911 rake front/rear - Page 4

island911 · 07-21-2003, 08:18 AM

Quote:

Originally posted by Mark Wilson
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.

Yeah, your right. After I posted, I thought I should have added the words "WRT (with respect to) aerodynamic considerations."

With out a doubt, I can say aerodynamic and stability were big considerations." . . .as were bumper height reg's, driveway clearances . . . it all has to come together and make a nice balance.

stlrj · 06:31 PM

Quote:

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.

From what I have read, considerable amount of aerodynamic consideration was involved with the introduction of underbelly pans etc to achieve a Cd of 0.32, at least on the 964.

No mention of speed bump or curb avoidance in any literature I have read.

My gut feeling is that if you lowered a 964 with the proper rake, you would likely raise the Cd considerably higher than the wind tunnel results achieved by exhaustive factory tests, possibly compromising high speed stability in the process.
.

Joe

BogieMan · 07-21-2003, 06:30 PM

Hmmm. Alot of interesting ideas about this one.

I've got some aeronautical experience including a M.S.A.A. and ten years working as an aerodynamicist at Boeing in applied research. My brother, who frequents this site, asked me to comment on this thread.

There are 3 essential phenomenom going on here. The car does indeed act like a wing, and this increase the total downforce as the nose is dropped, especially at the nose. It also creates a moment that shifts aerodynamic downforce forward, loading the nose more and unloading the rear wheels. Second, the proximity of the ground limits the wing effect as the gap is decreased. A 911 sized airfoil 1" from the ground would make roughly ZERO lift, and a smaller gap would actually lift the car up without changing the incidence. The third effect is that the upper surface of the rear of the 911 (like most cars) is separated, meaning that the air doesn't flow in nice smooth lines. At low speeds (say below 30mph), the air is pretty much separated over the entire rear deck. That's not a big deal since aero drag is pretty small (proportional to velocity squared) at low speeds. As the speed is increased, there is a change in ratio of momentum to frictional forces on the car (i.e. Reynolds number) which causes the separation to decrease. This is good in the sense that drag decreases, but bad because it creates aerodynamic lift on the rear deck and unloads the rear wheels. Duck tails and properly designed rear wings will inhibit this effect and keep the handling relatively consistent as speed is increased. The change in the angle-of-attack of a rear wing will be (at least) as significant as the change in lift on the body and should not be ignored. As the nose is dropped, the rear wing will create more downforce, so that's probably good. Dropping the nose will (slightly) exagerrate the unloading of the rear wheels as the speed increases.

Oh yeah, be careful about using Bernoulli or even a Venturi effect for airflow with friction/separation. It violates the formula, and should be used cautiously. Rear deck separation will decrease or even negate rear lift, so attached airfoil explanations are flawed. The change in rear lift with speed is probably more important for the driver anyway.

The underbody aerodynamics are really a 3D problem, with lots of flow coming around the sides and underbody vorticies (if you're lucky) behind the rear wheels. With all the junk underneath the car, there is no simple way to predict this. It is possible that dropping the nose will create some under-body vorticies that will create some nice rear down-force, but this will be dependent on the details of each model.

Well, that's my 2 cents worth. The bottom line is that it's probably not bad to drop the nose (aerodynamically), but be carefuly as you exceed highway speeds until you're comfortable with it.

speeder · speeder's Garage

BogieMan, Thank you for your reply! This stuff is interesting, and relates to safety in motorsports. Could you elaborate on the meaning of "separation", for a liberal arts major? TIA.

Tyson Schmidt · 07-21-2003, 07:06 PM

Quote:

Originally posted by Tyson Schmidt
It's true that by reducing the air under the car, you increase the air going over the car. The air going over the car is accelerated by the upward angle of it's shape. This will exagerate whatever characteristics the back half of the car has.
A 911 has a deeply sloping rear which causes lift. By lowering the nose, or adding a front spoiler/splitter, you will be increasing the rear lift. (That's why a rear wing is needed to balance out a front spoiler) Increasing rake though, will give the underside of the rear an upward slant, which will yeild a slight venturi effect which will reduce lift in the rear, effectively offsetting this effect. Ever looked at the shape of your rear bumper gaurds? The bottoms are steeply angled to get a little of this effect. The engine undertray on the 964 and 993 also have the upward slope to achieve the lift reduction.

