Functional flat underbody and rear diffuser design

[s4mmie]

Has anyone had any experience with the above?

After my last outing at Lakeside, it revealed a big problem with a combo of no weight in the rear and lots-o-lift.
Braking into the corners quickly you could feel the rear end get squirmy, and the rear also squirmed about hooking it along the straight around the dog-leg.

I like the idea of a rear wing for simplicity, but I wouldn't run it permanently and I'm trying to figure out a mounting solution that would let me return the car to stock if need be.

So in the mean time, I've turned my attention to the underbody and diffuser.
I'm currently trying to decide on the best material to use. Two thoughts: aluminium sheeting, or ABS plastic.
I would prefer to use the ABS plastic as it is potentially easier to work with.
Any opinions on if the ABS will have enough rigidity?

I'm also concerned with cooling. I'm thinking of running a naca duct at the front of the gearbox so that air can run down the exhaust tunnel and then out a vent in the rear..

I'm also contemplating removing the tub for the spare wheel and stitch welding a panel in the hole to allow room for a functional rear diffuser.


This is my end game --
http://www.vwwatercooled.com.au/forums/images/imported/2013/02/2422_31_zpsaf946786-2.jpg

[Stuwey]

I'd be looking closer at your driving technique, tyres/pressures, and suspension.

My brother runs a 1299cc Nb Mini under very tight class restrictions, and he's currently almost 5 seconds a lap quicker than me around Mallala. My first time out and he's done many, but you the the idea. 1299cc/10" wheels, 7,5" brakes/glass, full trim.. ZERO aero enhancements and just 145hp.

More experience, further adjustments first. Do you have a rear sway bar? Ditch it for one meeting to calm the rear, see how you go.

[VWindahouse]

It's more this track in particular Stuwey as all Polo's with no rear seats and no spare tyre experience what Sam has described above including me and I have all kinds of bracing, sways front and rear set on hard, coilovers with no visible gap between the guards and rubber, and antilift bushes, and wide sticky tyres to boot. What this track seems to exaggerate is lift in the rear that causes those feelings of the rear end becoming light and lose when turning on the main straight through the dog leg which has an uneven surface to begin with. Last time there I pinned it flat through there at 190kph and had a small heart attack as the car skipped over until it found grip again. Even Sean a very well seasoned track whore gets lift on the back straight when he pins it down the hill and he has a fully laden polo.

[shinygti]

Looks like a good idea and might help with stability. Thinking for easy use and light weight thinking the abs plastic is better idea but where do you get it from? Also have you thought about acrylic ?? You can buy it at bunnings and is cheap so you can experiment a bit with it.

[Eddy]

I'd be looking closer at your driving technique, tyres/pressures, and suspension.

More experience, further adjustments first. Do you have a rear sway bar? Ditch it for one meeting to calm the rear, see how you go.

Hoyhoy.

Spot on Stuwey.

[Stuwey]

Thanks, Eddy. I just think these cars have so much more potential, before you throw more money at it. The 'more more more' theory doesn't work!!!

[s4mmie]

Always willing to listen to those with experience.

I tend to agree with Ant on this one though.. it seems to be once you get over ~170 that the problems appear. I wouldn't say that you could adjust your driving style to the scenario Ant mentioned above, apart from slowing down and changing pants. I would prefer not to slow down. It's hard to argue with the aerodynamics showing how much lift these suckers have at the rear standard, let alone with ~50kg less in the rear.
In particular, hard braking before a corner almost feels like its going to mono.

I know there's a few schools of thought regards RSB. I feel that it's not necessarily the cause but more not helping the situation. I like it at slower speeds because I can force a bit of oversteer but it may be a little too skittish for the track. I'll give it a try this month, but like I say I think it's more accentuating another problem.
Like I mentioned, the car is very composed below ~170.

Funny you mention a Mini Stuwey, I'm pretty sure we have both (fellow ausminier?) and I know which one I'd rather be carrying speed in ;) Still can't beat it.

I'm not one to throw money at a problem, but from driving it, it feels like lift more than inadequate suspension or incorrect lines. I'm more than happy to take opinions on board :)

Also, re tyre pressures, I ran 36 psi instead if 38 and it made a great difference to rear end grip. Not sure I want to go much lower than that though..

