The missing link, or just missing a link. We consider the much maligned swing axle.
The swing axle is the first stop when considering how to make the movement of two rear wheels, previously attached to a solid axle, independent of each other. Simply pivot the shafts either side of the differential and have the two wheels bounce up and down, describing an arc around their respective pivot points. It’s a basic system with many shortcomings but, bearing in mind it dates back to the early days of the motor car, when it was patented by Edmund Rumpler in 1903, that is understandable.
It was also used on aircraft and, where high speeds on land only occur in a straight line, this was a fair system but, on a car, it was basically flawed. I actually viewed its errant behaviour close-up in my teens. Street racing with a friend (I’m not proud, but I was young and those were different days, etc …) both driving family cars, I followed his VW Beetle through a tight right hand corner and watched its left wheel tuck underneath the car, causing it to slide up on the kerb, buckling the wheel, breaking the driveshaft and, if not actually curing me totally of such hoonish behaviour, persuading me at least to keep it free of a competitive basis.
But, by then, I was already well aware of the shortcomings of the swing axle, ever since my Dad had returned from a trip to the US in 1966 with, in addition to the customary gift of motoring and hot rod mags, a copy of Ralph Nader’s Unsafe At Any Speed. The relevant section of this book centred on a particular villain, the first generation 1959 Chevrolet Corvair. In concept an admirable car that offered advanced solutions and elegant styling in a compact body, a stark contrast to Harley Earl’s chromey bloaters, it was lethally compromised by a mixture of management ignorance and cynicism at GM.
The cars that inspired the Corvair were small European, lightweight, rear-engined saloons, notably the VW Beetle and Renault Dauphine. Both fitted with swing axles, these cars were known for their handling vices when pushed, with lurid oversteer being available to the careless. Sticking a 2.3 litre flat six in the tail didn’t really improve this trait and, although Chevrolet’s engineers were aware of problems, they were frustrated by cost-conscious management in their attempts to improve matters. Simple diagrams and graphic accident descriptions in the Nader book showed what happened when this cheap and cheerless suspension set-up was pushed beyond its, usually quite low, limits.
Specifically the handling characteristics were very susceptible to changes in tyre pressure. To ensure more controllable understeer, the front tyres would normally be set at considerably lower pressure that the rear ones. In the case of the Corvair it was 15psi front and 26psi rear, this differential being chosen late in the car’s development and taking it outside the tyre manufacturer’s recommendations. The car’s behaviour then relied on the owner’s scrupulous monitoring of tyre pressures and their smooth driving style. Naturally many accidents occurred and GM fully deserved the shit that fell on their corporate heads, but less deserving of that was the 1965 Series 2 Corvair, actually a good and good-looking car with a decent fully independent rear end, but one that both GM and its customers had lost enthusiasm for.
But, if the swing axle was fatally discredited by the Corvair scandal, the Chevrolet’s rear end already had a multitude of less notorious cousins. Rumpler had produced his own car using his swing axle in 1921. The Tropfenwagen was a clever and advanced vehicle for its time but, like many admirable yet unconventional cars, it wasn’t a commercial success. In 1923, Benz engineers used Rumpler’s chassis design on their own 1923 Tropfenwagen, a mid-engined racer. Hans Ledwinka adopted swing axles for his 1923 T11 saloon car and subsequent Tatras and, in fact, today’s Tatra trucks still use swing axles. Whether Ferdinand Porsche’s Beetle was the result of plagiarism, or was just heavily influenced by Ledwinka’s Tatra V570, it certainly adopted that car’s swing axles, though Porsche had already used them in his terrifyingly glorious Auto Union Grand Prix cars, themselves heavily influenced by the Benz Tropfenwagen.
By the 50s and 60s, the swing axle was the independent system of choice for the German industry, being found on VWs, Porsches, NSUs, Borgwards and Mercedes. You also found it elsewhere, in the Renault Dauphine, the Triumph Herald and Fiat 600 to name but a handful.
Of all these, bearing in mind the manufacturer’s justified reputation for safety and innovation, it’s surprising to note that Mercedes were one of the swing axle’s longest running and final adherents. After the Tropfenwagen, the swing axle then took seed at Daimler-Benz, appearing in a variety of production cars and racers. For the 300 SL, often called the first supercar (though Alfa’s 8C 2900 is surely equally deserving), Mercedes introduced a raft of innovations, yet remained faithful to the swing axle when a De Dion set-up, of which they had experience, or even just a well-located live axle would have been a wiser choice. As such, glorious though it is, the Gullwing has a deserved reputation for being very unforgiving on the limit. After this, aloof yet diligent as ever, Mercedes improved the behaviour of their swing axle considerably by producing a rather inelegant looking but effective design which made the pivot radius as large as possible, with a single low pivot point for the right shaft and the diff twisting in parcel with the left shaft .
Later, a horizontal compensating spring was introduced and, by time of its last appearance in the large W108 saloons of the mid Sixties, although it could be argued that the design could not now claim to be truly independent since some forces must be transmitted from one side to the other, the suspension’s nastier characteristics were more or less expunged, so much so that it could even be used on the magnificent 300 SEL 6.3 up until 1972.
But that is no reason to mourn the swing axle’s demise, though if you do require more graphic illustration of its general viciousness, particularly when combined with a rear engine, do view this.