Put simply, the rake of a car is changing the angle of attack just like the whole car is a wing, and you are increasing the pressure on top, and reducing it on the bottom.

Nice work Bogieman. Would my previous post above be accurate? I know it's rather simplistic, and in laymens terms, but is it accurate?

nostatic · nostatic's Garage

Quote:

Originally posted by Mark Wilson
Mongo likes a little rake......

no, Mongo straight.

BogieMan · 07-21-2003, 09:43 PM

You can think of "separation" as "stalled out".

Separation is caused when friction overcomes momentum. Practically put, when the air accross a car or aircraft flows smoothly over the surfaces it is called "attached". When the angle-of-attack (meaning incidence relative to flow direction) is increased enough, you get large swirl patterns behind a surface. You can visualize this on a river behind bridge pilings or in a bathtub with a flat plate (sprinkle some coarse pepper or tarragon to see the patterns and move the flat plate parrallel and normal to the surface).

Tyson - I don't agree with some of your points. First off, I think the angle of the rear deck is more important than the amount of air diverted by lowering the front end. This will aggravate the change in rear lift with speed. Second, the venturi effect would tend to actually lift the rear car if the slope is increased. As the flow slows down energy is shifted from movement to pressure which lifts up on the under-body. Of course, venturi explanations aren't exactly accurate for production road cars due to all the stuff underneath and the 3-dimensionality of the flow. I've never crawled underneath a 996, so I don't know how to comment on the items noted above. They may very well have designed to improve things, but its hard to optimize ground effects and maintain good ground clearance. But that's just why things happen. I think we're in concurrence that lowering the front probably has beneficial effects.

Maybe it's just my aviation background, but I always figured a Porsche was a low flying vehicle that just happens to be touching the ground. My take is not to just think about the aerodynamics at a single speed but to consider how the (aerodynamic) wheel loading changes as the speed increases.

Let me know if any of you need specific ideas looked at. I've can run some computer simulations of the flow field on my home system, as long as you don't mind basic solutions.

Tyson Schmidt · 07-21-2003, 10:45 PM

The 964, 993, and 996 all have essentially perfectly flat bottoms, with a little upward slope at the trailing edge of the underbody.

I agree that by lowering the front, or adding a deeper front spoiler, you will be increasing the rear lift. that's what I meant by "exaggerate whatever effects the rear of the car has." Since the 911 has rear lift, then it will increase the lift.

What I don't understand is how the venturi effect could cause lift at the rear. If the underbody slants upward toward the rear, then wouldn't that create a low-pressure area under the rear of the car? I mean, if a given amount of air must now fill a larger space, then doesn't that create a lower pressure area?

BogieMan · 07-21-2003, 11:09 PM

Let's talk about the venturi stuff an drop the rest.

The upward sloping rear will increase the area underneath the car. Let's assume the flow coming in and out of the sides/tires is small (probably isn't, but what the hey). The same amount (mass) of air must flow all the way from the front to the rear (pretending that assumption is accurate). For this kind of flow, you can approximate it as incompressible which means that if mass flow rate is conserved, volume flow rate must also be conserved. That means that velocity times cross-sectional area is constant. Since the area increases, velocity decreases. When the velocity decreases, the air trades movement energy (kinetic) for pressure energy (potential), and the pressure increases. Higher pressure on the underside lifts the rear of the car up. Of course, the tires create stable vortex structures and there is a bit of 3-D stuff going on. I'm not trying to say you can't get a down force at the rear wheels due to lowering the front end, just that you can't get there with venturi effects.

Tyson Schmidt · 07-22-2003, 01:08 AM

I disagree. Yes it trades kinetic energy for potential energy, but that doesn't necessarily have to mean pressure. It can also mean vacuum. (Well, low pressure actually, it's all relative.)

If the air flowing under the car slows down as it enters a larger volume space, then there is even less of it to fill the space. The body of the car attempts to fill that space, and that provides downforce.

How a given mass of air trying to fill a larger space can cause pressure, is beyond me.

island911 · 07-22-2003, 07:51 AM

Ah geez Tyson . . post 911 for ya! . . .I think it must be time to retire that name.

