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Old 12-10-2006, 04:52 PM   #1
Hayes
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At the Track STi wing downforce at 120+ mph? and Auto Aerodynamics

The drag coefifence Cd is a number that tell us how slippery the shape is. How does the Cd relate to the frontal area or size of the vehicle? I ask this because I wonder how two vehicles with the same Cd can have a fuel consumption difference of 30 percent.
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Old 12-10-2006, 05:09 PM   #2
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They are two independent variables in the aero drag equation. They each have equal weight on the overall drag (form drag).

Drag in lbs force = ((air density) x Cd x frontal area x (Velocity in ft/sec) ^2 )/2

This is sometimes stated as Drag = (( .0025) x Cd x frontal area in ft^2 x (velocity in ft/sec)^2

The .0025 is simply the conversion from true mass units to lbs, and drops the 1/2 division. It is only accurate at standard temp and pressure.

By the way there is an often quoted formula for aerodynamic drag that uses MPH as the velocity value. It is incorrect (units do not work out properly) but it is a useful approximation of the true drag formula and handy to use. It gives results that are about 8% off of the correct version.

The simple way to deal with it, is all else being equal, the form drag will vary directly as the product of Cd x Frontal area.

A slick body on a big truck will have more drag than a crappy body on a motorcycle or small car. In that situation frontal area is proportionally the most significant variable.

Two cars of similar frontal area will be most concerned with their relative Cd's.

Larry

Last edited by hotrod; 12-13-2006 at 09:09 AM.
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Old 12-10-2006, 05:43 PM   #3
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Quote:
Originally Posted by Hayes View Post
I ask this because I wonder how two vehicles with the same Cd can have a fuel consumption difference of 30 percent.
They could have different:

Engines
Transmissions
Tires
Weights
BOVs

...
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Old 12-10-2006, 06:27 PM   #4
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Quote:
Originally Posted by Wiscon_Mark View Post
They could have different:

Engines
Transmissions
Tires
Weights
BOVs

...
what he said.

Cd is only one of many variables that contribute to different gas mileage. but if the two cars had all the above the same, and the same Cd, the one with a larger frontal area will do "worse".
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Old 12-10-2006, 08:08 PM   #5
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So Cd is a shape function compared to what? A round sphere?
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Old 12-10-2006, 08:13 PM   #6
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Quote:
Originally Posted by Wiscon_Mark View Post
They could have different:

Engines
Transmissions
Tires
Weights
BOVs

...
Nevermind the fact that the rest of the two cars could be designed completely differently. I am no aerodynamics expert but as I understand it the shape of the whole car, including the back, have an impact on the drag.
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Old 12-10-2006, 08:27 PM   #7
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Quote:
Originally Posted by Hayes View Post
So Cd is a shape function compared to what? A round sphere?
A Cd of 1 corresponds to a flat plate, where all the air hitting it would be brought to a stop
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Old 12-10-2006, 08:44 PM   #8
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I continue to bow to Larry's knowledge. Without him, we would be lost here.
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Old 12-10-2006, 08:47 PM   #9
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Quote:
So Cd is a shape function compared to what? A round sphere?
http://en.wikipedia.org/wiki/Coefficient_of_drag

Larry

Thanks unibomber -- I'm just a technical trivia freak.
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Old 12-10-2006, 09:23 PM   #10
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Why is this in off topic instead of Newbies and FAQ's or motorsports to name two better choices? Its a technical thread that deals with a topic of general interest to the auto enthusiast community not some random chat thread?

Larry
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Old 12-10-2006, 09:50 PM   #11
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Quote:
Originally Posted by Hayes View Post
The drag coefifence Cd is a number that tell us how slippery the shape is. How does the Cd relate to the frontal area or size of the vehicle? I ask this because I wonder how two vehicles with the same Cd can have a fuel consumption difference of 30 percent.
Frontal Area gets multiplied by Cd (among other things) to calculate total drag.

Oops, I see this was already well-answered so to add some value to the thread, I'll add that calculating the power required to overcome drag is:

Power = Drag * Velocity

Therefore, power consumed by drag is proportional to the CUBE of velocity.

Here's the plot of power consumed by aero drag only for a 2006 STI:

Last edited by nhluhr; 10-09-2008 at 03:29 PM.
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Old 12-11-2006, 06:13 PM   #12
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Bump --

Could one of the monitors please move this back into a technical forum where it belongs. Moving it here specifically violates the intent of off topic forum as posted in the forum title.

"Simpsons, politics, movies, etc. ANYTHING BUT SUBARU AND TECHNICAL RELATED!!!" as this thread is about a technical subject that has broad interest to auto performance and fuel milage. The move to off topic in effect killed the thread as in 24 hours it is now on page 9.

