A swing axle is a simple type of independent suspension first used in early aircraft (1910 or before), such as the Sopwith and Fokker, usually with rubber bungee and no damping. Some later motor-car 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. Volkswagens built before 1967 used torsion bars as their spring.
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
However, there are a number of shortcomings to 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 instability and in extreme cases lift-off oversteer
Mercedes-Benz addressed the inherent handling issues by producing swing axles with a single-pivot point located under the differential, and 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.
Swing axles were supplanted by deDion 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 Most rear suspensions have been replaced by more modern independent suspensions in recent years, and both swing and deDion types are virtually unused today.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.
The first production (1960–1964) Chevrolet Corvair used this design. The unsafe behavior of the Corvair was described in detail by Ralph Nader in his book Unsafe at Any Speed. Second Production Corvairs (1965–1969) used a true independent-rear-suspension (IRS )system.
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. But the pivot point of the axles is located not in the middle of the car but nearly on the other beam of the chassis, so the effect is far less hazardous.