# Lets talk Aerodynamics



## Tempes_TT (Oct 24, 2010)

:facepalm: We all know how no one was really interested in my debadging, so I thought Id get a real thread going about aerodynamics!

Educate me! :thumbup:

_P.S. I blame dogger with that awesome looking cupra lip and madmax with his limitless knowledge for taking my thread off topic!_ :laugh:


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## madmax199 (Oct 28, 2009)

Well for starters, let's get all the guessed numbers posted in the other thread out of the way.

This is the data that we have:

The 98 original MK1 TT has a *0.34 * tested drag coefficient(Cd in the US, Cx in france, Cw in germany) 

*copied and pasted from original specs*

_Exterior Length: 159.1 " exterior length 
Exterior Width: 73.1 " exterior body width 
Exterior Height: 53.0 " exterior height 
Front Tread: 60 " front tread 
Turning Radius: 17.1 ' turning radius 
*Drag Coefficient .34 drag coefficient *
Front Legroom: 41.2 " front legroom_


The *Frontal area (A) * is : 

A=1.9900 m2 (square meters)
A=3084.5062 in2 (square inches)
A=21.4202 ft2 (square feet)

*A* is the total surface area of the front of a vehicle that is exposed to the air flow.


The *Drag area *(CdA) is :

CdA = 0.6766 m2 (square meters)
CdA = 1048.7321 in2 (square inches)
CdA = 7.2829 ft2 (square feet)

The *CdA* expresses the aerodynamic efficiency of the vehicle and is measured by multiplying the drag coefficient (Cd) and the frontal surface area (A). The lower the drag area is the more efficient aerodynamically slick the car is, per say.

There is more to it as all the data above are in essence static nubers that give a general idea on how the car will cut through the air. 20v Master in his debate with DeckManDubs explained brillantly(as any engineer should) why drag coefficient and frontal area has nothing to do with high speed stability(post # 47 of the thread that gave birth to this one)

http://forums.vwvortex.com/showthread.php?5251141-I-joined-the-dark-side.../page2



When the TT is in motion, *downforce *and *lift* comes into play.

I don't have any Audi wind tunnel info (if anyone has the data please post).



*This is from ABT, showing the effect of their wing on the car when compared to OEM equipment.*
(take the result posted on their wing a grain of salt because they claim that it creates rear downforce with no cross effect on front lift, I raise the BS flag on that)




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## madmax199 (Oct 28, 2009)

After a little bit of research, I seems that ABT pulled all the numbers out of their ass, below I quoted from Audi's testing results before the recall that introduced the rear wing.

*"The TT has 148 pounds of lift at the rear axle at 125 mph without the spoiler, but only a 53-pound lift when the spoiler is fitted. By comparison, the BMW Z3 coupe generates 64 pounds of lift at 125 mph; the Mercedes-Benz SLK, 104 pounds of lift; and the Porsche Boxster, 68 pounds of lift."*

What I'm getting from this is that even spoilerless, the TT has acceptable amount of rear lift at such high speed. I suspect that the underbody's two rear plastic panels combined with the front undertray and the 4 tires wind deflectors does a great job at removing turbulances and smoothing out the airflow under the TT at speed. 

There is a TT design history book that has all the original numbers with pictures and schematics, anyone here has one?


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## ttwsm (Feb 27, 2011)

My awesome wife bought me a copy of The TT Story, by Lewandowski, Staretz, and Volker for Christmas this year. Is that the book? From page 63 (with heading "Look - no rear spoiler!"):

"An alternative approach had to be adopted in order to keep aerodynamic lift down to a minimum on a sports car that is, after all, capable of reaching speeds of up to 240 km/h.

After several hundred hours of wind-tunnel testing and computer analysis, the aerodynamics team succeeded in developing a most ingenious shape for the underside of the body, with large-area cover panels at the front and rear and a spoiler in front of the wheels to reduce wind resistance. These measures improved drag coefficient by about ten percent and at the same time reduced lift at both the front and rear axles.

The entire under-surface of the body was developed with the aid of a totally new wind-tunnel simulation technique originally used for racing cars. Despite its generously sized wheels, which themselves account for about 25 percent of total wind resistance, and the large openings needed to feed air to a hungry, powerful engine, the TT achieves an excellent Cd value for a sports car of 0.34 and has a frontal area of A=1.99 square metres.

