A swing axle is a simple type of independent suspension, almost always for the rear axles, designed and patented by Edmund Rumpler in 1903. 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 the Rumpler Tropfenwagen, later followed by the Mercedes 130H/150H/170H, the Standard Superior, the Volkswagen Beetle and its derivatives, the Chevrolet Corvair, and the roll-over prone M151 jeep amongst others.
Some later automobile rear swing axles have universal joints connecting the driveshafts to the differential, which is attached to the chassis. 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 1963 Jeep Wagoneer's front axle, upon its market introduction.
Swing axle suspensions conventionally used leaf springs and shock absorbers, though later Mercedes-Benz applications used coil springs.
It was also used in early aircraft (1910 or before), such as the Sopwith and Fokker, usually with rubber bungee and no damping.
The swing axle suspension has two advantages over the typical live axle:
Several engineering options can limit swing axle handling problems, with varying success:
Ralph Nader in his 1965 book Unsafe at Any Speed detailed accidents and lawsuits related to the shortcomings in 1960–1963 models of the first generation Chevrolet Corvair's swing-axle design. Nader identified a Chevrolet engineer who had fought management after the management had eliminated a front anti-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 true independent rear suspension (IRS) system.
The Hillman 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 a semi-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 much understeer and uneven tyre wear, and modifications were made to reduce the positive camber 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 the kingpin carrier relative to the wishbone.
Swing axles were supplanted in general use by de Dion tube axles in the late 1960s, though live axles remained the most common. Most rear suspensions have been replaced by more modern independent suspensions in recent years, and both swing and de Dion types are virtually unused today.
One exception is the Czech truck manufacturer Tatra, which has been using swing axles on a central 'backbone' tube since 1923 (model Tatra 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 effectively are 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, the A-arm suspension 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]