On a front air dam, i have basically reached the conclusion that the closer it is to the ground, the better, to keep turbulence down. But i am a little fuzzy on a couple of other things.

On a rear wing, how does one properly determine the rake and height? Is there any equation that can calculate the downforce at a given speed and given angle? or somthing?

regarding front dive planes, why are these used? I see them on a few le mans / gt / etc. cars but a good number of cars do not have these at all. I know they increase front end downforce, but why would a car manufacturer opt to use these instead of adjusting the front diffuser, which i would think would create less drag? (this obviously cannot be the case, since i am sure the engineers who put them on the cars are smarter than me by a long shot. could somone please explain?)

on the front and rear diffuser, how do the size and placment of the strakes effect the downforce? i know the purpose of the rear diffuser is to help air passing under the car to expand and create low pressure, causing a downward suction. I have no idea how it accomplishes this, however. I am pretty clueless as to the workings of a front diffuser, other than i know if you have a 1 piece seamless underbody panel it keeps turbulence to a minimum, but i am wondering why would you put strakes on the front diffuser?

if anyone could clear any of this up, i would very much appriciate it. thanks!!

 

The front splitter works on the principle that the air on the top splitter surface is at or close to the stagnation point on the car. The stagnation point is the point where the flow velocity is zero, and this will be the highest pressure region on the car (See Bernoulli's Law). So the pressure acting on the top surface is high pressure. So, A large force is pushing down on the top surface when you consider (i.e. sum up all of the pressure*area over the entire top surface) The larger the splitter the larger the downforce, although the downforce is increasing at a decreasing rate as the splitter length increases. This is because the area of the splitter closer to the air dam has slower moving air (lower pressure) than the air flow at the forward tip of the splitter (higher pressure).

The bottom splitter surface is an area where the air is moving very fast due to the small area opening the bottom of the car. The closer to the ground the splitter plate is located, the fasteer the air moves under the car up to a point. From Bernoulli's law this under surface is located at a low pressure region) Sum up all of the pressure*area on the bottom surface, and you get a smaller force magnitude then the upper surface force magnitude. The net result is net downforce.

Two problems need to be addressed:

1. You don't want the splitter to touch the ground because then no air goes under the splitter - hence, no low pressure region. Also, as the splitter gets closer and closer to the ground while braking, boundary layer effect start to pinch off this under splitter flow (low velocity flow on bottom surface), which results in reduced pressure on bottom surface. This is known as pitch sensitivity - i.e. a diminishing amount of downforce is generated as the car pitches under braking. Vice versa for acceleration where the splitter lifts off of the ground.

2. The air dam depth needs to be otimized. The air dam blocks flow that would otherwise go under the car. Since most cars have cluttered underbodies (exhaust systems, shift linkage, unit body undulations, bumper cover protrusions, ect), the air that flows from the front of the car towards the rear encounters all of these underbody objects that slow down the flow. This results in high drag and unwanted lift (once again, slow air is high pressure air that pushes up on the underbody). The air dam deflects some of the air that would otherwise go under the car. This deflected air goes around the sides or over the top of the body, which is less restictive to the flow. In effect, this wil reduce drag - up to a point.
The problem with the air dam is that the air right behind the air dam is low pressure air (either from flow separation or air that went through the radiator and was slowed significantly). The larger the area of the dam, the larger the net drag force will be acting on the dam. So there will be an optimal length of air dam that will deflect enough air from going under the car, yet the air dam exposed area is only as large as it needs to be.

This aero discussion to be continued when the Las Brisas Margaritas wear off.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Buttcrack &raquo;</TD></TR><TR><TD CLASS="quote">on the front and rear diffuser, how do the size and placment of the strakes effect the downforce? i know the purpose of the rear diffuser is to help air passing under the car to expand and create low pressure, causing a downward suction. I have no idea how it accomplishes this, however. I am pretty clueless as to the workings of a front diffuser, other than i know if you have a 1 piece seamless underbody panel it keeps turbulence to a minimum, but i am wondering why would you put strakes on the front diffuser?</TD></TR></TABLE>

I can give you a rough cut on this one.

The purpose of a rear diffuser is to allow the underbody air to rejoin the airstream as smoothly as possible. As Johnny Mac noted, the underbody air tends to be very turbulent, sluggish, and relatively high pressure. Without a diffuser, it tends to just tumble out the back. A properly-designed diffuser allows the air to smooth out and begin to accelerate while still under the car. Smoothing and accelerating the airstream lowers the pressure (see Bernoulli's law again), thus creating some downforce.

As for a front diffuser, it does basically the same thing. As Johnny Mac noted, the air flowing under the splitter is moving at high speed. If you let that air join the rest of the nasty mess under the car, you've kind of wasted that high speed (and low pressure). On the other hand, if you use a front diffuser to smoothly direct that air out the sides of the car and back into the airstream, you not only decrease drag, but also produce even more downforce since the low pressure acts over a larger area.