Detail refinement in the wind tunnel had other benefits too: despite the absence of rain channels at the sides of the windscreen, scarcely any water is deposited on the side windows."


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## madmax199 (Oct 28, 2009)

Yup, that's the book and thanks for posting.

Like I said, the under carriage is pretty well developed on the TT. You usually see that kind of aero conscious belly on high speed capable machines.

The only part that need improving is the rear bumper area. An effective rear diffuser would make the belly seriously effective at limiting unwanted lift at speed.

Any info in the book about airflow over the car?

I would love to see data on stall point on the front end, as well at air separation over the back without the wing.


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## AceOfSpades (Feb 26, 2003)

wow scary to think my belly pans messed from day one!

did the 3.2 model front bumper have any up grades to aero?


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## Tempes_TT (Oct 24, 2010)

AceOfSpades said:


> did the 3.2 model front bumper have any up grades to aero?


Im very interested in this question too. Did the 3.2 front bumper/lip really have any gains in aerodynamics, or was it just for looks? 

And about that rear diffuser, I think it could be done. If only I had CF fabricating skills, or fab skills in general, Id take a shot at it!


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## AceOfSpades (Feb 26, 2003)

also I take with the TT Panzer Plate
it does not have any of the aero or lips for the front tires


also what about the adding the front lip
does it help









also any one done work with this in the rear?
http://www.ttstuff.com/mm5/merchant.mvc?Screen=PROD&Product_Code=TTM1-DTMV-PC&Category_Code=T1BOE

or is it just looks....... which would be my guess


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## ttwsm (Feb 27, 2011)

Nope, no info on airflow over the car in the book.


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## QUA-TT-RO (Sep 4, 2010)

*lets talk aerodynamics*

Aerodynamics are for people who can't build engines.. Enzo Ferrari


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## madmax199 (Oct 28, 2009)

I was hoping for published data on airflow over the car but I guess we'll have to do without.

To answer a few of the questions, I don't see the cupra lip doing much in terms of reducing the amount of air going under the front end. Any effective lip would have to stick down a couple inches to have such an effect. The lip would also be a compromise between increasing the frontal area and the CdA to gain a more favorable anti lift effect. 

Imo, to improve on the front end, a lip would need to be paired with an effective splitter that's not crowded by the bumper. What I mean by that, is that the leading-edge of the splitter need to stick past the bumper and become the first thing making contact with the airflow. That would effectively create a controlled separation of the air going over and under the car, reducing lift at that end.



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If anyone has a pic of both bumpers next to each other please post. However, my guess is that it was a purely cosmetic change but I could be wrong on that.


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## madmax199 (Oct 28, 2009)

QUA-TT-RO said:


> Aerodynamics are for people who can't build engines.. Enzo Ferrari


Answers like that are for people are clueless.. Newbies

If you have nothing to contribute why bother posting?


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## PLAYED TT (Oct 17, 2010)

This makes me wonder why Audi did not address the rear? Perhaps because of the exhaust?


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## Doooglasss (Aug 28, 2009)

This makes me want to risk asking the dumb question: What is a rear diffuser really?

Is it a vent on the rear bumper that air traveling under the car can flow through hence planting the rear end better?

If this is so why not cut two rectangular holes in our rear bumpers?


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## idwurks (Oct 25, 2003)

QUA-TT-RO said:


> Aerodynamics are for people who can't build engines.. Enzo Ferrari


Yet how many hours does Ferrari spend in the wind tunnel for their F1 cars =)


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## idwurks (Oct 25, 2003)

DougLoBue said:


> This makes me want to risk asking the dumb question: What is a rear diffuser really?
> 
> Is it a vent on the rear bumper that air traveling under the car can flow through hence planting the rear end better?
> 
> If this is so why not cut two rectangular holes in our rear bumpers?


A diffusor increases the airflow speed and efficiency at the rear of the car, cleaning the airflow. If that underflow of the car is faster than the airflow OVER the car, it generates vacuum under the car, also known as downforce.

