MOTOR VEHICLES, COMMERCIAL. Development of commercial motor vehicles—lorries and omnibuses—began during the last decade of the 19th century, at about the same time as that of private motor cars. At first the commercial vehicle did not progress so rapidly, because in its field there was no equivalent to the enthusiasm of the pioneer amateur motorists to sustain the movement. Later on, however, it profited from much of the tech nical progress originally made in connection with private passenger cars, and in a decade and a half it attained a state of development which enabled it to play a part of great importance in the World War. A new chapter in commercial-vehicle history began with the introduction of what at the time was known as the giant pneu matic tire, shortly after the close of the World War. Up to that time motor vehicles had been used almost exclusively in local trans portation, in services for which horses were used otherwise; but with the adoption of pneumatic tires, the limiting speed of the ve hicle was practically trebled, it became suited to use in long-dis tance operations, and it entered into competition with the railways.
Petrol-engined vehicles of the four-wheel type predominate, and the engine usually is mounted over the front axle under a bonnet, hut in order to give the maximum length of body for a given overall length, in some cases makers have adopted what is known as the forward drive, in which the driver is seated along side the engine. The principal disadvantage of this arrangement lies in the engine being less accessible than in the bonnet type, but much ingenuity has been shown in overcoming this, and at least one maker (Pagefield) mounts the radiator. engine and clutch on a sliding sub-frame which may be withdrawn, like a table drawer, the weight meanwhile being partly supported on legs with castor wheels. The question of accessibility is of lesser importance to large users, who can install special appliances for dealing with maintenance, than is the case with the small user, who, having no large service organization behind him, must carry out adjustments as and when they become necessary.
As far back as 1898 Thornycrofts introduced a vehicle consist ing of a short four-wheeled wagon, on which the forward end of a long two-wheeled trailer was superimposed, the complete ve hicle constituting an articulated six-wheeler. It was in advance of its time. Interest in the articulated six-wheeler was revived in the United States about ten years later, where it is known as a tractor and semi-trailer combination, and where it has come into wide use. One of its advantages is that as the semi-trailer carries all of the load and can be readily uncoupled from the tractor, the latter, which is the more expensive unit, need not be idle during loading and unloading periods. Toward the end of the World War the rigid six-wheeler made its appearance, and it has come into wide use for the transportation of heavy loads. Its chief advantages are that driving effort is applied to four wheels in stead of two, and as the two driving axles are carried on a bogie, over which the frame and body are mounted, road shocks are greatly reduced. For instance, when one of the driving wheels passes over an obstacle, the frame and body are lifted only one half the height of the obstacle. It has an additional advantage in that an endless band or track may be fitted over the two driving wheel tires on each side, converting the drive into a creeper track or caterpillar, by which means soft or boggy land may be traversed which would usually be impassable for wheeled vehicles.