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Aerodynamic Comparison (was something else)
In a message dated 96-08-08 12:28:48 EDT, somebody writes:
<< Audi 5KCSTQ .30cd
Audi 5K .29cd
Volvo .34cd >>
Several years ago I read a series of SAE papers that were written by a group
of European engineers; each worked for a different car company or developer.
The thrust of the papers was a round-robin test of European wind tunnels,
using a range of cars. The drill was to test a number of the same cars in
each of the wind tunnels and look for correlation.
You guessed it. There was only poor correlation. That kind of takes the
wind out of aerodynamic claims or comparisons among different manufacturers.
Some of the wind tunnels tested consistently low, some were high and some
It's also important to know that the coefficient of drag (Cd) numbers you are
likely to find in normal consumer literature are "Marketing numbers". Nobody
ever reveals if the advertised number was done on a genuine production car,
just as it leaves the line. Some manufacturers close off the air intake
openings, tape the body seams, remove windshield wipers, use the narrowest
tires ... There are so many different ways to test, and so many variables in
testing and between facilities, that a comparison is worse than meaningless;
it is misleading. The four cars listed above are like comparing apples to
oranges to pears to peaches.
Finally, the coefficient of drag is only a number, it is dimensionless. To
really discuss aerodynamics, you need to include frontal area (A). Total
drag = Cd x A. And you really have to standardize for air density and speed.
Anything else is just falling into an endless morass of meaningless