Technical note of 20 February 1955 on the hydraulic brakes of the type 55-B truck

In this document

This two-page technical note, dated and stamped "20-02-55" in red ink p. 127, details the operation of the hydraulic brakes of the type 55-B truck, both front and rear.

Rear brakes

The 7-piston pump delivers 2 cm³ per revolution, i.e., 1.65 cm³ per engine revolution p. 127. It supplies a 360 cm³ accumulator inflated to 35 kg/cm², containing 263 cm³ of oil at 130 kg/cm²; at 600 rpm, its complete filling requires 15 seconds p. 127.

A table provides, for two axle ratios ("6 x 41" and "6 x 45"), the distance to be covered to replenish the volume required for one braking stroke (46 cm³) according to the gear engaged: from 12 m in 4th gear to 1.8-2 m in 1st gear p. 127. A second table p. 128 converts these values into the number of braking strokes available per 100 m, from 8.4-9.1 in 4th gear to 50-55 in 1st gear.

The note emphasizes that the accumulator "practically only acts as a 'buffer'" p. 127, with the engine maintaining pressure at every stop in town or during maneuvers. It insists on the fact that an excessively high number of braking strokes available after the red lamps light up would be dangerous, as the driver must not "rely on an uncontrollable reserve" p. 128: when the red light is on, the stop must be immediate.

Front brakes

A single-cylinder pump mounted on the transmission delivers 0.4 cm³ per revolution, i.e., 0.9 to 1 cm³ per meter traveled depending on the ratio p. 128. Two accumulators of 360 cm³ each, inflated to 35 kg/cm², offer a total of 526 cm³ available between 130 and 35 kg/cm² (including 386 cm³ between 130 and 75, and 35 cm³ below).

By estimating the volume required per front braking stroke at 23 cm³, the note calculates 23 total braking strokes available, including 16 after the lamps light up at 75 kg/cm². The truck must travel 25 m (6x41 ratio) or 23 m (6x45) to have one braking stroke available; when starting empty, it takes 400 m for the lamps to go out, but one braking stroke at 35 kg/cm² remains available from the first 25 meters p. 128.

Context

The acronym "55-B" and the mentions of "truck" place this document in Citroën's heavy-vehicle activity of the 1950s, distinct from Paul Magès' better-known work on the hydropneumatic suspension of the DS. We know from Magès' autobiographical notes that he joined Citroën in August 1925 and remained there for 49 years, notably holding a position as deputy head of the Super-Control (Draft of a letter from Paul Magès to Mr. Sarre and maintenance plan of the 11); this 1955 technical note falls under this same internal engineering activity, contemporary with the tests conducted that year on the 15-Six H (3-5 April 1954, Test of the 15-Six H ELV 412 by Paul Magès) and the presentation of the DS on 6 October 1955. The braking systems assisted by hydraulic accumulators, with warning lights, announce the principles that will be used on the high-pressure brakes of the DS, where Magès will play a central role. This connection remains, however, a simple historical parallel, not established by the document itself.

Archivist's perspective

This note is the result of fine-tuning work, where each braking stroke is quantified in oil volume and distance traveled — an approach characteristic of the engineering rigor found in other pieces of the Magès collection.

It seems that this document belongs to a comparative study phase between two axle ratios (6x41 and 6x45), probably to arbitrate a production choice for the type 55-B p. 127-128.

The concern expressed to avoid an "uncontrollable reserve" for the driver p. 128 testifies to a concern for active safety, which finds a more general echo in the 1945 manufacturing schedules: Citroën suspension, starter-launchers and brakes, another piece from the same file attesting to the longevity and continuity of work on brakes at Citroën.