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Originally Posted by radioranger
So lowering is the way to go then, just how to do it without ruining the ride and handling,
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Well this is the point: If you somehow managed to go so low to actually increase aerodynamic drag, the car would simply not be driveable anymore (not just from a riding point but more so from not being able to drive over bumps and such).
A movable ground isn't necessary to test a passenger car in a wind tunnel unless effects of lowering are to be measured: If lowering effects are to be measured a moving ground (road simulation) is imminent, since not doing so will increase the frictional drag underneath the car by a factor of 12 (compared to the 'real world') even if a boundary layer on the wind tunnel floor was missing (which it is not).
(And if frictional drag underneath the car wouldn't play a role, car manufacturers and hyper-milers wouldn't install smooth underbody panels).
Race cars with wing like structures on the underside of the surface wouldn't work in a wind tunnel without a moving floor as the wing like surface would simply stall (due to the low air speed underneath the vehicle provoked by the unnaturally high frictional drag including wind tunnel floor boundary layer).
Air is not being compressed at the speeds most cars and especially hyper-milers travel. At 120 km/h dynamic pressure (ram pressure) only reaches 0.6% of atmospheric pressure:
Dynamic pressure - Wikipedia, the free encyclopedia
The pressure difference (pressure drag) between front and rear of the car is primarily created by the 'negative pressure region' behind the car (wake). (And the size of the wake is also affected by what is happening in front and underneath the car).