Improving aerodynamics with aftermarket parts

[the bruce]

From what I see that car also has a Unibrace fitted.

....Are you sure? Wouldn't they have fitted their own Summit cross braces?

Yes, there's a bit of it to be seen on the left and right of the tunnel.



On a side note: You might also find some inspiration here:

http://www.mk5golfgti.co.uk/forum/index.php/topic,59674.msg669054.html#msg669054

[the bruce]

Interesting how they designed the brackets:

[danishmkvgti]

Interesting how the designed the brackets:

Seems they are trying to get the wing out in more undisturbed air

[Jaywoo-GTI]

^^^^
Thanking You!! 

Now where can I get one!? (if not too expensive). I don't really need one but I don't really need some of my other mods either.

Was thinking the same thing in fact i dont even want it in carbon.

[the bruce]

Yes, I told fuscobal to keep the templates so it will be easier to have a wing mounted later on.
I also want to have a rear diffuser but this is a costly job since I want to change the whole exhaust with the new diffuser.
But both these chapters require spending money and I prefer to spend the money on going to Hungaroring and/or Nurburgring instead. There is enough time to do all the mods...

The biggest challenge is you need to remove the rear exhaust muffler. This implies to
content oneself with a center muffler or even without any at all.

The diffusor itself is made with just some cheap parts (you already know this example):



The big advantage of a diffusor is it provides downforce without any drag penalty worth
mentioning (which a wing has besides the cost).

BTW, there's another 'cheap trick' which cannot substitute a wing, but will at least reduce
lift by some 10 N (at 200 km/h): a gurney flap. It's easily made by some piece of plastic
and fitted to the roof spoiler before you visit a track.
Unfortunately most of the known literature depends on mounting a gurney flap to wings.
Nevertheless it also works on a rear spoiler.


http://en.wikipedia.org/wiki/Gurney_flap

http://www.insideracingtechnology.com/tech104gurney.htm

http://www.dept.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf

[rex]

Yup, I know about that diffuser. That is my template also at the moment.
I know I have to get rid of the rear exhaust muffler, that is why I want a new complete exhaust.

Interesting read about the gurney flap.
As I can see it is even used on the diffuser above...

[fuscobal]

Good findings on the gurney flap. MAybe I'll make one on my wing as well. Let's take my wing and make some small assumptions :

- Wing > Reverie 150mm chord with 1300mm span
- Desired gurney flap > 1.25% to 5% of chord, meaning 1.875mm to 7.5mm (these are quite small and barely visible on the car)
- Downforce @ 160kmh & 12deg AOA with no gurney > 45kg
- Downforce @ 160kmh & 12deg AOA with 1.875mm gurney > +20-30% wich means about 54 to 58.5kg ( 9 to 13.5kg increase !!!)
- Downforce @ 160kmh @ 12deg AOA with 7.5mm gurney > +40-50% wich means about 63 to 67.5kg ( 18 to 22.5kg increase !!!)

If these values (taken from the last link Bruce provided) are not highly optimistic, then the downforce addition from these small devices is tremendous and easy to achieve !

It seems that I did not pay attention while ordering the wing since they also have carbon 5mm and 10mm gurney flaps available for 90-130L)

[the bruce]

According to Joseph Katz:

From an aerodynamic standpoint when on track with a production car rear downforce is
by far most important, then front DF and drag is neglicible. Even more with a Golf which
has more rear lift than most sedan style cars.
Rear DF allows for a more aggressive suspension setup which will help on small radius
corners (where you don't have DF worth mentioning).

Well, the gurney flap is one of the very few parts invented first in automotive racing (by
Dan Gurney) and then established in aviation. In the latter case such flaps are just needed
on take off and landing. On journey low drag and highest possible efficiency is what plane
designers are seeking for.

Contrary to this at car racing drag is nothing reace car designers worry that much. Most
prototype racers have a Cd of 0.7 to 0.8, open wheel cars have even more.
Even production car based racers have much more drag compared to it's stock brothers
(which as you all know are in the 0.25 to 0.35 area today).
Highest downforce you can get is faster than highest possible efficiency. That's the point.

This is why the same applies to a wing on a car. Downforce is more important than efficiency.
While you get the best efficiency with a wing without a flap a wing with a gurney flap will
give you more downforce at higher angle of attack without stall. Stall will occur later than
without the gurney.

Conclusion: It's better you fit a gurney if you want the most DF (and you will want DF).

There are some not-so-expensive CF lips on ebay, but any black plastic profile which is
not too brittle will do. Just take care to fit it to the upper (high pressure) side. No part of
it should disturb the flow on the lower (low pressure) side of the wing.

