Four-wheel Steering (mechanical)
For commercial vehicles it is quite common to have more than one steering axle. Yet, it most often concerns a dual front axle because one axle alone would carry too much weight. Also, there are vehicles with steering
rear axles, just think about the fork-lift truck. Throughout history, vehicles with both a steering front- and rear axle came and went. The picture above displays a mechanical solution.
How it works
A steering rear axle may have a dual purpose. If the front steering angle is small, it can reduce the oblique angle, meaning the rear wheels steer a bit into the same direction as the front wheels. The traction of the rear
axle in curves improves.
The construction assumes that full or almost full steering angles occur in combination with very low speeds, for example while parking the car. Manoeuvrability and a small turning circle
are the key issues. The rear axle shows almost the same steering angle as the front, just into the "opposite" direction. A small parking space may be occupied more easily this way.
It is not true that normal
modern rear axles are totally stiff, they may perform minor steering corrections if necessary. Already the rigid axle, mounted by trailing links, followed to some degree the movements of
the body performing minor steering movements. Modern five-link suspensions give even more way for such movements. What else is the purpose of the fifth link?