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[B] [C] [D] [E]
[F] [G] [H] [I]
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- D -
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.
- E -
EDI (Electronic Data Interchange)
The business-to-business interconnection of
computers for the rapid exchange of a wide variety
of documents, from bills of lading to build
tickets at auto plants.
Exempt Carrier
Company which transports commodities exempted
from Interstate Commerce Commission (ICC) economic
regulation.
EV (Electric Vehicle)
Vehicle powered by electric motor(s) rather than
by an internal combustion engine. Most common
source of electricity is chemical storage
batteries.
- F -
Fifth Wheel
Coupling device attached to a tractor or dolly
which supports the front of a semitrailer and
locks it to the tractor or dolly. The fifth
wheel's center is designed to accept a trailer's
kingpin, around which the trailer and tractor or
dolly pivot in turns.
Fixed Tandem
Assembly of two axles and suspension that is
attached to the chassis in one place, and cannot
be moved fore and aft.
For-Hire Carrier
Company in the business of transporting freight
belonging to others
- G -
GAWR (Gross Axle Weight Rating)
Maximum weight an axle is rated to carry by the
manufacturer. Includes both the weight of the axle
and the portion of a vehicle's weight carried by
the axle.
GCW (Gross Combination Weight)
Total weight of a loaded combination vehicle,
such as a tractor-semitrailer or truck and full
trailer(s).
Geared Speed
Calculated vehicle speed at the engine's governed
rpm in each transmission gear, or (commonly) in
top gear.
Gear Ratio
Number, usually expressed as a decimal fraction,
representing how many turns of the input shaft
cause exactly one revolution of the output shaft.
Applies to transmissions, power takeoffs, power
dividers and rear axles. Example: If 2.5
revolutions of an input shaft cause one revolution
of the output shaft, the gear ratio is 2.5:1.
Grade
Steepness of a grade, expressed as a percentage.
Example: A vehicle climbing a 5% grade rises 5
feet for every 100 feet of forward travel.
Gradeability
Vehicle's ability to climb a grade at a given
speed. Example: A truck with a gradeability of 5%
at 60 mph can maintain 60 mph on a grade with a
rise of 5%.
GVW (Gross Vehicle Weight)
Total weight of a vehicle and everything aboard,
including its load.
GVWR (Gross Vehicle Weight Rating)
Total weight a vehicle is rated to carry by the
manufacturer, including its own weight and the
weight of its load.
- H -
Hazmat
Hazardous materials, as classified by the U.S.
Environmental Protection Agency (EPA). Transport
of hazardous materials is strictly regulated by
the U.S. Department of Transportation.
Headache Rack
Heavy protective barrier mounted behind the
tractor's cab. Designed to prevent "headaches"
caused by load shifting forward from the trailer
and crushing the cab.
HCFC
Hydrochlorofluorocarbon.
Horsepower (hp)
Measure of power (the amount of work that can be
done over a given amount of time). One horsepower
is defined as 33,000 foot-pounds of work in one
minute. Example: Lifting 33,000 pounds one foot in
one minute, or lifting 3300 pounds ten feet in one
minute.
Horsepower, Gross Laboratory
Tested horsepower of a "bare" engine without fan,
water pump, alternator, exhaust system or any
other accessories.
Horsepower, SAE Net
Horsepower capability of an engine with full
accessories and exhaust system. Test procedures
per standards of Society of Automotive Engineers (SAE).
Hours-Of-Service
U.S. Department of Transportation safety
regulations which govern the hours of service of
commercial vehicle drivers engaged in interstate
trucking operations.
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