A swing axle is a simple type of independent 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 conventionally used leaf springs and shock absorbers. It was also used in early aircraft, 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 causes positive camber changes on both sides, which 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:
Anti-roll bar: As a design option, a front anti-roll bar which can ameliorate the swing axle car's handling—shifting weight transfer to the front outboard tyre, considerably reducing rear slip angles—thereby avoiding potential oversteer.
Single-pivot point: Mercedes-Benz addressed the handling issues by producing swing axles with a single-pivot point located under the differential, thus well below the axle. This configuration markedly reduced the tendency to "jack-up" and the later low pivot swing-axle equipped cars were praised in contemporary publications for their handling. The low-pivot swing-axle remained in production with Mercedes-Benz W108 280SE and 300SEL until 1972. It was fitted to the 300SEL 6.3, which was during the early 1970s the world's fastest production sedan. AMG-modified 6.3s were also raced with the stock swing axle.
Tyre pressure differential: The Renault Dauphine, Volkswagen Beetle and first generation Chevrolet Corvair used a tyre pressure differential strategy to eliminate oversteer characteristics of their swing axle suspensions—specifically low front and high rear tyre pressure—which induced understeer. The tyre pressure differential strategy offered a significant disadvantage: owners and mechanics could inadvertently but easily re-introduce oversteer characteristics by over-inflating the front tyres or by inflating all four tyres to the same pressure. The effectiveness of this option was criticized in lawsuits in the US during the 1960s.
Z-bar and roll-inducing springs: Mercedes-Benz introduced, to help their low-pivot swing-axle, a coil spring mounted transversely above the differential, which would transfer load from one side to the other, so as to force down one wheel when the other side went up. This coil spring increases the load bearing capacity of the rear suspension, so a new lower pressure set of springs was substituted for the usual ones to maintain ride suppleness. A similar effect was achieved by VW's Z-bar, as opposed to anti-roll bar. Both devices distribute the response of the rear axles to input on one wheel, thus reducing the tendency for excessive camber to occur on one wheel. These solutions represent a compromise between swing axle and beam axle characteristics, at the cost of decreasing ride quality.
Camber compensator: A transverse leaf spring is connected to the outer ends of the half-shafts and below the suspension assembly, resisting positive camber that could result from cornering forces or rebound. This was a successful solution introduced on the 1964 Corvair. Aftermarket camber compensators are available for Porsche, VW and early Corvairs.
Safety
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 used a true independent rear suspension system. The Hillman Imp designers learned from the problems with the Corvair, having crashed 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. 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.
Replacement
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 uses swing axles on a central 'backbone' tube since 1923 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 offroad chassis twisting can be a major issue.
Twin I-Beam
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 tyre rises and falls without affecting the position of the other, the parallelogram action of the A-arm suspension system is not present. Each tyre 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.
