A note on drag coefficient...

[sndprssr]

Ok, I was having this debate with someone today and have noticed on almost all car forums there are posts stating "this car has a better drag coefficient than your car so it must be faster/quicker/(insert whatever here)." The Drag Coefficient ≠ Actual Drag on the vehicle. This is basically just to clear things up and so you will have an answer if someone says this to you about your car. The "Cd" is just a number found in a wind tunnel used to figure out actual drag. The wind tunnel does not account for the frontal (reference) area of that which is being tested as the air travels the centerline of the vehicle.

The simplified drag equation states that drag is equal to 1/391 * Cd * (frontal area) * (air speed)

Now, I'm not doing the math because I hate it but using the TT and the 135i... The TTs frontal area is 2.08 m² with a Cd of .30 and the 135i is 1.86 m² (if I was told correctly) with a Cd of .33 which would put them around the same numbers, if not give the 135i a little edge.

"The amount of drag generated by an object depends on the size of the object. Drag is an aerodynamic force and therefore depends on the pressure variation of the air around the body as it moves through the air. The total aerodynamic force is equal to the pressure times the surface area around the body. Drag is the component of this force along the direction. Like the other aerodynamic force, lift, the drag is directly proportional to the area of the object. Doubling the area doubles the drag."¹

Hope this helps clarify some things for people and gives someone the right to give the finger to the next person that tells them the 135i is not as good as whatever car in any area solely based on it's Cd...

(¹ - Quote courtesy of www.grc.nasa.gov)

[john970]

Great info - Here's a great wikipedia article:

http://en.wikipedia.org/wiki/Automob...g_coefficients

Attachment 4203

where

Fd is the force of drag,ρ is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula. Air is 1.293 kg/m3 at 0°C and 1 atmosphere),v is the velocity of the object relative to the fluid,A is the reference area, andCd is the drag coefficient (a dimensionless constant, e.g. 0.25 to 0.45 for a car).

[Bringinthewolf]

"I was told there would be no math" - Chevy Chase

[john970]

Thank you very little.

[BMWeber]

thanks I think most people forget about frontal area. makes big difference

[hector]

Originally Posted by sndprssr

... drag coefficient...135i...Cd of .33...

)

in this era of manufacturers endeavoring to maximize the efficiency of their designs(e.g. mazda's "gram strategy"), 0.33 for a coupe/sedan is downright pathetic for it's detrimental effect on highway mileage not to mention, for those that don't care about fuel efficiency, it's effect on high speed acceleration and top speed. blame the upright styling meant to be evocative of the 2002 but which unfortunately sacrifices function for form, a luxury that we can no longer afford.

[sndprssr]

How is .33 pathetic? Most sedans/coupes are between .29 and .37 and in case you didn't read anything above your own post, the Cd is only PART of the real world equation to figure out things such as efficiency. You might should actually look up the Cd of most cars before posting that. Here are just a few, and the list goes on and on...

.38 New VW Beetle (32+ MPG)
.372 Ferrari F50
.37-.35 Z3 M Coupe
.36 Mustang and Honda Civic
.33 Honda Accord

[hector]

Originally Posted by sndprssr

How is .33 pathetic? Most sedans/coupes are between .29 and .37 and in case you didn't read anything above your own post, the Cd is only PART of the real world equation to figure out things such as efficiency. You might should actually look up the Cd of most cars before posting that. Here are just a few, and the list goes on and on...

.38 New VW Beetle (32+ MPG)
.372 Ferrari F50
.37-.35 Z3 M Coupe
.36 Mustang and Honda Civic
.33 Honda Accord

and the nuna, world solar challenge winner is 0.07, so what? btw, what model year accord are you referring to? the '07 coupe and sedan's values are 0.29 and 0.30 respectively and the '06 civic is 0.31 (the '01 civic had the 0.36 that you quote). and to use the bug for comparison.

[spudw]

My brain hurts...

[oneaabe]

Originally Posted by hector

and the nuna, world solar challenge winner is 0.07, so what? btw, what model year accord are you referring to? the '07 coupe and sedan's values are 0.29 and 0.30 respectively and the '06 civic is 0.31 (the '01 civic had the 0.36 that you quote). and to use the bug for comparison.

