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Maybe go and drive some cars that have a lot of aerodynamic lift? It's not a good experience at highway speeds, even with 10-15 per cent lift.
In fact, one car I have seen data for (14 per cent rear lift) had control problems on a public road. My mind boggles at how hard it would be at 50 per cent lift!
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Presumably you are talking about the mk1 audi TT, I don't believe you when you say that lift is the sole cause of the issue here, that is a simplification. The issue in that case was lift but also suspension issues.
The real issue is unequal lift and/or centre of pressures ahead of the centre of mass. Planes do not suddenly crash when they get to 50% of their weight on their tyres.
When you added those side fins on your insight, you got extra rear lift but because the centre of pressure moved rearwards the stability improved?
If unequal loading was the sole cause of the problem we wouldn't have cars with weight distributions up to 70/30.
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But it's easy enough to prove for yourself. Just put some high lift wings on your car and assess control and stability versus improved fuel mileage.
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But it isn't, because my cars aren't designed with aerodynamic stability in mind, they are aerodynamically unstable and rely on the tyres to stop the car from going sideways. This is a thought experiment about a car that is aerodynamically stable.
I have driven cars with front end lift, and I know it isn't pleasant, but then I have flown planes and gliders with much more lift and they have no directional stability issues. As I mentioned earlier the real issue is directional stability, rather than specifically lift.
Drop a plane and it will point in the correct direction, drop a car and it will tumble. Why? Aerodynamic directional stability.
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Either way, all the evidence is that any aero lift reduces stability, handling and braking
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On typical road cars maybe, but a plane becomes more stable as more lift occurs, due to more corrective aerodynamic pressure. Handling, again on a typical car, but this imaginary car can cut the lift, and has control surfaces so that evidence doesn't apply. Braking, air brakes would be employed, lift cutting and added downforce would be employed.
I am not saying that it would be in any way easy to do, or legal to do, but I still see no reason why a car specifically designed for it, such as in a university challenge like a solar challenge, wouldn't work.
Tyre grip is not linear no, but I was approximating it to linear because the linear region is where most car tyres are. However, the graph you provided suggests that halving the force on the tyres leads to only a 25% reduction in grip.