Lest I lose too many of you to a forbidding wall of body text, I'll begin with a little promotional film from Packard which gives at least a partial illustration of what it's all about: While I appreciate that some would deem the amount of time the inside of my head is occupied by suspension interconnection to be cause for pity, I do find it surprising that, out of an entire planet of people, so few share my fascination with the subject. My Facebook group on interconnected suspension has been no great success, I do not know whether because of the recondite nature of the topic or simply because my personality is all wrong for mobilising people around anything. No matter how strong my desire to get people fired up about an issue, it will be trumped by a deeper, possibly inborn, conviction that people ought to be left to care about stuff, or not, as they bloody well see fit. As a result, the group ended up being little more than a catalogue of my periodic ruminations — and as my brain-weirdness seems to include object-permanence issues inside of it, in that if a thought isn't before me I forget that I've had it, a way to remind myself that I've had this exact eureka moment four times already. The greater part of the people who have commented on anything are on the HAMB: UKAde, exwestracer, metalshapes, JamesD, volvobrynk, though only Ray (exwestracer) has posted a post of his own on the group. It's hardly the lively discussion I had hoped for. I had judged the topic a bit borderline for the HAMB proper, for though the three basic historic iterations of the concept, the Citroën 2CV and its derivatives, the BMC Hydrolastic and Hydragas cars, and the 1955-56 Torsion-Level Packards, all fit comfortably within the HAMB's focus era, only the Packards satisfy the country-of-origin requirement. Hence Facebook. The HAMB's new Off-Topic forum is perhaps a more appropriate place for this. To my mind the advantages of front-to-rear suspension interconnection are: Ride comfort due to pitch stiffness much softer than bump (or roll) stiffness, as illustrated in the above Packard film; Ride/handling balance due to the difference in wheel rates in pitch/single-wheel bump and roll/four-wheel bounce; Constant road contact over undulations/corkscrewing/etc. due to low or zero warp stiffness; and Virtual elimination of torsional loads on the frame or vehicle structure arising from corner inputs, also due to low or zero warp stiffness. The last one is particularly pertinent to hot rods as, free from crashworthiness standards which require modern cars to have a heavy body structure anyway, it allows a hot rod body to be completely non-structural and wholly within the construction capabilities of ordinary people. At the same time, lack of torsional rigidity ceases to be a dynamic limitation. But it is a small bowl of nuts to crack, the toughest of which turns out every time to be damping.
Historical question: it is known that the Citroën 2CV's basic suspension layout, front leading and rear trailing arms linked by tensile elements, was Alphonse Forceau's idea. But in the early TPV prototypes he had cables(!) acting on an arrangement of torsion bars under the rear seat. Who was responsible for the very elegant eventual solution, tensile "pots" containing coil springs acting against their ends via rods, the whole held between soft volute (later rubber) springs seated on frame outriggers? One source wants to suggest that it was Forceau and Marcel Chinon working together, but it remains inconclusive. Reading up on that history leads me to think that the development of the TPV would make for a good movie, especially the bit where Pierre-Jules Boulanger, André Lefebvre, and others foil the Nazis' attempts to get their hands on the prototypes and drawings. Unlike some (mainly) Americans who believe that excellence in automotive engineering can only be revealed in the most expensive of cars, and that the 2CV is therefore no more than a risible French failure to build a 1959 Cadillac, I have immense admiration for the team who developed the 2CV. Especially the suspension system is sheer genius: There are nevertheless three things I dislike about the 2CV "pots": They're welded together, so you can't get to what's inside; They don't allow for damping where it's needed; and The motion ratio, as installed, of ±0.22 is far from ideal. A more realistic motion ratio would make proper damping possible, but that would require access to the inside of the "pot". But how to arrange that without making the thing bulkier, as a bolted flange would? Screw-threaded end caps? A ring of radial screws? Tensile rods holding the end caps onto the tube would work. They'd run inside the coil springs but outside the damper body, so again, a better motion ratio would help us by allowing a smaller damper piston diameter and lighter rods, without making the tube diameter bigger. If anything, I'd prefer it to be smaller. The ±0.22 motion ratio doubtless comes from the 2CV's extremely long suspension travel. A ratio closer to 1 would have required longer coil springs, hence longer "pots" moving a greater distance fore and aft. All that could be reduced if suspension travel were closer to normal.
