Power Steering

Power steering is any system that uses an external source of power to assist the driver in turning the steered wheels, reducing the muscular effort required at the steering wheel. It became necessary as cars grew heavier and tyres grew wider, since the friction between a large contact patch and the road, especially when stationary or moving slowly, can demand more force at the wheel rim than is comfortable or, in some cases, manageable. By multiplying the driver's input, power steering keeps the car easy to control without resorting to extremely low-geared steering that would make many turns of the wheel necessary.

The assistance is most valuable precisely where unassisted steering is hardest: when parking and manoeuvring at very low speed, where the resistance of the tyres against the road surface is greatest. A well-designed system provides plenty of help in these conditions, so the wheel turns lightly, and then progressively reduces the level of assistance as road speed rises. This speed-sensitive tapering is important, because too much assistance at speed would make the steering feel light, vague, and twitchy, undermining the stability and feedback the driver relies on for confident high-speed control.

Two broad technologies deliver this assistance. The older approach is hydraulic, in which an engine-driven pump pressurises fluid that is fed, under the control of a valve linked to the steering, to a piston in the steering rack that pushes in the direction the driver is turning. It is effective and gives good feel, but it draws power from the engine continuously, even when the car is travelling straight ahead, with a small penalty in fuel consumption, and it relies on a pump, hoses, and fluid that require maintenance.

The modern and now dominant approach is electric power steering, in which an electric motor, mounted on the steering column or rack, provides the assistance under the command of an electronic control unit. Because the motor draws current only when steering effort is actually being applied, it is more efficient and eliminates the hydraulic pump, fluid, and hoses entirely. It also allows the assistance characteristic to be tuned in software and varied with speed and driving mode, and it provides the actuation needed for features such as lane-keeping and self-parking.

Whatever the method, the goal is the same: to make the car easy to steer at low speed while preserving stability and feel at high speed. Power steering is closely tied to the steering mechanism it assists, most commonly a rack and pinion, and the electric form in particular has become an enabling technology for the wider suite of driver-assistance and steering systems found on contemporary vehicles.