Power-weight

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Some observations on a possible future development

High speed performance and economy means streamlining, and streamlining means more weight, and more weight means less low speed performance and economy; it is all very difficult. The weight question can be met by light alloys and plastics, and there is no doubt that the war will have brought the commercial utility of both much nearer to the mass-production market: but, even allowing for that, there will remain a sufficient number of problems to make it unlikely that we shall see much of either for some little time after the breakdown of hostile relationships.

The merits of lightness are the obvious ones of better acceleration and/or economy. The objections to plastics have so far been no more than the lack of development, and the war has overcome that. The necessary dies arc, however, likely to put the extended use of plastics beyond the financial reach of any but mass-production manufactures for some considerable time. On the other hand, the remarkable development of certain rather secret, extra tough plastics may make it possible to use them for numerous stressed parts where metal has hitherto been considered essential.

The difficulties attendant upon light alloys are more numerous. Not the least lies in their expense, and while their widespread utilisation for belligerent purposes will doubtless reduce their cost, they will still inevitably be more expensive than iron and ordinary steel. Qualified authorities suggest an extra 10% as the likely financial penalty for building a car as much as possible of light alloys.

Aluminium alloys do not readily take to being put in a fire and straightened, should they be so unfortunate as to take a biff. It is therefore essential that any widely used alloy should not lose its tensile strength when heated and strengthened. Nickel-chrome steel and chrome-molybdenum are suitable for chassis and similarly stressed parts, but they are very expensive.

Noise conductivity is another serious drawback to the use of light alloys in motor-cars, though it is not, of course, a consideration in instruments of war. This increased conductivity can be met by careful insulation of the body from the chassis and engine, but this, again, all adds to the expense.

It will therefore be seen that the employment of light alloys in motor-cars is not “quite as easy as all that.”

It is also remarkable how little practical data there is to go upon. Efforts to lighten the power unit go back to 1902. when the 1,000-kg. racing formula brought forth engines of immense size arid lightness, though the actual power-weight ratio was poor. The modern use of aluminium in engines probably dates hack to 1916, when the Marmon Company of America produced a car whose engine had an aluminium cylinder block and cast-iron liners. Pioneers in this country were Napier and A.C. while the last word lies with the Cross rotary engine, which has no liners at all, the special flexible type of piston ring keeping the pistons entirely out of contact with the alloy bores.

But of all light construction the only serious practical examples have been the German racing cars of 1934-9 and the dozen or so all-aluminium motor-cars made in America, to designs by Mr. Pomeroy, senior, during the early 1920s.

The design, as a whole, both of the German and Pomeroy cars was dealt with, from the light alloy angle, by Mr. Pomeroy, junior, in the Motor of September 3rd, 1941, so that there is no point in enlarging on either of them here.

What is interesting however, is that the final Pomeroy machine, having a six-cylinder 4-litre engine and an 11′ 3″ wheelbase, was in all important respects what he himself described as “an aluminised and improved Packard Six.” It is therefore possible to see by direct comparison what saving in weight was effected in different parts of the machine, and the data provided by the following, hitherto unpublished, table is an exceptionally interesting pointer to future possibilities.

The actual performance of the Pomeroy car was a good deal better than the normal Packard, partly because of its lightness and partly because of the additional power resulting from lighter reciprocating parts. But it had an unnecessarily heavy cast-aluminium body, bringing the overall weight to some 27 1/2 cwt., and the power output was only 80 b.h.p., so that figures are not very illuminating, though it is creditable that the petrol consumption averaged 19 m.p.g. It is also noteworthy that all the cars – six-cylinder ones in particular – covered mileages in excess of 50,000 in normal use, without exhibiting any signs of structural weakness.

The chassis weight figures do, however, suggest that it would be perfectly practicable to make a 2-litre, 9′ 3″ wheelbase, light-alloy-cum-plastic streamlined saloon motor-car with an overall weight of 17 or 18 cwt ., as compared with the 24-25 cwt. which is now considered quite good. And even if the engine developed only 80 b.h.p. that would do to make it go quite some.

As to whether this saving will bring about the widespread use of light alloys and plastics in early post-war automobile manufacture anyone can have his own guess; but it will certainly be interesting to see what happens.

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