Little Red Devil

[gavs]

Little Red Devil

Thanks Gav, appreciate the input! Couple of things though and certainly not saying you're wrong here. Yes something like the Clio diffuser was my first intention. Minus the huge budget for part development and wind tunnel testing of course

I look at cost v's benefit with all this as well as being within the category rules and the rules say wheel well stays and a custom diffsuer can't go past the rear bumper. Plus I can't have a flat under body panel between the front and rear axles which isn't such a big deal as the Polo has two plastic panels running that length of the body either side of the exhaust tunnel and the exhaust itself is up quite high and not in the flow of air.

I also look at all the top hatchback time attack cars from all around the world to get ideas as they do spend massive coin on aero as there are basic principles when dealing with the very unique combo of FWD and hatchback chassis. Most of the fastest are sadly Civic's not Clios, Polo's or even Golf R's for that matter.

Now the biggest issue with any hatchback is rear end lift and drag. Usually caused by the same inherent design issue which causes a parachute effect at the rear from the air travelling under the car which there is a lot as these hatchbacks also have a very high COG. My Polo still has a high COG and it can't go any lower. In reflection banging on the wing was just masking the real issue as the lift is not caused by the top of the car.

So for your last comment can't say I agree. You are assuming the Polo's overall aero dynamics are well designed from factory. Well it's far from. Here are some wind tunnel testing of the MK5 Golf 3 door GTI (same chassis and shape as my car just a larger version)



So at 200kph the rear experienced 37kgs of lift and only 2kgs at the front.

And here are the other variants from VW that suffer the same massive lift in the rear end

Golf R32



36kgs of lift at the rear and 3 at the front

Mk6 Golf GTI



A little better at 33kgs at the rear and 5 at the front.

So there is one thing all these very similarly shaped hatchbacks have in common and that a very short and squat rear that catches the underside air flow like a parachute as I said before. This also causes a high pressure build up right where there should be any at all. Just for comparisons sake the Audi RS3 has only 21kgs of rear end lift



and the the best hatchback I can find is the Focus RS with only 4kgs lift at the rear



So VW obviously didn't spend a heap of time in a wind tunnel when designing the shape of hatchbacks in question.

My point is for what ever increase in rear end turbulence can't possibly out weight the drag and lift effect caused by entire rear bumper especially the lower valance plus the vertical bits of metal I have disposed of. They all act as a wind brake because on the Polo there are big gaps that air travels either side of the wheel well only to get caught up at the end. Cars that have a flatter underside that fills the gaps between the beam and the bumper won't have this issue.

Here is a sub 1 minute Civic at Tsukuba which has a well designed fiffuser that looks to be at the optimum 7-8* but because it also has provision for excess air to evacuate out of the bumper.



So until I win lotto and can afford a fluid dynamisit like Andrew Brilliant and a wind tunnel this crude yet very common hatchback mod will do the trick

No worries mate I wasn't really clear in what I was trying to say I guess, what I was saying is that the aerodynamic profile of the car is consistent, by no means is it efficient whatsoever the drag-inducing vortices are of a consistent pattern (thank you Melbourne weather an the 9n polo on the freeway this morning!) so by adding a volume of high-pressure air into the consistent wake of the car will change the balance, that's what I was trying to say...
If you can extend the sills of the car downward as well as the front bumper, this will help quite a bit in keeping low pressure air under the car but you have to try and get a flat, level floor. Not like the factory underpanels which duck down under the subframe. It doesn't need to run the full length of the car and cover the whole lot in, just so long as its smooth and flat...

[VWindahouse]

Pas d'escargot chez Walsh.

It's got to be this week though.

Turbo Trailer #2 Official 2013 Dreamworks Movie [HD] - YouTube

Haha I'm off to have me a Nitrous bath

Ant I can't actually imagine the Cfd thing would be that hard... What you are missing is the 3d model of the car... Solid works has a Cfd plugin which isn't hard to come by. The real challenge is developing the model to an accuracy to judge improvements. And of course being able to read the data. Alright so maybe its not easy, but certainly achievable.

Haha nothing good in life is easy!

No worries mate I wasn't really clear in what I was trying to say I guess, what I was saying is that the aerodynamic profile of the car is consistent, by no means is it efficient whatsoever the drag-inducing vortices are of a consistent pattern (thank you Melbourne weather an the 9n polo on the freeway this morning!) so by adding a volume of high-pressure air into the consistent wake of the car will change the balance, that's what I was trying to say...
If you can extend the sills of the car downward as well as the front bumper, this will help quite a bit in keeping low pressure air under the car but you have to try and get a flat, level floor. Not like the factory underpanels which duck down under the subframe. It doesn't need to run the full length of the car and cover the whole lot in, just so long as its smooth and flat...

