5-ton Maximum-load Single deckers Practicable
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Makers and Operators Have Little Interest in Integral Construction : Present Suspension Systems Regarded as Satisfactory A FTER describing the Nuneaton PA proving ground as an invaluable background which British manufacturers could now enjoy, Mr. Durrant said that, by strain-gauge analysis, it had enabled them for the first time to discover exactly how the stresses and strains of a vehicle structure were distributed. He had previously referred to the importance of weight reduction in 30-ft. by 8-ft. single-deckers, which was the type selected for comment, and he made suggestions why the target of an unladen weight of 5 tons or under was reasonable.
These included mention ot a British development in light-alloy integral construction, using a 5-litre mass-produced petrol engine, which had shown that a full-size bus could be built to weigh well under 5 tons. Such a vehicle could not withstand the impact of shunting, as practised in America, but fuel, lubricant and tyre costs were reduced by 10 to 20 per cent, and there was no reason to suppose that maintenance and life were seriously jeopardized Little Enthusiasm Mr. Durrant referred to questionnaires sent to 20 manufacturers in seven countries and 58 operators in 16 countries and said it was surprising that the replies had shown such little enthusiasm for integral construction.
The replies had also indicated a complete lack of enthusiasm for constructing the main framework with fabricated sections bolted together The advantages of this method, suggested Mr. Durrant, had not been fully appreciated, especially regarding the aspects of standardization, interchangeability and the employment of unskilled labour.
There was general agreement that the roof should carry the load and that the structure should be reinforced by stressed panels. Structures with stressed inner panels and readily detachable exterior panels had been approved almost unanimously by both operators and manufacturers. Improving aceess and lowering floor levels, said the author, were subjects that needed special attention. When reviewing the disadvantages of the leaf spring “ the large lump of metal we have inherited from the horse cart" —Mr. Durrant included the criticisms that the cost of maintenance was probably as high as the cost of engine overhauls, that replacement required great physical effort and, with regard to design, that the leaf spring was insensitive to small movements, restricted the turning circle if the base were normal and necessitated a very solid front axle.
Operators' opinions on suspension favoured the reduction of unsprung weight, but the only practical concession in this direction was the use of smaller wheels and low tyre pressures. Manufacturers considered present suspension arrangements, subiect to unsprung weight reduction, as satisfactory for the future.
Mr. Durrant asked whether the successful use of rubber suspension in the U.S.A. could be ignored, whether coils or torsion bars of steel would be used and whether springing could be performed by the medium of hydraulic power and compressed air. He said that it was necessary to eliminate long laminated springs and massive rigid axles, as these, combined with the benefits of smaller wheels and lighter brake gear, would reduce unsprung weight, and improve suspension characteristics.
Other advantages would be a lower floor level and reduced wheel-arches, improved stability and the bus would be self-trimming, so that the step height and riding qualities were constant, irrespective of load.
Braking problems would be simplified by the reduced weight and, according to present indications, the drum brake would give way to one of the non-fade disc type. Furthermore, the electric brake was already well established as a means for relieving the friction brake. There was even a visionary prospect of the development of a one-piece rubberand-flexible-steel tyre for bolting direct on the hub.
Differential on Vehicle?
The question was asked by the author whether the time might not be far distant when the differential unit and bevel gear would be mounted on the vehicle itself. In addition to reducing unsprung weight, this would also permit of a lower gangway. There appeared to be no reason why the gearbox should not be combined with the final drive.
With regard to steering developments, Mr. Durrant said that unless steering effort were reduced from the present figure of between 10 and 20 lb. to a maximum of, say, 3 lb., it would appear that power steering was inevitable. Modern designs had multiplied points of friction and wear, and it was necessary to ease the load on the "weaker limb." It was a pointer that several American car makers had standardized power steering.