Can you also post numbers of the frontal areas of those cars?
And then we can actually compute the drag force experienced with each car.

[sndprssr]

You mock the bug comparison but not the F50? :iono:

Anyway, the point is the Cd is only a fraction of a total equation so stating that .33 is pathetic due to "it's detrimental effect on highway mileage" is incorrect. That's like taking the formula for volumetric efficiency { (HP x 5600)/(RPM x Disp.) } and saying that a car has poor VE because it has 200 HP. Well...what's the RPM? What's the Displacement? You can't just discount all the other variables and make a blanket statement. As I stated before, most sedans/coupes are between .29 and .37 so how is .33 "pathetic"? I never said this car has the greatest Cd however, if you can't do the math correctly to back up your statement you shouldn't post just to start an argument...

[hector]

Originally Posted by oneaabe

Can you also post numbers of the frontal areas of those cars?
And then we can actually compute the drag force experienced with each car.

actually i don't have those numbers, the cds are from wikipedia. i do understand that the product of the cd and frontal area is the bottom line; i believe that my point will hold at least in the sense that the 1er's frontal area could have been reduced with a lower roofline.

[sndprssr]

I do see your point, but 1-2 in. won't make a real difference. Much more than that and you sacrifice headroom in an already small car.

The frontal areas are much easier to get by either a lot of online searching or calling the manufacturer. If you have a lot of time on your hands you can figure out the FA on your own...

The vehicle reference frontal area is defined as the area of the orthogonal projection of the vehicle; including tires and suspension components, but excluding vehicle protuberances, onto a plane perpendicular to both the longitudinal plane of the vehicle and the surface upon which the vehicle is positioned. Measurements of this area shall be computed to the nearest tenth of a square foot.

[Troyn]

Originally Posted by hector

actually i don't have those numbers, the cds are from wikipedia. i do understand that the product of the cd and frontal area is the bottom line; i believe that my point will hold at least in the sense that the 1er's frontal area could have been reduced with a lower roofline.

I think that your statement of "downright pathetic" was a bit inflammatory......

[hector]

[quote=sndprssr;70356]You mock the bug comparison but not the F50? :iono:
quote]
of course i noted the inclusion of the f50 on your list; am i to understand that you're disappointed that i didn't mock all of the absurdities? i was simply more amused by the bug comparison because it is after all a rolling jelly bean.

[sndprssr]

If you mock, you must mock completely. As long as the info. is out there, it's all in good fun anyway...

[hector]

Originally Posted by Troyn

I think that your statement of "downright pathetic" was a bit inflammatory......

maybe so but why go to the trouble of incorporating magnesium into an engine block and not maximize aerodynamics?

[sndprssr]

Because magnesium is light AND strong. New alloy development and lower costs, which are becoming competitive to aluminum, will further the number of automotive applications.

And who is to say they didn't try to maximize aerodynamics? It's not always about the Cd for gas milage...the Cd for an F1 car is anywhere from .7 to 1.1. Aerodynamics with respect to the Cd plays a major part in the downforce generated on the car and I would think a small 300HP RWD car could use a little downforce to keep the handling reasonable.

[Troyn]

Originally Posted by hector

maybe so but why go to the trouble of incorporating magnesium into an engine block and not maximize aerodynamics?

I think that your statement speaks for itself...


If anyone is able to post the more complete picture using the math - thank you in advance! :biggrin:

[B25 Jim]

Originally Posted by hector

maybe so but why go to the trouble of incorporating magnesium into an engine block and not maximize aerodynamics?

You will always sacrafice aerodynamics for something. Hell, the side mirrors create resistance, but we would be sacraficing safety to remove them. The car was not ultimately designed to be a race car, or even a high end sports car. It was designed to carry 3 people (2 full size, 2 pint size) and yet still be enjoyable to drive. And it's only the 128 that has the mag/aluminum block if I remember correctly (that rarely happens, so please correct me if I'm wrong).

[hector]

Originally Posted by Troyn

I think that your statement speaks for itself...

so what if it was intended to be provocative? grow up.

[atr_hugo]

Gentlemen, keep it civil please.