All Things Mk5 > Performance Modifications

Improving aerodynamics with aftermarket parts

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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 !!

 :happy2:

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

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