1.6. Double-decker Bus
These buses are used to transport large
numbers of people having little luggage for
short distances, usually in high-density traf-
fic. The double-decker bus (Fig. 1.11) occupies
the minimum amount of road space. These
vehicles require a stair space for people to
climb up to the upper deck (first floor).
The ground floor of the bus is arranged for
seating and standing provision of the pas- .
sengers. The size and quality of seats are
normally minimal due to short journeys. Visibility for passengers inside the bus is provided
sufficiently so that they can see where they are and where to get off. Most modern buses have
two sets of doors. Passengers can enter through the front side door and pay their fare, and can
disembark by the rear side door. The engine is normally located transversely across the back of
the bus or sometimes longitudinally to one side at the back.
Commercial vehicles used for the transportation of heavy goods are generally referred to as
lorries. These vehicles are grouped into two categories such as rigid trucks and articulated
These vehicles unlike articulated vehicles are constructed to have all the axles attached to
a single chassis frame. A simple truck (Fig. 1.12) has two axles and four wheels. More number
of axles and wheels are added to increase load-carrying capacity.
Classification of a Rigid Truck. The number of wheel hubs and the number of drive axle
hubs classify the rigid trucks as follows :
(a) A four-wheeler (4 x 2) truck with two driving wheels (Fig. 1.12).
(b) A six-wheeler (6 x 4) truck with four driving wheels (Fig. 1.13).
(c) A six-wheeler (6 x 2) truck with two driving wheels.
(d) An eight-wheeler (8 x 4) truck with four driving wheels (Fig. 1.14).
Articulated Tractor and Semi-trailer.
Articulated vehicles (Figs. 1.15 through 1.17) use a tractor unit for providing the propulsive
power and a semi-trailer for carrying the payload. The tractor uses a short rigid chassis and two
or three axles. The front axle carries the steered road-wheels, and the rear axle is the driving
(live) one. The middle axle may either function as an additional drive axle or for dual steering.
The semi-trailer has a long rigid chassis with a single-axle, tandem-axle, or tri-axle layout
at the rear end. All the trailer axles are dead axles. The front end of the trailer chassis is
supported on the rear of the tractor chassis. At this point it is free to swivel about a pivot known
as the fifth wheel coupling.
Fifth-wheel Coupling. The fifth-wheel coupling is the swivel mechanism ‘Fig. 1.15) used
to attach the trailer to the tractor unit. It contains a turntable, fixed to the rear of the tractor
unit, to support the underside front end of the trailer with a kingpin, which pivots between two
Fig. 1.11. Double-decker bus
half jaws. For hitching and unhitching of the trailer and the tractor, the half jaws are moved
either together to secure the kingpin or apart to release it.
Fig. 1.12. Rigid4x2 truck.
Fig. 1.13. Rigid 6×4 truck.
Fig. 1.14. Rigid 8×4 truck.
Classification of Articulated Vehicle. Different sizes of articulated tractor and trailer
are available which can be classified as follows.
(i) Four-wheeler and two-wheel trailer (rigid 4×2 tractor and single-axle 2 articulated
trailer) (Fig. 1.15).
(ii) Six-wheeler tandem-drive-axle tractor and four-wheel trailer (rigid 6×4 tractor and
tandem-axle 4 articulated trailer) (Fig. 1.16).
(in) Six-wheeler dual-steer-axle tractor and six wheel trailer (rigid 6×2 tractor and
tri-axle 6 articulated trailer) (Fig. 1.17).
Articulated Vehicles Compared with Rigid Trucks.
(a) The trailer and tractor units are interchangeable and a tractor can be immediately
coupled to another loaded trailer unit.
(6) Articulated vehicles have much smaller turning circles than rigid trucks of the same
(a) Less traction is available, because only the front end of the semi-trailer is supported
by the tractor.
<6) Tractor-and-trailer assembly have a tendency to jack about the fifth wheel under
certain steered and barking conditions.
(c * The tractor and trailer have only a small degree of pivoting in the vertical plane due
to which they behave unstable over rough ground.
id) Articulated-trailer wheels do not follow the same path as the tractor wheels, and tend
to cut in or across the road while turning a corner.