Ok sweet, do you know by any chance how far off the ground the front splitter and side skirts generally should be? I'm restricted a little by a 80mm class restriction and I'm already pretty low. I must say you've got me thinking about the diffuser again.

[gavs]

Little Red Devil

Lower is obviously better, 80mm is pretty high but if you were to raise the suspension a bit and tweak your roll stiffness to counter the high COG (maybe also do some fun things with ballast...) you could make your bodywork lower to allow better under-car airflow.

Just remember that a diffuser in whatever shape or form you go with, doesn't need to be the full width of the car, obviously wider is better but I you take your current exhaust point and work with that as your outer extremity off the centerline, you can still tailor a fairly simple undertray to work with it but like I said, skirts are needed to keep the under-car airflow nice and clean splitters also don't need to be the massive things you see on sport sedans and touring cars etc either, it just has to break the airflow.

[VWindahouse]

Thanks mate, I'll do some measuring and take some shots to see what's possible. I really need some decent coilovers with stiffer springs I think, so I can raise the car a bit and not add any body roll. The FK's are as low as they will go without badly scrubbing out the inner tyre on the liners, and mated with both front and rear anti-roll bars on their hardest settings it's only enough to keep stability where I like it.

Plus I'm a bit worried about aftermarket skirts for this Polo. The way that the chassis rolls under the car there is no way of telling just how much they will drop as a portion of them will stay above the lowest point of the sill. Plus there are no suppliers in OZ so would be luck of the draw from an OS supplier. Yet another challenge

[kaanage]

Ant I can't actually imagine the Cfd thing would be that hard... What you are missing is the 3d model of the car... Solid works has a Cfd plugin which isn't hard to come by. The real challenge is developing the model to an accuracy to judge improvements. And of course being able to read the data. Alright so maybe its not easy, but certainly achievable.

Modelling is easy if you assume laminar flow and no separation of the boundary layer and not too hard if you assume a constant position (or section for a 3D model) where the laminar boundary layer separation (and thus the beginning of the wake) occurs.

In the real world, where you get varying separation and turbulent flow, ACCURATE modelling is almost impossible due to the effects if seemingly small disruptions in the body shape like windscreen wiper nozzles and the wipers and arms. The turbulence from these disruptions ALWAYS lead to early separation of the boundary layer, which is extremely difficult to model as the turbulence will start and increase as speed increases in unpredictable ways which then makes the drag coefficient change (usually for the worse). If you know EXACTLY what you are doing, then the situation can be improved by the addition of upstream vortex generators to create a turbulent boundary layer which can stay attached longer than a disrupted laminar one.

Then we need to consider large disruptions to the flow like wheel wells, the wheels themselves, side mirrors...

They still use wind tunnels for aircraft design, F1 etc for good reasons.

Just remember that a diffuser in whatever shape or form you go with, doesn't need to be the full width of the car, obviously wider is better but I you take your current exhaust point and work with that as your outer extremity off the centerline, you can still tailor a fairly simple undertray to work with it but like I said, skirts are needed to keep the under-car airflow nice and clean splitters also don't need to be the massive things you see on sport sedans and touring cars etc either, it just has to break the airflow.

I still don't see a diffuser being effective on a Polo without removal of the wheel well. There is just not enough length available behind it for it to work. A good flat bottom tray and as much skirting as clearance allows to just to decrease drag is about the best you can do (and hence also increase the effectiveness of the front spoiler above the splitter). Conceivably, you could get some downforce by adding ducts from the base plate to the holes in the bumper as the low pressure in the wake could draw air from under the car - you'd have to test. Of course you could easily totally unbalance the car with masses of front downforce and not much at the rear.

That Civic pictured earlier has a massive rear wing mounted as far back as possible, again, for good reasons.

[seangti]

hey mate, the effect of adding downforce is to add directed (useful) drag/resistence. Given the "black art" of downforce, lots of time and money goes into it with variable results, or at least hard to quantify. Could an alternative approach be to focus on minimising drag and therefore focus on say the polo bluemotion which has a very low drag coefficient. Low drag = high speed.

My train of thought is that lakeside has one high speed corner, the dog leg. QR has two, turns one and two, so downforce during cornering on these 2 local circuits isn't a critical component of putting together a good lap. All other corners on these 2 circuits are sub 100km/h or thereabouts.

