During the course of instructing my High Performance/Racing Chassis class at the local community college, I frequently use my ERA Cobra as a project car. Some projects include bump-steer checks, cross weights and determing cg location. The laser Hunter alignment rack and Longacre digital scales make these projects much easier than using a carpenters square, level, plumb bobs and chalk lines.
Checking cg height is the most difficult simply because the front of the car must be jacked up and supported on its wheels with scales underneath. Fun.
Here's what we found about my 427 S/C ERA with 1/4 fuel load. (To be really accurate the tank should have been full or empty):
Weight distribution was 1384/1322 lb for a 51/49 front-to-rear weight distribution. With me behind the wheel, weight distribution changed to 49/51. Fill the tank and weight on the rear wheels goes up further and weight on the fronts goes down because of the cantilever effect.
About where the cg is: With a 1/4 tank of fuel, it's 44.2 inches behind the front wheels on a 90.5 inch wheelbase.
Now for the fun part--finding cg height: The car must be jacked up, the suspension restricted from moving, and all four corners weighed to get the numbers. Then a fairly complicated formula must be used. No easy task, but we did it.
Cg height came to be 18.62 inches above the ground with an empty car except for about 5 gallons of fuel in the tank. All things considered, not bad for a car with an FE engine. How do you get cg lower? One answer: Lower the car. Not easy considered the already restricted ground clearance. An all-aluminum engine would help as would relocating the battery lower, then to the rear while you're at it.
Oh, the ERA has really good bump steer. This is noticeable while driving over undulating or rought surfaces either in a turn or on a straight. Likewise, it eliminates the nasty transitional stuff such as while entering a turn. No steering-wheel corrections are needed.
Now that you have this information, what do you do with it?
ERA cg location--or who cares?
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