Anyone wanna chime in?


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## Tempes_TT (Oct 24, 2010)

DougLoBue said:


> This makes me want to risk asking the dumb question: What is a rear diffuser really?
> 
> Is it a vent on the rear bumper that air traveling under the car can flow through hence planting the rear end better?
> 
> If this is so why not cut two rectangular holes in our rear bumpers?




Well with my VERY limited knowledge, it does what it states above. "Cleans" the airflow underneath the car as it exits the rear. And think of a diffuser as a line of vertical canards underneath the rear bumper. Example


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## madmax199 (Oct 28, 2009)

idwurks said:


> A diffusor increases the airflow speed and efficiency at the rear of the car, cleaning the airflow. If that underflow of the car is faster than the airflow OVER the car, it generates vacuum under the car, also known as downforce.
> 
> Anyone wanna chime in?


This is how I can explain it in simple terms without going into Bernoulli' s effect, boundary layers and other complicated concepts - but I'm sure the resident engineer will chime in with a more technical definition.

Airflow travels under the car at faster speed than over body, but at some point that air has to separate from the rear bumper and join the air stream traveling over it. That separation, adding to the fact that the area under the bumper is not clean (smooth like the rest of the belly), creates a lot of turbulence. 

A diffuser, depending on design, has the potential to do two things. One is to create a smoother surface in the rear and the second is to control the air separation so there is less turbulence and reduced lift. Some diffusers also include vertical plates at the bottom to help cross sectional air flow and improve lateral stability.


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## raart (Aug 9, 2010)

madmax199 said:


> I was hoping for published data on airflow over the car but I guess we'll have to do without.
> 
> To answer a few of the questions, I don't see the cupra lip doing much in terms of reducing the amount of air going under the front end. Any effective lip would have to stick down a couple inches to have such an effect. The lip would also be a compromise between increasing the frontal area and the CdA to gain a more favorable anti lift effect.
> 
> Imo, to improve on the front end, a lip would need to be paired with an effective splitter that's not crowded by the bumper. What I mean by that, is that the leading-edge of the splitter need to stick past the bumper and become the first thing making contact with the airflow. That would effectively create a controlled separation of the air going over and under the car, reducing lift at that end


I was planing to install some front wings (see picture) on the front end. I just wait for the next month to place an order (tight on the budget) however after I read through here wondering if this wings will help or it will be just pure cosmetic... What do you think about? Do I need to get Seat Cupra lip too or not?


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## PLAYED TT (Oct 17, 2010)

How do they attach? As long as they are secure I would imagine they do something. Whether its good or bad I dont know


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## Converted2VW (Apr 8, 2009)

Agreed with a couple of answers above:

Diffusers can:

1. Redirect under-carriage airflow to match over-car airflow in order to reduce turbulence (more turbulence=more drag)

2. Generate downforce: airflow increases its speed under car when it travels from the front to the back, until it hits the diffuser where it is decelerated as airflow area is increased - The change in airspeed and pressure generates a vacuum under the car that sucks additional air in and generates downforce. 

Hope this makes sense... Illustration below:


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## madmax199 (Oct 28, 2009)

raart said:


> I was planing to install some front wings (see picture) on the front end. I just wait for the next month to place an order (tight on the budget) however after I read through here wondering if this wings will help or it will be just pure cosmetic... What do you think about? Do I need to get Seat Cupra lip too or not?


Those will not do much for aerodynamics due to the fact that it's not one piece and leave a void in the middle. That void will create some turbulance that would negate any positive effect the two wings may have had. Unless you like the way they look, I would no suggest investing money on them:beer:.


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## raart (Aug 9, 2010)

PLAYED TT said:


> How do they attach? As long as they are secure I would imagine they do something. Whether its good or bad I dont know


I don't know how they are getting attached but I was thinking anyway to purchase them... I just saw today at Audi Dealership (finally put new brake pads - Ferodo's and did brake flush) an S4 who has similar front wings however they are connected in middle with some kind of lip. I need to ask my contact at Audi if this piece can be separated....


