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Author: Subject: longitudinal and vertical CoG heights
Syd Bridge

posted on 1/5/08 at 07:31 PM Reply With Quote
Polar Moments not significant to handling??

Try driving a 911 hard in the wet and see if you change your mind.

Cheers,
Syd.

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v8kid

posted on 1/5/08 at 09:45 PM Reply With Quote
Chaps I'm surprised you don't think PMI is connected to ride quality. The acceleration of yaw and pitch are inversely proportional to PMI so a high PMI car will have a low acceleration pitching moment which is more comfortable to us. Daniels gives a very good explanation without any maths in his book "Car Suspension at Work" or if you like sums Milliken & Milliken cover it comprehensively if expensively. A free guide to the Physics is on the net search "the physics of racing"

On reading rpm's post a few times I think he is saying that with very low PMI cars the driver does not have the reactions to match the response time of the car.

Very valid for an everyday car where we let our concentration drift as we commute but surely a sports car is just that - a car for sport and hence it should be challenging and rewarding? After all we do not have to worry about litigation like a mainstream manufacturer who will design his car to be failsafe for the worst case driver

What I was, clumsily, trying to say in my previous posts is that we need to get the very basics right before drifting off into the esoterics of suspendion design. Low weight, low down, slightly to the rear, sticky tryes with lots of horsepower and it will go like s**t off a shovel, brake well and romp round corners even with very rudimentry suspension.

Cheers all

David

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rpmagazine

posted on 2/5/08 at 03:14 AM Reply With Quote
syd, if you read my posts you will see that I have said that I think the role of PMI is not as significant to other aspects.
When I interviewed Ron Tauranac just over a year ago he said his philosophy was that 'it is all about the tyres' and I can see the sense on that basic statement. The are many many other contributing factors in addition to PMI and some of the interactions between them are complex in the extreme.
V8kid your paraphrasing of my comments is mostly correct, however I would contend that driving on the road is inherently more challenging than on a race track given the variables and that concentration is but one part of a person's ability to govern the attitude of a car, particularly in transitions.

[Edited on 2/5/08 by rpmagazine]





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v8kid

posted on 2/5/08 at 07:33 AM Reply With Quote
Hmm I think we are discussing the motoring equilivent of "how many angels can dance on a pinhead".

I've never met this guru chappie you interviewed but for some reason he certainly is stating the bleeding obvious.

There must be an interim step between that and the incomprehensable surely?

On reflection quite probably not

Cheers all

David

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rpmagazine

posted on 2/5/08 at 08:27 AM Reply With Quote
Ron was the engineer behind all of the early Brabham and RALT cars.
He is still working and was designing a clubman for someone last I saw him at his place.





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Syd Bridge

posted on 2/5/08 at 08:48 AM Reply With Quote
I don't know if you're fishing for a story for your magazine, but I'll continue anyway........If this all ends up repeated in print, then at least I can point people to where it originated.

I would suspect, that Mr.Tauranac is being economical with the whole story. As most designers and engineers working in the racing industry are.

Also, if your OEM test drivers told the full story, or at least you reprinted it, then all would know that low PMI gives a faster car overall with better cornering power.

The high PMI is difficult to get to turn in, then difficult to straighten out. But, has softer reactions at the limit, and that limit is much lower than a low PMI car. It also needs different damping for the bumps, as the weight is closer to the axles. The dampers need to deal with the weight more directly, instead of the leverage arms that low pmi gives.

The low PMI car will corner much better, albeit with sharper responses which need sharper reflexes. It will have sharper turn in, and sharper exit behaviour. Downside is that at the limit in cornering, (and this is a limit higher than the high pmi car), the car will not have the progressive breakaway that the high pmi does. Breakaway will be more off/on, but with sharp reflexes is no less a disadvantage than the the high pmi. You've just got to have good feel in your backside. The limit, and response at the limit, can be made progressive with damper adjustment and Mr.Tauranac's tyre adjustments, and proper suspension dynamic characteristics.

If all the above is not so, then why do all the Locost racers, touring cars, V8 supercars et al strive to get the engines back as far as possible in the car?

Why do open wheelers and LM sportscars have the engine betwen the axles and as close to centre as possible?

Why do all knowledgeable racecar engineers strive to centralise all weight, and lighten the ends? Some are near to being obsessive about lightening the extremities.

