What is the drag coefficient of a Cobra roadster?

SunDude · #1 (**permalink**) 10-01-2009, 08:59 AM

Does anyone know what the coefficient of drag (cd) and frontal area (sq.ft.) were on an original 427 Cobra roadster?

Excaliber · #2 (**permalink**) 10-01-2009, 09:02 AM

Gutt feeling based on knowing the numbers for some other cars.

.41 or higher, a real brick.

RodKnock · #3 (**permalink**) 10-01-2009, 10:31 AM

Less than 1.

Excaliber · #4 (**permalink**) 10-01-2009, 11:23 AM

Here's an example of what I'm talking about "gutt feeling". First and Second generation Mazda Miata. Wheel base, size, weight all comparable to a Cobra. MUCH smaller frontal area, looks like it would have a pretty good CD. In fact Miata's are .38 which is pretty high, much higher than many modern cars. Therefore, Cobra's having even more frontal air and a much larger grill opening couldn't be any better. A .41 estimate is likely cutting them some slack!

patrickt · #5 (**permalink**) 10-01-2009, 11:46 AM

I've seen more than a couple of calculations that list it at .42 -- a brick it is.

Bob In Ct · #6 (**permalink**) 10-01-2009, 12:26 PM

All you have to do is ride in one to know that air does not go over or around the car very smoothly. Keeping a hat on is all but impossible. The steep windshield (by today's standards) does not help.

Bob

patrickt · #7 (**permalink**) 10-01-2009, 12:33 PM

A Hummer is about the only modern car I can think of off the top of my head that it beats. The lowly Prius comes in around .27, if memory serves.

Argess · #8 (**permalink**) 10-01-2009, 12:48 PM

Forgive my ignorance, but I take it the lower number is more aerodynamically slippery?

Hard to imagine a Cobra worse than a Miata based on the pic above. Just look at the giant side mirrors on the Miata and compare to the bullit pointy side-mirror on a Cobra.........LOL.

patrickt · #9 (**permalink**) 10-01-2009, 12:49 PM

Quote:

Originally Posted by Argess

Forgive my ignorance, but I take it the lower number is more aerodynamically slippery?

Yes, lower is better.

patrickt · #10 (**permalink**) 10-01-2009, 12:53 PM

Most people would bet that a Lamborghini Countach would not have the same CoD as the Cobra, when actually they're very, very close.

Silverback51 · #11 (**permalink**) 10-01-2009, 03:33 PM

It's better than a 4 X 8 sheet of plywood, but not by much.

greg schroeder · #12 (**permalink**) 10-01-2009, 05:02 PM

I figured it out a while ago based on rear wheel HP on a dyno and then HP loss at speed using a G-tech compition meter. It does look to be over .4 as mentioned. The usable HP at higher speeds has a huge loss from resistance. Over 120 or so mph it becomes a real challenge and even seems to require diffrent jeting there's so much load.

This is third gear with a few charts and assorted changes. It came out to be about 100 HP of energy loss around 110 mph. The faster you go the loss becomes exponential. The car on the dyno was making about 475 at the wheels when it made about 340 or 350 in motion of usable power.

Here's a formula.

http://www.gtechprosupport.com/support/AeroDragCalc.php

jon@harrison.ne · #13 (**permalink**) 10-01-2009, 06:56 PM

The drag coefficient Cd has nothing to do with the size of the car,only the shape. The total drag force is proportional to the frontal area times Cd times speed squared.

ZOERA-SC7XX · #14 (**permalink**) 10-01-2009, 07:23 PM

If I remember correctly, that's why Shelby developed the Cobra Daytona coupe. The roadster just didn't want to run much over 160 mph.

Excaliber · #15 (**permalink**) 10-01-2009, 07:24 PM

Just to help keep a VERY complex subject simple, I figured a Miata would be a good comparison. Similiar size, weight, shape with smaller frontal area.

BUT Patrickt brought up an interesting comparison with the Lambo, which apparently has very little frontal area and yet a CD similiar to a Cobra??

Which reminds me of the testing Shelby did when he first started working on the Ford GT-40. Overheating, terrible drag, low top speed. Due mostly, as I recall, to the many "air scoops" here there and everywhere. Which may be the Lambo's problem, big ass air scoops.

Thats some impressive work Greg!

eschaider · #16 (**permalink**) 10-01-2009, 07:43 PM

Here is some data out of the Bosch Automotive Handboook

We are probably pretty close to the entry for "Typical Roadster" with a Cd of 0.55

Sooo, that puts us in about the same class as a bus! Good news is we are not as bad as a truck.

Ed

Excaliber · #17 (**permalink**) 10-01-2009, 08:28 PM

Quote:

Good news is we are not as bad as a truck.

Oh I'm just jumping up and down over the good news...

lovehamr · #18 (**permalink**) 10-01-2009, 08:32 PM

Quote:

Originally Posted by eschaider

Good news is we are not as bad as a truck.

Ed

WoooHooo!!!!!

BTW, if memory serves, that huge CD for the Countach was when it was fitted with the huge rear wing not shown in the above pic.

Steve

Excaliber · #19 (**permalink**) 10-01-2009, 08:38 PM

Ah, good point. Same rule would apply to the fuel dragster which you KNOW is a pretty slick setup. It's the WING all right!

Man, wasn't Peter Brock something when "back in the day" with a limited understanding of CD he designed the Coupe!

JAC · #20 (**permalink**) 10-01-2009, 08:59 PM

Last time I saw this question someone posted a picture of a brick.