Determining CG height: An easier way

[BillyShope]

I've added a page (Page 39) which provides a relatively painless method for determining center of gravity height. No wheel scales or jacks are needed.
http://www.racetec.cc/shope

[bracketracer]

Can you explain that better????
Maybe add some instructions

[BillyShope]

Can you explain that better????
Maybe add some instructions

Sure. The spreadsheet makes the same sort of calculation the CAD software does as the engineer designs the different components of the car. The differences include a far more precise analysis of the part and its location and the CAD software does it for 3 dimensions, not just one.

To use the spreadsheet, you're going to click on one of those 4 buttons each time you add a part. For instance, you might consider the engine first. You'd put in a value for the engine weight and the approximate engine CG height. You'd then click the top button. You'd then see a value for CG height which is exactly that for the engine alone. You'd also see that this is but a small percentage of the total car weight. There are some other heavy components you're probably going to start with. In each case, you put in the weight and height and then click that top button. The spreadsheet keeps a running account of the change in the car's CG height.

Eventually, you'll move on to the tubing in the roll cage or in the main chassis structure. You'll then be using the second and third buttons.

Finally, the fourth button will be used as you consider the bodywork and window glass.

As you work, you'll notice that the CG height will begin to stabilize. In other words, smaller pieces of sheet metal might not change the calculated CG height as much as a tenth of an inch. Unless, you've missed something heavy enough to make a difference, this would be a good place to stop.

Don't forget to use the proper specific weight and, if you've made a mistake, you must leave the page, come back in, and start again. (You might be able to "erase" a mistake by repeating the individual step with a negative sign in front of the value for specific weight or, in the case of a component, the height.)
http://www.racetec.cc/shope

[bracketracer]

How can you accurately get the weight of the components with out removing them and weighing them individually?? I think it's easier to do some measurements and apply them to a program..

[fat bastard]

Hey guys,

You are talking way over the normal bracket racers head, we are
usually guys that work on cars not design them. I find all of the
posts on suspension confusing, please simplify so the non engineer
can understand them.

There has to be information that can be passed without being an
MIT grad?

I just want to hook and go fast!!!! and Win some races.


Fat Bastard

[BillyShope]

How can you accurately get the weight of the components with out removing them and weighing them individually??

How much accuracy do you need? When it comes to suspension adjustments, I don't believe attempts to achieve accuracy better than plus or minus an inch are justified. Engine and transmission weights are generally available online. Also, it must be remembered that the multiplicity of calculations has the effect of error cancellation. In other words, you might be a bit high with one component and a bit low on another. Calculations involving window glass, tubing, fenders, and such should be quite accurate. In short, the quality of the final result is directly related to the time and effort extended by the user.

I think it's easier to do some measurements and apply them to a program..

Don't understand this. This is exactly what you're doing with the spreadsheet.
http://www.racetec.cc/shope

[bracketracer]

Ok how can you find the weight of lets say a molly chassis??? A steel body that has been cut up to fit big tires?? All lexan for viewing?? Addition of wiring???Etc,Etc?????? Weight of an engine??? say mine has an aluminum pan and another has a steel pan with 4 more quarts of oil than yours...With out a scale there is no way to get it or even within a few pounds with out putting it on a scale...With out a accurate weight I can't see how it will give you an accurate CG..

[BillyShope]

Ok how can you find the weight of lets say a molly chassis??? A steel body that has been cut up to fit big tires?? All lexan for viewing?? Addition of wiring???Etc,Etc?????? Weight of an engine??? say mine has an aluminum pan and another has a steel pan with 4 more quarts of oil than yours...With out a scale there is no way to get it or even within a few pounds with out putting it on a scale...With out a accurate weight I can't see how it will give you an accurate CG..

Here's a "for instance":

Suppose total weight is 3500 pounds and the engine weight is assumed to be 700 pounds with a CG height of 19 inches. The remainder of the car (2800 pounds) is assumed to have a CG height of 20 inches. The total car CG height would be 19.8 inches.

But, suppose the actual engine weight is 800 pounds, with the remainder still at 20 inches CG height. The total car CG height would be 19.77 inches. So, an estimated weight error of 100 pounds resulted in an error of 0.030 inches.

Yes, all the errors could be in one direction, resulting in a very large final error. But, the probability of this occurring is extremely small. How small? Well, suppose you use the spreadsheet to make 20 calculations. The odds of all those lining up in one direction would be 1 in a million. That isn't really fair, of course, since all of those 20 calculations wouldn't involve items weighing as much as the engine, but I've just shown that a considerable error in engine weight doesn't affect the answer all that much.

