Steering


Steering
Part of car steering mechanism: tie rod, steering arm, king pin axis (using ball joints).

Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel (ship, boat) or 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.

Contents

Introduction

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

Basic geometry

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 travel.

Ackermann steering geometry

The steering linkages connecting the steering box and the wheels usually conforms 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.

Rack and pinion, recirculating ball, worm and sector

Rack and pinion animation
Rack and pinion unit mounted in the cockpit of an Ariel Atom sports car chassis. For most high volume production, this is usually mounted on the other side of this panel

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.

Older designs often use the recirculating ball mechanism, which is still found on trucks and utility vehicles. This is a variation on the older worm and sector design; the steering column turns a large screw (the "worm gear") which meshes with a sector of a gear, causing it to rotate about its axis as the worm gear 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 teeth of the worm and those of the screw; 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 much smaller degree of feedback with this design can also sometimes be an advantage; drivers of vehicles with rack and pinion steering can have their thumbs broken when a front wheel hits a bump, causing the steering wheel to kick to one side suddenly (leading to driving instructors telling students to keep their thumbs on the front of the steering wheel, rather than wrapping around the inside of the rim)[citation needed]. This effect is even stronger with a heavy vehicle like a truck; recirculating ball steering prevents this degree of feedback, just as it prevents desirable feedback under normal circumstances.

The worm and sector was an older design, used for example in Willys and Chrysler vehicles, and the Ford Falcon (1960s).[1]

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.

Power steering

Power steering helps the driver of a vehicle to steer by directing some of the it's power to assist in swivelling the steered roadwheels 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—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 the 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. The 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[citation needed], 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.

Four-wheel steering

Four-wheel steering (or all-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.

Sierra Denali with Quadrasteer, rear steering angle

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. Some systems, including Delphi's Quadrasteer and the system in Honda's Prelude line, allow the rear wheels to be steered in the opposite direction as the front wheels during low speeds. This allows the vehicle to turn in a significantly smaller radius—sometimes critical for large trucks or tractors and vehicles with trailers.

Many modern vehicles offer a form of passive rear steering to counteract normal vehicle tendencies. For example, Subaru used a passive steering system to correct for the rear wheel's tendency to toe-out. On many vehicles, when cornering, the rear wheels tend to steer slightly to the outside of a turn, which can reduce stability. The passive steering system uses the lateral forces generated in a turn (through suspension geometry) and the bushings to correct this tendency and steer the wheels slightly to the inside of the corner. This improves the stability of the car, through the turn. This effect is called compliance understeer and it, or its opposite, is present on all suspensions. Typical methods of achieving compliance understeer are to use a Watt's Link on a live rear axle, or the use of toe control bushings on a twist beam suspension. On an independent rear suspension it is normally achieved by changing the rates of the rubber bushings in the suspension. Some suspensions will always have compliance oversteer due to geometry, such as Hotchkiss live axles or a semi-trailing arm IRS.

Passive rear wheel steering is not a new concept, as it has been in use for many years, although not always recognised as such. For example, Jaguar independent rear suspension incorporated a small amount of passive rear wheel steering since 1961.

Recent application

In an active four-wheel steering system, all four wheels turn at the same time when the driver steers. There can be controls to switch off the rear steer and options to steer only the rear wheel independent of the front wheels. At slow speeds (e.g. parking) the rear wheels turn opposite of the front wheels, reducing the turning radius by up to twenty-five percent, 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. 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.

General Motors offers Delphi's Quadrasteer in their consumer Silverado/Sierra and Suburban/Yukon. However, only 16,500 vehicles have been sold with this system since its introduction in 2002 through 2004. Due to this low demand, GM discontinued the technology at the end of the 2005 model year.[2]

Previously, Honda had four-wheel steering as an option in their 1987–2000 Prelude and Honda Ascot Innova models (1992–1996). Mazda also offered four-wheel steering on the 626 and MX6 in 1988.

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.

