Power Assisted Steering - PAS
Long regarded as a feature of larger cars and an expensive option on the rest, power steering -P.A.S.- has become more common as the industry has moved more towards transverse engined, front wheel drive, cars where the placement of all of the drive components at the front of the car makes the necessary effort to steer much greater.
The majority of front drive cars now come as standard with power assistance, only those cars that are very small or light not needing such help. The result is much easier parking and, with the best systems, varying degrees of assistance relative to speed.
All such systems work by adding a hydraulic pump to the steering gear, the pump being driven by a power take off or belt from the engine and itself powering a piston which effectively increases the driver's leverage on the steering gear. A system of proportioning valves controls the amount of extra leverage applied to each side of the steering gear linked to sensors which detect both the rolling and turning resistance of the wheels.
A safety feature of most systems is the " sneeze factor" whereby a certain amount of slack is built in to prevent the steering being too twitchy.
A further factor of the system is that it is failsafe, should the assistance fail the car can be steered as normal albeit with heavier loadings and thus greater difficulty.
Article © Graham Benge 2007