Magès Minute: why a liquid spring cannot replace hydropneumatics (6 March 1961)

In this document

This handwritten minute, signed "Magès" and dated 6-3-61 p. 36, is explicitly titled "Following TAYCO proposal (12.12.1960) TAYLOR DEVICES" p. 34. It develops, point by point, a numerical comparison between the DS gas spring and an equivalent liquid spring.

The reference calculation (DS 19)

Magès starts from the front sphere of the DS: 750 cm³ inflated to 60 Kg/cm², operating pressure 100 Kg/cm² p. 34. For a load increase of 10%, he calculates a gas volume reduction of 40 cm³, corresponding to a piston displacement of 42 mm and a wheel travel of 95 mm p. 34.

The liquid spring equivalent

He posits that the same displacement of 40 cm³ would be necessary with a liquid p. 34. Relying on a bulk modulus of 8000 Kg/cm² for "special oils", in reference to "Comprzoil 118 Oil - Hydra Spring Division Wales-Strippit C[o], North Tonawanda NY - 1955 document" p. 35, he arrives at a requirement of 32 litres of oil, "instead of 0.750 [litre] DS" p. 35.

Conclusion and limitations

At point 6, Magès notes that by doubling the operating pressure the quantity of liquid would be divided by four, but judges that "8 litres of liquid compressed at 200 Kg/cm² in a sphere [...] is unthinkable" p. 35. At point 7, he concludes that the liquid solution is "not reasonable", "not even for a truck", and announces that Citroën tests are pointing towards an hydropneumatic suspension with 3 litres of suspension volume per wheel, at 150 Kg/cm² p. 36.

The general conclusion asserts that "A high-pressure gas suspension can only be conceived, for now, as 'hydropneumatic'" p. 36, and notes that "DOWTY has had liquid suspension patents since at least 1946 (Fr. Patents 931110 and 931111)" p. 36.

Context

The DS 19, launched by Citroën in 1955, remained famous for its hydropneumatic suspension designed by Paul Magès, combining compressed gas (nitrogen) and hydraulic fluid in spheres. In the context of the turn of the 1960s, several American companies (Tayco Developments, Rabinow) were proposing alternatives based on pure "liquid springs", without a gas phase. Wales-Strippit, mentioned in this document, was an American manufacturer of industrial hydraulic springs ("Hydra Springs"), already known to Citroën through earlier documentation.

Archivist's perspective

This document illustrates, probably better than any other piece in the collection, Magès's working method: a rigorous arithmetic demonstration intended to settle a technical choice. It responds directly to the commercial proposal from Tayco Developments of 12 December 1960, and relies on technical data already present in the Wales-Strippit collection, such as the Hydra News n°4 on the compressibility of Comproil oils, annotated by Magès himself.

The reasoning, by demonstrating the impracticality of an all-liquid suspension (32 litres of oil required, sphere deemed "unthinkable"), seems to constitute an a posteriori justification of the hydropneumatic choice already operational on the DS. The final mention of the 1946 Dowty patents also shows that Magès situated his reflection within a broader technical history of liquid suspensions, which he had known for a long time.