Suspension 

Conventional suspension systems suffer from the dichotomy between, on the one hand, the need for comfort and on the other hand, the need for good road holding and handling. A softly sprung car is comfortable but has poor roadholding and handling and furthermore suffers from attitude changes depending on the weight and distribution of the load carried. A firmly sprung car offers good roadholding and handling but is uncomfortable on anything but the smoothest of surfaces.

From its inception the car was intended to use either torsion bars or rubber suspension; the requirement being for a very supple system but without excessively long suspension arms since these, by virtue of an ever-changing wheelbase, would cause inconsistent handling.

Two solutions were achieved - that of the 2 CV and that of the D Series and subsequent hydropneumatically sprung Citroëns. The 2 CV solution is a very soft suspension and works by virtue of its front to rear interconnection and low weight. A number of VGD prototypes were equipped with 2 CV suspension but suffered from excessive body roll, pitching and load-related attitude changes.

Hydraulics 

In 1942, Paul Magès had an incredible idea. Magès was a junior engineer working on braking systems who concluded that hydraulic power could be applied to provide levelling of the suspension and operation of the steering and transmission in addition to the brakes. Even though the principles were well understood, putting them into practice presented numerous problems ranging from the incredibly fine engineering tolerances required through to the choice of materials and hydraulic fluid. Step by step these problems were solved.

Engines 

Walter Becchia was the man responsible for the 2 CV's flat twin and it was he who was given the task of developing a new engine. He designed a water cooled flat six of 1 806cc capacity producing 63,8 bhp @ 4 500 rpm. In 1948 following successful trials of the 2 CV's engine, the flat six was converted to air cooling although all else remained unchanged including power output which was adjudged to be too low. The project was abandoned in 1954 due to lack of power, excessive thirst and great weight despite the widespread use of aluminium. The flat six would have been mounted ahead of the front axle as in the 2 CV.

Becchia's flat six - here in water-cooled guise

AX scale model

Bodywork
By 1946, the prototypes looked unlike anything else on the road with a plunging bonnet, no radiator grill and an abbreviated Kamm tail. One of these protoypes was given the inelegant nickname l'Hippototame (hippopotamus). Aerodynamic cars like Chrysler's Airflow offered no clues at to the shape of la nouvelle Traction. Immediately after WW2, Citroën's chief stylist, Flaminio Bertoni started in secret to revise the dimensions of the VGD, lengthening the wheelbase and improving on both accommodation and aerodynamics. 

In December 1950, Pierre Boulanger was killed at the wheel of an experimental Traction and Robert Puisseux became Président-Directeur Général of Michelin who owned Citroën. He handed control of the VGD project over to Pierre Bercot, the new managing director of Citroën. Bercot agreed to a redefinition of the project, believing that here was the opportunity to create a car that would be as far ahead of the Traction as that car was of its contemporaries in 1934, even if that meant that the new car's launch would be delayed. André Lefebvre was given carte blanche yet again and thus was born Projet D.

The Traction's styling was looking increasingly dated - indeed Renault took to marketing its Frégate as la 11 CV Moderne and Peugeot with its 203 and Simca with its Aronde were making inroads into Traction sales with their "modern" styling. The French car market was protected from foreign competition and was totally dominated by Citroën, Peugeot and Renault (who had been nationalised on the grounds that the company had collaborated with the Germans during the war). Second division players included Simca, Ford, Panhard et Levassor, Hotchkiss, Delahaye and Facel Vega. Citroën's response was to offer the Traction in colours other than black.

	This particular drawing presages the "cab forward" layout that would be employed in the XM

From 1951 onwards, prototypes were clandestinely tested on the deserted roads of the Midi. In April 1952, l'Auto Journal published photos of one of these cars. In the June edition, they published accurate technical specifications. Bercot was furious and called in the police but the journalists refused to divulge their sources. Citroën improved security and a veil of secrecy descended over the new car, only to be lifted slightly with a preview in 1953 of hydropneumatic suspension in the 15 CV H. 

A "new" engine
Georges Sainturat, the man responsible for designing the Traction's engine was given the task of redesigning that engine so it could be used in the new car. He reworked the cylinder head of the 1 911cc engine (the Traction six cylinder was too long) and with various modifications managed to extract 75 bhp from it. The "new" engine was too tall to fit in front of the gearbox so the Traction layout was employed even though this meant that the engine protruded into the passenger compartment.

In an attempt to ameliorate this problem, designs incorporating the differential inside the sump were worked on.  This meant that the engine/transmission assembly could be shortened but financial constraints meant that the project was abandoned.

Disc brakes
Following Jaguar's 1953 win at le Mans with car equipped with Dunlop disc brakes, it was decided to equip Projet D with front discs.

