Quote:
Originally Posted by MikkiHuuru
Yes, SAE-approved is well beyond DIY. The values I produced from the measurements are good enough to see that Cd of 0.287 is wrong by a factor of 1.9 ± 15%. Any measurement have a margin of error, and you can tell how much approximately it is.
Also, the maximum speed achieved by the car tells that my values are quite correct. So does acceleration rate.
What comes to Prius, the Cd of 0.25 is not right either, not even close. It's pretty much impossible to have that low Cd when we know the range in electric mode that has been verified.
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Here are a few things to consider:
*The test car must be at its official EPA test weight (300-lbs over curb weight ).
*The test car must be driven for a half hour at 50 mph immediately before testing begins to stabilize all fluids and lubricants as well as tires and bearings at ambient equilibrium conditions.
*Tires must be inflated 'cold' to Mfgr's specifications.
*Test area must be absolutely level.
*Test area must be absolutely straight.
*Test area must be dry.
*There should be zero wind.If wind is present,its magnitude and direction must be monitored and recorded for each run for the entire test period.
*Accurate temperatures must be monitored and recorded for each run for the duration of testing.
*Station Pressure must be monitored and recorded for each run for the duration of testing.
*Actual air density must be paired to associated runs.
*Road surface must be evaluated for the test area.
*Velocities must be measured to 1/10-mph accuracy.
*Velocities must be recorded at 10-samples per second resolution with calibrated distance-measuring equipment.
*Test runs must begin above and coasting to 70 mph,then down to include 20 mph.
*A minimum of 10-runs must be made,back-to-back in alternating directions.
*Technically,the fuel tank must be topped up between every run to preserve a constant inertia.
*Polar moments of inertia should be measured for all rotating components of the car.They can be estimated,short of actual measurements.
*No 2-run 'pairs' may vary by more than a couple percent of each other.If they do they must be thrown out.
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Only after the successful coastdown test and data reduction will you actually know what the Rolling-Resistance portion of the Road Load Power is,from which to deduce the Aerodynamic portion,from which you may extract the Cd.