Idée Fixe [3]

In this final part, Steve Randle concludes his proposal for a latterday successor to the seminal Citroën DS. 

Image: citroenvie

Previously, we explored styling, power unit and drivetrain. Today, Steve Randle outlines his thoughts on body structure and vehicle dynamics.

Structure:  “Aluminium and magnesium would dominate the vehicle. The recycling problem with composites – particularly thermosets – are a concern. While both Aluminium and magnesium alloys are expensive in the first instance, they are easy to recycle.”

Driven To Write: Given the problems inherent with using aluminium for the upper structure of the vehicle – the necessity for much stronger and thicker pillars/smaller openings – how would you see this being resolved? What areas of the structure would the use of magnesium benefit from?

Steve Randle:  “Magnesium is good for seat frames, dash beams and so forth. With modern jointing techniques, a mixture of materials is no problem. Steel for door beams and pillars is a good solution. Castings are helpful where section nodes and hardpoints are required.”

Chassis:  “I would retain the gas hydraulic approach for both practical and cultural reasons. The system should be configured to eliminate warp stiffness – this idea was successfully introduced in the McLaren 12C, though steel road springs were retained. Use of a completely gas hydraulic system would provide further ride benefits, as would pitch interconnection. The high speed damping valves now available would be useful in optimising ride composure. There is still an issue with any gas or air suspension in that the spring becomes stiffer with suspension velocity (the adiabatic / isothermal issue), but we would be running lower ride frequencies which will help.”

Image: thecarhobby

“We also have a few ideas we could apply to further improve matters here. I would also introduce more longitudinal compliance into the suspensions – having the frequency below wheel hop gives significant benefits. We’ve developed multi-link suspension systems that can deliver this while still giving the necessary elasto-kinematic properties. Finally, I would like to see a return to higher profile tyres. 17” wheels were declared some years back by Michelin to be the point beyond which there was little point in proceeding unless you need huge brakes or styling demanded it. I don’t believe anything has changed since then. We’ve also been involved with some interesting research work into adaptive camber controlled suspensions – this allows a narrower, lower drag tyre to be run without loss of grip. I’d like to investigate this further.”

DTW: Can you elaborate upon ‘warp stiffness’ and how it manifests itself?

SR:  “Warp is when each axle is in roll, but they are in opposition. (like jacking up front right and rear left). A fully interconnected system where the four degrees of freedom are warp, heave (all wheels in phase), pitch (front & rear out of phase), and roll (left & right out of phase) would be an interesting approach.”

DTW: With wheel diameter limited to 17″, what about tyre aspect ratio? Would you advocate a taller sidewall than the 60-series which appears to be the most generous nowadays.

SR:  “I think 60 profile tyres are a reasonable compromise. I’d seek to work with Michelin on this.”

DTW: Regarding Oleopneumatics, would you advocate a totally analogue system, the electronics-assisted version used in the latter-day cars, or a full electronically governed system?

SR:  “Electronic control offers a greater range of possibilities and performance.”

DTW: I would assume braking would be handled by this oleopneumatic ‘ringmain’ as well. Is there any benefit to the driver in the original ‘solid’ brake, or would you apply an artificial travel as Citroën did latterly?

SR:  “The road car brake pedal is an interesting combination of displacement and force control. Clearly, even the original Citroën brakes had a small amount of displacement. Racing drivers rely pretty much exclusively on force. I think a firm pedal with an appreciable amount of motion is preferable. Enough to remind you it’s a Citroën.”

DTW: Given that light weight is likely to be a key element of this vehicle, are there any merits in revisiting the idea of composite wheels – a la SM?

SR:  “Carbon has its place, but I’m not sure it’s in a road wheel. I’ve seen some good looking solutions, but they’re not spectacularly light. They are however spectacularly expensive. Aluminium alloy wheels are typically design driven and are usually heavier than a steel wheel. Magnesium has been unfairly overlooked of late I think, as the corrosion issues have now been addressed.”

Steering:  “We are developing an electric steering system which will provide manual steering levels of feedback with the potential for adaptive vehicle dynamic tuning. I’d like to see this in the car.”

DTW: What is your position on the subject of four-wheel steering?

SR:  “There’s a lot we can achieve with front steer intervention, and with adaptive camber control. With both of those, rear steer becomes superfluous I feel.”

DTW: Have you reached a conclusion about the autonomy question?

SR:  “Autonomy is coming, but I think it’s a yes or no answer. I have grave concerns about partial autonomy (throttle and brake only for instance), or expecting the driver to step in if things get tricky. I really feel that is asking for trouble. I believe that something needs to change significantly in the cockpit if the car has selectable driver and autonomy modes (steering wheel folded away, seat turned around or similar). You can’t risk the driver forgetting who is in control. If you’ve got used to an automatic, it’s easy to forget to dip the clutch of a manual as you come to a halt. That’s not a life-threatening oversight though.”

DTW: Given that the turbine idea has so much merit (and appears such an elegant solution), why is everyone blindly piling into batteries? Is it the path of least resistance?

SR:  “Batteries are the bandwagon right now. They have their place as part of the solution – demand smoothing in particular. Energy density is still poor, as are their whole life cost and environmental impact. I would sooner carry as few as reasonably possible.”

As we draw our ruminations to a close, I ask Randle to reflect on such a vehicle’s likely reception with the motoring public.  “Having successfully put off 90% of luxury car buyers, I intend that this car will be the firm favourite of those that remain. Far better that than everyone’s third or fourth favourite.”

We began this piece with LJK Setright so it’s appropriate we conclude with that great iconoclast, wordsmith, aesthete, critic and commentator, who summed up the original, the only Déesse in inimitable fashion. His words apply equally here I feel.

