Discover Your Drag with Coast-Down Testing

In our December 2013 issue, we went in-depth on how to calculate your car’s drag by performing a coast-down test multiple times in both directions on the same stretch of road. A series of calculations based on the time your coast-down takes then allows you to see how many pounds of drag force your car is producing in total. That includes all drag, including aerodynamic drag, bearing drag, and rolling resistance.

Once you have that number, you can alter wing angles, install stickier tires, tweak tire pressures, add side skirts, leave the car in gear, or do whatever modification you want to test. Then perform the coast-down test again and calculate the force. This will allow you to see the difference. Of course, it’s ideal to perform one change at a time so you can pinpoint the drag added or removed with each change.

We’ve made it easy to calculate this drag force. Simply download the spreadsheet at this link and insert your own numbers. Happy driving!

(photo by Ken Neher)

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