CONCLUSION

In a conclusion, steering system is one of the most important feature in automobile. This system will control all of the direction and movement of the wheel of our vehicle. Besides, the components that are involved in this system are steering knuckle, centre link, anti-sway bar, upper control arm and adjusting sleeve. This components must always in good condition so that steering system will run smoothly and more efficient to provide the driver with good experience of driving on the road. For this group assignment we are given three different type of car manufacturer such as Cadillac, Subaru, and Citroen. Different type of car manufacturer has different characteristic in technology invented and has their own advantages and disadvantages. This characteristic is very important because when we want to buy a car we must know at least how this system works and how this feature can help us to enjoy a good experience of driving in our life.

First of all, in this discussion we are covering on Cadillac automobile. Cadillac type of car preferred on rear-wheel steering system that can control all four of its tyre by active chassis system. Cadillac’s rear-wheel steering system doesn’t just make the car easier to drive in low speed scenarios, it also increase agility at higher speeds. Other than that, by controlling all the tires this car or automobile is easier to handle and can provide more safety characteristic to the driver. The benefit of this active chassis control is switching between comfort, sporty and normal modes, handling performance can be influenced to suit the specific driving situation and driver. Furthermore, if power steering system in Cadillac fail to work the vehicle still can be steered using manual effort this condition can avoid unnecessary death from the failure of this system.

Second, Subaru is the other type of car manufacturer that is given to our group. For Subaru the rotational force that are transferred driver rotational force into a lateral one is the main function of steering system in Subaru’s type of car. Besides, the main characteristic of steering system is power assist. Hydraulic pressure provides added boost at low vehicle speeds to help the driver turn the steering wheel. At this point pressure is reduced when vehicle speeds is higher. Furthermore, rack and pinion steering system is the common one because this type of steering system is more precise because there are only few pivot points and parts. Thus, with only fewer parts this system is lighter and can save more room space for any modification. Although, it has its own advantages there are some disadvantages of using this type of steering system such as leakage, less durability and vibration.

Finally, the last car manufacturer is Citroen. The steering system that have been used is Diravi System. This system also known as “power steering with power assisted return” usually used for mainstream vehicles. 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. A hydraulic power safety prioritisation valve sets hydraulic fluid availability to each circuit in the Citroën system. In a hydraulic system failure, order of loss is first steering, then suspension, then brakes. The advantages of this system are its requires minimal physical exertion in the SM the steering wheel can be turned lock-to-lock with one finger when the car is standing and can be set up permitting vehicle to travel in a straight line without driver input on a constant camber road in still conditions while the disadvantages of this system are the driver must survey visual cues to determine approach of road holding limit and Cannot allow both hands to leave steering wheel when navigating curves  because of rapid self centering.

Based on the given points above its prove that different car manufacturer preferred different type of steering system. Cadillac prefer rear-wheel steering system and active chassis control while rack and pinion steering system is prefer by Subaru car manufacturer but for Citroen the car manufacturer stick with Diravi system. Each one of the steering system have their own disadvantages and advantages but for steering system the mutual characteristic is to provide good driving experience to the driver, make sure the safety precaution is the first thing to do and reduce the probability of system failure.

CADILLAC

CADILLAC

 .      Technology used (‘Active Chassis System’)

Active Chassis System means it is available to steer all 4 tires by adding rear-wheel steering system. The idea, which Honda has been a big proponent of in the past, disappeared from showrooms until Porsche equipped its new 911 GT3 with rear-wheel steering in 2014

.      System operation

a)      The rear wheels can turn up to 3.5˚ in either direction

b)      This system boosts

·         low-speed mobility (in effect shortening the wheelbase)  

·         high-speed stability (by lengthening it)

1              

                               Benefit of technology used

·         making the full-size four-door easier to move in tight spaces and city driving scenarios 

·         easier to drive in low speed scenarios

·         Active-on-demand all-wheel drive is also part of the Active Chassis System, making the most of handling and stability in all weather conditions

·         Cadillac’s rear-wheel steering system doesn’t just make the car easier to drive in low speed scenarios, it also increase agility at higher speeds.

SUBARU

DECADES AGO, SPORTS CARS ESTABLISHED THIS TYPE OF SYSTEM AS DESIRABLE DUE TO ITS PRECISION AND RESPONSE, AND TODAY MOST PASSENGER VEHICLES HAVE IT AS STANDARD EQUIPMENT.

                                           

Glossing over reference to rack-and-pinion steering in specification charts is easy to do. We tend to dismiss it because that’s what all vehicles seem to have. However, other systems do remain on the market. Understanding rack-and-pinion steering and its benefits will contribute to appreciating what makes a Subaru, a Subaru.