ANYWAY. . .

Quote:

Originally posted by Tyson Schmidt
I disagree. Yes it trades kinetic energy for potential energy, but that doesn't necessarily have to mean pressure. It can also mean vacuum. (Well, low pressure actually, it's all relative.)

When bogi says "For this kind of flow, you can approximate it as incompressible" he's saying the air is like water. It's something aero guys do often. It makes life much easier on them. To get this "incompressable air" they simpley go to the same suppy closet as where "massless ropes" and "frictionless pullies" are kept.

So, Tyson, what you are doing is trying to support a more complex (real-world) model in your head. (I'm not surpised, you need to for your work)
Now if you only had the big words.

(tic -that line.)

I'll just restate what I said earlier; that the guys making the smart aero decisions (for the cars on the track, or leaving the factory) are NOT modeling the aero considerations exclusively on wing theory. The effect of the ground is much too big of a consideration.

69911e · 02:03 PM

I asked my brother to chime in on this one, this post seemed to have a lot of guessing going on. I believe this board should be have a decent amount of solid science for any discussion.

Tyson: "How a given mass of air trying to fill a larger space can cause pressure, is beyond me.

I can help with this one, as I used to not understand this exact question:
A carburtetor works off the venturi effect. A tube with decreasing diameter, near the smallest portion of the tube the main jet passage is connected. The lowest pressure in the tube (below 1 atmospheric pressure) is at the smallest diameter, therefore the gas is sucked into the tube ( venturi).

The converse of your statement is" How a given mass of air trying to fill a smaller space can cause a decrease in pressure, is beyond me."
If this statement were true a carburetor would not work.

Hope this helps.

Eric Coffey · 07-22-2003, 11:54 AM

Hmmm,
That fact still remains that modern racecars do make significant downward (aero) forces from flat-bottom, upward-sloping-rear (venturi) ground effects. So, Bogieman, does that mean the only reason those work in the modern racecar world is because the undertrays/vanes/diffusers are designed to control the flow from nose-to-tail, without "spillage"?
Just a side-note: Most flat-bottom racecars using ground effects will have 1-3 "channels" that slope upwards in the rear (not one large piece). Is that were the difference is? Good info by the way.

ZAMIRZ · ZAMIRZ's Garage

This is the perfect stance IMHO (looks-wise at least):

BogieMan · 07-22-2003, 05:44 PM

Good call on the underbody channels. They create vorticies (like minature tornados) that suck the back of the car down. The air spins around real fast and trades pressure energy for movement (kinetic) energy (faster means less pressure).

You can get downforce by kicking the rear end up, but it is highly dependent on geometry details and the speed you are traveling.

Tyson - Sorry to hear you don't believe the venturi effect.

"Island911" states that the problem is to simple to be explained by either simple wing theory or simple venturi effects. I entirely agree. Side-body spillage, the presence of the rear axle, the effect of under-body air friction and the rear tires all play a huge roll for a production car. These are all critical parts of how a factory like Porsche tunes the underbody aerodynamics.

The comment about compressibility changing the venturi effect is ultra-bogus though. Compressibility effects for a car traveling at 100mph are worth roughly 0.7% of the total number. This is why I noted that the air is "pretty much incompressible". Don't believe that? Check out Chapter 3 of "Modern Compressible Flow" by Anderson where he shows that increasing the area of a pipe increases the pressure for anything below Mach 1. You may not like the answer, but that's how it is.

Practically speaking, try it out and see how it goes. Just be careful because it could get more squirrely after the change, especially as the speed is increased. You can also tape yarn strings to the rear deck or underbody to visualize the flow and view it from a video camera or chase car. The fact that race teams tilt the front down suggests that it will do something good.

350HP930 · 07-22-2003, 06:35 PM

As an aerospace engineer I have deliberately not posted in this thread since I have had the misfortune of describing everything from turbine and compressor theory to vehicle aerodynamics to non engineers and have come to realize that no matter how well you try to tell some people how things work they usually prefer to stick to their flawed perceptions of how they think things work.

Watching the noble actions of BogieMan and the other posts make me glad that I have stayed out of this till now.

