meh, I dunno about that. The spring rate on the rear of my car is 180 lbs/in. So if I add or remove 100 pounds it would lower/raise the back end by .56" My battery weighs 27 pounds, so that would lower the rear by .15" and raise the front by less than .15" , maybe about .1"? Nah, too much work for too little change. Lighter cars with softer springs might see a bigger difference in height for a given weight change. FYI my car weights about 2640 lbs with stiffer aftermarket springs.
Since I started this thread I've decided to leave it alone. I like the thought that a raised back end reduces the slope of the rear hatch. The tradeoff is that it increases the slope of the windshield. Overall I've heard the trailing edges are more important than the leading edges, but not by much. Luckily the transition from the front windshield to the roofline has a gradual taper, so airflow should stay attached either way. Unfortunately it's also the reason I can't see stoplights when I'm first at a light, the curve of the roofline blocks the view! Dang progress
I also like the thought that a higher back end creates lower pressure under the car, which should reduce drag. I know the car isn't lowered enough to keep air from sneaking in from the sides to fill the low pressure void, but I have to take comfort in something!
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