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Old 05-25-2009, 02:01 AM   #14 (permalink)
Ernie Rogers
Ernie Rogers
 
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Darin had two sets of data at his web site. I graphed the numbers for the first case (the Metro), then curve-fit them by eye and studied the results.

It appears from his description that the tests may have had a tail wind close to equalling the average speed in this coast-down test. If so, the aero drag is close to zero and only rolling resistance stopped the car. For the case of no wind, the rolling resistance coefficient is equal to the average slope of the road. Here are the Crr numbers I got for pressures from 20 psi to 55 psi--

20............0.0147
25............0.0131
30............0.0121
35............0.0114
40............0.0108
45............0.0104
50............0.0101
55............0.0099

These Crr values very closely fit the following formula--

Crr = .0071 + 0.15 /P

Ohh, my! Maybe I had the wind in the wrong direction. I can do that calculation but maybe someone else would like a turn.

Ernie Rogers

Quote:
Originally Posted by dcb View Post
Thanks for clarifying.

Darin actually did some coastdown testing based on psi, found the returns diminished sharply at a bout 45psi in his particular case:
http://ecomodder.com/forum/showthrea...ures-2721.html

"0.9%"
Yah, that is tough to discern on the street, even with accurate fuel consumption feedback.
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