roll centre discussion
with input from dennis
Cheers
Fred W B
[Edited on 28/10/07 by Fred W B]
Of course, all of that relies on you believing in 'Roll Centres' as a proven and true concept,..which they aren't. They have no basis
in maths nor physics nor practical observation.
Jay Novak comes close to the truth, without saying exactly what goes on. I only wonder if he really knows, or is quoting from other sources, just
like one or two wannabe's on here????
On the other hand, after a few bevies, leprechauns have been seen and engaged in conversation, as have fairies at the bottom of the garden. 
Cheers,
Syd. 
Hi Syd
So are you ever going to break down and explain to all us less bright people how it does actually work?
Even if a car does not behave as people think is does in the roll centre theory, surely it makes a convenient graphical way of comparing one
suspension to another?
Cheers
Fred W B
A lot of cars do just fine with imperfect chassis designs. There are many ways to do things and what's right for one person is wrong for another.
If a driver doesn't like the "perfect" chassis that was designed for him, what does that mean exactly? Either the driver's wrong
or the chassis is, or both. I'm not convinced there's One Answer, and it's nearly impossible to change only one variable, anyway.
Changing RC height to see how it changes things, without changing anything else, is nearly impossible.
[Edited on 10/29/07 by kb58]
Read the posts by Jay Novak in that thread.
He more or less calls 'rollcentres' for the BS that it is. Also read the rest of what he says, and think hard about it. Draw pictures if you
have to.
But, a car does not roll about those mythical centres. Geez, just by moving the shock upper mount, or the pushrod mount, you change the roll
behaviour. So, that 'rollcentres' fairytale is shown as the fertiliser it is.
Cheers,
Syd.
By this I assume you mean geometric RC's or do you include the force based ones in the bullshit category?
quote:
Originally posted by Syd Bridge
Read the posts by Jay Novak in that thread.
He more or less calls 'rollcentres' for the BS that it is. Also read the rest of what he says, and think hard about it. Draw pictures if you have to.
But, a car does not roll about those mythical centres. Geez, just by moving the shock upper mount, or the pushrod mount, you change the roll behaviour. So, that 'rollcentres' fairytale is shown as the fertiliser it is.
Cheers,
Syd.
I have spoke to an OEM engineer who works with Adams software and he agrees with the view of the geometric RC's. He also said that it was interesting that his work seemed to have a correlation with them all the same.
This may throw some more light on the force based approach.
I was about to reply that those who're interested should hunt down one of Bill Mitchell's articles on Force Application Points but Doug beat
me to it. The geometric roll centre is just an abstraction - it is not a "real" point about which anything actually happens. The Mitchell
article pointed to by Doug puts it all into pretty plain English.
Dominic
I've been involved in enough of these to know that I don't want to get into it deeply again.
None of the arguments in those texts above take into account the springs, and how they act on the system as a whole.
Stiffer springs change the whole system, and hence roll behaviour and weight transfer, and those mythical and ever elusive 'centres'. They
don't exist as single entities, and are ever changing.
Changing the angle and height of the coilover or pushrod makes greater changes to the roll behaviour, than anything you do with wishbone geometry.
This a plain and simple truth derived from simple mechanics and first principles.
It's also a good reason to take a hit on weight, and use pushrod/inboard suspension on closed high performance cars. Spring specs and behaviour,
and total car dynamics can be a lot more easily controlled.
Cheers,
Syd.
Mitchell's article is interesting though I would give it no more weight than some of the articles on opposing views, it is all part of the bigger
picture.
WRT push-rods, they are fine for limited suspension movement and light vehicles. It does also require an additional spend on good bearings/machining
and also very good dampers. The other aspect is that they are also a very high maintenance point on the vehicles that use them as tolerances are very
important. I had some good advice to stay away from them...for my application.
quote:
Originally posted by Doug68
This may throw some more light on the force based approach.
Two things I find that get lost in all these pseudo intellectual discussions, are the most important to car handling....
Keep the total mass as low as possible, and keep every possible part of that mass as low as possible.
Put all that Rollcentre crap where it belongs, stick with basics and you won't be far out.