I listed four advantages in the first post. In the case of the '55-'56 Packard, there is a 9' torsion bar on each side of the car. The torsion bar is nowhere anchored to the frame; it's supported by a few bearings and guides, but it's anchored only to the front suspension's lower control arm and to a short, inward-facing arm acting on the rear suspension's "truck arm". Let's leave aside the shorter "levelizer" torsion bars and the motor and mechanism attached to them for the moment, as they tend to get in the way of understanding the basic principle. Videos you find online imply that the Torsion-Level is all about the self-levelling system, when that is basically an add-on to the concept. Suppose we put the Packard on a lift. We push up on the left front wheel; the left torsion bar rotates without really twisting, the short arm at the back end of the torsion bar pushes down on the truck arm, and the left rear wheel moves down. Seen from the front, the torsion bar rotates anticlockwise. Push up on left the rear wheel, the truck arm pushes the short arm up, the torsion bar rotates clockwise, and the front wheel moves down. Now we put the car on the road. The weight of the car wants to turn the front end of the torsion bar anticlockwise, and the rear end clockwise, which twists the bar, which supports the weight of the car. Suppose we come to a bump in the road. The front wheel rides over the bump, twisting the torsion bar more. Nine feet of torsion bar makes for a very soft spring rate, so most of the force from the front wheel is taken up by twisting the torsion bar. Some of the force goes through to the rear wheel, raising the back of the car slightly. The car moves on until the rear wheel goes over the same bump, and the same process happens in reverse. The result is that the car doesn't pitch back when the front wheels go over a bump, nor does it immediately thereafter pitch forward when the rear wheels go over the bump. The car stays pretty much level, and just moves up and down a bit, which makes for a far more comfortable ride than a lot of pitching back and forth. That's advantage 1. Now suppose we go around a corner, and the car wants to roll to the outside of the curve. Now both outside wheels move up, and the ends of the torsion bar linking them are twisted more, in opposite directions. The bar is twisted from both ends, so it acts like two 4½' torsion bars anchored to each other. If we were to look we'd find that the midpoint of the torsion bar doesn't rotate at all relative to the frame. As the spring rate of a torsion bar varies inversely with its effective length, we've got double the wheel rate in roll as in single-wheel bump. Suspension twice as stiff in roll as over bumps is advantage 2. Remember how we could rotate the torsion bars without twisting them when the car was on the lift? The upshot of that is zero pitch stiffness. Brake and you're riding on the front bump stops; accelerate or put a load in the back and you're riding on the rear bump stops. That's why all these systems incorporate an additional system to control pitch, taking advantage of the fact that this can have spring rates even softer than the main suspension. In the case of the Packard that is coupled to a self-levelling system driven by an electric motor. Ideally we'd like the pitch control system to be interconnected left to right, but if the spring rates are really soft it's arguably splitting hairs. None of the production systems have that left-right pitch interconnection. Let's go back to the lift. We could move the left front wheel up and the left rear wheel down without twisting the left torsion bar, but now suppose we simultaneously move the right rear wheel up and the right front wheel down without twisting the right torsion bar? If you think about it, that's exactly what would happen if the car were parked on level ground, with the left front wheel on a ramp or something. All four wheels find their height relative to the frame, because they are balanced against each other, and the body takes up an attitude somewhere in between. The load on each wheel stays the same, regardless of the shape of the road. And this keeps happening while the car is moving: even while we're loading the suspension on one side of the car going around a corner, the front and rear wheels are still constantly finding their balance relative to each other. It's like having an open differential in the suspension. What this means for handling on wobbly, bumpy, odd-shaped real-world roads should be obvious, and is advantage 3. Now, consider this from the viewpoint of the frame. Instead of a separate spring on each corner imparting loads onto the frame, we have loads being passed to the opposite wheel in the interconnection, and only the constant mean load being imparted onto the frame. The suspension isn't trying to twist the frame all the time. That means that frames could be made much lighter — or conversely much better handling could (potentially) be got from a spindly Model A frame: advantage 4. It's all about Elastic Roll Moment Distribution between front and rear. Conventionally we do that by varying roll stiffness front to rear. Now we'd be doing it by varying leverage between the ends of the interconnections. (For instance, I said above that wheel rates in roll are double wheel rates in single-wheel bump, but if we're clever we can have, say, 120% at the front and 80% at the back, using different leverages.) I used the Packard as an illustration, but where the Packard uses torsion bars, the Citroën 2CV does the same thing with coil springs, and the Hydrolastic and Hydragas cars do it with hydraulics. Is that clearer or have I made it worse?