[Eddy]

:
The 'more more more' theory doesn't work!!!

Hoyhoy.

Spot on again Stuwey...:driver...:checkeredflag:

[Stuwey]

http://www.vwwatercooled.com.au/forums/images/imported/2013/02/800pxMazda_808_of_Tony_WallisJPG-1.jpg

I've done a 1:28. My brother has done a 1:23. This Mazda, does a 1:13. Improved Production is VERY limited. It's one of the lowest-tech cars in the country, but it's the fastest by a long shot.

Keep experimenting. Keep developing. Too many people rock up with a shopping list as long as their arm with the biggest, best, most expensive components. Doesn't work, though.

[Eddy]

it seems to be once you get over ~170 that the problems appear.

Hoyhoy.

So are we cornering @ this speed,,, I thought turn one @ Eastern Creek was the fastest available in Aussie.

[Eddy]

Also, re tyre pressures, I ran 36 psi instead if 38 and it made a great difference to rear end grip. Not sure I want to go much lower than that though..

Hoyhoy.

I adjust tyre pressure straight after each session & run down to 28 psi on the hardest working one, each track is different, this is with semi-slicks.

[Stan]

abs plastic can be bought in large flat sheets, i looked into it when considering making my own front body kit.
It's not that expensive, about $150 for over a square metre. Cant remember who the supplier was , but they were Aussie and delivered. I found them with a google search...
It can be heated and molded, cut into granules and cast, it's pretty versatile, and metho (or something similar, cant remember) can be used as a solvent to turn it into a liquid form to paint over forms for shaping...

you could use a standard rear spoiler as a form and make a wilder one with more aero...

Ahh, here it is, talk with these guys about abs plastic: http://plasticsfactory.com.au/

[team_v]

Hoyhoy.

So are we cornering @ this speed,,, I thought turn one @ Eastern Creek was the fastest available in Aussie.

There is a left handed kink on the main straight that you hit at about 170km/h (approximately 15-20 degree change).
There is also a left turn later on that changes camber halfway through so you feel a lot of lit-off on it.

[seangti]

I'm not denying the lift and tip toe feeling, but that's part of driving on the limit if adhesion. More track time on different circuits and driver training helps with different approaches to high and low speed cornering, for the kink i back off but get on the throttle very early so the cars as better balanced at speed rather than trailing braking late into the corner. Check some of the my vid and te g-forces, cant recall if ts clear or not. Would also be worth getting a reliable lap timer, creating similar segments amongst us so we can also compare to each others strengths and speeds.

But all keen on downforce stuff, extremely interesting. Though again without a reliable timer it'll be hard to show any tangible gain.

[h100vw]

Hoyhoy.

I adjust tyre pressure straight after each session & run down to 28 psi on the hardest working one, each track is different, this is with semi-slicks.

It could take months on Happy Laps to achieve this Eddy. You only get 30 minutes. If it was open pit or an all day session you could do it.

I have used an infrared gun on the tyres too in the past to check set up. A bit extreme for Happy Lapping really.

Gavin

[Guy_H]

I have done a fair bit of research into this as we are building a Scirocco R track car, we are also blessed with good friends that build some of the fastest time attack cars in the country & work with big $$ Aero engineers.

As much as its a great suggestion, I don't think a full flat bottom is going to give you positive benefits without a balanced wing / splitter / diffuser combo. Obviously you are trying to limit air getting under the car firstly, and that which you allow under, you then manage it. The rear wing is what is going to help with "weight" on the rear end.

Secondly, this is where front spring rates & damping (as well as sway bars) need to be set up to help limit & negate massive weight transfer (a two way differential also helps at this stage by keeping the front straight). A FWD car with a stiffer front end will be faster that one sprung the other way around.

If you are interested, I have a heap of links & you can purchase some very good books about it.

I have 900+ photo's I took of the Essen Motor Show in Germany, a lot studying the under-bodies of the cars (Including the Polo WRC, Scirocco wide body 24hr & the new Poland GTI cup) and the different approaches they use on the aero approaches. If you are around, pop in & I'll show you them :).

P.S. to get your true tyre temps, you can't do this with an infra red pyro, on a cool down lap & pull into the pits you lose a huge percentage of the working temperature. Invest in a proper probe that goes 5mm into the surface where the tyre is still at track temperature to see what is really going on.

