Four wheel steering is a vehicle system that lets the rear wheels turn as well as the front wheels. The rear wheels only turn a small amount, but that small angle can make a car feel easier to park, quicker to change lanes, and more stable at speed. This matters because a vehicle is always rotating, sliding, and gripping through four tire contact patches, not just the front two.
By controlling all four wheels, engineers can improve both low speed maneuverability and high speed control.
At low speeds, many four wheel steering systems turn the rear wheels opposite the front wheels to reduce the turning radius. At higher speeds, the rear wheels often turn in the same direction as the front wheels to make lane changes smoother and reduce body motion. Sensors measure steering wheel angle, vehicle speed, yaw rate, and sometimes lateral acceleration, then an electronic control unit calculates the needed rear steering angle.
Actuators at the rear axle move linkages or steering mechanisms to set the rear wheel angle within safe limits.
Key Facts
- At low speed, rear wheels often steer opposite the front wheels to reduce turning radius.
- At high speed, rear wheels often steer in the same direction as the front wheels to improve stability.
- Turning radius is the radius of the path followed by the vehicle during a turn.
- Approximate low speed turning radius for a bicycle model: R = L / tan(theta), where L is wheelbase and theta is front steering angle.
- Rear steering angles are usually small, often about 1 degree to 5 degrees depending on speed and vehicle design.
- The control system uses sensor data such as vehicle speed, steering angle, and yaw rate to command the rear steering actuator.
Vocabulary
- Four wheel steering
- A steering system in which both the front and rear wheels can change angle to help control the vehicle.
- Turning radius
- The radius of the circular path a vehicle follows when it makes a turn.
- Yaw
- Yaw is the rotation of a vehicle around a vertical axis, like the motion of a car turning left or right.
- Actuator
- An actuator is a device that converts an electrical or hydraulic command into mechanical motion.
- Electronic control unit
- An electronic control unit is a small computer that reads sensor inputs and sends commands to vehicle systems.
Common Mistakes to Avoid
- Assuming the rear wheels turn as much as the front wheels is wrong because rear steering angles are usually much smaller and carefully limited for safety.
- Thinking four wheel steering is the same as four wheel drive is wrong because steering controls wheel direction, while drive controls which wheels receive engine or motor torque.
- Using the low speed opposite steering rule at all speeds is wrong because many systems switch to same direction rear steering at higher speeds for stability.
- Ignoring vehicle speed in a four wheel steering problem is wrong because speed is one of the main inputs that determines rear steering direction and angle.
Practice Questions
- 1 A car has a wheelbase of 2.8 m and a front steering angle of 28 degrees. Using R = L / tan(theta), estimate the turning radius without rear steering.
- 2 At parking speed, a four wheel steering car has a front wheel angle of 30 degrees and the rear wheels steer 4 degrees in the opposite direction. What is the approximate total angle difference between the front and rear wheel directions?
- 3 Explain why opposite direction rear steering helps in a parking lot, while same direction rear steering helps during a fast lane change.