Rack and pinion steering
Steering column with flexible coupling
Reduction ratio 1/24.5

Turning radius beween kerbs:

  • 10.9m = 17.9ft (Saloon)
  • 11.8m = 19.4ft (Prestige)
  • 11.8m = 19.4ft (Estate)
     

    Turning radius between walls:

  • 11.8m = 19.4ft (Saloon)
  • 12.7m = 20.1ft (Prestige)
  • 12.7m = 20.1ft (Estate)
     

    Number of wheel truns from lock to lock: 4.5
    Steering wheel diameter: 410mm = 16.14in

    Optional: Rack and pinion power steering with powered return

    Reduction ratio: 1/13.5
    Number of wheel turns from lock to lock: 2.5
    Steering wheel diameter: 380mm = 14.67in

    This power steering operates on the same principle as on the SM, but has been improved (servo control block in the passenger cabin and separate from the steering system proper) and adapted to the vehicle (number of wheel turns from lock to lock: 2.5 against 2 on the SM).
     

    GENERALITIES

    The steering on a car is the eqipment allowing the driver to change direction at will as the road conditions dictate (how does one define self-evidence?).

    The stability of the vehicle as well as the comfort and safety of the driver depend on the steering system chosen. It is an  essential element needing careful design. To be perfect, a steering system must provide:
     

  • safety
  • ease of operation
  • precision
  • irreversibility
  • stability
  • compatibility of steering with suspension
  • apparently they forgot about reliability and maintainability...
     

    To satisfy all these requirements and to give the driver ease of control of a car with exceptional roadhandling, Citroen has created a new and completely unique steering system.

    The object of the steering is to improve safety at high and low speeds, to improve rapid handling, and to increase comfort by suppressing side-effects on the wheel. It is an advance in the automobile field at least equivalent to the introduction of hydropneumatic suspension.
     

    PROBLEMS OF CAR STEERING

    The comfort provided by a steering system may be characterised by the ease of directing the vehicle under any circumstances, with a minimum of effort and  fatigue and a maximum of pleasure and safety.
    What is tiring are the turning forces needed and the amount of wheel turning.

    In the case of mechanical steering, it is impossible to both reduce the turning force and wheel turning distance. This usually leads to geared down steering with the inconvenience of high wheel turning distance. This effect is particularly noticeable with heavier vehicles.

    With normal assisted steering, the turning effort when manoeuvering is reduced by the addition of a hydraulic ram which allows a reduction of steering gear ratio, but this system does not give firm steering at high speed with ease of parking.

    A new compromise is needed.
    In  a general assistance in manoeuvering is prefered, but in order to avoid abrupt change of direction at high speed due to over-light and under geared steering, a high ratio is retained although this necessitates much turning of the wheel.

    In other words, a normal assisted steering arrangement cannot satisfy the three requirements: easy manoeuvering, sure and firm steering and low geared steering.

    The solution adopted by Citroen for the CX resolves this problem and fulfills all driving demands at high and low speeds.
    The functions of turning, force on the steering wheel when manoeuvering and at speed and steering gear ratio have been dealt with separately.
     

  • Turning of the wheels
    This is effected hydraulically and controlled mechanically , the driver merely activating the servo control which determines the position of the wheels.
    In any case, the steering arm is actuated hydraulically which completely avoids any force due to shocks on the wheels being transmitted to the steering wheel, whether such shocks are caused by bad road surface (pot holes, ruts, etc...) or by accidentally driving over an obstacle, or even by a puncture.
    In actual fact (not visible in the pictures but in the Haynes manual), a tiny shock absorbing piston is part of the steering cylinder.
    In a word, the driver is always in complete control of the position of the wheels of his car. Naturally in the case of hydraulic failure steering can be mechanically controlled by the driver.
     
  • Force on the steering wheel
    The force on the wheel felt by the driver on turning or centring is produced by a cam linked to the wheel. This force increases with the angle of the wheel and with speed.
    When stationary and even for high turning angle the force onthe wheel remains low, giving pleasant and  easy driving even in town or twisty roads.
    The force on the wheel increases  due to a hydraulic servo controlled centrifugal governor. The curve of increasing force was defined so that the driver always senses an effort small enough not to be annoying but sufficiently high to remind him that the higher the speed, the less turning angle is allowable to remain safe.

    The centering force also increases in parallel and always returns the wheels to the straight ahead position even if the car is stopped.

    This is an important advantage which facilitates parking by allowing the wheels to return to the straight ahead position, although the use of the anti theft device allows the wheels to be locked turned on a slope.

    On muddy ground, snow, sand or slippery surfaces the driver can get his front wheels straight simply by letting the steering wheel retrun to centre.

    As speed increases the cam recentres the wheels more firmly.
     

    The picture shows the arrangement of the components of the steering system.
    To find out more technical details and to understand the unit please click on it.
    Details of other units will be added later.

    1    Steering Ram
    2    Steering Control Unit (Servo Control)
    3    Centrifugal Governor
    4    Safety/Priority Valve
    5    Brake Accumulator (through pipe)
    6    LHM Reservoir
    7    from suspension cylinders
    8    Vent
    9    High Pressure Hydraulic Pump
    10  Pressure Regulator

    HIGH PRESSURE FROM PUMP
    VARIABLE PRESSURE
    RETURN TO RESERVOIR (Return to Reservoir)
     

    Since the force on the steering wheel is independent of the force on the wheel, the steering does not get harder with tyre wear, as with other systems.