[VWindahouse]

Wow, hope there's a build thread in the near future for the Rocco Guy?!?

[Guy_H]

I hope so, I love so much the 24 hour GT widebody version, but to make it eligible for time attacks & production racing I can't go that far. We actually have two here donating to the project.

The donor is a brand new stat write-off so can't be registered - perfect for the job!

[Guy_H]

P.S. Sam,

here is some good reading for you:

Improving aerodynamics with aftermarket parts (http://www.mk5golfgti.co.uk/forum/index.php?topic=41887.0)

[VWindahouse]

I really hope it works out, a Rocco time attack car would turn a lot of heads! There is some pretty mad looking aero'd up roccos throughout this link for inspiration Guy!

Improving aerodynamics with aftermarket parts (http://www.mk5golfgti.co.uk/forum/index.php/topic,41887.75.html)

[Guy_H]

Yeah - I have a lot of close up photo's of the Polish GTI's - actually got to sit in one :)

http://www.vwwatercooled.com.au/forums/images/imported/2013/02/57j2fjmu-1.jpg

[VWindahouse]

Sounds like a very productive "business trip" then :P

[Stuwey]

I'm not denying the lift and tip toe feeling, but that's part of driving on the limit if adhesion. More track time on different circuits and driver training helps with different approaches to high and low speed cornering, for the kink i back off but get on the throttle very early so the cars as better balanced at speed rather than trailing braking late into the corner. Check some of the my vid and te g-forces, cant recall if ts clear or not. Would also be worth getting a reliable lap timer, creating similar segments amongst us so we can also compare to each others strengths and speeds.

But all keen on downforce stuff, extremely interesting. Though again without a reliable timer it'll be hard to show any tangible gain.

Slow in, fast out. Who'd have thought that would work in a FWD hot hatch.

Drive drive drive drive drive!! More super sprints/'time attack' track days. Less 'happy laps' bs!

[clicht]

Yeah - I have a lot of close up photo's of the Polish GTI's - actually got to sit in one :)

http://www.vwwatercooled.com.au/forums/images/imported/2013/02/57j2fjmu-1.jpg

Like :-)

[kaanage]

If you don't want to soften the rear roll stiffness or increase the front, raise your front ride height or lower the rear. With a torsion beam rear end/mac strut front end, this shifts the bias of the grip slightly towards the rear.

I did this after I spun at turn 1 Phillip Island with lift off oversteer and I only lifted off for a moment at turn in. Since then, the car is much more stable under brakes too (the dive under brakes exacerbates the situation).


Drive drive drive drive drive!! More super sprints/'time attack' track days. Less 'happy laps' bs!

+100 :thumbup: (and kudos for being brave enough to state what I feel)

[s4mmie]

Thanks for weighing in here Guy, that threads a good read.

Hmm that's interesting kaanage as I had actually lowered the front more since last time... I have a much stiffer front ARB and brace to go on the front, so hoping that helps.

I will say this thread has brought some interesting opinions.
Let me preface the following sentences with: I don't claim to be an amazing race car driver so I appreciate the feedback from those with more experience and I like to learn. And, I'm not having a go at anyone, we're all VW enthusiasts trying to help one another out. These are merely my observations.

With that in mind..

I find it curious that Eddy agrees with Stuwey about the 'more more more' approach not working, when the little blue girl has an insane amount of custom suspension and trickery...
I also find it curious that Stuwey posted a picture of a reportedly very quick Mazda, which has a whopping front air dam and rear ducktail... all aero mods, and at the end of the day that's what we're talking about here, whether it be in the form of a wing or diffuser.


Like I said, not having a go, just a little confused.




I fail to see how driving ability could change the physical grip limitations of the car at a certain speed following the right line through a corner. There is just logically a point where no matter who was driving it, you'll max out your current grip for a corner at a certain speed. Obviously slowing down will fix it, but that's not the point is it?

For example, would you rip all the aero off an F1 and when it goes spiraling off the track tell the driver he needed to practice more?

[VWindahouse]

Wow, this thread has gone a long away from the OP's question about what material would be best to make up a home made diffuser.

[Guy_H]

Best info above is practice, practice, practice.

