75 Rear diffuser project

[75evo]

Mats,

Instead of the diffuser, would it help cooling of the transaxle and brakes if the rear valence was cut so there is a bigger opening in the rear?

[Mezevenf]

You mean like this Z?

[BigSwede]

Maybe

Try it and measure before/after. There is no way to know for sure otherwise.
It will probably make it a cooler environment for the box/brakes but that would probably also mean more drag due to higher mass flow under the car. Again - Guessing.

[Craig]

G'Day Guys,

Downforce v Drag, what a Pandora's Box this thread will be , specially with the added requirement of cooling the gearbox and brakes .

As Mats has said, this will be very tricky.

For a couple of basic primers, see here and here.

First caveat, I do not race, have not raced and only know a little about Airflow.
Second caveat, YOU may already know all this.
Third caveat, I'm probably wrong

Some facts:

Downforce is a direct result of drag, you cannot have downforce without drag, however (on a car) you can increase downforce without increasing drag by altering something which already produces drag without (any/much) downforce.
Downforce is directly related to the speed of the car through the air.


Some suppositions:

Extra downforce on the straights is only useful if you have enough power to break traction while accelerating hard (except under braking).

So it would seem that for the average back yard racer that (within reason) reducing the downforce down the straight while increasing it as the nose dips under braking would be the best goal.


The tricky bits:

Changes in airflow due to braking, body roll, bumps in the track and wind direction, temperature (too hard, ignore).

All those lumps and bumps under the car and the need for cooling gearbox and brakes.
Minor changes could have huge effects (normally negative (ie. bad)).


So Pandora's box is open, let the interesting times begin.

[75evo]

AR4me has a transmission temp gauge and at Buttonwillow recently (slight wind, 70F ambient temp.) the gauge showed the transmission was hot (over 250F ? ). About adding drag by cutting out the rear valence, is it really a concern for some of us not so serious club racers? At some point it would, once one gets more serious, otherwise I would personally opt for more cooling.

[nizam]

Does anyone have access to a wind tunnel and some barometers we could borrow or rent? It would be interesting to put even a model of a Milano into a small wind tunnel and measure where the high and low pressure points are ... you know, at a rudimentary level to sort of understand the starting point.

[Craig]

Sounds like fun, but to get anything meaningful I think you'd have to go to about 1/10th scale. The body could be made from almost anything from balsa to a block of stone but I think you'd have to model the engine bay reasonably well (the engine could be pretty rough).

You would have to model a reasonably specific bodykit and would need to be able to model attitude (ie. under braking etc. wheel heights).

If you were going that far you might as well make a small wind tunnel at home, it would pay off in the long run.

[Duk]

Or you could use a manometer, place the sencing tube at various spots under the car and drive it in the real world.
You can get a digital manometer for reasonable price of Epay.

[BigSwede]

Craig: The drag/downforce rule always applies, but in this specific application we actually lower the drag a lot by adding a flat floor. That alone will give you a big slash in laptimes.
So in this case it would be more like smoothing the body then to actually add a piece to aid downforce. Downforce is a nice free lunch when smoothing the floor so to say.

Z: I totally agree, just as long as you know why you do it it's fine.

Anyone can gain access to a wind tunnel, with enough cash.
A scale model replicated in good enough detail to work as a real world test bed would be too expensive. Also, using a wind tunnel to simulate the floor aero is very difficult because the car is not actually moving realtive to the floor, there are ways to simulate that with rolling belts for instance but it will always be a bit different then in the real world (add to that the stuff that Craig mentioned about the car moving about in pitch/roll/heave). Did I mention it would be expensive?
For pressure distribution along a section of the body you can use any general section from a sedan available, they don't differ that much.

pressure measurements are indeed cheap and interesting, they do not tell the whole story however. But they are a nice tool.

[Craig]

G'Day Mats,

Quote:

Originally Posted by BigSwede Craig: The drag/downforce rule always applies, but in this specific application we actually lower the drag a lot by adding a flat floor. That alone will give you a big slash in laptimes. So in this case it would be more like smoothing the body then to actually add a piece to aid downforce. Downforce is a nice free lunch when smoothing the floor so to say.

[alfaspence]

just a thought (yeah i know, i don't do it very often )

Has anyone got access to pictures / tech drawings of the Alfa Corse cars?
Or the real things?

They had huge budgets to spend on aerodynamics, chassis's, engines, drivetrains etc....

And what ive seen on pictures the cars vary quite considerably with regards to bodykits etc

Perhaps there is no easy single option here, requirements will vary from circuit to circuit (short and twisty, long straights and fast)

If your serious, theres no point for the roads....

[BigSwede]

Craig: Yes, I noticed that part, it wasn't my meaning to imply anuthing else, just clarifying.

Spence: Usually cars like that are built to very strict rules and not seldom they would have looked very different had the rules been totally free.

[elogiov6]

[Craig]

Quote:

Originally Posted by BigSwede just clarifying.

[BigSwede]

elogiov6: Is this your car? Looks like styling more then engineering to be honest.

Craig: Exactly, it does kinda take time an money and shedloads of skill... :/