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Author: Subject: longitudinal and vertical CoG heights
rpmagazine

posted on 21/4/08 at 12:25 AM Reply With Quote
longitudinal and vertical CoG heights

I've just finished to days of work measuring and calculating to get the long/vert CoG heights (not fun) prior to suspension design. Has anyone else done this and compared to the finished product?





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speedyxjs

posted on 21/4/08 at 07:44 AM Reply With Quote
No





How long can i resist the temptation to drop a V8 in?

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Doug68

posted on 21/4/08 at 09:57 AM Reply With Quote
You mean like this

Sorry, couldn't resist

The model and real life so far are showing close correlation. Rescued attachment Properties.jpg
Rescued attachment Properties.jpg






Doug. 1TG
Sports Car Builders WA

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rpmagazine

posted on 21/4/08 at 10:50 AM Reply With Quote
yup, like that, though I used excel.
Now doing Bundorf analysis.





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matt_claydon

posted on 21/4/08 at 01:54 PM Reply With Quote
Must be useful to know the mass to the nearest milligram
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v8kid

posted on 21/4/08 at 02:15 PM Reply With Quote
When the CofG height gets close to the wheel center height it gets pretty tricky to measure it and confirm calculations.

I'd like to know how it can be managed to better than +- 10%.

Of course its much more accurate if the CofG is much higher than the wheel centers but that aint much of a sports car either I suppose.

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Syd Bridge

posted on 21/4/08 at 05:15 PM Reply With Quote
A simple tilting table is the best tool for a finished car. You can get CoM's within a few percent.
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rpmagazine

posted on 22/4/08 at 09:30 AM Reply With Quote
I am doing CoG prior to suspension design and will do tilt analysis after build to see how close I was.





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rpmagazine

posted on 24/4/08 at 12:01 PM Reply With Quote
data set placed on website





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v8kid

posted on 26/4/08 at 11:10 AM Reply With Quote
You chaps must have much more accurate locost scales than I have.

Using the tilt method produces very small changes in front wheel weights and all I could determine was that the CofG was around the wheel center height +- who knows? Certainly not better than 10mm.

See Milliken and Milliken section 18.2 for the maths.

Also note this maths is invalid if the wheels mave different rolling radii - can't remenber the SAE paper for it I lost my copy - wouldn't mind another copy if anyone has one to hand.

Cheers

David

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procomp

posted on 26/4/08 at 02:43 PM Reply With Quote
Hi dont forget to include the driver in the calculations. Yes i had to re do it the first time .

Cheers Matt






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Bob C

posted on 26/4/08 at 05:14 PM Reply With Quote
1) centre of mass is centre of mass - why would its height be different along or across the car?
2) When I was designing my braking system I tried to estimate height of COG by listing the big pieces of the car & their heights & combining them in excel. Came to 40cm IIRC.
3) I intend to try to balance the car on the wheels of one side & then the other. Trouble I need quite a few helpers to manage this & I keep forgetting... That should tell me the height of COG pretty accurately I reckon - I'll post on here if & when I remember to do it & compare it to the theoretical 40cm figure!
cheers
Bob

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kb58

posted on 27/4/08 at 05:01 PM Reply With Quote
I use a big spreasheet with the weights and x, y locations of every component. It's invaluable for calculating front/rear weight distribution and weight transfer. I really don't know how a car can be designed without.





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rpmagazine

posted on 27/4/08 at 10:07 PM Reply With Quote
centre of mass is only one figure, but each end of the car has components that can be placed in different positions.





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v8kid

posted on 28/4/08 at 08:42 AM Reply With Quote
The Sunday name is Polar Moment of Inertia.

Two cars can have the same CofG one with all the weight in the middle and one with half the weight over each axle. Although they have the same CofG the second car will act more like a flywheel and resist turning. Once turning it will want to keep on turning.

Great for a family car giving a smooth ride but poor for a sports car that needs to make quick changes in direction.

As well as varying longitudinally the weights can vary vertically making for different responses at each end of the car in yaw, roll and pitch.

Confusingly this will lead to different requirements for damping and springing in yaw, roll and pitch.

Best just to try to keep all the heavy stuff in the middle and as low as possible.

If any compromises are required, as they always are, keeping it low has the biggest overall effect.

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rpmagazine

posted on 28/4/08 at 11:44 AM Reply With Quote
I both agree and disagree with aspects of the above post, but this is not the place or time for such a discussion as I need to get the next issue of Race Magazine together.
I will post more base data following publication, which makes up the pre-kinematic design set.





