Commercial Motor Vehicles

The trolley bus is a type of vehicle to which much attention was directed during the years 1918 to 1928 as a means of solving the problems of tramway engineers who were faced with the necessity of replacing worn-out rails and rolling stock It is an electrically propelled vehicle which collects its energy by means of a trolley arm from a pair of overhead cables, instead of a single cable and rail return as used on many tramway systems. It is capable of being steered and brought to the edge of the foot path for picking up and setting down passengers, and in this respect has many advantages over tram-cars, but it lacks one of the chief advantages of motor-bus service in that it must necessarily follow the route of the overhead cable. Unlike a battery-propelled vehicle, however, it may be run at speeds com parable with those of petrol buses, has a high rate of acceleration and low energy cost per bus mile, but at best it is considered only as a vehicle for use in the transition stage from rail-bound tram cars to free-moving motor-buses. (See also INTERNAL COMBUS TION ENGINES; OMNIBUS; MOTOR TRANSPORT, COMMERCIAL.) Until about 192o, all except the lightest classes of commercial motor vehicles were fitted with solid rubber tires, which, to gether with the poor state of the roads at that time, limited speeds to is or at most 20 miles per hour. Development of the so called "giant" pneumatic tires for use on the heavier commercial vehicles began during the World War; their use on stock vehicles gradually extended, and in 1935 all of the commercial trucks and buses turned out by the factories were equipped with them. Av erage speeds were practically trebled by the change, which, of course, was accompanied by a corresponding increase in the horsepower output of powerplants, and it was only after the adoption of pneumatic tires that long-distance commercial transportation over the highways became really practical.

In 1939 pneumatic truck and bus tires were produced in sizes up to 13.5 in. in width. It was customary to equip large trucks and buses with single front tires and dual rear tires of the same size. As usually two-thirds or more of the total weight was car ried on the rear wheels, this tended to equalize the load.

As the mileage of hard-surfaced roads in the United States was increased, the motor truck came into direct competition with the railroads for freight business over longer and longer distances. To cut down the operating cost per ton-mile of transportation, it was necessary to increase the capacities of the trucks, which were generally limited by tire capacities and by the ability of the road surfaces to withstand shock. To extend these limits, six-wheeled vehicles were developed. In the original six-wheeled trucks there were two front steering, and four rear driving wheels. In these trucks the gross load was divided amongst six instead of four wheels, or amongst ten instead of six tires, and the capacity was therefore increased 5o per cent or more.

The "rigid six-wheeler" (as the vehicle described in the pre ceding paragraph is known in England) did not prove as popu lar, however, as the flexible six-wheeler, or the tractor-semi-trailer combination. This latter consists of a four-wheel motor tractor, similar to a motor truck of short wheelbase, which carries a bolster over its rear axle on which the forward end of a semi trailer is supported by means of a fifth wheel. The semi-trailer has wheels at its rear end only. When disconnected from the trac tor, its forward end is supported by props. In 1939 much of the long-distance transportation of freight over the country's high ways was by such combinations. One of their chief advantages was that the tractor, which was by far the most expensive part of the combination, did not need to be idle during periods of loading and unloading, as the trailer could be quickly uncoupled.

Development in Design.

The increase in the speed of com mercial vehicles called for more powerful brakes, and four-wheel brakes were largely adopted. These permitted of stopping in little more than half the distance required with rear-wheel brakes, pro viding the operator was capable of applying them to the point of practically locking the wheels. This, however, was beyond the strength of the average driver, and power brakes were applied on most of the faster commercial vehicles. One form of brake-apply ing mechanism, operated by the vacuum in the inlet manifold of the engine, was known as a booster. Generally the connections were such that the pull exerted on the brake linkage by a piston in a cylinder was directly proportional to the pressure exerted by the operator on the brake pedal. The operator's pressure on the pedal and the air pressure on the piston combined to apply the brakes. These brakes were used more particularly on the lighter types of commercial vehicle, while the heavier types were fitted with air brakes. With a vacuum-brake or an air-brake installa tion it was possible to apply brakes also to the wheels of trailers, and this was required by law in many states. Electromagnetic brakes for application to trailer wheels had also been developed.

Up to 1926 practically all trucks were equipped with four-cylin der engines, but with the demand for increased speed, engines of greater power were necessary, and many manufacturers then adopted the six-cylinder type. Another effect of the demand for increased speed was the adoption of multiple speed transmissions. Up to 192o the larger trucks generally were provided with four forward speeds. But if a truck was to be capable of high speed when light and at the same time capable of carrying a full load under bad traction conditions, it must have a range of gear speeds of about 8 to 1. Such a range could not be adequately covered with only four gear changes, and trucks designed for this wide range of adaptability were often provided with transmissions giving seven forward speeds.