Vehicle Road Testing – Part 1

Enhancing the performance of a vehicle isn’t always limited to making more power. Often more speed can be found by using less power to push the car through the air or along the ground.

The best way to evaluate the effect of a change on a car is via controlled testing.

We will need to know a few things about the car. The weight, and the frontal area.

Weight should be measured in “as tested” trim, and as precisely as possible. Truck stops, recycling centers, and city dumps have scales that may only be accurate to 100 lb increments whereas race scales may give tenth of a pound resolution.

Frontal area can be measured or approximated with a photograph taken head on of the vehicle, or via careful tape measure of “bounding boxes” on the car. More precision is better, but “close enough” is likely good enough. The frontal area number used affects the value of coefficient of drag when calculated. Much of this depends on what sort of testing you wish to perform, and what information you would like from the results.

Low speed coast down testing will allow the calculation of rolling resistance.

High speed coast down testing will allow the calculation of aerodynamic drag.

Ideally, the tests are done on flat ground, run in both directions, and repeated numerous times to validate the data. Running in both directions does remove some of the issues imposed by non-flat ground, but the test area should be as flat as possible. Notes should be taken on track temperature, air temperature, and atmospheric pressure as these can have an effect on the results. Having that data will let you compare results from different days/sessions more cleanly.

Data from coast down testing will allow you to determine aerodynamic drag and rolling resistance, but will then let you compare and re-test with different components. This additional testing will then let you calculate driveline inertia (including inertia from wheels, brake rotors, and tires).

We generally start by calculating the total drag force, using F= M x A. Acceleration data can be used from accelerometers or GPS data. The more precise, the better.

With total drag force, we can split aerodynamic drag from rolling resistance. (aerodynamic drag = total drag force – rolling resistance)

Rolling resistance is largely linear, and can be treated as constant for the sake of simplicity in calculations, whereas aerodynamic drag varies with the square of velocity. Treating rolling resistance as constant simplifies the calculations, but if sufficient data is collected, a curve can be fit to rolling resistance if desired.