As far as I know, strakes are primarily used to keep the air flowing smoothly. When the airflow separates from the surface, it becomes turbulent, increasing drag and producing less of a pressure change than if it remains attached. Strakes can be used to keep the air from separating.

Johnny Mac, welcome! Nice to have a practicing aerodynamicist here. Mmmm... Margaritas...

 

thanks very much for the info guys. I am very interested in this subject and am trying to learn as much as i can

 

Wow:

Great thread. Perhaps when the margaritas wear off Professors Mac & Smith can address the subject of the desired angle, height, placement and size (and manufacturers???) of the rear wings we are now seeing in H1 and USTCC.

From reading Carroll Smith last night, he suggests a preference for getting the rear "spoiler" right and adjusting the height of the air dam to balance the car (sedan). Nice for his world, but it seems easier for the rest of us to design the dam and splitter and then trim out the rear to match. Especially for the adjustable angle wings that most guys are using.

Also, I am curious about chassis set up for these cars. Am I thinking correctly that we fwd guys would go for more oversteer in general to get the car to rotate in the slow corners and then tune the aero package to give more neutral to understeer behavior in the high speed turns to keep the rearend from stepping out. (I'm thinking of turns 8 & 9 at Willow, Johnny.) Any knowledgeable discussion will be appreciated.

Thanks for indulging my ignorance.

Thawley

 

Yes, make the car mechanically oversteer, aerodynamically understeer. I've seen elaborate front aero set-ups on cars, air dams, splitters, canards, etc with NO rear aero aids. It amazes me. I understand that your governing body of peers has deemed rear wings unfashionable, but they are necessary. Unless, of course, you enjoy a nervous car in 120 mph turns.
As far as angle of attack, span and chord of a rear wing, that's at the discretion of the sanctioning body. I assume you have a height and width restriction on wing design. Unless you have a wind tunnel at your disposal, your just going to have to play around with different angles of attack until you feels you've reached the best downforce with the least amount of drag.
As far as an adjustable front air dam. Assume you use a sheet of HDPE for your air dam, slot the holes vertically where it attaches to the bumper cover. This allows for adjustment in the height of it as well. Yes, it is a bit of a pain in the *** to loosen and adjust the air dam, but you want adjustability, that's the price you pay.
In attaining the mechanically oversteering car, do so with spring rates and sway bars. Buy a well designed, hollow, adjustable rear sway bar. DO NOT remove your front sway bar. That maintains your front camber control, initial turn in bite, etc.

Understand that you can build the best car on the planet and still be slow as **** on track. Be sure to take time to read, study and work at making yourself a better driver. Read Carroll Smith's Drive to Win as well.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-cell &raquo;</TD></TR><TR><TD CLASS="quote">DO NOT remove your front sway bar. That maintains your front camber control, initial turn in bite, etc. </TD></TR></TABLE>

isn't there an equally valid school of thought that suggests no front bar and super-high front spring rates?

 

Does anyone have pics of a diffuser or floor pan on an Integra. Found a website which sells them for the DC5 but not the DC2

 

wow some honda-techers that have knowledge of good old fluid mechanics! all good info on this thread! Boundary layers and stagnation point were brought up!

Only thing that was not really mentioned was that ideally the rear spoiler works best with nice laminar air flow over the car, not the turbulent air that is coming off the roof. Only way you can find out the best place would be wind-tunnel work like mentioned above.

If you want to learn more about the subject pick up a fluid mechanics book or a car aerodynamics book and you will have more that enough info in there!

-nate

 

I like the higher spring rate no front sway bar theory. To a point sway bars should not really be needed if everything is set up correctly. On my street car I like the extra traction of the fully independent front with stiffer springs and dampers.

 

Whew! Those 'ritas were good, and no hangover (thanks to Cuervo Gold and not some no-name 'quila). Thanks Bernardo M. and Steve E. for the good company. See John, I don't drink alone, I had to drag a few names into the mess.

Also, thanks to Agent Smith, that was a great and concise explanation of the diffuser.

To continue the discussion on diffuser fences or aero fences in general. The fence on the diffuser does two things. One, it keeps the flow as 2D as possible (act like a guide vane or turning vane), which helps to reduce the chance of flow separation. In addition, the fence can act as a vortex generator that also helps to reduce flow separtation. Flow separation is something that you really don't want to occur.

