Suspension - droop question

[BillCobb]

I would like an explanation of what the comment about incorrect use of the "mass centroid axis" is all about, the before and the after.

Never mind, I found it.

His theory, was that TWO CG locations were in play during vehicle dynamic maneuvers, one at the front suspension reference point and one at the rear. Maybe in the days when cars were made out of jellow and connected somewhere with a soda straw. SO, cars are not all about dumbells, but the reverse is sadly true (in many cases).

[AG_]

forget the mass centroid axis and anything written about it.

Wow, thanks. Had no idea. TTW really had a few weird parts, especially in how antiroll was explained. Never thought too much about centroid axis though...

SO, cars are not all about dumbells, but the reverse is sadly true (in many cases).

Ive got to admit, a bit confused about what you mean by that. Are you saying cars were less torsionally rigid back then? and that now they are rigid to a point that it is basically one solid sprung mass?
If so, I guess the 2 mass center points at each axle I guess wouldnt make that much sense, but im not quite sold on your explaination just yet

[AG_]

Z, I'm a bit confused as to what exactly is the "inertia ellipsoid". I'm only started second year so some of the concepts Bill and you mentioned are a bit new to me.
Also, what is really the difference between the "mass centroid axis" and the inertia ellipsoid/ axies of rotation that you mentioned? This may be a meaningless question, coming from my very primitive knowledge in the subject of inertia.
As for roll and pitch axies - I've noticed that, how the roll axis is not constant throughout the motion. I figured best I can do is make the front change hight in the same way as the rear, but just start from a different position depending on the location of the supposed mass centroid axis - which I am still not fully grasping the implications of what you are explaining about it being irrelevant. I should probably say, thanks for taking the time to explain it. If I am asking questions about too simple physics, feel free to say "go read book xxxx and come back with some real knowledge" - I would accept that answer, and gladly go read. Although, reading it in this form seems much more interesting

[AG_]

Also, what is the parallel axis theorem? I've actually listened to Claude in person at Michigan, very interesting, but very much by the books.

[Charles Kaneb]

The moment of inertia measured by an axis through the CG is the lowest moment of inertia that can be measured about any parallel axis. To consider the inertia of an object measured on an axis away from its center of mass, you add the inertia of a point mass that distance from its center of mass.

It's a basic concept of point-mass models.

http://hyperphysics.phy-astr.gsu.edu/hbase/parax.html

[AG_]

Z, I was speaking to an alumni about it in the past, I've come to a conclusion that this supposed distance between the roll center and the mass centroid point at each end would mainly affect corner entry behaviour. But, the more I think of it now, the less that conclusion seems valid. Weight usually transfers diagonally, and apart from the roll moment there are so many things different for front and rear in terms of actual conditions, that even if I allign the roll centers with the mass centroid, it would probably make little to no visible difference. Especially without a fully tuned chassis, and a world class driver. But, that's based on my traditional knowledge...what you are saying is that this whole thing is invalid, but for other reasons

[AG_]

Charles - That sounds pretty straightforward, I'll read up on it though!

[onemaniac]

AG_, I got two things for you.

First, drop your pen, paper or CAD station, forget about the math and work on establishing 'proper' protocol for documenting design and communication.
If your team cannot get hold of someone who designed last year's car and even worse if you cannot find the documentation to allow you understand the design, your team will repeat the same mistake yet again. (I'd understand if you can't reach the guy who designed your 2005 car, not 2015 car)
Someone WILL drill wrong size holes, someone WILL install something backwards, someone WILL neglect the rule book.

Second, doing a 'proper' design does not mean complicating things. Simple design does not mean it isn't 'proper'.
It's funny how you mentioned 'adjustability' to get the real-world testing done for different characteristics.
How many parameters are you looking to adjust? at what increments?
Let's say your suspension design is heavily limited and you are only allowed to change the track widths (front and rear), spring stiffness (front and rear), damping (front and rear), tire pressure(front and rear), and front toe angle.
That's already 9 parameters. To make it even simpler, let's say you can only choose 2 levels - high and low - for each parameter.
Now you have 2^9 = 512 setup variations. How would you keep track of these changes? and How many more do you think you can handle? More importantly, how would you process your data?
In my opinion, the 'proper' way of designing a suspension is to get out there to test your current car and focus on one or two parameters at a time, and gather consistent data. You can THEN go backwards and figure out what does what and why.
I think too many students focus too much on Research and neglect Development. They should go together.