Geez, even the dinghies I raced as a youngster, down at St.George Sailing Club, benefited from light weight at the ends.

There must me something of effect and truth .

Cheers,
Syd.







[Edited on 2/5/08 by Syd Bridge]

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rpmagazine

posted on 2/5/08 at 09:42 AM Reply With Quote
fishing for a story? no I do not want for material and there is little in this forum that is print-worthy in a technical magazine.
The paraphrase of Ron is my own. There was of course much more to the conversation.
However I think you have mis-understood my point. As I have previously said PMI may well be overshadowed by many other aspects of design.
Dampers 'see' two masses, the unsprung and sprung, please explain the PMI effect on damper performance.
Rereading your post for the third time I can see that you have not understood what Ron was saying. Virtually ALL dynamic aspects are about making the tyres function to the best of their ability.


[Edited on 2/5/08 by rpmagazine]

[Edited on 2/5/08 by rpmagazine]

[Edited on 2/5/08 by rpmagazine]





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v8kid

posted on 2/5/08 at 10:21 AM Reply With Quote
Steady on rpm I thought Syd stated the case precicely and succincitly - as well as correctly in my opinion. You must admit it would be very readable in a Mag.

It sounds cheap to trash others opinions we are all here to exchange views based on our experiences and deductive powers afterall.

Can you elaborate on your points regarding the damping? I assume you were alluding to the load transfer on the contact patches from the PMI via the dampers which kind of reinforces my point.

Cheers all

David

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rpmagazine

posted on 2/5/08 at 11:28 AM Reply With Quote
I am not here to trash anyone's view but I am very short on time and have an allotted 2 minutes or so as a break in layout. I cannot devote much attention to this ATM so excuse the point format:
Simply put *everything* you adjust on the car to affect handling has its effect through the tyres.
The control issues re recovery etc are in Milliken and Dixon and I cannot explain them better
Dampers do not in any way deal with 'weight', the springs do. The dampers deal with mass/velocity, and unless you have some very fast transfers and some very advanced dampers high low speed adjustment) you will not be able to address weight transfer. I disagree with some of the other points as described as they need covering statements such as 'all other things being equal'...which is kind of a moot point in vehicle design and suspension design.




[Edited on 2/5/08 by rpmagazine]





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Syd Bridge

posted on 3/5/08 at 04:50 PM Reply With Quote
Mr Tauranac is not wrong in saying that it all happens through the tyres.

BUT, how you control the tyres is what it is all about. Has Mr.Tauranac told you how to do this yet, explicitly and simply?

I doubt it, and I also doubt he ever will, as I've previously commented about designers and engineers. They won't tell you something you can't find somewhere else, they will tell you what is easily found out though.

Oh, and forgive me for mixing weight and mass, but most people understand weight, but get lost when you start talking about masses. This really isn't the place for a discussion on mass dynamics and how they effect PMI, and vice versa.

This is supposed to be a forum for amateur builders, and they don't need to get embroiled in a lot of unnecessary theory, and application. They just want to do it, simply and easily.

Just like Locost racecars!!

Cheers,
Syd.

[Edited on 3/5/08 by Syd Bridge]

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rpmagazine

posted on 3/5/08 at 11:50 PM Reply With Quote
yes, this above all places is the place for a discussion about such things, otherwise we perpetuate poor understanding and application of a limited range of theory.
How do we progress a hobby without progressing understanding and skill?
You had discussed weight transfer, PMI and the effect on dampers had you not?


[Edited on 3/5/08 by rpmagazine]





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Kaspa

posted on 4/5/08 at 02:42 AM Reply With Quote
i must admit , i find some of these threads going from one extreem to another, starting on PMI etc is fine if your building a formula 1, which most of us are not,
i think we all no, or should no the basics as far as PMI goes and that is the more central you can have the bulk of the weght mass in your car the better it will handle, the main reason why mid engined cars were designed and built and handle so well.
but a front engined car or for that matter a rear engined car can be made better by transposing as much auxillery weight as posible to offset the engine mass
ie set the engine back as far as you can pracitibly then place things like battery fuel tank etc in areas to counter ,
i dont wish to rain on anyones parade here but ive been building and racing all sorts of cars for to many years to remember, and although i'm learning new tricks every day quantum physics just isnt one of them, and personaly i dont think the guys on this forum need to go to that extreem, i no i dont
JMTBW
cheers Kaspa





understeer= when you hit the wall front first
oversteer=when you hit the wall back first
HP= how fast you hit the wall
Torque= how far the wall moves when you hit it

