The demand for greater loads and increased speeds has neces sitated better braking facilities. These have been secured by fit ting brakes to front as well as rear wheels, or by the addition of power-multiplying devices, operated by mechanical, hydraulic or vacuum means, or by a combination of both methods.
For operations in con gested areas, the acceleration of a vehicle, that is, the rate at which it can get up speed, is of importance. Following are average times required by modern British lorries to reach a speed of 30 m.p.h. from a standstill, with full load: i ton pay-load, 17sec.; 3 tons, 25sec.; 5 tons, 32sec., Ic) tons, 48 seconds. In 1938, Brit ish lorries of different pay-load ratings could be brought to a stop from a speed of 3o m.p.h. on hard, level pavement in the fol lowing average distances: I ton, soft. ; 2 tons, 52ft.; 5 tons, 57ft.; 10 tons, 62 feet. For the o-ton lorry, at least, the legal speed limit is 20 m.p.h., and from that speed it could be stopped in 27.5 feet. The fuel economy of a commercial vehicle usually is ex pressed in gross ton-miles per gallon of fuel consumed. This item usually increases with the size of the vehicle, chiefly because in the larger vehicles the engines are more heavily loaded at normal speed, and therefore operate more efficiently; but also because the larger vehicles are not so fast, and therefore require less power to overcome air resistance. In the case of petrol ve hicles the economy varies from about 4o gross ton-miles per Im perial gallon for 1-ton vans, to i oo ton-miles for 8-ton lorries. Oil-engined lorries of 5 tons pay-load rating and over may be expected to show a fuel economy of around 170 gross ton-miles per Imperial gallon. The foregoing performance figures are based on the results of numerous tests conducted on British lorries by the staff of The Commercial Motor.
The spring-suspension system presents great difficulties because of the widely varying loads which have to be dealt with. To meet these and to ensure smooth riding, springs are generally made of semi-elliptic shape and of considerable length and breadth. The combined length of one front and one rear spring in the case of a large passenger vehicle is rarely less than 65% of the wheel base. Springs must of necessity be made comparatively stiff and of small camber in order to avoid wide differences in platform height be tween the laden and unladen conditions, and to minimize these dif ferences auxiliary springs or rubber buffers are frequently fitted.
Wagons.—Great Britain is the only country in the world in which the manufacture of steam-propelled commercial vehicles ever attained considerable proportions. A fleet of steam omnibuses was in service in London during the decade 1910-20, and heavy steam lorries were produced in considerable numbers until the early 193os, but thereafter the rate of production de clined rapidly, as indicated by the fact that the number of new registrations of steam wagons in Great Britain decreased from 423 in 193o to 8 in 1938. The steam wagon used solid fuel, and its fuel cost was low, which enabled it to compete on even terms with the petrol lorry. About 193o the high-speed, heavy-oil (Diesel) engine became available for use on motor lorries and omnibuses. It uses gas oil as fuel, which at the time cost only about one half as much per gallon as petrol, and the consumption of the engine was only about 6o% as much as that of the petrol engine for the same power output. Hence the fuel cost of internal-combustion engines was greatly reduced, and several firms which had been pro ducing steam wagons turned to the manufacture of Diesel-engined lorries.
Electric vehicles which depend upon a storage battery for the source of current, although limited in their sphere of action by the battery capacity, have been developed and used to a considerable extent. There appears to be no standard form of chassis design, as is the case with petrol ve hicles, neither is there any agreement on the relative merits of transmission by one or two motors. The motors, however, are usually of the series-wound type, driving the road wheels through spur gearing or by chains. The battery capacity ranges from 3o to 35 miles per charge, and the maximum speed is 14 to 15 m.p.h. for small vehicles, and 9 to Io m.p.h. for the heavier type, the restricted range and low speeds necessarily limiting the scope of application to such purposes as factory and railway platform trucks, municipal vehicles and town delivery vans.