Citroen DS 21 Cabrio, 1970

The Citro�n DS (also known as D�esse, or Goddess, after the punning initials in French) was an automobile produced by the French manufacturer Citro�n between 1955 and 1975. Citro�n sold nearly 1.5 million D-series during its 20 years of production.The DS is well-known for its futuristic, aerodynamic body design, and for its innovative technology (including its hydropneumatic self-leveling suspension system).

Technical innovations

Hydraulic system
The hydraulic system of the DS 19 was a revolution. Previously hydraulics had been restricted to use in brakes and power steering; the DS used them for the suspension, clutch and transmission. The later ID19 had manual steering and a simplified power braking system.

Suspension
At a time when few passenger vehicles had caught up with the four-wheel independent suspension of the Traction Avant, the application of the hydraulic system to the car's suspension system to provide true self-levelling was a stunning move. This application - 'hydropneumatic suspension' - was pioneered the year before on the rear of the top of range Traction Avant 15CV-H.

Each wheel was connected not to a spring, but to a hydraulic suspension unit consisting of:
* a sphere of about 12 cm in diameter containing pressurised nitrogen
* a cylinder containing hydraulic fluid screwed to the suspension sphere
* a piston inside the cylinder connected by levers to the suspension itself
* a damper valve between the piston and the sphere

A membrane in the sphere prevented the nitrogen from escaping. The motion of the wheels translated to a motion of the piston, which acted on the oil in the nitrogen cushion and provided the spring effect. The damper valve took place of the shock absorber in conventional suspensions.

The hydraulic cylinder was fed with hydraulic fluid from the main pressure reservoir via a height corrector, a valve controlled by the mid-position of the anti-roll bar connected to the axle. If the suspension was too low, the height corrector introduced high-pressure fluid. If it was too high, it released fluid back to the fluid reservoir.

A control in the cabin allowed the driver to select one of five heights:

• normal riding height.
• two slightly higher riding heights, for poor terrain.
• two extreme positions for changing wheels.

The DS did not have a jack for lifting the car off the ground. Instead, the hydraulic system enabled wheel changes with the aid of a simple adjustable stand.

Source and reserve of pressure
The central part of the hydraulic system was the high pressure reservoir, which maintained a pressure of between 130 and 150 bar in two accumulators. These accumulators were very similar in construction to the suspension spheres. One was dedicated to the brakes, and the other ran the other hydraulic systems. Thus in case of a hydraulic failure (a surprisingly infrequent occurrence), the first indication would be that the steering became heavy, followed by the gearbox not working; only later would the brakes fail.

Hydraulic fluid
The original hydropneumatic system used a vegetable oil (LHV or liquide hydraulique v�g�tale) similar to that used in other cars at the time. Very soon, Citro�n changed to using a synthetic fluid (LHS or liquide hydraulique synth�tique). Both of these had the disadvantage that they are hygroscopic, as is the case with most brake fluids. Disuse allows water to enter the hydraulic components causing deterioration and expensive maintenance work. The difficulty with hygroscopic hydraulic fluid was exacerbated in the DS/ID due to the extreme rise and fall in the fluid level in the reservoir, which went from nearly full to nearly empty when the suspension "got up" and the 6 accumulators in the system filled with fluid. With every "inhalation" of fresh moisture- (and dust-) laden air, the fluid absorbed more water. In August 1967, Citro�n introduced a new mineral oil-based fluid LHM, or liquide hydraulique min�rale. This fluid was much less aggressive on the system and it remains in use to the present day.

Gearbox and clutch
The mechanical aspects of the gearbox and clutch were completely conventional and the same elements were used in the ID 19.

The gear change control consisted of:
* Hydraulic gear selector.
* Clutch control. This was the most complicated part. The speed of engagement of the clutch was controlled by:
* A centrifugal regulator, sensing engine rpm and driven off the camshaft by a belt
* The position of the butterfly valve in the carburettor (i.e. the position of the accelerator)
* The brake circuit: when the brake was pressed, the engine idle speed dropped to a rpm below the clutch engagement speed, thus preventing friction while stopped in gear at traffic lights. When the brake was released, the idle speed increased to the clutch dragging speed. The car would then "creep" much like automatic transmission cars. This drop in idle throttle position also caused the car to have more engine drag when the brakes were applied even before the car slowed to the idle speed in gear, preventing the engine from "pulling" against the brakes.

DS in the US
While the DS was a hit in Europe, it seemed rather odd in the United States. Ostensibly a luxurious car, it did not have the basic features that buyers of that era expected to find on such a vehicle - fully automatic transmission, air conditioning, power windows and a reasonably powerful engine. The DS price point was similar to the contemporary Cadillac luxury car. Also, people at the time wanted only the newest models, which changed every year, like fashion, yet the DS appeared vaguely derivative of the 1950 Hudson Hornet step-down design.

Outdated US legislation also banned one of the car's more advanced features, aerodynamic headlamps, now common in US automobiles. Ultimately, 38,000 units were sold. The first year of the aerodynamic glass over the DS' headlights along with driving lights turned by the steering, was also the first year these features were outlawed in the US.

Design variations
The DS always maintained its size and shape, with easily removable, unstressed body panels, but certain design changes did occur.

A station wagon version was introduced in 1958. It was known by various names in different markets (Break in France, Safari and Familiale in the UK, Wagon in the US, and Citro�n Australia used the terms Safari and Station-Wagon). It had a steel roof to support the standard roof rack.

In September 1962, the DS was restyled with a more aerodynamically efficient nose, better ventilation and other improvements. It retained the open two headlamp appearance, but was available with an optional set of driving lights mounted on the front fenders. In 1965 a luxury upgrade kit, the DS Pallas (after Greek goddess Pallas), was introduced. This included comfort features such as better noise insulation, leather upholstery and external trim embellishments.

In 1967, the DS and ID was again restyled. This version had a more streamlined headlamp design, giving the car a notably shark-like appearance. This design had four headlights under a smooth glass canopy, and the inner set swivelled with the steering wheel. This allowed the driver to see 'around' turns, especially valuable on twisting roads driven at high speed at night.

However, this feature was not allowed in the US at the time (see World Forum for Harmonization of Vehicle Regulations), so a version with four exposed headlights that did not swivel was made for the US market.

The station wagon edition, the Break (called the ID Safari on the UK market) and "Familiale", was also upgraded. The hydraulic fluid changed in all markets (except the US) to the technically superior LHM (Liquide Hydraulique Min�rale).

Rarest and most collectible of all DS variants, a convertible was offered from 1958 until 1973. The convertibles were built in small series by French carrossier Henri Chapron, for the Citro�n factory. In addition, Chapron also produced a few coup�s, non-works convertibles and special sedans (DS Lorraine for instance).