A SHORT HISTORY OF SUSPENSION  

Part Nine

CITROEN SAFETY

What are the essential features of safety in a car ? The road holding and the brakes. We have seen the vital influence of the suspension on the road holding. The importance of the brakes is evident. Obviously, the faster the car, the more powerful its brakes must be. This is why manufacturers, the world over, have now adopted the hydraulic brakes that Citroën was one of the first in Europe to install as a standard feature in 1934. 
 

It is also the reason why disc brakes - far more reliable and efficient than the drum type - seem to be adopted as a general rule on fast cars.  Here too, Citroën set the example, as they were the first to fit disc brakes (at the front) on mass produced cars: the DS19 and ID19.

Finally, this is the reason why improvements have been made on the conventional hydraulic system, the latest of these being the introduction of a servo brake, or its equivalent, in the form of an accumulation of
potential energy to replace any effort on the driver's part.

The DS19, and more recently the ID19, are fitted with a hydraulic servo brake. Instead of the driver having to tire himself by exerting his energy on the brakes, the hydraulic system does the work and the pedal only serves to open the valve to the appropriate degree, which does not demand a great effort.  Undoubtedly. this is an achievement. But concern for constantly improving safety conditions leads one to investigate the problem of ground grip and in particular the question of making full use of the weight, the distribution of which (displacement of the centre of gravity) varies as a function of the speed of deceleration - or braking. 
 

It therefore appeared desirable, or rather necessary, to control the distribution of the braking effort in proportion to the weight distribution. We are aware of its influence on the springing, and vice versa the influence of the springing on it. Now on the DS19 or ID19, there was both a hydraulic suspension system and a hydraulic braking system. This resemblance was to enable Citroën engineers to establish the essential connection between the brakes and the suspension. 

Thus, although other designs for the hydraulic control of the interaction between the brakes and the suspension are conceivable, it is logical to come round to the idea of a centralized hydraulic plant, to the exclusion of any other hydraulic application.

This centralization of two vital functions brings out the superiority of mechanisms, which behave as slaves with brains of their own and replace the driver, to act in his stead with a perfection and speed he could never attain. The DS19 has a double braking circuit: the front and rear braking circuits are independent, each with its own pressure supply. In other words, if one of the circuits fails, the other will still work (and save the passengers' lives). Even today, the DS19 is one of the few cars to comprise this feature though it is unanimously advocated by all safety specialists.

When the driver puts his foot on the brakes, a dispenser, controlled hydraulically by the pressure in the rear suspension spheres, distributes the power between two separate distributors. These distributors, lined up on the high pressure system, route fluid to the brake cylinders with a proportionate force. The dispenser automatically determines the braking force on each axle, on the basis of the load, i.e. the weight on the axle.

Though somewhat lengthy to describe, this action is of course instantaneous. On the ID19 a device based on the same principle achieves the same result.
 

Thus, the Citroën suspension not only coordinates the many parameters which make vehicle suspension problems so complex, but also controls and determines the braking force. This is an additional advantage which gives it the envied position of the safest suspension of our time. 

It is also a convincing example of the complete safety and genuine functional simplicity obtainable by reasoned application of hydraulics to car design.

At the same time, it is the key to the superiority of Citroën's suspension and the reason why it cannot be compared with any other type. The Air-Oil hydropneumatic system is a brakes and suspension assembly unique in the world.

Photographic credits in original Citroën book:  F. Mortimer-Rapho, R. Burri-Magnum, Louis-Frédéric-Rapho, R. Viollet, Giraudon, R. Roche, Citroën, A. Martin, Dollfus collection. 

Cover: Giraudon, Viollet, Hutin, Martin, Roche, Dollfus collection. 
Delpire editeur. 

This piece prepared for our fellow Citroën friends by Bruce Kennett and Julian Marsh, using an original book owned by Dick May.
 

It is difficult to describe the softness of the Citroën suspension, as comparisons fall short of the truth,and only a road test can really bring out the truth. However, for the first time ever no doubt, this technical fact can be illustrated by a graph. Three lights were fixed to a DS19 and to a metal-sprung car: one on the centre (hub) of the rear wheel, the others at different heights on the body. The cars were then photographed going over the same obstacles. A comparison of the light curves reveals the behaviour of the respective suspensions. [metal-sprung above, DS19 below]