It's an interesting concept. Obviously, with the modern concept of McLaren hydro and GM's magnaride damping, this gets even more complex. I am a neophyte on suspension and handling. So this is just a WAG thought process. It seems that first, the most often used for high end handling is the double a arm or SLA at each corner. Various Japanese and European luxury makers use many links that appear to be computer modeled to locate the spindles/wheels. Stuff like 2 ball joints on 2 different links. Those seem strange to me, but they must have some advantage. I am not versed enough to be able to visualize the hows and whys of this adjusting road/tire contact to optimize handling/ride. Considering that the supercars all tend toward the Double A arm, my guess is that the wilder stuff is more for ride, or possibly to handle the larger heavier loads. So while your thoughts on interconnected springing is interesting, I'd expect that this has been modeled and chosen not to be used, instead using the interconnected damping I mentioned. Why? I have no idea, it's above my level of knowledge. I'd expect that it's not safety (single point failure causes entire side to drop) since modern electrical faults are getting pretty bad, 'bricking' some stuff. Packaging and crush zones might have something to do with it, although it should be packaged below passengers and within the wheelbase and modern cars love their big, heavy sills. Being a Mopar guy, I have a warm spot for torsion springing. That coil spring inside a tube looks overly complex. Typical American thought, keep it simple unless it's a big improvement. I can see where a rising rate linkage like a motorcycle swing ram (or F1) could be incorporated. EDIT: I'll also mention that your What If? posts are always thought provoking, but seem to be always asking open-ended questions about stuff that could be explored with the currently available CAD modeling programs and some finite analysis testing. Superfast matt did a bunch of this with his latest LSR car, and it wasn't a huge burden, although he has the skills to do the programming and got some free assistance with the heavy airflow testing.
A very basic explanation of Mini Cooper suspension https://classicmotorsports.com/articles/quick-explanation-bmc-hydrolastic-suspension/ A YouTube vid
Atleast here in America on our roads I can’t see where this type of suspension would be an advantage. Factor in the complexity of it and it seems even less advantageous for a car maker to go through the expense of it all.
The OEMs' heads aren't anywhere near this stuff because the dominant paradigm, lately entrenched in crashworthiness requirements, involves a big, stiff, heavy, complex, investment-intensive unibody anyway, which makes a lot of the advantages of interconnection redundant. How it got this way is a complicated bit of history with political-economy aspects: suffice it to say that the OEMs are happy as clams at high tide with the current arrangement. Hence the idea of interconnection getting limited to details around the edges. Even the F1/GTP/FSAE types are this way. They like to get into the tricky stuff and don't struggle to grasp the concepts, but they are coming from a history of a huge amount of cumulative development around space frames, monocoques, and the like, so again there is little incentive to go to radical interconnection. The only person I've seen champion radical interconnection in those spaces is an irascible Australian known only as "Z". Interconnection does require a different way of thinking, and it's understandable that people who have a thorough understanding of conventional suspension struggle to get their heads around it. If you look closely it becomes obvious that even Bill Allison, who came up with the Packard system while he was at Hudson, only saw the thing through a glass, darkly, as it were. That's why the Packard has anti-roll bars, which partially defeat the object: and moreover the system could be considerably simplified by having short arms attached to the torsion bars at mid-length, acting on a pair of pitch-control springs interconnected left to right, and dispensing with the "levelizer bars". Too many interconnected systems have been ripped out and replaced with conventional systems because people didn't understand how it works. A lot of people know what to do to conventional suspension to shift the ride-handling balance to the handling side, but faced with doing the same with an interconnected system they scratch their heads trying to figure out what does what, and finally junk the system in favour of what they know. It's a pity that nobody ever really explored the high-performance potential of the principle, because that might have inspired a generation of car nuts to think it through.