[jasn78]

Car sounds good mate

[VWindahouse]

hey mate, the effect of adding downforce is to add directed (useful) drag/resistence. Given the "black art" of downforce, lots of time and money goes into it with variable results, or at least hard to quantify. Could an alternative approach be to focus on minimising drag and therefore focus on say the polo bluemotion which has a very low drag coefficient. Low drag = high speed.

My train of thought is that lakeside has one high speed corner, the dog leg. QR has two, turns one and two, so downforce during cornering on these 2 local circuits isn't a critical component of putting together a good lap. All other corners on these 2 circuits are sub 100km/h or thereabouts.

Wise words mate. And that was the exact decision I came to a few pages ago when concentrating mainly on reducing rear end drag instead of going a very complicated rear diffuser design.

For the Blue Motion the only changes I know of from our cars are the front upper grille deflecting air over the bonnet (not an option as I want as much air to hit the upper part of the front mount and Rad.
The front lip which seems to channel air flow uniformly under the centre of the car as there are minimal restrictions getting in the way of the flow of air (I think at the speeds I'll be travelling a small splitter that also acts as an underbody tray to about the start of the front underbrace will do). Once again the category rules only allow for a 50mm distance from the front bumper so that's fine and means I don't have to bolt it up to the chassis to make sure it won't fly off.
The side mirrors I believe are the same.
And finally the weird looking rear roof spoiler which is meant to reduce turbulence close to the rear of the car reducing drag (I have one of the best designed rear wings on the market for added downforce yet minimal drag so good compromise)
APR Performance - New GTC-200 - CFD Data
And the side skirts which I'm looking into

However Sydney Motorsport Park is a very high speed track so down the track I may have to reconsider a few things.

Cheers.

[VWindahouse]

Car sounds good mate

Haha thanks mate! Not too loud for morning traffic?

[VWindahouse]

Sound clip

[team_v]

Sounds really deep and throaty, can't wait to hear it at Happy Laps

[Lito]

Sounds tough as bro!

[h100vw]

Too quiet?

Gavin

[gavs]

Little Red Devil

hey mate, the effect of adding downforce is to add directed (useful) drag/resistence. Given the "black art" of downforce, lots of time and money goes into it with variable results, or at least hard to quantify. Could an alternative approach be to focus on minimising drag and therefore focus on say the polo bluemotion which has a very low drag coefficient. Low drag = high speed.

My train of thought is that lakeside has one high speed corner, the dog leg. QR has two, turns one and two, so downforce during cornering on these 2 local circuits isn't a critical component of putting together a good lap. All other corners on these 2 circuits are sub 100km/h or thereabouts.

Irrespective of if Anth goes with a diffuser at some point, the point I was making previously was that at the expense of less drag, the addition of high velocity, positive pressure into the negative pressure wake of the car would more than likely make the rear of the car unstable and render the wing useless. What I was suggesting was that he explore the addition of a flat floor on the car, with or without a basic diffuser to eliminate the drag issues cause by the rear bumper. I've seen many a Soarer or supra lose their expensive, votex rear bumpers on drag strips due to the drag from the bumper and I've also seen how just covering that area in can solve the issue cheaply, effectively an without the cutting of holes.

Undercar aero tuning does not increase drag and is why so many exotics now no longer sport the garish wings present during the 90's. you can have highly efficient negative lift without the traditional sacrifice of drag caused by wings and spoilers.

Anth I'll try to draw my thoughts up at some stage for you.

[MountainBikeMike]

Re: Little Red Devil

Irrespective of if Anth goes with a diffuser at some point, the point I was making previously was that at the expense of less drag, the addition of high velocity, positive pressure into the negative pressure wake of the car would more than likely make the rear of the car unstable and render the wing useless. What I was suggesting was that he explore the addition of a flat floor on the car, with or without a basic diffuser to eliminate the drag issues cause by the rear bumper. I've seen many a Soarer or supra lose their expensive, votex rear bumpers on drag strips due to the drag from the bumper and I've also seen how just covering that area in can solve the issue cheaply, effectively an without the cutting of holes.

Undercar aero tuning does not increase drag and is why so many exotics now no longer sport the garish wings present during the 90's. you can have highly efficient negative lift without the traditional sacrifice of drag caused by wings and spoilers.

Anth I'll try to draw my thoughts up at some stage for you.

+1. Been having a good read of this thread. Interesting stuff!