Production cars with active four wheel steering

  • BMW 850CSi (optional)
  • BMW 7-Series (2009 onwards, part of sport package) [3]
  • Chevrolet Silverado (2002–2005) (high and low speed)
  • Efini MS-9 (high and low speed)
  • GMC Sierra (2002–2005) (high and low speed)
  • GMC Sierra Denali (2002–2004) (high and low speed)
  • Honda Prelude (high and low speed, mechanical from 1987 to 1991, computerized from 1992–2001)
  • Honda Accord (1991) (high and low speed, mechanical)
  • Honda Ascot Innova (1992) (high and low speed, computerized from 1992–1996)
  • Infiniti FX50 AWD (option on Sports package) (2008–Present) (high and low speed, fully electronic)
  • Infiniti G35 Sedan (option on Sport models) (2007–Present) (high speed only?)
  • Infiniti G35 Coupe (option on Sport models) (2006–Present) (high speed only)[4]
  • Infiniti G37
  • Infiniti J30t (touring package) (1993–1994)
  • Infiniti M35 (option on Sport models) (2006–Present) (high speed only?)
  • Infiniti M45 (option on Sport models) (2006–Present) (high speed only?)
  • Infiniti Q45t (1989–1994) (high speed only?)
  • Mazda 929 (1992–1995)(computerised, high and low speed)(all models)
  • Mazda 626 (1988) (high and low speed)
  • Mazda MX-6 (1989–1997) (high and low speed)
  • Mazda RX-7 (optional, computerized, high and low speed)
  • Mazda Eunos 800 (1996–2003) (Optional, computerized, high and low speed)
  • Mitsubishi Galant/Sigma (high speed only)
  • Mitsubishi GTO (also sold as the Mitsubishi 3000GT and the Dodge Stealth) (Mechanical) (high speed only)
  • Nissan Cefiro (A31) (high speed only)
  • Nissan 240SX/Silvia (option on SE models) (high speed only)
  • Nissan 300ZX (all Twin-Turbo Z32 models) (high speed only)
  • Nissan Laurel (later versions) (high speed only)
  • Nissan Fuga/Infiniti M (high speed only)
  • Nissan Silvia (option on all S13 models) (high speed only)
  • Nissan Skyline GTS, GTS-R, GTS-X (1986) (high speed only)
  • Nissan Skyline GT-R (high and low speed)
  • Renault Laguna (only in GT version of 3rd generation which was launched October 2007, GT launched on April 2008)
  • Subaru Alcyone SVX JDM (1991–1996) (Japanese version: "L-CDX" only) (high speed only)
  • Toyota Aristo (1997) (high and low speed?)
  • Toyota Camry / Vista JDM 1988–1999 (Optional) [5]
  • Toyota Celica (option on 5th and 6th generation, 1990–1993 ST183 and 1994–1997 ST203) (Dual-mode, high and low speed)
  • Toyota Soarer (UZZ32)

Articulated steering

A front loader with articulated steering.

Articulated steering is a system by which a four-wheel drive vehicle is split into front and rear halves which are connected by a vertical hinge. The front and rear halves are connected with one or more hydraulic cylinders that change the angle between the halves, including the front and rear axles and wheels, thus steering the vehicle. This system does not use steering arms, king pins, tie rods, etc. as does four-wheel steering. If the vertical hinge is placed equidistant between the two axles, it also eliminates the need for a central differential, as both front and rear axles will follow the same path, and thus rotate at the same speed.

Rear wheel steering

A few types of vehicle use only rear wheel steering, notably fork lift trucks, early pay loaders, Buckminster Fuller's Dymaxion car, and the ThrustSSC.[6]

Rear wheel steering can tend to be unstable because in turns the steering geometry tends to decrease the turn radius (oversteer), rather than increase it (understeer). A rear wheel steered automobile exhibits non-minimum phase behavior.[7] It turns in the direction opposite of how it is initially steered. A rapid steering input will cause two accelerations, first in the direction that the wheel is steered, and then in the opposite direction: a "reverse response." This makes it harder to steer a rear wheel steered vehicle at high speed than a front wheel steered vehicle.

Steer-by-wire

The aim of steer-by-wire technology is to completely do away with as many mechanical components (steering shaft, column, gear reduction mechanism, etc.) as possible. Completely replacing conventional steering system with steer-by-wire holds several advantages, such as:

  • The absence of steering column simplifies the car interior design.
  • The absence of steering shaft, column and gear reduction mechanism allows much better space utilization in the engine compartment.
  • The steering mechanism can be designed and installed as a modular unit.
  • Without mechanical connection between the steering wheel and the road wheel, it is less likely that the impact of a frontal crash will force the steering wheel to intrude into the driver's survival space.
  • Steering system characteristics can easily and infinitely be adjusted to optimize the steering response and feel.