For all of you armchair experts, cop this,.... I've just now finished the wishbone geometry for a track car. The outer wishbone separation is
250mm vertically, and the inner pivot separation is 133mm. The upper wishbone is longer than the lower. These are long wishbones, relatively, and will
be travelling quicker than anything you lot will build in your sheds!
Cheers,
Syd.
[Edited on 31/10/07 by Syd Bridge]
the problem with RCs as i see it is that its a gross oversimplification, you can have a 'normal' looking set up with a given RC height, and design something thats blatently diabolical with the same RC height. Ive only thought about it with static RCs so maybe working it out at different positions/rolls would change that, but for me, it seems a lot more constructive to just work out camber change over your own specific bump and roll figures and take it from there.
Okay, I've snapped:
Syd, as I said, I'm all ears - I want to learn - but all I hear is how clueless everyone is. What's with this?:
-"pseudo intellectual discussions"
-"all of you armchair experts"
-"and [what I'm designing] will be travelling quicker than anything you lot will build in your sheds."
Is arrogance the price of knowledge, Syd?
Why allude to being some kind of Suspension Oracle, yet view with distain our attempts at understanding? You won't explain anything, yet insult
our attempts to do so on our own.
Why are you here? I'm disappointed that it's apparently not to educate or enlighten. Answering everything with, "Wow are you guys
wrong", is worst than useless.
Cheers.
[Edited on 10/31/07 by kb58]
SO far all I have seen from you Syd is smoke, arrogance and other peoples work and I too tire of your expressed arrogance Syd. I think it is time to
put up or shut up.
I would like to know your formal qualifications and experience before I give any weight to your opinions.
As for the comparisons of what we are building, well simply it does not reflect well on you, nor is it even the point of building your own car.
Wow, raw nerves or what??
I don't profess to be a suspension 'oracle', or any other type of expert. Suspension design is very simple and relies more on simple
geometrical constraints than anything else.
What you read as arrogance, I write as common sense and straight talking. I don't believe anything until proven.
'Roll centres' theory assumes that they exist, then all these professors write articles trying to prove how they exist. None of the articles
and book sections I've ever read, clearly defines 'roll centres' in a precise and accurate engineering manner.
I come at it from the other direction. I don't believe the theory and assumption, then look for an answer that shows me I'm wrong and that
'roll centres' are real.
If you lot can't open your minds, and think about this for yourselves, then I'm not about to write a treatise on the subject, only to find
my words repeated with someone elses name to it. Plain and simple.
I come across a few well known names in my work on occasions, and have asked about these mythical points, only to find that the people I'm
talking to come at the situation the same as me. What I have had explained to me however, makes an awful lot of sense.
What I'm trying to get you lot to do, is look at it from the same perspective as me, when it was all new to me. It didn't make good
engineering based sense, ...and still doesn't. The many variables in the system negate any mathematical model which may come close to
approximating the situation in a definitive manner.
What I find curious in all this, is that the most heated and smokey people are the ones who are writers. They seek the attention of the world. All
I'm doing is quelling their fire a bit, with a bit of doubt cast on their beliefs, which are derived from someone elses published work anyway.
I've explained myself and my learnings to a couple of people on here. They have understood and thanked me, privately. They also won't be
seen replying to this thread either.
Cheers,
Syd.
[Is arrogance the price of knowledge, Syd? ]
If you see it as arrogance, then your problem.
The price of knowledge???Years of bloody hard graft, and I'm not about to distribute the fruits gratis! Someone wants my knowledge, they pay for
it, pure and simple!
After all, you do 'sell ' your books, don't you? Or are they now free to anyone who wants one? The same as Mr.
rpmagazines little tomes, and he doesn't give them away either.
When you fellas start giving away for free your publications, I might consider writing something which you can also distribute freely. But, I'm
not holding my breath!!
If you're going to make money off my back, then I'll have a whopping great chunk of it, .............cash,....up front.
