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See my post responding to KB58, immediately above: what's important is the single roll moment formed by the perpendicular distance between the geometric roll axis and the centre of gravity of the sprung mass.

And if you read that statement after working through David Gould's calculations in Competition Car Suspension, you might realise what I meant about there being an error in his workings, but I'm going to shut up at that point, 'cos I'm giving you too many clues.

This moment around the geometric axis is resisted by the springs (and ARB's, and tyres...), but the fact that it is not resisted equally at each corner of the car means that the chassis does NOT simply roll around the geometric roll axis like a pig on a spit.



Firstly, not all chassis and suspension designers - even professional ones - fully grasp what's happening any better than a lot of us enthusiastic amateurs on the forums, so not all designers place sufficient importance on fixing the roll centres.

But quite a lot do, and by a roundabout route, they do give attention to Olley's Rule, in deed, if not in word.

I've had the opportunity to study and analyse suspension geometries created by some of the 'greats' - well known F1 designers, for instance - and I can tell you that their roll centre locations are always far too well constrained for it to be mere coincidence.

Of course Olley's rule allows you to calculate a fixed roll centre position exactly, but in doing so it will also dictate the rest of the geometry exactly, and there are often practical reasons why you don't want this to happen - roll centre isn't the only factor you have to consider!

What you will find is that quite a lot of designers take great care to locate the roll centre as closely as possible, within the limitations of the other parameters they have to work with.

As an example, this very evening I'm working on a suspension design where - having taken into account all the other constraints I've got to work with - the best I can manage is to locate the roll centre within 1.35mm relative to the sprung mass, for my full range of suspension movement.

With Mr Olley's help, I could have calculated this down to nothing (and to be honest, the OCD part of my personality is a bit cheesed off that I couldn't get it below 1.0mm, at least)... but it would then have fouled up one of my other constraints. And I'm pragmatic enough to know that the magnitude of the shifts in weight transfer caused by a roll centre movement of a couple of millimetres are so small as to be simply not worth worrying about.

There used to be a guy on another Locost forum who made it clear that everyone else's ideas of suspension were wrong - yet he wouldn't give his magic recipe. All I know - and likely many others here - is that there's more than one way to achieve "good enough", and tire wear after a trackday event is a excellent indicator achieving success - that fact speaks for itself. As said above, this is getting on the verge of argument, where seemingly everyone's comment is met with "um, no, wrong again", so I'm going to bow out and watch from the sidelines

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No magic recipe, Kurt - I've fully explained the principles above, if you make the effort and have the wit to understand them. If you haven't got the wit, then I'm happy to continue to try to explain to the best of my limited ability, but there's not much I can do to force you to make the effort.

I'm not going to hand over my spreadsheets and the formulae they contain, for two reasons:

1) Because a lot of donkey work went into developing and refining them and I rely on them to make me money - which they won't do if I gave them away. Since you're in such a philanthropic mood on my behalf, perhaps you'd like to lead by example and e-publish your books here on these forums, for everyone's free benefit?

More importantly:

2) As I believe I've said on this forum before, there's limited value in simply giving people a tool to use if they don't understand the underlying principles. The preceding discussion on Vsusp on this thread should demonstrate the folly of that.

As I've said before, David Gould's published chapter in Competition Car Suspension gives the formulae and procedures for calculating this approach to a full and functional level; certainly enough to give you an understanding of the key principles. I do the maths a bit differently, and I've developed things to start taking account of tyre rates, damping and a few other bits, but they're bolt-ons: the principles and the end results are broadly the same (at least if you stick to steady-state and wooden tyres ). It would be an evening's work for me to create a spreadsheet based directly and solely on Gould's calculations, but I'm not even going to do that for you because - as I've said before - I think it's the process of working through the calculations and turning them into a spreadsheet which will lead you to actually understanding them. And so far as forums are concerned I think it's the sharing and disseminating of knowledge that's the important function, not doing free calculations for people who are too lazy to do it for themselves.

Though it's surprising how many people must have blindly worked through those calculations without actually thinking about what the results mean in practice. You are, in fact, calculating the weight transfer loads on each corner, so it should be self evident even to a relative dimwit that a spring of known rate will compress by an easily calculable amount in response to a known load; and >>>since the chassis is supported directly on the springs, its posture will then necessarily conform to the spring deflections<<<.

