Hydraulics Quiz No. 3 — hydropneumatic suspension and frequency calculation

Third part of a series of six Citroën training quizzes on hydropneumatic suspension, combining multiple-choice questions on spheres and height correctors, and a suspension frequency calculation exercise.

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Understanding this document

In this document

This document is the third in a series of six Hydraulics Quizzes, to be returned "no later than fifteen days after receipt" in a stamped envelope p. 28. The questionnaire is presented as a tick-box training tool, with the instruction "better to leave blank than to guess" p. 28.

Questionnaire on hydropneumatic suspension

Seven multiple-choice questions cover the principles of hydropneumatic suspension: bump absorption according to suspension flexibility (Q1), the role of the sphere in storing shock energy (Q2), pressure distribution between spheres on the same axle or between axles (Q3), the installation of the height corrector (Q4), the operation of height correction during load variation (Q5-Q6), and the filling of a sphere according to its residual nitrogen pressure, with proposed thresholds of 15 bar and 65 bar (Q7) p. 28.

A telephone number ("49 45 43 56") and a fax are provided for any questions, with the instruction to add "16 1" from the provinces p. 28.

Frequency calculation exercise

Page 29 offers an applied mechanics exercise: calculating the frequency of a mechanical spring suspension using the formula f = 1/(2π) × √(K/m), with a mass m = 500 kg and a stiffness K = 33,360 N/m p. 29.

Question	Proposed values	Page
b) frequency after adding 300 kg	2 Hz / 1.3 Hz / 1 Hz	p. 29
d) stiffness to maintain the same comfort	33,360 / 53,376 / 20,850 / 53,321 daN/m	p. 29
e) case of a gas spring with +300 kg	increases / decreases / remains substantially the same	p. 29

The document ends with a space for "remarks concerning this chapter", a box to indicate time spent, a signature, a place and a date, as well as the note "Keep the carbon copy" p. 29.

Context

The hydropneumatic suspension, invented by Paul Magès at Citroën — who joined the company in August 1925 and whose suspension plans date from 17 February 1944 (Hydropneumatic suspension plans), after the first tests on the 2 CV in 1946 (Notebook of the first 2 CV tests) — introduced in 1954 and then generalized on the DS presented on 6 October 1955, replaces metal springs with spheres containing compressed nitrogen separated from the oil by a membrane. The principle of the height corrector, mentioned here, is one of the central elements of this system, allowing the car to maintain a constant ground clearance regardless of the load — unlike a conventional mechanical spring suspension, whose natural frequency varies with the onboard mass, as shown by the proposed calculation exercise.

Archivist's perspective

This quiz is likely part of a distance learning system intended for technicians or salespeople in the Citroën network, judging by the postal submission method and the correction system mentioned p. 28. It probably complements other modules in the same collection, notably Hydraulics Quiz No. 1 devoted to the basics of pressure and braking, and may have been accompanied by the welcome booklet for the Hydraulics Expert Notebooks.

The comparative exercise between mechanical spring and gas spring p. 29 pedagogically illustrates one of the major advantages claimed for Magès' invention: the stability of the suspension frequency regardless of the load, a central technical argument for hydropneumatic suspension.