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Steering is the term applied to the collection of components, linkages, etc. which will allow for a vessel (ship, boat) or vehicle (car) to follow the desired course. An exception is the case of rail transport by which rail tracks combined together with railroad switches provide the steering function.

Part of steering mechanism: tie rod, steering arm, king pin.

Contents

[hide] 1 Introduction 2 Rack and pinion, recirculating ball, worm and sector 3 Power steering 4 Speed Adjustable Steering 5 Four-wheel steering

o 5.1 Recent application o 5.2 Production cars with active four wheel steering

6 Articulated steering 7 Steer-By-Wire 8 Safety 9 Cycles 10 See also 11 External links

12 References

[edit] 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 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 tanks usually employ differential steering — that is, the tracks are made to move at different speeds or even in opposite directions to bring about a change of course.

[edit] 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 kingpins 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"; it also does not normally have any backlash, or slack.[citation needed] 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). 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 steering linkage 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 traveling 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 worm and sector was an older design, used for example in Willys and Chrysler vehicles, and the Ford Falcon (1960s).[1]

[edit] Power steering

Main article: Power steering

As vehicles have become heavier and switched to front wheel drive, the effort to turn the steering wheel manually has increased - often to the point where major physical exertion is required. To alleviate this, auto makers have developed power steering systems. 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 assist level 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.

[edit] Speed Adjustable Steering

An outgrowth of power steering is speed adjustable 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, 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-adjustable power steering systems reduce the pressure fed to the ram as the speed increases, giving a more direct feel. This feature is gradually becoming commonplace across all new vehicles.

[edit] 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.

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 for 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.

[edit] Recent application

In an active 4ws 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.

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 will not offer the technology on the 2007 update to these vehicles.

Previously, Honda had four-wheel steering as an option in their 1987-2000 Prelude, and 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.

[edit] Production cars with active four wheel steering

BMW 850CSi (optional) BMW 7-Series (2009

onwards, part of sport package) [2]

Efini MS-9 (high and low speed)

GMC Sierra (2002) (high and low speed)

Honda Prelude (high and

Mitsubishi Galant /Sigma (high speed only) Mitsubishi GTO (also sold as the Mitsubishi

3000GT and the Dodge Stealth) (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)

low speed, fully mechanical from 1987 to 1991)

Honda Prelude (high and low speed, fully electronic from 1991 to 2001)

Honda Accord (1991) (high and low speed, mechanical)

Infiniti G35 Sedan (option on Sport models) (2007-Present) (high speed only?)

Infiniti G35 Coupe (option on Sport models) (2006-Present) (high speed only) [3]

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 626 (1988) (high and low speed)

Mazda MX-6 (1989-1997) (high and low speed)

Mazda RX-7 (optional, computerized, high and low speed)

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 SVX JDM (1991-1996) (Japanese version: "L-CDX" only) (high speed only)

Toyota Aristo (1997) (high and low speed?) Toyota Camry JDM 1990-1992 Camry

Prominent (Optional)(high and low speed)[citation needed]

Toyota Celica (option on 5th and 6th generation, 1990-1993 ST183 and 1994-1997 ST203) (Dual-mode, high and low speed)

Toyota Soarer (UZZ32)

[edit] 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.

[edit] 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 similar technology is in used in military and civilian aviation applications.

[edit] 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. Non-collapsible steering columns very often impale drivers in frontal crashes. Audi used a retractable wheel system called procon-ten but has since been discontinued.

Collapsible steering columns were invented by Bela Barenyi.

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.[4]

[edit] Cycles

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

[edit] See also

Bump Steer Caster angle Camber angle DIRAVI Dry steering Power steering Steer-by-wire Steering law Steering ratio Steering wheel (ship) Steering wheel cover tiller

Skid steer

SimHydraulics 1.5

Power-Assisted Steering Mechanism

The model represents a simplified version of a power-assisted steering mechanism showing all its

major parts: double-acting hydraulic cylinder; rotational valve; fixed- displacement pump; pressure-

relief valve; mechanical load consisting of a reduced to a driving shaft inertia, damping, and

stiffness; and torsion bar installed between the steering wheel and the pinion of a rack-and-pinion

mechanism. The rotation of the steering wheel causes the torsion bar to twist with respect to the

pinion position. The deformation of the bar is transformed into opening of the rotational valve, which

connects ports of the cylinder to pressure or exhaust lines depending on the direction of rotation. If

deformation exceeds 9 deg the wheel is connected directly to the pinion through the hard stops

installed in parallel with the torsion bar. The cylinder moves the steering rods and, at the same time,

twists the torsion rod in the opposite direction until the valve is in neutral position.

