Improving aerodynamics with aftermarket parts

[fuscobal]

Following Bruce's idea with the OEM parts, I thought we should also have a brother topic for aftermarket parts. If the aerodynamics of an OEM vehicle is more about improving mpgs, the goal of aftermarket parts is to bring more downforce while keeping the drag as low as possible. Generally there is a consensus regarding parts that should be added to a vehicle in order to increase downforce and thus cornering grip. The contenders are :

1) Front air-dam > can be part of the front bumper or an add on to the lower side of the bumper. This part alters the ratio between the high pressure slow moving air over the car and the low pressure fast moving air under the car. The lower a car it is, the more effective an air dam will be.
2) Front splitter > can be used with or without the air-dam to increase the downforce even more. It acts similar to the air dam by creating a high pressure area above it and low pressure area under it wich sucks the front of the car to the ground (see Bernoulli principle).
3) Front wing > this one's not practical for usual sedans but worth mentioning since it's highly effective on F1, Pike Peaks or similar other cars.
4) Front canards > these are small wings mounted on the sides of the front bumper in order to bring a little bit of downforce and help with the fine-tune of the front-to-rear downforce ratio. They can also produce vortices of low pressure air wich help to reduce drag.
5) Flat underbody > Flat panels installed to the underbody of a car cover the car's various mechanical parts and smoothen the air flow, keeping the low pressure and again sucking the car to the ground. They also help with the mpgs and reduce drag.
6) Rear diffuser > acts as a way of speeding up airflow (lowering pressure) and then slowing down airflow (increasing pressure), in an aid to create downforce and smooth out turbulent air exiting the rear of the car, matching the outside high pressure air while minimising drag.By providing what is essentially an expansion chamber at the diffuser's exit, the airflow has to flow and expand back to ambient pressure in the diffuser.
7) Side skirts > these make sure the low pressure air under the car moves to the rear only and doesn't interfere with the air outside the car.
8 ) Vortex generators > Vortex generators were firstly developed for the aircraft sector, this technology has made it's way into Motorsport and car design. The main function of this device is to delay air flow separation. Air flow separation is when the airflow of a object detaches from the surface and creates eddies and vortexes. This basically means that the car will result in more drag and will reduce top speed and potentially downforce due to the turbulent air entering other aerodynamic device ( rear wing for example) and the wake of air left behind the car.

So by positioning vortex generator over the rear of the roof, you effectively help to reduce drag and increase downforce via the rear wing. This will have the effect of reducing the overall drag created by the car travelling through the air at speed and the faster and faster you drive the more effect this device will have. Especially effective in speeds in excess of 60 mph (100 KMP) speed ranges.
9) Rear wing > this is essentially a reversed aircraft wing and acts to increase downforce while also increasing the drag. They usually have multiple mounting angles so that a good balance between downforce and top speed can be achieved.

This is only a short story of it but you can read more in detail articles :

http://www.rapid-racer.com/aerodynamic-upgrades.php

http://en.wikipedia.org/wiki/Bernoulli's_principle

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

[fuscobal]

Now, starting from the list above, let's see what we can do on our Golfs to improve downforce for track days :

1) Front air-dam > there are many add-ons to the OEM front bumper but no downforce data available. Most are for cosmetics only, few have been developed on track but no data for these also. The answer I got from VwRacing for the only air-dam developed on track was a little dissapointing. They said they did not test the air-dam on a wind tunnel but their drivers said a difference can surely be felt on track with it on.
2) Front splitter > same as above, many carbon splitters to add under the bumper but too small to have a real effect on downforce
3) Front wing > I think we can skip this one.
4) Front canards > I think we can skip these too as they are not too aesthetical on our cars. No one makes them for us and the trouble of making custom ones would be too big for a small revenue.
5) Flat underbody > This has also been covered on Bruce's topic of improving aerodynamics with OEM parts. While the front of the car can be easily covered by the long tray, the rear is a little more difficult to to in one piece as there's no available part.
6) Rear diffuser > highly effective part if properly done. Can be seen OEM on several cars like EVO 10 or Clio RS but again is quite small to be very effective. A real diffuser goes very deep under the car and at an angle of 10-15deg to the ground. A real difficulty in making an effective diffuser would be avoiding the exhaust.
7) Side skirts > Many on the market but again no data
8 ) Vortex generators > Can also be seen on EVO 10 mounted as an OEM part but don't know how many would be willing to try them on their Golf. These would probably be quite easy to install (universal ones should be just good). Also, don't know how effective they could be on hatch-backs as they would be too close to a presumptive rear wing !
9) Rear wing > Although there are several world renowned rear wing manufacturers, no one makes a true wing for our Golfs but VwMotorsport wich again sells it through Bildon in US only (very weird decision of not making them available through an european dealer as it would be much cheaper for us this way)

PS : I bolded only the ones I considered useful for our cars

Waiting for your replies with thoughts, links of parts available for our cars and aerodynamic data that some of you may have !