WHEN YOU TURN THE STEERING WHEEL

With the action of turning the steering wheel to cause a vehicle to change direction, a number of components come into play. Although enhanced by other systems, here are the basics of rack-and-pinion steering:

The steering wheel rotates the steering column.

The teeth on the pinion gear at the end of the steering column engage with teeth on a rack in the steering gear assembly.

The rack assembly moves the tie rod assemblies to which it’s connected.

The tie rod assemblies push or pull on the steering knuckles connected to the front wheels, causing the direction in which the wheels are pointed to change from one side to another.

The point of the steering system is to transfer the driver’s rotational force into a lateral one that alters the direction in which the vehicle is traveling. The steering force from the teeth in the rotating pinion gear becomes a lateral force via the gear teeth on the rack.

All Subaru steering systems have power assist. Hydraulic pressure provides added boost at low vehicle speeds to help the driver turn the steering wheel. Pressure is reduced at higher vehicle speeds.

electrical rack and pinion.

BENEFITS

One of the main differences between rack-and-pinion steering and other types – mostly recirculating-ball steering – is the direct connection of the components. Recirculating-ball systems transfer steering forces through a series of gears and ball bearings that tend to diminish the feel of the tires on the road. That feel is transmitted better by a steering rack. (Recirculating-ball steering systems are better suited to heavy-duty vehicles, because they can be engineered to bring greater forces to bear for turning the front wheels.)Rack-and-pinion steering also is more precise. A small degree of movement with the steering wheel brings immediate results on the road. Other steering systems tend to have some play in the steering wheel when traveling straight down the road. The rack-and-pinion system has proven more active and fun to drive.Plus, the simpler rack-and-pinion steering systems are easy to service, which contributes to a lower cost of ownership.

                                                       

CARING FOR YOUR STEERING SYSTEM

• Important things to note about steering system care and maintenance:
• ü  When checking fluid levels under the hood, don’t overlook the power steering fluid reservoir. (Check it monthly.)
• ü  Markings on the reservoir indicate maximum and minimum levels for the fluid when hot or cold.
• ü  Do not hold the steering wheel at the fully locked position (left or right) for more than five seconds, because it may damage seals within the system.

CITROEN

DIRAVI

In the DIRAVI system invented by Citroën, the force steering the wheels comes from the car's high pressure hydraulic system and is always the same no matter what the road speed is. Turning the steering wheel moves the wheels simultaneously to a corresponding angle via a hydraulic cylinder. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to centre position. The amount of pressure applied is proportional to road speed, so that at low speeds the steering is very light, and at high speeds it is very difficult to move more than a small amount off centre. Citroën engineer Paul Magès invented the system as part of its effort to engineer a practical high horsepower, front wheel drive car.

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, and was known as 'VariPower' in the UK and 'SpeedFeel' in the U.S.

While DIRAVI is not the mechanical template for all modern power steering arrangements, it did innovate the now common benefit of speed sensitive steering. In the late 1960s, General Motors offered a variable-ratio power steering system as an option on Pontiac and other vehicles.

Citroen DIRAVI type closed-center arrangement.

The SM DIRAVI is based on the DS Dirass with a closed-center arrangement, speed powered steering wheel and forced return. The CX DIRAVI differ structurally with respect to the SM DIRAVI by another control command. Herein, the hydraulic arrangements with a different kind of differentiaalregelaar are grouped together next to the axis. XM DIRAVI equals the CX DIRAVI with the difference that the eccentric disk has no recess in the straight position. Furthermore, the straight-ahead position with a rotary knob to adjust under para vane. This is done because it DIRAVI wheelhouse is accessible from the ZPJ (4) V6 engine compartment.

Functioning

                                                                                           Neutral position  

1. Principle of control when cornering
A very small revolution of the driving shaft makes piston T1 slides, chamber B is then related either to incoming pressure or to outgoing pressure: chamber B develops hydraulic volume variations.
The size of the hydraulic section to chamber B and the onset of the pressure depends on how fast the driver turns his steering wheel.

The piston of the rack hydraulic command deals with two opposite forces of the same values:

                                         The balance is triggered by valve T1.

fig.1

Left turn - Fig. 2

The volume variation in chamber B (increase) pushes on S of the piston a dominating force pushing the rack to the left

This efficient control lets the driver a very small effort to give to the steering wheel. This gives a vehicle that is very easy to drive in a city and enhance very low speed manoeuvres

                                                               Fig.2

Because of the cam design, the effort increases with the cornering angle ; this allows the driver an even higher precision for path modifications.

2 - Principle of the self-centering steering gear on straight lines

The very little effort to apply to the steering wheel when cornering requires a perfect centralizing system to keep the car with an efficient stability.
To preserve this stability whatever the speed, the return effort to set back the steering wheel is applied on the cam by the roller increases with the speed through the centrifugal regulator.