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## madmax199 (Oct 28, 2009)

raart said:


> I don't know how they are getting attached but I was thinking anyway to purchase them... I just saw today at Audi Dealership (finally put new brake pads - Ferodo's and did brake flush) an S4 who has similar front wings however they are connected in middle with some kind of lip. I need to ask my contact at Audi if this piece can be separated....


Again, if function is the priority, I would not spend money on these.

This is what a functional front splitter should look like. The splitter sticking past the bumper to effectively split the airflow going over and under as well as having a negative angle of attack. (I know , I know , the poor brakes are being overworked and could use some cooling)





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*This what I have in the works for the TT*





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*The end product would be something like this:*





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I already de-spoilered the back and will have a bolt on rear wing for when I go race. I will also have a diffuser for the bottom, I just need to find time to work on the many projects I got for the TT.

The wing and diffuser will be, less dramatic TT versions of these:




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*De-spoilered back*





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## 20v master (May 7, 2009)

Tell me you didn't put a Flex Fuel emblem on the rear of your TT. :laugh:


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## madmax199 (Oct 28, 2009)

20v master said:


> Tell me you didn't put a Flex Fuel emblem on the rear of your TT. :laugh:


Gotta rep the E85 army
I got lectured at a pump by a fire marshal for not having the stupid sign, a yellow gas cap and an EPA certified conversion kit :screwy:.

I told him it's a Madmax kit that's pending certification and that I will get the yellow cap(he kept my original cap). I figured the sign not being there was a dead give away that I wasn't a flex car.


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## 20v master (May 7, 2009)

Because that E85 is going to auto ignite and burn your car down. Gasoline would never do that. Wait.....:laugh:


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## PLAYED TT (Oct 17, 2010)

Haha


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## TheDeckMan (Sep 26, 2004)

Splitter looks good :thumbup:


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## PLAYED TT (Oct 17, 2010)




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## Marcus_Aurelius (Mar 1, 2012)

Forum has been dead for a while now, so let's bring back some cool threads and topics from the deep end. :wave:

I have been playing with aero on my track roadster for a while now and have gathered a decent amount of empirical data. Here are some pics/video I have compiled together. These really helped with making educated decisions in the building stages of what I was doing. 


1) Coupe silhouette pressure zones simulation without rear spoiler 












2) Coupe silhouette pressure zones simulation with stock rear spoiler












3) Coupe silhouette pressure zones simulation with aftermarket rear wing











4) Static shot of airflow around the car in motion











5) Coupe silhouette simulation video
https://m.youtube.com/watch?v=GZwk0ztZPy0


Pic 1-3 shows high and low pressure zones and illustrate faster-moving vs slower-moving regions on the car's body. These are very helpful if you're going to add vents in the hood, do front splitter and rear spoiler work, as well as adding block-off plates to unused holes in the front fascia. 

Pic 4 says a lot about how bad the rear end is. Lots of rear flow separation, low pressure wake, and excessive turbulence in the air from underneath re-entering the air stream.

And finally #5, the video. Short of some wind tunnel data, this is the best and most useful piece of data for our car that I've come across yet. If anyone for some reason have some data of the original Audi wind belt tests on the car please post (you'll get a free gift from me). Discuss!


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## Converted2VW (Apr 8, 2009)

The TT can use a long tail...


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## Gonzalo1495 (Feb 2, 2015)

What software did you use to model this?! This is awesome. I would be curious to see what a functional splitter and a diffuser would do to help remedy some of the turbulence especially in the rear like you mentioned. And how long and deep do the vanes have to be for it to start actually being functional.


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## MCPaudiTT (Nov 26, 2006)

Max, I tried to do some flow modelling a while back in SolidWorks. Unfortunately I couldn't get a good model of the car to get accurate data. Would LOVE to have the model of the TT in that video... Of course, there are likely some good models on the Web now that weren't available when I did my work - need to look in to that! Would be interesting to model the stock car, and then each of your mods....


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## MCPaudiTT (Nov 26, 2006)

There are a few out there now. This is a good start but still needs a good bit of work...


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## MCPaudiTT (Nov 26, 2006)

Found a few more, including an open topped Roadster! Just need to scan you and your helmet in ; )


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## ttwsm (Feb 27, 2011)

Does SolidWorks do fluid flow? From the name, I would have thought it was focused more on FEA. We use Fluent at work for flow and mixing simulations - that's the only CFD package I'm familiar with. I wonder about a few things - are the wheels turning? Is the road surface moving?


