Quote:
Originally Posted by jimepting
....I've heard from others that Cd by the coastdown method is very difficult, so I'll be anxiously awaiting your feedback...
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You are absolutely correct about coast-downs being somewhat of a challenge.
I drive a reasonably flat section of road on the way to work. According to Google Earth, the road is flat within several feet, up until a slight 5 foot rise at the end.
There are now well over 50 coast down tests accumulated from the winter to late spring, using a hand held GPS on this section of road, always going the same direction.
I like math and use it allot at work, and when analyzing the data, it's absolutely amazing to me that the coast down testing reflects every single dip in the road with uncanny realism.
Unfortunately all these dips have to be accounted for in the data stream. Even a foot rise in the road shows up in the dV (delta-Velocity) testing as a slight change in velocity, which another way of saying, an acceleration.
If the road bumps are not accounted for, then the decleration data due to air and Crr are skewed, and one can easily be mislead or come to the wrong conclusion regarding Cd of the car.
After spending many hours analyzing the data, the numbers are still revealing their secrets!!
I have a ways to go before making any concrete conclusions however.
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Regarding baseline runs, I would follow Metro's lead and simply drive at a constant speed, and simply monitor the fuel used over a stretch of road, just like Metro did when he extended his tail.
Driving to a constant MPG does work, but since the air velocity changes, comparing apples to oranges is harder when trying to figure out how much better the aero tweak worked out.
Jim.