Poor Man's Ground Effects -- I need some input

[k9handler]

How does that floor jack hold up the entire front of the car...and so level?

[Craig 930 RS]

I like Pete's 2nd paragraph idea - if it is possible, it will be effective.
Do try it!

You still must reduce airflow (especially *turbulent airflow* caused by the objects protruding in front) from under the car - the entire car - to make any of this work. I believe Jack's car is quite stiff already.

[Yellowbird RS]

Drag and Lift
Drag:

Aerodynamic efficiency of a car is determined by its Coefficient of Drag (Cd). Coefficient of drag is independent of area, it simply reflects the influence to aerodynamic drag by the shape of object. In theory, a circular flat plate has Cd 1.0, but after adding the turbulence effect around its edge, it becomes approximately 1.2. The most aerodynamic efficient shape is water drop, whose Cd is 0.05. However, we cannot make a car like this. A typical modern car is around 0.30.

Drag is proportional to the drag coefficient, frontal area and the square of vehicle speed. You can see a car travelling at 120 mph has to fight with 4 times the drag of a car travelling at 60 mph. You can also see the influence of drag to top speed. If we need to raise the top speed of Ferrari Testarossa from 180 mph to 200 mph like Lamborghini Diablo, without altering its shape, we need to raise its power from 390 hp to 535 hp. If we would rather spend time and money in wind tunnel research, decreasing its Cd from 0.36 to 0.29 can do the same thing.

Since air flow above the car travels longer distance than air flow underneath the car, the former is faster than the latter. According to Bernoullis Principle, the speed difference will generate a net negative pressure acted on the upper surface, which we call "Lift".

Like drag, lift is proportional to area (but surface area instead of frontal area), the square of vehicle speed and Lift Coefficient (Cl), which is determined by the shape. At high speed, lift may be increased to such an extent that the car becomes very unstable. Lift is particularly serious at the rear, you can easily understand, since a low pressure area exists around the rear screen. If the rear lift is not adequately counter, rear wheels will become easy to slip, and that is very dangerous for a car travelling at something like 160 mph.

Air Tech-S. 2002

[Craig 930 RS]

Excellent.



And a major problem with the 911 shape is twofold:

The windshield is very upright and high, with a sharp transition from windshield to roof. Not good.

Airflow off the roof and onto the window and decklid area is NOT laminar - it is VERY turbulent! The RS 3.8 wings, contrary to pop belief do not flow much air at all between the deck area and the wing element.

This is why the VERY high wing is truly necessary for true downforce - as opposed to REDUCING LIFT - which is what most of the 911 wings accomplish.

Signed, 2 much time inthe windy tunnel,

[Wil Ferch]

Craig...don't start me on the verbal difference on REDUCING LIFT as being different than CREATING DOWNFORCE......

It's all a matter of if the *added downforce* creates NET downforce....when all is said-n-done. If it doesn't provide NET downforce....people here tend to say it REDUCES LIFT. If it does provide NET downforce...people tend to say it CREATES DOWNFORCE....

Let's not go there....see the archives.....

Wil

[Yellowbird RS]

993 tt aero-test


link:http://www.autospeed.com/cms/A_1065/article.html

[Craig 930 RS]

Wil, what you mention is really a problem when discussions arise - the terms get all screwed up! So yes, I agree.

I presented an aside of the problems of the 911 shape with briefly delving into differences - we are essentially saying the same things.
It does get ugly if one delves into definitions.

Vehicle/shape/tomato (or whatever) at certain speed generates a certain amt of lift or downforce (*change in weight at a measured point, compared to the difference from the at-rest measurement*)

So, you have either:

A CHANGE in the measured qty of two at-speed measurements

- or -

The DIFFERENCE between static (non moving) weight and the overall measured difference after changes.

I prefer the latter! ;-)

[Randy Webb]

I would also see if I could fab some sheet Al to keep air from ging in front of the muffler and behind it in front of the rear panel. You obviously want to be careful re heat here.

I would duct the air from the horn grills somewhere or block them off with solid panels.

Then there is cutting off the headlights....

It'll cost you about $5 to make a manometer....

Wool tufts and tape are even cheaper. You should do some testing on where the air is actually going and at what pressure....

[wevoid]

It's clear to see this thread can easily be de-railed (sorry 356 guys), but going back to Tysons input on the first page, the idea is founded in simplicity - aim low and achieve - almost certainly - a very worthy and reliable approach that leads to progress.

Jack has made a step with his front underbelly fairing and it seems the goal is to search for similarly easy - even if inexplicable - improvements. This time at the rear.

I dont consider myself an expert, but with 100's of hours in scale model wind tunnels with racing sports cars and open wheel cars I have learned 2 hard facts.

1) Dont assume the results of any configuration.
2) Even if it "looks" right - dont be surprised if it is not.


Conceptualization of aero ideas is founded in creativity and perception of the invisible. Often the creativity far outweighs the perception, so due process is the only reliable mechanism to work through ideas.

Even though I detect an element of brinkmanship and symantics in the language, you are all talking about approximately the same thing, only granularity of purpose or sense of ownership of the ideas separating each post.

The only idea I have not seen explored yet in this thread is the expansion of a diffuser element in plan view. The illlustrations being used and the ideas being projected are all basically geometry in elevation, extending accross whatever pratical car width exists.

The energized air under the car is happy to expand left and right as well as vertical and a diffuser design exploring this can be made much more suited to the constraints of the engine cooling and basic rear engine packaging. The diffuser works primarily as a function of expansion ratio.

Whatever simple solution is the outcome of this discussion, the results will be determined by the necessity of unscientific testing.


