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It then generates it's own model for velocity (V model) based on entered constants and initial guesses for Cd and Crr. The spreadsheet averages data from all 6 trials to create a single data set representing velocity (V actual) as a function of time. #Fsae tire coefficient of friction downloadDownload the spreadsheet I created Drag_Coefficient.xls and enter all your data following the instructions included. All these values will be averaged for a more accurate analysis.ħ. ![]() Repeat the test in both directions twice more (6 trials in all, 3 in each direction). Repeat the test in the opposite direction.Ħ. #Fsae tire coefficient of friction freeUsing a free program such as Avidemux ( ) you can play the video back on your computer frame by frame and view the timestamp at desired speeds.ĥ. Simply have your passenger record a video of your speedometer during the coast down tests, or find some way of mounting the camera so you can do the recording without an assistant. ![]() You don't need any equipment except the digital camera. Aside: If you have a digital camera capable of recording several minutes of low resolution video (as most people seem to have these days), the process is much easier and more accurate. The passenger should record this value next to each time. #Fsae tire coefficient of friction driverThe passenger should indicate every 10 seconds after that and the driver should call out the current speed to the nearest whole km. At this time (t=0) the passenger should start the clock. The driver should indicate when the speed drops to exactly 70 km/h. Have the driver accelerate up to above 70 km/h or so, and shift into neutral.Ĥ. Have the passenger ready with stopwatch and paper to record data.ģ. Drive to a flat road with little traffic or wind.Ģ. Note that Crr will not be pure rolling resistance but will include some drive-train resistance as well.ġ. You can determine Cd and Crr from the same set of test data by measuring velocity with respect to time as your vehicle coasts in neutral. This adjustment can be done manually, by overwriting the values of Cd and Crr with new values till the model matches the data, or it can be done using a "Solver" function. The model values for Cd and Crr can thus be adjusted until the model matches the data. I created a spreadsheet based on these formulas to generate a model of velocity vs time that can be compared to actual data. Note that the acceleration is negative indicating that air and wind resistance will cause the velocity to decrease. Combining these formulas with a bit of algebra gives us the acceleration due to air and wind resistance as a function of velocity: This is why driving at high speeds is much less efficient than driving at low speeds. Note also that Fd is increases as the square of velocity. Note that both Fd and Frr are negative indicating that these forces act opposite to the direction of the velocity. G is the gravitational acceleration constant which is 9.81 m/s^2Ĭd is the vehicle's drag coefficient we want to determineĬrr is the vehicle's coefficient of rolling resistance we want to determineįd = -Cd*A*0.5*rho*V^2 (formula for force due to air resistance or drag)įrr = -Crr*M*g (formula for force due to rolling resistance)į = Fd + Frr (total force is the sum of Fd and Frr) Rho is the density of air which is 1.22 kg/m^3 at sea level ![]() M is vehicle mass including occupants in kg Fd is the force on the vehicle due to air resistance (drag) in Newtonsįrr is the force on the vehicle due to rolling resistance in Newtonsį is the total force on the vehicle in Newtons ![]()
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