|MadSci Network: Physics|
Stability, in itself is a broad design topic, intimately involved with fluid dynamics and some good practical engineering sense, encompassing a whole library of information. I will try to synopsize all of this information as best I can with a focus to racing. Please settle in with a couple snacks as I tackle your question...If you are just interested in the spoilers, just scroll to the end:)
There are 3 important design consideratons in racing: 1) Speed (velocity) 2) Stability/Traction 3) Regulated Designed Tollerances-mass distribution and overall shape tollerances.
There are three axis of stability: 1) Roll x-axis 2) Pitch y-axis 3) Yaw z-axis For the purposes of my answer, put the front of the car facing the positive end of the x-axis and the roof of the car facing the positive end of the z-axis, with the center of gravity (cg) of the car in the origin of the 3-d coordinate system. Now...despencing with the formalities.
Weeding out some Assumptions
Basically, you want to go faster than everyone else! Focusing on stability on the race track, let's nail down some assumptions. Assuming your engine is as powerful as the next guy's engine, what's left for the desing team to play with? The size of your racing tires are usually strictly controlled, therefore we should assume everyone's tire to have the same cross-sectional width. Spoilers and skirts, if allowed, are always in the same location as required by your racing division, leaving the designer the ability to modify the airfoil characteristics of the skirt and spoiler. I believe the physical cg of the race car is strictly confined within tight tollerances, so we chould assume that to be fixed.
This leaves the designer with a few available changes: 1)Tires and side-slip yaw stability 2)Shocks, struts, roll-bars, stiffeners, dynamic stabilizers, etc...this category covers a large and diverse area beyond my personal experience and the desired scope of your answer. Suffice to say that in racing circles, especially at high speeds, smoother and tighter is better. Without a good dynamic response system you wont be able to take a quick turn, let alone maintain a straight course.3)Aerodynamic foil characteristics-the heart of your question-primarily deal with pitch stability and assisting the race car's traction.
TIRES and Side-Slip
First, your tires...well, as you indicated, your tire's surface area is an important factor in its interaction with the road. However, in most races your tire sizes are strictly controlled. Now, assuming everyone in the race has the same size tires, the desired friction or "grip" comes into play. Do you want your tire to be "soft"-sacrificing its surface to mate with every crevace and particle on the road-or do you want a trade- off for more durability and less traction?
The desired traction and durability of your tire are both very important considerations, not only in the area of high-speed stability, but for your racing strategy as well, i.e. how many pit-stops do you want to make?
Traction results from the interplay of the friction characteristics of the road AND the tire. On a high-speed turn, "side slip"-the sliding motion of the front and/or rear of your vehicle-becomes very important. Side slip results when the angular momentum overcomes the friction forces fighting to keep the vehicle on the desired path.
Now...finally, the aerodynamic reasons for spoilers and skirts...
Spoilers and skirts do a few things to reduce the drag profile-a 3-D messure of the laminar(non-turbulent or "flat" air) and turbulent air pressure on the race car which acts as a friction force against the car- and increase pitch stability, while also aiding the traction. Spoilers and skirts act in similar fashion to creat lift and to stabilize air flows at opposite ends of the vehicle.
Picture the cross-section of a race car, cut from front to back. In it's most base form, a race car is like a frisby that doesn't spin; flat on the bottom and slightly curved on top.
Now picture a block of air infront of the car. As the car moves "through" the block of air, the air is split over and under the car into two parts. There are two parts of the original block of air. Both parts, according to the laws of the conservation of mass and momentum and the incompressible nature of air at sub-sonic speeds, must meet up as a uniform block of air at the end of the car. For simplification sake picture this end block of air with the same size and shape that characterized the original block of air.
Due to the curved top of the car, the air moving of the top surface of the car has a longer distance to travel than the air below the car, moving over a flat surface. Therefore, in order for the two parts to merge as one block at the same time at the end of the car, the top part must move faster than the bottom part. This results in a lower pressure on the top than on the bottom of the car, result in lift, a general upward push.
Yes, that is a brief discussion on lift:) Anyway, picture two much smaller versions of the car model at different ends of the car, one above and behind usually with only a slight or non-existant curve facing the ground, and one below and in the front shaped similar to the car.
The rear device is the spoiler which spoils the lift generated by the car. This is done by changing the aerodynamic profile of the race car with the additional raised surface. Due to the spoiler, air pressure builds at the back of the race car since the top of the car no longer "seams" to be as curvy. This means the air doesn't have to travel as quick as it would in the original profile to mate up with the air travelling under the car. Therefore, the spoiler helps hold the car down to the ground for better tire traction especially important in high-speed turns.
The device in the front is the skirt which acts just like the original car model, with the added benifit of streamlining the air beneath the car for a smoother ride. With out interaction the air would flow relatively smoothly; laminar for our example. However, due to the ground surface interacting with the air, called ground interaction, the air under the car spins into a bumpy turbulence, and at high speeds can begin to buffet the car and significantly reduce traction. The ground skirt's interaction with the underbody current is also called ground slip, and provides a small downward force on the nose of the car as well.
At high speeds it becomes especially crucial to properly configure the shape and location of the spoiler and ground skirt. They must be properly distanced from the center of gravity for the race car to prevent one surface from flipping the car as has happened recently in some la Manze (spelling?) races.
In short, your tires give you traction and aid in yaw stability (side slip) and are dependent on road conditions and material characteristics. The spoiler reduces the lift generated by the race car and improves pitch stability and traction. The skirt stream lines the ground interaction and, if properly placed, can help ballance the effect of the spoiler at exceptionaly high speeds.I hope that answers your question, if you have any further questions, I'd be happy to tackle those as well.
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