[the bruce]

Thank you, interesting topic again !!

 

I' going to try providing some useful data and at least my opinion while you already have listed the main points.


First let's have a look at the aerodynamic lift we have on our cars at stock condition:


From German magazine 'SportAuto' Supertest Golf V GTI

Wind tunnel: aerodynamic balance

    * A: 2.23 m²
    * drag coifficient (cd): 0.33
    * index (cd × A): 0.74 (more than Mercedes S-Class)

    * front: 2 kg lift
    * rear: 37 kg lift

 @ 200 km/h (!!!)


Golf V GTI:  lift 2 kg front and 37 kg rear



Golf R32: lift 3 kg front and 36 kg rear



Audi TT: 23 kg + 24 kg (wing position lifted)



Golf V: sadly no data



Focus RS: currently no data - will add them later > 8 kg front downforce; 4 kg rear lift




So in the case of the Golf (like almost all hatches) rear lift is by far more an issue than front.


Even more it's interesting to know what parts of the car do cause what amount
of drag (and most likely lift):





From reading books about aerodynamics I know underside and rear of the body are
most important. Rear roof + side spoilers (look at the Golf24) are the way to go.


But what are we able to achieve with a modded GTI?


The Scirocco R Cup has a downforce of 200 N (~ 20 kg) @ 200 km/h total both axles:



It's gonna be extremely difficult to get even more downforce than this kit car with
flat underside an rear wing.

So next question is what we have to do to reach this?


Without any doubt it's easiest not to make own developements but have a look at
Golf based race cars instead:















(of course no rear muffler fitted, but have a look for the Mk.VI Milltek which is extremely sleek)


Some high performance street cars:

997 GT3 RS:



Panamera:



Ferrari F355:



Nissan GT-R:



Mercedes SLR:




All of these have a flatter underside compared to a Golf. Some have a diffuser that
may work. But don't get confused: these cars do not have real downforce. They just
have less lift !!


1) Front air-dam:

Fuscobal, Im with you. I think the VWR is the most efficient. The Rieger looks the same
in shape. I run the Volkswagen/Votex which seems a more elegant alternative to me.


2) Front splitter:

Kerscher make one:



Of course before purchasing one (or making one on your own) I'd think about the
question if you really need one.
Another idea is to make a splitter as part of a complete underbody tray including a
rear diffuser.


6) Rear diffuser:

To me the Clio RS diffusor seems to have a proper shape:






9) Rear wing:

If you don't want to fit a huge wing some roof spoilers may be a less efficient but
more elegant alternative:



> www.vary.co.jp



There's so much I like to say/write about it, but that's it for now. ;)

[mkviken]

the clio 197 diffuser definitely reduces lift. i think it ads 20kg of downforce or something.

when i head my clio it would suck leaves etc up off the road and stick to the underside and back of the car so I'm sure it was working.

[mkviken]

these reckon 70kg of downforce! doubt that but I'm sure it was 20-40kg or something

http://www.autoexpress.co.uk/carreviews/firstdrives/68538/renaultsport_clio_197.html

[the bruce]

Thanks for this link.  

Bespoke body panels add a sporting, muscular image to rival the Clio V6 - especially from the rear, where it has a huge aerodynamic diffuser under the bumper. Those custom panels are bigger than on the standard version, too - although they need to be to house the 10mm longer wheelbase and 5cm increased track of the 197.

The changes aren't only for visual impact. That diffuser really works, and can add up to 70kg of downforce.

Well, they speak about "up to 70 kg", so it seems this is at top speed (or downhill).
Next question is it the rear axle downforce value (which I don't believe) or just the
amount which the diffuser adds to whole aerodynamics. So there may be some lift
at the rear roof (as always on a hatch) and up to 70 kg downforce at the diffusor
which may give a total near zero lift.  

A by far better value than on a Golf of course.  

[mkviken]

its definitely better. the RS 197/200 is a seriously good handling car and the brakes actually give some confidence, unlike the golfs!

the amount of debris you get under the car is crazy on the 197/200 though. i had to remove mine a few times to clean all the wildlife out from it!