Image: fotki

 “No car had ever been cleverer. No car was ever braver. The DS should have inspired the world to embark on a new course of motor engineering, to accept and advance the new standards that Citroën had set. All it did was gratify the desires of 1.3 million people, to stimulate a lot of arguments, to expose a great deal of ignorance and to stand as a lasting reproach to the rest of the industry whenever we compared what they were making with what, on the evidence of the DS, they should have been making. If it achieved no more than that, it was not the fault of Citroën, it was the fault of everyone else.”

©Driven to Write. All rights reserved.

Author: Eóin Doyle

Founding Editor. [Dis]content Provider.

4 thoughts on “Idée Fixe [3]”

  1. Interesting. I’ve always been bothered with the high roll angle on the DS, and wished for them to have interconnectedness left and right and not only front and back. As I understand it, the old oleopneumatic non electronic system required one sphere per wheel plus one for the brakes, five spheres in total. And that one sphere ccould only be interconnected to one other, thus giving a choice to dampen either pitch or roll. And that the Hydractive system was just adding another complete set of spheres giving ten in total or so. In addition to the four connected front and back, another four connected left and right.

    It seems for solving every problem, they just doubled up on complexity. So, why couldn’t the four wheels be connected to one large and central sphere, thus evening out any unbalance on any wheel with the mass of all the others? It seems to me the only way to dampen warp, heave, pitch, and roll at the same time would be to have all the wheels interconnected with all the others at all times. But yes, this is a re-active system that depends on actual physical action/reaction to function, to have a pro-active system that thinks ahead one must control all the factors independently. I can only see that with sensors that can see the road ahead and take action before it happens.

    Concerning the engine, the DS experience has never been about the engine, mostly because the only engine they had was a modernised Tracta unit with a crossflow top and five bearing main. A part from that possibly antediluvian in origin. I’ve spent hours contemplating other engines in counter factual thought experiments, and I’ve always wondered what would’ve happened if they had bought their engines from GM, I’m thinking the boxer 6 from the Chevrolet Corvair. Then they could’ve had their aircooled boxer six and the top of the line would’ve been called DS24 and DS27.

    My point is, for an experiment like this, they not buy the entire engine and drive train package from another car maker? I’ve always wanted to retrofit a restored DS with the drive train from a Toyota Prius. And why not? Why not have a hybrid drive train? They always said about the DS that one could run the entire car from the pressure generator. Why not turn it around and have the generator run the entire car? Switch the generator to an industrial grade electric engine and have the ICE unit only feed the batteries. Citroen just didn’t think that far ahead sixty years ago. The point is, the choice of engine really doesn’t matter, as long as it is adequate enough. Why not think out of the box on that one?

    1. Ingvar: Better technical minds than mine may be able to clarify, but my reading on the hydractive version of the oleopneumatic system simply added one additional suspension sphere to the original compliment – certainly as fitted to the original XM. Hydractive 3 may have been a different matter however.

  2. These are very interesting thoughts on the suspension, although the information is still a bit sparse for me to grasp it all. I like how new, unorthodox thinking comes in, and still the Citroën tradition can be respected. And thank you for opting for real tyres instead of rubber plated metal billets. The challenge here is to incorporate this in a way that doesn’t make the car look old-fashioned.

  3. What an excellent piece! Thanks for taking the time, Eoin. I remember seeing a Randle rear suspension photo in the previous interviews on Jaguar, and it’s pretty obvious that he is a suspension nut. Couldn’t be a better person to have a think about a new big Citroen design on that front.

    It seems to me that for the oleo, oil hydraulic part of the suspension that a central control block could be designed, where the oil displacements from the suspension units could be combined by a series of valves to be routed in certain ways from one unit to any other. Electronically connecting these valves and fed with sensor information as to cornering, braking, steering, and bump forces, then oil could be sent wherever it’s needed. Side-to-side and front to back and diagonally, all at the same time.

    The control strategy might be complicated or it might turn out to only need to be relatively simple. The Tenneco system used on earlier McLarens sort of does his, but doesn’t use air/nitrogen as the springing medium. Using air springs allows one to “weigh” the car and occupants by monitoring static pressure after the car rolls a few feet from rest to shake out rubber suspension bush friction forces. Then you can tailor the spring rates to the load for a comfortable ride under all conditions. The oil rushing about through the dampers and through the control valve block can be used as a substitute for anti-roll bars, tailor warp resonse, front to rear response. Think of that ridiculous new Audi A8 mechanical nightmare suspension we looked at last week, and think how much simpler this would be. Two oil lines from each suspension unit, above and below the oil piston, connected to a central control block fed from a main reservoir and held at a given pressure by a hydraulic pump.

    Fore and aft suspension compliance at the level of the control arms and bushes to the body is a bit of a black art, since sideways compliance ideally should not be affected. Geometrically unlikely multi-link suspensions have been the solution to that problem for 25 years, torturing rubber bushes in the process, but the quality of results depend on acual configuration and tuning.

    Both Audi and Ford (who churn out a million alloy bodied F150 pickup trucks a year) don’t seem to have run into JLR’s problem with requiring massive structural cross sections and consequently small door openings, so alloy seems to have a bright future in vehicle bodies. In addition, Honda has developed methods for welding steel and aluminum together to avoid galvanic corrosion problems, so door openings, etc could be made stronger and lighter by judicious choice of materials. Many vehicles already have magnesium beams in the dash tieing the A-pillars together.

    Excellent series of articles. Unfortunately PSA is likely to do nothing so daring as the ideas discussed. A pity.

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