Truck Equipment Glossary - D

Deflection Rate 
Used in rating springs to determine the number of pounds necessary to deflect a spring one inch. For torsion bars, it is a one-inch deflection of the control arm. 
Department of Energy
A governmental department charged with advancing the national, economic and energy security of the U.S.; promoting scientific and technological innovation; and ensuring the environmental cleanup of the national nuclear weapons complex. Visit to learn more.

  • 21st Century Truck — A DOE Vehicle Technologies Program partnership aimed at developing technologies for the nation’s trucks and buses that could safely and cost-effectively move larger volumes of freight and greater numbers of passengers while emitting little or no pollution and dramatically reducing dependence on foreign oil. Click here to learn more. 
  • National Renewable Energy Laboratory (NREL) — A DOE facility dedicated to the research, development, commercialization and deployment of renewable energy and energy efficiency technologies. Visit to learn more. 
The gear assembly on the drive axle that permits one wheel to turn slower or faster than the other when going around corners.
Differential-No Slip or Limited Slip
This type of differential will not allow one wheel to spin while the other is motionless, such as when a truck is stuck on ice or in mud. Torque is transmitted to both drive wheels for better traction. This type of differential is available on almost all cars and trucks today as an option and is standard equipment on some.
Drive Cycle
A series of data points (usually presented as a simple line graph) that plot vehicle speed versus time. In some cases, engine RPM is also incorporated to account for conditions such as stationary power take-off operation, off-road operation and operation of power equipment at low road speeds.
The displacement of an engine is the volume through which the head of the piston moves multiplied by the number of pistons in the engine.
Diverter Valve
Auxiliary valve to provide hydraulic power from the hydraulic pump by switching the oil flow from its usual passageways into additional pieces of equipment such as snowplows or other lifting cylinders. Also called Diversion or Selector Valve.
Dolly (Towing)
A four-wheel carriage often used in towing to support the trailing end of a vehicle.
Abbreviation for Department of Transportation, the agency composed of several federal agencies dealing with regulations concerning the manufacture and operation of motor vehicles and motor vehicle equipment.
Double-bottom Trailer
A combination consisting of a tractor pulling a semi-trailer with a full trailer in back. Also called Tandem Trailer.
The total system drive shaft consisting of universal joints, slip yokes and flanges between the transmission and axle(s).