A pretty much standard '70s sight was a Fad-T with a tube axle in front and Jaguar IRS at the back. That was the occasion of a lot of local stubbornly-getting-it-wrong, but that is at present besides the point. Fad-Ts weren't all about handling, and the typical tyre spec involved a lot of rearward grip bias, so the combination's inherent flaw of greater camber recovery at the front than at the back was never a real factor. When considering a more serious chassis than a Fad-T, with much more front grip, the prospect of gaining camber in roll at the back only makes me rather nervous, which is why much of my suspension thinking has involved DeDion axles. I've been leery of solid front axles with IRS. This is not to say that that combination is unprecedented in OEM use. It was a feature of the Steyr XII, which was popular as a taxicab in Vienna between 1925 and 1929, and the Tatra 11/12/54/57 of 1923-49. Hans Ledwinka designed the latter, and had a hand in the former. Both of them featured Ledwinka's signature swing-axles with longitudinal differentials using dual concentric pinions each driving a ring gear, which eliminated the need for U-joints or CV joints, at the price of a high rear roll centre and a propensity for jacking — and rather aggressive camber recovery. Since then there have been a number of efforts to make independent suspension behave in some respects like a solid axle. Two which spring to mind are the Dax system of around 2000, and Torix Bennett's Fairthorpe system of the '60s. The Dax system is based on SLA thinking and is conceived primarily as IFS. I do not believe I have ever seen it applied to IRS, but there is no reason why it can't be. The Fairthorpe system is based on a trailing-arm paradigm, and is expressly conceived as IRS, without any provision for steering. There are doubtless other efforts. Perhaps some of you know of some of them? @Kerrynzl ? It sounds like something you might have come across. Both of these have been around for long enough for patents to have expired. Both seem to have died quiet deaths. If I were to hazard a guess as to why, I'd say that it's because of a lack of setting-out information; a lack of a rule-of-thumb for the relative lengths and angles of things for the system to work as intended. The people who figured it out already aren't talking. I've repeatedly modelled versions of both systems, and I can't discern a consistent principle of proportion. If anyone else wants to have a go at it, please do! I must say that of the two, the Fairthorpe system is the more forgiving: the roll centre is consistently on the axis of the trailing arms, and it is very hard to set up a version which won't go to slight — and symmetric! — negative camber gain in roll. The trailing-arm geometry makes for a low-height installation: even more if it is articulated as a leading-arm system, which is entirely possible. Worth looking at?
Interesting stuff, and there have been a number of multi-link front and rear suspension systems produced in recent times. With modern modeling, I'd guess that once an optimum tire location through suspension travel, cornering and weight changes is devised, the parts used to locate the hub could also be optimized. I'd imagine also that this would change somewhat depending on weight, CG and as usual, intrusion into the car's envelope and stiffness in the structure. I would think that high priced halo cars would explore this, but it may be a case of not wanting to go off the beaten path. It seems like all of them use SLA, with springing and damping being the difference. I haven't looked into the big luxury cars, perhaps some of them do use more links. Something like the Czinger is probably modeled further than I could even dream. But it sure looks like SLA. Yes, this is the front. There is another chassis that's been shown for a couple years that was designed and optimized by stress modelling that is like nothing else but perhaps bone structure. Very different and I can't seem to find it right now. It's all like that upper steering/caliper mount in the picture. EDIT: I think it was an early prototype of this same car. I'd also venture that any new design suspension would load the tires as evenly as possible, but still end up requiring custom tires to outshine the others. Shades of Bugatti's expensive sets! https://www.slashgear.com/1898243/tires-bugatti-use-cost/
To my old school thinking, Lotus had it right; Simplify, then add lightness. Both of the systems you mentioned go the opposite direction. I think also that the pneumatic springing and damping you have brought up before hasn't been fully explored properly. Not only in suspension travel, but also in the bushings. There have been hydraulic engine mounts for years for NVH. Consider having the SLA arm mounting points also allowed to flex in designed ways, like rear steering under load, which has also been explored with multi-link. A great deal of pressure would be required, which I find complex, but could be better than the current ram rear steering. I can picture the parts being very expensive wear items, like the McLaren dampers.
From the Wikipedia page on William B. Stout: "Stout is remembered for his engineering credo, "Simplicate and add more lightness." This would later become best known as the adopted maxim of Colin Chapman of Lotus Cars. It actually originated with Stout's designer Gordon Hooton." I'm not sure that I agree entirely, at least not unqualifiedly. Simplicity is not a simple concept! Which is simpler, a cuckoo clock (a very large number of very simple parts) or a cellphone (a handful of extremely complex parts)? I know that I prefer cuckoo clocks over cellphones, for a whole variety of reasons.
