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 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.
Comparison
Advantage
The swing axle suspension has two advantages over the typical live axle:
It reduced unsprung weight since the differential is mounted to the chassis
It eliminates sympathetic camber changes on opposite wheels
Shortcomings
A great amount of single-wheel camber change is experienced relative to beam axle designs, since the radius of the jointed half-shaft is less than half that of the whole axle assembly. "Jacking" on suspension unloading (or rebound) causes positive camber changes on both sides, which (In extreme cases) can overturn the car. Change in camber due to cornering forces can cause loss of rear-wheel adhesion leading to oversteer—a dynamically unstable condition that can cause a vehicle to spin. This is an especially severe problem when a swing axle is used in a rear-engine design, because of the greater side-g forces on the rear wheels from the mass of the engine. Camber changes during deceleration can increase the severity of lift-off oversteer.
Solutions
Several engineering options can limit swing axle handling problems, with varying success: