TRAILERS AND SEMITRAILERS
Section I. SEMITRAILERS
shaped goods, such as poles, pipes, or structural
38-1. General Description.
A typical semitrailer
members. In this case, the pole, or boom, forms the
chassis (fig. 38-1) consists mainly of a frame, spring
trailer frame. The pole is attached to a turntable
suspension, axle, fifth-wheel connection, and a landing
mounted on the tractor in much the same manner as a
gear. It resembles the conventional truck chassis in that
fifth wheel. The trailer axle unit is attached by adjustable
its frame is made of two pressed-steel side members
clamps to the other end of the pole.
with several crossmembers and has laminated leaf
The wheels and tires are
A truck tractor used to haul a semitrailer must be a
interchangeable with those of the tractor used for hauling
special design. Its wheelbase is shorter than that of a
the trailer. Figure 38-1 illustrates a semitrailer chassis
standard truck, and the engine and transmission units
with tandem axles for carrying heavy trailer loads.
are designed to produce the necessary power for pulling
a loaded semitrailer. Truck tractors may be designed
Early semitrailer frames were built of straight side
with the cab over the engine, or may be of conventional
members, which meant that the rear of a level trailer
truck design, with either single or dual rear axles. The
body had to be at a considerable height above the
flexible hose connecting the brake system of the tractor
ground to clear the rear wheels of the tractor. Such a
to the trailer is shown clearly at the rear of the tractor cab
high frame is not acceptable because it heightens the
center of gravity, making it easier to overturn the trailer.
A kickup (fig. 38-1) in the semitrailer frame permits a
38-2. Kingpin and Plate.
A heavy steel plate
lower center of gravity without reducing the necessary
clearance space above the rear wheels of the motor
assembly (fig. 38-3), known as the upper fifth-wheel
plate, is attached securely to the underside of the front
end of the semitrailer frame. This plate serves as the
Crossmembers are located where the greatest strains
bearing, or front end, support of the semitrailer when it is
occur to the trailer frame. They may be tubular, channel,
coupled to a tractor. The front edge of the plate is turned
or box-shaped, with gusset plates riveted or welded to
up approximately 45 degrees to form a skid that slides
the side members to make a rigid, strong frame.
on the lower fifth-wheel plate (mounted on the truck
tractor) when the semitrailer is being hitched to the truck
Another type of semitrailer with a variable wheelbase is
tractor. In the center of the upper fifth-wheel plate
the pole trailer used for transporting long or irregularly
Figure 38-1. Typical Semitrailer Chassis.