A Swing Axle
system is an inexpensive method of suspending the driving axle of an automobile
or other four (or more) wheeled vehicle, particularly those where the engine
are bolted directly together. Swing axles are very simple. A universal joint
is located on each half-shaft
just outside the differential
. This permits each wheel to respond to bumps somewhat independently. The swing axle isolates the engine and transmission from the bumps in the road. Which in turn means that the engine and transaxle don't have to move or absorb the impacts.
Swing axles have two signficant downsides. First of all, they provide zero control of camber. Ideally, a wheel should always be perpendicular to the road surface, in order to maximize the tire's contact patch. Because the swing axle's motion describes an arc the tire will necessarily move from positive to negative camber reducing contact patch when the road is less than perfectly smooth, a condition not met even on the race track. The result of uncontrolled camber is very twitchy handling, with the car capable of shifting abruptly from understeer to an oversteer condition. This leads to unpredictable handling.
The second problem is that when a car with a swing axle crosses a bump or obstruction, the suspension motion has a jacking efffect, which raises the car's center of gravity. The reason for this is that the wheel moves only in one direction, the arc described by the length of the halfshaft and permitted suspension travel. When a wheel hits a bump it will tend to raise the car by pushing against the differential attachment, lifting the differential which if fixed in the car. At speed this effect can be substantial, and lead to a potential rollover. It is for this reason that Ralph Nader condemned the early Chevrolet Corvair's swing axle suspension in his book Unsafe at Any Speed.
Though the design permits some independent wheel movement, in no way is a swing axle any form of independent suspension, for it provides only for very limited control of wheel motions. Swing axles are for bump isolation, not wheel location. They are inexpensive, which is why they were popular in older cars such as the original Volkswagon Beetle, the early Corvair, and the Mercedes 300 SL. However, their drawbacks have led to the near abandonment of swing axles by modern designers.