We dive beneath the skin.
Irrespective of whether Citroën’s Bureau d’Études was acting in concert or as alleged, in a contrary and fragmentary fashion, there were a number of engineering imperatives which for them would prove sacrosanct. The first of these and perhaps foremost was the mode through which drive forces would be transmitted.
The second and if anything, just as much a prerequisite would be the use of Citroën’s centralised engine-driven, high-pressure hydraulics for damping, steering, braking, levelling and attitude control. This highly innovative and technically ambitious oleo-pneumatic system was developed by Paul Magès and first employed for the rear suspension of the 1954 15 h model, prior to it being rolled out in fully fledged form in 1955’s DS 19.
Assisting Magès on Projet S was Hubert Alléra, who had amongst his other palmarès, designed the hydraulically actuated gearchange for the DS. Suspension-wise, the SM didn’t depart radically from existing practice, in fact a great deal of DS thinking (and hardware) was almost literally carried over; largely for cost reasons, but also because in the opinion of Jacques Né, not only were they strong enough to cope with the car’s additional performance potential, they were entirely appropriate to a car of the SM’s potential.
Né pointed out to author, Jan Norbye, that 100% of the SM’s roll stiffness was provided by the anti-roll bars. The main alteration from the DS saw the front suspension upper and lower arms mounted in a trailing orientation, primarily to help comply with US Federal impact standards due to come into practice in 1973.
However, Magès and Né had envisaged a mechanically actuated active suspension system, which by 1975 was running in experimental prototypes, incorporating hydraulic control of roll and pitch. However, this hydro-mechanical system was deemed too complex and expensive to productionise by Citroën’s masters, and was put on the shelf until such time as suitable electronic control units became available.
The major departure from previous Citroën technical practice was Magès’ radical DIRAVI (DIrection a RAppel asser-VI) speed-sensitive, self-centring rack and pinion steering, introducing, in concert with the geometrically pure centre-point steering, an unprecedented level of control. By the time DIRAVI made its debut, it had already been fitted to a number of Projet-S prototypes, the first in 1962. Such was its directness, an unpowered system would have rendered the car undriveable.
To alleviate this, Magès’ engineers developed a speed-sensitive variable servo, which provided maximum assist at very low speed (manoeuvring or parking) with a gradual easing of assistance as speeds rose, with zero assistance at high speeds when straight line stability was of greater importance. Coupled to this was powered self-centring, which came into effect once the handwheel was deflected from the straight ahead position, aiding high speed stability and reducing driver fatigue.
This was controlled by a gearbox-driven dual chamber hydraulic regulator, employing a centrifugal mechanism with bobweights, operating slide valves which increased or reduced hydraulic pressure in the larger chamber (the smaller chamber remaining constant) acting on the steering circuit. A cam regulated the amount of hydraulic pressure, set up to reduce the level of assistance at high speeds. The regulator was connected to a separate self-centring servo which applied hydraulic force upon the cam.
Citroën engineers determined from their exhaustive tests of the system that the smaller movements required upon the handwheel with the DIRAVI setup combined with the ultra-quick steering ratio, saved important time during direction changes, making for quicker and safer manoeuvring; similar perhaps to the siting of brake actuator and throttle pedal in the same plane, which led to often crucially quicker reactions from the driver. Another safety aspect of the DIRAVI system was that it was impervious to the otherwise potentially malign effects of a sudden tyre deflation or encountering a pool of standing water, for instance.
From an architectural point of view, Né and his team was somewhat hamstrung; first by the fact that he was not operating with an unlimited budget, and second, because there was a definitive way of approaching things at the Quai Andre Citroën, which Né, as an André Lefèbvre loyalist would have been bound to uphold. Furthermore, since the S-vehicle programme was intended to explore the upper reaches of front-wheel drive performance envelope, there was a compelling case to be made for continuity.
Hence, the SM body structure and chassis layout closely reflected that of the existing DS saloons, with a large pressed steel ‘punt-shaped’ base unit, to which the structural elements of the SM bodyshell were welded. One notable advantage of the DS layout was that the engine’s fitment well inside the wheelbase not only aided weight distribution, but allowed for a more favourable dash to axle ratio, so beloved of exterior designers, which a fore axle FWD layout would have made impossible.
However, this layout, with the engine mounted aft of the gearbox and final drive, was not particularly space efficient, and despite the engine being tucked snugly against the bulkhead, there entailed a long frontal overhang – fortunately another Citroën tradition. A further advantage of this layout was a low polar moment of inertia, which aided stability and balance.
All of these factors would entail that the SM was a longer car than it might otherwise have been, more so even than the far more commodious DS saloon. Citroën engineers made use of the space ahead of the front wheels by siting most of the hydraulic units there, the car’s penetrating nose precluding the siting of the spare wheel in this section of the engine compartment.
The SM’s gearbox was a five-speed version of the unit fitted to contemporary DS models, with gear ratios optimised for the Modenese V6 power unit. In the autumn of 1973, a version of the 3-speed hydraulic torque converter-equipped Borg Warner Type 35 automatic gearbox was made available for the US market.
The SM also brought a new sophistication to something as prosaic as lighting, hydraulics being employed to provide automatic levelling for the six quartz halogen Cibie headlamps; the inner pair also turning in harmony with steering inputs. The outer pair of lamps provided main beam, while the middle pair supplied dipped beam.
Following DS practice, the SM also employed a pressure sensitive brake button, which operated the four wheel discs, the front brakes mounted inboard, alongside the final drive unit. The handbrake operated upon the front discs, employing separate calipers, mounted above the main units. As a failsafe, the hydraulic system accumulators stored sufficient fluid reserves to allow between 40 and 50 brake applications, should the system fail for any reason.
So while the technicalities of the SM were Quai de Javel distillate in its purest form, its engine was quite literally another country. It is here that we will turn our gaze next.
Sources and references:
Robert Opron L’Automobile et L’Art: Peter I Pijlman
Citroën SM : Jan P. Norbye
Citroën SM : Brian Long
Sa Majesté – Citroën SM : Peter I Pijlman/ Brian Cass
Citroën SM – Accidental Death of an Icon : Stuart Ager
André Citroen – Engineer, Explorer, Entrepreneur – John Reynolds
Maserati- The Citroën Years 1968-1975 : Marc Sonnery
 The Hydractive system made its debut in the 1989 XM, reaching its series production apogee with the 1997 Xantia Activa model. [Source: Jan Norbye]
 The DIRAVI steering fitted to the SM employed a ratio of 9:4:1 with a mere two turns lock to lock. [Source: Jan Norbye]
The SM gearbox and final drive was not only fitted to the Maserati Merak, but also the Lotus Esprit in conjunction with Hethel’s slant four unit until 1987, when it was replaced by a Renault-sourced unit.