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AERO- DUCT by Strong-Strut
BMW parts manual refers to them as "aero duct" so we're told they have something to do with the air flow or air currents near the wheels. Many other models of BMW's also have aero ducts including the ///M5. BMW said they divert the air flow in front of the rear tires and help prevent hydroplaning in the wet. Also has something to do with noise reduction but in a convertible, its noisy anyway ! But, we inquired with BMW NA and here is their reply "Thank you for writing to BMW of North America, LLC. The air ducts change the pressure in the wheel house, ideally to improve aerodynamic resistance & axle buoyant force. Wheel/ wheel house account for about 30% of total aerodynamic resistance. As such, we always use all possibilities to improve air flow in this area."
One thing we do know for sure is that the plastic factory parts last about 6 months and then they require replacement. The Phoenix/Scottsdale, AZ BMW dealers price these at $28 each plus local tax.
The Strong-Strut version of the aero-duct is fabricated from aluminum alloy and the parts are riveted together for a lifetime of service. The production version pictured here, is machine made and hand assembled. It is powder coated with a durable, baked on, textured black finish.
Pictured below is the plastic OEM aero duct after 8 months of service. The plastic is split, cracked and deformed and is beat to death from road debris impacting it. Replacing these every year at approximately $60 per set may not be the best thing to do. The Strong-Strut aluminum version is $99.00 & domestic postage is $15. Our Aero Ducts will last the lifetime of your car with reasonable care. See pictures of production version further down the page.
Below.....factory plastic units, badly deteriorated
Below.....Alloy Aero-Duct by Strong-Strut