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#2160 - 11/26/07 08:29 PM Downforce Rearend  
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You cannot limit downforce just on front end for that changes under/oversteer quite a bit so also install a good rear wing that is adjustable to set the wing angle for the racing your doing as too much angle causes more drag, hence losing top end speeds as in a ORR or too little wing on a short course has no downforce.

On my 1994 ZR-1 for Open Road Racing:

[Linked Image]

On our Snakeskinner ZR-1 :

[Linked Image]

On our 1999 Racer

[Linked Image]


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#2161 - 11/27/07 10:01 AM Re: Downforce Rearend [Re: teamzr1]  
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Ever stick your hand out the window at highway speed?
Do it palm-first,and the air pushes your whole arm backward. Hold your hand out palm down and you can control arm movement, even against the air pressure.
Tilt your hand upward, and air pressure forces your arm higher. Go the opposite way and tilt downward, and your hand gets pushed downward. You've just experienced the aerodynamic properties of drag, lift, and downforce. Air flowing over, under, and around a car acts the same way.

Drag and Frontal Area

Palm-first, your hand has lots of resistance. That's why air pressure pushed it back. Engineers call this drag. Measured in pounds, total dragis a combination of the vehicle's frontal area, dynamic pressure (air density and velocity), and a shape factor that defines how slippery it is. The shape factor is a dimensionless numerical expression called the coefficient of drag (Cd).
Think of Cd as an efficiency factor, like horsepower per cubic inch (hp/ci) is for engines: A go-cart engine and a supercharged big-block Chevy might make the same hp/ci (be equally efficient), but the Rat motor's total power output is vastly greater because it has more displacement. Likewise, the Goodyear blimp and a pencil have a similar Cd because they have a similar shape. The difference lies in the overall size of the object (also known as frontal area) that is pushing through the air. In car terms, a Corvette has both a much lower Cd (it's swoopier) and less frontal area (it's smaller)than an 18-wheeler.

Also, aero forces go up with the square of the speed, meaning that if you double the vehicle speed, you increase the total drag resistance by four times. What's worse from a gearhead standpoint, AeroDyn's Gary Eaker points out that horsepower requirements to reach a given speed goup with the cube of velocity.
At 200 mph it takes eight times as much power to push the car through the air as it does at 100 mph. If your car needs 80 rear-wheel horsepower to run 100 mph, for it to see 200 mph requires 640 hp. Gives you more respect for those 300-plus-mph Bonneville racers, doesn't it?
Therefore, if top speed is the goal,anything that reduces frontal area and makes the vehicle's shape more streamlined is crucial.

Lift and Downforce

Returning to our palm test, your upwardly inclined hand experienced positive lift; tilted downward it saw negative lift, or downforce.
Nearly all stock cars have natural positive lift. Older cars with blunt front ends generated lift kind of like your hand did--air gets underneath and pitches the nose up.
Pre-'68 Corvettes, old Cobras, and classic-era GT-40s were notorious for front-end lift at speed. But even modern, fuel-efficient, bubble-cars have lift, paradoxically caused by their very efficient wing-like shape.

Positive lift is bad; it makes a car float and hard to control,especially at speeds over 100 mph. It's far better to have downforce to push the car down against the pavement, increasing its traction.
"It is important to be able to steer the car at the speed you want to run at,"says engineer Chuck Jenckes. "You don't want a buoyant car.
"Unfortunately, many of the techniques used to increase downforce also raise drag.
As Jenckes puts it, "You need one to generate the other." It becomes a fine juggling act--aero drag reduction from better streamlining versus increased drag from more downforce--and which one to emphasize is dependent on the vehicle's inherent design, its intended use, and (if engaged in organized racing) the sanctioning body's rules restrictions.

More Downforce or Less Drag?

What's more important: going as fast as possible, or being able to turn corners?
The answer will dictate whether you want more downforce or less drag. For anything that must turn, generating downforce usually outweighs any additional drag that might be generated.
"You can make up more time in the corners than on the straights," says Jenckes.Road-racers always want lots of downforce.
With oval-trackers, it depends on track length, whether the rules permit radical downforce mods, and if the cars are power-limited.
Today's Nextel Cup teams pay close attention to streamlining because the rules require apower-limiting restrictor plate and closely limit downforce-generating mods to those that equalize competition and ensure vehicle safety.
Short-track cars benefit from downforce enhancers, especially because they are usually traction- limited, not power-limited.

On the other hand, for land speed racing, ultimate straight-line topspeed is the goal, so reducing drag reigns supreme in all cases. These cars need only enough downforce to hold traction and to keep the frontend planted.
Finally, in drag racing you want to add some downforce for traction, but the majority of drag-car aero mods are to enhance top-end vehicle stability and improve performance past mid-track and through the traps.
"A drag-car should have no more downforce than needed or you're wasting your potential," says Racecraft's Mark Wilkenson.

