Anyone know the location of the CG in third gens (preferably hard or T-top)

I'm looking for distance from the front and rear wheels, and height from the ground.

thanks

 

Find a weigh scale. Measure the weight on the front tires. Measure the weight on the rear tires. A simple calculation will tell you where the center is between the front and rear axle. As for the height, normally it's calculated at the camshaft height. There's no way to accurately measure the CG height.

You have a 101" wheel base. If you have 1800 pounds on the front and 1600 pounds on the rear, the car then weighs 3400 pounds and has a weight ratio of 53/47. So 53% of the vehicles weight is on the front tires. 53% of the distance back from the front tires is very close to the CG. 53% of 101 is 53.5". So 53.5" back from the front wheels or 47.5" forward of the rear wheels would be the front to back center of gravity.

My car is nose heavy with a 60/40 ratio. With 1800 on the nose and 1200 on the rear. My wheelbase is 102" since my diff is moved back slightly. My CG is 61-1/4" back from the front wheels or 40-3/4" forward of the rear wheels.

 

Unfortunately that aproach requires that I currently own a car lol.

Thanks for the numbers, that'll help.

I'm doing some vehicle dynamics simulation and still need a CG Height, any wild Guesses (the less wild the better)

 

That's not correct...

Put 99% in place of 53%, and it'll be pretty obvious what the problem is.

You can't really calculate the CG for an object of non-uniform density, when you don't know the exact mass distribution.

 

You can calculate the CG position front to back knowing that the sum of the moments about the front or rear wheels is 0

For the case given with
wheelbase= 101"
Front weight= 1800
Rear weight= 1600
Overall weight 3400

let x be the horizontal distance of between the CG and front wheels.

0= 101"(1600lbs) - X (3400lbs)
x=47.5"

So the CG will be 47" rearward of the front wheels. (at some unknown height)

Unfortunately without the height (can also be calculated, but not with the values we have) I won't be able to sim a camaro properly.

 

Using the above example, mass distribution is 53/47. It's always calculated when the vehicle is at rest.

At 99% then most of the vehicle's weight is on one axle. The only way this will happen is if you pull the front wheels off the ground high enough that all the vehicle's weight is on the rear tires. The vehicle's CG is basically static but can move around as the vehicle moves dramatically. Under a hard launch, the front end rises. Less weight on the front wheels means more weight on the rear. This means the CG moves backwards. Once a vehicle starts moving, there's less torque being felt and the CG will move forward again.

When calculating 4-link setups, you need a bunch of different weights. As I've mentioned above, front and rear axle weight is close enough to calculate a front to rear CG. Density has nothing to do with it. CG means a neutral distance between an object that has a different weight at either end. Take a yardstick and tape a weight on one end. The CG now isn't at 1-1/2'. To properly set up a 4-link, you must also know how much unsprung weight the vehicle has. A car's CG is normally very low. That's why camshaft height is normally used as the height.


I used some 4-link software to calculate my ladder bar adjustments. For a CG height, I use my camshaft centerline of 18.5" but I've also lowered my engine. I have no idea what the cam height is in a SBC equipped stock third gen. For chassis setup, you need to find out where the suspension instant center is in relation to the anti-squat line. Different setups react differently to different track conditions.

 

18.5" it is.

Simulating only for the purpose of getting some marks in my Vehicle Dynamics class, so that'll do fine.

Thanks

 

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

 

I would think the car would weight less then 3400. Because it still measure some of the weight up front doesnt it?

Maybe I'm off, I'll just watch n read.

 

I only used those numbers as an example. I know the front weight is close to 1800 pounds and even though I have a BBC, there's enough weight stripped off the front of my car to still be around 1800 pounds. That's why I use V6 springs. 3400 pounds is a rough estimate of a street legal car so the rear weight is just a calculation. Since too many factors can change the weight, only driving across a scale will give you accurate numbers. An empty 18 gallon fuel tank is 25 pounds. Fill it right up and it's about 130 pounds. Power option door are very heavy compared to manual doors. Depending on the model and what options are in it, the weight can be moved around quite a bit. An Iron Duke or V6 engine won't have as much weight on the nose as a V8 will.

Good formulas on that page. I bookmarked it for further reading.

 

stephen, I'm not considering the engine..

What I'm saying is if you weight the rear tires alone, then the front tires alone. Add them up, IMO it would add up to more then if you weight the car as a whole.

It partly reads the weight off the other half of the car, even though its not on the scale.

I dont know how to really say what I'm thinking..

 

But, then that weight won't be weighted when the other tires are on the scales, and vise versa.

Think about corner-weighting a car, you jack the front right up, it gains weight on the scale, but the left rear loses. Even though you have moved what wheel is supporting the weight, the total is still the same. You have just redistrubuted it.

 

Weighing each axle is how it's done. I drive my car onto a truck scale every spring. Drive the front axle on, get a weight. Drive the whole car on, get a weight, drive off until just the rear axle is one, get a weight. The front and rear weight is eual to the total weight. It really depends how far you drive on. I drive the car on until the front wheels just get on the scale. When I drive off, I stop just before the rear wheels come off. This prevents any serious overlap in weight and seems to work fine.

My last weigh slip reads:
1785
2976
1190

That's front, total, rear. Total weight is only off by a pound. At the bottom of the slip is a total which adds up all the weights and says 5952 so the extra pound is somewhere in the front or rear but I'm not worried about 1 pound. I have a total weight plus a front and rear weight which equals the total. I can then calculate the front/rear ratio.

That's how tractor trailer trucks have done it for years. They don't make a scale big enough to put the entire truck on so they weigh axles or axle groups at a time. They're allowed so much weight on each axle but also have a maximum GVW allowed. Adding up all the axle weights gets a GVW.

Corner weighing a car is the best way but very few people have access to corner scales and drive on scales make it hard to do. When you figure a driver estimated at 160 pounds is on the left side of the car that it's going to be heavier on that side however the engine/tranny is slightly offset to the passenger side. They don't sit directly in the center of the frame.

A 50/50 weight ratio is always a goal to aim for but some cars make that hard to do. I could improve the weight ratio by adding weight to the rear but adding weight is a step backwards. I always want the car lighter.

 

Unless you have errors in your scale, the vehical isn't level, or you have changed something about the vehicle (ie moved luggage, left a door open while weighing the fronts, and closed it for the rears etc.) the front and rear weights (or the 4 corner wights) will always add up to total vehical weight.

 

ok, so my thoughts are wrong. Carry on.

 

ever heard of relocation barckets, adjustable lca's, or an adjustable torque arm? If you're doing any "performance" driving be it corners or straight line you should have the aftermarket suspension parts...

 

What does that have to do with the discussion of the location of the CG on our cars? Which will change per car, each car is going to be slightly diffrent.

 

doesn't relate to the actual center of gravity, just answering the question on how to reset your suspension according to your CG.

- actually, who ever had put that in edited it out. - Some one had put in that knowing these numbers is great when designing a car, but means nothing on an actual car because you have no way to change it(not exact words) so that was my response...

- sorry to have been off topic -