Steering

Steering is the control of the direction of motion or the components that enable its control. Steering is achieved through various arrangements, among them ailerons for airplanes, rudders for boats, cylic tilting of rotors for helicopters, and many more.

Aircraft

s are normally steered when airborne by the use of ailerons, spoileron, or both to bank the aircraft into a turn; although the rudder can also be used to turn the aircraft, it is usually used to minimize adverse yaw, rather than as a means to directly cause the turn. On the ground, aircraft are generally steered at low speeds by turning the nosewheel or tailwheel or through differential braking, and by the rudder at high speeds. Missiles, airships and large hovercraft are usually steered by a rudder, thrust vectoring, or both. Small sport hovercraft have similar rudders, but steer mostly by the pilot shifting their weight from side to side and unbalancing the more powerful lift forces beneath the skirt. Jet packs and flying platforms are steered by thrust vectoring only.
Helicopter flight controls are steered by cyclic control, changing the thrust vector of the main rotor, and by anti-torque control, usually provided by a tail rotor.

Automotive

A conventional automotive steering arrangement allows a driver to control the direction of the vehicle by turning the direction of the front wheels using a hand–operated steering wheel positioned in front of the driver. The steering wheel is attached to a steering column, which is linked to rods, pivots and gears that allow the driver to change the direction of the front wheels. The mechanism may include a rack and pinion mechanism that converts several turns of the steering wheel into a large linear displacement. Alternatively, it may use a recirculating ball system. The mechanism may be power-assisted, usually by hydraulic or electrical means.
The use of a variable rack was invented by Arthur Ernest Bishop in the 1970s, so as to improve vehicle response and aim to allow for more comfortable steering, especially at high speeds. He also created a low cost press forging process to manufacture the racks, eliminating the need to machine the gear teeth.
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, where the tracks are made to move at different speeds or in opposite directions, using the clutch and brakes, to achieve a change of direction.
Common steering system components include:

The basic aim of steering is to ensure that the wheels are pointing in the desired direction to move the vehicle as required. 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, which tends to make the steering self-centered in 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 travels in 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 in the vertical plane, known as camber angle, also influences steering dynamics as do the tires.
Steering wheel turning is often measured in terms of number of full 360-degree turns to go lock-to-lock. This is when the steering input mechanism is restrained at its mechanical limit from the full right-turn stop to the left-turn stop.

Rack and pinion, recirculating ball, worm and sector

Many modern cars have a steering mechanism called a rack and pinion. The steering wheel turns a pinion gear, which moves a rack back and forth to steer the wheels. This mechanism converts the circular motion of the steering wheel to linear motion, which is applied to the wheels of the car via tie rods and a steering knuckle.
Rack and pinion steering has several advantages, such as a direct steering "feel". This means that the driver can feel the road better and have more precise control over the car's movement.
BMW was one of the first manufacturers to adopt rack and pinion steering systems in the 1930s, with many other European manufacturers following suit. Auto manufacturers in the United States began to use rack and pinion steering 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 the 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 gives a driver a greater mechanical advantage, resulting in its use on larger, heavier vehicles, while the rack and pinion would originally be limited to smaller and lighter ones; due to the almost universal adoption of power steering, however, this is no longer considered an important advantage, leading to the increasing use of rack and pinion mechanisms 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 eliminated because it will produce 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. To reduce friction, the sector is replaced by a roller or rotating pins on the rocker shaft arm.
Generally, older vehicles use the recirculating ball mechanism, and only newer vehicles use rack-and-pinion steering. This division is not very strict, however, and rack-and-pinion steering systems can be found on British sports cars of the mid-1950s, and some German carmakers did not give up recirculating ball technology until the early 1990s.
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 attached directly between the steering column and the steering arms, and the use of cable-operated steering linkages is also found on some home-built vehicles such as soapbox cars and recumbent tricycles.

Power steering

Power steering helps the driver of a vehicle to steer by directing some of its engine 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 uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Electric Power Steering 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 driver preference. An added benefit is the elimination of the 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

Speed-sensitive steering allows for highly assisted steering at low speeds for maneuverability, and lightly assisted steering at high speed for stability. The first vehicle with this feature was the Citroën SM with its DIRAVI system, first sold in France in 1970. The hydraulic steering system applied force on a centering cam which pushed the steering rack and wheel back to the straight-ahead position. The centering force increased with speed, requiring more effort to turn the wheel at greater speeds.
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. For example, it was used on a production pickup truck, the Tesla Cybertruck, in 2023.