A multiuse street car must be a jack of all trades: stable at high speeds, sticky in the corners, versatile enough to survive driveways and gas stations, and tough enough for weekend race duty.
Realistically, there are limits to what you can do to an average car without going off the deep end. Unless you go the route of extreme customization--chopping, channeling, narrowing, and sectioning--it's hard to significantly improve a production car's drag coefficient.
You could remove the driprails, molding, chrome trim, and antenna; shave the door handles; lose the windshield wipers and side mirrors; racer-tapeall the body seams; meticulously polish the vehicle's surface; close the side windows; lay back the windshield; and block airflow through the radiator grille (to the extent the car doesn't overheat, but quite practical in a short-duration drag race).
These things will lessen drag a little bit. However, generating more downforce, even on a street-driven car, is entirely practical, and some of the things that add downforce can also aid aero efficiency.

Spoilers and Air Dams

"The two hotspots where there's the most room for improvement on mostcars are the lower front and upper rear," says AeroDyn's Gary Eaker.
Racers realized this as far back as the mid '60s and developed rear spoilers and front air dams to help their cars maintain traction. A rear spoiler is a device attached to the car's upper rear surface (usually the trunk lid) with no gap between it and the bodywork. If it has a gap,it's considered a wing, which is far more complex.

As its name implies, a spoiler's purpose is to "spoil" the fast, smooth,low-drag airflow coming off the roof. By sticking up into the airflow, aspoiler causes the airflow to detach and separate, reducing its velocity and creating a pressure rise that decreases the rear lift tendencies.
Although a rear spoiler primarily adds downforce, in some situations it can also decrease drag, depending on the spoiler's height, angle, and length of extension off the deck. The more vertical the spoiler's angle,the more the downforce at the price of increased drag.
Exact results vary per vehicle and can only be determined by cut-and-try testing, but some studies suggest that the most downforce is achieved with a spoiler height that's about 8 percent of the car's wheelbase. That means about8-9 inches for a 106-inch wheelbase. For any drag decrease, the spoiler height will usually have to be less than 1 inch, but up to 2 inches there is usually no drag penalty.

Air dams vary from simple lower-front-valance slab-like extensions to complex cowcatchers. An air dam's purpose is to reduce the amount of airflowing underneath the car, which has a number of benefits. Most cars do not have smooth underbodies--exhaust, drivetrain, and suspension hangdown, creating considerable additional drag.
In this situation, a frontair dam can improve aerodynamics even though it increases frontal area. More importantly, a front spoiler reduces air pressure underneath thecar. Any air remaining under the car is turbulent, just like air behind the rear of a vehicle.
That's the principle of drafting in NASCAR; guys get hauled along behind the lead car by riding inside its low-pressure wake. A front air dam creates a similar effect under the vehicle,essentially sucking the car into the ground.

Spoilers and air dams work for most vehicles without complex experimentation. But for the vehicle to perform properly there must be a balance of forces at the front and back. If too much downforce is applied to the front, the rear might get light; conversely, too much rear downforce may result in front-end pitch.
Most front-engine,rear-wheel-drive cars with correctly balanced suspensions work best with slightly more downforce at the rear than at the front.

Wings

It stands to reason that if you turn a wing-like shape upside down with the flat surface on top and the curve on the bottom, it will work the opposite way it does on an airplane--it should generate downforce.
Done right, upside-down, rear-mounted wings can generate huge amounts of downforce, but unless the wing is integrated into the vehicle's overall body-shape (as on classic Dodge Daytonas and Plymouth Superbirds),there's usually a big drag penalty. However, even though a bulky wing may reduce ultimate top speed, it increases a car's cornering ability so much that overall lap times are dramatically lowered.
The problem is implementation. There are literally thousands of potential airfoil shapes, and the wing's interaction with the rest of the body is extremely complex.
You cannot just take a generic wing, turn it upsidedown, haphazardly mount it, and expect it to work. Yet if the wing truly is effective, it can seriously unbalance the car, actually unloading the front tires. At a minimum, a huge cowcatcher-style front spoiler isusually needed to regain balance.

Bearing this in mind, there are several things to consider when planning a wing. Thicker and/or more highly cambered shapes generally generate more downforce, but also more drag. On any given wing design, mounting the wing at a lower angle of attack (flatter) has less drag, but is also not as effective.
A steeper angle (with the rear higher than the front)yields more downforce, but also more drag. Adding a small 90-degree wicker or "Gurney flap" to the rear edge of an existing wing can add downforce with little drag penalty, and varying height wickers can serve as a fine-tuning aid for different tracks and conditions.
Vertical sideplates at each end of the wing keep air from spilling over the sides,sometimes increasing the wing's effectiveness as much as 30 percent.There are also multi-angle and multi-element wings, which are good ifwing size is limited by class rules, as is often the case.

Ground Effects and Diffusers

Even the best air dam can't fully remove the effects of tires, which cause over 40 percent of an open wheeler's total drag. Even on a fendered car, the wheels and tires remain a major lift and drag source.The tires' rolling resistance makes up a component of drag, and this resistance increases at higher speeds.
Air accelerating over the top of a spinning wheel reduces pressure on its top surface, creating lift.Tires also add to the frontal area, but no one wants to trade off the stickiness of wider tires for more aero-efficient skinny tires--unless they're racing Bonneville.
But you can run the tires at higher inflation pressures to reduce rolling resistance, enclose them in fenders, and get air out of the fenders as efficiently as possible. If there are no rules to obey, bulge out the fronts of the fenders, and tuck in the rears slightly.

