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Old 05-17-2009, 11:02 PM   #5 (permalink)
Ernie Rogers
Ernie Rogers
 
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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I see a lot of good ideas here. Here are my additional thoughts--

I just did mine with a stopwatch that collects multiple times (actually, my sport watch). I timed at 10 mph intervals from 70 to 10 mph. You really need a road with no traffic and no cops. A smooth, level mile is long enough. I averaged the results and then graphed.

Graph speed versus time, and the slope of the curve is the acceleration. Measure the acceleration (from the graph) at a high speed and also at a very low speed. Then get your Cd and Crr from the equation--

F = Ma = Cd A 1/2 rho V^2 + Crr M g

I did the calculations in metric because I felt more secure there. Important: the first term with Cd dominates at high speed. The second term dominates at low speed. My procedure is this--

You can guess a Crr that seems fair. plug that into the equation with your numbers for the high speed, and calculate Cd. Then, plug that Cd into the equation with the numbers for the low speed and calculate the Crr. Repeat the steps, put that Crr into the high-speed case, etc. After a few cycles, the Cd and Crr stop changing. Those are your answers for both.

My view is to not trust anybody's number, like the car weight (M g), unless you verify it. I got my car weight at a commercial scale. (They shouldn't charge you.) They did the weight both with me in the car and with me out. The difference had to match my weight.

Actually, it probably doesn't matter if you don't know the frontal area accurately. It's the product, Cd A, that is useful anyway.

Be very sure the roal is truely level. Pavement quality will affect the Crr measurement.

Ernie Rogers
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