Copyright 2008-2009 The MathWorks™, Inc.

Power steering mechanism Document Type and Number:United States Patent 4296673

Abstract:

A power steering mechanism includes a control valve which acts both to control the flow of pressurized fluid to and from a pair of working chambers of the mechanism and as a pressure relief valve to limit the maximum pressure which may be communicated to the working chambers. The valve includes a stack of plane annular discs which are relatively rotatable and arranged coaxially with the control shaft of the steering mechanism. The discs are resiliently biased together and define valve ports and compartments which cooperate to control the flow of pressure fluid dependent upon the relative rotational positions of the discs. Rotation of the control shaft causes rotation of one of the discs relative to the others to correlate the ports and chambers so as to supply pressure fluid to one chamber and to vent fluid from the other chamber. If the fluid pressure within the compartments of the control valve should exceed a predetermined value, the resilient bias holding the discs together is overcome and the discs separate slightly to open communication between the fluid supply ports and the vent ports.

Another embodiment of the mechanism includes a linkage arranged to separate the discs and an abutment which cooperates with the linkage when the mechanism reaches either end of its range of travel. The working chambers are incapacitated when the linkage separates the discs so as to prevent damage to the mechanism from a too-high fluid pressure.

Steering Mechanism

Jack the front of the car up and place securly on axle stands.

Grab each of the front wheels at each side and shake the wheels vigourously. There should be no free play and the steering mechanism should be free to turn.

Check that the wheels do not wobble indicating play in the wheel bearings, ball joints or suspension mountings or bushes.

Repeat these tests grabbing each wheel from the top and bottom to check for any vertical play.

Spin the wheels and make sure that they turn freely. There may be some friction from the brake pad. If this is the case then touch the brakes and repeat. If the wheel tends to lock up, this can be an indication of worn wheel bearings along with any grating noise heard while the wheel is spinning. Again make sure that this is not the brakes.

Repeat these tests to the back wheels after the back of the car has been jacked and placed on axle stands.

Power-Assisted Steering Mechanism

The model represents a simplified version of a power-assisted steering mechanism showing all its major parts: double-acting hydraulic cylinder; rotational valve; fixed- displacement pump; pressure-relief valve; mechanical load consisting of a reduced to a driving shaft inertia, damping, and stiffness; and torsion bar installed between the steering wheel and the pinion of a rack-and-pinion mechanism. The rotation of the steering wheel causes the torsion bar to twist with respect to the pinion position. The deformation of the bar is transformed into opening of the rotational valve, which connects ports of the cylinder to pressure or exhaust lines depending on the direction of rotation. If deformation exceeds 9 deg the wheel is connected directly to the pinion through the hard stops installed in parallel with the torsion bar. The cylinder moves the steering rods and, at the same time, twists the torsion rod in the opposite direction until the valve is in neutral position.

Copyright 2008-2009 The MathWorks™, Inc.

Steering mechanisms

One of the most difficult things to design and build on a kart is the steering mechanism. The steering mechanism is used to turn the kart around the bends in the track. There are many different ways in which this can be done.

Steering controlsThis is how the kart driver controls how much the wheels will turn around any corner. Most kart

designs use handlebars to steer the kart like the ones used on bikes, but some use a steering wheel or long rods with handles that can be pushed forward and back to turn the wheels, as you can see in the photographs (right).

But how do we convert the turning of the wheels or handlebars, or the pushing and pulling of handles, into the right movement of the wheels? - We use a steering linkage.

