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Page 39
CENTER OF GRAVITY HEIGHT

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, it 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 method presented here 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. Finally, you could include an "object" with a weight equal to that in the "Weight unaccounted for:" output and place it at a height which you feel appropriate. 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.

All that's required for this spreadsheet is a knowledge of the weights of the major components, a tape measure, and a little time. The final accuracy is dependent upon the proximity of "Percentage of total weight:" to 100.

"object," in the button field directly below, is a major component, such as engine, transmission, wheels and tires, battery, etc. "round tubing" and "rectangular tubing" are, as you might have guessed, the tubing in the car. "sheet" could be a fender, window glass, etc.

The specific weight table lists those pounds per cubic inch values for the materials listed. Obviously, you're free to insert any values you like.

When you make a selection, the circle will not be filled, as you might expect. Don't worry about it.

Also, don't be surprised if an item fails to make a change in the displayed value for center of gravity height. The value is rounded off to the nearest tenth of an inch.


object
round tubing
rectangular tubing
sheet

total weight (including driver) =

COMPONENT:

component weight =

component CG height =

ROUND TUBING:

outer diameter =

wall thickness =

length =

CG (midpoint) height =

material specific weight =

RECTANGULAR TUBING::

outer dimensions: x

wall thickness =

length =

CG (midpoint) height =

material specific weight =

SHEET:

dimensions: x

material thickness =

CG height =

material specific weight =

SPECIFIC WEIGHT TABLE
Material Specific Weight
Steel0.25
Aluminum0.1
Glass0.1
Reinforced Plastic0.03

CENTER OF GRAVITY HEIGHT:

Percentage of total weight:

Weight unaccounted for :

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