As to the means for application, thequestionnaire had revealed the logical preference for mounting at wheel level in place of mounting at the steering column. One could speculate, said Mr. Durrant, on the possibility of using shafts with constant-velocity joints between the wheel and a rack-andpinion gear at hub level.
If the rigid axle were discarded a gear with only four joints subject to friction and play could be used. This simplification should lead to longer periods between overhauls, easier maintenance and space saving. Continuous 'lubrication should take care of friction and wear.
Future of Turbine
Mr. Durrant said that it was of interest that both the President of the British S.M.M. and T. and the chairman of the Board of Transportation of New York City had expressed their faith in the gas turbine, which was only about a quarter of the weight of the piston engine and the other parts it replaced. It was expected that the use of the turbine would enable maintenance costs to be reduced and the floor level and gangway height to be lowered.
In the meantime the development of the oil engine must be continued. Neither operators nor manufacturers seemed prepared to make any substantial concession to weight reduction by employing a small engine with low torque and a multiplicity of gear ratios. They were also opposed to gaining extra power by making use of the two-stroke cycle or supercharging, but were in favour of the " square " engine.
Operators had shown considerable interest in dry-sump lubrication, but doubt had been expressed by manufac turers about it. Both considered it necessary to match engine torque characteristics to road speed.
Automatic Transmission The author said that the popularity of planetary gears increased as the stops per mile increased to five. Operators favoured preselection and manufacturers automatic control, but both agreed that an automatic transmission should be provided with an overriding manual control.
Electric transmission was out of favour, the hydraulic torque converter was popular, but criticism was implied of its range, and there was a demand for an automatic step-ratio gearbox with overriding control.
The economies resulting from the long life and reliability of a planetary transmission were, said Mr. Durrant, such as to warrant its use irrespective of its other qualities. After tracing the history of the Wilson gearbox, it was stated that the box could be adapted to contain a built-in overdrive as part of the main running gear.
B20 In a given bus the use of a planetary gearbox enabled a smaller and lighter engine to be employed. The small loss in efficiency of the fluid coupling tended to detract a little from its appeal, but what appeared to be an effective and reliable form of centrifugal clutch which would eliminate fuel loss, had been developed.
It was surprising, said Mr. Durrant, that in the light of the better figures given by British double-deckers, so many operators were content to obtain less than 100,000 miles from engines and transmissions. The conservative expectations of engine life might, in some cases, be the result of using the indirect-injection type of engine. British experience had demonstrated that it was inferior to the direct-injection unit in so far_ as engine life was concerned.
It was a reasonable expectation that developments in direct-injection engines would lead to mileages of 150,000 to 200,000 being obtained and that planetary gearboxes would achieve over 200,000 miles.
In a survey of double-decker design, the author said that the way was now open to approach the double-decker as a subject for integral light-alloy construction, incorporating the experience gained with strain-gauge technique. Advanced methods of orthodox chassis and body construction had already led to a reduction in weight of 2 tons, and a reasonable target for the unladen weight of a 60-64-seat lightweight double-decker would appear to be under 6 tons. There was no advantage in the underfloor engine for rear-entrance double-deck buses.
The extra emphasis on low gangways demanded a still more compact bevel box, even to the extent of doublereduction with epicydic gears in the hubs. The British Lodekka design showed how a really low floor level could be achieved by eliminating the central transmission shaft and the normal type of back axle. The overall height of 13 ft. 2 in. could be regarded as the ultimate in low height for a double-decker.
Standardization Needed
In his conclusions, Mr. Durrant said that the conception of the future bus that had been envisaged would remain visionary unless operating authorities supported large-scale standardization. Individuality inevitably denied the manufacturers the opportunity to standardize design with the prospect of quantity production at competitive prices. Operators should get together and tell them what they wanted.
In Britain, certain large operators were also manufacturers, or were so big that they could specify their precise requirements when ordering vehicles. This brought the operating, maintenance, design and manufacturing experts together with beneficial results to the product and the prospects of future development.