improving aerodynamics with OEM parts

[the bruce]

Well, I like my coil over, my ARBs, my wheels and so on much more,
but these underside parts are some bit cheaper, Robin.

Placing the battery isn't easy, I know. There is a nice place for the
battery in the rear in the 4-motion models that we sadly don't have.
That's why I replaced my spare wheel with the compressor kit. One
more reason: saving little weight.

Second is the cabling. Using OEM cables is very expensive.

[RedRobin]

Well, I like my coil over, my ARBs, my wheels and so on much more,
but these underside parts are some bit cheaper, Robin.

Cheaper yes, but what benefits do these aero underbits actually have in reality? - Did you read cmdrfire's reply about downforce and diffusion?

Placing the battery isn't easy, I know. There is a nice place for the
battery in the rear in the 4-motion models that we sadly don't have.
That's why I replaced my spare wheel with the compressor kit. One
more reason: saving little weight.

Second is the cabling. Using OEM cables is very expensive.

....Personally, even though some of my mods have saved weight, I'm not on a mission to save weight. If I really wanted to save overall weight I'd do at least a partial strip out. And I certainly don't want to compromise my load carrying ability.

[cmdrfire]

If you want to save weight, don't buy a Golf...

[Snoopy]

Is a fixing kit you dot need to use It though just use some self tapping bolts m8 thread

I had the bolts from VW, but don't hesitate and use some standard ones.
Just take care it's zink plated or similar.

....I think stainless steel bolts would probably be best of all.

I try to avoid shiny bolts on, in and under the car (with the exception of exhaust).

....Any particular reason?

My thinking would be to achieve the best durability. Titanium might be rather expensive!

Thanks guys. I will use stainless steel bolts as we use loads of them at work so they are easy for me to get a hold of. Plus the underside of my car has loads of waxoyl on so there shinyness will not show after i do them too.

[the bruce]

Cheaper yes, but what benefits do these aero underbits actually have in reality? - Did you read cmdrfire's reply about downforce and diffusion?

This one?

As a note, maybe 10-15% of an "eco" car's efficiency gains are from improved aerodynamics on the underside.

Remember though that increases in aerodynamic downforce also result in increases in drag, making the vehicle less efficient and and reducing vmax.

Undertrays designed to reduce drag will not, in my opinion, improve high-speed stability.

I don't agree with this.

The stock underside of the GTI creates both, drag and lift.

But before explaning the reasons in bad English it would be easier you read some
scientific material. I've read the very most of it:


http://www.informaworld.com/index/780901753.pdf

http://elib.tu-darmstadt.de/tocs/133153142.pdf

http://www.buecher.de/shop/dynamik/aerodynamik-des-automobils/hucho-wolf-heinrich-hrsg-/products_products/detail/prod_id/14466476/

http://www.atzonline.de/Fachmedien/Buecher/4/132/Aerodynamik-des-Automobils.html

http://libros-en-pdf.com/descargar/aerodynamik-des-automobils-3.html

http://ieeexplore.ieee.org/iel5/4588253/4601965/04602075.pdf?arnumber=4602075

http://www.spiegel.de/auto/aktuell/0,1518,673321,00.html

http://www.fl-automobil.de/page-m_ge_die_kraft_Was_ist_Abtrieb-12.phtml


Feel free to find even more of this stuff in English. The conclusion will be the same:

A flat underside will always reduce drag and lift. Believe me. 


Without a wind tunnel we cannot state the amount of improvement but it it will be
an improvement. Flatter is better in this area.
We cannot exactly state the resulting lift front and rear (but I promise at 200 km/h
it's around 0 front and 250 N rear). No downforce of course, but better than stock.

[RedRobin]

^^^^
Crikey! That's an awful lot of bedtime reading!

Unfortunately, in this country we don't get so many opportunities to reach 200 km/h.

[Richn83]

That is a lot of reading! might help pass the friday afternoon at work    I havent read any of it yet but my understanding is that the Aero drag only starts becoming affective from 60+ mph and increases exponentially (or at least in a non-linear fashion) beyond that point.  For example if you double the speed you will increase drag or wind resistance by a factor of 4 or greater. 

So these modifications would only provide any benefit at motorway speeds in the UK or track use, but even if you get a 10-15% reduction in drag this could result in a 10% reduction in fuel use at 70mph that could equate to a 3mpg decrease in consumption, but with the in-efficiencies of the combustion engine to use the chemical energy in fuel into kinetic energy in the car (read something this morning that said about 60% wastage of energy - heat, noise, un-burnt hydrocarbons) you might just save enough to cancel out the resistance of your air-con pump!   

[the bruce]

As I said at the beginning: don't expect too much. 

Imho this package is practical:

- lowering by springs oder coilover
- VWR- or Votex lip or similar
- Zender winglip or Mk VI rear spoiler
- undertray parts

The undertray is just one aspect of the story.
c_w x A may be reduced from 0.72 to 0.68 I bet.
Btw, the most helpful mod ist dropping the the car. 

[Richn83]

Yeah I dont want to lower my car as for my usage standard is more than adequate and would just be for show, yet I imagine that lowering and fitting a lip that forces the air around the car rather than under will significantly reduce the drag, I know the VWR lip they run on their race cars states this as its main use, for me doing this mod would be about a small improvement in fuel efficiency at motorway speeds, as unfortunately thats my main use of the car travelling off at weekends around the country.

[the bruce]

Ok:


- lowering by springs oder coilover
- VWR- or Votex lip or similar
- Zender winglip or Mk VI rear spoiler
- undertray parts

[Richn83]

Ok:


- lowering by springs oder coilover
- VWR- or Votex lip or similar
- Zender winglip or Mk VI rear spoiler
- undertray parts

Yeah those would be the possibilities.

Out of interest Bruce what sort of mpg or km/l do you get at 130kph with your aero mods if you know.

[alackofspeed]

Is there a stock solution to combining the handbrake cable guide with the stone guards? I have the guards, and have offered them up to the trailing arm, noting the clash with the cable guide. Presumably there is an audi part that works with the guard.

[the bruce]

Think Audis do not have this part.
I'd like to have a look under a A3 or TT (FWD).

Ric, I don't know. But some mates report of a reduction of 0.2 - 0.4 l / 100 km.
Maybe this seems to much, but that's what they told me.

[the bruce]

http://www.wagenstad.com/index.php?modid=231&page=8-825-15

[Richn83]

Think Audis do not have this part.
I'd like to have a look under a A3 or TT (FWD).

Ric, I don't know. But some mates report of a reduction of 0.2 - 0.4 l / 100 km.
Maybe this seems to much, but that's what they told me.

Thanks Bruce that seems about 8-10% which over a tank could be 30-40 miles so not insignificant for the money spent on parts