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rpmagazine

posted on 4/5/08 at 03:09 AM Reply With Quote
There are frequently too many generalisations of theory, particularly theories that are taken out of their original context, which in reality is where they have meaning. None of what we are discussing is close to complicated in the sense of quantum mechanics and not complicated in the sense of suspension design by a suspension engineer. Many builders I converse with know this sort of stuff backwards and the conversations are such that I have difficulty following it. These are the guys who win and build good cars and I would contend they are the people we should be learning from.
I recently had the sobering experience of speaking to a suspension engineer who described what he saw as the process for designing the kinematics of my cars suspension. It was sobering as it indicated that what I previously saw as best practice was deeply flawed.
Or should we continue to discuss design of suspension and handling using terminology and understand as per what Staniforth wrote 30 years ago?
The irony of this comment above is that Bundorf released his paper in 1976 or so...so it has been around for quite a long time.
I should add that I feel that the principle advantage that a manufacturer such as Lotus has is its relationship with the tyre manufacturer and it's development team.

[Edited on 4/5/08 by rpmagazine]

[Edited on 4/5/08 by rpmagazine]





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v8kid

posted on 4/5/08 at 07:05 AM Reply With Quote
Err you've lost me - what does Bundorf say then?
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rpmagazine

posted on 4/5/08 at 07:18 AM Reply With Quote
http://en.wikipedia.org/wiki/Bundorf_analysis
http://en.wikipedia.org/wiki/Automotive_suspension_design

Note the engineer I spoke to said that the wiki bundorf section needs a lot of work, but it gives the basics.





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Lippoman

posted on 4/5/08 at 11:51 AM Reply With Quote
quote:
Originally posted by rpmagazine
Dampers 'see' two masses, the unsprung and sprung, please explain the PMI effect on damper performance.


Actually, a damper 'sees' no masses, it merely reacts to the speed it is forced to move at... The body of the vehicle 'sees' the forces transmitted through the suspension and reacts to these. It is here the PMI of the vehicle in all axes come into play.

The work needed to negotiate a turn are:
- shifting the direction of movement of the vehicle (mass of vehicle)
- rotating the vehicle to the new direction of travel (PMI)

Tires are usually the factor that the car designer has the least influence over so these are usually a key design factor. But generally tires 'like' gradual load changes and as small force demanded as possible .

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rpmagazine

posted on 4/5/08 at 12:13 PM Reply With Quote
I was actually thinking more of mass/vel as in acceleration, but you are correct, a damper is as its names suggests a damper of mvt.
The ratio of damper valving is partly set/influenced by the two masses in that the bump valving reflects the vehicle mass and the rebound the unsprung mass, or at least this is how I have reasoned it.

[Edited on 4/5/08 by rpmagazine]





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cymtriks

posted on 10/5/08 at 05:21 PM Reply With Quote
CoG heights

These heights, at the front and rear axle lines, define the preferential axis about which the total suspended mass will want to rotate about.

There is of course only one true (as in the strict definition of CoG)CoG height but the term gets used in defining the two points through which the axis mentioned above must pass.

The true CoG lies somewhere on the axis defined by the front and rear CoGs, the position depending on the weight distribution.

Faults with Staniforth etc

Well they can't be that serious as a great many very good cars have been made using these principles.

Perhaps the biggest "obvious" critisism is that the method assums a solid tyre. This can be compensated for in a spread sheet by adding a tyre displacement versus load function to wheel movement. The effect is to make the effective roll centre a bit higher.

Another critisism is that the method assumes freely suspended masses. They're not! the springs and dampers make sure of that. As the springs and dampers get harder the whole system gets closer to four tyres bolted to a rigid block. Again the effect is to slightly increase the "true" roll centre. A completely rigid suspension would place the roll centre at the CoG, it couldn't be anywhere else.

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Doug68

posted on 10/5/08 at 11:54 PM Reply With Quote
quote:
Originally posted by cymtriks
define the preferential axis about which the total suspended mass will want to rotate about.


