Is a rigid chassis needed w/ actively adjustable suspension?

[xpensive]

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?

I believe that depends on the level of perfection in the active-hydraulic system and how it's programmed.

If you imagine an infinitely fast and corner-independent servo-assisted hydraulic suspension with feed-back, or "is-value", from four ground-clearance sensors in each corner of the car, then the chassis flex wouldn't matter much unless the system is programmed for keeping the same ground-clearance in all the corners.

[DaveW]

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.

My memory (almost) failed me. I assumed that the "only real Active Ride" system referred to Terrain Following. This was a system originally developed by Ferranti for TSR-2, & refined on Tornado. I had quite forgotten the body bending dampers implemented on the B-1. I recall that TF systems included a "gain" demand which was used to exchange "ride" with following accuracy.

[Tommy Cookers]

/quote]

I believe that depends on the level of perfection in the active-hydraulic system and how it's programmed.

If you imagine an infinitely fast and corner-independent servo-assisted hydraulic suspension with feed-back, or "is-value", from four ground-clearance sensors in each corner of the car, then the chassis flex wouldn't matter much unless the system is programmed for keeping the same ground-clearance in all the corners.

Unless the chassis was quite stiff you couldn't adjust the handling aka 'get a balance'

In engineering terms, you maximise the roadholding by adjusting the relative front:rear roll stiffness so as to optimally distribute front:rear the overall cornering 'weight transfer'. For this the chassis must be a lot stiffer than the suspension.

Tim Wright said this (better?) yesterday.

[xpensive]

I think that as long as we're talking "active suspension" in terms of maintaining a constant ground-clearance, chassis stiffness should not matter that much, but of course an infinite stiffness would still help, again depending on the way it's programmed?

[Tommy Cookers]

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

My memory (almost) failed me. I assumed that the "only real Active Ride" system referred to Terrain Following. This was a system originally developed by Ferranti for TSR-2, & refined on Tornado.

IMO all Terrain Following (other than the B-1B) is reactive ride

[DaveW]

If you imagine an infinitely fast and corner-independent servo-assisted hydraulic suspension with feed-back, or "is-value", from four ground-clearance sensors in each corner of the car, then the chassis flex wouldn't matter much unless the system is programmed for keeping the same ground-clearance in all the corners.

That is not a bad description of the Lotus system, except that it didn't use ride height sensors, for various reasons, and it didn't attempt to control camber, castor, etc. In a sense the system was a simulation of the banned Lotus 88. Actuators had a wide (but not infinite) bandwidth, and were capable of feeding back (and destabilising) structural modes, one reason for requiring good (but not exceptional) structural integrity.

[xpensive]

Problem with hydraulics however is that it's not that fast, the perfect system would probably be with electromagnets, though they would be terribly inefficient power-wise. But other than that, you could theoretically make the chassis stable as a rock vs the ground, even playing with the rake at will if you wish?

[DaveW]

IMO all Terrain Following (other than the B-1B) is reactive ride

Apologies, but you will have to explain to me why that is not an exercise in semantics. There is no doubt that the B-1B is (was) a very good aircraft, but what is it that makes it so special (apart, perhaps, from the natural frequencies & damping ratios of its body bending modes).

[bhall]

As defined here, it seems to me that the only vehicle with a truly active suspension is a steamroller. Otherwise such systems are inherently reactive.

[xpensive]

Naah, a steamroller will still go up with a bang over a bump, just like the F2012s front, while an active suspension should compensate to maintain ride-height.

[bhall]

Active as in it prepares the surface upon which it will run. Truly active.

[jon-mullen]

Chassis compliance adds extra degrees of freedom that can be modeled (and controlled within reason) but cannot be eliminated.

This. If you had a deformable (enough) chassis, you'd have to account for it in your dynamics and it'd probably come out strongly coupled in the equations and non-linear. If it added poles/eigenvalues that were slow or lightly damped you could have a really bad time designing the controller to get the transient response and steady-state error that you're looking for.

[olefud]

If it added poles/eigenvalues that were slow or lightly damped you could have a really bad time designing the controller to get the transient response and steady-state error that you're looking for.

Would that be the chassis itself with its own response time and resonance?

If we’re only considering ride height for aero, perhaps it wouldn’t be too bad. But with another spring in the system and a changing reference for the various suspension parameters, a good, rigid chassis would seem to be an elegant static answer for problems that servos don’t like to deal with.

[DaveW]

A quick resume of the Lotus Active Suspension System>

Lets start with xpensive's proposal:

If you imagine an infinitely fast and corner-independent servo-assisted hydraulic suspension with feed-back, or "is-value", from four ground-clearance sensors in each corner of the car, then the chassis flex wouldn't matter much unless the system is programmed for keeping the same ground-clearance in all the corners.

As I noted above, that is a good description of the Lotus System. I only need to add four coil over springs (used for fall back in case of a failure and for sharing the load with the actuators to reduce power consumption).

Now, each suspension actuator was controlled by an EHSV (electro-hydraulic servo valve) whose function was to control the actuator to move with a linear velocity proportional to the current driving the valve. It is fair to state that the drive current/velocity was not exactly invariant, but it was close enough not to matter over a frequency range of, perhaps, 100 Hz.

With that information, it is possible to imagine an accelerometer attached to the upright (the wheel side of the actuator) measuring the vertical component of upright acceleration. If the signal generated by the accelerometer is integrated, to produce a signal proportional to absolute velocity, and that velocity is scaled to generate current that is used to drive the "local" EHSV, then the actuator can be made to faithfully (more or less) move to exactly reflect the velocity of the upright. In that case, the actuator will transmit no additional load to the sprung mass, regardless of upright motion (with caveats). The sprung mass will then remain stationary in space. That should satisfy xpensive's proposal (again with caveats), and the sprung mass could be made of cheese, or anything else that springs to mind.

That is not a bad starting point for thinking about Active Suspension. It could be made to work in that way, but it would not form a realistic suspension solution for various reasons:

1. It would extract no energy from the tyre/unsprung mass mode, which would therefore be severely underdamped.
2. It would dissipate no energy from unsprung mass motion.
3. It would quickly run out of actuator travel (drive straight into hillsides, etc.)
4. It would do nothing to control load distribution (such as that mentioned by Tim Wright).

It is an interesting thought experiment, however, because it demonstrates (hopefully, & again with caveats) that "preview" is not absolutely necessary for a good active suspension.

[Tommy Cookers]

Preview was the point of the B-1 comment .

Without preview, a system can be called anything, it will have more limitations ? Should all such systems be called Predictive ?

Terrain Following is 'low-fi', so is TF + variable smoothing of that TF

I assume the B-1 uses stored ground maps for TF , as does anything newish ?

The B-1 was intended to spend much time supersonic hedge-hopping, then was simplified for easier flight conditions made available with improved stealth.
The original (crucial) ride smoothing was based on preview (of the state of the air, not the ground).

Weak (reactive ?) smoothing' is used in (recent) airliners every day. Nothing else is allowed.


Fortunately F1 doesn't need realtime preview for Active Ride, the system can map the track and store the mapping, a synthetic preview? Surely this will always give better results than a system without any kind of preview ?

Road cars can't do this, they should either have real preview eg radar? or not claim F1 type Active Ride ?


There's a danger of losing the subject of the thread !