Quote:
Originally Posted by aerohead
I've never heard of a 'roll down' coefficient.Can you expand on that?
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It is a sophisticated (i.e., technically challenging) EPA technique that takes a car to at least 70 mph and measures how long it takes to reach 15 mph, SAE J1263:
- Standard day temperature, ~60 F.
- Very level, +/- a few feet, straight at least a mile track.
- Wind and other climate limits.
- 16 bi-directional runs.
From the roll-down metrics, you can develop three drag coefficients:
- A - lbs per mph squared
- B - lbs per mph
- C - lbs
With these three coefficients, you can plot the drag force as a function of speed or more useful, the drag power needed as a function of speed. Combine that with the drivetrain efficiency, you can make a curve showing the MPG or kWh per mile as a function of speed.
My technique is to use at least three benchmarks to derive the quadratic equation. Two have to be on either side of the local minimum or maximum, and the third out into the high-speed region. Then use a quadratic solver to derive the A, B, and C coefficients. This is much easier and less complicated than the SAE J1263 procedure.
Using this procedure, we can test changes to improve a car's performance, either acceleration and/or range.
Bob Wilson