    Steering Gear Ratio
    The ratio is chosen to give the best handling possible. 2 1/2 turns of the steering wheel are needed from lock to lock (3 turns on the DS, 3 to 3,5 turns for normal assisted steering, 4.5 turns for mechanical steering, 2 turns on the SM).
    This reduced gearing is an essential comfort factor since it limits the arm movement needed and allows the hands to be placed correctly on the wheel for town or twisty road driving. Allied to a small diameter steering wheel this gives incomparable handling worthy of the high quality of the CX. (Sounds lovely, doesn't it?)

    The low gearing is also a safety factor since it gives the driver the manoeuverability needed to avoid unexpected obstacles, which he could not do so rapidly with higher gearing.

    This rapidity is consistent with the "minimum reaction time" principle Citroen introduced 20 years ago as a safety improvement when the zero movement braking system was evolved on the DS.
    Summing up, the CX hydraulic steering system with servo-return gives the driver:
     

  • exceptional manoeuverability
  • more comfort and reduced fatigue
  • unique control of wheel position
  • realistic impression of increased car speed
     

    These characteristics make this steering system original and may, therefore require a short period of adaptation which, when acquired, will allow the driver to benefit from all the advantages of the CX.
     

    DESCRIPTION

    (now, this part is heavily edited by me and should leave no secrets)

    A differential piston type hydraulic ram actuates the steering gear.

    A pressurised fluid distributor activates the ram.

    A control box:

  • connects the steering gear to the wheel with an angular lag controlling the distributor
  • controls the centering system
     

    A pressure regulator (centrifugal governor) modulates the centering force as a function of speed.
     

    To make this more understandable to the non-automotive engineer, let us have a closer look at the picture below:

    Click on the picture for more detail.

    The differential piston is the one in the steering ram. Differential because of the different surface area of the piston on both sides. When you look closer at the picture you will notice that the diameter of the rod is different on both sides of the piston which influences the piston's surface area. In fact it seems to be calculated in such a way that the surface area of the left side is twice that of the right one.

    Remembering Physics - force F equals pressure P multiplied by the surface area A:

    Chamber one: F1=P1 * A1

    Chamber two: F2=P2 * A2

    When no whee no wheel movement is required, the forces F1 and F2 must be equal.

    F1=F2

    Now we know that by design, the surface area of the left piston is about two twice the surface of the right one .

    In order to maintain the balance of the two forces the equations must look like:

    F1=1/2 * P1 * A1 and

    F2=P2 * 1/2 * A2

    so

    F1=F2 =1/2*P1*A1=P2*1/2*A2

    That means that the pressure in chamber 1 must be half of the pressure in chamber 2 to maintain an equilibrium of the steering forces F1 and F2. This also explains the mysterious "hp/2" found in the Haynes Manual.
     

    For this equilibrim to be held, the pressure in chamber one must be isolated from the rest of the system, otherwise the car would start steering to the right. This is done by a slide valve 3 in the "steering control unit".
     

    Steering action

    Steering to the right:
    For any movement of the steering ram, the equilibrium of forces F1 and F2 must be upset. This is done by sliding the valve 3 out which causes lower pressure fluid in the left cylinder to be pressed into the return pipes (yellow) by the forces of the high pressure fluid in the right half of the ram. (You may hear the famous slurping sound) The ram will move to the left and the car will steer right. Returning the valve will then restore equal forces F1 and F2.

    Steering to the left:
    For this, F1>F2, the ram moves right and the car left. The slide valve must now move down and fluid under high pressure passes into chamber 1 and gradually builds up pressure. Equal pressure in both cylinders however means different forces because of the different surface areas of the piston, so the ram will move right, turning the wheels left.

    The slide valve moves by an ingenious mechanism which must be achieved by having a different gear ratio of the front and rear sprockets of the second shaft 5 in the steering unit. This causes a precise movement of the slidevalve 3 proportional to the turning of the steering wheel.
     

    Self Centering

    The Self Centering technique in fact is much simpler than I first expected. Because I have problems with my steering unit, I thought much about how it is mean to work. A hint was given in the Haynes manual, which is a copy of the material published here (originally by Citroen).
    The basic principle is very simple and the magic word is "eccentric cam".
    This eccentric cam is located at 5 in the previous picure and rotates proportionally to the turning of the steering wheel. A roller in between, displaces a piston for the self centering mecahnism.
    The system is best understood if one just forgets about the hydraulic pipes, pressures and the governor in the first place. Pure mechanics does the job.
    The following picture will illustrate the function.

    The piston is spring loaded and excerts a force over the roller to the eccentric cam. This force will cause the steering wheel to return to its centre position where a little notch will keep it there.

    To increase the self centering force, the force on the cam needs to be increased. Because there is only one spring to give a basic self centering, additional force must be provided by using hydraulic fluid pressing at the piston and thus adding to the force on the cam.
    This force should be dependent on the vehicular speed to give safe handling. This is achieved by the Centrifugal Governor, which is a similar valve as found in the Steering Control Unit. A high pressure pipe goes into the unit, modulated pressure comes out and of course a return pipe. Depending on the road speed (driven from a special "speedo" cable), little wings in the unit rotate with a speed proprtional of that of the vehicle. When speed increases, the wings are forced to fold down with the aid of increased "centrifugal" force, pushing the valve mechanism down and allowing fluid with higher pressure to act on the self centering piston. (See details  in the following pictures.)
     
     


    The Flywheel weights fold down.


    The governor valve opens and increases pressure on centering cylinder.
     

    The feel of the steering in your CX is thus almost absolutely artificial!

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