How To: Measure your vehicle's Cd without a million-dollar wind tunnel
At highway speeds, your vehicle is fighting an invisible foe trying to hold it back. The nemesis is wind resistance, and it is an automobile's worst enemy. Overcoming the resistance of the air is the key to increasing acceleration, top speed, and improving fuel economy.
Wind resistance is often measured with sophisticated computers in multi-million dollar wind tunnels. The result is usually expressed as a number called the "drag coefficient" (Cd). While having a low drag coefficient is important, the size of the vehicle (expressed as "frontal area") is also important. In general, as drag coefficient and frontal areas decrease, a vehicle becomes easier to push through the wind. Automakers are well aware of this, so they go to great lengths to ensure a low Cd on vehicles designed for high efficiency. Some approximate Cd values for well-known vehicles.
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.25 Honda Insight
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.26 Toyota Prius
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.27 Nissan GT-R
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.34 GMC Acadia
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.36 Bugatti Veyron (spoiler retracted)
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.36 Chevrolet Suburban
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.65 Tractor Trailer (with fairings)
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.75 Formula 1 Car (downforce = high drag)
There are a couple of ways to determine the Cd of your vehicle. If your vehicle is stock, simply look it up on the Internet. If you have modified your car with wider tires, spoilers, roof rack, or changed the ride height, then your Cd has changed. To determine whether your mods are hurting or helping you at the pump (or the track), why not calculate it yourself with a few household items and an Excel spreadsheet? Of course, this requires a bit more effort... but, if you have any geek in your bloodstream, this will really get your juices flowing. The process is too long to detail here, but check out the simple instructions, grab your household items, and let us know what your results are. Thanks for the tip, Farris!
[Source: Instructables]
Reader Comments (Page 1 of 1)
Yar 11:07AM (4/22/2008)
Is this as reliable as the do-it-yourself tracheotomy kit?
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epilonious 1:17PM (4/22/2008)
maybe not as reliable... but definitely less painful and bloody (assuming you don't run into something)
Hank 11:31AM (4/22/2008)
Cool, I'll have to check it out. An interesting tidbit to add to that list is the first generation Mercury Sable, which had a Cd of just .29, making it one of the most aerodynamic production cars in the world in 1986.
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jon 11:34AM (4/22/2008)
sweet! my suburban has the same wind resistance as the veyron!
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Derek 12:19PM (4/22/2008)
Cd is only one part of the wind resistance equation. If you want to compare aero drag between two vehicles, Cd x frontal area is what you want. So, while the two vehicles have en equally efficient shape (equal Cd), the Veyron has much less frontal area and thus less drag.
Lee Gibson 12:10PM (4/22/2008)
Fail. Frontal area * Cd = drag.
Golferal 1:13PM (4/22/2008)
I feel much better about my Tahoe now too!
nezromatron 11:39AM (4/22/2008)
I'm not sure what the value of knowing your Cd is. Besides the Cd is a constant without units and must be multiplied by your frontal area to be of any use.
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Derek 12:20PM (4/22/2008)
The spreadsheet includes Crr as well, I am impressed. Many techniques overlook Crr.
So, you can calculate the change in drag by switching tires/pressures as well as change in aero drag from any body modifications.
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icu812ru469 3:48PM (5/06/2008)
Holy bat smoke, a diY Cd calculator using regular household products. What will they think of next? Hey, why do the manufacturers spend millions testing it with a tunnel then, save that money using these $10 items. Ha ha. I just did a calculation and my new Chrysler T&C has the same Cd as a school bus.
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Derek 12:24PM (4/22/2008)
Being unitless is far from being useless on its own. It is very useful as a comparitive value if someone makes modifications to their car and wants to quantify the change. Rear spoiler, bellypan, changed grille opening, etc.
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JZeke 12:53PM (4/22/2008)
Will the little piggy be helpful in making these calculations?
hehe.
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Matt 1:13PM (4/22/2008)
The Ford Flex will be somewhere between a tractor trailer and a F1 car...
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epilonious 1:20PM (4/22/2008)
I love that the test car was a 1992 Geo Metro and he found it had some awful drag value like 0.35.
Showing once again that the best way to save gas is to buy something light with a small engine.
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MikeW 1:37PM (4/22/2008)
The Veyron has higher drag coefficient than 0.36
http://www.bugatti-configurator.com/content/pdf/Veyron_en.pdf
Don't forget to use Cd not Cx.
Cd is usually 10-11% higher than Cx
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Michael Harley 11:13PM (4/22/2008)
Correct, but when the Veyron drops down to its most aerodynamically "clean" shape (at 220 mph+), its Cd is .36 (it reduces downforce to hit its top speed).
- Mike
Eric 8:11PM (4/22/2008)
My 2000 Passat with a Cd of 0.27 FTW(well, i guess a tie with the GT-R is still pretty respectable)
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Josh 10:44PM (4/22/2008)
"Frontal area * Cd = drag"
That isn't really correct...drag is dependent on many factors.
The drag equation is actually closer to the following: Cd * ((Density of air * velocity squared)/2) * reference area.
Reference area would be the cross-sectional area.
Other factors that need to be considered is the body's inclination to the air flow, how laminar the flow of air is across the body, the air's compressibility, and other aerodynamic qualities. It is incredibly complex to mathematically find the drag of an object, which is why most professionals use wind tunnels.
Josh
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not4no1 4:15PM (4/23/2008)
What do a MINI Cooper & a Suburban have in common?
.36Cd
Size doesn't matter after all.
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