This centralizing effort increase enhances the precision when rapid course changes.
The regulator speed connected to the front of the gearbox depends on the speed of the vehicle.

Neutral position 50 km/h - Fig. 3

The pressure given by the centrifugal regulator pushes the return piston and puts the system at its center position.

              Neutral position 200 km/h - Fig. 4

Because of the centrifugal force, the leads move apart, piston T2 moves and increases the pressure on the return piston increasing the effects of the centralizing system.

Feature

• Fully hydraulic (no direct mechanical connection between the steering wheel shaft and the steering pinion during normal operation).
• Power operated unlike conventional power assisted steering systems.
• Rapid self centering to straight ahead position - whenever the engine is running the steering wheel will return to center, even when parked
• Artificial feel inbuilt - centering force varies in proportion to vehicle speed and/or steering wheel deflection.
• Power for the system from a regulated high pressure hydraulic pump which also operates the brakes and suspension system.
• The steering is operated by a rack and pinion system which normally only works as a feedback loop.
• The rack takes the form of a double acting hydraulic ram, but is capable of taking over full steering function in the event of a hydraulic failure. During normal operation the rack and pinion merely provides a position indication to the steering control valve through the pinion shaft. The rack and pinion do the actual steering only in the absence of system hydraulic pressure.
• A hydraulic power safety prioritisation valve sets hydraulic fluid availability to each circuit in the Citroën system.
• In a hydraulic system failure, order of loss is first steering, then suspension, then brakes.

Advantages

• No steering kickback - blowouts, potholes, ruts, etc. cannot affect the steering wheel or the direction of the steered wheels which can only be moved by steering wheel input, since apparent feedback is entirely artificial and bears no relation to the actual forces on the front wheels from the vehicle's inertia and the roadway
• Requires minimal physical exertion - In the SM the steering wheel can be turned lock-to-lock with one finger when the car is standing
• Very fast (minor steering inputs equal large front wheel movements)
• Can be set up permitting vehicle to travel in a straight line without driver input on a constant camber road in still conditions

Disadvantaged

• No feedback to the driver - apparent feedback is entirely artificial and bears no relation to the actual forces on the front wheels from the vehicle's inertia and the roadway
• Driver must survey visual cues to determine approach of road holding limit
• Requires familiarization - novices find DIRAVI too fast and sensitive
• Cannot allow both hands to leave steering wheel when navigating curves - because of rapid self centering

REFERENCE

i. http://www.xmclub.nl/forum/diravi-direction-%C3%A0-rappel-asservi-het-doel-en-nut

ii. http://smclubdefrance.org/english/self-centering-steering-gear.html

iii. http://www.citroen.co.uk/new-cars-and-vans/ds-limited-edition/1955-limited-edition

POWER STEERING

For decades, the steering wheel remained nothing more than a wooden circle, mounted inside the car and through which the driver controlled the directional movement of the vehicle. It had and it served no other purpose. As you might imagine, steering with early wheels was not an easy task, as the whole procedure was done mechanically: the driver pulled the steering wheel to the left or right. Of course, the wheels resisted the commands, and the friction with the surface below made steering a difficult task at times, especially when the car was stationary

Several attempts to introduce power steering to the automotive world were made. G.W. Fitts received a patent for a power steering gear all the way back in 1876, while a vacuum-based system was patented in 1904; in 1902, Frederick W. Lanchester patented a hydraulic power system in the UK. Yet, none of them made it into production.

It was in the 1920s when experiments with what was to become the precursor of power steering began. Francis W. Davis, an engineer with the Pierce Arrow Motor Car Company, trying to make steering for truck drivers a little easier, ended up inventing the first power steering system to be fitted into a car.

Coincidentally or not, it was the nautical industry which sparked the advent of power steering in cars, as it had done with the steering wheel itself. Davis based some of his early work on the power steering system used in ships, only that with hydraulics. He slowly worked his way around several problems he encountered and managed to fit his system into a Cadillac.

Between 1931 and 1943, Davis received patents for five different inventions which made up the power steering system. His invention was acknowledged by GM, who made a contract with Davis to fit it into future Cadillac models. The contract was eventually scrapped in 1934 due to the economic crisis.

In 1936 the Bendix Corporation took note of Davis' work and signed a deal with him to build and promote the product. By 1939, ten models using Davis' hydraulic power system had been built and only two sold. Again, coincidentally or not, GM bought two systems to fit them into experimental Buicks. And then the war broke out...