[fuscobal]

Wow, very good info Bruce. It's for the first time I see the downforce numbers for the Golf GTI. The 2kg of lift on front could probably be negated by the VwRacing air-dam (wich I've already ordered by the way) and maybe even have a little downforce. Does anyone have an idea of how much downforce can an air-dam bring to a street car ? (I know it may vary a lot from car to car but a medium value would give us an idea). s for the 37Kg on the rear, we probably need a large wing to negate that and add downforce. I tried contacting VwMotorsport and Becker Carbon but no answer so I would look into Simon McBeath wings (made by Reverie & DJ Racecars). Some links for these :

1) http://www.djracecars.com/carbon-fibre-wings-and-aerofoils.asp > 100mph, 1400mm span wing downforce data > http://www.sm-designs.ndo.co.uk/images/singleDfvalpha.gif

2) http://www.reverie.ltd.uk/225mm_chord_wingdata.php > very nice data of downforce vs drag vs BHP absorbed vs angle of attack vs span ...

3) http://www.sm-designs.ndo.co.uk/ > Simon's website from UK > This guy seems to have a huge background in wing design. Look at the cars he did until now !!!

PS : The downforce of the Scirocco Cup can be without doubt bettered because those cars are limited by their category. The wing cannot be installed higher than the roof of the car or further back than the rear bumper wich limits its effectiveness. They may also have a limited span. I would do a wing wich looks more like this one >

[mkviken]

I quite like the look of the gurney strip for the spoiler. Wonder is any European manufacturers make one?

http://www.vary.co.jp/reife.html

I like th look of this too but don't know what the quality would be like and if it would actually make a difference

http://www.ebay.co.uk/itm/110717469037?redirect=mobile&autoredirect=off

[the bruce]

Fuscobal,

You know I had 5 kg + 35 kg in my mind, but compared to 1.4 to of a complete Golf it's almost the same.
Many people tell about the rear feeling light at high speeds. When braking it is light on rear !!  


I strongly believe I already have (very small) downforce on front with my Votex + undertray + lowering.
I guess the VWR should give you ~ 5 kg. You are already lowered as well, so you'll have some downforce then.


Yes, the Scirocco R Cup uses the OE Votex rear wing which has to be mounted in a TÜV conform position.
It does not givve the amount of downforce on these cars possible with a higher position and steeper angle.


Keep in mind a diffuser is more efficient than a wing. A well designed diffuser provides both downforce +
reduction  of drag at the same time. Any wing will allways produce some drag as well.

But there's also a good point on this: drag produced on top of the car will lift the front (by a small amount).
So not only the wing's downforce, but even it's drag will optimize front/rear balance.


If you like to mod your exhaust (eliminate rear exhaust for example) I'd consider a diffuser as well. The Clio
RS diffuser seems even small enough to combine it with the Milltek Mk.VI exhaust:







If you watch the GTI24 pics carefully you will recognize the small 'spoilers' on the sides
of the rear window:



All Sciroccos come with it from factory:



and even many other cars:





Renault Scenic and many more as well . . .


Why not the Golf?



ps:
I was looking for a roof spoiler similar to the Vary/Reife, but never could find one.
If you have a Mk.VI be happy to have a much more efficient rear roof spoiler from
factory than the M.V came with.

[fuscobal]

My trouble with the big rear wing would be how to fix it to the roof of my car. The one seen on the Scirocco 24 pic posted by me seems to be fixed only by some adhesive (I can't see no screw/bolt there). If I have the wing, I think I can replicate those black aluminium pieces (I will have to cut a cardboard on my car's OEM wing and use it as a mold for the aluminium part (the OEM wing does not have perfectly straigh shapes so I will have to take the mold on the cardboard first and then make the aluminium part after the cardboard). There are 2 question here :

1) Will it be enough for me to use an adhesive to hold the wing into place (50-70Kg of downforce is quite some weight). The 2 aluminium pieces catch on the OEM wing like a claw (from under and above) but the cantact area is really thin (like 5mm or whatever the width of the aluminium board used is) !

2) Supposing I find a strong enough adhesive, will the OEM wing hold the big rear wing + 50-70kg or will it break > I don't know if the metal boot lid has a reinforcement that goes inside the OEm wing as well or the OEM wing is a simple plastic add on to the metal boot lid with no metal reinforcement !

