Steering is the collection of components, linkages, etc. which allows any vehicle (car , motorcycle ,
bicycle ) to follow the desired course. An exception is the case of rail transport by which rail tracks combined together with railroad switches (and also known as ‘points’ in British English) provide the steering function.The primary purpose of the steering system is to allow the driver to guide the vehicle.
Bell-crank steering linkage
Rack-and-pinion steering linkage
The most conventional steering arrangement is to turn the front
wheels using a hand–operated
steering wheel which is positioned in front of the driver, via the steering column , which may contain universal joints (which may also be part of the collapsible steering column design), to allow it to deviate somewhat from a straight line. Other arrangements are sometimes found on different types of vehicles, for example, a tiller or rear–wheel steering. Tracked vehicles such as bulldozers and tanks usually employ differential steering — that is, the tracks are made to move at different speeds or even in opposite directions, using clutches and brakes , to bring about a change of course or direction.
Wheeled vehicle steering
Ackerman steering linkage
kingpin pivot line and gray area indicates vehicle’s tire with the wheel moving from right to left. A positive caster angle aids in directional stability, as the wheel tends to trail, but a large angle makes steering more difficult.
Curves described by the rear wheels of a conventional automobile. While the vehicle moves with a constant speed its inner and outer rear wheels do not.
The basic aim of steering is to ensure that the wheels are pointing in the desired directions. This is typically achieved by a series of linkages, rods, pivots and gears. One of the fundamental concepts is that of caster angle – each wheel is steered with a pivot point ahead of the wheel; this makes the steering tend to be self-centering towards the direction of travel.
The steering linkages connecting the steering box and the wheels usually conform to a variation of Ackermann steering geometry , to account for the fact that in a turn, the inner wheel is actually travelling a path of smaller radius than the outer wheel, so that the degree of toe suitable for driving in a straight path is not suitable for turns. The angle the wheels make with the vertical plane also influences steering dynamics (see camber angle) as do the tires.
Rack and pinion, recirculating ball, worm and sector
Steering box of a motor vehicle, the traditional (non-assisted), you may notice that the system allows you to adjust the braking and steering systems, you can also see the attachment system to the frame.
Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a linear gear that meshes with the pinion, converting circular motion into linear motion along the transverse axis of the car (side to side motion). This motion applies steering torque to the swivel pin ball joints that replaced previously used kingpins of the stub axle of the steered wheels via tie rods and a short lever arm called the steering arm.
The rack and pinion design has the advantages of a large degree of feedback and direct steering “feel”. A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the only cure is replacement.
BMW began to use rack and pinion steering systems in the 1930s, and many other European manufacturers adopted the technology. American automakers adopted rack and pinion steering beginning with the 1974 Ford Pinto .
Older designs use two main principles: the worm and sector design and the screw and nut. Both types were enhanced by reducing the friction; for screw and nut it is the recirculating ball mechanism, which is still found on trucks and utility vehicles. The steering column turns a large screw which meshes with nut by recirculating balls. The nut moves a sector of a gear, causing it to rotate about its axis as the screw is turned; an arm attached to the axis of the sector moves the Pitman arm , which is connected to the steering linkage and thus steers the wheels. The recirculating ball version of this apparatus reduces the considerable friction by placing large ball bearings between the screw and the nut; at either end of the apparatus the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus, thus they are “recirculated”.
The recirculating ball mechanism has the advantage of a much greater
mechanical advantage , so that it was found on larger, heavier vehicles while the rack and pinion was originally limited to smaller and lighter ones; due to the almost universal adoption of power steering, however, this is no longer an important advantage, leading to the increasing use of rack and pinion on newer cars. The recirculating ball design also has a perceptible lash, or “dead spot” on center, where a minute turn of the steering wheel in either direction does not move the steering apparatus; this is easily adjustable via a screw on the end of the steering box to account for wear, but it cannot be entirely eliminated because it will create excessive internal forces at other positions and the mechanism will wear very rapidly. This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage.
The worm and sector was an older design, used for example in Willys and Chrysler vehicles, and the Ford Falcon (1960’s). To reduce friction the sector is replaced by a roller or rotating pins on the rocker shaft arm.
Other systems for steering exist, but are uncommon on road vehicles. Children’s toys and go-karts often use a very direct linkage in the form of a
bellcrank (also commonly known as a
Pitman arm ) attached directly between the steering column and the steering arms, and the use of cable-operated steering linkages (e.g. the capstan and bowstring mechanism) is also found on some home-built vehicles such as
soapbox cars and recumbent tricycles .
