Note on "Servo-controlled anti-roll for motor vehicles" by Paul Magès (P 2908)

In this document

The initial observation

Magès starts from a general principle of automotive comfort: the more flexible the suspension, the more the vehicle leans in corners under the effect of centrifugal force p. 1. Anti-roll bars (or "stabiliser bars"), mounted at the front, rear, or both axles, correct this defect but impose a compromise: being rigid, they limit roll but transmit more shocks p. 1.

The three conditions set by the experiment

After "several years of study and testing", Magès formulates three conditions for a truly servo-controlled anti-roll system p. 3:

1. The system must trigger before the appearance (and before the cancellation) of centrifugal force;
2. It must tilt the vehicle towards the inside of the corner, with a limit inclination that is always identical regardless of the corner radius and speed;
3. The speed of tilting must depend on the speed of the driver's action on the steering wheel.

The technical principle adopted

The device relies on stabiliser bars connected by hydraulically operated jacks to the wheel arms, allowing their setting to be modified p. 3. Sending pressurised fluid, to the right or left, tilts the vehicle and creates a counter-torque to the roll p. 3-4.

The control "signal" is directly the power steering pressure: as soon as the driver steers, pressure appears in the corresponding circuit, proportional to the speed of their movement p. 4. This solution meets the first condition because a lag naturally exists between the steering effort and the actual appearance of centrifugal force p. 4-5.

A mechanical valve, actuated by the relative travel of the wheels of an axle, cuts off the pressure supply once the limit inclination is reached p. 5. The straightening is done by venting the valves when the steering pressure is cancelled p. 6. Finally, the front and rear jacks are hydraulically connected in pairs to maintain a constant ratio between the restoring torques, regardless of the flatness of the ground p. 6.

Context

This note is a direct continuation of Magès' work on the hydropneumatic suspension developed for Citroën, where hydraulic pressure and hydraulic power steering (Diravi, which appeared with the DS) were already available technological building blocks.

Historically, active anti-roll — tilting the vehicle towards the inside of the corner rather than simply limiting roll — would remain a major automotive research focus for decades, long after this 1961 note.

Archivist's perspective

Dated precisely 17 July 1961 at the top of the first page, this handwritten/typed note bears the traces of work in progress: erasures, corrections ("cylinders" replaced by "jacks"), interlinear additions p. 3, 6. It seems to constitute a theoretical development of the principle, probably intended to serve as a basis for a patent filing (the document is explicitly designated as such, p. 3: "this patent").

The interest of this piece lies in the clarity with which Magès articulates an engineer's reasoning: starting from the limits of passive solutions, setting functional conditions, then proposing a solution exploiting a pressure already present in the vehicle (that of the power steering) as a control signal — an economical and elegant approach.

This text finds a visual extension in the technical diagram plate P.5043, which probably gives graphic form to the circuit described here, as well as in the photographic report of the installation on a test vehicle, contemporary with this note.