Theoretical Framework for the Estimation of the Coefficient of Drag for a Car
Posted on 5/2/2008 1:07:42 AMIf we are to assume the rolling resistance as well as intertial effects due to component rotation to be negligible, then the rate at which the velocity of the car declines when on a level surface is a function of the drag of the car body. As a result, we may calculate an estimate of drag as well as a constant coefficient of drag if we know the vertical plane projected area of the car and its mass.
In order to perform this experiment appropriately, the road must be as level as possible, and for this derivation which I have done below, there can be no wind. It is important to notice that the final equation is not just CD, it is the product of CDA. This is due to the difficulty in determining the cross-sectional frontal area, A. It is also important to note that this parameter has the dimensions of length squared or area.
On this level surface, accelerate to some given speed, V
0, and maintain it there. The speed should be rather high in order to decrease the relative significance of rolling resistance on the results
1. In the event of this experiment being conducted on a high speed vehicle this speed should not exceed Mach 0.3. Take your foot off of the accelerator, and record the time in seconds that it takes for the car to coast to some lower speed, V. The parameter m is the mass of the car and rho is the air density. This latter parameter changes with elevation, but is generally around 1.2 kg/m
3.
1 White, Frank M., Fluid Mechanics, McGraw-HIll Higher Education, 2003, 5th Edition, Boston, MA.