Preliminary design of hydropneumatic suspension for a FIAT vehicle (Mr. Petitdidier, 12 May 1971)

In this document

This four-page technical note, signed by Mr. Petitdidier and dated 12 May 1971, is a preliminary design for a hydropneumatic suspension established by the Citroën design office for a rear-wheel drive FIAT vehicle p. 157.

General characteristics

The document starts from a provided plan specifying: total unladen weight 1,300 kg, total fully laden weight 1,780 kg, unsprung weight 40 kg per wheel, wheelbase 2.750 m, track 1.420 m, centre of gravity height 0.55 m p. 157.

Front axle

The calculations determine a piston section of 3.7 cm² for a maximum pressure of 115 kg/cm², with a cylinder 35 mm in diameter (9.62 cm²) and an arm ratio R=320 mm / r=123 mm p. 157. The maximum inflation pressure is set at 68 kg/cm², for a pneumatic block volume of at least 390 cm³; the final choice is a 500 cm³ block inflated to 53 kg/cm², shared with the rear p. 158.

Rear axle

Same procedure: section of 3.35 cm², identical 35 mm cylinder with R'=320 mm / r'=112 mm p. 158. The calculated maximum inflation pressure is 43 kg/cm², for a minimum volume of 465 cm³; the selected block is 500 cm³ inflated to 40 kg/cm² p. 159.

Anti-dive and anti-squat

For the front axle, only braking can be compensated on this rear-wheel drive vehicle; the calculation gives an arm axis convergence angle of 14° p. 159-160. At the rear, the angle required to fully compensate for braking would reach 45°, deemed "impossible" to achieve p. 160; on the other hand, for acceleration, an angle of 11° is considered "more easily achievable" p. 160.

Context

In the early 1970s, Citroën, then linked to Fiat within a joint industrial project (PARDEVI), explored the possibility of equipping vehicles from other manufacturers with its hydropneumatic suspension, an iconic patent born with the DS in 1955. This document illustrates this logic of technology transfer: the design office adapts the same principles (pneumatic blocks, inflation pressure, anti-dive correction) to a foreign chassis, here a rear-wheel drive vehicle — a configuration different from that of traditional front-wheel drive Citroëns.

Archivist's perspective

This document offers a valuable example of the standardized calculation method implemented at the Citroën design office to size a hydropneumatic suspension: the process (section, cylinder, block, pressure, anti-dive angle) seems to be a genuine reproducible protocol, likely transposable to other projects.

The partial failure noted on the rear axle (45° angle deemed unrealistic, p. 160) is revealing of the concrete limits of the technology when faced with mechanical architectures different from those initially designed for the DS.

This FIAT project is in line with the reflections on hydropneumatic suspension initiated as early as the 1959 notes and continued via road tests in the early 1960s, such as that of April 1962. It shows that, fifteen years after the DS, the design office continued to export and adapt this expertise to architectures outside the Citroën range.