07-10 BMW 328i Rear bumper lips are the minimalist way of giving your 328i the look of a complete custom body kit. With a simple installation that involves drilling screw holes and using double sided tape, they are the easy choice for quick and easy styling. There are also other advantages, by virtue of their design, they often end up in a lower position than a full Rear bumper replacement would be, making them look more aggressive and more effective at hiding large gaps if your car is not too low. There also happens to be a lot of clean styles available that are clean without being too ricey or gaudey.
More importantly though, with a fiberglass Rear end, hit one bad speed bump or dip and there goes your bumper and it's paint job. A BMW 328i lip on the other hand, has a bit more play to it, so it won't just shatter to pieces as soon as it kisses concrete. Also, even if it does, replacing it with another 328i lip is pretty quick, easy, and affordable, especially if it is left racer style and installed without being painted.
Some bumper modification required for proper installation.
