Because the springs in a BMW are made from higher quality materials and have a higher spring rate (lbs. per sq. in.). Your springs sag, because your springs are old. Get new springs for the front and rear. If you get just the front, then you will have new firm springs in the front and saggy old springs in the rear. Your car will still sag in the back on hard acceleration and have a lot of body roll in the rear during hard cornering. Aftermarket springs will have a higher spring rate which helps eliminate this on all four corners. Buy progressively wound springs, not linear. Buy a well known manufacturer to ensure good quality. Then have fun driving like a maniac.
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