The cargo trailer is hooked to a horseshoe-shaped coupling
device called a fifth wheel at the rear of the towing
engine that allows easy hook up and release. The trailer
cannot move by itself because it only has wheels at the
rear end, hence the name semi-trailer: it only carries half
its own weight. The vehicle has a tendency to fold at the
pivot point between the semi and the trailer when braking
hard at high speeds. Such a truck accident is appropriately
called a jack-knife, or jack-knifing.
Semi trucks use air pressure, rather than hydraulic fluid
to actuate the brakes. This allows for ease of coupling
and uncoupling of trailers from the tractor unit, as well
as reducing the potential for problems common to hydraulic
systems, such as leakage or "brake-fade" caused
when overheated brake fluid vaporizes in the hydraulic lines.
(Brake fade may also occur when the lining of the braking
unit becomes severely overheated. This has no connection
to the fluid lines.)
The "parking brake" of the tractor unit and
the "emergency brake" of the trailer are applied
when air pressure is released, and disengaged when
air pressure is supplied. This is an emergency feature which
ensures that if air pressure to either unit is lost, that
unit will not lose all braking capacity and become uncontrollable.
The trailer controls are coupled to the tractor through
two "glad-hand" connectors, which provide air
pressure, and an electrical cable, which provides power
to the lights and any specialized features of the trailer.
"Glad-hand" connectors are air couplers, each
of which has a flat engaging face and retaining tabs. The
faces are placed together, and the units are rotated so
that the tabs engage each other to hold the connectors together.
This arrangement provides a secure connection, but allows
the couplers to break away without damaging the equipment
when they are pulled, as may happen when the tractor and
trailer are separated without first uncoupling the air lines.
Two air lines control the trailer unit. An "emergency"
or main air supply line pressurizes the trailer's air tank
and disengages the emergency brake, and a second "service"
line controls the brake application.
Another braking feature of semi-trucks is the "engine
brake", colloquially known as the "Jake
brake". This feature uses the engine to slow the
vehicle, which allows trucks to travel down long grades
without overheating their wheel brakes. Due to noise concerns,
some locales have prohibited or restricted the use of engine
brake systems inside their jurisdictions
Because of the wide variety of loads the semi may carry,
they usually have a manual transmission to allow the driver
to have as much control as possible. A special driver's
license is required to operate a semi-trailer in most countries.