Power steering is a system for reducing the steering effort on cars by using an external power source to assist in turning the wheels. Power steering was invented in the 1920s by Francis W. Davis and George Jessup in Waltham, Massachusetts. Chrysler Corporation introduced the first commercially available power steering system on the 1951 Chrysler Imperial under the name Hydraguide. Most new vehicles now have power steering, although in the 1970s and 1980s it was the exception rather than the rule, at least on European cars. The trend to front wheel drive, greater vehicle mass and wider tires means that modern vehicles would be extremely difficult to manouevre at low speeds (e.g. when parking) without assistance.
Most power steering systems work by using a belt driven pump to provide hydraulic pressure to the system. This hydraulic pressure is generated by a rotary-vane pump which is driven by the vehicle's engine. As the speed of the engine increases, the pressure in the hydraulic fluid also increases, hence a relief valve is incorporated into the system to allow excess pressure to be bled away.
While the power steering is not being used, i.e. driving in a straight line, twin hydraulic lines provide equal pressure to both sides of the steering wheel gear. When torque is applied to the steering wheel, the hydraulic lines provide unequal pressures and hence assist in turning the wheels in the intended direction.
Some more modern implementations of hydraulic systems also include an electronic pressure valve which can reduce the hydraulic pressure of the power steering lines as the vehicle's speed increases (Variable assist power steering).
In the DIRAVI system invented by Citroën, the force turning the wheels comes from the car's high pressure hydraulic system and is always the same no matter what the road speed is. As the steering wheel is turned, the wheels are turned simultaneously to a corresponding angle by a hydraulic ram. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to center position. The force of the centering device increases as the car's road speed increases. As long as there is pressure in the car's hydraulic system, there is no mechanical connection between the steering wheel and the road wheels. This system was first introduced in the Citroën SM in 1970.
Electric Power Steering, such as those found on the Acura NSX, the Saturn VUE V6, and on most FIAT and Lancia cars, uses electric components. Sensors detect the motion and torque of the steering column and a computer module applies assistive power via an electric motor. This allows varying amounts of assistance to be applied depending on driving conditions. Most notably on FIAT group cars the amount of assistance can be regulated using a button named "CITY" that switch between two different assist curves (boost curve), while on Volkswagen/Audi group cars, the amount of assistance is automatically regulated depending on vehicle speed.
In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. The software in the computer module enables the flexibility of "tuning" the characteristics of the electric power steering system to suit the preference of the vehicle designers. The "feel" is often set a bit on the light side so a criticism commonly expressed is a lack of steering "feel".
Electric power steering is limited to smaller vehicles. This is because the 12 volt electrical system is limited to 80 amps of current which, in turn, limits the size of the motor to less than 1 kilowatt. (12.5 volts times 80 amps equals 1000 watts) Vehicles such as trucks and SUVs require a larger power output. A new 42 volt electrical system standard may enable use of electric power steering on larger vehicles.
Electric systems have a significant advantage in fuel efficiency because there is no hydraulic pump constantly running, whether assistance is required or not, and this is the main reason for their introduction.
Audi offers speed-dependent power steering, in which the amount of servo assist depends on road speed and thus provides even more comfort and convenience for the driver. The amount of power assist is greatest at low speeds, for example when parking the car. The greater assist makes it easier to maneuver the car. At higher speeds, an electronic sensing system gradually reduces the level of power assist. In this way, the driver can control the car even more precisely than with conventional power steering.
So called "hybrid" systems use the same hydraulic assist technology as standard systems, with the hydraulic pressure being provided by an electric motor instead of a belt driven one. Those systems can be found in Volkswagen, Audi, SEAT, Skoda, Suzuki, MINI and some Mazda cars.
- Car handling
- Center of mass
- Electronic Stability Control
- Inboard brake
- Suspension (vehicle)
- Unsprung weight
- Vehicle dynamics
- Weight transfer