[Edited on 1/11/07 by Syd Bridge]
Syd Bridge:
In Issue no.66 of the magazine "Race Tech" there is an article named "Suspension Masterclass Part 1" written by Peter Ellray one
of the designers of the Le Mans winning Bentley that proves the existence of the geometric roll centre using the "Kennedy-Arronhold
Theorem". Is the vehicle your currently building using the "there is no roll centre theory" going to be faster than the Bentley? Now I
don't belive that the roll centre is a point that the car rolls about so roll centre is a bad name for the point were talking about but it is an
important factor in understanding the way the weight travels around on the car.
You also state that nobody takes springs into concern and that moving the coilover mount has a bigger impact on the cars roll behavior than any change
in wishbone geometry. That might very well be true but that is because it changes the motion ratio of the tire in addition to changing the roll
resitance of the axle meaning that the tire travells a different amount in regard to the change of load that it experiences from the cornering.
Changing the wishbone geometry is a way to change the roll resitance of the axle without chaning the motion ratio of the tire.
Finnaly I would like to give my personal view on the roll centre. I don't think that the placement of the roll centre is that important... Keep
it fairly low and you will probably be alright. Optimize for camber change and then fix the weight transfer distrubution with anti roll bars.
Hope that made any sense!
And Syd. No hard feelings, just want to here your take on what I written above and what Peter Elleray say. If you know something that I don't
I'm all ears, willing to admit I'm wrong and learn
[Edited on 2/11/07 by varg]
[Edited on 2/11/07 by varg]
'Nuff been said already.
If you are indeed a genuine 'newbie', then my apologies.
Cheers,
Syd.
Syd
Why are you here. purelly to sell your expertise? or just upset those you think are less able or experienced than you.
Straight talking is not how your posts are coming across.....
'Armchair experts' are one of your quotes along with all but stating that our shed built cars are rubbish compared to yours.
These are your comments not ours and smack of a 17year old troll.
I am quite sure this is not the person you are (having seen other posts from you) and you probably have forgotten more that I will ever know about
suspension design (I certainly hope so anyway)...... but please do not patronise and belittle those that are trying to learn and build the best car
they can.
PS I paid an expert to design my geometry based on a proven race car.
Syd you are quite correct that I sell Race Magazine and it sells well.
However I have seen nothing from you in words, ideas or actions that I would pay you money for...perhaps you should show me something so that I can
take you even a little bit seriously.
I also asked you a simple enough question in that I asked for your formal qualifications and experience and you have ignored it...why?
I have no problem with the concept that geometric roll centers are not the only or best design tool, but you are simply lambasting a theory with no
proof or evidence and claiming secret knowledge...with as far as information or evidence demonstrated so far - no basis in fact or action.
Neil, I'll meet you in person one day soon, possibly. I'll be in Sydney in the New Year.
Then you might change your mind, and also apologise for your personal statements towards me.
My qualifications? Sufficient for the purpose, then some. Exactly what they are needs not be publicised. You'll find out on the day. I don't
see a need for a public 'Outing', as such.
What I want you lot to do is put aside all you've read, think about what hapens when a car Starts cornering, then goes into and through the
corner.
Where are the forces applied? What are the forces? How do those forces change and interact?How does the mechanical configuration affect, and is
affected by, those forces?
Then you go to a racetrack, and one driver wants a soft setup, and another hard, both have differing camber and associated settings, and they both do
the same times.
At that point all the theory goes out the window.
Cheers,
Syd.
What personal statements Syd?
I've commented on your actions as per this thread and forum and no more.
Until I see some evidence I remain sceptical and why should I not?
Syd,
I appreciate what you are trying to do here, you I believe are trying to get people to think about what they’re doing with their design and to
challenge the conventional wisdom.
Both are laudable ambitions IMHO
However when you opened your input to the thread you opened with…
“…I only wonder if he really knows, or is quoting from other sources, just like one or two wannabe's on here????”
Now that is not an encouraging statement is it? Essentially telling the reader they’re not up to the task, it is not going to encourage them seek out
further words of wisdom on the subject or too give any weight to the person giving that information. Additionally in conveys prior baggage in the
argument clearly this is a subject you’ve been through before and are getting tired of the “Hoary Chestnut” coming up again, except the majority of
people in the conversation I doubt have been privy to all that gone before.
Now if we look at your last post you say…
“…What I want you lot to do is put aside all you've read, think about what hapens when a car Starts cornering, then goes into and through the
corner.