Larry
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Old 12-11-2006, 06:20 PM   #13
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I would do it, but i don't have any magical mod powers in OffTopic
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Old 12-12-2006, 11:23 PM   #14
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I give up !!! --- now were in a forsale forum ----- ARRRRRRRGGGGGGGGHHHHHH


Technical discussions belong in Technical forums!

Do we have any moderators here that can stop this silly game of hopscotch to the wrong forums.

Larry
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Old 12-13-2006, 12:09 AM   #15
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Alright it seems the consensus is that this goes best in Motorsports so here goes-

Last edited by Mulder; 12-13-2006 at 07:01 AM.
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Old 12-13-2006, 08:32 AM   #16
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Ok...its open for business...not that motorsports is much of a technical forum...but OK...

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Old 12-13-2006, 09:05 AM   #17
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Quote:
not that motorsports is much of a technical forum
Thanks! At least the motorsports folks (I hope) will give this red headed step child thread a home where folks can discuss some of the technical issues involving aerodynamics and its impact on car performance.


nhluhr:
Your comment about the cube power law is very important for folks who are looking to go faster than about 140-160 mph, as that is when most cars run out of power to go faster. Very small improvements in top speed above that range come at a high cost for power. The inverse of this is true as well. At high speed simply lifting the throttle can provide substantial braking effect due to aero drag, so lost time in shifting, a simple throttle lift to avoid running over another car etc. becomes very significant, where at slower speed it has minimal impact on the cars average speed.


Quote:
Originally Posted by emorphien
as I understand it the shape of the whole car, including the back, have an impact on the drag.
Yes in fact the rear of the car is more important to over all drag than the front in many cases.


Larry

Last edited by hotrod; 12-13-2006 at 09:12 AM.
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Old 12-13-2006, 10:07 AM   #18
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I can add a factoid I heard once...Open wheel race cars have a Cd around .7-.8.
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Old 12-13-2006, 12:41 PM   #19
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The power vs. velocity graph is very interesting. It looks like you don't lose appreciable power until you're north of 80MPH. Maybe I don't need to be quite so concerned with the aero shortcomings of my rally car, after all!
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Old 12-13-2006, 01:56 PM   #20
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Quote:
The power vs. velocity graph is very interesting. It looks like you don't lose appreciable power until you're north of 80MPH. Maybe I don't need to be quite so concerned with the aero shortcomings of my rally car, after all!
That is one of the major puzzles in racing and aerodynamics. Where do I spend my effort. In a racing application you have to balance 3 things, total power to the ground, air drag at speed and grip in the corners.

Your max power and air drag will set an absolute top speed, and dominate how the car accelerates at high speed. The course will determine which is more important, grip in the turns, or top speed. In some very fast courses which have curves which are either slow or not too challenging, top speed and acceleration trump grip in the corners due to aerodynamic forces. On tight courses, with short straights, where you have some challenging turns that everyone fights with and your corner speeds are higher 80+ mph then you have significant lap time due to corner speeds and you can trade top speed you will seldom if ever see for a faster average time in those faster corners.

This is one of the reasons ( aside from exposed wheels) that F1 and other open wheel cars have such high aerodrag. They have stupid amounts of power, and to get it to the ground, they need to maximise traction in the corners. They use relatively high drag but high down force designs and trade 270 mph top speeds they will never hit, for 2+ to 4+ g cornering due to the high down force.

The stock bug eye wrx is drag limited at about 148 mph in stock trim. That means that the sum of drive train, and rolling resistance and aerodrag come out to about 227 hp at 148 mph. Since drive train and rolling resistance is only about 40 - 50 hp at speed, then total aerodrag at 148 mph is around 160-170 hp. This is why acceleration is so sluggish at high speed. Your really only accelerating with maybe 20 surplus hp at speeds north of 100 mph. In the case of the typical rally car, you will never see the cars aero limited speed anyway, so you trade down force and aerodynmic stability in jumps and turns where you are turning for top speed. Stability when the cars direction of motion does not match the normal front on angle to the air flow also is very important. This is one of the reasons they are using the grid fin design rear spoilers with multiple vertical support elements, as that design gives considerable directional control even when the car is at a high angle of attack ( ie sideways) at speed.

This is also evidenced by the STi's much worse Cd compared to the earlier designs like the bug eye body.

Larry

Last edited by hotrod; 12-13-2006 at 02:08 PM.
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Old 12-13-2006, 02:11 PM   #21
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Quote:
Originally Posted by hotrod View Post
Thanks! At least the motorsports folks (I hope) will give this red headed step child thread a home where folks can discuss some of the technical issues involving aerodynamics and its impact on car performance.

Larry
But of course....I wish I had enough money to get my car aero tested..


