A swing axle is a simple type of independent (rear wheel) suspension designed and patented by Edmund Rumpler in 1903. This was a revolutionary invention in the automotive industry, allowing wheels to react to irregularities of road surfaces independently, and enable the vehicle to maintain a strong road holding. 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, and the Chevrolet Corvair, 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. Swing axle suspensions traditionally used leaf springs and shock absorbers. 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, swing-axle design, and identified a Chevrolet engineer that had fought management who eliminated a front anti-roll bar for cost reasons. 1964 models were fitted with a front anti-roll bar as standard equipment, in addition to a rear transverse leaf spring, thus improving emergency maneuver stability. 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 one at a relatively low speed, and designed their rear-engined car with 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. 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 deDion types are virtually unused today.
One exception is the Czech truck manufacturer Tatra, which uses 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, else being equal. This is especially manifested in long 6+ wheel vehicles where offroad 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". Though it is an independent suspension system, as each tire rises and falls without affecting the position of the other, the parallelogram action of the A-arm suspension system is not present. Each tire moves in an arc, but, due to the longer arms, camber changes are proportionally smaller than in powered swing axles for the rear wheels listed above. The pivot point of the axles is lower and located not in the center of the car, but nearly on the opposite beam of the chassis, so the effect is far less hazardous.