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chicane67
08-16-2004, 11:29 AM
ddennis68
Registered User
Posts: 332
(8/7/04 9:43 pm)
Reply Center of Gravity
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I've read through Adam's, Puhn, and Smith's books (I even ran a search at c-c.com). None have indicated, that I can find how to find CoG for a given chasssis. Does anybody have a fairly accurate way without a million dollars in equipment?
Dennis-

check out progress of Bondobucket

chicane67
Registered User
Posts: 205
(8/7/04 9:57 pm)
Reply | Edit Re: Center of Gravity
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I believe you will have to do the independant suspension systems and weights of the chassis in question, to findout how they couple first, and then go from there.

I have never done both front and rear intersects at the same time without a large drafting table or the expensive software.

o1mrquick
Registered User
Posts: 2116
(8/7/04 10:00 pm)
Reply COG
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Im not too sure about the exact center but you can figure out the CGH(center of gravity height). You'll need to put the car on scales. let me see what I can dig up....Did you get any Steve Smith books? Vince

davidpozzi
Moderator
Posts: 1271
(8/7/04 11:06 pm)
Reply
Re: COG
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Front and rear weights will give you CG location F/R but not the height.
To calculate height you need another front end weight but with the rear jacked high as possible, the shocks need to be removed and solid links up front, probably the rear too. If no solid links are used the front suspension will compress, the rear extend due to weight shift, and mess up the calculaton.

There is another way I've seen photos of, you raise one whole side of the car until it teeters and measure the angle! The tires must be pumped up very high and DON'T let it FALL OVER!

The books have the formula to calculate the CGH.

Polar moments can be measured too, but the car must be hung from the "celing" and swung back and forth!
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/

Edited by: davidpozzi at: 8/7/04 11:08 pm

Mean 69
Registered User
Posts: 100
(8/8/04 10:20 am)
Reply Estimate
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One way you can estimate the height of the CG is to determine the height of the camshaft at ride height. I have heard several folks using this as an estimate, and most people will agree that it is pretty close.

Front to rear bias can usually be found by searching, someone out there has the info. You might not get completely accurate numbers, but it close enough to design around with reasonable accuracy. I am guessing this is for calculating the geometry figures for the new rear setup?

Mark

ddennis68
Registered User
Posts: 335
(8/8/04 11:36 am)
Reply Re: Estimate
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HOLY [email protected] height!!! That's like 30" or so. I thought most production cars ran like 20-24" and I am about 3" lower that stock, I was hoping for around 20" or less if possible.
You are correct Mark, it is for plotting numbers for the rear setup. I'd like to find a front program for long-arm/short-arm front suspension too.
Dennis-

check out progress of Bondobucket

davidpozzi
Moderator
Posts: 1272
(8/8/04 1:36 pm)
Reply
Re: Estimate
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I have factory data showing a CG height of 20" for a stock 67 Camaro with one passenger. I'm not sure since it doesn't say, but it could be for a 6 cyl Camaro, but I'd imagine the CGH would be pretty close for either engine.
The specs are titled "Inital Design Parameter Specifications" From GM Engineering.
Unsprung weight FT 179lbs
Unsprung weight R 265lbs
One passenger load, 2866 lbs. I assume this is the gross weight minus the unsprung weights. Note the tire is probably a stock 5" wide X 14" steel wheel with a 7.37X14 Uniroyal Laredo tire.

The car the way "we" normally use them would be lowered two inches putting it at 18" CGH.
I would think most mid-size GM models would be within an inch of that, so 17" to 19" is my best guess.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/

Edited by: davidpozzi at: 8/9/04 9:53 pm

Mean 69
Registered User
Posts: 102
(8/8/04 7:49 pm)
Reply Sheesh!
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Dennis,

I estimated the CG height of my 69 as being right around 18" or so. This is with a drop of about an inch (ish), and again, it's an estimate, but a decent one. David P gave some gut check agreement with his car some time ago, and checks in again, above. My guess is that your cool 'Camino is pretty close to this height as well.

30"? Better check that ruler!

M

ddennis68
Registered User
Posts: 338
(8/8/04 9:14 pm)
Reply Re: Sheesh!
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Quote:
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My guess is that your cool 'Camino is pretty close to this height as well.
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Uhhmmm-OK. Thanks for the kind words-my kids say somebody needs to pimp my ride . I don't think it's as funny as they do. I'll re-check my cam height tomorrow with a tape instead of my optical gauge.


