911 rake front/rear

[Tyson Schmidt]

Oh, and Island? Quit showing off. We know you're smart.

[island911]

heh-heh. . .I was just trying to say ground effects and wings should not be confussed.

A cylinder, sucking in air, is more like the rake/ground effect. . . .Maybe think about how a Wankle (rotary) engine pulls in air . .. kind of an opening wedge.

Changing the 'rake' (angle of attack) of a wing in free-stream (not near the ground) is qiute different.

[RoninLB]

Originally posted by Tyson Schmidt
Oh, and Island? Quit showing off. We know you're smart.

quote:
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Originally posted by IROC
Changing the rake of the car won't affect CG - the CG is "where it is" regardless of the orientation of the car.

I always suspected the rake thing was aerodynamically driven. Maybe it's for looks. :>)

Mike
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excuse my wording..
the hard info said something about air movement /lower psi maxed out under the dash..........Ron


__________________
Ronin LB
'77 911s



hey Guys.. don't confuse me..

I trying to still figure if I'm 1/2 correct.. and now I think Tyson would say I'm 1/2 smart.. that's because Island is Full smart..

[Randy Webb]

Bill, Where did you get the 2 diagrams? Is the triangle showing the CG added by you or put there by PAG (if it's a PAG diagram).

Also, have you read something re the CG on the 911 being similar that of the 996? Or is it just your estimate? If the former, do you recall the cite?

BTW, if anyone has a tilt table, you can dtermine the CG on your (modified) car using the methods outlined in "Race Car Vehicle Dynamics."

[Randy Webb]

Joe, Besides the experimental analogy suggested by Tyson (which isn't particularly analogous anyway), think about it this way: the air going under the car is being compressed between the car and ground. The air going over the car is being compressed (less) by the wing like shape of the car's upper body. It's less because there is a free air surface above the car, but a rigid body below, and because the bottom of most cars is very rough, inducing eddies and turbulence that slow the air. That's the reason that a few recent cars have smooth undersides, air channels, etc.

[RoninLB]

Originally posted by Randy Webb
That's the reason that a few recent cars have smooth undersides, air channels, etc.

... then it becomes more of a wing..

so that somehow sounds like more lift

[Eric Coffey]

Originally posted by RoninLB
... then it becomes more of a wing..

so that somehow sounds like more lift

Yep, except that "lift" is inverted and given a new name: down-force. On racecars, flat-bottom ground effects/rear diffusers make up for a significant percentage of the down-force/aero grip.

[masraum]

I would say the front/rear location of the center of gravity would align with a plane through the jacking point. So the only unknown then is the right/left distribution and the height.

[Tyson Schmidt]

Originally posted by Randy Webb
Joe, Besides the experimental analogy suggested by Tyson (which isn't particularly analogous anyway),

No, not perfectly analogous. But, simple enough so that someone misguided enough to think that raising the front of a car would reduce lift, could maybe understand it.

[Randy Webb]

re analogous ...

The main defect in your analogy was that there is no rigid body beneath a hand held out in the air stream. Also, the hand can vary its shape.
You can simulate a stall tho, by raisng the angle until lift vanishes.

I can think of many situations in which raising the front of a car could reduce lift. Aerodynamics is a very complicated subject.

[Bill Verburg]

The 3 lift diagrams were from a discussion by Prof. Robert White and published in Pano in 1977

The 996 cg was provided by a 996 owner that frequents the Audi lists

The cg is easily calculated if you know the corner weights. Mine is 35.2" forward of the rear axle(good guess Steve) it is just forward of the jacking point.
The side to side position can also be easily calculated, I just never bothered.

f/r lift changes where the effective cg is. Which also affects the cars dynamic properties.

[Randy Webb]

Thanks, Bill. I lifted the diagrams and will add the cite.

Mongo likes a little rake......

[Bill Verburg]

Bernouli says that the faster the flow the lower the pressure

A wing has a curved top and relatively flat bottom

The air flowing over the top of a wing has to be flowing faster than the air under the wing because it has to travel a further distance in the same time as the air under the wing. this creastes lift and drag(they usually go hand in hand)

A sailboats sails and keel operate on the same principle, they are curved, and the air on the leeward surface must flow faster than the air on the weather surface because it has to travel further, this creates lift and drag, enabling a sailboat to sail faster than the true wind(it generates its own wind called apparant wind). The sail is particularly good at this because you can change the amount and position of the sails curvature to generate the ratio of lift/ drag that is approprite for the sea and tactical conditions present(think Chapperal). The keel on the other hand is not flexible and can only have different angles of attack to generate its lift(much like the hand out the window analogy or a fixed spoiler)

A car does the same thing ideally but in practice there are issues at the front where the clean air is split and rear edges where laminar(attached) flow becomes turbulent, so a car uses aero aids to better manage these transitions . The aids that Porsche uses are not true spoilers because they do reduce lift but do not increase drag. By definition a spoiler does both. However the nomenclature has stuck so we continue to use it.

Air under the front end increases lift at that end so everything the can be done to stop that flow should be done, this includes air dams, rake, lowering, minimizing holes in the front bumper etc. Any air entering the front is best routed out the top, not the bottom

At the back there are additional issues w/ vortices. These can be controled w/ several strategies which include the Kamm back and
spoilers

Here is an interesting article on 993TT aero charicteristics

[stlrj]

Air under the front end increases lift at that end so everything the can be done to stop that flow should be done

Bill,

I understand what you are saying, however, it seems the factory is doing everything just the opposite. Look at the nose up high angle of attack on a stock 964, 993 and even the 996. Every inch of these cars seem to attract more air flow on the undersides.

Has the factory gone nuts or do their engineers know something that we don't?

Just some observations...

Joe

[Bill Verburg]

They are still using a nose down attitude on race cars. There are too many odd ball legal/marketing stricytures on street cars.

They are using integrated shapes and duct arrangements
Here are a few examples



Notice the oil cooler ducting out the top, the radiator ducting out the sides?
The wheels also are designed to suck air out from under the car!

They are also using true wings rather than the aero aids used on street cars. Look at the GT3RS.

[masraum]

Don't forget the vent out of the top of the GT2 nose, and the fact that the Turbo, GT3, and GT2 have deeper front lips than the regular models. Less air under the car is a good thing unless maybe you have ground effects which require air for their effect. I think several ferrari's and other supercars are using undercar ground effects to create downforce.

[Bill Verburg]

If you can use a tunnel under the car you can create a faster stream of air which will force the car down because of the higher atmospheric pressure above the car. Most cars can not use these tunnels either due to class rules or production constriants.

Here is a pic showing the exit of the tunnels on a 962


and a GT1

[island911]

Originally posted by stlrj
. . .
Look at the nose up high angle of attack on a stock 964, 993 and even the 996. Every inch of these cars seem to attract more air flow on the undersides.

Has the factory gone nuts or do their engineers know something that we don't?. . .

my guess is they are giving the car better 'manners.'
Raking the nose closer to the ground, cuts off air flow. This can make a car very pitch sensitive, as the Venturi effect (acting on the rear) starts ramping up. (so to speak)

Originally posted by island911
my guess is they are giving the car better 'manners.'
Raking the nose closer to the ground, cuts off air flow. This can make a car very pitch sensitive, as the Venturi effect (acting on the rear) starts ramping up. (so to speak)

I suspect the nose high attitude of newer cars (with factory settings) has to do more with keeping the nose from dragging speed bumps and hitting curbs rather than any aerodynamic consideration.