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MODERN STANDARDS AS

25th August 1939, Page 38
25th August 1939
Page 38
Page 39
Page 40
Page 38, 25th August 1939 — MODERN STANDARDS AS
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BLISHED BY ROAD TESTS

ALREADY the Commercial Motor Show in November next is beginning to occupy the minds of all concerned with the manufacture, distribution and operation of commercial vehicles. The marking point, as it were, of the two-year epoch of our industry is approaching. Therefore, the moment is appropriate for a reappointing of standards, if *to do so be a practicable task.

From the data supplied by the road tests carried out by this paper, a vast amount of useful comparative information may be amassed, and, with the object of arriving at some sort of guide from which to judge various performance characteristics and design tendencies, we have collated and averaged a large number of figures relating to specific matters under these two general headings.

We do not aim-at attempting to define anything in the nature of a criterion, but at presenting representations of average modern standards. Accordingly, an operator can compare the performance of a vehicle, in so far as fuel-consumption, acceleration and braking are concerned, with our figures, and may weigh any superiority or inferiority thus shown against other considerations, with definite bases to help him in forming an opinion. He can also study certain of its features of design, as opposed to performance, again comparing them with our indications of average tendencies.

Each of the accompanying charts shows graphically the average figure, in a specific respect, for 16 types of vehicle. These are indicated by the letters A to P. The types, it will be observed by reference to the Analysis Key, are comprehensive with one outstanding exception. No figures appear for the maximum-load six-wheeler.

We have not carried out a test of this type since a road-test analysis was last published. Presumably this is because no marked developments have taken place in this class that were not represented in another and, in the particular circumstances, more interesting model..

Therefore, for an assessment of performance and design characteristics of vehicles of about 13 tons payload capacity, we must ask our readers to form their own deductions from those of the machines just above and below, or to refer to the earlier analysis which appeared in our issue dated July 17, 1936.

This raises the matter of progress, and, from certain graphs given on these pages, it will be observed that, in some respects, big strides forward have been accom plished. Probably the most notable advance is in braking. It indicates the great effort the manfacturer has made to assist in accident reduction, and coincides, more or less with the introduction of the wedgeactuated brake.

B4 No doubt because there was more scope for improvement among the bigger machines, it is in these classes that the progress is most marked. The theoretical horizontal line, however, has not yet been reached, Fundamentally, it should be possible for all vehicles to have equal retardation, irrespective of weight, and evidence of this is to be found in specific cases, where heavy machines are unusually well equipped, in this respect, and can be brought to rest from 30 m.p.h. in, say, 40 ft., which may be regarded as excellent for any class.

Nevertheless, the curves we have plotted, from the by no means uniform data available, definitely indicate that efficiency at the upper end of the scale of pay-load ratings has increased from 33 per cent. to 47 per cent., and at the lower end from 58 per cent. to 62 per cent. In all cases of taking averages there is a risk that the rue average may be an incorrect valuation of the iorrnal. This risk is, of course, accentuated when the lumber of figures, of which the average is required, is mall. We have inevitably been labouring under this lifficulty in compiling these graphs because our availtble data was limited and, as mentioned in the preceding )aragraph, by no means uniform.

To leave out of the reckoning the one vehicle out of our, say, which is abnormal (possibly for some quite atisfactory reason) and which therefore makes the .verage figure appreciably and misleadingly higher or lower than the normal figure provided by the three orthodox vehicles, is wrong in principle.

However, in considering these charts (the matter applies less in the case of graphs because the smooth curve practically disregards extremes) allowances need to be made on this score. A notable case is in columns A and B in several charts, where A has been influenced in one direction by one or more models and B in the reverse respect.

Turning now to the braking charts we find a stopping distance of 30 ft. for the 6-cwt. class. This is 100 per cent. efficiency, and is not an isolated case. Two vans that passed through our hands for test during the past two years returned this figure.

As they definitely establish the practicability of such retardation, we separated the 6-cwt. class from the 8-cwt. class, instead of grouping them together and taking the average for the lot.

Apart from this, the retardations do not vary widely, but the tendency, indicated more clearly by the curve, is apparent.

Unlike the progress in braking, improvement in acceleration is most marked in the maximum-load 30 m.p.h. class. Because of its popularity, this type has been the subject of more attention by designers than any other—hence this advance. Also, the position has been influenced by the fact that a popular proprietary engine, suitable for 5-tonners and 6-tonners, has an output rather kigher than that of the corresponding units of two years ago.

