Center of Gravity Calculations

[sirhk100]

Hopefully someone can possibly explain the outcome I've had or disprove it cause to me it doesn't make sense. We're doing center of gravity studies on vehicles here at my work comparing stock to lifted. Obviously a lifted vehicle is going to have a higher center of gravity. We're using scales and lifting a vehicle and doing some calculations as described in the following webpage.

http://www.longacreracing.com/articles/art.asp?ARTID=22

Here's a situation... Let's say you bolt on a generic 6" lift kit to a F/S chevy 4wd, Fabtech, RCD, Rancho, whatever... Throw on some 35" tires. This is just a scenario. Let's say that truck is now sitting 9" taller overall, 6" from the suspension and 3" gained on the tires. Is it possible that the COG can be raised more the 9" in this scenario? We're getting numbers like 11.5" higher then stock on a vehicle sitting 9" higher then stock. This doesn't make any sense to me. Is it error in our calculations or procedure or is that right? If that could be right please explain how the COG increase could be greater then the height lifted cause I don't understand that.

Also, same procedure doing a body lift has yielded that the COG has actually gone lower on the vehicle!!!! Is that at all possible??????

Thanks,
Khris

[67BAJA]

I did this calculation for a project for school. To test my methodology, I used a brick and some wood for a model. I ran the calculations then tested the brick, and it came out as expected. Maybe doing a model will give you the answer, I can look and see what equations I used, I just need to dig them up.

[prerunner1499]

Hmmm funny thing physics is aint it.
Im thinking in simple terms here, and not getting all Newtonian on y'all.
The COG is not a constant figure that is in dirrect relation to the mass moving upward.
The COD will change faster (more) the higher you go. This I believe is gue to the force of gravity getting higher as you move higher in the atmosphere. Kind of a compounding effect if you will.
It will not be a constant ratio of change the higher you go. I think FOG is 9.86 or someting like that, college was too long ago and its not like I use physics every day.

Now with the body lift only, you are moving the "vehicle higher" but not really the COG because the COG is actually "lower" now on the vehicle that it was before. This is due to the mass that is still the same position as it was before, your not moving the ENTIRE mass upward, just part of it, so your COG figures will be a little askew.
Basically unless you move the entire mass upward, you wont change the COG much.

[ntsqd]

Khris,
Are you locking out the suspension somehow? If not, your measurements are in vain. Even if you are, I've been told that getting that method to yeild repeateable results can be tricky. Maybe tilting in all four directions and taking the average CoG height?

[sirhk100]

LOL, Thom, I was actually hoping you'd be one of the ones to chime in on this... You seem to always know your stuff!!!

Yes we are bolting an adjustable rod that can be clamped solid in place of the shocks.

LOL, I totallly know what you mean about college being too long ago. Heck, I almost decided to major in physics but what was I going to do with that degree?

I didn't expect the COG to go up 9" with a 9" increase in height but I didn't expect it to be greater then 9" I expected it to be somewhat lower then 9" because the Diffs and some other heavy mass items with these style of lifts are only going up the added height the tires add. That and the added weight of crossmembers is WAY low on the vehicle. I was expecting something like 8" up.

We're talking a plane here though, not the actual point. We're just doing front to back tilting, nothing side to side. All these COG measurements are describing a horizontal plane XX"s above the ground, not in relation to a point on the vehicle.

I could see how the COG would be lower with the body lift in relation to where the passengers are sitting but in relation to a plane in space measured off the ground I would expect it to rise somewhat because the body and mass associated with it is rising. Sure the major mass of the vehicle is staying stock height but the body does have some weight to it...

Khris

[McClintock]

And throw some passengers in the vehicle when the truck is body lifted, and the COG gets even higher. Nice discussion!!

[prerunner1499]

This is exactly why I always referred to Physics and Phylisophy as "brain masturbation" LOL

I did not assume you were measuring in planes? Duh! silly me,,though that will make things a bit different.
If you measure the side to side plane COG differences I think you will find a more desired/expected result.
The side/side COG plane is smaller in relation to the relative change you are making when raising said plane. The front to back COG should in effect change in a lesser amount until you get so high that the % change or height is more of a component of the area plane itself. Did that make sense??? Hmm I guess so.

[WannaB-class5]

First of all you are finding CGH not CG and commen sense is harder to apply. Also they are using the most simple method which often means least accurate. I think your number are accurate and CGH is what is throwing you off. If you think about it, lifting a truck then putting it at an angle like that, more weight will be moving down tword the fornt. Think of a stock truck going down a very steep drop off, then think of one with say 8" of lift, then picture a disco 22" lifted F-250. The 250 will roll where the others may not (I know weight distribution is a factor....just trying to help visualize).

No sure if you are an equation guy but looking at it, if you get Tan-theta (the angle) wrong your numbers will be off. I know you dont find the angle, you calculate it but this becomes a big issue if you dont do what they asked with the tire pressure. Even at full psi it may flex throwing the angle off. I would recomend only lifting the minium of 10" since that will transfer the least weight to the front OR doing the whole thing at many angles/heights. This is prob not your problem.

BTW I am still in college so its pretty fresh but I could be totally wrong. As we all know "In theory" doesn't mean "in practice." I really think its that you are testing CGH but thinking in terms of CG.