I think this may be true providing you've just gone over a cliff. The rest of the time an object will want to rotate about the points at which the input force is being reacted against.

To demonstrate this take an object on your desk and push it over.

The other thing is the method which we all use and swear by is not the same as that the SAE accepts as the correct method. Unfortunately unless you are a pro or a PHD uni student you won't have access to the tools to work that method.





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rpmagazine

posted on 11/5/08 at 03:16 AM Reply With Quote
Oh sure we could start using forced based RC's etc, but as you note there is no cheap way of working these numbers as yet. No doubt we will get there, probably about the same time we are forced to drive 1lt diesels!





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Doug68

posted on 11/5/08 at 04:53 AM Reply With Quote
You're quite right, I think we need to recognize though is that the state of the art in calculation and proper understanding of the problem has moved on from what an amateur can reasonably expect to wrap their brains around.

I for one would struggle to get going doing a force based analysis even if I did have a copy Adams to hand. As a result I've flogged the death out of Susprog3d instead hoping the approximation of what we think is happening and whats actually happening are the roughly the same thing.

After all not yet understanding the fundamental nature of matter hasn't yet stopped us making things from it.





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rpmagazine

posted on 11/5/08 at 10:43 AM Reply With Quote
Greg the guy I have doing my suspension uses Adams every day, but he will still use his own software for Godiva. It would be possible to do a susprog with force based kinematics...but is the market there for the costs of a complicated redesign? I suspect not so we get what the market allows and we must accept that we cannot take the geometric kinematics too seriously.





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cymtriks

posted on 11/5/08 at 01:35 PM Reply With Quote
quote:
Originally posted by Doug68
quote:
Originally posted by cymtriks
define the preferential axis about which the total suspended mass will want to rotate about.


I think this may be true providing you've just gone over a cliff. The rest of the time an object will want to rotate about the points at which the input force is being reacted against.

To demonstrate this take an object on your desk and push it over.

The other thing is the method which we all use and swear by is not the same as that the SAE accepts as the correct method. Unfortunately unless you are a pro or a PHD uni student you won't have access to the tools to work that method.


I don't think you've read my post properly!

The CoG height define the acis that the sprung weight wants to rotate about. However this weight is restrained by the suspension which is restrained by the contact patches.

To go back to your "push something over" analogy try pushing something that is suspended from something. It won't move in the same way that a solid object does before it finally falls over.

As I pointed out in my earlier post the basic "classic" approach assumes a floating suspended mass, i.e. one that truly moves about the points defined by the interaction of its mass and its supporting links (suspension!)

But in reality the suspended mass is semi-solid as the springs and dampers make the suspension behave part way between a floating mass on links and a rigid box with wheels on.

This raises the roll centre a bit.

The other factor left out of the clasic approach is the effect of squashy tyres. This can be added to the analysis fairly easily by assuming and extra displacement at the wheel for each unit load placed on it due to bumps or cornering.

This also raises the roll centre a bit.

How much is "a bit"?

Well I've read some threads on force based roll centres and I've seen quotes around the half inch to one inch for race cars. Road cars would probably have greater effects as the suspension and tyres are that bit softer so I'd guess at the true Roll Centre being about 1 or 1.5 inches higher than the classical approach.


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Syd Bridge

posted on 11/5/08 at 07:32 PM Reply With Quote
You people would be surprised at how much, or in reality how little, of all that theory actually goes into a top end racecar.

The numbers may be crunched as an after thought, but the basics are all that are needed. Keep everything as light as possible, as low as possible, and as central as possible.

Suspension is a simple geometrical exercise. Springs and dampers need a little maths, but certainly not a supercomputer!

From then on it's about packaging. Same goes for roadcars, but costs are the major factor in roadcar design. And speed of build. Every minute saved on the production line can be, and is, costed.

Cheers,
Syd.

By the way, if suspension was rigid, then the car would probably roll about the outer contact patch in cornering. Maybe???

[Edited on 11/5/08 by Syd Bridge]

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rpmagazine

posted on 11/5/08 at 11:04 PM Reply With Quote
Sure Syd, you CAN make it as simple as you like, but things are usually constrained by the rules and available time.
However OEM's do a lot of work on suspension and they do take in all of the variables if they can, which is why cars have progressed so much in the past 15 years.





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