As for the car's underbody . . . clean it up!
If you can, make a full bellypan. At a minimum, extend the front air dam horizontally with aflat plate back toward the front axle and oil pan. This tends to reduce the turbulence and accelerate what air that does get by the air dam,lowering its pressure, which increases front downforce.
In a perfect world, nothing would hang lower than the bottom of the framerails. A flat, full underbody lowers drag, but according to Jenckes won't alter overall lift tendencies, which are dictated by overall vehicle shape.
In other words, adding a full bellypan neither appreciably helps nor hinders downforce--unless you take advantage of the Bernoulli Principle,which says that air speeds go up when air is forced to flow through atube of reduced cross-section. That's how a carburetor's venturi works and the same principle can be used underneath the car.

So once you've got the floor flat and the car low, add a slight nose-down rake--1-2 degrees is sufficient. This makes the entire underbody act like a simple venturi: There's now a narrow throat under the front--which accelerates the air and produces low pressure--and therest of the body acts like a long diffuser, permitting rapid (through gradually decelerating) air to flow toward the rear. And out back, curve the under-panel upward to form a duck-tail integrated with the outer rear taillight panel. This helps the rear low-pressure wake suck air outthe back.

Diffusers are a lot more complicated (look at the underside of an IndyCar), and are beyond the scope of this story.

Air can be either your enemy or your friend, and what works on one car won't necessarily work on another. Drag can limit the car's top-speed potential, but correctly managed, drag in the form of downforce can produce tremendous improvements in traction to the point that the entire suspension must be stiffened and retuned.
Many techniques that help improve drag and aero are relatively inexpensive if you have access tobasic metalworking equipment and the patience to cut, try, test, and experiment. And that's the essence of hot rodding.



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#2748 - 08/17/08 04:25 PM Re: Downforce Rearend [Re: teamzr1]  
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Calculating downforce with a wing upper surface of 68 inches by 12 inches.

From speeds of 130 to 180 MPH wth wing angles of 0, 5, 10 and 15 degrees

Results are in pounds of downforce

Attached Files downforce.jpg

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#3129 - 08/27/09 02:39 PM Re: Downforce Rearend [Re: teamzr1]  
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What you are probably already familiar with is the concepts of drag and downforce. Drag being bad, and down force generally being good.
The best design possible will try to minimize drag and maximize the downforce. This being said, in many cases the amount of downforce you get is heavily dependent on the amount of drag a car sees.

The basic logic behind the whole ordeal of how drag and downforce come about are based on two main principles. Boundary layers and pressure differences.

The boundary layer concept stems from the idea that if you were to be on a surface with air flowing past it, the closer you get to the surface the slower the air will be moving past it.

The air molecules at the surface can best be assumed as being stationary. But the farther away from the faster the air will be moving.

Generally, you want a nice smooth even boundary layer, if there is something that causes vortexes (little twirls of wind), then there will generally be an increase of drag.

These vortexes form from a number of things. Things including surface roughness, sharp angles, the angle at which a slope diverges from an air way, and even the speed at which the surface is moving relative to the air.

To test and optimize a vehicle aerodynamically a couple main methods are used. The first is CFD (computational fluid dynamics) wherein you use a computer model of a vehicle and use programs to calculate how the air will flow around the various features.

This is a very lengthy process and requires a lot of engineering know how and experience to optimize these models. But with enough computers/engineers you can rapidly develop a mathematically optimized car shape.

The second is by using a wind tunnel. This is perhaps the most accurate method to test, but is also perhaps the physically hardest to test as well.
Not only do you have to manufacture all of the components for every little change you make, but you also have to create an ideal test chamber, hopefully with a treadmill floor to imitate road movement.
In these tests Engineers generally use smoke trails and dip them around the various parts of the car looking for swirls of air, once found it's back to the drawing board to try and get rid of them.

Now onto downforce. Downforce is the byproduct of varying pressures around the vehicle.
Every psi of pressure difference can make a huge impact on the overall downforce on the car. Especially when it is applied over very large surfaces.

A couple ways to influence the air to go into a high or low pressure zone are as follows.

To create a low pressure zone, the air needs to spread out. For example if you have a channel under the car that starts out small and spreads out into a wide free flowing tunnel, you will create a low pressure zone under the car when the air expands in your path.

Say if you manage to lower the pressure by 1psi, and an air funnel has an approximate 3' by 8' surface area, the corresponding 3456 square inches of surface area will create 3456 pounds of downforce!

To create a high pressure zone, the air needs to be compressed. Air is compressed when the air flows over a spoiler, or over the windshield, or any other inclined or narrowing surface(s). A high pressure zone over the vehicle will again help force the vehicle down.

However to produce any large changes in pressure, there will also be increases in drag, this drag is created because it takes energy to compress and expand air from it's natural state. And as such will slow you down. The best you can hope for is to minimize the vortexes over these pressure inducing surfaces.


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