A New Recipe for Roadholding

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Signalman D. C. Hings sends the following suggestion for a six-wheeled racing car, following intelligent observation of Army vehicles. He is at the mercy of our technical experts and it will be most interesting to learn how his idea is received. – Ed.

This article is intended to give “special’ builders something to think about for the next Shelsley Walsh. The author, being at present rather out of touch with the motoring world, is, however, not at all sure whether competitions committees would approve of the proposed vehicle. Three-wheeled cars are barred, no doubt, because they come under a different class of taxation, but is there anything to prevent a six-wheeler being entered? Anyway, if this article provides only a few minutes’ thought or discussion among enthusiasts it will have achieved its purpose.

Having watched the amazing cornering abilities of the standard American General Motors six-wheeled truck when handled by a particularly reckless d***ie driver, it is felt that perhaps there is something to interest us in this chassis from the point of view of roadholding. From a purely theoretical point of view it would seem that there are three possible advantages.

The first is that on any surface which is not dead smooth (and what road or track is?) after hitting a bump even the best suspended wheel loses adhesion and may become “airborne.” Using twin rear axles with the usual cantilever-type springing, when the forward wheel encounters the bump, the shock is transferred to the rearmost axle as increased downward pressure, as well as being absorbed by the spring itself. It should perhaps be pointed out here that the spring is not fixed rigidly to the chassis, but is free to rock on its mounting. While the forward wheel is airborne, due to its inertia, the spring, in returning to its normal shape, maintains this extra pressure on the rearmost wheel and increases its adhesion. Naturally, the same action takes place when the rearmost wheel encounters the same bump, increasing the adhesion of the forward wheel. Thus we have the admirable state of affairs whereby quite severe bumps have little effect on rear-end adhesion, although in practice where there are many little irregularities, some adhesion will be lost. Also, when one wheel traverses a hollow in the road the action is not quite the same, but at least 50 per cent. of the original weight is still applied to the road. Obviously both rear axles must be mounted so as to reduce lateral movement to a minimum, to give good results. It would seem, therefore, that twin rear axles might Materially improve adhesion on a bad surface.

The second advantage also brings up the main snag, which is increased overall weight. However, as each rear axle will have to stand up to only about half the stresses and strains of a conventional single axle, each can be of considerably lighter construction. This is where the advantage as regards roadholding comes in, for each axle will have a lower unsprung weight than the conventional single one, with all the benefits to roadholding that this will bring about. If, therefore, each axle considered alone is better than a single one, surely with two such axles the advantage is doubled? Another point which may not be generally realised is that the cantilever spring further improves the unsprung weight in comparison with the normal 1/2-elliptic. With the latter, almost the whole mass of the spring is fixed to, and moves with, the axle. With the cantilever, and for that matter, the 1/4-elliptic spring, the heaviest part of the spring is fixed to the chassis. An indirect benefit also gained from this design is that there should be less likelihood of the chassis distorting, because, on one wheel hitting a bump, the shock applied to the chassis is only half that which would be experienced with a single rear axle. It should, therefore, be possible to employ a somewhat lighter chassis, this factor balancing out to some extent the extra weight of the additional axle and wheels, so far as total weight is concerned.

The third advantage is related to tyre distortion when cornering, which causes sideways movement of the car, away from the centre of the turning circle.. This effect is not due to skidding but is a factor depending on tyre pressure and the tyre itself. Under centrifugal force due to the car cornering, the tyre tread in contact with the road moves in a different plane from that of the wheel itself, causing the sideways movement mentioned above. This is usually measured as the angle between the path of the wheel at any instant when cornering, and its plane of rotation. This angle is known as the “slip angle.”

It is usually desirable for a car to have under-steering properties, which means that the rear slip angle must be smaller than that at the front wheels. With the proposed twin rear axles, the sideways thrust when cornering will be shared by four tyres instead of two, thus decreasing the distortion and slip angle at the rear end, assuming the same tyre pressures as used with a single axle. Twin rear axles thus increase the under-steering properties, which is desirable if not overdone. On the other hand, if it was not necessary to reduce the under-steering effect, lower tyre pressures could be used at the rear, giving better adhesion under certain conditions for braking and accelerating. It also seems possible that the magnitude of this slip angle has some connection with the point at which a skid starts, so our twin rear axles, by lessening tyre distortion and slip angle, might be less prone to skid. This seems to be borne out by observation of the aforementioned C.M.C. truck which, at least, appears to get round corners at speeds which would send similar four-wheeled vehicles off the road.

So far, only the conventional twin real axle layout has been considered. Its sole purpose on motor lorries is to enable greater loads to be carried, not to enable them to “dice” round corners. We are, however, concerned only with the latter, so we could probably improve the design for our purposes in the light of existing knowledge on the subject of roadholding. The best system, where low unsprung weight is most necessary, would employ two De Dion-type axles at the rear. The cantilever springing system appears to be better than independent suspension for these four wheels, as the latter would eliminate the first advantage mentioned, namely, the transfer of weight from one axle to the other on rough surfaces. Some problems may arise in the design of the drive shafts and universals, as the wheels will move to a greater degree relative to the chassis with this articulated suspension than with a single axle, unless movement is restricted by design. Another factor to be considered is weight distribution; it will obviously be necessary to move the weight towards the rear so that the loading on front and rear wheels is approximately equal, or as desired. Thus mounting the engine in the centre of the chassis, behind the driver, seems to be the ideal.

The drawing shows the suggested rear suspension together with the relative positions of the main components. The cantilever springs are held by U-bolts to bearings on extensions of a tubular chassis cross-member between the rear axles, so that they are free to rock. The beam axles are mounted a la Frazer-Nash (front) with two friction-type shock-absorbers forming the lower radius arms. De Dion-type axles being used, the differentials are mounted on the chassis cross-members. The forward differential, gearbox, and engine could be one unit. As there is no propeller-shaft in the way, the driving seat may be even lower than shown, giving minimum overall height. Lancia-type front suspension is suggested, although not shown. There is plenty of room behind the engine for fuel, oil, and water header tanks.

It does seem, therefore, that a six-wheeled car might have quite a considerable advantage over its four-wheeled counterpart. Of course, all the above theories could be applied to the front end as well, but perhaps that would be too much of a good thing! The author is wondering whether an Auto-Union might be found under a haystack somewhere over the Rhine, as he feels that such a car, modified as above, might do great things.