I have read Sqd.1Ldr. Marris’s article, “Speculations on Fast Car Suspension,” with great pleasure and interest, and must express my complete agreement with your description of it as a masterly survey.
It is important to emphasise that the characteristics of a car as regards. overor under-steering are finally dependent upon the tyres themselves and the conditions in which they operate. The outstanding example of this, of course, is the way in which the handling characteristics of a car are completely altered by a partially deflated tyre. The front and rear tyres of a G.P. car differ normally in loading, section and pressure. I should expect that the greater rear-wheel loading (50 per cent. in the case of the Mercedes) and larger section tyres would give an over-steering tendency which would reduce by a large amount the understeering tendency inherent in the suspension system.
It would be very interesting to know whether the tyre manufacturer can control the slip angle by varying the construction without affecting other properties of the tyre. The reason behind the Michelin broad base tyre, which was fitted to immediately pre-war Citroen and Peugeot models, is presumably to reduce the slip angle without decreasing the resilience. The position of the roll centre for any independent suspension is quite easily determined graphically from the layout of the linkage, as it is at the inter-section of the two lines drawn normal (i.e., at right angles) to the line of motion of the point of contact between the wheel and the road. This explains why reducing the tyre scrub lowers the roll centre, as reducing tyre scrub means that the wheel moves more nearly at right angles to the road and therefore the line normal to it lies at a more acute angle to the road surface. It also follows that when equal transverse links are used, the roll centre is only on ground level when the links are parallel to the ground, and that when
the links are very short, as on the early Mercedes G.P. cars, the roll centre must change in position very appreciably with wheel deflection.
I disagree with Sqd./Ldr. Marris’s suggestion that the distortion that takes place in Porsche suspension arms is mainly twisting, as, due to the overturning couple on the wheel, together with the side thrust, the horizontal loading on one link can very considerably exceed the vertical load. If the distance between the two arms is half the radius of the wheel, which is roughly the proportion on the E.R.A., the horizontal thrust in one arm will be two-and-a-half times that at the road surface. Also, with the steering layout normally adopted, horizontal distortion will affect the directional stability, whereas vertical distortion will only result in additional wheel deflection in bump. The steering can be laid out so that it is not so affected, but only at the expense of simplicity. I imagine that to obtain the rigidity necessary in a racing car, it is essential that these arms be as short as possible, as otherwise it would be difficult to make them sufficiently stiff, since they are splayed out as well as extending rearward to the hub.
For these reasons, I am inclined to think that the later Mercedes system has more of a future. This, it will be remembered, consists of two equal transverse wishbone links of pressed steel, which are ideally situated to cope with transverse loading. Incidentally, the links are some 10 in. to 12 in. long and can therefore hardly be described as being short: The unsprung weight of the Mercedes could be slightly reduced by ball jointing the link ends directly to the stub axle flange, as is done on the Citroen and the Porsche, so eliminating the king pin and its attendant parts. When considering rear suspension, it must be remembered that any axled system, whether of normal or De Dion type, can cause underor over-steer quite apart from that arising from tyre slip, due to the slewing of the axle which occurs with rolling owing to the finite length of the radius rods. With springs or radius rods anchored in front of the axle, this will cause a tendency to oversteer. It may be that the reason for the success of the Bugatti reversed quarter
elliptic layout lies in its tendency to give under-steer.
As a matter of history, I have always understood that the earlier Mercedes cars had swing-axle suspension, and that the De Dion axle was not adopted until the last year of the “under 750 kilo.” cars.
I have always been rather curious to know how Auto-Union have arranged their De Dion axle. Presumably the beam passes over the final drive casing, as there would be insufficient clearance underneath, and to pass it behind as on the Mercedes, would mean a very large bow in it as the gearbox is mounted behind the final drive.
There is a certain amount of doubt in my mind as to whether the German G.P. cars were originally designed as a result of a complete understandmg of the factors involved. If they were, the German technicians kept their findings very much to themselves, as a very large part of our present knowledge of these problems has come from research carried out in America in the middle ‘thirties.
In this connection it is easy to put forward other reasons than that of increasing the moment of inertia for the Mercedes chassis layout. For instance, a saving of weight comes about by combining the gearbox and final drive, and a roomier cockpit results. In any case, the weight of the gearbox is not very large compared with that of the 40-odd gallons of fuel at the rear end. The engine could not be moved forward until i.f.s. was adopted, and its forward position may have been to get the maximum of machinery into the minimum of chassis, and also to counterbalance the weight of fuel at the rear.
In any case, I am not convinced that the moment yf inertia should be increased in a vehicle, part of whose raison d’etre is to go round corners quickly. The Auto-Union was as fast as the Mercedes, although its moment of inertia must be less rather than more than that resulting from the normal layout as the main masses (the engine, the fuel and the driver) are all concentrated centrally. I am, Yours, etc.,
R.E.M.E. J. S. MOON (Capt.).