by Jim Kerr
The rear cover of a popular automobile publication recently featured a
full-page advertisement for the Chrysler 300M. Speed sensitive steering
was highlighted among the available features and described as giving the
driver the best feel for the road. This option is available from many
automobile manufacturers under a variety of names, and electronics have
made it possible.
Most systems operate in a similar manner. Ford calls their system VAPS
which stands for variable assist power steering. When the car is driven
at low speed, the power steering gives full assist so the driver can
turn the steering wheel easily. Parking or turning tight corners can be
done with little effort.
As vehicle speed increases, the mount of power steering assist is
reduced and the steering wheel becomes harder to turn. This gives the
driver more feedback about road conditions through the steering wheel
and better vehicle control. If an emergency arises and the driver needs
to turn the steering wheel quickly, the full power assist is provided
almost immediately after the wheel is turned only a few degrees.
In a conventional power steering system, the power steering pump
provides constant oil pressure for the system. The oil pressure is high
so turning the wheel can be done easily when the engine is idling. The
same high pressure is produced as the engine speeds up and the same
effort is required to turn the wheel even at higher vehicle speeds.
Most speed sensitive steering systems vary the oil pump output and the
amount of power assist by rapidly turning on and off an electric control
solenoid mounted on the pump. High assist is provided at low speeds but
the assist is gradually reduced as vehicle speed increases to give the
driver more road feel.
Another type of speed sensitive steering is used on some General Motors
cars. Magnasteer, an all-electronic system, uses a variable strength
electromagnet to vary the effort required to turn the steering wheel.
Oil pressure from the power steering pump is not varied. The system is
compact and mounts inside the top of the steering rack.
Honda uses electric power steering on its S2000 and Insight models. An
electric motor, mounted in the steering rack, assists the driver in
turning the steering wheel. A computer can easily control steering
effort with this type of system.
On all types of variable assist steering, an electronic module controls
the amount of assist provided. Two input signals are used by the module
to determine how much steering assist is required. One input is a
vehicle speed signal provided by the vehicle speed sensor.
Almost all vehicles use electronic speed sensors mounted on the
transmission instead of a speedometer cable. The sensor sends
electrical pulses to one of the vehicle’s computers. It is then sent on
to other computers such as the engine computer, cruise control, ABS
computer, and instrument panel cluster. The electronic steering control
module uses this signal as well.
The second input signal comes from the vehicle’s steering column. Ford
vehicles with variable assist steering have a small notched disc mounted
on the steering shaft under the dash. The disc rotates when the steering
is turned and a sensor mounted beside the disc measures how fast and far
the steering wheel is being turned. Other manufacturers use similar
Turning the steering wheel rapidly for more than a few degrees signals
the steering control module to turn on full power assist. This ensures
the driver will have full assist during emergencies.
Finally, if the electronics should fail, the systems are designed to
provide maximum steering assist at all speeds. Variable assist steering
makes parallel parking easy and keeps the vehicle stable at higher
speeds. It is features such as this that help make current automobiles
handle much better than their predecessors.