[fuscobal]

Yes, seems pretty clear I should add it. At 1-2% of chord the gurney adds no drag enalty at all and at 2-5% it adds increasing drag but still nothing to worry about if you compare to what it brings. I've read some technical papers and it seems that a 4-5% of chord gurney can add up to 40% downforce. For me, even adding a conservative 25% would mean about 10-12Kg @ 160kmh ! Not bad at all !

[fuscobal]

Did some approximations on the real downforce level with my wing at max downforce position. For this I used the following data :

- OEM car rear lift > 37kg @ 200kmh (as posted by Bruce some time ago from a wind tunnel test)
- Reverie wing downforce > 30kg @ 160kmh (could be 35 but better be conservative so let's say it's 30)
- I've considered a 25% downforce addition from a gurney flap wich is 4-5% of the wing's chord
- Formula used to extrapolate from the 2 values I have was DF @ new speed = DF @ old speed x (new speed ^ 2 - old speed ^ 2) wich mean practically downforce increases with the square of the speed

Speed in kmh > Stock lift > Wing DF > Total 1 (Wing DF - Stock lift) > Wing + gurney DF > Total 2 (Wing + gurney DF - Stock lift)

  80 > 5.92kg > 7.50kg > 1.58kg > 9.38kg > 3.46kg

100 > 9.25kg > 11.72kg > 2.47kg > 14.65kg > 5.40kg

120 > 13.32kg > 16.88kg > 3.56kg > 21.09kg > 7.77kg

140 > 18.13kg > 22.97kg > 4.84kg > 28.71kg > 10.58kg

160 > 23.68kg > 30kg > 6.32kg > 37.5kg > 13.82kg

180 > 29.97kg > 37.97kg > 7.99kg > 47.46kg > 17.49kg

200 > 37kg > 46.88kg > 9.88kg > 58.59kg > 21.59kg

[rex]

If there is stock lift and wing dowforce, in order to see the benefits you should add the 2 values (or improvement = WingDonwforce - ( - Stock lift)).

[fuscobal]

Lift and downforce have opposite directions so you simply add them (one is vertical up and has -, the other is vertical down and has +). The bolded values are the final result wich is a small amount of downforce, knowing the wing bring a little more downforce @ a certain speed thn the lift of the OEM car @ same speed !

[the bruce]

First: Think we should a link here to your pictures:

http://www.mk5golfgti.co.uk/forum/index.php/topic,12484.msg698714/topicseen.html#msg698714


To me it's better to read this way:


speed  >>> stock lift >>> wing DF >>> Total 1 >>> wing + gurney DF >>> Total 2
in km/h . . . . . . . . . . . . . . . . . . . . . (wing DF - stock lift) . . . . . . . . . . .  (wing + gurney DF - stock lift)

   80 >>>> +5.92 kg >>>>> -7.50 kg >>> -1.58 kg >>>>> -9.38 kg >>>>> -3.46 kg

 100 >>>> +9.25 kg >>>> -11.72 kg >>> -2.47 kg >>>>> -14.65 kg >>>>> -5.40 kg

 120 >>> +13.32 kg >>>> -16.88 kg >>> -3.56 kg >>>>> -21.09 kg >>>>> -7.77 kg

 140 >>> +18.13 kg >>>> -22.97 kg >>> -4.84 kg >>>>> -28.71 kg >>>>> -10.58 kg

 160 >>> +23.68 kg >>>> -30.00 kg >>> -6.32 kg >>>>> -37.50 kg >>>>> -13.82kg

 180 >>> +29.97 kg >>>> -37.97 kg >>> -7.99 kg >>>>> -47.46 kg >>>>> -17.49 kg

 200 >>> +37.00 kg >>>> -46.88 kg >>> -9.88 kg >>>>> -58.59 kg >>>>> -21.59 kg


Added a '+' to the lift values and a '-' to the downforce ( negative lift) values.



Most interesting - What to conclude?

I'd guess depending on track most important turns are between 80 and 140 km/h. I'd say:

1.
At 80 km/h it may be of lesser significience if you have 6 kg of lift on rear or 3.5 kg DF.

2.
At 140 km/h it definitely matters if you have 18 kg lift or you have a downforce of 10.5 kg
instead, even more if you turn in and have still your foot on the brakes.

3.
If you apply the brakes at the end of start/finish rear grip will be vastly improved.
37 kg lift compared to 22 kg downforce - it's a difference of almost 60 kg at a moment when
dynamic corner weights are often below 100 kg !!! Rear wheels now have a chance to take
part of deceleration.

[rex]

Oh, fuscobal, now I understand... We were both right!
You were calculating the DF you have at various speeds and I was looking to see the gains over the stock setup...

[fuscobal]

Rex, gain over stock is exactly the columns wing df or wing + gurney df :)