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An Analysis of
Page 90
Page 91
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GOODS -VEHICLE PERFORMANCE
Unique Data in Respect of Various Classes of Chassis. Summarized Information Obtained from our Series of Comprehensive Road Tests
I
N our Special Passenger Vehicle
Number we gave an analysis of the
performances of modern passenger vehicles which had been subjected to the standard road test of The Commercial Motor. On the whole, it was found that the performances in any particular class were of equal value within fairly narrow limits.
In the case of goods vehicles the difference between one make and another in the same rated load capacity is much more marked. This is largely due to the fact that there are calls for more types. In one case a moderate average speed is all that is needed, but low fuel consumption may be of vital importance. On the other hand, a machine having rapid acceleration, a high maximum speed and braking considetably above the average are required for special work.
Special High-speed Types.
Generally speaking, the modern goods vehicle can meet all demands as regards average speed, but special types are produced for those users who pay more attention to high average than to considerations of the legal maxima.
No factor is more vital to the goods-vehicle operator than fuel consumption. We reproduce on this page an unusually interesting chart showing average fuel consumptions for various classes of vehicle. These c28 _. include oil-engined machines of 6 tons and 10 tons. On the whole, it is found that compression-ignition engines effect a volumetric economy from about 80 per cent,, ranging up to so much as 120 per cent. These figures ignore lubricating oil which is burnt as fuel.
It will be noted from the chart how the rate of increase of fuel used per mile rises in steps according to the pay-load capacity. One point stands out pre-eminently after an inspection of individual results, viz., that it is not necessarily costly in fuel to use a large engine—frequently the reverse is the case—partieularly in hilly districts, where it is necessary to have frequent recourse to lower gears when the engine is small in relation to the load it has to handle.
As an indication of the differences which exist in connection with fuelconsumption rates it is interesting to note that in many classes there are variations as great as 13 per cent. above or below the average for the category.
As exemplifying the view that it is cheaper to haul than to carry, the results obtained by the Beardmore tractor-trailer outfit should be noted in the fuel-use chart which is reproduced.
Braking Facts.
To-day no subject is of greater importance than braking. The results of our many tests disclose some
interesting facts. Generally speaking, from the point of view of stopping distance, two-wheel braking can be almost as efficient as four-wheel braking, provided the roads are dry ; but on wet surfaces, or rough ones—when wheel bouncing occurs—four-wheel braking is definitely superior and tends to greater safety.
Vehicles from 5-cwt. up to 5ton pay-load are, within close limits, fairly equal as regards braking efficiency. Vehicles carrying greater loads show a tendency to require increased distances in which to pull
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up. The graphs show that large steam wagons are not so rapidly braked at low speeds as petrol vehicles, but the braking improves in the higher speed ranges. "
With regard to rigid six-wheelers there are instances where considerable improvement is required, particularly when a trailing axle is adopted. This appears to be due largely to the effect of variation of bogie wheelbase upon the brakeoperating mechanism. This remark is not a general one, as there are instances in which a multi-wheeler equals the four-wheeler or is even better.
An interesting fact is that the mechanical booster scores at low speeds, thus showing that ordinary mechanical operation allows an appreciable time lag. Good results have been obtained with one of these devices on a Unic 2i-tonner, as will be seen in the appropriate chart At high speeds a servo device shows to the best advantage. There are still a few instances in which the leverages are unsuitable for braking by physical effort if the engine should stop, thus making inoperative the vacuum servo, but this trouble has, in the vast majority of cases, been overcome.
Where self-energizing 'shoes are employed it is necessary, as a rule, for the driver to give a final additional pressure on the pedal just before the vehicle comes to rest, otherwist there is a tendency for the braking efficiency rapidly to decrease below about 5 m.p.h.
Speed Capabilities.
Turning now to the question of speed and acceleration, reference should be made to the appropriate series of curves. In general, vehicles up to 3-ton pay-load capacity are capable of well over 40 m.p.h. and several of the lighter machines can exceed 50 m.p.h., as can such highspeed types as the Garner and Reo 31-tonners. All these machines maintain their rate of acceleration fairly consistently up to about 32 m.p.h., whilst larger petrol-engined types gather 'Speed in an even manner up to about 26 m.p.h. Large steam vehicles accelerate steadily up to 20 m.p.h., but only slowly above that speed. " It is important to note that in cases in which the speed is purposely restricted the maintenance of a good average is not appreciably affected. There is mach to be said for the principle of governing the maximum speed to about 70 per cent. above the legal figure. This gives adequate performance without allowing " flatout " driving which is a common cause of rapid wear. This scheme is adopted with success on the Studebaker Pierce-Arrow 2-toie•chassis.
If the recently introduced speed limits be enforced consistently there is little doubt that engines will tend to revert to the type which develops its maximum torque rather lower in the speed range than has lately been the practice. The flexibility and economy evinced by a power unit of the medium-speed variety are well suited to goods-carrying work. The fewer the gear-changes that have to be effected the better.
With modern designs of clutch and gear-change mechan'sm the limiting factor in the speed of change is usually the weight of the flywheel, particularly with oil engines. A minor improvement which in some cases might be arranged is a clutch stop, which, on coming into action, gives warning of its presence by adding slightly to the pressure required upon the pedal. It is often difficult to tell whether the pedal has been pressed down far enough to free the clutch when changing down without bringing into action the unwanted stoP.
As to choice of indirect reties, this matter is usually satisfactory, but with engines of the high-speed variety there is occasionally a tendency towards the use of too high a third gear, thus demanding too early a change down to second gear on hills. As optional direct-drive ratios are usuallY available the purchaser can suit his requirements with ease.
The modern tendency towards making the first gear one for real emergency is good, the driver normally only employing second, third and top ; up-to-date clutches are generally well up to the work involved. Hill-climbing ability in the larger categories is usually well up to industrial requirements, but more engine power is desirable in the case of some 1-ton and 30-cwt. machines. These are to-day often employed over very poor surfaces in hilly districts, and optional engines might, therefore, be offered as standard.
Four-wheel Drive.
While on the question of effective tractive effort the marked advantages of four-wheel drive for work on bad surfaces or very steep hills became apparent in our recently published test report on the Hardy 4/4 model. This principle not only vastly extends the scope of a lorry, but reduces the risk of final-drive failure.
Concerning suspension, this has almost invariably proved to be as good as can reasonably be expected without recourse to individual springing of the wheel. In the 30-35cwt. class there are instances of a tendency towards rolling ; this might be obviated by the employment of a wider "spring-base," by the use of auxiliary springs which come into action at two-thirds maximum load or by the employment of rebound dampers.
In the past five years steering systems have shown a marked improvement ; except in the case of the use of an unsuitable ratio between the column and drop-arm shaft—only one instance has occurred—the driver's task is made as easy as the gross weight permits.
Ease of engine starting is uni. formly good, but large compression ignition units which rely upon elec tric power call for the general adoption of a 24-volt circuit ; for the haulier the ability to start without electric aid is of considerable advantage, as his battery-maintenance facilities are usually not so extensive as those of the passenger-fleet operator.
Those interested in the perform ance of goods vehicles of any type can assess the merits of any particular model by comparison of the figures in respect of it with those given in the accompanying graphs. The information is set forth in a manner which is not readily obtain able elsewhere.