Rear wheel steering (RWS) is an advanced vehicle technology that allows the rear wheels to turn slightly, working in conjunction with the front wheels to influence the vehicle’s direction of travel. This system represents a significant departure from conventional steering setups, where only the front wheels are mechanically linked to the steering wheel. By actively controlling the direction of all four wheels, RWS aims to enhance a vehicle’s agility at low speeds and improve its stability during high-speed maneuvers.
The Basic Concept
Rear wheel steering functions as an electromechanical system that independently controls the angle of the rear wheels. The core components of this system include an electric motor, an actuator assembly, and various sensors, all managed by a dedicated Electronic Control Unit (ECU). The ECU constantly receives data on vehicle speed, steering wheel angle, and yaw rate to determine the precise movement required from the rear axle.
The physical steering of the rear wheels is achieved through an actuator, often an electric motor that drives a spindle or moves a rack-and-pinion mechanism at the rear axle. This motion applies an adjustment to the rear wheels’ toe angle, the extent to which the wheels point inward or outward. The maximum steering angle for the rear wheels is significantly smaller than the front, typically ranging from 1 degree up to a maximum of 10 degrees, depending on the vehicle’s design and intended use.
Speed-Dependent Operation
Rear wheel steering operation is entirely dependent on the vehicle’s speed, which dictates the control logic the ECU employs. The system operates primarily in two distinct phases: opposite-phase steering at low speeds and same-phase steering at higher speeds. The transition between these modes often occurs around speeds of 30 to 40 miles per hour (approximately 50 to 65 kilometers per hour).
At lower speeds, the system engages opposite-phase steering, where the rear wheels turn in the direction opposite to the front wheels. When the front wheels turn left, the rear wheels turn slightly to the right, which has the effect of shortening the vehicle’s effective wheelbase. This action allows the vehicle to rotate more quickly around its center, significantly reducing the turning radius and making the car feel more compact and nimble.
Conversely, when the vehicle is traveling at higher speeds, the system switches to same-phase steering, where the rear wheels turn in the same direction as the front wheels. This parallel movement effectively lengthens the vehicle’s wheelbase, which maintains stability during rapid lane changes or sweeping highway curves. By steering all four wheels in the same direction, the vehicle changes direction with a smooth, crab-like motion, reducing the side-slip angle and dampening unwanted rotation around the vertical axis.
Impact on Driving Dynamics
At low speeds, the opposite-phase steering drastically enhances maneuverability, a benefit particularly noticeable on vehicles with a long wheelbase. For example, the system can reduce the vehicle’s turning circle by up to six feet or more, making tight parking situations or three-point turns substantially easier to execute.
At higher speeds, the primary benefit shifts from agility to stability and control through the use of same-phase steering. When traversing a curve or performing an emergency avoidance maneuver, the system minimizes the vehicle’s yaw rate, which is the speed of rotation around its vertical axis. This active stabilization results in a more composed and secure feeling for the driver, providing better response and greater confidence during quick steering inputs.