the bug is like a giant airfoil, especially with it's flat bottom. put a front spoiler on it and lower the car then instead of high pressure under the car it should be a lower pressure. i think the reason the rear end feels so light at high speeds is because of how rounded out the back end it, there's probably a very great area of low pressure there, pulling the rear end up. if the front is lower than the rear that should help keep the rear end down too because it creates low pressure in the rear of the car, that's basically what a spoiler does though. check out how airplane winds work with airfoils.
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