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High-Pressure Hydraulics by Citro?n

jeanlouis

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I worked hard under my Citroën CX for changing some hydraulic pipes when I thinked you could be interested to know how is working this damned machine.
For happy the document I found is translated in English. When it was published, at the end of seventies, it was the golden age of this technology. Citroën GS and CX were producted at nearly 400000 units in 1978, more of 60% of the total production. Don't be afraid it is more simple than a computer or a flat screen. ;)

















 

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How high-pressure hydraulics adventure began



At the beginning it was the new small utilitary the TUB, derived from the "traction", that had a brakes issue. It was a big weight variation on rear axle between the empty and the loaded van. Mechanical systems of repartition were not enough efficient. Paul Mages, the "father" of high-pressure hydraulics by Citroën (on the right photo), designed a specific device working with an hydraulic power central station. Unfortunately this efficient system was to expansive for a small serie like the TUB. Andre Lefebvre the manager of Research Department (on the left photo) had the idea to install this device on a big serie car that had to replace the "traction", the future DS. To pay off the cost of an hydraulic power central, brakes, direction, suspension, gear shift would be managed by this hydraulic station.


The starting of any hydraulic high-pressure install, here the first DS


Brakes diagram, the first hydraulic project designed by Paul Mages, here the first DS.
In the case of a classic suspension, an utilitary for example, the roulette is moved by a mechanical system.
 
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jeanlouis

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At the 1955 motorshow, the Citroën DS appeared. "For you this car is working all by itself" said the advertising.
 

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In 1954 the first hydropneumatic suspension was on trial on the rear axle of old 15/6 (big six in UK). Only 3000 saloons were made on a total of nearly 48000 15/6 since 1938. For according front and rear axles, the length of front twist bars was increased.
 
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jeanlouis

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Simplified hydraulic circuits with only one function, brakes or suspension




Presented in 1965, the new Citroën truck used the brakes system of the DS



Presented in 1973, the new van C35 used a high-pressure hydraulics brakes
described higher at the second post. (GS or CX type)


Presented in 1956 the first ID19 kept only the hydropneumatic suspension of its elder sister DS. Later hydraulics of both will come closer, power steering and brakes for ID19 and a mechanical gear shift becoming possible on the DS. Some drivers prefered this basic system, not me ;)
 
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Citroën and Total created new mineral hydraulics fluid LHM in 1966 (green liquid)





The main development of high-pressure hydraulics power station is this new fluid LHM (liquide hydraulique minéral) near of a motor-oil but very more fluid. With this green hydraulics liquid, devices reach an outstanding endurance and it solve embarrassing leak issues of the first DS.
Starting from this date, the constructor can envisage big serie cars working with high-pressure hydraulics technology. It will be the GS in 1970 (suspension and brakes high-pressure) and the BX in 1982 (suspension, brakes and power-steering on some versions), both models that saved the firm.
 
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3 power-steering models by Citroën





In 1955 Citroën DS 19 is one of the first european cars to be equiped with a power-steering. At the opposite of a classic assistance, the jack is incorporated to steering rack. The wheel and high pressure hydraulics check front weels orientation and lock it. Mechanical connection is kept as a security. It's working as a shutter control in aeronautics.






In 1970 with the SM the assistance of the steering vary with the speed of the vehicle. Sweet in town, the wheel become harder on road and highways. Only two turns of wheel are necessary bump-stop to bump-stop. This system will be used again on the CX five years after (2.5 turns)



On the BX in 1983 it become possible to order a model equiped of a power-steering.
It is just a classic assistance adapted to hydraulic power station of the BX by a flow distributor.
The jack push and pull the rack as you can see on the diagram. Standard and bought outside, distributor valve and jack are less expansive than some Citroën composants. Without centering servo-mecanism used on SM and CX, the driving of this BX is also nearer of the one of any other car.
 
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From air cushion to flying carpet, how Citroën communicated about its amazing hydropneumatic suspension







Oil underneath, air on top and the rubber band at the middle
At the beginning with ID19, the most important was to reassure traditional customers. The model is presented as a "traction 57", a traction on air cushions with only some meters pipes in more ! ;)






 
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This is why GH is still the best, there is food for the brain as well as for the cock. You won't find an article like this on JUB or LPSG, only here.
Most manuals say 'refer to your dealer' for anything more difficult than changing a wiperblade. Here they don't just tell you exactly how it works but also how and why they chose to design it in this brilliantly eccentric way.
 

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The third sphere



At the end of eighties, Citroën invented a development of its hydropneumatic suspension named "hydractive" where processors help hydraulics to control better body movements according the driving. With an additional sphere by axle, flexibility of suspension become variable because this sphere is now actived, now by-passed. Five captors and a computer in board manage the use of this third sphere. The driver can choose between two driving style, sport or straight. In 1987 this system is put on trial on 15 CX named REGAMO sold to selected customers driving a lot. Today it seem that these rare cars has been transformed back in ordinary CX.





