Tudor (Ted) Miron
|
| posted on 25/2/04 at 10:01 AM |
|
|
Ron,
I thought that I have to add some complication to this  . In reality car DOESN'T roll around it's geometric RC. There's
"true" force based RC - which is usually little higher than pure geometric.
Ted
|
|
|
|
|
rontyler
|
| posted on 25/2/04 at 03:22 PM |
|
|
Ted,
In your first reply... “Another thing is in racing we don't usually see that big difference in bump and rebound forces as in
"normal" street car.”
This is about the only thing that I feel addresses the question. However, I can’t help feeling that there’s “more”
Then you post...
“In reality car DOESN'T roll around it's geometric RC. There's "true" force based RC - which is usually little higher
than pure geometric.
Now this has my attention. Please share with us what you know or point us to a book or example. Question... what did you mean by “higher than
geometric”? The force is higher or the location is higher?
Thanks for you time.
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
Tudor (Ted) Miron
|
| posted on 25/2/04 at 04:17 PM |
|
|
Ron,
I thought that first part of my post was addressing your question. It doesn't really matter is there big difference in rebound/bump shock
forces or not. Point is that part of WT is via springs, bars and DAMPERS and part is via RC. RC part may change significantly in transient if RC is
messing around. It may cause the car to change it's attitude from understeer to oversteer - there and back during one turn and even one phase of
turn. There are also jacking forces generated straight through A arms from instant centers. I was suggested to think about those jacking as same as
anti - dive but lateraly.
This forces may have significant effect on CG height, roll (and bump sifness during turn - loaded wheel) stiffness and ultimately tire loading.
Here's a copy of letter from one VERY well educated person:
"I would like to respond to the many discussion points on the subject of roll center & RC migration. Sorry to be so long winded but this is
a complex subject.
I have spent many years working within Ford Racing to study NASCAR, Indy Car & sports car suspension geometry & the associated effects. This
has included design, modeling & testing both on the track & in laboratories. Within Ford Motor Co. we have laboratories that can actually
measure the forced based effects of roll center height & RC migration to a very accurate degree.
Here are a few interesting points:
1. Suspension models in general are not forced based but are relatively simple geometric models.
2. Geometric models are very good for defining the basic character of rigid suspensions (heims etc. no rubber bushings). These include camber gains
both jounce & roll. Roll center placement & migration, roll rates, steering geometry etc.
3. Geometric lateral roll center migration, in general, increases as the geometric roll center gets closer to the ground.
4. Geometric roll centers are an approximation of the real world of forced based roll centers. (what happens on the race track & how the vehicle
transfers weight from tire to tire)
5. Forced based roll centers can be modeled in ADAMS or other such complex models.
6. Force based effects can be measured in the laboratory very effectively.
7. Force based roll centers can & do migrate & this can have a very significant effect on weight transfer, jacking forces & the resulting
COG height and tire contact patch loads.
8. Vehicles with relatively high roll centers (2" - 3" or more above ground statically).
In forced based testing those vehicles that exhibit significant geometric RC migration, have higher tire contact patch weight transfer than do
vehicles with the same roll center height & little or no lateral roll center migration
9. Vehicles with low roll centers (very near the ground).
In forced based testing these vehicles do not have siginificant weight tranfer associated with roll center height & or roll center migration.
10. The forced based roll center is always higher than the geometry indicates. (for all vehicles we have tested)
11. Many NASCAR suspension geometries have roll centers that migrate (geometrically) this results in greater force based weight transfer & a
resulting reduction of grip at the limit.
in summary:
1. Very few of us have the ability to conduct forced based modeling or testing.
2. Use geometric models to define your basic geometry requirements.
3. Do your best to reduce geometric lateral migration of the roll center without compromizing the important parameters.
4. If your roll center is very close to the ground lateral migration of the RC is MUCH LESS important than with higher roll centers. To say this the
other way; cars with higher roll centers should have MUCH less lateral roll center migration.
Again sorry to be so long winded & I hope that I have not confused the issue.
Jay Novak
Also found this paper: http://www.ee.ic.ac.uk/CAP/Reports/2001/ASCInteractions.pdf
Also I can recomend a new book " Introduction to race car engineering" - great stuff!
I have access to ADAMS/Car but currently I don't do much with it - it's VERY complicated ( at list for a such dumb guy like I am) - how
ever after aerodinamic study of my project will get more or less clear I'll come back to it and hope to finally learn how to get real profit
(not money ) out of this great tool.
Thank you
Ted
PS: Here's another informative words from another VERY good engineer " The biggest problem with above ground front RCs is the attendant
jacking effect. While it will cause quicker front transfer reactions, it also makes the front very sensative to ride height changes, and can cause the
car balance to change dynamicaly a LOT during cornering. As a consequence, it is a lot harder to get a good setup that is stable, predictable, and
doesn't eat front tires if things are off just a tiny bit.
