MrDude_1

whats the drag Co and frontal area of my car?

its a 91 camaro CONVERTIBLE.

i cant find the numbers anywhere.... im just looking for somthing so i can compare my vert to the coupes....



also, feel free to toss up any Co and frontal areas for any other 3rdgen you have...

smithtc

iTrader: (5)

I have some old engineering books from school (I'm not an automotive eng., these books are just some transportation books used for calculating road vehicle performance)

Using this non-thirdgen information. I will guess the following...

Drag coeff. 0.32-0.35.
frontal area ~22 square feet.

Just approximations

Edit:
For comparison purposes: Drag coefficients
1968 Chevy Corvette 0.50
1968 Volkswagon Beetle 0.46
1968 Mercedes 300se 0.39
1978 Triumph TR7 0.40
1987 Honda Civic DX 0.35
1987 Acura Integra 0.34
1987 Porsche 944 Turbo 0.33
1987 Ford Taurus 0.32
1987 Maxda RX7 0.31
Ford Probe V (experimental 4 door sedan) 0.137

rustybluebird

There are some drag co. #'s on the bottom of this page.
https://www.thirdgen.org/newdesign/tech/techdb.shtml

MrDude_1

note the lack of later camaro info. and the total lack of vert info.



smithtc: im sure that info is great for the "average" shaped car, but 3rdgens are kind wierd in that they're big, but the rear glass makes up for alot of it........

im just wondering how ditching the glass (with the vert) will effect it


i'll be running windows and top down.

smithtc

iTrader: (5)

85 IROC-Z28: .34 Cd
I would think this would be extremely close to your convertible. What body changes did they make. Maybe add a hundreth to the coefficient for it being a convertible.

The info I gave you was not for an "average" shaped car.
What rear glass are you talking about? makes up a lot for it?
ditching what glass? the windshield? The rear glass helps the drag coefficient, not hurt it. More transition is not good.
Gonna wear a helmet and cover the interior with fiberglass to the spoiler except where your body comes through?
What are you using this info for?

I have some fluid dynamics information to use for all kinds of drag, pressure variation, form drag, skin-friction drag, tables, reynolds number, etc. determining Cd
I will study this a bit more later for intellectual fun and highlight a few importants later. For now, a few more vehicles' drag coefficients from another table I have.

1932 Fiat Balillo 0.60
Volgswagon "Bug" 0.46
Plymouth Voyager 0.36
Toyota Paseo 0.31
Dodge Intrepid 0.31
Ford Taurus 0.30
Mercedes-Benz E320 0.29
GM Sunraycer (solar vehicle) 0.12

Edit:
On a vehicle beginning w/ a Cd of 0.363:
Rolling down the windows will add about 0.018 to the Cd. or 5%
Windows up and t-tops off will add about 0.026 or 7%.
T-tops off and windows down will add about 0.084 or 23%.
Disregard what I said earlier about 0.07 and the top down, etc. I will edit that part.

MrDude_1

im not changing anything from stock with the exception that i'll have to have a rollbar, and i'll have a helmet on.

i may shave the door handles for other reasons, and i know the wheels have a big impact, and i may be changing rims, but im mostly intrested in stock vert drag..

f-crazy

iTrader: (-1)

i know that the C5 C/O is .27 and thte 91-92 stlye birds are .31...id imagaine the verts arent to far from that

smithtc

iTrader: (5)

I would use 0.35 and add 0.1 to it for top and windows down. Might as well say 0.50.


I’ve got 2 sources to research and present some information from. I’ll present info 1st from Principles of Highway Engineering and Traffic Analysis by F. Manning and W. Kilareski. I’ll do this for my benefit as well as anyone interested, since I would also like to do some road racing in the future and this info may come in handy.

Aerodynamic Resistance (Ra):
Over 85% by turbulent flow of air around the vehicle body, a function of the shape of the vehicle…particularly the rear portion.
~10% by friction of air passing over the body
~3% from air flow though vehicle components such as radiators, air vents, intake ducting, etc.


Ra=(p/2)*Cd*Af*V^2

p = air density in slugs/feet cubed. Examples: 0.002378 for 0 altitude and 59 degrees F, 0.002045 for 5000 alt. And 41 degrees F
Cd measured by wind tunnel or field tests
Fa = frontal area. I’m estimating 22sf. Will measure my car later to verify.
V = speed in feet per second relative to prevailing wind

Since power is a product of force and velocity:

Power Ra = (p/2)*Cd*Af*V^3 (velocity is cubed)

Since 1 hp = 550 ft-lb per second,

Hp Ra = (p*Cd*Af*V^3)/1100
Example: (Cd=0.45, Af=22, p =0.0025) 71 hp needed to maintain 100 mph (146.67 fps) against aerodynamic resistance

If your speed is doubled, 8 times more power is required to overcome the resistance.

You can measure the Cd by field tests where you declerate from a known speed with other sources of resistance (rolling & grade resistance) accounted for. I will research a bit on the rolling and grade resistance, and use this to figure out about what our cars will have...and figure out a decent field (road) test to do.

My 2nd source of info is more complex (Drag & lift) Engineering fluid mechanics.

Let me know if this stuff is worth posting…

rx7speed

there are two different cd numbers for the 87 rx7
you put he one up for the base, GXL, LX, and turbo.
there is also the sport so to make an addition to your list


87 RX7 sport 0.28


which is what I have

f-crazy

iTrader: (-1)

Whatever dude...want a medal or something?

pvt num 11

Don't forget the first gen w-bodies, the Grand Prix had ... .299 I think. If only they made them RWD...

smithtc

iTrader: (5)

I'm not a fan of the RX7. My wife digs the new RX8, though.
What were the changes to the body that enables a 0.03 decrease? Kind of a big jump on one particular car...Gotta picture of the difference?

smithtc

iTrader: (5)

Some more information from my 1st source:

The 3 major sources of vehicle resistance are 1) Aerodynamic res. 2) rolling resistance & 3) grade or gravitational resistance.