In 1989 Citroën present CX replacement named XM. Only high-scale models are equiped with this new suspension. At starting, sport style position is just the by-pass of this famous third sphere, a few time later, with Hydractive ll, this position is more nuanced and response times of the computer decrease.
 
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The SC CAR (Citroën active control rolling system) of Xantia Activa in 1994



Xantia is Citroën model that replace BX series in early 1993. Like XM, this car is available with the Hydractive ll suspension in high-scale versions.
The constructor show it can go still farer about fight against rolling body movements. It is Xantia Activa. (See farer at # 94)
 
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Hydractive lll and lll+ : the end of hydraulics power station



In 2001 Citroën presents the C5 equiped with new hydraulics systems.
There is no more power station and high pressure circuit is kept for Hydractive lll suspension only. It is now hydro-electro-pneumatic. Spheres and fluid are new. Only high-scale models have the suspension Hydractive lll+ with variable flexibility (3 spheres by axle) and the choice between 2 driving styles, sport or straight. Circuits become separated. Steering power and brakes assisted by depression are classical, some standard PSA products.


Presented in 2006, the C6 remind me of the CX. It has the same technical base than the C5, but with Hydractive lll+ suspension only.
 

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In 2008 Citroën presents a new C5. Hydractive lll+ suspension with variable flexibility is kept only for high-scale models. A basic metallic system is assembled on a majority of vehicles. Today C5 saloon and estate are no more producted, Hydractive suspension is left and Citroën is become a constructor like another... :(
 

jeanlouis

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This is why GH is still the best, there is food for the brain as well as for the cock. You won't find an article like this on JUB or LPSG, only here.
Most manuals say 'refer to your dealer' for anything more difficult than changing a wiperblade. Here they don't just tell you exactly how it works but also how and why they chose to design it in this brilliantly eccentric way.

Thank you very much for your comments Dargelos. It could seem kinky to make a megathread about a technology that no more exist in automobile. I like it not only because it realized by my favorite constructor. ;)
I like mechanical, pneumatic or hydraulic automatisms because it is based on some physical principles that never change and it is relaxing for the mind. :thumbs up:
Inside any hydraulic device, there is a perpetual balance on the left and on the right of the drawer and when it move for any reason, the balance is straight away restored. It's always the same. I chose for example the pressure regulator with its accumulator assembled on the first ID19-DS19.


It is the simplest device I found, but an hydraulic block for automatic clutch and gear shift of the DS is the same thing with many more drawers.
 
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dargelos

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Once I had to go a long way in a CX Pallas, which was lovely but it was difficult for me to avoid falling asleep. The ride was so quiet, so smooth, so comfortable, that it was not like being in car, it was more like sitting on a bed gliding over a placid lake. Perhaps a car can be too luxurious.
One aspect of the design that I like is the simple idea of non canceling indicators. You switch them on and then you switch them off, not like on ordinary cars where they switch themselves off too soon and you have to remember to switch them back on again.
 

jeanlouis

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Once I had to go a long way in a CX Pallas, which was lovely but it was difficult for me to avoid falling asleep. The ride was so quiet, so smooth, so comfortable, that it was not like being in car, it was more like sitting on a bed gliding over a placid lake. Perhaps a car can be too luxurious.
One aspect of the design that I like is the simple idea of non canceling indicators. You switch them on and then you switch them off, not like on ordinary cars where they switch themselves off too soon and you have to remember to switch them back on again.

The well known Citroën image of a DS on a lake with balloons instead of wheels seem exact. This air cushion feeling could make sick passengers with the body movements. Anyway comfort notion is subjective. Medicaly we know now that firm seats and suspensions are preferable especially for the back, like the mattress for example. We have to replace these cars in 60s and 70s at the time where (french) people drove fast on bad roads. In these conditions hydropneumatic suspension was really efficient.
 
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The first market interested by special features of hydropneumatic suspension was the one of ambulances, as soon as first ID19 estates appeared on the roads in 1959-60. In front of an increase demand, Citroën integrated these special cars to its catalog. Ambulances DS and CX were the very last series to be producted, two years after saloons stopping. In 1991 when the last ambulance CX was assembled, Citroën lost a market of several thousands cars in spite of the interesting XM estate base.


GS








DS


CX


HY
 
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jeanlouis

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Radio-TV transmission and movies travelling were the cases where low frequency hydropneumatic suspension was the most difficult to replace. Today there is the "drone".






 

jeanlouis

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Double, triple, quadruple hydropneumatic rear axles by Pierre Tissier are a concrete application of the principle seen at the beginning of this thread. Tisser company transformed all kind of DS, SM, CX from 1971 to 1991.


DS stage triple rear axles

Special luggage van for parcels service Hollander SA, news papers fast-logistic between Paris and northern Europe by highway (about 20 units)




SM stage with 4 rear axles
 
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