Four-wheel steering, 4WS, also called rear-wheel steering or all-wheel steering, provides a means to actively steer the rear wheels during turning maneuvers. It should not be confused with four-wheel drive in which all four wheels of a vehicle are powered. It improves handling and helps the vehicle make tighter turns. Production-built cars tend to understeer or, in few instances, oversteer. If a car could automatically compensate for an understeer /oversteer problem, the driver would enjoy nearly neutral steering under varying conditions. 4WS is a serious effort on the part of automotive design engineers to provide near-neutral steering. The front wheels do most of the steering. Rear wheel turning is generally limited to half during an opposite direction turn. When both the front and rear wheels steer toward the same direction, they are said to be in-phase and this produces a kind of sideways movement of the car at low speeds. When the front and rear wheels are steered in opposite direction, this is called anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter turn.
|Fig. Four wheel steering system|
ADVANTAGES OF 4WS
1. The vehicle‟s cornering behavior becomes more stable and controllable at high speeds as well as on wet or slippery road surfaces.
2. The vehicle‟s response to steering input becomes quicker and more precise throughout the vehicle‟s entire speed range.
3. The vehicle‟s straight-line stability at high speeds is improved. Negative effects of road irregularities and crosswinds on the vehicle‟s stability are minimized.
4. Stability in lane changing at high speeds is improved. The vehicle is less likely to go into a spin even in situations in which the driver must make a sudden and relatively large change of direction.
5. By steering the rear wheels in the direction opposite the front wheels at low speeds, the vehicle‟s turning circle is greatly reduced. Therefore, vehicle maneuvering on narrow roads and during parking becomes easier.