I stand corrected! Thanks! Simple can be fewer parts (sundial!) or each part doing more than one thing, which Mr. Chapman was also fond of. Simple can be a straight link between two pivots (see the lower arm to spring pivot) but then because of packaging may end up being a curved part. Add in stresses and forces and that simple spring pivot becomes quite complex for weight reduction! The CC&AR uses 6 links and 2 bell cranks to replace a pair of single upper arms. Add in many pivot points. I'm no math guy, so I wonder how much gain is there. What is the total suspension movement, what is the total hub angle change through that movement with an upper arm compared to the CC&AR? With that difference, what is the tire load/heat/grip change? I'd expect different tracks would make this more or less an issue (oval track VS road course). It would also be subject to wear, so what is the cost to gain? I enjoy your thought provoking posts, but feel like a child trying to grasp college level stuff. For your entertainment, I offer this. www.youtube.com/watch?v=m7uZu_Wsdis
A lot of what OEMs call "multilink" comprises efforts to eliminate the shortcomings of semi-trailing arm IRS, like unfortunate toe changes, by adding control links — mainly because they were already invested in a semi-trailing arm structural and packaging regime. As an approach it's amply precedented: turn a configuration which doesn't work into one which works well — even entirely differently — at the price of a little bit of complexity. In fact, you could call SLA suspension a way to eliminate the shortcomings of a swing-axle setup by adding camber-control links. Those links change the geometry radically, making the system quite different. I can see the principle of the Dax system, but every attempt I've made to model it has failed in at least one suspension mode. If it maintains camber in bump it does odd things in roll, etc. Everything out there about the WOBlink includes a formula for the necessary proportions of link and arm lengths, and much includes a warning about the severe multiplication of force that may be expected, iirc at the middle pivot of the arm: If anything like this exists for the Dax system, I haven't found it. Figuring it out by trial and error, even in simulation, becomes stale very quickly. By contrast, the Fairthorpe system is geometrically rather transparent. Barring a few configuration conditions it will always go to negative camber in roll, which is arguably always benign. The amount of negative camber increase depends primarily on the vertical distance between the ends of the cross links: the larger the distance, the smaller the increase. How much negative camber increases, I'd submit, however, is close to trivial, within reason. I can even propose a rule of thumb: the lower cross-link pivot should be on the longitudinal rotation axis of the hub carrier. This is not to say that the system won't work if the rule isn't applied; merely that if it is applied you're golden — because it eliminates any possibility of positive camber increase in roll. And it is extremely simple, if the hub carrier has an integral longitudinal beam with a ball joint at its forward end. Then it needs only two links per side: the cross link and a link to triangulate the trailing arm. The former is ordinarily in compression and the latter in tension. For some reason, links suddenly appear more complex, and possibly even more numerous, the minute they pivot on the suspension on the other side of the car and not on the frame. I do not understand why some experience this illusion. The downside to the Fairthorpe system is that it is likely to place the trailing arm pivot axis poorly for launch purposes. It would typically be too low, and therefore produce pro-squat geometry. Raising it might eat into space earmarked for other stuff, and introduce unwanted toe-out in roll. One way around this I've mentioned above: turn the whole thing around so that it basically comprises leading arms. That allows a low pivot axis behind the rear axle to produce initially substantial, but diminishing, anti-squat geometry, without appreciable toe changes. Another way would be to include portal hubs — assuming you've got another good reason for using them. Introducing torque reaction at the hub carrier defines the zero-squat line differently, meaning that that low trailing arm pivot axis is now in a very good position for anti-squat geometry.
Taken to an extreme, who says linkage length or mounting points have to be fixed? With linear actuators as control links and moving pivot points perhaps software can continuously adjust and reconfigure a suspension's geometry on the fly. Of course, this all falls into the category of even if you could, should you?
In that vein but simpler, Pete Aardema had adjustable ride height by locating the chassis end of his coilover shocks on a pivot, which had a lock/unlock feature and a hyd actuator. Kind of like a seatback, you could pull the handle, move to where you want, then release. This was in the 80s! The changing pivot points was what I was suggesting before in post #3. You could for instance move the top front arm point to change caster, camber and a bit of toe.
Multiple mounting points are fairly common. What I envisioned was mounting locations that actively moved in three planes under software control. Doable? Sure. Practical? Maybe not so much.
There are all kinds of reasons not to, but they aren't technological These speculations arise out of a desire to engineer outside the realm of digital control.
The Jalopy Journal
