Roll rates in RCVD

[Jay Lawrence]

Z,

Your childish attack on Tim's post aside ("MAN UP boy (!!!) ..." really?), can you explain how you could have non-linear pitch springing whilst creating a stable aero platform and having soft twist mode? Given that torsion bars can't have variable rate (?) I'm assuming you'd set up your 'lever arms' at each end such that they are falling/rising rate. Doesn't this mean that you have to travel to the extremes of your lever arm to get sufficient stiffness for pitch (without bottoming out), which would involve significant chassis movement to achieve (and hence non-stable aero platform)?

[Z]

Jay,

One day when you are old, and tired of hearing the kiddies moaning,
"Aww, but it's just TOOO haaard... And something really bad might happen to me. And [whinge, whine...]..."
then you will learn that everyone is happiest shortly after you pick up the kiddies and throw them in the deep-end. Sure, there is a little spluttering to begin with, but after that lots of FUN!
~o0o~

... can you explain how you could have non-linear pitch springing whilst creating a stable aero platform and having soft twist mode?

So simple ... and all explained before. But once again...

Consider UWA's recent "diamond-springing" (reg. TM?) cars that have front and rear beam-axles, with a spring at the centre of each beam that together control Heave and Pitch. (Roll is controlled entirely independently with a central Lateral-U-Bar+2-x-Balance-Beams, with the BBs forming the aero-tunnels). The beam-axles-with-central-spring are effectively Lateral-Z-Bars. So, overall, the car is completely Twist-soft (except for some friction, etc.).

The beam-centre-springs, namely the "W-springs", serve also to horizontally locate the beams wrt body, but ignore this for now, and just consider a vertical spring at the centre of each beam. It should be obvious that this spring can be implemented in any of countless different ways. Steel-coils, multiple-pre-loaded-steel-coils-in-series-and-parallel, torsion-bars-with-crazy-linkages, rubber-bands-with-cables-and-funny-pulleys-and-cams-and..., etc.

A lot of these can be very simple to make, yet also have very non-linear rates. They are everywhere! (Ask your teachers, or ask for your money back!) The non-linear rates can make the Pitch behaviour of the car very stable, yet also very good at absorbing bumps. Furthermore, separating the Heave and Pitch spring-rates of an UWA-like car is quite straightforward. I have covered all this many times before.

Bottom line, all this is easily doable. Just because you can't think of a way to do it right now, does not mean it is impossible. Or even "TOOO haaard...".

Z

[Jay Lawrence]

Thanks Z, when I wrote my question I had the Packard image in my head and couldn't reconcile the variable rate idea with it.

[ben]

So just to clarify Z on the aero tunnels acting as balance beams. So if you move the longitudinal position of the U-bar attachment along the beam you change the roll stiffness distribution?

Ben

[Pete Marsh]

So just to clarify Z on the aero tunnels acting as balance beams. So if you move the longitudinal position of the U-bar attachment along the beam you change the roll stiffness distribution?

Ben

That's correct Ben. ALL elastic RMD is distributed by the central U bar, and is distributed F/R according to the drop link location, which is of course adjustable. Future iterations will have driver adjustment, allowing balance trim on the fly. The kinematics of the link, and hence RMD, is very stable through all suspension motions.

[ben]

So warp soft with tunable LLTD...

Much as Z rubs people up the wrong way I actually have as big a problem with people denying something can be done by just stating "we were really smart yet didn't manage it"

Ben

[dynatune]

So where is this discussion now going to .... ? 3 Weeks after having stepped out of this discussion and more than 2 weeks after the last comment I cannot see what was all the fuss about. Besides the excellent contributions from Tim and his conclusions that the already anticipated disadvantages are fully confirmed - much progress has not been made .... - and if I may say so .... what is wrong with "we were really smart yet didn't manage it" ..... ? Some guys (like Isaac Newton & Leonardo Da Vinci showed us what we can do - or cannot do) ... I have seen here "just" a lot of "this should work" but no hard facts .... at all, just ideas ......no hard facts ... no car ... no nothing ..... show us ... that it works .... show us that we were incompetent 20 years ago and make me eat my words

Cheers,
Dynatune, www.dynatune-xl.com

PS... this is a motivation ....

[Z]

So where is this discussion now going to .... ?

Dynatune,

Tim last said that he will be giving more details soon. I have the patience to wait...

He also said he will be comparing an all-Z-Bar suspension with the other more conventional suspensions, and with his fully-modally-independent suspension. And he has already confirmed that a stiff Twist/Warp-mode makes a thorough mess of LLTD when the car is on any slightly uneven surface (ie. per MILLIMETRE of Warp!).
~~~o0o~~~

... the already anticipated disadvantages are fully confirmed...

I hope this is NOT the reasoning you use in your professional capacity!

Tim gave ONE PARTICULAR EXAMPLE (4 corner-springs + 2 longitudinal-Z-Bars) that had some disadvantages. A slightly different choice of spring-rates would have eliminated those disadvantages. An all Z-Bar suspension would be even better. A fully-interconnected set-up even better again.

By your reasoning, all conventional suspensions can be deemed "fully confirmed" catastrophic disasters, simply by choosing a SINGLE BAD EXAMPLE of such a suspension.

Very sloppy thinking.
~~~o0o~~~

I asked you quite some time ago, several times, if you know how to build a suspension with zero-rate Twist-mode, but also with fully adjustable LLTD. As Pete confirmed to Ben above, the STUDENTS at UWA know how to do that. They have known how to do that for at least 10 years.

