dcn@ihuxl.UUCP (Dave Newkirk) (03/02/84)
The drag coefficient is the total aerodynamic drag of a car divided by its frontal surface area. It shows how well a car is designed to reduce its air resistance independent of its size. Some cars that look very sleek have poor drag numbers, such as the Porsche 928 (~.45) and the old Chevy Corvette. Good drag coefficients for new cars are between .35 and .4, and some are even lower. Graphs that depict all the sources of resistance to forward motion in a car clearly show that air resistance becomes important at 40 mph and increases rapidly with speed. A low drag number also means reduced wind noise and better fuel economy at highway speeds (as well as higher top speeds!). But don't place too much importance on the drag coefficient - it's only one way to improve performance. Dave Newkirk, ihnp4!ihuxl!dcn
rmiller@ccvaxa.UUCP (03/07/84)
#R:ihuxl:-94700:ccvaxa:4900037:000:861 ccvaxa!rmiller Mar 5 18:13:00 1984 pity, the poor 928 and a drag coefficient of .45! probably those damn exposed headlights! (mind you, my aerodynamics were learned from sailplane pilots, where sticking just your fingers out the little canopy side window added 20-50% to the aero drag on the aircraft!) i really wonder how rain gutters, door and window fittings (especially the windshield!), and unsealed panel openings such as hood and trunk lids affect the cd of a car, anyone got any ideas? and what games do the manufacturers play for advertising? i still find a .35 for the new thunderbird a bit hard to believe. another spot (and probably the one that got the late 70's vettes) is the rear window and frame of a notchback styled car. those vettes and the fiero would pay dearly for that, while the rx-7 and the new vette get a .34 cd with a smoother rear quarter. uiucdcs!ccvaxa!rmiller
seifert@ihuxl.UUCP (D.A. Seifert) (03/07/84)
The 928 was designed before Cd became a big advertising
game. *Maybe* (anybody *know* ?) the designers traded
off some drag for downforce. This is definitly the case
in the M1. BMW went for the most stable design they
could get, and sacrificed some drag to get it.
Anyone who thinks drag is more important than stability
is invited to drive a VW Beetle (or better yet, the bus)
on a windy day.
Yes raingutters, body seams, etc. count, at least a little
bit. Look at the Audi 5000 with it's super-flush side
windows (they had to use a sunroof type mechanism to
lower them), and various other tricks. Whether fixing
up little tiny details is worth the extra cost is another
question. Another problem which crops up as we approach
super-aero designs is providing sufficent cooling for
engine, brakes, exhaust, etc.
Yes, low-drag is a good thing. The designers just have to
be careful not to throw away something more important
to get it.
And yes, there are 'tricks' to get a low number for advertising.
Lower the car, remove mirrors, antennas, etc. GM did a *lot*
of this to get the numbers they did for the new Firebird a
couple years ago. Not all windtunnels give identical figures,
either. The car mags had articles on all this about the time
the new Firebird came out.
keep the shiny side up,
--
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