Regards


Hayden

[Randy Webb]

How do you see a diffuser working on an air cooled rea engine car like this?

Are you thinking of splitting off some of the air flow and sending it up into the eng. comp. itself?

Or just doing some channeling of the air as it picks up heat by flowing past the exhaust? He'd need some metal bending work wouldn't he? See his post re cheapo hardware store level funding.

Or some other idea?

[wevoid]

Randy,

I am not specifically interested in the design of the parts, more the process leading to the design.
For those enjoying the challenge of conceptualizing some solutions to test, I could see that plan form of the diffuser was so-far overlooked.
The sketches, dimensions, illustrations so far have all been in elevation, if you add the plan view of the same components it will be clear how a diffuser can be expanded on both planes.
This might allow better management of air needed for the exhausts, but it may also be beyond the manufacturing resources Jack planned to use.
Tyson did intend to do it in one evening, so aiming low will be rewarded.

Regards

Hayden

[Jack Olsen]

Originally posted by petevb
... but like I said I’ve tried the basic low splitter/ diffuser approach others are advocating, and I think that would require you to stiffen the suspension up in order to insure the car doesn’t get scary as the ride height changes. I’d clean up the entrance to the underbody at the front while I was at it, btw.

How stiff? Are we talking about 1000# and up srings?

[Jack Olsen]

Originally posted by k9handler
How does that floor jack hold up the entire front of the car...and so level?

600# springs.

[Jack Olsen]

Originally posted by wevoid
...aiming low will be rewarded.

You've nailed my situation right on the head, Hayden. All of you guys with wind tunnel experience have to act like you're dealing with a mildly retarded guy on this.

I located an aluminum supplier, today. I picked up a few pieces. But I'm going to proceed slowly.

Maybe some of you can tell me what's wrong with this plan:

1) Modify the chin splitter to mount on my bumper.
2) Add a sheet of thin aluminum to cover/smooth over the front AC condenser area.
3) Affix side skirts that run straight down along either side, leaving 2-3 inches of clearance.

Maybe stop for some testing? Then:

4) Fabricate an engine undertray based on the 964 unit, about 36 inches wide, with additional aluminum to connect forward to the end of the existing plastic sheet, and with 3-4 vertical fins in the back, each 3 inches above ground.

It would be great if there were a simple way to test this, without driving down to where Hurricane Ivan hits land. Any ideas?

[Tyson Schmidt]

Jack, I can't remember if the stock 964 engine undertray will clear the B&B headers or not. I don't see why they wouldn't fit.

I mean, the Factory went through all that R&D, so why not go with that? The trans undertray piece could easily be modified to fit your car, and it would connect the existing underbelly piece we built to the engine undertray.

Sideskirts, etc. would look rather silly. I think it would be wise to simply mimic the 964 and call it a day.

[Jack Olsen]

But I have $20 worth of aluminum, now.

Shouldn't I be able to get another second with that?

(The sideskirts, etc, would be items I throw on at the track, and unscrew before heading home.)

[project935]

Hmmmm ... I used to have a 911 stereolithography model file somewhere I could use to set up a CFD analysis. Just need to find out where I put my CFD software ...

[Craig 930 RS]

Nothing is wrong with your ideas - hell it can't hurt (t00 much) let 'er rip!
(Just get that front splitter LOWER) :-)

[petevb]

Originally posted by JackOlsen
How stiff? Are we talking about 1000# and up srings?

Jack,

I think the spring stiffness you need would depend a lot on it low you run the splitter and how fast you get going. When I played with this a few years ago (not on a 911) we tried a splitter lip a little bigger than the one you’ve got, and I think we ran it with about 2.5” of static ground clearance. I think the front spring rate worked out to around 600 lbs/ inch. At 130 mph the car started to feel twitchy on the right roads (it was dead solid before the mod). By 160+ the front end could suck down to the point that it would bottom out, cutting off airflow. I’m guessing the front needed to be generating around 1000 lbs+ of downforce to do this? In any case, when the front bottomed or came close to bottoming over bumps the airflow was cut to the raked bottom and diffuser, meaning the underbody/ back wasn’t making downforce, and the car would start to try and change lanes or swap ends by itself.

Because of the above it seems critical that you maintain ride height with this type of setup- you either always or never want to be flowing air. I’d guess that the real danger area would be when the splitter gets within about an inch of the ground at high speeds (100+). If you’re within an inch you’re already cutting lots of flow to the underbody/ diffuser, so the center of pressure is moving around depending on front ride height (bumps, braking, etc) and this is *very* spooky.

If I had to make an educated guess on spring rates I’d say that if you run the thing 3.5” clear of the ground and never saw above around 130 then maybe 500 lb front springs would be fine. If on the other hand you ran at 2.5” or less of clearance and were getting to 160 then you’d want way stiffer springs than we ran- over 1000 lbs per inch I’d estimate. Get the splitter lower and I think you’d need to go much stiffer.

I’m going from memory on the heights, etc, so they are probably off. You get the idea, though. My experience gave me a healthy respect for the danger of moving the center of pressure forwards of the CG, and of the sensitivity to ride height. My personal guess is that you did the opposite with your ABS mod and moved your center of pressure rearwards- this made the car less twitchy at speed, giving you the confidence to drive it quicker in high speed sweepers.

BTW, what did end up working was a flexible air dam (no splitter) and side skirts that just tried to block all the air to the underbody. This cut drag, but wasn’t making anything close to the splitter downforce…

[bigrubberjeep]

Does anyone see any ideas/input from the pictures on this thread?

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