The Porsche 924 Turbo's Rear Suspension

I had just switched off a TV programme called “Did Darwin Get It Wrong?” the other night when it occurred to me that my description of the Porsche 924 Turbo’s rear suspension, in last month’s road-test report, might have been rather ambiguous. So may I make it plain, before someone else points it out, that this excellent sports-coupe has independent rear suspension by semi-trailing arms, using torsion bars as the springing medium, for its transaxle, as indeed I stated on page 471, but it does not use a De Dion back axle?

The De Dion system of carrying the final-drive unit as sprung weight on the chassis, so that jointed drive-shafts to convey the turning motion from the final-drive to the back wheels were required, these wheels being joined by a light tube, dates from the steam-carriages of Amedee Bollee and Trepardoux, but was used on the very early De Dion Bouton cars in order to obviate failure of the “bicycle” tyres of those days under the pounding that a heavy unsprung mass of a conventional back axle would have imposed on them.

As those first De Dions were rear-engined, power-plant, gearbox and final drive were all conveniently mounted on the chassis, ahead of De Dion tube. The De Dion Bouton Company retained this form of back-axle right up to the days of its big V8 cars, but by then the gearbox was in the centre of the chassis. When this form of rear-end was revived for racing cars, historians recalled these ancient De Dions and the name stuck, for this kind of back-axle. Before then the Edwardian-style chain-drive monsters had achieved good handling qualities and a reasonable consumption of tyres because, with chains, the heavy gearbox-cum-differential and final-drive gears were bolted to the chassis frame, the side chains simply driving the rear wheels, which were mounted on a light, dead, beam axle — the first “transaxles”.

There were light-cars with the final-drive and differential-unit made as part of their back axles, such as the celebrated AC and the first Singer Tens, etc. But because such axles were not chassis-mounted, the unsprung weight of them was often all too evident! I suppose technically they too were “transaxles” — the term presumably derives from “transmission-in-the-axle” — but not in the modern sense.

The point is, however, that a transaxle, with the gearbox and final-drive unit on the chassis, does not have to be used in conjunction with a De Dion tube, and it isn’t in the case of the Porsche 924. On the other hand, a De Dion back axle can be and usually is, used with only the final drive differential unit attached to the chassis, because these days the gearbox is normally in unit with the engine. This is how Rover did it on their old 2000 models. In modern times the use of independent rear suspension, in which obviously the final-drive unit has to be on the car’s frame, and a De Dion rear axle, has little to do with easing the load on the tyres but is a matter of improving handling and traction by reducing unsprung weight and attending to the camber-angles assumed by the tyres. This raises the point that although many pundits list the De Dion System with independent rear suspension, in fact because the back wheels are coupled by the De Dion tube, rear wheel movement is not entirely divorced, off-side to near-side, as with true i.r.s.

When Mercedes-Benz went over to “De Dion” back axles for their 1937 W125 and subsequent Grand Prix cars, and Auto-Union adopted this from 1938, it was to improve the handling characteristic from oversteer to understeer, which the drivers of these fast and very powerful cars were better able to control. There are many facets of the design and construction of a De Dion tube and drive, which there is no space to discuss here but which, as the case may be and the draughtman’s pencil run, may or may not follow closely the original pot-joints and tube-linkage of the first De Dion Bouton application.

The late Laurence Pomeroy goes into this pretty thoroughly in his great discourse on the evolution of the Grand Prix racing car. He points out that a “De Dion” axle was probably adopted for early steam-vehicles as convenient to their physical layout, a theory produced by Kent Karslake. He remarks that when Harry Miller made FWD racing cars in 1924 he used a “De Dion” front end, which was later adapted to his rear-drive racing cars but no doubt more as a convenience than from an appreciation of the other advantages to be gained. Auto Union used the system for their Horch production cars in 1935 but it was the use of a “De Dion” type axle by Director Wagner for the fabulous 5.6-litre 646 b.h.p., 166 to 200 m.p.h. W125 Mercedes-Benz GP cars of 1937 that again drew attention to this form of back-axle.

It must be remembered, too, that apart from the above-listed advantages of independent (and De Dion) rear suspension, these layouts enabled softer springing-mediums to be employed, with increased comfort for passenger cars and a further aid to better handling of racing cars. So the point I want to make is that the Porsche 924 has a transaxle (which Porsche claim has merits of its own) but it does not use a De Dion back axle. Indeed, the sophisticated De Dion-type rear-end is now found on Aston Martin V8/Lagonda, some Alfa Romeo and Volvo 343/345 cars, while front engined (and of course rear-engined) Porsches employ the even more sophisticated trans-axle. Whereas ordinary i.r.s. with various springing arrangements and linkages figures on models from Fiat, Skoda, all BMWs, Colt, Datsun, Ford (for the Granada), the big Peugeots, Subaru, all Mercedes-Benz and Rolls-Royce/Bentley cars, the top Opel/Vauxhall models, AC 3000, De Tomaso, all Ferraris, Lamborghini, Maserati, Jaguar, Lotus, TVR, and Lancia Monte Carlo. Class dismiss! — W.B.