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TEST DAT., A. REVEAL PROGRESS , RN DESIGN

20th August 1948, Page 44
20th August 1948
Page 44
Page 45
Page 46
Page 44, 20th August 1948 — TEST DAT., A. REVEAL PROGRESS , RN DESIGN
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THE Commercial MotelShow is beginning to occupy the minds of all concerned with the manufacture, distribution and operation of commercial vehicles Although little progress in design was made during the war, most of the vehicles exhibited this year will bear a marked difference from those seen at the last pre-war Show in 1937. Therefore the time is appropriate for a realignment of standards

From the data supplied by the road tests reported in "The Commercial Motor," useful comparative information may be compiled, with the object of arriving at a guide from which to judge various performance characteristics and design tendencies This information has been collated and averaged, and is presented in graph form under these two general headings.

I have not attempted to define a criterion, but to show average standards of vehicles produced since the war. A graph has also been drawn to indicate the performance of pre-war vehicles and provide a comparison with present-day standards. With the aid of these graphs an operator can compare the fuel consumption, acceleration and braking characteristics of various makes and types, and may weigh any superiority or inferiority in these respects against other considerations, with a definite basis to help him in forming an opinion. He can also study certain features of design as opposed to performance, again comparing them with other indications of average tendencies.

Each of the accompanying graphs, which, with the aid of the analysis key, is self-explanatory, shows average results in a specific respect for 16 types of

vehicle. These are indicated by the letters " A " to "P.' These types, it will be observed by reference to the analysis, cover new models which have been developed and tested since the war.

From the comparative graph, it can be seen that general' progress has been made in brake design

Experience gained during the war is reflected in present design, and further assistance has been given to manual effort to provide a greater force at the brake shoes. It is recognized that there is a useful limit to the force which may be applied, and to increase the tate of retardation it has been necessary to improve on previous brake-drum and brake-facing specifications.

As vehicles weighing over 3 tons unladen are likely to be permitted to operate at a maximum speed of 30 m.p.h., the stopping distance of both the light and heavy classes of vehicle should be alike. From the graph, however, it will be seen that the vans and lighter classes have a braking performance comparable with that of a private car. but the braking curve rises when the medium and heavier classes of vehicle are introduced This need not cause consternation, because drivers of the heavier vehicles have due regard to weight when driving "laden," whereas when operating without a load the braking efficiency of a " heavy " is at least equal to that of a lighter machine Perhaps a progressive load-efficiency brake may be seen as a later development. Since 1939 engine capacity in the classes up to 7 tons has remained practically unchanged Acceleration, in some cases, is perhaps slightly slower than in previous models, because of robust chassis construction. Among the " heavies" there is a tendency towards the fitting of larger-capacity engines, thus bringing their acceleration more in line with that of the medium and lighter vehicles. The latest trend in engine design shows that the prospect of the 30 m.p.h. limit for all heavy vehicles is regarded as a foregone conclusion, and the most recent tests have been inade with this prospect in mind. The greater tare weight of all classes is particularly noticeable, especially in the heavier range of vehicles This additional chassis weight, in the case of the medium and heavy vehicles, may be permanent, because cast-iron crankcases in oil engines and heavy-duty differential casings appear to be more in favour on account of their inherent rigidity and correspondingly longer life. There is a general tendency towards higher fuel consumption, according to prewar standards, although notable improvements in the design of petrol engines, especially in the 7-ton range, have shown some advantage in economy. Heavier consumption is an outcome of increased tare weights. It must be appreciated that models tested in the past three years have been rather restricted in class and id number, and it is possible that if all the models to be exhibited this year could be tested and the results incorporated in the graphs, a different story might be told.

To do full justice to engine designers, the graph of -gross ton-miles per gallon does show the recent models in a favourable light. There is again a marked improvement in the figures of the medium range,

especially within the 5-7-ton payload limits. The heavier models of 10-14-ton capacity also show a slightly better result than earlier models.

A comparison of power-to-weight figures reveals a definite bias towards higher ratios for vans and lorries up to 2-ton capacity. In. specific Cases the acceleration

of the vans is faster This, to a large extent, may be because of the introduction of synchromesh and Constant-mesh gearboxes, which permit swifter gear changes to be made. The medium range of oilers of 5-6-ton capacity has a lower ratio than previously, but acceleration is more or less the same.

. Improvements in the design of petrol engines for 7-tonners arc most apparent in the graphs, and the power to weight ratio and acceleration are superior to those of earlier models. With the introduction of larger

engines for the "heavies," the power to weight ratio has become higher, and acceleration shows a corresponding improvement.

There are two schools of thought on the use of larger engines. One side considers that fuel consumption will perhaps suffer, but this belief is disproved by the results of our tests. The large-capacity engine should also show to advantage in maintenance.

When considering the b.h.p.

developed against the gross load of the vehicle, by reason of a higher tare weight, the petrol engines in 4-tonners show a slight disadvantage.

• Because of improvements in design, the lightweight petrol-engined chassis, together with the medium and heavier vehicles, reveal a slight improvement, compared with earlier models.

The output-capacity graph affords a good summary of engine performance, which has advanced appreciably since the war. Independent figures of some of the petrol engines of contemporary models, especially in the case of overhead-valve engines, show a marked improvement. In oil engines, too, particularly those of 7-litre and greater capacity, research conducted since 1939 has been responsible for definite improvements in performance, and in design features making for greater ease of maintenance

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