Again, the result is only as good as the input. If you have Lexan windows instead of glass, you need to input the specific weight for Lexan. It's fairly easy to identify those items which will make the greatest contribution and it logically follows that the greatest care should be taken in assigning values for those items. The "beauty" of the spreadsheet is that it allows you to immediately see the effect of lesser items. When you see that the calculated CG height value remains unchanged with the lighter items, it's apparent that your job is done. Accuracy of plus or minus an inch is certainly achieved without making it your life's work.
http://www.racetec.cc/shope

[BillyShope]

I want to thank you, bracketracer, for your comments, for they've caused me to direct further thought toward the value of my spreadsheet. I'll be adding the following (or something similar) to the instructions:

As you're aware, there are two methods commonly used to determine the car's center of gravity height. One is to measure the camshaft height (pushrod V8 engine is assumed) and assume that to be the same as the CG height for the entire car. The other is to tilt the car forward on its front tires to a specific angle and calculate the CG height, noting the change in front wheel loading between the horizontal and tilted positions.

Surprisingly, the first method will often yield a fairly accurate result. The engine is the heaviest single component and a number of other heavy components have CG heights essentially the same as the tire radius. It just happens to work out that, when the engine is mounted in a "typical" position, the camshaft height is quite close to the car's CG height. But, there are always exceptions. The Ramchargers' first car was a '49 Plymouth powered by (of course) a hemi. The valve covers were pressed right up against the underside of the hood. This is, admittedly, a radical exception, but it illustrates the weakness of the method.

The second method can yield quite accurate and reliable results. There is a certain amount of danger involved, but this problem can be overcome with reasonable caution. If the result is to be accurate, if follows that care must be taken to accurately record the wheel scale readings and to accurately measure the angle of tilt. Also, some equations being used assume front and rear tires to be of the same radius. This might not be a valid assumption. Page 31 has a spreadsheet for this method which allows input of individual (front and rear) tire information.

The tabular method (Page 39) can be considered to be an extension of the first method, with the intent of minimizing the first method's weakness. Instead of considering only the CG height of the engine and instead of assuming a "typical" relationship between engine mounting and wheels, you can expand your investigation as far as you desire.

Perhaps you will include only a few of the heavier components. These might include engine, transmission, driver, battery, and radiator. You might then lump together the weights of suspension components, axle, wheels and tires, and consider them all to have a CG height equal to the tire radius. This might seem crude, but it's a whole lot better than using the engine camshaft height alone, which wasn't all that bad an assumption in the first place.

If, on the other hand, you're working with a sprint car, you might want to carry the process much, much further, taking into account every piece of tubing and every piece of sheet metal. This is at least possible with a sprint car, but virtually impossible with a production unibody car.

Thanks again, bracketracer.
http://www.racetec.cc/shope

[bracketracer]

Billy, Im still having a problem with this as I just can't figure out how to do it ......It's just confusing ....Anyone else work this program????

[BillyShope]

Okay, here's a "for instance":

The numbers I'll be using have absolutely no relationship to any car, living or dead.

We'll keep the total weight at the default value of 3500 pounds. The engine is estimated to weigh 700 pounds and its CG height is estimated to be 19. (This is NOT the camshaft height, but an estimate of the actual CG height, taking into consideration the steel crank, aluminum heads and manifolding, accessories, etc.) After these numbers are input into the proper boxes and the "object" button is clicked, the CG height calculated is, of course, 19 inches and the remaining weight is shown to be 2800 pounds.

We'll assume the suspension components and wheels and tires weigh 500 pounds and have a CG height of 14 inches (the tire radius). After these numbers are input into the proper boxes and the "object" button is clicked, the CG height is calculated to be 16.9 inches and the remaining weight is 2300 pounds.

That remaining weight is assumed to have a CG height of 22 inches. After these numbers are input into the proper boxes and the "object" button is clicked, the CG height of the total car is calculated to be 20.3 inches.

Obviously, you're going to do a better job than this. You'll be doing everything possible to minimize the effect of that final "guesstimate" involving the remaining weight. So, you'll be taking into consideration the radiator, fuel tank, battery, frame rails, and...don't forget...the driver and you might as well take some measurements of sheet metal and window glass. As a simplification, you might want to consider the doors as "objects."

In short, do what you can to drive that 2300 pound figure closer to zero.
http://www.racetec.cc/shope