The body design was still undecided six months before the new car's launch. Bertoni eventually came up with the definitive shape - a dashboard moulded out of plastic, a single spoke wheel, air vents, rear indicators mounted above the C pillars were all realised at a very late stage as were the precise angles of front and rear screens. 

Thursday 5th October 1955
At 9 o'clock, the new Citroën was unveiled at the Paris motor show. Minutes later, dozens of Projet D - now officially named DS 19 were driven out of the factory gates and into the Paris traffic. By 09:45 that morning, Citroën had taken 749 firm orders and by the end of the day, 12 000 orders had been placed, the vast majority by people who had never seen the car. The Traction effect had been repeated. Unfortunately, so had the problems. No-one had the faintest idea how these cars worked. The workshops had no manuals. The salesmen had no publicity material. The obsessive need for secrecy had worked against the company. early cars were less than totally reliable. Many was the owner who found himself stranded with no steering, no brakes, no clutch, no gearchange , no suspension and a big pool of fluid under his car. The local garagiste had no idea what to do. The company quickly mobilised itself, providing the agents with the necessary workshop manuals and training to allow it to honour its guarantees. But when it was working, the DS was undoubtedly la Reine de la Route offering novelty and modernism in addition to unprecedented levels of comfort, road holding, braking and safety. Little by little, the bugs were solved. The hydraulic fluid formulation was improved to reduce oxydisation caused by the intensely hygroscopic properties of the early fluid and eventually, the DS became as reliable as any of its conventional contemporaries - if maintained properly.

Variations

Over its 20 year lifetime, the DS was refined and improved and the range was extended to include numerous new variants. The DS was built in England , Belgium, South Africa and Australia as well as in France.  

In 1957, a "dry" DS was introduced. The ID 19 retained the body and suspension of the DS but was equipped with a conventional braking and steering system and a normal clutch and manual gearchange. Despite marginally lower power, the ID 19 offered similar levels of performance thanks to a reduction in the load placed on the hydraulic pump. The Traction ceased production that year. The purists at Javel considered this car to be a "sous-produit". 

In 1958, the ID Break (estate car) was launched, available as a 7 seater, 9 seater Familiale or Commerciale.

In 1960, the 6 volt electrical system was replaced by a 12 volt system. 
In 1961, power output was upped to 83 bhp. 
In 1962, the front end of the car was tidied up resulting in a 8 kph/5 mph improvement in top speed. 

Quartz halogen auxiliary driving lamps were introduced in 1964. 

Two new engines were introduced in 1965 - a 90 bhp short stroke 1 985cc which replaced the long stroke Sainturat engine in the DS 19 and a 109 bhp short stroke 2 175cc engine which powered a new model - the DS 21. 

The front end of the car was redesigned again in 1967 with four lamps mounted behind transparent faired in panels and on top of the range models, the inner pair of lamps swivelled with the steering while the outer pair were linked to the suspension maintaining a level beam irrespective of whether the car was accelerating or braking.

In 1968, the DS21 gained an extra pair of horses while the DS 19 gained thirteen. Along with the additional power came a name change to DS 20. IDs became D Spécials if equipped with the old engine or D Super if fitted with the DS 20 engine. All cars gained a new dashboard too. 

In 1969, the millionth DS was built and to celebrate, the DS 21 was offered with Bosch electronic fuel injection boosting power to 139 bhp. 
In 1970, a five speed manual gearbox was offered. 
A Borg Warner fully automatic gearbox was made available in 1971. 
In 1972, the DS 21 was replaced by the DS 23 equipped with a 2 347cc engine developing either 124 bhp when fitted with a carburettor or 141 bhp when equipped with fuel injection. The D Super was available with the DS 21 engine and 5 speed gearbox and was called the D Super 5-21. 
 

Other variants included the Prestige which was fitted with a glass panel separating the chauffeur from the owner and the Decapotable, both of which were built by Henri Chapron .

Another variant was the Pallas which featured superior external finish and improved interior including leather upholstery. On 24th April 1975, production of the DS ceased.

Total production of all models came to 1 455 746. Its replacement, the CX was nowhere near as innovative, being evolutionary rather than revolutionary and that car's successor, the XM continues this trend, representing the refinement of existing concepts rather than the redefinition of those concepts. 

It is unlikely that the world will ever see such a fundamentally new car as the DS again. It is true that many manufacturers show concept cars that are truly innovative but a combination of international legislation and economics ensures that these are never put into production. Had the DS merely been such a concept car, it would have been sufficient for an entry into any history of the automobile. The fact that it made it into production renders it unique. 

The DS was an incredibly successful rally car and was also used as the basis for the incredible Maserati powered SM .

The DS was sold worldwide and met with a mixed reception - ranging from stupefied incomprehension to unalloyed adulation.

In the United States of America, protectionist Federal regulations imposed fixed headlights without the glass nacelle and ultimately led to the demise of the marque in that marketplace.