OK, I re-checked cam height, it's 17". I really need to get the opti-gauge checked.
Dennis-

check out progress of Bondobucket

Edited by: ddennis68 at: 8/9/04 10:15 am

Ripper
Registered User
Posts: 315
(8/9/04 11:58 am)
Reply
Re: Sheesh!
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Quote:
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The car the way "we" normally use them would be lowered two inches putting it at 18" CGH.
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Ap ap ap - just because you lower the car 2 inch doesn't mean that cgh drops with two inch.. Don't forget the unsprung mass that doesn't move..

EDIT: David; where did you get the factory data from? The only car I've seen numbers of CG is Honda NSX (that's Acura i US right?)


Edited by: Ripper at: 8/9/04 12:00 pm

davidpozzi
Moderator
Posts: 1275
(8/9/04 12:39 pm)
Reply
Re: Sheesh!
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Ripper,
Good point, I thought about the unsprung weight but was too lazy to mention it. I figured the unsprung weight "should" have only a small influence on CGH change since it is a relatively small amount, but I haven't done the calculations to see the effect.

If you were going to use CGH to calculate F/R roll-couple distribution then you want to use unsprung mass and CGH since the unsprung mass is what is acting on the suspension when cornering.

I haven't thought about this stuff in quite a while so maybe I'm a little fuzzy on the details... I usually review the books before doing a calculation.

The source of the info is a 1982 special Camaro issue of "Quicksilver Supercar Series CAMARO - Three Generations Of Premier Performance Cars"
By Martyn Schorr.
ISBN 0-940346- 13-3

It was given to me by Pdq67 and has lots of techincal test data from GM in it.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/

chicane67
Registered User
Posts: 211
(8/9/04 1:23 pm)
Reply | Edit Easy schmeesy.......
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Finding the CG and/or GCH can be done in any of several ways, none of which are accomplished very easily and without some work. I will try and explain a simple method that I learned from the ole' man Steve Smith.

To duplicate it you will need a set of blocks at least 20 inches high to support the weight of the rear wheels and a set of scales. It would be nice to have a chassis pad to do this on, but not everyone has a 6 foot deep cement slab just lying around....but just make sure it is as flat as you can find.

First you will need to find the CG along the wheel base. To do that find the total weight's of the front and rear end. Second, add them for a total. Then divide the total weight into the rear. The result is the percentage. Multiply that percentage times the wheelbase and this will give you the distance behind the CL of the front wheels that the CG is located.

Draw a scale of the car for your graphical representation and for your notes to follow.

Next, jack up the rear end to be placed on blocks at least 20 inches and weight the front and rear end again and be sure to subtract the block weight. On your drawing, project a new imaginary line for the wheelbase that is parallel to the first when at static ride. Using the hypotenuse method for the next series of calculations would be the best. Record your intersects on the drawing. The wheelbase is now noted as being shorter and re figure the distance that the CG lies behind the front wheels using the new front and rear weights, on the 'new' wheelbase measurement.

Back to the drawing, draw the car up on blocks and then draw in the hypothetical wheelbase figured by the hypotenuse method. On top of the wheel base line draw a line perpendicular to the wheelbase at the point where the true CG lies along it. Next draw a line perpendicular to the new wheelbase line through the 'new' CG. Extend those lines until the two CG lines intersect. Measure the distance from the intersect point to the ground level, then convert it from the scale drawing into actual inches.

This is your actual CG and CGH and as stated by many before, it should lie somewhere in between 13 to 18 inches. I am guessing the hard part is if you dont have any scales to perform this work on.

Edited by: chicane67 at: 8/9/04 1:30 pm

Ripper
Registered User
Posts: 316
(8/9/04 1:42 pm)
Reply
Re: Easy schmeesy.......
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About the way measuring CGH;
Springs must be locked and brakes should not be locked to get a good value.



Norm Peterson
Registered User
Posts: 174
(8/9/04 5:49 pm)
Reply Re: Measured Vehicle Parameters
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CG height data for a variety of vehicles is presented here, from which you can download this spreadsheet.

The list is by no means complete, but briefly, the CG height for most cars falls in the range between about 20" and 23.5". Only a couple of listed cars (Datsun's 280ZX and Toyota's MR2) come in under a 500mm CG height (19.7").

Obviously, this data is for vehicles in their OE condition, but at least it gives some data points from which to start in the absence of making your own one-custom-car-at-a-time CG height measurements.

Norm
'79 Malibu (too unstock for Street Mod)