As of 2007 there are no production cars available that rely solely on steer-by-wire technology due to safety, reliability and economic concerns, but this technology has been demonstrated in numerous concept cars and the similar fly-by-wire technology is in use in both military and civilian aviation applications. Removing the mechanical steering linkage in road going vehicles would require new legislation in most countries.

Safety

For safety reasons all modern cars feature a collapsible steering column (energy absorbing steering column) which will collapse in the event of a heavy frontal impact to avoid excessive injuries to the driver. Airbags are also generally fitted as standard. Non-collapsible steering columns fitted to older vehicles very often impaled drivers in frontal crashes, particularly when the steering box or rack was mounted in front of the front axle line, at the front of the crumple zone. This was particularly a problem on vehicles that had a rigid separate chassis frame, with no crumple zone. Most modern vehicle steering boxes/racks are mounted behind the front axle on the front bulkhead, at the rear of the front crumple zone.

Audi used a retractable steering wheel and seat belt tensioning system called procon-ten, but it has since been discontinued in favor of airbags and pyrotechnic seat belt pre-tensioners.

Collapsible steering columns were invented by Bela Barenyi and were introduced in the 1959 Mercedes-Benz W111 Fintail, along with crumple zones.

This safety feature first appeared on cars built by General Motors after an extensive and very public lobbying campaign enacted by Ralph Nader.

Ford started to install collapsible steering columns in 1968.[8]

Cycles

Steering is crucial to the stability of bicycles and motorcycles. For details, see articles on bicycle and motorcycle dynamics and countersteering.

Ship and boat steering

Ships and boats are usually steered with a rudder. Depending on the size of the vessel, rudders can be manually actuated, or operated using a servomechanism, or a trim tab/servo tab system.

See also

References

  • Encyclopedia of German Tanks of World War Two by Peter Chamberlain and Hilary Doyle, 1978, 1999

External links


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Look at other dictionaries:

  • steering — index guidance, leading (guiding), management (supervision) Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

  • Steering — Steer ing, a. & n. from {Steer}, v. [1913 Webster] {Steering wheel} (Naut.), the wheel by means of which the rudder of a vessel is turned and the vessel is steered. [1913 Webster] …   The Collaborative International Dictionary of English

  • steering — ► NOUN ▪ the mechanism in a vehicle, vessel, or aircraft which allows it to be steered …   English terms dictionary

  • steering — A mechanism for controlling the direction of a vehicle. See absorbing steering ackermann steering adjustable steering all wheel steering assisted steering axle and steering ball and nut steering ball steering …   Dictionary of automotive terms

  • steering — [[t]stɪ͟ərɪŋ[/t]] 1) N UNCOUNT The steering in a car or other vehicle is the mechanical parts of it which make it possible to steer. 2) ADJ: ADJ n A steering committee or a steering group is a group of people that organizes the early stages of a… …   English dictionary

  • steering — noun ADJECTIVE ▪ heavy, light (both BrE) ▪ power, power assisted (esp. BrE) ▪ precise VERB + STEERING ▪ …   Collocations dictionary

  • steering — n. power steering * * * [ stɪ(ə)rɪŋ] power steering …   Combinatory dictionary

  • steering — noun (U) the parts of a car, boat etc that allow you to control its direction: power steering | The steering on this car is lighter …   Longman dictionary of contemporary English

  • Steering — Steer Steer, v. t. [imp. & p. p. {Steered} (st[=e]rd); p. pr. & vb. n. {Steering}.] [OE. steeren, steren, AS. sti[ e]ran, st[=y]ran, ste[ o]ran; akin to OFries. stiora, stiura, D. sturen, OD. stieren, G. steuern, OHG. stiuren to direct, support,… …   The Collaborative International Dictionary of English

  • steering — noun Equipment used to control direction, something used to steer. I hit a rock with my car and broke the steering …   Wiktionary


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