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 (emulated by the Mercedes 170), the Standard Superior and the Volkswagen Beetle.
Some later automobile rear swing axles have universal joints connecting the driveshafts to the differential, which is attached to the chassis. They do not have universal joints at the wheels: the wheels are always perpendicular to the driveshafts. 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.
This type of suspension was considered better than the more typical live axle for two reasons:
- It reduced unsprung weight since the differential is mounted to the chassis
- It eliminates sympathetic camber changes on opposite wheels
There are several shortcomings with this arrangement:
- A great amount of single-wheel camber change is experienced, since the wheel is always perpendicular to the driveshaft
- "Jacking" on suspension unloading (or rebound) causes positive camber changes on both sides, which (In extreme cases) can overturn the car.
- Reduction in cornering forces due to change in camber can lead to oversteer — a dynamically unstable condition where a vehicle can lose control and spin — and in extreme cases lift-off oversteer.
These problems were evident on the Mercedes-Benz 190SL and 300SL, Porsche 356, Triumph Herald, Vitesse and Spitfire, Tatra T603, Renault Dauphine, Volkswagen Beetle (until rear suspension modifications, c. 1967) and others.
A number of engineering options could ameliorate swing axle handling, with limited results:
- 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 tire, 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 70s the worlds fastest production sedan. AMG-modified 6.3s were also raced with the stock swing axle.
- Tire pressure differential: The Renault Dauphine, Volkswagen Beetle and first generation Chevrolet Corvair (1960–1964) used a tire pressure differential strategy to eliminate oversteer characteristics of their swing axle suspensions — specifically low front and high rear tire pressure — which induced understeer. Nonetheless, the tire pressure differential strategy offered a significant disadvantage: owners and mechanics could inadvertently but easily re-introduce oversteer characteristics by over-inflating the front tires (that is, to typical pressures for other cars with other suspension systems) or by inflating all four tires to the same pressure. The effectiveness of this option was questioned 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 transversally 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, a roll-bar, as opposed to anti-rolling bar. Both devices increase the rolling tendency of the rear axles, thus reducing the oversteering tendency. Aftermarket roll-inducing springs from the likes of SCAT, also called camber compensators, are available for Porsche, VW and early Corvairs, which reduce oversteering tendencies at the cost of increasing suspension harshness.
Ralph Nader in his 1965 book Unsafe at Any Speed highlighted accidents and lawsuits related to 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 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 and a central 'backbone' tube instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off road.
Another use of the swing axle concept is Ford's "Twin I-Beam" front suspension for trucks. This has solid axles (so they do not transmit power). Though it is touted as an independent suspension system in that 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 in fact moves with a similar camber change to that of the powered swing axles for the rear wheels listed above. The pivot point of the axles is located not in the center of the car, but nearly on the opposite beam of the chassis, so the effect is far less hazardous.