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Old 06-06-2022, 10:23 AM   #320 (permalink)
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how many how much

Originally Posted by Phase View Post
Okay so I finally found a farm road during my Idaho road trip to do coast down testing

There was one stretch of flat road that was perfect but not very long

Found 2 marker signs and did a base line test with windows up

Test started around 73 mph and when I reached the second marker, the speed was 68 mph

Did the windows down test, and it was 66 mph at the second marker

So 2 mph drop thanks to drag

Did my cardboard wheel skirts, speed dropped from 73 down to 69 mph

Only a 1 mph change. But it clearly worked. The results show that all 4 windows rolled down causes more drag than the gain you get from wheel skirts ( obviously!) also- with the wheel skirts, acceleration out of the parking lot felt more stable and actually faster. Hard to explain.

So what Iím asking is- how many cd points does the windows down add to your average vehicle, or how much drag percent is added based on the 2 mph slower test I just mentioned. Once we find that out, we could half that and say thatís how many cd points Iíd be lowering with rear fender spats?

Math anyone?
1) While you're experiencing a 'trend' in the data, the test methodology isn't rigorous enough, nor data points of sufficient extent to enable the use of the mathematics.
2) Coastdowns must be conducted just as proscribed in the SAE Handbook.
3) Every rotating component in the car must be quantified for moment of inertia, or estimated according to SAE instructions.
4) You need your own weather station, or, perhaps timely AWOS ( automated weather observation service ) updates from a local airport, adjusted to raw barometer. ( I paid CAR and DRIVER road warriors to test my CRX at the Chrysler Proving grounds for top speed, and coastdowns. They threw in skidpad testing for free ). Chrysler has a sheltered straightaway, designated specifically for coastdown testing. Cost was equivalent to 1-hour in a wind tunnel, in 2014-2017 dollars ( DARKO or A2 ).
5) We don't know what happens to the IONIQ's engine BSFC when the cars' drag is reduced at a constant test speed.
6) We don't know anything about it's actual rolling resistance.
7) We don't actually know if it's published Cd was @ radiator shutters fully-closed, zero-yaw, crosswind-averaged, etc.. We do know that the wheels were rotating in the wind tunnel.
8) MPG data would only help, fully-warmed, at a constant speed, without any hybrid function, in both directions, just the engine operating at a constant speed, with weather monitoring. And at an accuracy you're comfortable with.
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