Trucks and other
transportation equipment/vehicles have become bigger and bigger over the years
as the need to transport over dimensional loads arise and engines and systems
can now provide the power levels which are much greater than in the past. There
are certain basic requirements to meet in using vehicles for super loads or
over dimensional loads. One of these is that all these vehicles must not exceed
the load weight of each tyre (tire) which touches the ground so as to avoid any
damage to the roads. There are multi wheel combinations because there should be
minimum distances between these wheels. The rules and regulations specify the
limits but these rules and regulations would depend on the geographic area like
which state, province or country.
A semi-trailer
also called a semi-tractor-trailer, truck and trailer, transfer truck,
18-wheeler, mack truck, big rig, transport, artic, and lorry is a vehicle
having permanent or semi-permanent pivoting joint which allows the it to turn
sharply and it has a towing engine and a semi-trailer which carriers the
freight. A semi-trailer does not fully
tail the towing vehicle but it joined at a point that it is just forward of the
rear-most axle of the towing unit. A fraction of the weight is carried by the
prime mover with most of that at the rear axles of the prime mover and a small
amount at the front axles. Most semi-tractors have two-three axles and are
commonly called tractor-trailers in North America.
They are used for hauling
over dimensional loads. These vehicles
can legally haul payloads of 40,000 – 50,000 pounds depending on the
configuration of the vehicle and configuration of payload not to be over the
80,000 pounds gross weight. The dimensions of a trailer differ a lot depending
on the amount and type of cargo it was intended to haul. Nowadays the use of
two single, wider tires called super singles on each axle is used. The
advantages of this are reduction of fuel consumption from less energy wasted
flexing tires side walls, the lighter tire weight lets a truck to be loaded
with more freight and the single wheel covers less of the brake unit thereby
reducing the fading of the brake and making the cooling fast. The disadvantage
probably would be the lack of redundancy. In case a tire gets deflated or
destroyed there won’t be another tire fixed to the same hub to sustain the
dynamic stability of the vehicle. With the dual wheels, the remaining tire may
be overloaded but usually it makes the vehicle stopped safely or can be driven
to a repair shop.
Many have accepted
a new innovation known as the skirted trailer for hauling over dimensional
loads. The air moving under the trailer is an aerodynamic drag because the area
between the road and the bottom of the trailer was left open before. When
airflow is block under the trailer the fuel economy would improve. With the
design of the skirted trailer the fuel economy has improved based on a test
made in 2009.
