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Old 04-29-2020, 06:19 PM   #2 (permalink)
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I will present the throttle-stop data here as I measure it. Unless otherwise stated, all data is for my Gen 1 Honda Insight that has the following modifications:
  • electronic height-adjustable air suspension
  • full ABS undertrays front and back
  • rear diffuser
  • rear acrylic fins
  • rear GOE222 profile aluminium wing
  • MoTeC dash including 10Hz GPS speedo
  • No IMA turbo, water/air intercooled and MoTeC ECU

Windows open/closed
Windows closed 104 km/h
Windows open 98 km/h
Calculated change in CD = 12 per cent

(Katz J, Automotive Aerodynamics, Wiley, 2016, Page 292 - gives a range of 10-16 per cent change in CD with windows open.)

Ride height
Ride height low - 105 km/h
Ride height high - 102 km/h
Calculated change in total drag - 6 per cent

(Rob Palin, former Tesla aerodynamicist, said to me of this value: I'm surprised by the small size of the ride height one. Ride height tends to be pretty linear (especially if you maintain a constant reference area!), with the deltas rising more sharply if you have very ugly wheels that you're now hiding better. The Insight had decent wheels, and the side cover at the back, so maybe it just isn't that sensitive.)

Rear wing angle
Some scatter in measurements around plus/minus 10 degrees wing reference angle, but the line of best fit shows an 8 per cent increase in car drag from a wing angle of plus 10 degrees to minus 10 degrees. That seems too much for the wing alone so I assume it is also influencing the wake and/or action of the diffuser.

At a wing angle of 80 degrees (ie air brake), car CD increases over the slipperiest position of the wing by 25 per cent.
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