Center of Gravity "cg height"

[scottsee]

hello all. i was going to post this in the suspention section, but i figured this is more a general question. has anyone run across (for the stock 2g gsx) the Center of Gravity Height or "cg height" for short? i bought a book called Auto Math, alot of suspention and weight distribution equations use the cg height. so i was hoping somone might know the answer to this one becasue its not exactly easy to find. heres a link to the equation.

http://www.racerpartswholesale.com/longtech6.htm

thanks everyone.

 

[Defiant]

Wow. I don't even know where you'd go to find it. Road & Track don't show it on their road test, and they're about as technical as anyone around (although they aren't as much so since they dropped Tapley data from their tests). I also have the factory Press Release books, tech training books, and New Release books, and I'll be damned if I can find mention of it in there.
Which leaves you with a wild-assed guess. Unless one of the guys who ralleys or autocrosses knows.

 

[scottsee]

yeah, its somthing i've never heard about before. but now that i know about it, i feel its as important as knowing your spring rates. not veary common indeed.

 

[tstkl]

first you will need 4 scales, a lot of wood, and some weightless material that can support your car on its side

put the for scales under the board, and put the car on top of the board

move the scales around until they are all equal, this will take forever, because its close to impossible to find the right spot, i know it took me a long time...

now after that, find the center of where the scales were, thats the z and x component of the center of gravity of your car, now here comes the tricky part....

using that weightless material that can support your car on its side, balance your car on its side and put it so the flat side of the material is down..... now using two scales move them around so taht the weights are the same, and make sure your staying on the correct x plane as the one found in the first steps.... now once you have them balenced, you have your center of gravity height, and from the eariler step, you have your full center of gravity

if it helps at all, (even tho im a 1g) my measurements from the front drivers side farthest point was 20ft, 2 ft, 7 ft, for HxWxD


seriously, thats the easiest way i can think of, and im guessing you dont have a weightless material that can support over 3000 lbs if you have a 2g awd....

 

[Defiant]

Actually, if it were possible to put the car on its side, all you'd need to do is put two fulcrums under it and slowly move them together. Gravity would demonstrate the CG of its own accord.
Test this with a yardstick- put your two fists out in front of you, extend your index fingers, lay the yardstick across them and bring your hands slowly together. The yardstick will self-level, and will show you the CG of the stick.

Which does shit-all for figuring it on the car.

I don't know, I suppose figuring it's a unibody -no low frame rails- and how the engine/transmission are laid out, maybe around 18-20". And on a GSX, the driveline might drop it just a bit below that.

 

[tstkl]

good luck moving two fulcrums near each other, even if you used something with wheels on the bottom and top (non ball bearing or something with a somewhat large amount of friction, just less compaired to a flat surface with 3000 lbs on it rubbing on the ground), theres still the problem of the car on its side...

maybe if you made something with adjustable heights, it would work, either way turning the car on its side is close to impossible and just plain stupid, if it falls, even if its not on something or someone,.... have fun paying for a new roof, or if your lucky, maybe youll just f*ck up the axles....

 

[Defiant]

"if it were possible"
Which it isn't, but thank you for pointing that out to anyone who might actually be more dim than you, and still -somehow- remembers to keep taking consecutive breaths.

 

[Enki]

Here's what I would try first:

Buy 8 mechanical bathroom scales (most I've seen go up to 350 lbs). Next cut 2 lenths of wood that are each as wide as 4 of the scales. Next cut 2 lengths of wood that are as long as your car, wide enough to fit the tires on, and equal in height to the short boards + the height of the the scales.

Drive your car up onto the 2 long boards, then far enough forward that the 2 front wheels are on the scales. The weight should be distributed over all 8 scales by the boards on top. Since each scale is made to hold ~350 lbs they should be able to hold the weight without being crushed as long as less than 1400 lbs is going to be on each wheel.

Measure the weight of the front. Jack up the back about a foot, measure again. Subtract out the weight of the crossboards and there you go. Then return the scales (if they're not crushed) for a full refund.

Sorry if this sounds stupid and won't work but it's almost 5 am, I'm on a caffeine binge, and I'm a dumb noob.

 

[wret]

Mods, move this one to Handling Tech. A question as good as this one deserves some of the numbers guys over there to comment on it (not me, I failed algebra).

 

[L2RTSiAWD]

Moved to handling tech.

 

[terefic181]

You can find one answer to locating the "center of gravity height" of a vehicle in "RACE CAR Engineering & Mechanics" by Paul Van Valkenburgh on page #131.

It entails placing scales under the rear wheels and raising the front of the car with say an overhead hoist. You then plug the number into the formula.

To perform this, you will need, a flat surface to perform it on, to remove all fluids from the vehicle, a scale device, a hoist or other lifting mechanism, (W) the total vehicle weight, (L) wheel base length in feet, (X) raised height of the front wheels in feet.

H= height of C.G. above the rear axle centerline in feet.
AW= the increase in rear scale weight.

If you raised the vehicle to the point that the full weight is reflected on the scales, then that is the CG height. But you don't need to raise it all the way just to a comfortable level and take the measurements from the scales and height raised and plug it into the formula. The higher you raise the vehicle the more accuracy for the number, but the more un-nerving it might be.

I haven't done this to my car as yet, but had looked into it some time back.
As I have said a number of times before, I'm no engineer but I'm sure there are a number of them on this board that can better explain this exercise.

My drawing and the formula are below.

 

[jtmcinder]

To do more serious modeling, you actually need the CG for the front half of the car and the CG for rear half. (Actually, you want the CGs between the two pairs of wheels, but you get the idea.) There is one person on this list that claims to have these numbers and even promised to post them about four months ago. Too bad he has been playing "I've got a secret" and won't follow through with his promise.

When I get the other computer back from my wife, I'll post the values that I have been using. These are probably only within an inch of being correct, but it will allow you to move on.

- Jtoby

 

[terefic181]

To do more serious modeling, you actually need the CG for the front half of the car and the CG for rear half.
- Jtoby

I know I've seen additional material in my track set up books somewhere, but I didn't find any of them this morning. I'm pretty sure I've seen instuctions on getting front to rear and side to side CG pin pointing. I'll post these if and when I find them.

These are probably only within an inch of being correct, but it will allow you to move on.

Any of these numbers should change a bit per different vehicles because of modifications gutting etc., but as a reference any decent number should be a great starting point.

Tom

 

[scottsee]

how to find the CG height of a car is in the Hyperlink at the bottem of my first post. .
Secrets are retarted when it comes to DSM's. ban his ass. if i dont find out what the stock 2g gsx ch height is by the end of June i'll head down to a local hot rod shop and pay them to use their scales to figure it out. i'll probably just end up doing it monday. i dont like to wait. if its more then a hundred buck i might change my mind.

 

[terefic181]

scottsee > how to find the CG height of a car is in the Hyperlink at the bottem of my first post. .

DANNNNNNNG!!!

I wished I'd seen that. It would have sure saved me some time today. From the rest of the following post, I thought the formula was what you were looking for.



Tom

 

[jtmcinder]

I've been using 20" for the sprung weight and 12" for the unsprung. But these were estimated, not meaured.

- Jtoby