No great increase would be expected in the deliveryvan classes, where, for constant stopping and starting, acceleration has always been important, nor among the biggest types, because there have been no marked recent developments in the output of large engines.

In our previous analysis we gave petrol-consumption curves, showing pay-load ton-m.p.g. for 1934 and 1936. A considerable improvement was revealed. This year's corresponding curveis almost identical near its ends with that for three years ago, but indicates a further gain in the intermediate stages.

There were, in 1936, insufficient data for the inclusion of an oil-engine curve of any value. This year plenty is available. Indeed, no petrol vehicles of over 81 tons capacity have been tested by us.

Probably the ntost interesting feature of the graph is the clearly defined step-up from petrol to oil in pay-load m.p.g. At the 5-ton mark the mileage of the petrol vehicle is 66.6 per cent. of that of the oiler, whilst, if the lower curve were extended, the percentage would apparently be about 50 at the line for the biggest class.

On the same graph is a curve denoting a current design tendency in connection with the power-to-weight ratios of all ratings of goods vehicles. It differs little from that we plotted an the earlier occasion. There is a slight rise in powering in the big 30 m.p.h. classes, probably due to the same influence as the acceleration increase referred to already.

In view of the limitation on weight imposed here, an upward movement 'constitutes a tribute to the manufacturers concerned. With regard to the absence of change in the big classes, incidental to it is the fact that some ground has been gained recently by the direct-injection unit, which is the more economical type, but slightly inferior in b.h.p. output.

The remaining curves, showing power in relation to piston displacement, are of interest mainly because they clearly reveal a distinguishing characteristic between the two types. When considering the significance, however, of these two curves, it must not be forgotten that the oil engine runs more slowly than its petrol confrere. Not only should r.p.m. be included, in making a com parison, but also torque.

Column 0, in the chart relating to power and displacement, calls for mention. The vehicle, a solitary representative of its class in this case, was a special type —a mechanical horse and semi-trailer. This explains the discrepancy. On the engine output from 2.75 litres the foregoing paragraph has a bearing.

Another special type represented the 1-ton class (column C), viz., an imported machine, small for its rating and powered by an engine of correspondingly low dimensions.

Studying, in conjunction with this chart, that giving ton-m.p.g., one can find an answer to those who hold that economy is promoted by the practice of underpowering chassis. Were this the case, the proportions of the latter chart would be expected to conform inversely far more closely with those of the former. Comparing columns N and 0, for example (9-10-tonners and articulated 10-tonner), the engines are respectively of 6.92 and 2.75 litres, but the consumptions are as close together as 151 and 157 gross ton-m.p.g. So far as consumption is concerned, it seems, from the two charts, that little affects this besides weight and engine type. Admittedly column A, in the m.p.g. chart, is abnormally high and column B too low, but beyond this the petrol and oil columns, respectively, graduate well.

A comment prompted by the acceleration chart, on which are also shown power-to-weight ratios, is the fair degree of uniformity in the relationship of one to the other. No allowance, however, .is made for gear ratio, and this must obviously exert an influence. Where a low b.h,p, per cwt, produces a quick attainment of 30 m.p.h., it is probable that this desirable characteristic is oil-set to some extent by a lower maximum speed.

Finally, we come to the weights chart. In this the percentages of pay-load to gross-weight are the points of principal interest. Although no great progress is evidenced by the chart, achievements are constituted by the fact that oil engines are now commoner, larger tyres are more frequently standardized and margins of safety generally are wider. This is a much more healthy form of development than rating up chassis to freak figures.

Throughout the foregoing we have made no direct reference to cost. Possibly in this factor further explanations of performance discrepa,' rscies_are to be found, but it is more likely to be manifested in matter's of greater complexity not normally assessable by road tests of 70 miles or thereabouts.

It is always difficult to convince prospective buyers why they should spend twice as much on a certain lorry as is being asked by the maker of another apparently similar machine. It is equally difficult to introduce the cost factor into a consideration of performances and tendencies such as this. The answer to the question involved is summed up in two words—maintenance and longevity, and these we are compelled to ignore.

Nevertheless, these graphical representations and specific analyses, imperfect as they are, serve a practical purpose as well as being of general interest and constituting a record of progress.

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