[the bruce]

Ford Focus RS:



Fahrzeugstirnfläche (A): 2.350 m²
Luftwiderstandsbeiwert (cw): 0.36
Luftwiderstandsindex (cw × A): 0.86
Vorderachse: 8 kg Abtrieb (front downforce); Hinterachse: 4 kg Auftrieb (rear lift)


Ford Focus RS Raeder Motorsport:





Fahrzeugstirnfläche (A): 2.350 m²
Luftwiderstandsbeiwert (cw): 0.35
Luftwiderstandsindex (cw × A): 0.82
Vorderachse: 0 kg Abtrieb (front zero lift/DF);  Hinterachse: 9 kg Auftrieb (rear lift)

Während der Serien-Ford Focus RS im Windkanal an der Vorderachse mit leichtem Abtrieb (rund 80 Newton) gemessen wurde, zeigt die Raeder-Variante bei der Messung an gleicher Stelle eine neutrale Null. Eine etwas stärkere Auftriebstendenz offenbart sich auch an der Hinterachse, die bei 200 km/h nunmehr um 85 Newton (Serie: 40 Newton) entlastet wird. Der tiefere Grund für die geringfügige, gleichwohl ohne Konsequenz bleibende Verschlechterung dürfte in der Tieferlegung zu suchen sein. Trotz der breiteren Bereifung fällt der cW-Wert mit 0,347 (Serie: 0,36) tendenziell aber etwas besser aus.

http://www.sportauto-online.de/supertest/raeder-ford-focus-rs-im-supertest-fronttriebler-wird-zum-porsche-schreck-3383831.html?show=2


Golf Mk.VI GTI Edition 35:





Fahrzeugstirnfläche (A): 2.230 m²
Luftwiderstandsbeiwert (cw): 0.31
Luftwiderstandsindex (cw × A): 0.69
Vorderachse: 0 kg Auf-/Abtrieb (front zero lift), Hinterachse: 31 kg Auftrieb (rear lift)

In aerodynamischer Hinsicht hat sich in der nunmehr sechsten VW Golf GTI-Generation einiges getan. Obwohl der cW-Wert signifikant verringert werden konnte, und zwar von vormals 0,33 auf jetzt 0,31, sind auch die Auftriebswerte verringert. Die statisch mit 882 Kilogramm belastete Vorderachse bleibt bei 200 km/h unberührt, das heißt der Auftrieb liegt bei null.

Der cW-Wert hat sich signifikant verringert (von 0,33 auf 0,31) und der Auftrieb erfährt eine Verbesserung: Bei 200 km/h geht er gegen Null.


Golf Mk.VI R:



    Fahrzeugstirnfläche (A): 2.250 m²
    Luftwiderstandsbeiwert (cw): 0.33
    Luftwiderstandsindex (cw × A): 0.75
    Vorderachse: 5 kg Auftrieb (front lift), Hinterachse: 33 kg Auftrieb (rear lift)

Der cW-Wert fällt mit 0,33 nicht so windschlüpfig aus wie erwartet. Das lässt sich aber sowohl mit der großen Stirnfläche von 2,25 m² als auch mit den großen Kühlluftöffnungen in der Front und den breiten Reifen leicht erklären. Während der Auftrieb an der Vorderachse mit 47 Newton vernachlässigbar gering ist, steigt er an der Hinterachse bis 200 km/h auf 326 Newton an, was einer Entlastung von etwa 33 Kilogramm entspricht.


Audi RS3:



Fahrzeugstirnfläche (A): 2.180 m²
Luftwiderstandsbeiwert (cw): 0.36
Luftwiderstandsindex (cw × A): 0.78
Vorderachse: 21 kg Auftrieb (front lift), Hinterachse: 1 kg Abtrieb (rear very slight DF)

Aus der Stirnfläche von 2,184 m² und dem cW-Wert von 0,355 errechnet sich ein Luftwiderstandsindex von eher mäßigen 0,78. Die ungleichmäßige Gewichtsbalance von 60 zu 40 Prozent (Vorder-/Hinterachse) wird durch die aerodynamische Konstellation etwas gemildert. Der Auftrieb an der Vorderachse beträgt bei 200 km/h 203 Newton. Die Hinterachse wird mit Abtrieb bedacht - auch wenn es nur vernachlässigbare 14 Newton sind.

[fuscobal]

Weird...why does the Raeder Motorsport Focus (wich is has lower ground clearance) has more lift than the stock car ? Shouldn't be exactly the opposite ?

[rex]

Yes, and it is loosing downforce on both ends  .

[the bruce]

It has 265 mm wide tyrers (at least 275 mm in real). Wide tyrers disturb aerodnamics.

Pardon me for the German passages. it's the genuine SportAuto text.


Reifenmarke:    Dunlop Direzza 03G
Reifen:    265/35 ZR 18
Felgengröße (vorn):    9,5 J x 18

1.4 g instead of 1.2 g stock.