[sirhk100]

Yeah, my mistake... It's the center of gravity height we're after. We've done it with the vehicle at 10" and also at 20". We can't really go any higher because of lack of equipment. We've already had a bad rack experience in the past so we're not using it to lift the truck up to 45 degrees...

Am I totally off... We're thinking that raising the front or rear and doing the math is giving us a flat plane a certain height off the ground. Could you elaborate on why you think a side to side tilt would be more beneficial. Lamens terms here if possible?

WannaB, I follow you on the formulas... We've been pretty thurough with the truck setup.


1497, thanks for the enlightenment... Mind blowing stuff you have to add there. LOL, if people only knew the amount of body lifts sold and the $$$ made on those things they probably wouldn't laugh. I know I was SHOCKED!!!!

[Kritter]

throw in side wall flex for 35s as well as that you are lifting the front more then you are lifting the rear usually since the the trucks come raked from the factory and you will have some margin of error.

[Bryan_D]

We've already had a bad rack experience in the past so we're not using it to lift the truck up to 45 degrees...



Do tell!! Ok maybe not

[ntsqd]

I'm thinking the bigger the angle, the less error in the trig part of the calcs. BUT that screws up the tires.
What about replacing the wheels and tires? Most of us have seen those stands made for show car guys that bolt on in place of the wheel. Could build 4 of them, and design them to hold iron weights so that they weigh the same as the wheel/tire combo they replace.

What other method could be used to determine this? Other than weighing each part and measuring it's distance from the ground.

[ChuckH]

hmmm lets see what Nasa has to say about this, even though i was told they don't know what their taking about here :p yes a PHD at NASA said dampening.

a lot depends on where the main Mass is, you would need to weigh about everything and figure the CG out of each of them...or you could tip it up on its side and see where it tips over at, stock and jacked up all flat biller like :D

http://http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html

[sirhk100]

LOL, don't tempt us on the tipping thing!!! We've managed to do it once on a rack... Hint hint "bryan..." Gotta love insurance!!!

[ntsqd]

Phd = Piled higher & Deeper, btw the Phd who wrote my college text on Measurement & Insturmentation calls them "damping" functions. So the moral is that there's no agreement there either. I'll continue to call it "damperening". :)

Why does microsoft hijack your link? Does for this machine anyway. Can you post it text-only?

[FABRICATOR]

http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html

I always thought that stood for post hole digger cause I have a real one of those.

Hmmm, haven't crunched this yet but the CGH cannot raise more than the actual height increase.

This was discussed in depth a while ago. It became clear that finding it out physically by balance was about as easy and almost fool proof.

From the NASA site:
"For a general shaped object, there is a simple mechanical way to determine the center of gravity:


If we just balance the object using a string or an edge, the point at which the object is balanced is the center of gravity. (Just like balancing a pencil on your finger!)

Another, more complicated way, is a two step method shown on the slide. In Step 1, you hang the object from any point and you drop a weighted string from the same point. Draw a line on the object along the string. For Step 2, repeat the procedure from another point on the object You now have two lines drawn on the object which intersect. The center of gravity is the point where the lines intersect. This procedure works well for irregularly shaped objects that are hard to balance."

[Gordon]

If you only have the truck jacked up 20 inches the scales need to be very acurate. You might be OK if you are using race car scales, but if you are using a truck scale, your measurments will be pretty inconsistant.

[Stephen]

The couple of times I did this we had to raise the truck up a long way to make the numbers anywhere near accurate and even at that small variations in many variables made a big difference in the final result. You can do it, but you'll have to be careful and it might be a good time to refresh your statistics and find out how close your final number is to the truth.

[Kritter]

Chuck, I love when people call damping...dampening...you already know why!

[Kritter]

[QUOTE=ntsqd]Phd = Piled higher & Deeper, btw the Phd who wrote my college text on Measurement & Insturmentation calls them "damping" functions. QUOTE]

Thats because college professors usually are more well versed then private sector scientists...but at the same time the professors who cant hack the profession of being a professor end up working for govenrment agencies such as Nasa...go figure.

I guess every dynamometer manufacturer for shocks is wrong too as well as every MAJOR manufacturer of shocks...

Sales and marketing is why we have damping referred to as dampening...when you dont know what you are talking about...dampening sounds right! And...if you read it on the internet it must be true!

[ChuckH]

Looks like it was a good day to catch a whopper :D

http://www.coyabajamaica.com/photos/foodfish.jpg

Those who can, do. those who cant, teach :p :D

[sirhk100]

Yes, we're using race car scales doing it here in the shop...

[ntsqd]

Something to ponder: http://www.jeepaholics.com/tech/cog/
I've not read thru it, no idea if it's any help. It's written for jeep drivers, so it's likely very basic but maybe there's a nugget of value in there somewhere.

[toddz]

Yes, we're using race car scales doing it here in the shop...

Wow-all this high tech stuff going on in Chino Valley! :-). Glad to see it.

Todd Z.

[sirhk100]

LOL, yeah, we gotta push the horses outta the way to get it done but occasionally it works!!! YEEHAW!!! You should see what we can kill and cook with that $1.5mil. lazer cutter we gots up here!!!!