Is a rigid chassis needed w/ actively adjustable suspension?

[g-force_addict]

For theory sake let's suppose FIA allows actively adjustable suspension.
You can actively adjust camber, caster, toe in/out, anything.

Would you still require a rigid chassis?
Would active suspension adjustments compensate chassis flex?

[xxChrisxx]

If anything you'd want a more rigid chassis with active ride, if you are directly controlling the degrees of freedom of an object. You don't want to introduce a second, uncontrolled, source of freedom (ie a flexing chassis).

It would be like having a gyrostabilised gimbal tea tray carrier to prevent your freshly brewed hot tea from spilling. Then making the tray from jelly.

If the chassis were to flex in a predictable manner it could be compensated for.

[Jersey Tom]

It would be like having a gyrostabilised gimbal tea tray carrier to prevent your freshly brewed hot tea from spilling. Then making the tray from jelly.

Perhaps the best analogy I've heard.

[DaveW]

What an interesting post. I would like to put my own interpretation on it.

It is worth pointing out that there is no such thing as a rigid chassis.

In the years I have spent rig testing a wide range of race vehicles, I have encountered several vehicles that have suffered from excess compliance. Such vehicles are almost never competitive. The signature of a problem is often what the team has done with suspension set-up, presumably to try to mitigate the consequences of the compliance. It is usually the case that those "enforced" changes do not actually improve performance.

I have also rig tested some good "stiff" race vehicles. Such vehicles respond logically to set-up changes and, others things being equal, are normally competitive.

I think true to say that chassis compliance is not often measured, & when it is, interpretation is somewhat primitive. Gaining control over compliance is not easy, & neither is it "free" (it costs weight & or volume). Largely because of this, I suspect, many designers ignore compliance, & those that do impose limits, do so in a fairly "global" way.

Without question, I would suggest that devoting some of the megaflops currently used on CFD to the design of a well controlled chassis would pay dividends. This would not be a completely straightforward task, because "well controlled" needs to be defined in detail, and details matter. For example, the upright-upright torsional stiffness of a open wheeler is divided, typically, more or less equally between the chassis and the suspension links.

[hardingfv32]

In the case of the chassis, why is it not treated like an additional spring in the system? The chassis flex is consistent is it not?

Brian

[Tim.Wright]

Yes, but its an undesirable spring with undesirable and uncontrolled side effects. On top of that, it is an undamped spring.

You can easily engineer out all of this crap by making the chassis stiff enough.

I dont drink tea, so I will give another analogy regarding chassis compliance. One major aspect of a suspension design is the way it shares the rolling forces between the front and the rear axle to achieve an "optimum" or target load transfer.

It does this by effectively passing a torque through the chassis. If the chassis is made of cheese, there can be no communication of torque front/rear which means you cant tune the load transfer using the suspension.

No amount of suspension or aero trickery can fix that problem.

Tim

[marcush.]

flex is not slack and I think Dave is implying there is a whole lot of installation stiffness lost with myriads of joints along the loadpath .screws and bolts needle bearings roller pairs and what have you.A wheel is not a very stiff thing as well let alone a wheel bearing ..Question is whére is the weak link in it ...is it the base design or a component which is just not having the proper quality or tolerance ...
It´s all too easy to lay out the cF fibre carpet ....but when I happened to have contact to a quite competitive LMP team I always voiced my concerners about their installation stiffness..I have the feeling Dave would know who I have in mind ...
not only in le mans one could see from where the understeery behaviour of the thing came from ..

[GSpeedR]

Chassis compliance adds extra degrees of freedom that can be modeled (and controlled within reason) but cannot be eliminated. The level of control depends on the active system, but 'full' control can't be achieved. Another example is the control of tire deflections: active suspensions can control unsprung motion to a significant degree, but there is nothing that can be done to prevent the mean (DC) tire deflection from aero load.

[DaveW]

Brian: I'm afraid the world is a little bit more complicated than you have assumed, as g-force hinted in his post.

Tim, Marcus: agreed...

GSspeedR: I agree. Another example occurred when OEM's became interested in active suspension for road cars. There was a widely-held belief that it could be used to cure all ills. One car we were offered had a design for which, at one frequency, the apparent front sprung mass was around 10 percent of static (courtesy of the coupled response of the power train & the structure). The customer was very unwilling to accept (despite the evidence) that the only way improving matters was to re-design the engine installation. Incidentally, that problem occurs even on passively suspended vehicles (you might correctly guess).

[strad]

If anything you'd want a more rigid chassis with active ride

Absolutely,,,you have to have an dead rigid base.

[hardingfv32]

Brian: I'm afraid the world is a little bit more complicated than you have assumed, as g-force hinted in his post.

Not doubting your position, just trying to elicit a little more understanding of the issue from you.

Brian

[Tommy Cookers]

Since nothing is truly rigid, and there will be a level of chassis stiffness that behaves essentially the same as the theoretical perfectly rigid chassis, shouldn't that level be (usefully) the definition of 'rigid' ?

Surely a chassis torsional stiffness about 1 order of magnitude greater than the end-to-end torsional stiffness of the whole car (that being dominated by tyre and suspension compliance) can thus be regarded as 'rigid' ?
Nothing stiffer is needed, handling balance will respond faithfully to changes in front-to-rear roll stiffness. A chassis stiffer than this just won't work any better ? This is all calculable.

Since 1981 F1 has had such hard suspension for aero loads that it was initially almost impossible to meet the above requirement (this compelled the use of CF reinforced composite).
Dave W, I know how stiffness design is always eaten away by unrecognised practical detail, and so often falls short.

Surely the main point of active ride in F1 was to eliminate the need for such hard suspension, having continuous real-time compensation for aero loads the suspension could do its proper job (as could the aero)? In effect (sometimes also in reality) the 'springing' for aero loads was independent from the springing for bumps.

Surely with less hard suspension a less stiff chassis would be needed.
So Active Ride suspension appears not to need such a stiff chassis as passive suspension.

Since the banning of even simple active ride F1 has ingeniously got the tyre to do much of what we thought was the suspension's job, the suspension is still very hard and the chassis (apparently) must be very stiff.

The case for active ride is still the same ? (also a less stiff chassis could be slimmer, giving more scope for aero ?)


I am not advocating active ride as a magic cure-all.
I am not really advocating it at all !

Active Ride should be called Reactive Ride ; the only real Active Ride vehicle ever is the Rockwell B-1B (although F1 cars were developing the technological infrastructure to synthesize AR ? )

[strad]

Surely the main point of active ride in F1 was to eliminate the need for such hard suspension

The drivers hated the active suspension cars because the were so stiff and rode so hard they were beated and bruised by the end of a race and just praying for it to be over.

[DaveW]

...the only real Active Ride vehicle ever is the Rockwell B-1B

I don't think that is quite correct... I believe the Tornado terrain following system had similar characteristics, & was probably earlier.

The drivers hated the active suspension cars because the were so stiff and rode so hard they were beated and bruised by the end of a race and just praying for it to be over.

Again, sorry to disgree, but the Lotus Active system had a good ride. Senna remarked that he no longer had to bandage his hands, & I ran a road vehicle fitted with the 'T99 system for development purposes, and it was acceptably comfortable. I can't comment on the Williams system, however.

[bhallg2k]

...the only real Active Ride vehicle ever is the Rockwell B-1B

I don't think that is quite correct... I believe the Tornado terrain following system had similar characteristics, & was probably earlier.

It wasn't earlier, but I'm not entirely sure that was the reference anyway.

The B-1's forward vanes are automatically and independently adjusted to smooth out low-altitude flight.