Mini — including Cooper — suspension came in two variants, "wet" and "dry". The Hydrolastic or "wet" suspension had rubber bags filled with a water/glycol medium interconnected front and rear, acting on rubber doughnuts. The "dry" suspension had aluminium cones in place of the bags. I think there are three reasons why "wet" Minis were often converted to "dry": As noted in Post #12, the "wet" system was poorly understood, so people didn't know how to go about performance-tuning it; The Hydrolastic units are unserviceable, and factory stocks ran out early. They can be refurbished, but only up to a point and only if they weren't too far gone to begin with; and The "wet" system solved the problem of getting a small car to ride decently, causing people to think of it as the "comfort" version and the "dry" as the sporty one. In reality they handled the same, and the "wet" possibly better under certain circumstances. Interestingly, the last Mini model to have the Hydrolastic system before it was phased out in 1971 was the final Cooper S. Also, it seems that despite being close friends, Alec Issigonis and Alex Moulton were thinking at cross purposes around this stuff. Issigonis was always about torsionally stiff structures, while Moulton was making torsional stiffness a lot less important. The Hydrolastic/Hydragas systems didn't really show their worth in the Issigonis cars. It's a pity the complicated history of British Leyland didn't allow the overlap, as one car which would have been perfect for Hydrolastic/Hydragas was the Triumph Herald and its derivatives, with its light backbone frame.
I wonder if a Cooper style hydraulically connected rear suspension would be helpful in increasing traction during acceleration aka drag racing ? Dan
The BL/Rover Metro had basically the Mini's rear suspension, but interconnected left to right, with unconnected coil spring suspension at the front. This gave it zero rear roll stiffness, and made it effectively a reverse trike because the two rear wheels together then act like a single rear wheel. This is similar to what every decent-handling small hatchback with torsion-beam rear suspension does, i.e. lift the inside rear wheel off the ground in a tight turn taken at speed, making it also effectively a reverse trike. Left-right interconnection can help launch traction with irs, by equalising loads on the rear wheels. With a live axle we're dealing with torque reaction, and often part of the rear suspension's job is to keep the car off the right rear bump stop, which left-right interconnection would make impossible. That is why for instance the traditional parallel ladder bar setup effectively turns the rear axle into a very stiff anti-roll bar. But it raises an interesting question. I wonder if anyone has tried an offset rear roll centre in drag racing. It would be tricky to determine exactly how far to the right to put it, though, as you want the torque reaction to be cancelled by the unequal weight load leverage on the rear axle, and the torque isn't constant. It would be easy enough to do (one way that comes to mind is a Watt's linkage with the centre link well to the right of the car's centreline) but how to do it well is another question. And I wonder how that kind of setup would handle on the street. This is regardless of how well the launch geometry is worked out otherwise, in other words how far the rear suspension's instant centre is from the zero-squat line. With irs (or my favourite, the DeDion axle) the torque reaction problem goes away, but the launch geometry is different in that the zero-squat line runs through the centre of the rear wheels and not through the contact patch. I think the DeDion axle has a lot going for it in a drag racing situation. I can imagine an alternative history timeline in which DeDion axles became a common conversion in some drag racing classes somewhere in the '70s. Whatever the case, with proper launch geometry pitch stiffness becomes irrelevant to squat, and can be as soft as other requirements dictate. Speaking of launch geometry, I can't for the life of me figure out how pitch was controlled on the Hydrolastic/Hydragas cars. There doesn't seem to be anything there to do the job. Geometry can only go so far. The rear instant centre in braking is probably close to the zero-lift line, but without having studied it in any detail I'd expect the front instant centre to be at infinity on the ground. Any load in the back would have the headlights aimed at the moon. That is probably why Mini station wagons, panel vans, and pickups all came with the "dry" suspension.
The setup you started with would be something like 2 torsion bars geared together in the middle so when the front is subject to a change, the rear changes in an equal but opposite direction, right? That makes me wonder about a frequency change at different speeds and the washboard test roads we've seen in old films. www.youtube.com/watch?v=MTE4On69kXQ
Yeah, same result, different mechanism. Still concerning. Also, in and out would require tuning for lever length and weight difference F and R. One of the advertised features of the magnaride was that the computer was so fast, it could sense a bump in front, calculate the force and the vehicle speed and change the rear shock damping in real time. Not sure I believe this, but it was described. My mistake, I spelled it wrong. https://en.wikipedia.org/wiki/MagneRide
Thank you @Sharpone . I enjoyed that immensely. 17 minutes and 11 seconds very well spent. @Ned Ludd , this system from BMC sounds like it should have taken the world by storm. Why didn't it???