The problem is isolating the issues you are experiencing. My suggestion (seeming you are in Brisbane) is to do one of the Paul Stokell driver training days at QR or at Lakeside.

This will give you:

A- some driver tuition one on one on braking points, lines, balancing the car for corners & when to get on the gas.
B- You can let one of the instructors drive your vehicle & give you valuable feedback on where they think your car is lacking or has limitations.

Paul himself has won several national championships in FWD cars.

Probably your best investment of cash at the moment.

Track & Training Days (http://drivingevents.com.au/performance-driver-training/track-and-training-days)

[VWindahouse]

Best info above is practice, practice, practice.

The problem is isolating the issues you are experiencing. My suggestion (seeming you are in Brisbane) is to do one of the Paul Stokell driver training days at QR or at Lakeside.

This will give you:

A- some driver tuition one on one on braking points, lines, balancing the car for corners & when to get on the gas.
B- You can let one of the instructors drive your vehicle & give you valuable feedback on where they think your car is lacking or has limitations.

Paul himself has won several national championships in FWD cars.

Probably your best investment of cash at the moment.

Track & Training Days (http://drivingevents.com.au/performance-driver-training/track-and-training-days)

I totally agree with that. I did a morning of driver training theory at Lakeside last year and it was very good as they went over each corner in detail etc. However the instructor did say you don't let off the accelerator through the kink near the end of the straight which by then I'm in 5th gear and last outing hit about 190. Here is a map of Lakeside for those who have never been there

http://www.vwwatercooled.com.au/forums/images/imported/2013/02/Lakeside_zps57b11b32-1.jpg

It's taken a few outings to Lakeside to muster up the courage to do pin it flat at those speeds through there as there is a big concrete wall not 5 meters or so off the right hand side of the track to great you if you get it wrong and many have. The G forces pull towards that direction which is not a nice feeling. To add to that there are imperfections in the track surface right at the turning point which actually lift you off at the moment those G forces start to act. In my limited opinion and experience when this is combined with the natural lift the polo encounters (as seen in the thread I posted about aero which shows the rear end lifts at 36kgs at 200kph added to the fact we've taken out another 50 plus kgs from the rear) it all combines for a brief holy crap moment. With speaking to a couple of people who attend happy laps that do regular timed events there and clock very impressive times they both say until they got their aero sorted ie. front splitter, adjustable rear wing and wind vortex generator they suffered the same thing.

[kaanage]

Wow, this thread has gone a long away from the OP's question about what material would be best to make up a home made diffuser.

Sure, but a diffuser is going to do SFA in solving the instability issue - you have listed the extent of aerodynamic changes needed in your post above which is far more dramatic than a rear diffuser alone.

In your situation, the solution is to either man up and hold it flat (easier said than done, I know since I don't do it at turn 1 PI where I probably should) or to lift a BEFORE the corner and then get back on the gas as you turn in.

Stability vs agility is an area where you are always compromising on a track setup and the optimal for a fast flowing track is not the same as for a tight twisty one.

[s4mmie]

Thanks Guy, those track days look fantastic and well worth the money. I may have to bring it up with the Minister of W&F.
No doubt about it, the more practice I can get the better, it's a ball to drive.

Just to be clear, I'm not driving like an absolute lunatic and expecting modifications to make up for my lack of driving ability or judgement for putting it too hard into a corner. I've noticed that when I'm carrying speed, the rear feels very light after losing a lot of weight from the rear. The problem was nowhere near as bad before. The way I see it, if I can potentially add the amount of downforce equivalent to the weight I've removed, then at speed it should effectively be the same as fully laden and I'm back to square one. But hopefully faster being lighter..

Is that the wrong way to go about it?

I'm not trying to be argumentative in any way, I don't think anyone is, just trying to make sure everyone's on the same page.
I don't mind the tangent this thread has taken, as it seems we all have the same end goal: to be better and faster drivers.

[kaanage]

The basic problem is that the car is hugely unbalanced with all the stuff removed from the rear and the aero package needed to fix it would be dramatic and probably impractical for street use.

I'd try putting the rear seat back in and filling the tank up to see how it feels but without doing proper timed track sessions, you will never really know which setup is faster unless you can get accurate splits for that sector with a timing device.

[clicht]

Best info above is practice, practice, practice.