As it did in so many fields of human existence, the war pushed power steering development into high gear. The driving force behind it was the Military one, who wanted, obviously, easy controllable war machines. Bendix-Davis systems saw action for the first time in 1940, after being fitted into Chevrolet armored vehicles built for the British Army. By the end of the war, over 10,000 vehicles equipped with power steering were roaming the battlefields.

After the war, Chrysler began developing its own power steering, based on Davis' expired patents. The system was featured on the Chrysler Imperial and was named Hydraguide. Since competition is the driving force of the industry during peace time, GM made a deal with Davis for the system and by 1953, 1 million vehicles using it were built. The success was immense and instant: by 1956, one in four cars on the roads had power steering. By the next decade, 3.5 million power steering systems were sold.

Since Davis, several types of power steering systems have been developed. Depending on what is used to power the steering wheel, the systems can be hydraulic, as Davis', electro-hydraulic, electric and so on. Some manufacturers, like Citroen and AM General, patented their own technologies (DIRAVI and Servotronic, respectively).

Today, the ratio has turned against non-power-assisted vehicles. Yet, despite a new role for the steering wheel, that of making the driver's task easier, the steering wheel itself remained in design as simple as it got. The only added function was the introduction of the airbag in the 1970s.

                                                           

INTRODUCTION

The steering system in an automobile is the system that allows the driver to modify according to his will the trajectory of his vehicle according to the circumstances.
The vehicle stability, comfort and security will also depend on the steering system used on the car. The steering system is a main issue. Careful design has to be provided for this element. A perfect steering system has to fully comply with the following elements:

·         Security,

·         Smoothness,

·         Precision,

·         Irreversibility,

·         Stability,

·         Compatibility with the suspensions.

To comply with all these requirements and to allow the driver to use without any difficulty a car with a such exceptional road handling and a car with a high performance engine, Citroën created a new steering system which has no equivalent in the world.
The steering system goal is to enhance the security at high speed, and to speed up the movements at low speed and also to increase the comfort in removing any reaction. This breakthrough in the automotive domain at least equivalent to the hydropneumatic suspension breakthrough.





STEERING SYSTEM

The steering system consists of a rack and pinion with a hydraulic variable rate power according to the vehicle speed, with centraliser device.

Ratio: 1 : 9,4.

·         Number of steering wheel revolutions (max left to max right): 2.

·         Steering wheel column in two parts using two double universal joints.

·         The steering wheel can be adjusted in both angle and depth.

Turning circle

·         between bands: 10,50 meter

·         between walls: 11,50 meter

History of steering System

The steering wheel of a car is one of those instruments which we pretty much take for granted. We mean, who ever heard of a car with no steering wheel? When buying a new car for instance, we bring hell upon the head of the sales person, asking all kinds of more or less ridiculous questions about the fabric on the seats, the number of bolts holding the wheels in place or the composition of the paint. In very rare cases, we think to ask about the steering wheel.

Some of us even think the steering wheel magically came to be at the same time the car did. That it was somehow already in the mind of the inventor as the perfect tool to make the new contraption work. That's not entirely true. The steering wheel did not came to be at the same time with the car, but was adopted later, as it became obvious its shape is perfect for the task.

You must take into account that at the turn of the 19th century, when the idea of the automobile germinated in the minds of the time's inventors, there was really a single man-made machine the man himself controlled: boats.

There was no way to steer a train, as its tracks guided it from point A to point B without human input. Turning a carriage only meant pulling the harness left or right, depending on your intentions, so there was no need for an additional mechanical device to be created.

All of the above meant that the car's inventors had really one source for inspiration when trying to figure out how to steer a car: boats. Boats used human activated rudders, controlled by means of tillers, to turn, so the idea appealed to the car's creators.

By 1894 however, the use of a tiller to steer a car became more and more ineffective. Taking inspiration from the same nautical industry, car builders began replacing the tillers with ship-inspired helms. Simpler and smaller than their nautical counterparts, the steering wheels in the car made their mark during the Paris-Rouen race, when the Panhard model driven by Alfred Vacheron was first recorded using a steering wheel to turn.

The ease of operation shown in the 1894 race meant that by 1898, all Panhard et Levassor cars came equipped as standard with steering wheels. The principle quickly caught on and similar systems sparked across the world. In Britain, Charles Stewart Rolls bought a Panhard from France and implemented the steering wheel into his designs. By 1899, the steering wheel fever expanded to the US, where Packard introduced the steering wheel on one of its models. By the time the Model T arrived, the steering wheel was an essential part of the car.

After that moment, the steering wheel stuck with the car, with its most common shape, that of a circle, unchanged for more than a century now. What did (and still does) change however is the purpose the steering wheel serves. As humanity crawls its way through the 21th century, the steering wheel is quickly leaving behind its established role of "helm of the car" and becomes more and more of a command hub for the entire vehicle. But let's take it one step at the time.