Passenger and Goods Vehicles
Page 46
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of To-day and To-morrow Experts Review Design Features and Future Possibilities AT the Institution of Civil Engineers last Friday three interesting papers were read, covering private cars. (Mr. Maurice Platt, M.Eng., M.I.Mech.E.), public service vehicles (Mr. D. M. Sinclair, general manager, Birmingham and Midland Motor Omnibus Co., Ltd.), and goods vehicles (Mr. V. W. Pilkington, M.B.E., M.Eng., M.I.Mech.E., director of engineering, Leyland Motors, Ltd.).
Mr. Sinclair introduced his paper with a brief history of the development of the public service vehicle, and said that it was not until 1924 that we had -a chassis specifically designed for passenger-carrying work. The " driver-besideengine " type was introduced in 1926, twin rear wheels were used for the first time in 1928, and, in 1.930, six-cylindered engines, developing 105 13 h.p., were being employed. The foregoing were in vehicles designed and constructed by the • "Midland Red" concern.
In 1935 a major change came about with the introduction of the compression-ignition engine, and, said Mr. Sinclair, chassis had to be modified to deal with the particular characteristics of this prime mover.
The passenger vehicle, as we now know it, he said, is basically the same as in 1939, and is capable of running 100,000 Miles oimore between complete overhauls. The average annual Mileage is 50,000, which tepresents about 2,500 running hours.
Mr. Sinclair Went on to deal, at some length, with the present design position, and sketched the high spots in engines, transmission systems,. brakes, suspension and frames, and concluded with remarks on bodies, equipment, and tyres.
The auth3r was of the opinion that the future holds many major changes in store, and he first referred to larger engines up to at least 10 litres capacity. Supercharged two-stroke units and air-cooling are also poss;bilities, he said.
Crankcases in light alloy in unit construction with the cylinder block, cast-iron crankshafts and camshafts, vacuum or pneumatic types of governor for fuel pumps, and fully automatic hydraulic transmission systems were also envisaged.
Mr. Sinclair considered that there is every likelihood of the hypoid bevel gear rear axle coming to the fore and that the leaf spring will eventually go into honourable retirement, particularly so far as front axles are concerned, in favour of coil springs, hydraulic units, torsion bars or rubber, or, maybe, a combination of these arrangements.
The so-called chassisless method of construction, said Mr. Sinclair, will take the place of the separate body and chassis, and light alloys will be more generally employed.
Goods Vehicle Design
Mr. Pilkington's paper, dealing with goods vehicles, opened with an historical review of road transport.
Concerning lorries carrying up to five tons payload, the author said that, for classification purposes, this type still lies within the definition of a motorcar, for which reason the majority of such vehicles is manufactured by the leading motorcar makers. The power unit, and in some cases the transmission, bears a direct relationship to the pattern used in the heavier types of car manufactured particularly for export.
It is important to note this relationship, he went on. because of the economical means for production of such units. Fr high-quality and high-quantity production, the highest capital value of any plant is generally associated with the engine, and this, therefore; has a major bearing on the range of vehicles offered by certain makers of cars.
Referring to what the author called medium lorries—those coming in the class under three tons with a payload capacity up to about seven tons—he said that, as a general rule, the *hole of the units are specifically designed for their pats ticular application and usage. He pointed out that it is in this range that the compression-ignition engine makes its appearance. It is in the heavy-lorry category, Mr. Pilkington explained, that the most marked change in design features can be seen. In the conventional two-axle design, the maximum gross weight permissible is 12 tons, and as a well-designed machine in this class will weigh slightly over four tons, the net payload can be something in the order of only seven-and-a-half tons. This does not compare favourably with the payload which can be carried on the "medium "-type lorry. It is for this reason that such two-axle vehicles, as a rule, operate with a trailer, when the gross weight of the combination can be 32 tons. The permissible payload can then be up to 18-20 tons.
Variations in the axle arrangements are available, such as a single driving axle with four steered wheels, a single steered axle with a double-drive bogie unit at the rear, and an eight. wheeler which can go to a gross weight of 22 Ions, the payload being about 15 tons
Weight Ratios
After his brief outline of the types of goods vehicle in normal use in this country, the autt or, in dealing with design features, called attention to the relationship between the pay, load and tare weight, which, be said, is in the order of two to one. In the lighter machines it is much higher, probably two to five, and in heavy vehicles it may go down ió one to eight.
Referring to back-axle drives in the heavier machine, Mr. Pilkington said that the worm-drive unit has the feature of being easy to adjust, is quiet-running, and gives the facilities of driving in tandem as with a bogie-type vehicle. This becomes more difficult with the double-reduction gear, and, on grounds of standardization the worm drive appears to be the most popular.
As to the degree of reliability of the oil engine, Mr. Pilkington declared that this is almost astounding. Whereas the best specific consumption figures for a petrol unit lie between 0.55 and 0.6 pint per b.h.p.-hour, the modern oil engine has a specific consumption as low as 0.34 pint per b.h.p.-hour.
The maximum horse-power of the engines fitted range up to about 130, and although the light lorry requires about 10 b.h.p. per ton load, the heavy lorry figure falls as low as 5-6 b.h.p. per ton. This power-weight ratio enables vehicles—in this country at least—to perform satisfactorily.
In touching on brake design, the author pointed out the limitation imposed on drum diameter and width. Because of these restrictive features, there have, he said, been marked improvements in the brake-facing materials utilized. These give immunity from 'fade" under high-temperature conditions, and a stable facing to withstand theshigh pressures now normal.
In concluding his paper, the author foresaw little change in the general design of the goods vehicle for some time to come. One manufacturer has adopted an "engine amidL ships" design, and, whilst it has its advantages, it does, he said, place a limitation on the use of the vehicle, particularly in the case of short-wheelbase models for tipping work.
Sporadic attempts have been made to design vehicles in light alloys, and one maker undertakes to build a vehicle wholly in such metals, the chassis and body being constructed as a unit. Whilst this is very laudable, said Mr. Pilkington, it limits the use of such a vehicle to A specific type.
When considering matters of weight reduction in a chassis, the necessity of providing for high mileages without breakdown must be borne in mind; the actual amount of material to be dealt with is a comparatively small proportion of the gross weight, and, therefore, there is a tendency to deal with materials of which we know the fatigue properties very well, rather than to be spectacular and take a few hundredweight off the weight of the vehicle at the expense of