Center of Gravity

DPpatrol

Well-Known Member
Posts
269
Reaction
1
OK, this question is probably geared toward the engineers, but I'm sure a few of the fabricators will be able to answer this.

How does one calculate a cars center of gravity? Also how does one calculate the car's "mass centroid axis"? In what I have read so far, much of the varibles which describe and control how the suspension behave are based on these 2 measurements, but so far there has been no discussion of how these are calculated.

jason
 

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
Unless you have software to do it(any good design software does it) or are willing to take use of calculus you can only estimate it. Most of the popel who refer to COG with reference to car design either know how to use the software and get the actual COG as in the material you are reading, design ars using 3D software that does it for them, or are blowing smoke up the arse with estimates or even worse flase assumptions.

Centroid, COM, and COG are synonymous.



An easy example of how to do it on a smaller scale. get an object and hook a string to it and two levels perpindicular to each other now keep moving the string until you find the location where the object has leveled out on both levels, now rotate the object, the string, and levels and do it again this will give you an estimate of the x,y,and z coordinates for the COG.



Kris

"A signature always reveals a man's character -- and sometimes even his name. "
 

Dylan

Well-Known Member
Posts
232
Reaction
0
You can measure it empirically on vehicle scales. You can calculate the for/aft position by the front to rear weight split when the vehicle is level. The height of the CG is really what we want though to do this you take weight measurements front and rear when the vehicle is level and again when one end of the vehicle is elevated. Form the weight transfer due to the attitude change the CG height can be calculated. The weight transfer is not vary drastic so accurate measurements must be taken. In addition the suspension must be locked so that weight transfer does not change the suspension’s ride height. What you now have is the CG for the entire vehicle but what you want is the CG of the sprung mass. From here you can subtract out the individual unsprung masses from some given mass moment reference point (M*D^2)total -(M*D^2)unsprung. I have a work sheet on how to do this somewhere. If I can find it I will scan it and post it. It’s also described in the book by Milliken. I did this process on 3 different vehicles for a 4-link design project that I did at school.

If your designing a vehicle from scratch you may have the X,Y,Z points of your tube chassis. If you don’t have the whole thing in a solid model you can even set up an Excel spread sheet to sum up all the segments of the chassis and approximate the other components as point masses. I think you can even find info on the CG and radius of gyration of components like engines and such that you can put in you spread sheet.

I have even seen a huge pendulum that you could put a whole vehicle on and swing it to get the polar moment of inertia. My friend built a similar apparatus for measuring the moment of bicycles for single track vehicle analysis.
Dylan
 

BradM

Well-Known Member
Posts
618
Reaction
0
Dylan is right. Do it with scales and a little trig. If you don't have a set of scales, many shops will scale a car for a nominal fee. Re-read his post and take note of the parameters he outlined.
 

FABRICATOR

Well-Known Member
Posts
5,147
Reaction
107
There is another high-tech device for locating CG. It is accurate to within about 1.5", and is called a floooor jaaaack. Accuracy can be enhanced with another special tool called a-piece-of-square-or-rectangular-bar-stock. This will bring it down to within about .5".

Roll center in off-road racing is very dynamic in nature and is dependent on what the car is doing at that moment. Traditional (technical) roll center is applicable, at best, only to smooth turns. And even then it is affected by traction. Actual roll center is moving all the time and for all practical purposes has no "center".

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
THe jack only gives 2 dimensions unless I am not seeing it right. Fore and aft dimension and side to side. What about the height dimiension?

Kris

"A signature always reveals a man's character -- and sometimes even his name. "
 

BradM

Well-Known Member
Posts
618
Reaction
0
I may not have been paying attention all this time but I think <font color=yellow>roll center</font color=yellow> and <font color=yellow>center of gravity</font color=yellow> are two totally seperate ideas. LOL! Or were you just trying to induce a state of confusion?

<P ID="edit"><FONT SIZE=-1>Edited by BradM on 04/10/02 11:17 AM (server time).</FONT></P>
 

Waldo

Safehouse
Posts
5,455
Reaction
122
Just go to the LBC and ask the D - O - DOUBLE G and he will give you the 411 on the COG or COM or whatever!

BRAAAAAAAAP!
 

