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Originally Posted by niky
How do you control for wind in that? SAE? I thought the coastdown was dyno only, to get rolling losses, then aero numbers were plugged in.
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See here And go to the attachment I, on page 25. All the gory details.
In short: wind speed, air temperature, and barometric pressure must be recorded for each coastdown run.
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The test road or test track should be straight, smooth, and level for a sufficient distance to obtain the necessary data. The necessary distance will vary depending on the vehicle type, the test surface grade, and the ambient conditions; however, the anticipated minimum necessary road or track length is 0.5 to 1 mile (1 to 2 km).
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Tests must be conducted on the road or track in both directions with minimal interference from other vehicles during the data collection periods. During the data collection period, the track surface and vehicle should be dry, the track should be free of obstacles or significant irregularities. The absence of intermittent wind barriers near the road or track surface is preferred to reduce positional wind variations.
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Coastdown runs are pairs in opposite directions one immediately following the other. At least 5 paired runs are averaged. Wind, grade, tire temperature, and air density are corrected for mathematically.
This procedure yields a road load equation. The coastdown time between 2 speeds, 55 to 45 mph for instance, can be found. Then the dyno load can be adjusted by dyno coastdown tests between 55 and 45 mph so the dyno load matches the road load at 50 mph.
-mort