The development of the 1955 D-Series had actually begun in the late 1930s, when Pierre Boulanger, then Citroen's patron, approved work on a Traction replacement. Because of the war, however, prototypes weren't built until the late 1940s.
One early proposal called for a 4.5-liter engine, very large by European standards, though that was later reduced to a 3.5. Predicting the Tucker -- not to mention the later Chevrolet Corvair and Porsche 911 -- an air-cooled flat-six was also tried.
In the end, though, engineers settled for a reworked version of their existing "stroker" four (78 x 100 mm), applying a new aluminum head with higher compression and valves inclined at 60 degrees above hemispherical combustion chambers. The result was a modest 75 horsepower from 1,911 cubic centimeters, a mere 116.6 cubic inches.
The D-Series' most notable engineering achievement was unquestionably the central high-pressure hydraulic system controlling suspension, steering, brakes, clutch, and (except on the downmarket ID models) the gearchange, too.
It was quite literally the car's heart and life-blood. A broken hydraulic pump or a leak in one of the system's many lines could render a D-Series as dead as if it had suffered a coronary. At least the car warned you of possible cardiac arrest via a big red "master alert" light on the instrument board.
The suspension was especially intriguing. Introduced on late Tractions starting in 1954, it was a long-travel, all-independent setup that replaced conventional shock absorbers and steel springs with hydropneumatic spheres that interconnected front to rear on each side (much like Packard's new 1955 "Torsion Level Ride").
Actually, this "Citro-Matic" system was oleopneumatic in that each sphere contained pressurized nitrogen gas above a supply of synthetic vegetable oil (later changed to mineral oil), separated by a flexible diaphragm.
A piston in the bottom half linked each sphere to its wheel; piston movement was dictated by pressure within the sphere. A pump driven by belt from the crankshaft pressurized the gas and fed fluid to the spheres as needed. Valves and sensors regulated fluid flow in response to "height corrector" valves, operated by a rod from the center of the front and rear anti-roll bars, to maintain ride height, which could also be altered manually by an interior lever.
As mentioned, the central hydraulics also powered the steering, which normally required very little effort but became impossibly heavy if the engine wasn't running or system fluid was low.
The same applied to the hydraulic brakes, which comprised inboard-mounted discs at the front and conventional twin-shoe drums at the rear. Instead of a normal pedal, however, brake actuation was by a near-zero-movement button mounted right on the floor, which with the low effort made for a good many accidental "panic" stops until the driver got used to the feel of it.
Quirkiest of all was the column-mounted fingertip shift mechanism, which gave the four-speed unit quite leisurely changes because the hydraulics were slow to respond to lever movement.
However, there was no clutch pedal because the clutch engaged and disengaged automatically depending on throttle position, which was driven hydraulically by a low-pressure pump in the water pump.
Due to its powered suspension, the 1955 D-Series offered one smooth ride. Read on to learn more about this car's suspension system.