Where are the forces applied? What are the forces? How do those forces change and interact? How does the mechanical configuration affect, and is
affected by, those forces? …”
Now this is a statement that most people will engage with, there are direct statements about points you feel need thought, that’s something that can
be worked with. If you had opened your participation in the thread with the last comment rather than the first I’m sure the tone of the conversation
would have been a whole lot different.
Now if you really wanted to drive the point home you might give an example of an actual vehicle that follows your given method and clearly would be
all “wrong” if looked at from the traditional roll-center point of view. For example the Williams shown on the link is clearly “odd” but given F1’s
extremely small vertical suspension movements and fixation with aerodynamics it may not be that relevant to the discussion here?
Also please stop using the phrase “you lot” it’s derisory and rubs people up the wrong way, which is fine if you want to rub people up the wrong way
but probably most of the time you don’t.
I hope you take the above in the good spirit it is intended in, I am sure you would much rather be discussing what people have discovered after
carefully thinking and doing the math on the subject rather than what’s going on now.
Sorry, I've always had trouble speaking polite pommy english. Comes from a rough bush upbringing.
And, if anything disproves,(or disregards, anyway) the theory, it's an F1 car. Suspension is in the tyre sidewalls for the major part, and aero
stability is the only priority at the front and overall. But the rear ends warrant some scrutiny.
Cheers,
Syd.
I'm away to work now 'til next week.
[Edited on 5/11/07 by Syd Bridge]
Sounds like an underwear problem 
I don't understand any of it so I'm going to carry on pretending it doesn't exist.
de-wedging... the action of taking de wedge out from under de door.
de-wedging
perhaps syd is talking about stuff like this?
http://phors.locost7.info/contents.htm
I find when it comes to suspension design it doesn't harm to go back to physics.
I program for my day job and have always been interested in simulating dynamics and such. So i've tried many formulea's in my spare time, to
try and simulate the movement of a car ( im a geek 
).
An interesting formulae to play with is 'Pacejka's Magic Formulae', its used in all the latest racing games ( Grand Turismo, Forza
Motorsport etc ) to model the amount of friction force applied to the ground by a tire given its current variables.
Afterall, thats what suspension is all about, grip.
The force applied against the ground allows the body of the car to turn and therefore get us around corners. The higher/optimum the force, the faster
we can go around a bend without breaking trackion.
Its a very accurate model and i think its worth a look.
I don't know much about suspension design, but its probably a start
-Josh
josh when I last spoke to Ron Tauranac he said it was nearly all about the tyres.
Now the issues here of course is that he is designing racing cars and not compromised small trucks designed to carry more than one person...it is the
balance of compromises that is the tricky part.
quote:
Originally posted by rpmagazine
josh when I last spoke to Ron Tauranac he said it was nearly all about the tyres.
Now the issues here of course is that he is designing racing cars and not compromised small trucks designed to carry more than one person...it is the balance of compromises that is the tricky part.
the point about compromised small trucks, as I see it , is that road cars have to work in s much wider dinamic area... they do not have to be as good
or precise as racing cars, though...
I did a lot of design work on my chasis, spending much time on the uprights... and ust as I was so proud of brake packaging which left a lot of room
to place the uprights and joints well insidee the wheel, I realised that for my wheel travel, which is 70 mm bump + 25 mm bump stop + 80 mm rebound,
and for 25 deg of max steering angle, I had major clearence issues with the interference of the A arms and the wheel...
then I wanted to minimize the bending on the A arm that results from the spring forces.... well I think I did that, but I can not have the lower
joints mounted in double shear any more... you can't have it all...
quote:
Originally posted by CaptainJosh
quote:
Originally posted by rpmagazine
josh when I last spoke to Ron Tauranac he said it was nearly all about the tyres.
Now the issues here of course is that he is designing racing cars and not compromised small trucks designed to carry more than one person...it is the balance of compromises that is the tricky part.
I didn't really mean to come back to this thread, just thought I would bump it into a more constructive direction, but...
I don't understand your answer, what do you mean "compromised small trucks"?
Sorry, im quite confused.
-Josh