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## MCPaudiTT (Nov 26, 2006)

If you buy the Flow Simulation add-in, yes ; )

https://www.solidworks.com/sw/products/simulation/flow-simulation.htm

It does heat flow with the same add-in as well.


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## l88m22vette (Mar 2, 2006)

opcorn:


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## sciroccohal (May 4, 2005)

*looks like*



Gonzalo1495 said:


> What software did you use to model this?! This is awesome. I would be curious to see what a functional splitter and a diffuser would do to help remedy some of the turbulence especially in the rear like you mentioned. And how long and deep do the vanes have to be for it to start actually being functional.


FLUENT or RAMPANT.


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## carsluTT (Dec 31, 2004)

wasn't there a company back in the day that did a down-force comparison of different spoilers on the back of a mk1 TT? does anyone happen to have the info still?


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## Gonzalo1495 (Feb 2, 2015)

carsluTT said:


> wasn't there a company back in the day that did a down-force comparison of different spoilers on the back of a mk1 TT? does anyone happen to have the info still?



Are you referring to this.


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## MCPaudiTT (Nov 26, 2006)

Gonzalo1495 said:


> Are you referring to this.


I have seen this photo tons of times, but never have I seen anything to back up how these numbers were generated. Anyone can draw a picture and put some numbers on it. This "graph" is even more worthless than a Dyno plot without more background to substantiate the claims. It was this image, btw, that prompted me to do the analysis in SolidWorks myself...


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## Gonzalo1495 (Feb 2, 2015)

MCPaudiTT said:


> I have seen this photo tons of times, but never have I seen anything to back up how these numbers were generated. Anyone can draw a picture and put some numbers on it. This "graph" is even more worthless than a Dyno plot without more background to substantiate the claims. It was this image, btw, that prompted me to do the analysis in SolidWorks myself...


Same here, but this is also the only aero "test" done with the rear spoiler that I have seen, so I wanted to clarify if CarSluTT was asking about that. By no means does me posting the picture indicate I believe it to be accurate or legit. I'm on your side actually, it looks like it was poorly made in paint with basic copy pasting lol. :laugh:


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## MCPaudiTT (Nov 26, 2006)

No personal nap offense intended, comments directed at the data only. Apologies if it came across otherwise.


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## Gonzalo1495 (Feb 2, 2015)

MCPaudiTT said:


> No personal nap offense intended, comments directed at the data only. Apologies if it came across otherwise.


No worries man.


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## MCPaudiTT (Nov 26, 2006)

Looky what I found...


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## TheDeckMan (Sep 26, 2004)

Nice!!!


Sent from my iPhone using Tapatalk


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## TheDeckMan (Sep 26, 2004)

What format is that model in Mike?


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## MCPaudiTT (Nov 26, 2006)

SolidWorks. It's on GrabCAD.


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## TheDeckMan (Sep 26, 2004)

MCPaudiTT said:


> SolidWorks. It's on GrabCAD.


:thumbup:


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## MCPaudiTT (Nov 26, 2006)

TheDeckMan said:


> :thumbup:


I was wrong, not SW, but STEP. It think it is all just surfaces as well:

https://grabcad.com/library/audi-tt-rs-dtm-race-car


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## Marcus_Aurelius (Mar 1, 2012)

MCPaudiTT said:


> I have seen this photo tons of times, but never have I seen anything to back up how these numbers were generated. Anyone can draw a picture and put some numbers on it. This "graph" is even more worthless than a Dyno plot without more background to substantiate the claims. It was this image, btw, that prompted me to do the analysis in SolidWorks myself...


Agree, this chart always has been questionable IMO. Reduced rear lift values with no cross effect in the front? I think not!


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## Marcus_Aurelius (Mar 1, 2012)

MCPaudiTT said:


> Found a few more, including an open topped Roadster! Just need to scan you and your helmet in ; )


Oh, could it be that we're getting somewhere? It would be nice to have some solid aero simulation. I have been doing things the hard way with trial and error, so anything is great help.


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