Quote:
Originally Posted by freebeard
No reference, but from memory rolling resistance is linear and aerodynamic resistance is exponential.
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This term is commonly misused. Aerodynamic resistance is a
power function (an independent variable, in this case v, raised to a constant, 2), not an exponential function (where the independent variable is an exponent).
That squared term in aerodynamic force equations is derived from the
dynamic pressure of the flow, which is proportional to its energy. If you remember back to physics, kinetic energy is proportional to the squared velocity of a body since it is an integral of its momentum (P = mv --> K = [1/2]mv^2).* For a fluid of constant density, like the airflow around cars, dividing both sides by a reference volume gives
dynamic pressure = (1/2)(density)v^2
Multiplied by a reference area gives aerodynamic force. Dividing the reference area by a convenient/arbitrary fixed area, such as the cross-sectional area of a car or the plan area of a plane, gives a dimensionless drag coefficient.
*In reality, K = (1/2)mv^2 is an approximation of a more exact quantum equation. That wasn't known until the 20th century; the macro approximation was discovered by a Frenchwoman in the 1700s. Prior to her experiments it was thought that kinetic energy varied linearly with velocity.
Quote:
Originally Posted by Cd
I keep seeing this comment over and over.
I remember a comment that Darin made about how important that aero is even at 45 MPH.
Can someone suggest a direct link to some info on this ?
It gets frustrating.
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It's not a very useful comment; it depends on the size, mass, and drag of the car, and terrain, and road surface, and....
Say you have a car driving on a flat asphalt road that has equal rolling and aerodynamic drag at 40 mph, like a large SUV or something. If you change something and reduce the aero drag by 10%, is that not "hav[ing] an effect"?