Quote:
Originally Posted by 3-Wheeler
Jim,
You learned what the rest of us have while attempting to discern while performing coast downs.
It's really difficult without substantial time and patience.
Jim.
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Jim, Having tried coast downs, I think it is substantially worse than time and patience. The Insight is extremely sensitive to environmental conditions, particularly wind and temperature, however slight. If a set of coast down experiments goes on for any significant period of time, say a couple of hours, then wind or temperature are highly likely to change. Then the modder is left with trying to figure out how to compensate for changes caused by the environment, not the experiment itself.
Very high speed coastdowns might help, but those can't be conducted on public roads.
No amount of sophisticated data gathering is going to compensate for shifts in the environmental conditions during the test. I don't think we have any accurate models for environmental shift other than perhaps averaging the two "A" segments, which sandwich the "B" segment, but then that violates the primary philosophy of the A-B-A test itself. What am I missing?
Even the famous Aerodynamist Kamm made rather gross errors when trying to make sense of coast down data. From coast down data on his K3 he derived a Cd=.24. Some years later the car was tested in the Volkswagen wind tunnel and found to be Cd=.37. (Source "Aerodynamics of Road Vehicles, 4th ed" by Hucho, page30). If such a brilliant engineer didn't get it right, then it's a problem.