While the basic concepts of gas and fluid dynamics are simple, these simple things combine to cause complex behaviours that are usually difficult for the average layman to grasp.

My compliments to BogieMan in taking the effort of doing a very good job at breaking this down for those who are willing to learn, and to Tyson, throw out your assumptions about how things work and realize that what this man is telling you is the bona fide truth from and expert and none of this should be beyond you.

Tyson Schmidt · 07-22-2003, 08:26 PM

But I DO believe in the venturi effect! I do I do I do!!!!

That was my whole point.

The venturi in a carburetor necks the the opening down. On the down-flow side of this neck, is a low pressure area where there are inlets for fuel to be drawn in by the the low pressure and mixed with the incoming air. I'm extremely familiar with carburetors, I assure you.

If the front of the car is lower to the ground than the rear of the car, then the front is the venturi, and the low pressure area is the area behind it.

Tyson Schmidt · 07-22-2003, 08:49 PM

Quote:

Originally posted by 69911e
I asked my brother to chime in on this one, this post seemed to have a lot of guessing going on. I believe this board should be have a decent amount of solid science for any discussion.

Tyson: "How a given mass of air trying to fill a larger space can cause pressure, is beyond me.

I can help with this one, as I used to not understand this exact question:
A carburtetor works off the venturi effect. A tube with decreasing diameter, near the smallest portion of the tube the main jet passage is connected. The lowest pressure in the tube (below 1 atmospheric pressure) is at the smallest diameter, therefore the gas is sucked into the tube ( venturi).

The converse of your statement is" How a given mass of air trying to fill a smaller space can cause a decrease in pressure, is beyond me."
If this statement were true a carburetor would not work.

Hope this helps.

The lowest pressure area in the carburetor is the area just after the venturi. That's where the fuel inlets are.

I'm not sure how I got branded as not believing in the venturi effect, when the whole point of my argument was that the upward sloping rear of the underbody caused the venturi effect and would generate downforce.

It's like the expansion valve in an A/C system. High pressure before the restriction, and low pressure after. The restriction on the car being the close proximity of the nose of the car to the ground, and the low pressure area that results after it.

350HP930 · 07-22-2003, 09:00 PM

The only thing you need to take away from a venturi is this one simple fact.

When air is flowing through ducts or around shapes it can change the speed and direction of the air flow.

When the air is sped up to a speed that is faster than the average air flow speed, its pressure drops relative to the slower moving air. This is what happens in the small part of the carburator. The air must speed up to pass though the small venturi, sucking in gasolene due to the low relative pressure.

When the air is slowed down, it does exactly the opposite which is cause an increase in pressure.

Now in the case of a car moving through the air at 100 MPH we have no venturi but the same laws apply. Faster air has lower pressure, slower air has higher pressure.

At the nose of the car we have high pressure for obvious reasons. The air only has four directions to go, and for obvious reasons most of the air goes over the car. To pass over the car the air will need to go faster than the undisturbed air flow since the car is displacing most of the flow over itself. For this reason most of the upper area of the car is going to be covered with low relative pressure air causing lift.

The limited amount of air that goes under the car is never going to compensate for this pressure force and that is the reason why any normal car body will create lift at high speeds.

Thats the reason why crazy fast sports cars use wings and tunnels. These devices are there to create forces using real wings and tunnels to route pressures from one area to another in an attempt to counteract this unavoidable problem with ground vehicles.

Well, I know I said I was not going to try to explain anything, but I hope this helps.

MotoSook · 07-22-2003, 09:03 PM

Wow! What an interesting thread.

Tyson, in the 3-D world, flow would migrate under the car from the sides too. If the car had side skirts that also minimized flow as the lowered nose will in the increased rake position, then any flow that gets under the car can be better managed, with channels at the rear.

And to stir things up a bit, the under side of our 911's are relatively flat. I always thought that skirts could be made to mount under the car (not on the sides like..say an 80's Camaro, but inconspicuously) to minimize flow from the front and the sides of a 911 for less lift.

You know I recall reading something about the long nose cars having less frontal lift than the later impact bumper cars, because flow impinging on the sloped nose of the car generate a force downward. There's something to think about!