Steering linkagesA steering linkage changes the movement of the driver's handle bar or steering wheel into the turning of the wheels to go round corners. Steering linkages on a kart are usually made from a system of rods and pivot points (moving connections) between the handlebars or steering wheel to the front wheels of the kart.

Some karts put the wheels in a diamond shape, with a single front wheel for steering. On a kart with a single front wheel, like a bike's, the steering linkage can be very simple, as there is only one wheel to turn. This can be done by having a set of handlebars connected to the front forks exactly as you would do with a bike. If the driver is sitting further back from the front wheel, two connecting rods can be used to steer the wheel either using simple handles, as shown in the photograph above, or handlebars as in the drawing below.

Having only three wheels can make the kart unstable when going round corners, increasing the risk of it falling over.

You can connect two front wheels to a single pivot point in the centre of the front axle, and use the same mechanisms. But this can cause the wheels to jam on sharp bends and it also makes the kart unstable when turning.

The kart is much more stable if it has two wheels at the front of the with their own pivot points, but the steering linkages get more complicated.

The names of many of the parts in a steering linkage using two front wheels are shown below.

The Pitman arm is used to connect the steering column to the tie rods. The arm changes the rotational (turning) movement of the handlebars and steering column to a sideways motion in the tie rods.

The tie rods are connected to the steering arms, which then change the sideways motion back into a rotational movement, which turns the wheels.

By changing the length of the Pitman arm you can change the amount the wheels turn.

The steering arms are connected to the wheels using a stub axle, or bicycle front forks in most cases and a pivot point called a king pin. It is called a king pin because in early vehicle designs, it was a large pin that held all the other parts together.

These linkages show how the kart driver can make the wheels turn from turning the handlebars or steering wheel, but by how much should we turn them to get round a corner?

Turning angleYou may think at first that both front wheels of the kart have to turn to the same angle to get round a bend, but that isn't correct.

If a kart goes around a circle or bend, the outside wheels have to travel further than the inside wheels – try running around a circle with a friend at different distances and see who has to run faster to keep up!

To make the wheels go round these different distances they need to turn by different amounts.

If we draw an angle between the rear axle of the kart, the centre of the circle and the centre of each front wheel, we see that this angle is different for each wheel. Using trigonometry, we can see that these angles are also the angles by which each wheel needs to turn into the curve. The inner wheel always turns more than the outer wheel; the difference between these two angles (Θ1 minus Θ2) is called the 'toe-in' of the wheels.

So, for example, if the inner wheel turned by Θ1 = 45°, and we assume the kart dimensions to be x = 1m and y = 2m, then;

r = y/tan (Θ1) = 2/tan(45°) = 2/1 = 2m tan (Θ2) = y/(x+r) = 2/3, so Θ2 = 33.6° Toe-out = Θ1 - Θ2 = 11.4°

So how do we get the two front wheels to turn by different amounts?

Ackermann steeringAckermann steering is named after Rudolph Ackermann, who designed a solution to the turning problem in London in 1817. The idea is to angle the steering arms of the steering linkage towards the centre of the kart so that the tie rods change the wheel angles by different amounts.

Calculating of the exact angle of each steering arm is complicated but angling the steering arms so that a line drawn from the centre of each arm meets at the centre of the rear axle gives a good result.

ConclusionsUsing a design with four wheels at each corner makes the kart more stable but makes the steering more complicated.

The front wheels should turn at different angles to get round a bend.This can be achieved by using Ackermann steering i.e. angling the steering arms towards the centre of the rear axle.

You know that when you turn the steering wheel in your car, the wheels turn. Cause and effect, right? But a lot of interesting stuff goes on between the steering wheel and the tires to make this happen.

In this article, we'll see how the two most common types of car steering systems work: rack-and-pinion and recirculating-ball steering. Then we'll examine power steering and find out about some interesting future developments in steering systems, driven mostly by the need to increase the fuel efficiency of cars. But first, let's see what you have to do turn a car. It's not quite as simple as you might think!

Turning the CarYou might be surprised to learn that when you turn your car, your front wheels are not pointing in the same direction.