Main article: Power steering
Power steering helps the driver of a vehicle to steer by directing some of its power to assist in swiveling the steered road wheels about their steering axes. As vehicles have become heavier and switched to front wheel drive , particularly using negative offset geometry, along with increases in tire width and diameter, the effort needed to turn the wheels about their steering axis has increased, often to the point where major physical exertion would be needed were it not for power assistance. To alleviate this
auto makers have developed power steering systems, or more correctly power-assisted steering, since on road-going vehicles there has to be a mechanical linkage as a fail-safe. There are two types of power steering systems: hydraulic and electric/electronic. A hydraulic-electric hybrid system is also possible.
A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel.
Electric power steering (EPS) is more efficient than hydraulic power steering, since the electric power steering motor only needs to provide assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly. In EPS, the amount of assistance is easily tunable to the vehicle type, road speed, and even driver preference. An added benefit is the elimination of environmental hazard posed by leakage and disposal of hydraulic power steering fluid. In addition, electrical assistance is not lost when the engine fails or stalls, whereas hydraulic assistance stops working if the engine stops, making the steering doubly heavy as the driver must now turn not only the very heavy steering—without any help—but also the power-assistance system itself.
Speed sensitive steering
An outgrowth of power steering is speed sensitive steering, where the steering is heavily assisted at low speed and lightly assisted at high speed. Auto makers perceive that motorists might need to make large steering inputs while manoeuvering for parking, but not while traveling at high speed. The first vehicle with this feature was the Citroën SM with its
Diravi layout,  although rather than altering the amount of assistance as in modern power steering systems, it altered the pressure on a centring cam which made the steering wheel try to “spring” back to the straight-ahead position. Modern speed-sensitive power steering systems reduce the mechanical or electrical assistance as the vehicle speed increases, giving a more direct feel. This feature is gradually becoming more common.
1937 Mercedes-Benz Type G 5 with four-wheel steering.
Quadrasteer, rear steering angle.
trolleybus demonstrating its four-wheel steering on front and rear axles (2006).
telescopic handler using crab steering.
Hamm DV70 tandem roller using crab steering to cover maximum road surface (2010).
Agricultural slurry applicator using crab steering to minimise soil compaction (2009).
Four-wheel steering is a system employed by some vehicles to improve steering response, increase vehicle stability while maneuvering at high speed, or to decrease turning radius at low speed.
Active four-wheel steering
In an active four-wheel steering system, all four wheels turn at the same time when the driver steers. In most active four-wheel steering systems, the rear wheels are steered by a computer and actuators. The rear wheels generally cannot turn as far as the front wheels. There can be controls to switch off the rear steer and options to steer only the rear wheels independently of the front wheels. At low speed (e.g. parking) the rear wheels turn opposite to the front wheels, reducing the turning radius by twenty-five percent, sometimes critical for large trucks or tractors and vehicles with trailers, while at higher speeds both front and rear wheels turn alike (electronically controlled), so that the vehicle may change position with less yaw, enhancing straight-line stability. The “snaking effect” experienced during motorway drives while towing a travel trailer is thus largely nullified. [dubious ]
Four-wheel steering found its most widespread use in monster trucks , where maneuverability in small arenas is critical, and it is also popular in large farm vehicles and trucks . Some of the modern European Intercity buses also utilize four-wheel steering to assist maneuverability in bus terminals, and also to improve road stability. The first rally vehicle to use the technology was the Peugeot 405 Turbo 16. Its debut was at the 1988 Pikes Peak International Hill Climb, where it set a record breaking time of 10:47.77.  The car would go on to victory in the 1989 and 1990 Paris-Dakar Rally , again driven by Ari Vatanen .
Previously, Honda had four-wheel steering as an option in their 1987–2001 Prelude and Honda Ascot Innova models (1992–1996). Mazda also offered four-wheel steering on the 626 and MX6 in 1988. General Motors offered Delphi ‘s Quadrasteer in their consumer Silverado / Sierra and
Suburban / Yukon . However, only 16,500 vehicles were sold with this system from its introduction in 2002 through 2004. Due to this low demand, GM discontinued the technology at the end of the 2005 model year.
Nissan / Infiniti offer several versions of their HICAS system as standard or as an option in much of their line-up. A new “Active Drive” system is introduced on the 2008 version of the
Renault Laguna line. It was designed as one of several measures to increase security and stability. The Active Drive should lower the effects of under steer and decrease the chances of spinning by diverting part of the G-forces generated in a turn from the front to the rear tires. At low speeds the turning circle can be tightened so parking and maneuvering is easier.