Track & Training Days (http://drivingevents.com.au/performance-driver-training/track-and-training-days)

Any suggestions on equivalent in Melbourne....thanks

[s4mmie]

...or to lift off BEFORE the corner and then get back on the gas as you turn in.

Yep, that's the approach I take at the moment. It beats meeting the wall :P

Fair point about the rear diffuser not being as effective as a rear wing. The main reason I'm pursuing the diffuser is it's a little more subtle. I would like to run a wing solely for track days, but I'm still trying to come up with a mounting solution that is completely removable. Until then, I thought it would be worth pursuing or even bring up the idea of a rear diffuser/flat pan combo. The way I understand it is if you can get the air moving faster under the car then above, it effectively sucks it to the ground..
Even smoothing out the rear area so it's not acting like a damn parachute!
My thoughts were if it was cost-effective it might not be a bad way to go. Bang-for-buck seems to be in the wings favor tho.


I think there's merit in both sides of opinion, but personally I don't want to learn how to drive a skittish car fast. Putting the weight back in or removing components may help alleviate the problem, but then I lose the benefits of those in other areas. Like I mentioned in my last post, I feel that a bit-o-aero will give me the best of both worlds. At least then I can feel confident to practice and push harder. A few years ago I would just sack up, but I have a wife to come home to people! haha :toothless:

[s4mmie]

Just to throw another Q out there; How low would be too low pressure in the rear with low profile tyres?
IIRC low profile tyres need higher pressures than normal to help protect the wheel itself which is why I havent ventured too low..
Currently gone from 38 to 36psi with a marked improvement.

[Guy_H]

QR is a way better track to do your driver training on, then build up to Lakeside.

Tyre pressures? - that will depend on a huge bunch of factors - that is something the driver trainers can help advise on :)

Clicht - I think John Bowe does similar days in VIC John Bowe Driving Defensive Driving Courses & High Performance Driving Courses (http://www.johnbowedriving.com.au/)

[kaanage]

Any suggestions on equivalent in Melbourne....thanks

Just get on to a track day and drive, Clint. You've done open wheelers in the past which is far more than most of us here, so don't bother with driver training unless you feel something is "missing" when you get out there.

[team_v]

Just to throw another Q out there; How low would be too low pressure in the rear with low profile tyres?
IIRC low profile tyres need higher pressures than normal to help protect the wheel itself which is why I havent ventured too low..
Currently gone from 38 to 36psi with a marked improvement.

I thought for circuit driving you wanted higher pressures as opposed to low pressures for drag racing.

[Guy_H]

I thought for circuit driving you wanted higher pressures as opposed to low pressures for drag racing.

Depends on your tyre. An "R" compound will have a sidewall 4 to 10 times stiffer than a road tyre, so will work better with lower pressures. edit: and a drag tyre will be considerably software in the sidewall over a street tyre too.

[Stan]

just a thought, but wouldnt it be better to make a series of single changes instead of a heap all at once - that way each change can be evaluated on its merits.

e.g. first try different anti sway bar settings, then put seats in, then rear wing, then splitter, etc and work out which things in your case make marked improvements, vs which things make little or no gain. An extensive process, but the end result is more informed...

[team_v]

just a thought, but wouldnt it be better to make a series of single changes instead of a heap all at once - that way each change can be evaluated on its merits.

e.g. first try different anti sway bar settings, then put seats in, then rear wing, then splitter, etc and work out which things in your case make marked improvements, vs which things make little or no gain. An extensive process, but the end result is more informed...

I think the problem is that there are so many variables (air temp and humidity, tyre wear, amount of fuel, potential boost leak from DV etc) that it is almost impossible to tell the differnece that one change will make.
You also have to bear in mind tht the more you drive the same circuit you are going to be more comfortable with it so you may end up going quicker by your own practice rather than any modification.

[Stuwey]

just a thought, but wouldnt it be better to make a series of single changes instead of a heap all at once - that way each change can be evaluated on its merits.

e.g. first try different anti sway bar settings, then put seats in, then rear wing, then splitter, etc and work out which things in your case make marked improvements, vs which things make little or no gain. An extensive process, but the end result is more informed...

And, ultimately faster. After all, isn't that what we're trying to achieve?