Any approach that attempts to predict the physical 'roll centre' of the chassis, or the consequent camber angles that the linkages impose on the wheels, without taking account of the springs is pretty much a waste of time.

...If you're incapable of making that leap of understanding, then my providing spreadsheets that do the sums for you isn't going to help one jot.

My earnest hope is that one day, someone will come back and say 'I see what you mean about Gould's calculations: he did XXXX, which is really unnecessary because YYYY... there's this cleaner and purer way of approaching it, isn't there?'. That way I will know that I've helped at least one person to a full and proper understanding by their own efforts, rather than having just blindly plugged numbers into a formula (or spreadsheet) without thinking about the implications of the results they're generating. But if you never get further than grasping the implications of Gould's calculations, as they stand, then it will be enough.

In case the thought of maths is putting you off, don't let it: they really are very simple formulae; I don't think there's anything in there more complicated than multiplication and division, with the occasional use of 'pi' and 'squared'.

Don't worry about Kurt, he always seems to be in a bad mood.

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

Threads like this make my head hurt...

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30 years old, not sure what I want to be when I grow up…

They shouldn't, honestly - it's not that complicated when you look at it from the right angle (pardon the pun)!

Look at it this way:
* We all know that cars lean when they go round corners, right?
* We all know that when you lean on a spring, it squashes, but that a stiff spring will squash less than a soft one?
* Most of us on this forum will be aware that the springs have different stiffesses at one end of the car than the other?

...so when the car leans on its springs, it will squash the springs at one end more than it squashes the springs at the other, and as a result it will sit at a funny angle as it leans around the corner.

Fundamentally, that's all we're really saying here... the rest is just trying to work out the numbers to quantify it, and even that's not too hard once you've grasped the basic idea.

The difficult bit is understanding how some people out there still manage to believe that you can work out what the funny angle is without knowing how stiff the springs are.

I hate to see this discussion degrade; there is too much at stake here. Every noob that likes a 7 and wants to build one is immediately stymied by where to locate suspension pick up points, no?

So, I spent most of last night deriving energy formulas to solve this. The potential rotational energy of the external cornering forces (lateral g forces) and their subsequent resistances (tire reactions at the ground level) must equal the rotational spring energy stored in the rotated chassis. And yes, I plan on sharing.

Today it is back to the garage toy. If I can get some accurate instrumentation I would like to map the chassis roll with the lateral forces for three different suspensions layouts. And map where the fulcrum point is for each (fulcrum point is the best definition I have heard so far). This too I will share.

I don't mind that everyone has their race car secrets that they won't share. Like hackers, it's just a matter of time before we find out. And when we find out your "secret", you better be right. If we find out you are wrong, we won't tell you, we will just pass you in the curves!

ps. I'm learning some stuff but most of the value I am getting is the literal hands-on feel of things happening when a chassis is subjected to a variety of forces by tugging on my vertical and horizontal load cords. When I get done screwing with the numbers, I already have a gut feel where to start in laying out the suspension pick up points. In my designs, I design in flexible mounting locations and I'm very comfortable that the range of positions that I designed in will give me something ready for fine tuning on the track. That's my secret. I would like to share my design with all (actually it has been posted) and challenge everyone to build one and find out the best suspension for their particular track. I'm up for the competition.

You shouldn't need to. It's not that complicated.

You don't need 'rotational energy formulas' to work out what energy is stored in the springs: it's a simple matter (and I mean simple) of leverages.

Really, truly, if you need much more than force x distance to work out what's happening with weight transfer, then you're doing it wrong!

And I repeat: everything you need to know is already out there in the public domain, most of it neatly distilled into one, short chapter of a book that everybody with an interest in suspension design ought to have on their bookshelf already.

The clever bit is merely grasping the implications of those simple calculations.



Well, I'll share with you one of my secrets, though it's one that I've shared, repeatedly, before, apparently to no avail:

The one thing that makes what's happening horribly complicated to analyse is if your geometric roll centres are moving around. You end up with iterative calculations that are continually chasing a roll centre that's moved somewhere else.

