Note "Reflections on braking" by Paul Magès (20 February 1955)

In this document

Energy principles and the role of "power"

The note posits that to stop a vehicle of weight P travelling at a speed V, a certain amount of energy must be expended, and that as soon as the notion of "time" is involved in obtaining a given deceleration, one must reason in terms of "power" p. 1. The author points out that, in conventional brakes, this power is provided in part by the driver — which poses a problem because "not everyone is capable of developing the necessary 'power'" p. 1.

Observations on drivers and accidents

Recordings have allegedly shown that even modest braking pressures (30 kg/cm² on 11 CV) are not always reached quickly by certain drivers, and the author notes being "surprised to see the number of accidents in which braking marks are barely visible" p. 1.

Towards a fully servo-assisted brake

These observations lead to the consideration of a fully servo-assisted brake, capable of releasing its maximum power quickly even for a less vigorous driver. The note compares this adaptation to that which a driver of a B‑2 had to make when braking a 7 CV Traction Avant for the first time p. 1.

Air / hydraulic comparison

A response time table is provided, measured on a P.38: time between the action on the pedal and reaching 17 kg/cm² in the rear receiver cylinders.

The author notes that the air servo-brake provides a pleasant "softness" but is detrimental to safety, whereas the hydraulic servo-brake allows for all desired braking values after an adaptation period linked to the voluntary reduction of pedal effort p. 1.

Critique of the decelerometer and conclusion

Page 2 extends the reasoning: an air servo-brake (vacuum or pressure) cannot be abrupt due to the elasticity of the motive fluid, which lengthens the pressure rise time. The author then criticizes the method of checking braking with a decelerometer, which neglects the time required to reach deceleration, whereas "the main goal of braking" is the minimum stopping distance p. 2. A brake that is too progressive would be "pleasant" but would increase the stopping time and distance p. 2. The text is dated "20-2-55" and bears a handwritten signature p. 2.

Context

This note is part of Paul Magès' work at Citroën at the time of the development of the DS 19, presented in 1955, one of the striking systems of which will be precisely front disc power braking, powered by the high-pressure hydraulic circuit of the hydropneumatic suspension. The P.38 cited as an example refers to an earlier model in the Citroën range used here as a comparative test bench. The reference to the B‑2 and the 7 CV Traction Avant evokes, as a pedagogical comparison, two generations of Citroën vehicles with very different driving behaviours.

Archivist's perspective

This text appears to be a preparatory reflection note, prior to or concomitant with the design of the DS 19 braking system: Magès builds a theoretical argument (energy, power, time) before translating it into concrete technical solutions p. 1-2.

It is likely that this note fed into, or accompanied, more operational documents in the same file, such as the brake distributor plan and its control or the sketch of the disc brake block, which date from the same year, 1955.

The precise date of "20-2-55" p. 2 confirms and refines the dating initially established at the day level.