A below ground front RC does require a much larger front roll bar than an above ground RC, but the thought that that is a handicap is a fallacy. Below
ground RC's actually transfer weight to the inside tire for a instant upon initial turn-in, which can dramatically help turn-in response. Also,
the lack of jacking effect makes the front VERY stable, especially in high speed corners and over undulations, as changes in ride height don't
upset the roll couple balance as much as an above ground RC. Trail braking is also enhanced because of that stability.
Softer compounds can also be used without killing them immediately, but the decrease in geometric transfer does make it harder to get heat into the
tires initially. Higher spring rates can be utilised to control aero stability without abusing the tires - assuming the shocks are set up correctly,
of course. "
And another one from him: " Obviously, the F1 guys have data resources that we can only dream about. All we can do is to first get a thorough
understanding of the way the forces work, and then spend a ton of time and money experimenting. I wish I had the resources and didn't have to
worry about next month's rent payments!
Sorry, but there is no such thing as a car that will lean into the turn with the CG above ground, the RC at a reasonable height above (or below),
and "conventional" suspension. Physics doesn't work that way! Even the old Bugs and Triumphs showed normal roll to the outside as
they jacked way up onto the outside tire.
However, that said, it IS possible to "trick" the roll to going negative thru some fancy and complicated linkages - it's been done
in the past. Of course, it can also be done thru active hydraulic rams in place of the shocks and springs or as a moveable pickup point for the
coilovers.
The problem with all of this is that almost no matter what you do, the "stiffening" in roll from the jacking forces also affects the bump
stiffness while cornering, at least that of the outside tire. But, this is also part of the reason that they can hit the curbs so hard without tossing
the car into the infield (along with possibly accelleration sensative shock valving).
There are aero reasons for sure in just about everything the F1 guys do suspension-wise nowadays - keeping a stable aero platform is the holy grail to
them."
Sorry for such a long post.
[Edited on 25/2/04 by Tudor (Ted) Miron]
[Edited on 25/2/04 by Tudor (Ted) Miron]
[Edited on 25/2/04 by Tudor (Ted) Miron]
|
|
|
rontyler
|
| posted on 25/2/04 at 04:35 PM |
|
|
Ted,
Good stuff... Thanks!
So, it appears that the forces that "roll" a car about its RC, percentage-wise, are MUCH higher than any differences in wheel rates...
Agreed?
Thanks Again.
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
Alan B
|
| posted on 25/2/04 at 05:08 PM |
|
|
quote:
Originally posted by Syd Bridge
.......But we're supposed to be building replicas of a '50s sportscars, aren't we??........
Not in this part of the forum Syd... ....however your points are well taken...
I'm off...this thread is severely hurting my brain...
|
|
|
rontyler
|
| posted on 25/2/04 at 05:31 PM |
|
|
"There was a discussion on this rollcentre thing a while back"
I'm unable to find it 
"The major dynamic in cornering is the force of the CoM..."
CoM ????
Thanks.
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
Tudor (Ted) Miron
|
| posted on 25/2/04 at 06:05 PM |
|
|
If you can't beat 'em with science, baffle 'em with bullsh1t! (Syd Bridge)
Thank you Syd 
There was a discussion on this rollcentre thing a while back. Search it out and see what a couple of well known designers said about it in a
discussion I was privileged to be party to. (Syd Bridge)
Would you please post a link?
As for F1 and GT's.. Aerodynamics is the ONLY consideration. They must keep even flow over the wings. A car moving up and down detaches flow.
(Syd Bridge)
This IS very interesting “ detaches flow when moving up and down” - that’s what I call “science “
Therefore, suspensions are pegged to stop any droop at all, and bump is no more than 20-30mm. With so little movement, there is little need for any
fancy RC claptrap, and indeed, no way to design it in. (Syd Bridge)
F1 designer Steve Nichols was interviewed and he was asked how critical suspension geometry is these days. 'Not absolutely critical - not like
there is a vast amount of time to be had in it' was his reply. Of course, he was fired a few months later.
Why do you think they now run such extreme cambers? (Syd Bridge)
I thought that it’s some how mysteriously related with radial tire camber sensitivity – Oh sorry for starting ballsh1ting again
Because the suspension can't move to compensate in cornering. Go back a few years, and have a look at the suspension geometry. You'll find
things very different. The rear suspensions today, show more than a passing resemblance to those of a few years back, and for good reason. (Syd
Bridge)
The major dynamic in cornering is the force of the CoM acting against the contact patch. From there on, gravity takes over, and the suspension
geometry moves. (Syd Bridge)
– This is again very interesting – suspension moves because of gravity!
