How do you get any aero downforce/less lift out of a ST185?

[OnAll-FOUR]

Is that rear bar custom or is that diffuser off another car? and how far forward will you extend it?

it is a photo shop effort with the evo rear diffuser that some people in the states are looking at.

personally I would like to extend it further forward, smoothing out the transition from the fuel tank to the rear bar there is a big hole there otherwise. I think this would leave me with a diffuser sitting lower than what is show here but would hopefully attach better to the air flowing under the car.

[Roundy]

and the fuel tank in the way

thats one thing i don't like, i look at the back of my car thinking diffusers and all that but realise i will never get a suitable angle without a smaller fuel tank, or a silly long back bar.

or the other option is to not integrate it into the rear bar, but i would prefer it to be a lil subtle

[OnAll-FOUR]

More photos for more Ideas.

[Crazyracer]

I was running a Datsun 1600 race car that gradually progressed from my every day road car to an unregistered Sports Sedan over several years. During that time I went from no aero to a front spoiler with under tray and a huge rear wing.

Initially I put a reasonably deep front spoiler on and the improvement to the high speed stability was immediately obvious, but only at speeds over 150km/h or so. It was most noticeable down the front straight at Phillip Island. The car would wander around the track anywhere near as much with the spoiler fitted.. Adding a splitter/undertray made marginal improvements on that. It probably also helped the times in the twisty bits, but not as noticeably through the wheel.

The addition of a rear spoiler and later wing was less convincing. The final wing was up at the same height as the roof. The car was probably slightly more stable through some of the fast corners but change was nowhere near as obvious as the front spoiler.

As for full undertrays, here's a pic of the underneath of the current sports sedan...

[craigsimon]

Underbody diffusers only generate a small decrease in preasure around 1-2psi difference but it is the large plan area that they work on that generates large downforce values their effectiveness is reduced by increasing ground clearance due to high pressure leaking under the car. At 40mm ground clearance the downforce potential is just about halved and by 100mm it is very minimal, they are also very sensitive to flow obstructions/ turbulence so really need a completely smooth underbody preceding them to function well. As mentioned previously skirts were used in F1 to stop the ingress of high pressure and improve the efficiency of the difusers but due to sticking skirts and the usual complete loss of control of the car following were quickly banned by the FIA. Since the banning of skirts aerodynamicists have used tricks by adding vortex generators down the sides of the cars to create an "air' barrier to attempt to minimise the high pressure migration trying to get back some of the downforce lost to the rule makers.
Most wings on race cars are low for a few reasons and attempts usually made to the body work in front of them to provide smooth airflow to them. Wings generate more lift (downforce) for their drag when they are within their chord width of the bodywork and also they improve the efficiency of the underbody difuser by drawing the air out (this only occurs when the wing and diffuser are in close proximity) F1 cars have the second set of wings down low below the main elements for this reason.
Decreasing the amount of air that flows under the car by adding a front airdam will usually decrease drag on production cars because the irregular surface under the car generates a lot of turbulence as a extra bonus they create front downforce with the addition of a splitter once again though it is a matter of ground clearance, that said if air flow is completely blocked off at the front the downforce generated will reduce to zero leading to unpredictable handling.
What speed regime are you trying to increase down force for? or how much top speed can you afford to loose to drag? you can generate down force at relatively low speeds the FSAE guys that run wings are making 50-100kg of downforce at 100km/h using multi element high camber wings, their top speed is limited by the courses they run on to not much more than this so they can afford the drag generated.

[OnAll-FOUR]

My track aims are local Sydney super sprints so Eastern Creek and Wakefield Park and dutton rally style events. But it is still my road car so I realise there are limitations.

I guess I'm after some simple wins if that is at all possible considering the Gt4's are originally designed as a rally car and they predate a lot of recent aero developments.

Building a front and rear under tray to tidy things up doesn't seem like a bad idea to me. The GT4's originally come with a undertray at the front but I believe it's function was more to protect the engine from stones. The under tray does slope upwards from the front edge leading air into the radiator. This would also cause a negative pressure to be developed wouldn't it? The tray then goes through a range of folds and creases.

I was planning to try build one to mimic this sort of shape but smooth out the design?

[Lachie]

FSAE guys that run wings are making 50-100kg of downforce at 100km/h using multi element high camber wings, their top speed is limited by the courses they run on to not much more than this so they can afford the drag generated.

Yeah, but what about the size of the wings? I'll see if I've got any pics of Monashes car later, but there wings are the size of the car

Also, 50-100kg's of downforce is great on a car that weighs 250kg max (wet weight, without driver) but how much will this help a plus 1000kg car? I'm sure the general public don't have the exstensive knowledge of fluid dynamics that engineers have to be able to designthem to be really effective.

I always thought your inlet area to your intercooler/radiator should be a third the size (33%) of the heat exchanger size, not 25% (i'd do some research to get the correct figure).

Just let me put on my flame proof pants before I write this next bit

As far as testing goes, why not make a samll wind tunnel and get a 1/24th scale model of a celica for testing?

It would only have to be a little tunnel, and you can calcualte all your air flow and pressures etc so its an accurate representation of the full car.

All you need is a large tube, and some way of making a laminar flow of air through it. Granted I've never done anything like this beofre but the wind tunnel at my uni doesn't see to complicted. The tunnels walls are wood for fooks sake!

[craigsimon]

using multi element high camber wings

yeah I know FSAE wings are large as I said, I was repsonding to "downforce doesn't work below 150km/h" It can but it does so at a massive drag cost severely limiting top speed. That is why I asked what speeds he expected the car to be functioning at. 100kg down force would be very useful on any car as most production cars are generating lift by 150km/h but you could achieve more than 100kg with a well though out wing for quite low drag penalty operationg between 100 and 250km/h.
I think the easiest solution to implement on a production road car is to run a front air dam with splitter running at the lowest ride hight you can afford without grounding under maximum bump and a well thoughtout single element rear wing. There are plenty of low speed airfoils that will generate useful downforce at the speeds seen on a race circuit (think ww1 aircraft that have flight envelopes around 100-250km/h airspeed)
there are a couple of good books for further reading which give are a good place to start

Competition Car Downforce - Simon McBeath
Race Car Aerodynamics: Designing for Speed (Engineering and Performance) by Joseph Katz
and this one is data from american government funding of airfoil study in the early 1900's and has massive amount of data on lift and drag numbers for lots of NACA 4 and 5 digit airfoils
Theory of Wing Sections: Including a Summary of Airfoil Data (Dover Books on Physics) by Ira H. Abbott and A. E. von Doenhoff

Craig