A swing axle is a type of rear axle design that has the center section mounted rigidly to the vehicle's chassis and uses universal joints to connect the drive axles to the center section. Unlike a modern independent suspension, the axles do not use a universal joint at the wheel; therefore the wheel remains perpendicular to the drive axle at all times. The swing axle was first installed on early World War I aircraft using bungee cord as a spring and shock absorber. When used on automobiles, the swing axle typically used a transverse-mounted leaf spring and shock absorber suspension system. This axle configuration is basically unused in manufacturing today.
Many problems plagued the swing axle when used in automobiles, the most famous being the difficulties surrounding the 1963 Corvair manufactured by General Motors (GM). The Corvair, with its swing axle and lack of front anti-roll bar, was found to have roll-over issues. In 1964, it was equipped with an anti-roll bar in an attempt to save it from being dropped from GM's automobile line-up. The ploy did not work and the Corvair was dropped from the new car offerings. Volkswagen (VW) also used the swing axle in the early VW Bug, however, a true independent rear axle was soon added to the vehicle.
The main problem facing the use of the swing axle is the unloading of the rear tires after encountering a bump in the road. Unlike an independent rear suspension, when one side of the swing-type axle encounters a bump, it goes up and over the bump; however, as it comes back down to the road, it causes the suspension to unload. This means that as the rear of the vehicle bounces up after hitting the bump, the entire rear of the car unloads or pulls both of the rear tires up and away from the roadway. This causes the tires — that must remain perpendicular to the drive axles — to tip on their edges, which often causes a loss of traction.
By mounting the center of the axle to the chassis, un-sprung weight was reduced on the suspension; however, the lack of universal joints between the axle and the wheels was the demise of the swing axle. Designers soon learned that the addition of a universal joint between the axle and wheel allowed the wheel to remain straight up and down when it traveled over a bump. This created a suspension that was deemed much safer and offered better performance than the swing axle.
