How is the drag coefficient measured?
05-19-2006, 04:17 PM
#1
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How is the drag coefficient measured?
This might be an odd question, but was the .34 CD number measured with the headlights up? I've concluded that CD numbers make no sense... the 928 S4+ with its incredibly sleek body and smooth lines has a .34, while my friend's brick-like BMW has a .31. What's the deal with that?
05-19-2006, 04:28 PM
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Like I continue to say aerodynamics are NOT intuative and often long sleek lines do NOT work as well as more box like ones where you take the hit all at once and get it over with so many things affect CD and even that is meanlingless until you factor in frontal area. A 747 might have a CD lower than a 928 !
05-19-2006, 04:36 PM
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Micheal;
Most people think of cd as air hitting the car, it is also a function of air leaving the car. i.e. think tadpole shape for an idea.
Mercedes just built a car based on a fishes shape, they found it had a lower cd than their SLR.
Most people think of cd as air hitting the car, it is also a function of air leaving the car. i.e. think tadpole shape for an idea.
Mercedes just built a car based on a fishes shape, they found it had a lower cd than their SLR.
05-19-2006, 04:38 PM
#6
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Looks can be deceiving. The Concord is sleek, but was designed for compressible, supersonic flow at the nose. All subsonic jets will have a blunt nose and would seem to be less aerodynamic - but are more efficient.
Another example is the 911. It would be intuitive to believe the spoiler-less 911 would be more aerdynamic, but vehicles with the more blunt shaped rears (e.g., most sedans) are more efficient. And more stable in yaw.
Another example is the 911. It would be intuitive to believe the spoiler-less 911 would be more aerdynamic, but vehicles with the more blunt shaped rears (e.g., most sedans) are more efficient. And more stable in yaw.
Last edited by tomcat; 05-19-2006 at 06:04 PM.
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05-19-2006, 04:58 PM
#9
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Originally Posted by mspiegle
This might be an odd question, but was the .34 CD number measured with the headlights up?
For comparison, the Corvette C6 Coupe has a CD of .29; the C5 Z06 .31.
05-19-2006, 05:05 PM
#10
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Somebody should test a Kamm tail design on a 928. According to Kamm's theories the 928 tail is all wrong. When Peter Brock applied the Kamm tail design to the Cobra Daytona Coupe, he found that top speed and fuel economy went up about 25% IIRC.
Along the same lines, others have said if we turned the 928 around it would be much more aerodynamic, consistent with Kamm.
Along the same lines, others have said if we turned the 928 around it would be much more aerodynamic, consistent with Kamm.
Last edited by Bill Ball; 05-19-2006 at 05:26 PM.
05-19-2006, 05:07 PM
#11
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Bill that's essentially a 911 duck tail, and essentially therefore, an "S" trim tail for the 928 ... riiiight? 
... http://www.astonmartins.com/np6c/db6.htm
... http://www.astonmartins.com/np6c/db6.htm
05-19-2006, 05:16 PM
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Remember the Nordstadt Golf - the Rabbit that was built on a 928 chassis? Well, it was faster than a standard 928.
The 928s problem has always been its beautiful rear end. Not good for aerodynamics.
When they did the wind tunnel testing, they put an early 928 in backwards, and it had much lower drag coefficient. However, it also hat a lot of lift... which our cars don't have.
The 928s problem has always been its beautiful rear end. Not good for aerodynamics.
When they did the wind tunnel testing, they put an early 928 in backwards, and it had much lower drag coefficient. However, it also hat a lot of lift... which our cars don't have.
05-19-2006, 05:33 PM
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Originally Posted by heinrich
Bill that's essentially a 911 duck tail, and essentially therefore, an "S" trim tail for the 928 ... riiiight? ... http://www.astonmartins.com/np6c/db6.htm
... http://www.astonmartins.com/np6c/db6.htmYep, the DB6 is a good example. Many cars from Alfas, Ferraris, NSU, have observed Kamm's ideas in tail design. The Daytona Coupe:
05-19-2006, 05:34 PM
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Heinrich the best example of Kamm back is a STATION WAGON ! Which is one reason why the Southern California Timing Association the boys who do land speed record runs on dry lakes do NOT ALLOW any station wagons, it makes the coupes look bad !