You all say the Mazda has heaps of aero. Don't forget the hi-tech leaf springs and recirculating ball steering!

I think you lot need to sort some things out.
-Is it a race car, or a street car?
-Can you actually afford to do this? Both monetary and emotionally (if you get it really, really wrong)
-if you can't afford to leave it in a crumpled mess at the track, you can't afford to take it there!
-Sam, there really is something incredibly satisfying about driving essentially average cars, very fast.

Change one thing at a time. Money and stick-on things won't make your car fast, developing it over time with well thought-out additions and adjustments will.

[s4mmie]

Good points Stuwey, and obviously some of the limiting factors when trying to build a street x track car.
I think that before going any further, my money would be best spent on getting a driver in the car with more experience and seeing how that goes.

Who knows, maybe I just need to learn to drive it the way it is? I like how it handles at the moment. It could maybe do with a little bit more turn in. I figure you want the car to want to rotate rather than understeer like a pig.

[Redliner]

If you don't want to soften the rear roll stiffness or increase the front, raise your front ride height or lower the rear. With a torsion beam rear end/mac strut front end, this shifts the bias of the grip slightly towards the rear.

I did this after I spun at turn 1 Phillip Island with lift off oversteer and I only lifted off for a moment at turn in. Since then, the car is much more stable under brakes too (the dive under brakes exacerbates the situation).



+100 :thumbup: (and kudos for being brave enough to state what I feel)


Totally agree.
When I installed my Kw coilovers the front was lower than the back.
I took the car out for a test drive and the tail was all over the place.
Played around with heights and the handling changed dramatically.

If the car is too low it could also affect how it handles. (spring pretension, roll centres etc).
I would think that fine suspension tuning would have the greatest effect on lap times
combined with testing and practice.

[Eddy]

Hoyhoy.


http://youtu.be/zwFg5Rad0cA

Folks watch this video of Warren Luff (he can drive) in Guy Hardings new Polo @ 2011 June Motor Magazine Shootout, he ended up doing
a damn good time.
You'll see what I mean in the first 1.5 minutes.
It behaves just like the Little Blue Girl, Its only after I got that sort of behaviour in the Pog that I got my fastest times.
If I go out too hard on the first lap, I'm a goner, I know, been there many times, thats why I like Wakefield Park.
It has run off everywhere, good for OLD FARTS like me, plus it lets ya go 110% & taste the dirt, but after a wash all is good.
Me started late in life with tracking, 2007 in the Dutton Rally I achieved 1'18" @ Waky in a powerfull car, now with practise
I've obtained a sub 1'10" in a wee Little (Blue) Polo.

What I'm trying to say is,,,,, things can get scary out there,,,,,PRACTICE.

[seangti]

Those with coil overs, perhaps also have a look at corner weighting so you know where the weight is sitting. I believe there is a place down at slackscreek with corner weighing ability. They may also be able to offer input as to rebound/spring rates. Some changes could assist balance.

I'm a fan of making incremental changes and measuring. From total stock to adding lowered springs gave me less consistent lap times, but much the same and few more "moments", added the bilstein struts and dropped 2secs a lap on flogged out continental tyres but had great adjustability. Put kumho ku31, lost 1 sec a lap and some on the limit adjustability. This was at QR sprints on their timer, not the gtech. If I did tyres and suspension together I wouldn't know how each affected the other. I also know my max braking g-force and cornering force so can compare forces with speed and time as I make changes to the car and my driving.

It's really just trying to optimise your current set up and ensure you can get everything from it.

Hope this isn't a rant, sorry if it is.

[clicht]

Just get on to a track day and drive, Clint. You've done open wheelers in the past which is far more than most of us here, so don't bother with driver training unless you feel something is "missing" when you get out there.

Yeah, you right...its just the polo is rather different from my race cars, but the basics are the basics....sure I'll see you soon on the track :-) or a good Hill climb

[kaanage]

A road car will be a lot more forgiving and less responsive than an race car so you don't have to worry about being caught out as long as you don't leave the pits and then try to brake and turn in on the first corner like in the open wheeler ;)

seeya out there soon, I hope

[pologti18t]

Have a look at the autospeed.com site. Julian has done a few stories on aerodynamics. He has a Skoda Roomster and sometimes posts in that forum.