Worse than that: if the roll centres are shifting then, by definition, so is the weight transfer. And unless you are very clever in managing that (much, much cleverer than me, for sure), then sooner or later it's gonna bite you in the ass. Sometimes literally.

Closely fixed roll centres (as you're already aware, since you quote Olley's Rule) require careful attention to the pickup positions. Having multi-adjustable geometry is probably the easiest way there is of confusing things beyond all hope of analysis.

My advice is to get it right on the drawing board and if you find you haven't, go back to the drawing board and do it again properly rather than trying to tinker with basic geometry at the trackside.

And my 'secret': whatever else you do, make sure that the geometric roll centres don't move (at least not more than a couple of millimetres), under any possible range of your suspension movement, relative to the sprung mass.



There's a lot more complicated stuff to come, for sure: we haven't considered jacking, yet, or damping and its effects on transitional behaviour, but lets get a grip of the basics, first?

Just for everyone's benefit (and clarity) we're speaking of David Gould's portion of Chapter 9 (pages 149-159) in Allan Staniforth's (R.I.P., Allan) book, correct? That book being ==> Competition Car Suspension: A Practical Handbook, Haynes Publishing, 2006

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Yus, that's the badger!

Sam, I am figuring it out. No more games; I will soon catch up with you and pass you if I don't get distracted onto another subject. I have played around enough with my toy and I know the "secret" of just where my fulcrum point is for any geometry I set and how it changes with roll. Ain't nothing like hands-on. But now I want to quantify it with my numbers. I gotta have some basis for my paper that will make me famous in the road racing world(!?). Confounds such as tires, opposing ends, etc. will follow if I want to keep going with this.

The energy solution resulted in a open loop with no real solution short of getting into least virtual work theorem, which means calculus. Um, not tonight, Dear, I have a headache.

Why not a vector analysis? For those out there that believe it is all in the springs, vectors are merely compressed springs. I started this last night, drawing all my little force arrows while we watched T.V. Finally, Wife leans over and says "Why don't you draw some little bows to go with all your little arrows, Dear?"

Onward!

Constructive critiques welcomed. I'll take a snipe or two but be constructive.

I think I should have been a little more description of finding the "secret" fulcrum in my model. And this was an experience everyone should actually "feel".

For a given lateral load and corresponding roll in my model, I added an additional lateral load cord acting at various heights of the model. When my additional lateral load cord reached a height where it no longer rolled the chassis and the chassis "locked up" in roll, there was my fulcrum height. Yes, it varies but I would need some more precision in my model bearings and tools to measure - it don't vary much. Note that this concerns vertical heights to the fulcrum; lateral were not measured.

It's difficult to be sure, since your model operates under the wholly erroneous assumption that the sprung mass rotates around the fixed point of the CoG (when in fact the sprung mass/CoG tries to rotate around the roll axis, but is resisted to differing degrees by the springs at each end)...

... but I think that what you may have 'discovered' is that if your geometric roll centre is at the same height as your CoG, there will be no roll.

Keep up the good work: I'm sure it's only a matter of a few more months, at most, before you 'discover' that if you move your roll centre above the CoG, the chassis will lean INTO the corner.


Extra points if you can tell us what happens when the roll centre is below the CoG at one end of the car, and above it at the other.

You will be subsequently shocked by the atrocious handling, as the only way to achieve this is to design the suspension so it jacks under lateral load. You still transfter weight to the outside tire, but it is supported by the jacking action of the control arm rather than the spring.



You get a whole lot of pitch in corners. I think the VW Golf is designed this way.



I think the whole confusion that people have with software like Vsusp is trying to use it to model dynamic behavior of the chassis. That's not what its for. Its used to model things like camber curve and geometric IC and RC under suspension deflection, but linking these to chassis roll under cornering forces is not in the capability of the software.

If you want to really throw in complexity, try to figure how the relationship between static camber, caster, SAI, bump steer, steering angle, speed, and body roll all add up to keep the tire happy in a corner.

I think this is why Colin Chapman said, "Any suspension will work well if you don't let it."