### Re: What devices help race cars take turns at faster speeds? And how?

Date: Tue Oct 10 12:34:10 2000
Posted By: Kenneth Chivers, Grad student, B.S. Aerospace Engineering, In school for MBA:Management of Information Systems, NAWCAD, Lakehurst, NJ
Area of science: Physics
ID: 970515989.Ph
Message:
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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
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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