Thanks for that. In all my researches I've never come across this graphic. It solves what was to me a mystery, showing the pitch bars at the rear. My first thought would be how easy it would be to rig a self-levelling system by rotating the inboard ends of the pitch bars.
Add to the three reasons I listed in post #13 4. It wasn't possible to open up the displacers e.g. to modify the damper valving etc., which probably suppressed an enthusiast community developing around those kinds of modifications; 5. The cars to which the systems were fitted were well engineered but poorly made, tending to rust at an alarming rate. Because the systems were poorly understood they got part of the blame for that. The Hydrolastic system was used, primarily for promotional purposes, on an Indy car in 1964: Though the car was called an MG, the only BMC parts it contained were the Hydrolastic system. The displacers can be seen above the transaxle in this pic. It is possible to emulate these systems using airbags. Air spring manufacturers approve the use of water-glycol media in industrial applications where the bags are to be used as variable mounts rather than springs. As the same kinds of maximum pressures apply as with air i.e. ±100-120psi, the motion ratios would have to be much lower than in the Hydrolastic/Hydragas, which ran about 260-280psi. A diy Hydragas would be a hydropneumatic accumulator mounted onto each bag, with a valve block between them. The interconnection would be taken from the bag side of the valve block. The accumulator would be specced and sized to use water-glycol and shop/forecourt air, or air from an onboard compressor. Because of the lower working pressure all the components would be a bit bulkier, but it would be possible to have a short piece of large-diameter pipe between a bag and its accumulator, which might help with packaging. It would be possible to vary ride height either hydraulically or pneumatically, though any change at a front displacer will be echoed by a similar change at the corresponding rear displacer. A self-levelling system can be used to vary the car's attitude in pitch. That's part of the mental shift: instead of thinking "front" vs. "rear", we'd be thinking "up-down" vs. "dive-squat".
This is the same thing brought up to editors, make sure to spell check and follow proper grammar. If the reader sees a bunch of errors, it calls into question the message you are trying to convey. If the car falls apart after a couple years of normal use, the entire design and manufacture is called into question.
More good stuff from Grassroots Motorsports, don’t know if you’ve seen this or not. https://grassrootsmotorsports.com/forum/grm/interconnected-suspension-how/237536/page1/ links to more stuff etc. Now you have me hooked! Dan
Guess what I've been doing the past 24 hours? It turns out accumulators for our diy Hydragas aren't a triviality. Industrial accumulators are rated at 3000psi+ and are accordingly heavy and expensive. Water-pressure accumulators are a possibility. The little plastic ones used on boats and RVs should work on smaller cars (say <3000lbs loaded) but they are ugly. I'm immediately thinking how to replace the casing with something better. Spun aluminium? There is a company here that does small-run custom spinnings. It should end up being about 3¾" in diameter and 9" tall. The bottom end of the domestic water supply accumulator range should be sufficient for heavier cars. Those tend to be bladder rather than diaphragm type, so they need to be mounted upright. They're around 6½" in diameter and 12" tall. Careful, though, as some are up to 100-120psi operating pressures and some aren't.
The main reason the wet suspension was replaced by dry in race Minis here in Australia was weight. The old rubber suspension was a lot lighter than the wet system. These cars were very light and shedding 50lb of fluid and bladders had a huge impact on their performance as back in the day a 120FWHP Cooper S was a very good and fast racecar. Was the Citroen DS a linked system?
Also, the Mini shell was plenty stiff, so conventional suspension tuning was feasible, especially given typically smooth circuits. The DS etc. system wasn't conceptualised as interconnected. The spheres were connected left to right, though, but the interconnection was counteracted by hefty anti-roll bars. I suppose the interconnection was merely about simplifying the control valving to the hydropneumatic system: one valve at the front instead of two doing exactly the same thing, and likewise at the back. It also made it easy to operate the self-levelling valves from the midpoints of the anti-roll bars.
I came across this: It's about 5" in diameter and 8" tall. There is a version rated at 232psi working pressure. It is compatible with aquaeous glycol media. It has a 1" pipe connection. It is a diaphragm type, so it can be installed horizontally. A company local to me lists it at about US$50, including shipping to South Africa and all the applicable taxes and duties.
The Jalopy Journal