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Syd Bridge

posted on 29/4/08 at 09:46 AM Reply With Quote
Why disagree? I see little, if anything, that is incorrect in V8kid's statements????

Plain and simple physics and mechanics!

Cheers,
Syd.

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tadltd

posted on 29/4/08 at 09:15 PM Reply With Quote
quote:
Originally posted by Syd Bridge
Why disagree? I see little, if anything, that is incorrect in V8kid's statements????

Plain and simple physics and mechanics!

Cheers,
Syd.


Hear hear!





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Steve.
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rpmagazine

posted on 30/4/08 at 07:25 AM Reply With Quote
my how we jump to conclusions.
I cannot see how PMI has anything to do with ride quality.
I am also increasingly unconvinced at the supposed low PMI benefit with limit handling given the feedback loop available to a driver, particularly one such as all of us with relatively limited skills on a public road. Low PMI cars have low recoverability in yaw and only a relatively minor incremental advantage in outright grip.
With recent developments in my own project I realise how limited text books, including Milliken, are at address all of the possible variables WRT design.

[Edited on 30/4/08 by rpmagazine]





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Tralfaz

posted on 30/4/08 at 09:57 AM Reply With Quote
quote:
Originally posted by rpmagazine
I cannot see how PMI has anything to do with ride quality.




I would think a vehicle with the mass over the wheels would get jostled around less than one with the weight at the CG.

A teeter totter is much more resistant to outside influence with a kid at each end than with one fatso on the pivot...

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rpmagazine

posted on 30/4/08 at 10:23 AM Reply With Quote
were I balancing a car on a fixed pivot and wobbling it by hand I might agree with you.





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kb58

posted on 30/4/08 at 01:33 PM Reply With Quote
quote:
Originally posted by rpmagazine
my how we jump to conclusions.
I cannot see how PMI has anything to do with ride quality.
I am also increasingly unconvinced at the supposed low PMI benefit with limit handling given the feedback loop available to a driver, particularly one such as all of us with relatively limited skills on a public road. Low PMI cars have low recoverability in yaw and only a relatively minor incremental advantage in outright grip.
With recent developments in my own project I realise how limited text books, including Milliken, are at address all of the possible variables WRT design.

[Edited on 30/4/08 by rpmagazine]

I guess I missed it, did someone say PMI affects ride quality? It does but by a tiny amount. PMI is all about slew rate in yaw and I can attest that its effects are very noticable in my mid-engine Mini. Fast transitional turns are great fun.

As a great demonstration, next time you're at the market, move all the stuff in the cart to the rear and see how easy it is to change direction. Now push it all foward and try to turn. It makes a big difference - but not for ride.





Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book - http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html

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rpmagazine

posted on 30/4/08 at 10:32 PM Reply With Quote
The response may be faster or rather feel faster, which is not always the same thing. However the controllability at grip limits and recovery is often poorer. So in short it feels great but when it 'goes' i.e. starts to slip etc then you need to be much much better to catch it and you cannot necessarily recover as well. Curiously this opinion is not mine but that of two test drivers for a local OEM. There was much more technical discussion of this, but as I said this is not the place.





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Tralfaz

posted on 1/5/08 at 01:00 AM Reply With Quote
quote:
Originally posted by rpmagazine
were I balancing a car on a fixed pivot and wobbling it by hand I might agree with you.


Perhaps the analogy was poor.... However my belief remains that on a softly sprung/damped car having more mass moved from center may well affect ride.

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rpmagazine

posted on 1/5/08 at 04:33 AM Reply With Quote
PMI is a very small contributor to many aspects of ride and control - relative to other issues. I suspect that unsprung mass is much more significant in it's contribution to NVH and ride, though it is less significant to track performance assuming certain track conditions. If we were talking about Australian tracks, which is what I know a bit about, I suspect that unsprung weight and dampers would have an additional importance to what I have seen of in-car footage of overseas./ Local tracks can be quite bumpy in braking areas.
PMI is one of the aspects of design that we can 'optimise' (WRT our cars intended use) in order to offset the limitations of our design and manufacturing capability and lack of development (after all we don't have the test rigs, prof engineers and test drivers and exhaustive damper techs and tyre racks etc etc).
The one aspect that I feel we don't often acknowledge is that we effectively trade NVH performance for 'handling' and this is a very big advantage we have over a manufacturer.

[Edited on 1/5/08 by rpmagazine]





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