Drag coefficeints

[Takid455]

Wondering what production cars have for Cd values. Looking for the drag coef. for a 1978 trans am in particular. any input would be appriciative. also feel free to list other makes and modles as well for comparison.

[David Pozzi]

Here is a neat page for measuring aero drag. It even has a downloadable spreadsheet in Excel.
http://www.geocities.com/plymouthsd1...ragforces.html

Here is a huge table of Aero drag, but no Firebird listed: http://www.mayfco.com/tbls.htm

[Takid455]

dave

that site has an overwhelming amount of information. very useful. thankx

[David Pozzi]

With a coast down test you can measure your actual CD, - if you have a running car. There is a test method where you take two coast down measurements, one at high speed, the other at a lower speed. This separates the rolling resistance from aero drag. Or rolling drag can be estimated.
Here is an online calculator that does it: http://web.archive.org/web/200601042.../Utilities.htm

They updated their site and did away with the calculator, so I used the "wayback" machine site which has the old site archived.
David

[BluEyes]

Wondering what production cars have for Cd values. Looking for the drag coef. for a 1978 trans am in particular.

Check out the March issue of Hot Rod. There's an article called "Wind Camp" where they do some wind tunnel testing on some 2G Camaros. They got a Cd of .49 or so on a lowered '70 and then did a few mods to drop that by nearly half.

[jbone]

so is this based on a scale of 0-1?

[CraigMBA]

You know, my mom has a 1950 Plymouth. It's the first car my grandparents ever owned. They had pickups before this Plymouth, but never a car. It's still in the family.

Thanks for the link.

[Norm Peterson]

so is this based on a scale of 0-1?

Not really. Cd expresses a relationship between the shape of an object and the drag it will develop. Obviously, the lower bound of zero is a 'hard' boundary. But Cd can exceed 1.0 by a rather large amount. Here's a link to a few tables.


Norm

[jbone]

ok....forgive me for being slow but how do you know when a vehicles cd is good or bad? aside from comparing it to other averages (which may not be available)

[Norm Peterson]

For an automobile, 0.3 is still a pretty good number, and I think that the older cars that are of interest on this site are more like 0.4 or a little higher in OE trim. IIRC, the early 3rd gen Firebird was right at 0.3 with the headlights in the 'down' position and about 0.32 or 0.33 with them up.

Arguably, a better measure is Cd * A, where A is the frontal cross sectional area. A car with a smaller value of "A" can make up (at least in part) for having a slightly poorer Cd. Trouble is, it's harder to come by the "A" term than Cd.


Norm

[KWIKND]

Speaking of cd. How does a wind tunnel work? Everyone has seen the smoke trail over the roof but how does one measure what you are seeing? What instruments are used? I suppose you are creating a sort of venturi with an obsticle in the air path which could be measured in pressure. What's the skinny?

Dan

[David Pozzi]

This was posted over on Team Camaro reciently, a new square foot number for a 69 Camaro. I thought it would be good to reference it here for our archives.

QUOTE
"Camaro CD info: http://www.camaros.net/forums/showthread.php?t=16452
Hi, I'm new to this message board, I am bringing this post back from the dead (7 years, is this a record maybe? heheh) to bring you guys some useful information regarding 1969 Camaro aerodynamics. The biggest error I've seen published is in frontal square feet. Most figures people are finding is 25 sq. ft. This is incorrect...

The flat square of the width x height = 25.85 sq. ft. Now all the empty space must be subtracted to get the true frontal area.

The method I used was a basic silhouette using a bright light a large distance from a 1:25 model set at stock ride height with a sheet of paper directly behind it taped to the wall. The light was a single source LED placed parallel to the wall inline with the car and centered to the geometric middle of the car. My recordings are accurate to +/- 5% as I used a 1" real size grid to measure it.

These numbers include two stock side mirrors and all use 245 section width tires front and back.

STOCK: 19.64 sq. ft.
1" drop: 19.50 sq. ft.
2" drop: 19.36 sq. ft.
3" drop: 19.22 sq. ft.

Our cars are actually very good for this measurement, ranging from 76 down to 74% of full square. (lower is better)
Oh, with this good news comes the bad: CD is at a minimum, 0.4x."

[Norm Peterson]

Thanks for posting that, David. Particularly that ~75% factor for net to gross frontal area. I've used somewhat higher percentages, albeit for cars of less "rounded" frontal profiles, so this gives me some idea as to the sensitivity of this parameter.

. . . but how does one measure what you are seeing? What instruments are used? I suppose you are creating a sort of venturi with an obsticle in the air path which could be measured in pressure. What's the skinny?

Dan

Just speculation, but I'd guess either load cells under a plate supporting the whole car or plates under just the contact patches to measure forces in at least the fore/aft and vertical directions. Fore/aft will get you Cd; vertical will allow determinations of CL (lift). Direct measurement is the simplest and most accurate method.

I suppose that working from pressures is theoretically possible, but that would involve measuring both pressure (which varies wildly from place to place on the car body) and direction (ditto) over the entire surface area of the car and then integrating the combination of pressure and direction over the area of the car. Including all the messy underside and underhood stuff. No thanks.

Load cells that measure in all three directions would be the way to go if you were interested in the response to crosswinds.


Norm

[David Pozzi]

Speaking of cd. How does a wind tunnel work? Everyone has seen the smoke trail over the roof but how does one measure what you are seeing? What instruments are used? I suppose you are creating a sort of venturi with an obsticle in the air path which could be measured in pressure. What's the skinny?

Dan

Here is a small aircraft wind tunnel, air flow is from right to left. the inlet has a stack of drinking straws to straighten the flow, then air is accelerated by reducing the flow area by aprox 50%.

Note the balance scale for lift, there is a pointer on a graph plate on the lower left, it is attached to the wing being tested, as the aluminum test wing rises, the pointer moves upward, as drag increases, the pointer moves to the left. Air speed is measured by the pitot and static tubes on the lower left with red fluid in it.
David

[BillyShope]

A value of a little more than 0.45 is the drag coefficient for a '60 Valiant. I mention this particular car as it serves as an excellent example of the difficulty in estimating the aero drag. Few of you are old enough to remember, but, in 1960, there was a "showdown" race for the compacts on the road course at Daytona. Those of us at Chrysler were pretty confident that our Valiant would come out on top, but then we heard a disturbing rumor: The '60 Falcon was so much cleaner than our Valiant that it could match our speed with only 140 horsepower. We were getting over 200 on the dyno and were planning on something around 190 for the race and we began to sweat. Well, by NASCAR standards, the TV coverage was terrible. In fact, NASCAR considered cancelling plans for future TV coverage. The cameras were kept on the leading pack of half a dozen cars...all Valiants. The only time Falcons and Corvairs were seen was as they were being passed. Later, tests were run on the Falcon at the Chrysler Proving Grounds and everyone was shocked to learn that the aero drag was actually HIGHER on the Falcon! Whoda thunk it? It certainly looks cleaner.

[David Pozzi]

Mr Shope!
Welcome to Pro-Touring.com , and thanks for participating in our forum.
David

[chicane67]

Here is some food for thought... if you chase the links around... you can kill a week or two just trying to read some of this information.

http://purplesagetradingpost.com/sum...m#Aerodynamics