Just my $0.02

[AG_]

Onemaniac, I'm fully with you on the communication issue. I've mentioned the time management issue in my initial post, but the really knowledge transfer was absolutely terrible. Sure I have the SW of the design, but unless I understand the reasoning for it (which is hard by just backtracking), I would like you said make the same mistakes over again - or just different ones too. I've started a onenote design notebook to track my work, super convenient so far. Rest of the team? I'd have to constantly remind them probably haha. I will also try to reach the designer. For adjustability, of coarse there are the standard things, but I was thinking more in depth- leverage ratio (or motion ratio, if you prefer that name) behaviour - a linear setup as a basis, with different gradients of progression as adjustment holes. Is it necesary? I don't know, perhaps it is all so minor of a difference that it won't be noticed. I guess I just want to find out on track. Testing the current car - for sure, haven't done as much as I'd want to. This is why the redesign for this year is a minor one, in order to get that testing I need for a properly redesigned suspension.

[Z]

AG_,

Two very SANE points made by Onemaniac! They are so good that I will restress them here.
~o0o~

1. The very proper path for an FSAE Team is good "Knowledge Transfer". In fact, it is your School that should be setting up the right conditions for such, and then ensuring that there is lots of it going on. After all, that is their job!

The main repository of KT are the theses of all the previous Design-Leaders (assuming they did their FSAE work as a thesis/major-project). These theses should be compulsory reading for future Design-Leads. At the very least, a quick read of these theses' abstracts/intros/conclusions should give the following year's students a good idea of what the thesis writer thought most important.

Similarly, each Team(or Section)-Leader should write some brief notes (a few pages at most) stressing what they considered most important to get right, and what "fashionable must-haves" they found unnecessary. For example, a Suspension-Lead might write "MOST important is toe-stiffness. It (edit: Toe-Compliance) MUST be below (whatever...). On the other hand, motion-ratio is NOT very important, and can be anything between ...".
~o0o~

2. Don't take this the wrong way AG_, but your team is currently taking baby steps. If you run out onto the stage (ie. attend your next comp) and try to do a triple back-flip with half-twist, you are all but guaranteed to fall flat on your face. Even the experienced teams struggle to get their simple tumble-rolls right.

So a highly adjustable suspension might be something you play with away from competition, when, and if, you have lots of free time. But your comp-car should be fairly simple. And stiff, toe-wise! Otherwise you just get lost in the never-ending permutations and combinations of the adjustability...

Similarly, at this stage it pays to keep your theoretical analysis fairly simple too, least you get lost in the never-ending mumbo-jumbo of Voodoo Dynamics.

Given your apparent understanding of the subject so far, I suggest you try doing an accelerating(or braking) + cornering weight-transfer calculation of a car with four-wheels/suspensions, but only a POINT mass at the CG. Say, wheelbase = 1.6 m, F&R tracks = 1.2 m, with CG = 0.3 m high and at some given F:R%. Try to include the effects of "anti-roll" and "anti-pitch" (*) from the suspension geometries, in addition to the weight-transfer that comes from spring stiffnesses. (* These also known as "RC/PC heights", or "lateral and longitudinal force-line-slopes", or "n-line slopes", and covered briefly in this Jacking Force thread.)

Much, much later, you can move on to Eulerian Dynamics with its continuum-mass-distributions, principal-axes, MoIs, gyroscopic-couples, and so on...
~o0o~

Finally, as easy weekend reading, here is a 2004 thread about Vehicle Dynamics. This thread is easier reading than the Jacking Force thread, and it covers "front and rear CGs". You can think of it a bit like a soap-opera on the state of the art in modern VD. Yes, it was 2004, but nothing much has changed...

Z