Keep to the basics and you can't go wrong. Keep CoM as low as possible, and configure the suspension arms to keep the contact patch as stable as
is possible. (Syd Bridge)
Good point – seriously
The rest is theoretical (and some practical) fertiliser. (Syd Bridge)
True in a sense that keeping the basics right is important. Main thing to keep in mind is to try and don’t SCREW anything badly. How ever I’d like
keep my right to try and learn what’s really happening.
Then there's driver preference and 'feel' to build in....... (Syd Bridge)
But we're supposed to be building replicas of a '50s sportscars, aren't we??(Syd Bridge)
Are we really?
Here I have to apologize for bollsh1ting people and promise to stop doing this – sorry.
Ted
|
|
|
Tudor (Ted) Miron
|
| posted on 25/2/04 at 06:07 PM |
|
|
CoM = center of mass
|
|
|
rontyler
|
| posted on 25/2/04 at 06:14 PM |
|
|
It seems that several "languages" are bieng used... CoM=CoG?
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
flak monkey
|
| posted on 25/2/04 at 06:19 PM |
|
|
CoM and CoG are essentially both the same thing....but centroid is not....
CoM / CoG are the important ones though, both are where all the weight of the object, in this case the car, seems to act as a point mass. Which is
very useful when modelling things...but im sure you all know that anyway so now i will be quiet
Cheers
David
[Edited on 25/2/04 by flak monkey]
Sera
http://www.motosera.com
|
|
|
pbura
|
| posted on 25/2/04 at 06:37 PM |
|
|
quote:
Originally posted by Syd Bridge
But we're supposed to be building replicas of a '50s sportscars, aren't we??
Dunno about you, Syd, but I'm building a Supercar... :O 
Fast, supple, and handling like an extension of my own body! Err, maybe better!
Kidding aside, I'd appreciate a link to that thread you mentioned, as I didn't have any luck searching.
I agree with you that, for the home builder who does not have access to a cyclotron for testing, the tried and true is the best way to success. I
also have tremendous respect for the Seven design as having brilliant suspension geometry. Before computers, even!
However, anti-roll bars are legitimate modifications to the original design, IMO, unless the builder is seeking authenticity (an honorable goal, btw).
The Seven relies on firm springing at the front and a high roll center at the rear to limit body roll, both of which become negotiable once ARBs
enter the picture.
Oh, yeah, we're talking about middies, aren't we? Well, in that case, ANYTHING goes! As far as Mr. Novak's opinions go, I
have seen his postings in another forum, and I would put my money on his knowing his hockey, and that his assertions would be worth investigating
further. His advice for the home designer is very sensible.
I also respect Ted for reasoning out his design process. I'm sure that when he's done, he'll have a good foundation for his
decisions and not just someone's say-so.
Pete
Pete
|
|
|
rontyler
|
| posted on 25/2/04 at 06:51 PM |
|
|
"As this subject strays into areas where I derive some of my income, we'll leave it be."
I can respect that.
"Just stop looking for a problem and answer where they don't exist in the first place."
Can you REALLY say this in good conscience? From my point of view, this thread has turned into an attempt at understanding how the suspension
design effects weight transfer... in my experience, this is one of the more important aspects of vehicle handling. I don't see it as
dissmissable.
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
giel
|
| posted on 25/2/04 at 09:23 PM |
|
|
it seems like I have stirred up some things with my question, never expected it to unleash such a load of knowledge!!
Anyway, I'll keep following what comes up on this subject, also in other threads and forums.
If anything, the comments in this thread have brought me to the decision to keep the front RC low with little migration vertical and lateral, and add
a ARB if necessary. Although some of you have given some reasons to use a long SAL, I keep having the feeling that the camber compensating effect of a
short SAL is very beneficial, despite of the undesired camber change in bump (can be reduced with anti-dive geometry!) .
I will keep the SAL fairly short until I have proof that it should be lengtened, not too late until it's welded together, right?
BTW I like these threads with some theoretical depth, better than just building by the book without knowing why.
|
|
|
Alan B
|
| posted on 25/2/04 at 09:43 PM |
|
|
quote:
Originally posted by giel.........BTW I like these threads with some theoretical depth, better than just building by the book without
knowing why.
Good job too...it's all we have in the absence of one single definitive book on middy building....maybe it's because we ARE on our own to
certain degree that it is so appealing........who knows..
|
|
|
rontyler
|
| posted on 25/2/04 at 10:32 PM |
|
|
quote:
Originally posted by giel.........BTW I like these threads with some theoretical depth, better than just building by the book without
knowing why.
Giel,
Sorry to have ransacked your thread... it does illustrate the diversity in suspension thinking... whats important to one person is drivel to
another and yet they can both be right! Keep us posted on your progress.
Regards, Ron Tyler
"Nothing is ever accomplished by a reasonable man."
|
|
|