So, once again;
Do you know how to do it?
Did you know how to do it 20 years ago?
Will you ever give us the details of what you did 20 years ago, when you "failed miserably"?
Or, otherwise, how are we to draw any conclusions regarding the reasons for your failure?
~~~o0o~~~

"I have seen here "just" a lot of "this should work" but no hard facts .... at all, just ideas ......no hard facts ... no car ... no nothing ..... show us ... that it works .... show us that we were incompetent 20 years ago and make me eat my words."

Once again, all the wooden-carts, and farm-tractors, and earth-movers, and +++..., FOREVER, have soft Twist-modes. And the 2CV, and Packard, and all ride-on lawn-mowers, and fork-lift trucks, and +++..., FOREVER!!!

I built my first 3 x Z-Bar car ~35 years ago. It worked very well indeed.

Racecars from F1's "active-suspension" era had soft Warp-modes (in their definition). And they worked very well indeed.

UWA's cars that have won several FSAE events had soft Twist-modes (different to Warp). They worked very well indeed. (And so will their current cars, when they manage to train their students NOT to leave small pebbles INSIDE the brake M/C! )

Dynatune, you are in a tiny minority. You have locked yourself in a small cupboard, and closed your eyes. You should get out more.
~~~o0o~~~

"... what is wrong with "we were really smart yet didn't manage it" ..... ? Some guys (like Isaac Newton..."

Newton, who, in the modern era made some of the greatest advances in our understanding of how things work, said,

"... to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.", and,

"If I have seen further it is by standing on the shoulders of Giants."
~~~

A century earlier, Michelangelo, who as a young man carved the statue of David, and in his middle age painted the ceiling of the Sistine Chapel, and in his final years designed the dome of St Peters (still the largest such dome in the world), said on his deathbed, at age 89,

"I regret ... that I die just as I am beginning to learn the alphabet of my profession."
~~~

Dynatune, who "failed miserably" to make a mechanical COPY of the pre-existing semi-active-suspension cars, said,

"I ... with some of the brightest vehicle dynamics engineers ... did look at all the options known to man-kind..."

That is the height of arrogance.
~~~o0o~~~

Students,

BEWARE THE EXPERT WHO THINKS HE KNOWS IT ALL!

Z (<- A nobody who knows nothing, but likes driving his soft-Twist-mode tractor... )

[BillCobb]

It's the sprung mass that rolls. There is additional roll inducement from tire overturning moments acting through the roll by camber kinematics of the suspension. (In many cases, this can be worth 10% extra induced roll). The tire ovtm is also a track modifier. (not race track but lateral axle dimensions) Also, there is an interesting roll inducement via the aligning moment camber characteristics of the tires. This is what makes a vehicle waddle in the pavement troughs created by heavy semi-trucks in warm asphalt. Yes there is extra roll in the entire vehicle coordinate system because of tire spring rates.

If you ever plan to study vehicle roll-over, all these factors plus the dynamic effects of roll velocity will allow you a complete enough understanding of roll gains . That means damper performance in both velocity directions and both ends of the vehicle. Keep in mind that dampers usually have different compression and extention rates, so this complicates the 'roll center' location(s) during a maneuver. Once you've been in the presence of a dynamic K&C machine, all this will be evident. You don't even need a 'dynamic' K&C machine. Running a static hydraulic powered machine in a square wave instability mode will jostle the vehicle enough to see all these things.

BTW: With 'slow' road speed and high steer angles, the influence of caster angles must be factored in, too. This alters the TLLTD estimate.

PS. I'm fascinated by the explanations of 'slow slip angles' and spikey things in the step response signals. You really ought to try a little simulation of various understeering and oversteering and neutral steering system models. All this will then be engineering science and not mystery.

[dynatune]

Z,

So you now need to relay on Tim to prove that you are right , I would have expected to see from you some of his calculations way earlier to convince people, that would have given your statements some added credibility ......

Professionally I am used to look at hard facts, in order to make the correct decisions. This does by no means mean, that I am ignorant or too arrogant to see any pro's of such a system . My point of view -as stated weeks ago - is that, if the system brings more disadvantages than advantages, I would not go for it.

Let me repeat some key points (again for a passive analog mechanical system only, for using in a race environment) that are still standing :

Advantages:

Higher Comfort - Confirmed, yet not needed in race cars
Better "Diagonal" Grip - To be confirmed for a mechanical system (active F1 cars confirmed)


Disadvantages:

Increased Pitch sensitivity - Problematic for Braking/Acceleration especially on aerodynamic pitch sensitive cars. To be corrected (?)
Increased Components and Complexity of Installation. Added linkages (or Hydraulic lines --> if going to hydraulics one can take the next step and become full active)
Increased Complexity in Car-Setup and Tuning.

And I have not even looked at factors like costs, durability and so on that in for automotive OEM's will be important ... those items above are just the "functional" pro's and con's

Then, as I mentioned when "we" looked at it 20 years ago in F1 we found next to drivers complaining about a sluggish response of the car one other major disadvantage. The car was never faster than a "conventional" car ...... and this was not just "us" but all other teams who were trying to reproduce a mechanical system that would be as good as the active system. And ever since those days the racing environment (where still a lot of money is being spend on finding lap time has failed to incorporate this concept ... I wonder why ...).

So my dear Students,

Be open minded, but do always ask the right (and hard questions). Be critical to others but mostly to yourself and your ideas. Listen and find guidance from experiences made in the past by your colleague's and reflect. In your future professional life your manager will ask you sometimes hard and provocative questions to get the facts out of you. You will need to be prepared and convince him by facts, not fairy tales. Reacting emotionally, telling him, that he is arrogant, ignorant and not professional is most likely not going to help your case but will probably be a great way to limit your career .....

I guess I will be back in 3 weeks or so

Cheers,
Dynatune, www.dynatune-xl.com