One of my reference texts is "Fundamentals of Vehicle Dynamics" by Thomas D.
Gillespie. There is a complete chapter dedicated to vehicle drag. One of the
figures in it has information on the components of drag as it relates to
normal road vehicles. I have converted that data into percentages so that it
might be more generic and sorta applicable to other cars. Use it at your own
risk, your milage may vary...
Taken from Figure 4.11, fundamentals of Vehicle Dynamics
Drag Component Percentage of Total Drag Rank
Afterbody 33.33%
1
Wheels and Wheel Wells 21.43% 2
Underbody 14.29%
3
Forebody 11.9%
4
Skin Friction 5.95%
5
Cooling System 5.95%
5
Drip Rails 2.38%
6
Window Recesses 2.38%
6
External Mirrors 2.38%
6
As can be seen, the rear end part of the car is the largest contributor to
total drag. The next figure in the text indicates that the afterbody drag
is minimized when the slope angle of the roof line is approximately 22
degrees with a slope length of 45 percent( ie, if the car length is l and
the slope lenght is m then m/l = 0.45).
The next largest drag component, Wheels and Wheel Wells, is caused by
turbulent recirculating flow in the cavities. This is a complex flow to say
the least. The data indicates that decreasing the clearance between the
underside and the ground and minimizing the wheel cavity decreases the drag.
One of the conditions that occurs is that as the air moves around the car it
flows by the wheel well opening (front) and the edge of the wheel well
causes separation of the flow. This essentiall makes the car frontal area
appear to be wider than it is.
Aahhh, the underbody. This is a high drag area because of the frame members,
exhaust pipes, suspension, rear end and all the other bits and pieces that
hang down under the car. The air under the car undergoes the most stressful
flow because the air at the road surface is standing still (so to speak)
while the air at the underbody is also standing still, but the vehicle is
moving. This causes a large shear stress and that takes power to accomplish.
Aids to reducing the shear stress are the use of air dams in the front to
keep the air from under the car (danger here also, because you can add more
dam than is needed thusly raising the drag!) and installing a smooth pan to
eliminate the flow around the pieces under the vehicle.
Skin friction is a fair chunk of the total drag. It comes from the air
moving accross the skin. This can be reduced in 2 ways, maybe more. Reduce
the amount of skin through surface analyses, and waxing the snot out of it!
Make it really smooth so that the air is not tripped and glides accross it.
The cooling system is the last major element in drag. The air flow through
the radiator impact all of the junk under the hood and passes out through
the under body (also adding drag). Putting stuff in the way of the air flow
would seem to work by keeping the air from entering the engine compartment.
I do not subscribe to that line of thought however, I prefer to let the air
out of the cavity in the smoothest way possible. Duct work if possible.
A final thought: since the drag coefficient is a linear component of the
power required, any change directly reduces the power requirements. So get
out there and look at your car!Do what you can do that falls under the rules
of your class to reduce your drag and you will go faster!
Again, your milage may vary, use with a grain of salt, Danger, Danger Will
Robinson......
mayf, the red necked ignorant desert rat hooligan in Pahrump
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