Test no. 219.084 (1949): comparison of Vacuum 45 and VP 100 oils for Citroën shock absorbers

In this document

Purpose and scope of the test

This report, dated 19 July 1949, bears test number 219.084 and originates from the André Citroën Laboratories, 53 rue Balard in Paris p. 9. It is intended for the "Studies" department and mentions Mr. Magès in the margin p. 9. The document responds to an internal request "Studies no. 42.401 Z" p. 9.

The stated goal is to "determine the general influence on shock absorber calibration of the oil change sometimes practiced in Sweden: Vacuum 45 instead of VP 100 (standard)" p. 9.

Method

Three series of tests were conducted p. 9-10:

1. recording of viscosity curves for both oils;
2. comparative calibration of a G.22 P standard (AR PV) shock absorber, at 100 cycles/min (100 mm stroke) and 600 cycles/min (30 mm stroke);
3. endurance testing on a bench of 6 shock absorbers (3 H.20 O, 2 G 22 R, 1 G 22 P) over 300 hours, including 150 hours with each oil.

Viscosity results (p. 10)

Braking and sealing results

In compression, no significant difference between the two oils; in tension, Vacuum 45 brakes less strongly than VP 100, with the gap narrowing as temperature increases: 11% at -4°, 9% at +30°, 7% at +70° p. 9. The report establishes braking equivalencies, for example VP 100 at +30° ≈ Vacuum 45 at +10° p. 9.

After 150 hours of endurance, oil losses were higher with Vacuum 45 (average 7.5 g, compared to 2 g for VP 100) p. 10. The graph on page 11 shows the maximum braking force in kg as a function of temperature, for both oils, in tension and compression.

Context

Hydraulic oil shock absorbers were fitted to classic Citroën suspensions at that time; the code G.22 P designates a standard model intended in particular for rear axles. The question raised—oil substitution according to climate—is consistent with concerns regarding exports to cold countries like Sweden, where lubricant viscosity directly influences road handling.

This document dates from before Paul Magès' invention of the hydropneumatic suspension, the first plans for which date from 17 February 1944 (hydropneumatic suspension plans) and which would be introduced on the DS presented on 6 October 1955: this 1949 report still relates to the improvement of classic hydraulic shock absorbers, even as Magès worked in parallel, at the design office, on gas and liquid suspension.

Archivist's perspective

This test report illustrates the meticulous, comparative daily work of the Citroën Laboratories at the end of the 1940s: it appears to be a simple quality control aimed at validating or rejecting a local practice (the oil replacement in Sweden), rather than a search for a technical breakthrough.

The presence of Magès' name in the header p. 9 confirms that during this period, when he was a technical writer and then deputy head of the Super-Control after joining Citroën in August 1925 (autobiographical notes), he followed very concrete issues of shock absorber calibration and sealing—a useful field of experience for his later research on fluids and pressure transmissions, as evidenced by the technical note on the hydraulic transmission of high pressures from 1948.

Note also the chronological proximity to the notice excerpt on the oleopneumatic suspension of the Panther motorcycle (1949), which testifies to a similar context of interest in pressurized fluids, although the two documents do not deal with the same technical object.