The natural deformation of the tire as it goes over the road accounts for 90% of the total rolling resistance. Also contributing are pavement surface penetration, and corresponding compression at about 4%. Frictional motion such as tire slippage, air circulation around the tire and the wheel (“fanning effect”) account for about 6%.

A hard, rigid tire, inflated, and at a higher temperature will reduce rolling resistance.

To approximate the rolling resistance, use the vehicle weight and a friction coefficient f(r).

f(r) =0.01*(1+V/1.47) use this for typical tires & typical road pavement. Don't have other information on it from this source.

V = speed in feet per second.

Rolling resistance (Rr) = f(r) * W

Horsepower to overcome Rr = ( f(r)*W*V) /550

So we have aerodynamic res. (Ra), rolling resistance of tires (Rr), tractive effort of tires (F), and weight (W). I’m going to disregard grade since if you can find a nice stretch of road then the grade is negligible, or zero.

F=ma+Ra+Rr

I'm going to try to put 60 mph into the equations to come up with some way of driving down the road at 60mph, pop the gear into neutral, and see if what happens to the speed to come up with a deceleration rate. The mass is known; therefore, maybe I can determine the deleration rate enough from 60 to some lower speed, use that and find Cd.

F = tractive effort. Gotta get that figured out, now. Do that later

rx7speed

not fond of the rx8 myself

it's not an rx7 but was never made to be one


reall the only differences was the addition of a spoiler out back (and you guys think they only cause drag) as well as a little piece in front of each wheel that help air move around the tire and a small air dam


the only picture I can find right now is the one in my sig


you might be able to see them in there

smithtc

iTrader: (5)

Think I have got this figured out without too much mental masturbation. To be as accurate as possible in finding the car’s Cd, a Calculus equation would be needed due to the changing variables in the equation, or the fact that the speed, resistances, and deceleration are not constant. I’m going to avoid that by making some assumptions…and also state that you would need to do this road test a number of times to get a good, consistent number, or at least have access to Excel program where you can graph the speed, resulting resistances, and deceleration approximate. Having said all that here is what I plan to do:
Without Calculus, the faster you drive to do this test, the more accurate it will be, I think. But I’m not advocating breaking the law.

The equation is:
F=ma+Rr+Ra+Rg

Where F is tractive effort (rear wheel torque for rpm at relative velocity*gear ratio) all divided by wheel radius in feet.
We can ignore this…make it zero…since popping the gear into neutral makes this zero, no available torque to rear wheels.
If you find a flat stretch of road Rg = 0 also.

New eq. Is –ma=Rr+Ra

Since my GTA weighs about 3450lbs+driver (170)…dividing by 32.174 gives the mass at 112.5 slugs

If I get on the interstate and go 80 mph on a flat stretch of road…then:
Rr = f(r)*W = 0.018*3620lbs = 65.16 lbs
Ra = (p/2)*Cd*Af*V^2 = (0.00235/2)*Cd*22sf*(117.33^2) = 355.86*Cd lbs

So:
112.5*(deceleration)=65.16+355.86*Cd

Deceleration = change in velocity over time. (X mph *1.4667)/ X seconds
Such as 10 mph or 14.67 fps over 8 seconds would be 1.834 feet per second squared average deceleration
Say I did that from 85mph to 75 mph. Using the 80 mph Rr and Ra and the deceleration from 85 to 75 over 8 seconds…what would that do?

112.5*1.834 =65.16+355.86*Cd
206.325-65.16=355.86*Cd
141.165=355.86*Cd
141.165/355.86=Cd
0.397 =Cd


And yes…you could just ignore the math, you use the same stretch of road, consistent tire pressure, and consistent air density, see how long it takes to go from 100 to 60 mph in neutral, make a change to your car, and then see if takes more time to check if the body change made improvement.

Clear as mud??

Just need to verify the frontal area…and go do a few tests. Again, I think the faster speeds will give more accurate results due to the increased aerodynamic resistance, but with a smaller range of mph change (120-115mph), I think. Also…the deceleration needs to be as steady as you can make it (constant)…and timed accurately, also.

smithtc

iTrader: (5)

Placement of the spoiler to improve the transition and reduce turbulence can help the drag. But what the ricers do with the spoilers is a Real Big Waste of Money in most cases due to improper angle placement, and increased drag.

Edit:
If angle placement and design are correct, proper negative lift will help stability and traction.

rx7speed

and I think between the wing and the aero pieces in front of each wheel the Cl is reduced close to half

not sure on that though would need to look a little further
but I think close to

kdrolt

TTT, then for comparison see.

askulte

Back calculating it, your V was 1.176 to give a .018 f(r)...

I'm coming up with:
V (80 mph) = 117.33 ft/s, which gives me .808 for f(r), and 2925 lbs drag... Ballpark f(r) of .015 * 3620 lbs = 54.3 lbs drag

What am I missing?

smithtc

iTrader: (5)

80mph = 117.33 ft/sec
f(r) = 0.01*(1+(V/147)) where V is in ft/sec (80*5280/3600=117.33).
f(r) = 0.018

R(r) = f(r) * W
R(r) = 0.018 * 3620 = 65.16

Not sure what you're getting at. I don't think you should get 0.808 for the f(r).

askulte

Aha! Dividing by 147 instead of 14.7. That'll do it!

A.