Bill
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Old 12-13-2006, 02:14 PM   #22
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Quote:
Originally Posted by fliz View Post
I can add a factoid I heard once...Open wheel race cars have a Cd around .7-.8.
truth... i know the cd for open wheels cars are pretty high because of all the wings and stuff..
plus the cd is influenced a lot by having exposed wheels turning in the airstream
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Old 12-13-2006, 03:30 PM   #23
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interesting reading.....but holy headache........
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Old 12-13-2006, 04:14 PM   #24
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Quote:
Originally Posted by fliz View Post
I can add a factoid I heard once...Open wheel race cars have a Cd around .7-.8.
yes but they also make enough downforce to drive upside down if they were so inclined
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Old 12-13-2006, 04:15 PM   #25
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Default STI & EVO aero study

I know these pics were sort of posted elsewhere, but I have archived them and am rehosting them to make sure they stay available. Anyway, the first really blurry pic is a test of the EVO 7 versus the STI v7. What this test shows is that the EVO has very poor downforce characteristics. Notice the first test, with the wing set to its minimum position, the car has literally 4 times the aero lift of the STI. Then, with the wing in its maximum position, it has similar rear lift, but now 6 times the front lift (thanks to cantilevering the rear downforce across the rear tire fulcrum point):


Prodrive did some testing of their own regarding the bugeye WRX with stock parts and with their aero parts, as well as with a 1" drop. The results include front lift, rear lift, and drag power required to maintain speed at 100mph. Notice that lowering the car (1" front and rear) reduced drag and reduced rear lift. If you lower the car more in the front than in the rear, you can expect exactly the opposite to occur. Front lift will be drastically reduced, but drag will be increased.

Code:
Tested at 100mph
Vehicle Setup:                           Front Lift     Rear Lift   DragHP
WRX with no wing                         59.2           70.0        60.5
WRX with stock wing                      71.4           50.7        60.5
WRX with Prodrive wing and no lip        75.0          -10.0
WRX with stock wing & Prodrive lip       65.8           51.3        57.7
WRX with Prodrive wing & lip             20.8           21.8        58.7
WRX, prodrive lip/wing, 2cm lower        20.8           14.3        54.5
Now we have the 2006 iteration of the STI and EVO. The biggest changes for the STI include the rear underbody diffusor and the roof vane. These are significant improvements, whether you believe it or not and the test data shows. The v7 STi wing (and the whole rear end, actually) is identical to the 2006 except for the roof vane and diffusor, so the difference shown in the test is QUITE dramatic.

First, the STI vs EVO airflow off the roof into the wing. Note that the EVO's airflow is quite turbulent (seen in the smoke getting diffuse and swirly) which makes the rear wing far less effective, thanks to it being in a useless airflow:



Next, the STI vs EVO airflow over the hood and front part of roof. Not much difference shown here:



The STI vs EVO airflow over the 'hoodscoop' area. What we see is a huge disturbance in airflow right where the 'vent' on the EVO hood sits. This is probably exiting airflow from the radiator inlet blasting up out of the vent and disturbing the flow. On the Subaru, no such disturbance exists and in fact what we see instead is a nice clean scoop of air being directed into the intercooler:




Continuing on, we have the Lift characteristics measured for both cars. This shows the rear end of the subaru with massive downforce compared to lift on the EVO's rear end:


Looking at this you might think "but the evo has the better front" but what you'd be neglecting is that the Scooby, with the massive increase in rear downforce causes the front end to experience 'lift' as well, leveraged across the rear tire fulcrum. Notice in that older test that I posted up above, when you have the EVO with its wing on the "LOW" setting, the front lift is apparently low, but when you raise the rear wing to the "HIGH" setting, the front lift is HUGELY increased. The fact is that the 06STI has a tiny bit more front lift in this test but massively more rear downforce... if you remove some of the rear downforce, you'd see a huge reduction in front lift... but the better option is of course to add the front lip which not only reduces the front lift, but improves the drag as well.

Finally, we have the Yaw/Roll/Pitch characteristics of the EVO and STI. I have no idea what most of this chart means so I can't say if the higher or lower numbers are better, but one part of the chart I can interpret is the Cd and Sf shown to the lower right of each car. The Drag Coefficient is slightly higher on the subaru but it has a smaller frontal area so the total drag is actually lower. Making downforce means creating drag, but the less drag you add the better your design is. I think it's clear that the Subaru is more efficienct at creating downforce than the Mitsubishi since it has more downforce and less total drag.



Also, from the Prodrive testing it seems obvious that the appropriate mod for the 06+STI is the V-Limited front lip, but this also begs the question, on the 07 Limited, which has an almost nonexistent rear downforce scheme (joke of a wing, no roof vane), but has the V-Limited lip in place, it must feel a little light in the rear at very high speeds.

Last edited by nhluhr; 10-09-2008 at 03:31 PM.
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