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Aswing axle is a simple type ofindependent suspension designed and patented byEdmund Rumpler in 1903 for the rear axle of rear wheel drive vehicles. This was a revolutionary invention in automotive suspension, allowing driven (powered) wheels to follow uneven road surfaces independently, thus enabling the vehicle's wheels to maintain better road contact and holding; plus each wheel's reduced unsprung weight means their movements have less impact on the vehicle as a whole. The first automotive application was theRumpler Tropfenwagen, another early example was the 1923Tatra 11 later followed by theMercedes 130H/150H/170H, theStandard Superior, the pre-faceliftVolkswagen Beetle and most of its derivatives, theChevrolet Corvair, and the roll-over proneM151 jeep amongst others.
Many later automobile rear swing axles haveuniversal joints connecting thedriveshafts to thedifferential, which is attached to thechassis. Swing axles do not have universal joints at the wheels — the wheels are always perpendicular to the driveshafts; the design is therefore not suitable for a car's front wheels, which require steering motion. Nevertheless, a simplified variant, wherein the differential remained fixed to one of the halfshafts, was offered optionally on the 1963Jeep Wagoneer'sfront axle, upon its market introduction.
Swing axle suspensions often usedleaf springs andshock absorbers, though later Mercedes-Benz applications used coil springs and the VW beetle swing axle was torsion bar sprung.
One problem inherent in the swing axle concept is that it almost inevitably results in a very high roll centre which causes detrimental jacking effects and camber change when cornering and lateral cornering forces are applied. Its simple geometry limits design freedom to a great extent.
Swing axles can also be used as a low cost and durable independent suspension solution for non-driven front or rear axles, theTatra 17 which had swing axles front and rear being an early example. It was also used in early aircraft (1910 or before), such as theSopwith andFokker, usually with rubberbungee and no damping.
The swing axle suspension has two advantages over the typicallive axle:
Several engineering options can limit swing axle handling problems, with varying success:
Ralph Nader in his 1965 bookUnsafe at Any Speed detailed accidents and lawsuits related to the shortcomings in 1960–1963 models of the first generationChevrolet Corvair's swing-axle design. Nader identified a Chevrolet engineer who had fought management after the management had eliminated a frontanti-roll bar for cost reasons. The 1964 models were fitted with a front anti-roll bar as standard equipment, in addition to a rear transverse leaf spring, thus improving stability during emergency maneuvering. Second-generation Corvairs (1965–1969) used a trueindependent rear suspension (IRS) system.
TheHillman Imp designers learned from the problems with the Corvair, having crashed[4] one at a relatively low speed, and they designed their rear-engined car with asemi-trailing arm suspension at the rear. To attain correct handling balance, they actually used swing-axle geometry at the front, with the steering pivots mounted at the outer ends of single swing wishbones. These caused too muchundersteer and uneven tyre wear, and modifications were made to reduce the positivecamber of the front wheels by lowering the swing-axle pivot points.[5] Aftermarket kits were also available to do this, and an inexpensive alternative was to insert a tapered shim to change the inclination of thekingpin carrier relative to the wishbone.
Swing axles were supplanted in general use byde Dion tube axles in the late 1960s, thoughlive axles remained the most common. Most rear suspensions have been replaced by more modernindependent suspensions in recent years, and both swing and de Dion types are virtually unused today.
One exception is the Czech truck manufacturerTatra, which has been using swing axles on a central 'backbone' tube since 1923 (modelTatra 11) instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off-road. There the inherent reduced stability on roads is compensated by an increased stability on rough terrain, allowing for higher off-road speeds, all else being equal. This is especially manifested in long 6+ wheel vehicles where off-road chassis twisting can be a major issue.
Another use of the swing axle concept is Ford's "Twin I-Beam" front suspension for trucks. This system has solid axles, and may transmit power in four-wheel-drive versions, where it is called "Twin Traction Beam". It is an independent suspension system, as each tyre rises and falls without affecting the position of the other. Although each tyre still moves in an arc as in a standard swing-axle suspension, the lower control arms are effectively lengthened by attaching the axle pivot point to the bottom of the opposite frame rail (i.e., the left lower control arm pivots on the right frame rail and vice versa). The lowered pivot point and longer arm length reduce the change in camber and the effect is far less hazardous than powered swing axles for the rear wheels listed above, where the pivot point is approximately on the same side frame rail.[6] The Twin I-Beam suspension includes an additional radius arm link on each side to control caster.[7]
Although the camber change is reduced with the Twin I-Beam suspension, theA-armsuspension system constrains the wheel into a parallelogram motion, further minimizing camber changes throughout suspension travel.
The 1956 Series 1 Lotus Eleven sports racers used a swing-axle front suspension, derived from the Ford E93 sedan.[8]