For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

There are a couple different types of steering gears. The most common are rack-and-pinion and recirculating ball.

Rack-and-pinion SteeringRack-and-pinion steering is quickly becoming the most common type of steering on

cars, small trucks and SUVs. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.

The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle (see diagram above).

The rack-and-pinion gearset does two things:

It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.

It provides a gear reduction, making it easier to turn the wheels.

On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right).

The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio.

Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response -- you don't have to turn the steering wheel as much to get the wheels to turn a given distance -- which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.

Some cars have variable-ratio steering, which uses a rack-and-pinion gearset that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel's turning limits.

Power Rack-and-pinionWhen the rack-and-pinion is in a power-steering system, the rack has a slightly different design.

Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist.

We'll check out the components that provide the high-pressure fluid, as well as decide which side of the rack to supply it to, later in the article. First, let's take a look at another type of steering.

Recirculating-ball SteeringRecirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system.

The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel --

without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.

Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is provided by supplying higher-pressure fluid to one side of the block.

Now let's take a look at the other components that make up a power-steering system.

Power SteeringThere are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism.

PumpThe hydraulic power for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.

As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds.

The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped.

Rotary ValveA power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called the rotary valve.

The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has.

As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel.

Animation showing what happens inside the rotary valve when you first start to turn the steering wheel

When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

It turns out that this type of power-steering system is pretty inefficient. Let's take a look at some advances we'll see in coming years that will help improve efficiency.

The Future of Power SteeringSince the power-steering pump on most cars today runs constantly, pumping fluid all the time, it wastes horsepower. This wasted power translates into wasted fuel.

You can expect to see several innovations that will improve fuel economy. One of the coolest ideas on the drawing board is the "steer-by-wire" or "drive-by-wire" system. These systems would completely eliminate the mechanical connection between the steering wheel and the steering, replacing it with a purely electronic control system. Essentially, the steering wheel would work like the one you can buy for your home computer to play games. It would contain sensors that tell the car what the driver is doing with the wheel, and have some motors in it to provide the driver with feedback on what the car is doing. The output of these sensors would be used to control a motorized steering system. This would free up space in the engine compartment by eliminating the steering shaft. It would also reduce vibration inside the car.

General Motors has introduced a concept car, the Hy-wire, that features this type of driving system. One of the most exciting things about the drive-by-wire system in the GM Hy-wire is that you can fine-tune vehicle handling without changing anything in the car's mechanical components -- all it takes to adjust the steering is some new computer software. In future drive-by-wire vehicles, you will most likely be able to configure the controls exactly to your liking by pressing a few buttons, just like you might adjust the seat position in a car today. It would also be possible in this sort of system to store distinct control preferences for each driver in the family.

In the past fifty years, car steering systems haven't changed much. But in the next decade, we'll see advances in car steering that will result in more efficient cars and a more comfortable ride.

steering system  

in automobiles, steering wheel, gears, linkages, and other components used to control the direction of a vehicle’s motion. Because of friction between the front tires and the road, especially in parking, effort is required to turn the steering wheel. To lessen the effort required, the wheel is connected through a system of gears to components that position the front tires. The gears give the driver a mechanical advantage, i.e., they multiply the force he applies, but they also increase the distance through which he must turn the wheel in order to turn the tires a given amount. Various types of gear assemblies, none with any decisive

advantages over the others, are used, although some manufacturers prefer a rack-and-pinion system. In faster, heavier cars the amount of force required to turn the tires can be very great. Many of these cars use a power-steering system. The system contains a hydraulic booster, which operates when the engine is running and supplies most of the necessary force when the driver turns the wheel. When a vehicle turns at a rate exactly proportional to the rate at which the steering wheel is turned, it is said to have neutral steering; if it turns at a slower rate it is said to understeer; if it turns faster it is said to oversteer. While any vehicle can react in any of these ways under extreme conditions, most automobiles are built to understeer. Racing vehicles are often designed for neutral steering; few vehicles are built to oversteer, since this is considered hazardous by many authorities. As a safety feature in many modern cars the column on which the steering wheel is mounted will collapse if the driver is thrown against the wheel in a collision.