there is a "static" cg, which exists when the car is at rest. now this
static cg is influenced by many other factors when the car is being flogged
down a raceway, and the static cg now exists in a constant state of change
due to the "action / reaction"- newton's 3rd law of motion principle. some
examples of the factors which change the cg are 1. downforce created by
the wing or spoiler increasing with speed 2. wind drag of the overall
vehicle, the air ramming the windshield and the vacuum behind the car tend
to load the rear tires,which change with speed 3. acceleration and
deceleration, thats one reason why drag cars can wheelstand under hard
acceleration ,even grabbing the next gear or popping the chute changes the
cg. 4. chassis design influences where the power of the engine is
translated into the vehicle, and thus the cg. this is where 4-link setups
can be helpful. 5. even the overall shape of the vehicle may cause it to
act as an airplane wing , shifting the cg. 6.even things such as fuel slosh
and fuel consumption affect the cg. so if one steps back and looks at some
of these factors, it becomes difficult to calculate it on paper. , and when
the car powerslides sideways at 200 mph, that actual cg is still changing.
now on the topic of the low pressure or vacuum area behind the car, one
must realize that air has mass, it actually weighs something. sure, its not
as heavy as water or steel or lead, but consider the sheer amount your
forcing to go around your vehicle at speed. so it doesn't weigh alot, but
there is alot of it. now all this air, which has mass, tries to violently
return around the back of the quarter panels and trunk and spoiler to try
to fill the hole the vehicle just punched thru it. so, if the back of the
vehicle tends to be more streamlined , it is easier for this air to return
to the back of the vehicle, it doesent have to take a sharp turn around a
spoiler or sharp edge ( and, the spoiler causes the rear of the vehicle to
look bigger, so there is a bigger hole to fill). whats really happening is
that the mass of the air causes it to not want it to return to behind the
vehicle, and so there is less air behind the vehicle. less air= lower
pressure= suction on the back of the car= drag. getting technical, but a
spoiler can be a double edge sword. the sharp edge may have a tendency to
trip the laminar layer of air flowing around the vehicle and cause it to
act like a vortex generator. ( a good example of this can be seen by
looking at the new drag bike and indy car helmets, which have a distinct
ridge along the top).whats happening her is that your basically stirring
the air(causing turbulence) and it now is easier for it to return behind
the vehicle because it is now moving in all different directions. after
writing my thoughts on this i now realize why i'm always driving down the
highway with the window down on the way home from work with my hand out the
window moving it around seeing this stuff in action. -everything is a
tradeoff.
hope this helps-
regards,
doug ferguson
black radon engineering
groton, ct
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