rpmagazine
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| posted on 21/4/08 at 12:25 AM |
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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?
www.racemagazine.com.au
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speedyxjs
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| posted on 21/4/08 at 07:44 AM |
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No 
How long can i resist the temptation to drop a V8 in?
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Doug68
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| posted on 21/4/08 at 09:57 AM |
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You mean like this 
Sorry, couldn't resist
The model and real life so far are showing close correlation.
 
Rescued attachment Properties.jpg
Doug. 1TG
Sports Car Builders WA
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rpmagazine
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| posted on 21/4/08 at 10:50 AM |
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yup, like that, though I used excel.
Now doing Bundorf analysis.
www.racemagazine.com.au
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matt_claydon
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| posted on 21/4/08 at 01:54 PM |
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Must be useful to know the mass to the nearest milligram
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v8kid
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| posted on 21/4/08 at 02:15 PM |
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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
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| posted on 21/4/08 at 05:15 PM |
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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
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| posted on 22/4/08 at 09:30 AM |
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I am doing CoG prior to suspension design and will do tilt analysis after build to see how close I was.
www.racemagazine.com.au
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rpmagazine
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| posted on 24/4/08 at 12:01 PM |
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data set placed on website
www.racemagazine.com.au
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v8kid
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| posted on 26/4/08 at 11:10 AM |
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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
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| posted on 26/4/08 at 02:43 PM |
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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
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| posted on 26/4/08 at 05:14 PM |
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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
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| posted on 27/4/08 at 05:01 PM |
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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.
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
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| posted on 27/4/08 at 10:07 PM |
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centre of mass is only one figure, but each end of the car has components that can be placed in different positions.
www.racemagazine.com.au
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v8kid
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| posted on 28/4/08 at 08:42 AM |
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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
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| posted on 28/4/08 at 11:44 AM |
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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.
www.racemagazine.com.au
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Syd Bridge
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| posted on 29/4/08 at 09:46 AM |
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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
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| posted on 29/4/08 at 09:15 PM |
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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!
Best Regards,
Steve.
www.turnerautosport.com
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rpmagazine
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| posted on 30/4/08 at 07:25 AM |
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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]
www.racemagazine.com.au
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Tralfaz
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| posted on 30/4/08 at 09:57 AM |
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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
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| posted on 30/4/08 at 10:23 AM |
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were I balancing a car on a fixed pivot and wobbling it by hand I might agree with you.
www.racemagazine.com.au
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kb58
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| posted on 30/4/08 at 01:33 PM |
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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
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| posted on 30/4/08 at 10:32 PM |
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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.
www.racemagazine.com.au
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Tralfaz
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| posted on 1/5/08 at 01:00 AM |
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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
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| posted on 1/5/08 at 04:33 AM |
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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]
www.racemagazine.com.au
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