Lighter Buses Meet Overseas Needs
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L. J. Cotton Reviews Trend of Design : Importance of Correct Weight Distribution : Advantages and Disadvantages of Underfloor Engines „ LTHOUGH the lighter types
of bus have been built to fit in with the economics of transport in Britain, they have many . potential applications in overseas markets." This observation was made by Mr. L. J. Cotton, technical editor of "The Commercial Motor," in a paper dealing with trends in commercial vehicle design which he read last Monday to members of the Edinburgh Centre of the Institute of Road Transport Engineers.
Weight in vehicle design, he said, followed a vicious spiral. The 10-litre engine which was formerly required for high performance, demanded .a heavy transmission system, 'a heavier frame, larger tyres and more robust steering
gear. Credit was, therefore, due to those makers who had taken steps to put the process in reverse.
The relatively smaller engine used in. the lighter chassis gave an equivalent ,performance with greater economy and without a marked sacrifice of passenger comfort. The appeal of lighter buses to overseas markets would be strong, because in the rapidly expanding road traniport systems abroad, the roads had deteriorated badly.
Guaranteed Markets
Whilst the underfloor-engined passenger chassis was the most popular at present, there would always be markets for the front-mounted vertical-engined 28-35-seater and small buses for use in urban and country areas and for feeder services.
There were, however, certain technical reasons why this type of chassis could not be considered ideal for passenger work. Among these were incorrect weight distribution, lack of accessibility of the engine, encroachment on passenger space by the bonnet, and the restricted size of the door and gangway which it was possible to employ in a forward-entrance body.
In its favour could be mentioned the low frame height, giving an ideal entrance and floor line, and when the driver's compartment was separate from tile saloon, visibility for the driver at night was greatly improved.
To show that the front-mounted engine need not encroach on passenger space, Mr. Cotton referred to an Italian chassis in which the engine was slung horizontally below the front overhang. A' point in favour of this arrangement was that the same type of chassis could be used for both passenger and`goods vehicles, it being possible in the latter case to fit a body longer than normal.
Given a suitable cab, an engine mounted in this way could be more accessible than one carried vertically at the front. As a bus or coach operated in Italy, the vehicle concerned was restricted to a gross running weight of 14 tons, of which weight 39 per cent. was carried by the front axle on E22 tyres.
Mr. Cotton said that horizontal engines had been mounted amidships to increase carrying capacity and pas senger comfort. Designers believed that with the engine well behind the axle, weight distribution would be improved and that 33 per cent. would be supported by the front axle. Operators, however, demanded a frame suitable for a wide, front entrance, which brought the position of the front axle more towards the centre, and in contemporary British designs an average of 43 per cent. of the load was carried by the front axle.
In 15 different makes of British underflnor-engined chassis the proportion of weight carried by the front axle varied from 40.3 to 47 per cent. French designers discovered this in their prototype machines, and by moving the batteries, fuel tank, steering wheel and brake servo-gear forward, they were able to balance the loads carried by the front and rear axles and to use single tyres alt round.
With 40-45 per cent. of the total load on the front axle, said Mr. Cotton, certain braking problems arose. Some braking systems had insufficient initial power to give a maximum efficiency at the front wheels.
Power Steering
The rate of wear on the front-brake facings was high and the steering effort required was fast reaching a point where power assistance was becoming necessary. The introduction of the lightweight chassis had overcome many of these problems.
The rear-cngined passenger vehicle offered the advantage of correct weight distribution with twin tyres at the rear. It had a short transmission line, the engine and transmission components were grouped in accessible positions and, provided that the bodybuilder exercised care in design, less noise was heard by the passengers and driver.
Reviewing the possibility of using the underfloor engine in goods vehicles, Mr. Cotton said that there must be no components above the engine which might be subject to failure or require attention, because it would not be prac
ticable to move the load to reach them. If the engine were mounted between the axles in a chassis of a short-wheelbase tipper or tractor, other difficulties might be experienced because of the short transmission line.
To preserve engine life when the engine was mounted under the floor. greater care was needed in filtering the intake air. Sometimes cooling presented a problem, because it was known that. with certain types of body a partial vacuum was created behind the front panel which caused a reversal of air flow through the radiator.
In Continental designs, great trouble was taken to ensure a cool, clean air supply to the engine intake, and means for regulating the temperature of the cooling water were provided in the form either of a manually controlled blind with automatic shutter control or an electrically controlled fan.
Automatic transmission systems, which were found in most American coach anclbus chassis, were not popular in this country. Chassis exhibited at Earls Court showed little change from standard practice and the only noticeable trend was a swing towards the conventional constant-mesh gearbox with hydraulically Operated clutch. There was a general tendency to dispense with the worm-driven axle in favour of the hypoid-bevel or double-reduction spurbevel drive.
Develop Disc Brakes
Mr. Cotton thought that -British manufacturers should develop the disc brake as quickly as possible. The drum brake, he said, was fast reaching its limit of constant efficiency.
Because there was a limit to the size of front-wheel operating cylinders, the triple-servo-vacuum system Was known to possess certain disadvantages for application to underfloor-engined chassis, and the lightweight compressedair system with diaphragm-operated shoe units might provide the best means.
Looking into the future, Mr. Cotton said that in the 30-cwt.-2-ton group of vehicles there was a possibility of the oil engine supplanting the petrol unit, and as a long-term prophecy he thought that the automatic transmission might be included. With the possibility of a low-priced oil engine being introduced during the next two years, compression. ignition engines might become more widely used in the 5-ton class.
Were the 20 m.p.h. limit relaxed, there might be additional makes of oilengined 7-8-tonner, alterations were likely in braking systems, and there would be a greater trend towards multispeed transmission systems.
The trend towards lighter passenger vehicles was almost certain to continue, although not to the extinction of the heavier types for city operation and for export. Mr. Cotton was doubtful whether the crush-load single-decker would ever find general favour in this country.