Flow separation: Consider undisturbed flow over a wing that is say positioned at a -10 deg. angle of attack. The flow over the bottom surface will start out laminar for a small percentage (usually around 5 to 15 percent) of the chord length. If the wing was at a smaller angle of attack, the laminar flow would be up 30 percent of chord (this is typical for one of the NACA 6 series airfoils, eg NACA 64-415). The streamlines near the bottom surface encounter what is known as an adverse pressure gradient. This adverse pressure gradient occurs when the flow velocity increases in the direction of the flow, which is due to the shape of bottom surface of the wing. At the same time, the boundary layer gets thicker along the wing surface. If the adverse pressure gradient is bad enough, the air will laminarly separate right away. Bad! This results in large loss of downforce and large drag. You can see this scenario on a typical drag polar (Cl vs. Cd) and the massive separation occurs at angle of attacks larger than the wing stall angle.

Reference:Abbot and Doenhoff, Theory of Wing Sections, Dover Repub ~ $12

Strakes are used on delta wing aircraft to generate leading edge vorticies (LEV), which one of two mechanisms responsible for generating lift on swept wings. The dive plate is similar to a strake, but dive plate tend to be very inefficient downforce generators (low downforce/drag ratio) whereas strakes tend to be relative efficient. Use dive plates if you really need more front downforce after the efficiency (or rules requirements) of the splitter has tapped out.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Nate &raquo;</TD></TR><TR><TD CLASS="quote">
Only thing that was not really mentioned was that ideally the rear spoiler works best with nice laminar air flow over the car, not the turbulent air that is coming off the roof. Only way you can find out the best place would be wind-tunnel work like mentioned above.

-nate</TD></TR></TABLE>

Bummer. I don't have a wind tunnel, currently. How about a "Top Ten Tips on Tufts" thread?

Thawley

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-cell &raquo;</TD></TR><TR><TD CLASS="quote">DO NOT remove your front sway bar. That maintains your front camber control, initial turn in bite, etc. </TD></TR></TABLE>

Here's another good new-thread subject. I agree with the theory of using the front bar, but in practice most of the fast guys in SoCal aren't running them. I took my car into Porgress Group who make the most commonly used ITA suspension bits for West Coast racers and essentially said, "Do it." I wanted the best race set-up they were selling. Got the car back, and no front bar. Handles great, but I don't have the benefit of comparison between the best front bar set up against the best NO front bar set up. Has anyone tested both ways (changing spring rates appropriately)?

Thawley

 

does anyone know at what speed does aerodynamics begin to play a large roll? my ballpark guess would be around 80 mph? anyone have an idea?

 

First, you may want to define what you mean by "large role" as it is quite vague.
Aerodynamics would play a role even on a typical autox course.

 

Here are some pictures of some aero parts on the front of some LMP cars from a couple of years back showingthe state of the art at the time.

Champion Audi R8 front view


Champion Audi R8 side view



Cadillac LMP front end


Corvette C5R rear wing and diffuser


Audi R8 front end


Panoz LMP front end with extra long splitter


A good site to look at aero stuff is:
http://www.mulsannescorner.com/

And now how do you aply all this to a Honda in the absence of a wind tunnel???

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by descartesfool &raquo;</TD></TR><TR><TD CLASS="quote">And now how do you aply all this to a Honda in the absence of a wind tunnel???</TD></TR></TABLE>

test test test

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by descartesfool &raquo;</TD></TR><TR><TD CLASS="quote">
And now how do you aply all this to a Honda in the absence of a wind tunnel???</TD></TR></TABLE>

There are two low cost ways to determine a qualitative understanding of the flow properties.

1. Tufts - that is yarn tufts. Tufts help us determine where flow separation takes place. You can also determine where the turbulence is severe with tufts. At points downstream of where separation occurs, some of the tufts will be pointing in the direction opposite of the flow direction. This is called reversionary flow and is found primarily at locations of adverse pressure gradient - like the rear window, the bottom of a racecar wing, ect.

Take a 2" long piece of yarn and tape it the area on the body where your interested in the state of the flow. Sure, you could put the tufts over the entire car, and that would take quite awhile. One obvious place for closed body cars is on the rear window and on the bottom surface of the racecar wing.

2. Mini smoke bombs cartridges. Just tape these puppies on the car and point the video camera from a distance. This technique works better using a chase car so that you can get far enough away from the test car to see really what's happening. Make sure the smoke bomb is enclosed in a streamlined container so that the flow aft of the smoke bomb isn't hugely disturbed. This will give you an idea of what is happening at the placement of the rear wing.

If you want more quantitative info on the flow field, then there are two relatively inexpensive ways of measuring flow velocity. One is called hot-wire anemometry and the other is using a Cobra pitot static tube. HWA uses a short hot platinum wire that's exposed to the flow field. The method relies on the rate of heat transfer from the wire into the flowfield to determine the flow speed. Must be calibrated.