His aim was for improve economy though
AutoSpeed - Trialling a Rear Undertray (http://www.autospeed.com/cms/article.html?&title=Trialling-a-Rear-Undertray&A=110872)
AutoSpeed - Modifying Under-Car Airflow, Part 1 (http://www.autospeed.com/cms/article.html?&title=Modifying-UnderCar-Airflow-Part-1&A=2455)
AutoSpeed - Modifying Under-Car Airflow, Part 2 (http://www.autospeed.com/cms/article.html?&title=Modifying-UnderCar-Airflow-Part-2&A=2456)
AutoSpeed - Reducing the drag on square-back cars (http://www.autospeed.com/cms/A_112656/printArticle.html)

[blackbetty]

Have a look at the autospeed.com site. Julian has done a few stories on aerodynamics. He has a Skoda Roomster and sometimes posts in that forum.

His aim was for improve economy though
AutoSpeed - Trialling a Rear Undertray (http://www.autospeed.com/cms/article.html?&title=Trialling-a-Rear-Undertray&A=110872)
AutoSpeed - Modifying Under-Car Airflow, Part 1 (http://www.autospeed.com/cms/article.html?&title=Modifying-UnderCar-Airflow-Part-1&A=2455)
AutoSpeed - Modifying Under-Car Airflow, Part 2 (http://www.autospeed.com/cms/article.html?&title=Modifying-UnderCar-Airflow-Part-2&A=2456)
AutoSpeed - Reducing the drag on square-back cars (http://www.autospeed.com/cms/A_112656/printArticle.html)

As i have a huge interest in land speed racing (i especially hope that they can approve racing on lake Omeo in country Vic) i very much enjoyed reading these articles, thanks for posting the links.

I have articles at home on aero tricks for cars - i'll try to go through them on the weekend and find some relevant stuff for this thread.

As the GTI is a hatch the sudden drop ver the rear window is probably the single biggest area to make aero gains. the undertray would be second (mainly because of the tray thats already there from the front bumper). the polo bluemotion (not sure if that was the model name) also had some good areo ideas

[VWindahouse]

Very true. Here is a link to some wind tunnel testing for the Bluemotion 9n testing.

POLO BM IN THE WIND TUNNEL - YouTube (http://www.youtube.com/watch?v=fySDCXrY0e4)

[blackbetty]

i did some reading up on the weekend about undertrays and general aero tricks. to make a one piece under body tray i reckon is over kill. the general consensus is to tackle individual sections such as a cover for the rear suspension, the rear boot - spare wheel - fuel tank etc. this is a very (time) expensive experiment. but it would be easier if tackels with a section at a time.

some general tricks to improving your top speed (and i mean top speed only):
keep the front of the car as close to the ground as possible (street cars obviously cant do this) - front air dams, fron wing extensions etc
wheel discs - but this will mean ur brakes get very hot
cover anything that could couse drag - the grill is prob a big improvement area. enough air enters below the number plate for cooling, so you could block the grill off totally BUT please keep an eye on your temp gauge :)
give the car a slight rake (rear end higher)
vent the pressure that builds up in the engine bay - that could mean a couple of lovers in the bonnet, or even lifting the rear of it up in relation to the front just enough to allow the air to get out. venting the hot air also helps with natural airflow passing through the radiator so in effect helps with cooling.
remove anything you dont need that could cause drag - wipers, mirrors, badges.
Tape up any seams Mcgyver style.
Keep the car very clean and waxed (NASCAR have proved a cars top speed is best when the cars are spotless, something like 3 to 5 MPH at 180+ MPH).

Also, keep in mind that HP and top speed is a "to the power of 3 factor"...for example lets say it takes 100 HP to go 100 KPH, then it would take 400 HP to go 200 KPH, 800 HP to go 300 KPH. So to improve drag/airflow even a tiny bit to gain that little bit extra top speed, is heaps cheaper (and more reliable) than pulling the extra HP.

to illustrate, my torana with a stock 179 six cylnder had a top speed of 145-146kph with a 3.36 diff gear and 3 speed automatic (super high tech stuff haa haa). after some of the aero cheats above (the single biggest gain was in grille and headlight covers), the car just touched the max of 160 on the speedo. But you cant drive around with a blocked off grille on a 40 deg day.
Plus it looked sh*t.