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Vehicle Road Testing – Part 2

In part 1 of this article, we discussed the principles for gathering coast down data. There are a number of things that can be determined from that data.

We can calculate the aerodynamic drag and rolling resistance components of the total drag. The large component at high speed will be aero drag. It will vary with the square of velocity.

Total Drag = ( (Coefficient of Drag)  x (Air Density) x (1/2) x (Velocity ^ 2) * (Frontal Area) ) + (Rolling Resistance)

If we plot Total Drag vs Speed, we can identify a “floor” at very low speed. In the last few MPH, that floor is rolling resistance. Subtract that from Total Drag, and we have a plot of Aerodynamic Drag versus Speed, and can then use our estimate of Frontal Area from Part 1 to calculate the drag coefficient.

In making changes to the vehicle for evaluation, the aerodynamic results are often easier to spot. We can verify our drag coefficient by doing simple tests, like removing side view mirrors, and subtracting that frontal area from our calculated frontal area, and re-run the tests to see if we end up with the same drag coefficient.

We also need to be careful in making changes to other bits and the “measured” effect on rolling resistance. For example, changing to lighter or heavier wheels or brakes should not impact the rolling resistance or the aerodynamic drag in a large or measurable way. It will, however, add to the inertia of the vehicle, and more specifically on those two, to the rotational inertia.

We can use that information to calculate the driveline inertia influence. We can also measure the inertia of the wheels, tires, brake rotors, and hubs, and use the radius of the wheel to calculate/verify the above.