ROAD TRACTORS The road tractor (or tractor-truck, as it is often called in the United States) was developed from the lorry. It differs from the latter only in having a shorter wheelbase and in that, instead of being provided with a platform or other body for carrying loads, it is fitted with a bolster on the frame over its rear axle which carries the forward end of a semi-trailer through a fifth wheel. Whereas in the conventional lorry nearly all of the useful load is carried on the rear wheels, in a tractor-trailer combination it can be equally divided between the rear wheels of the tractor and the two wheels of the trailer, so that a much greater load can be carried. Moreover, by far the most expensive part of the combi nation is the tractor, and this need not be kept idle during periods of loading and unloading, but can be kept busy all the time, pro viding a sufficient number of trailers is available. When the semi
trailer is being loaded or unloaded its forward end rests on jacks or folding supports with caster wheels. Coupling of the semi-trailer to the tractor is effected automatically by simply backing the tractor up to the trailer, and uncoupling is an equally simple operation. Road tractors for hauling semi-trailers have been built with electric, steam and petrol power-plants.
(P. M. H.)