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
WORD

Kris

"A signature always reveals a man's character -- and sometimes even his name. "
 

BradM

Well-Known Member
Posts
618
Reaction
0
Kris - It is a multi step process. You have to first consider the four corner weights on a level surface. That data can be used to locate the CG in two dimensions. Then do the same calculation for an inclined axis. If you raise one end and recalculate the distance to the CG. The change in distance (say from the rear axle to the CG) on an inclined plane, along with the angle of inclination can be used with the tanget function to solve for the height of the CG relative to a level ground plane.
 

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
I understand that I jsut dont understand how simply a jack alone can do 3 dimension

Kris

"A signature always reveals a man's character -- and sometimes even his name. "
 

geoff

Well-Known Member
Posts
356
Reaction
1
Fabricator -- care to elaborate?

how to find the height of the CG with a floor jack? weve got the side to side and front to back idea down pat... thanks

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
Re: Fabricator -- care to elaborate?

Here is an easy formulae I use to calculate CG of a vehicle
1. The front wheels are set on scales and the rear wheels are set at the same height using a spacer.
2. A reading of the weight is taken from the scales
3. The rear axle is raised lifting the vehicle to an exact height
4. A new reading is taken from the scales and the weight difference is calculated

Centre of gravity height = HCOG

Wheelbase distance = dWB

Weight difference = WDIFF

Weight total = WTOTAL


Kris
"Buy American before it's too late"
 

FABRICATOR

Well-Known Member
Posts
5,147
Reaction
107
Re: Fabricator -- care to elaborate?

It simply involves suspending the car from a single point. A single jack can be used to lift one end of a beam placed through the car, and spaning between two benches or stands. This has a chain or other hookup to a cross beam (piece of tubing) attached across the inside of the chassis. The height of the cross beam and where it hangs when the car becomes neutral (no longer adds to or resists tilting) will be the CG point. It is important that the cross beam and all attachments be symetrical. If they are, they will have no effect on the reading. A hoist or forklift is even better, and you won't need the large beam at all.

All fluids must be drained or filled so there is no weight transfer. The result will be within millimeters, depending on how careful you are. Suspending a full bodied car could be a nightmare, but suspending a tube framed vehicle is quick and accurate.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

FABRICATOR

Well-Known Member
Posts
5,147
Reaction
107
Re: Fabricator -- care to elaborate?

The simple floor jack under the car (or hoist from above) will give you "2 dimensions" right away, including a balance plane across the car. The cross tube simply goes on that plane, also across the car. All you need to find is the height.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

geoff

Well-Known Member
Posts
356
Reaction
1
Re: Fabricator -- care to elaborate?

right, im just not following how to go about finding the height, without using corner weights on an incline. thanks.

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

FABRICATOR

Well-Known Member
Posts
5,147
Reaction
107
Re: Fabricator -- care to elaborate?

You should be able to guess the height within 4 to 6 inches. Then place the cross tube through the chassis. One way is to use a couple of flat plates of metal or even wood clamped vertically to each side of the car on two longitudinal frame tubes. Each plate needs a hole in the middle of it for the cross tube. The plates can be placed on the inside or outside, but must be equal in shape and size. They also must be placed symmetrically upon the car so they do not affect the CG. Once the car is raised, you can guestimate at how far off you are by tilting the chassis. If the lift point is high the car will try to stay level. If the lift point is low the car will try to stay tilted. Adjust the plates and try again. A tripod arrangement coming up from the floor is even faster, but you must factor in the weight of the tripod.



<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

Porterrace

Active Member
Posts
26
Reaction
1
Re: Fabricator -- care to elaborate?

This shop tech thread is cool, but let's talk about something that I think is the most important thing in off road racing. Good old fashion common sense. I truly do not mean to make light of any engineering education some of you obviously have, but don't loose sight of the goal. Off raod cars are not Indy cars and thank god they are not nascars, they never see the same direction, hole , speed, or anything more than once. The driver can't even see where to point the car most of the time. 95% of the cars I see in the desert have had tons of money put into them and a whole hell of a lot of hard work. But it just seems a few things can't see the forrest for the trees. My point is every once in while it's o.k. to put down the keyboard, look at something, think of where the stress and impacts are coming from and use your brain, and find a strong way to solve the problem. Just a thought.
 
Top