Glossary of Semi Truck Crash Terms

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Dead-Heading: Operating a truck without cargo.

Demountable Rim: Multi-piece steel wheel rim assembly which is bolted to a spoke hub. Demountable rims are still in use, though they have been replaced in many applications by the simpler disc wheel. (see Cast Spoke Wheel)

Disc Wheel: Single-piece rim/wheel assembly of stamped and welded steel or forged aluminum, anchored by 8 or 10 nuts to a hub. A "Budd wheel" is a ten-hole, stud-piloted disc wheel; a design originated by the Budd Corporation.

Displacement (Piston Displacement): Sum of the volumes swept by an engine's pistons as they travel up and down in their cylinders. Based upon bore (diameter of cylinder) and stroke (distance traveled by piston). Expressed in liters or cubic inches.

Doubles (Twins, Twin Trailers): Combination of a tractor and two semitrailers connected in tandem by a converter dolly.

Driveline: All the components which together transmit power from the transmission to the drive axle(s). These consist of at least one driveshaft (propeller shaft) with a universal joint at each end.

Drivetrain (Powertrain): All the components, excluding engine, which transmit the engine's power to the rear wheels: clutch, transmission, driveline and drive axle(s).

DRL (Daytime Running Lights): System that automatically turns on a vehicle's low beam headlights when the parking brake is released and the ignition is on.