The Cobra probe uses three angled total pressure ports (=30,0,-30 degrees) and one set of static ports (The Pitot static tube uses one static and one stagnation port to determine pressures and then using Bernoulli law, allows one to determine flow speeds) The Cobra probe is attached to three manometers (measures pressure). The probe is rotated until the two ports (-30,+30) record the same pressure. Then, the 0 degree total pressure port is measuring flow speed at the correct flow velocity orientation. Hence, with this probe, you are able to measure both the direction of flow (at that point in space) and flow speed. This probe is helpful in determining where to place the rear wing. Or course, you want the wing in the highest velocity position and minimum turbulence (i.e. away from large scale eddies). The reason velocity alone is not enough is because vorticies can cause havoc with the flow separation on the wing. So it would be nice to place a wing high enough and back far enough where vortical influence is minimized.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Arsenal &raquo;</TD></TR><TR><TD CLASS="quote">Does anyone have pics of a diffuser or floor pan on an Integra. Found a website which sells them for the DC5 but not the DC2 </TD></TR></TABLE>

I like the concept of running diffusers, even on road cars. Properly designed diffusers can help in both downforce and drag reduction. Although I won't be able to run a diffuser on my H1 integra, I like the thought of producing a carbon fiber and kevlar diffuser for the DC2. Typically, diffusers show a drag reduction at inclination angles of less than 6 degrees. Downforce can usually be increased for diffuser angles up to an sometimes exceeding 14 degrees. Of course, the performance of any diffuser will depend on the car.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Johnny Mac &raquo;</TD></TR><TR><TD CLASS="quote">
There are two low cost ways to determine a qualitative understanding of the flow properties.
</TD></TR></TABLE>

How about a more mundane plan to test aerodynamics, without tufts and Pitot tubes. Let's say I have a rear wing and a front air dam and splitter. Rear tuning options are rear wing height above the rear deck, and angle of attack, with a fixed profile and width and chord length, as well as end plates. Front options are air dam height above ground and splitter length forward of air dam with fixed flat bottom to centerline of front wheels. I am not changing the shape of the car, and vehicle height is as low as I can get it based on all available compromises, whether rules, tire rubbing, etc. Aside from the obvious lap time changes, what would be a good test plan and methodology to optimize performance. Reading a few good books on aero has not done it for me in terms of an aero test plan. What do you measure?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-cell &raquo;</TD></TR><TR><TD CLASS="quote">Yes, make the car mechanically oversteer, aerodynamically understeer. </TD></TR></TABLE>

&lt;--- stands up and claps.

thank you , sir.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-cell &raquo;</TD></TR><TR><TD CLASS="quote">DO NOT remove your front sway bar. That maintains your front camber control, initial turn in bite, etc.

</TD></TR></TABLE>

again , standing ovation.

my hat is off to you , I'm forced to constantly argue these points on this board.

 

thanks very much for all the great info guys!

i was looking at some jgtc cars and saw a couple of interesting things. in this picture http://216.117.199.231/Toyota-...1.jpg (warning it is a big mama, 1600x1200) what is the purpose of the three small fins that are next to the front tire in the sideskirt? also i assume the small scoop near the rear tire is a brake duct. what is the significance of its shape? personally, i think thoes types of scoops compress air, but i am not entirly sure. could somone explain?

This hood design creates front downforce by ducting the air up to the hood, where it pushes down on the panels...? or is it just cooling vents?


Modified by Buttcrack at 11:34 PM 3/30/2004

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Buttcrack &raquo;</TD></TR><TR><TD CLASS="quote"> what is the purpose of the three small fins that are next to the front tire in the sideskirt? </TD></TR></TABLE>

[educated guess] The fins are designed to direct the flow entrained by the tire's motion and spill it overboard to maintain as low pressure as possible under the car. [/educated guess]


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">also i assume the small scoop near the rear tire is a brake duct. what is the significance of its shape? personally, i think thoes types of scoops compress air, but i am not entirly sure. could somone explain?</TD></TR></TABLE>

That inlet is a NACA inlet first used in the '40s, I believe. It works by creating two vorticies that curl up into the inlet, creating a low pressure region, pulling in outside air. It's not the most efficient inlet but it's very low drag. It also creates a turbulent airstream in the inlet, which is probably why you see a more conventional inlet used for the engine/radiator air just above it.

As for the hood, I'll let the more practical guys answer that one, because I'm not sure which mechanism they are going for there (there are a couple).

Andy

 

we drive hondas.........not LeMans 900 cars!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hoochie the Haggard Clown &raquo;</TD></TR><TR><TD CLASS="quote">we drive hondas.........not LeMans 900 cars!

</TD></TR></TABLE>

yeah but the main ideas can be implemented with astoundingly positive results!

the idea with the hood vents is to let the air flowing under your air-damn to have a place to escape rather than be trapped and cause lift, also letting hot air out of the engine. this is why some LMP cars have louvers over the wheel wells to remove all possibilities of lift.

-nate