human ear's sensitivity to various frequencies.
de Dion suspension: a suspension system in which the rear, driven wheels are bolted to a transverse, lightweight, rigid member. Power is delivered to the wheels by universal-jointed half-shafts attached to a body-mounted differential.
dead pedal: a footrest found to the left of the leftmost pedal. It provides a place for the driver to brace his left leg during hard cornering.
detonation: a condition in which, after the spark plug fires, some of the unburned air-fuel mixture in the combustion chamber explodes spontaneously, set off only by the heat and pressure of air-fuel mixture that has already been ignited. Detonation, or "knock," greatly increases the mechanical and thermal stresses on the engine.
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.
dive: the dipping of a car's nose that occurs when the brakes are applied. Dive is caused by a load transfer from the rear to the front suspension; this transfer occurs because the car's center of gravity, through which all inertial forces pass, is higher than its contact patches, the points where the braking forces are exerted on the ground.
DOHC: double overhead camshaft: a DOHC engine has two camshafts in each cylinder head; one camshaft operates the intake valves, the other actuates the exhaust valves.
downforce: a vertical force directed downward, produced by airflow around an object: such as a car body.
drag coefficient: a dimensionless measure of the aerodynamic sleekness of an object. A sleek car has a drag coefficient, or "Cd," of about 0.30; a square, flat plate's is 1.98. Also signified by Cx.
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.
drivetrain: all of a car's components that create power and transmit it to the wheels; i.e. the engine, the transmission, the differential(s), the hubs, and any interconnecting shafts.
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.