Fender vents, what is the purpose?

BMW E46 M3 CSL fender vents

What is the purpose of fender vents?

The main purpose of fender vents is to relieve built-up air pressure from inside the wheel arches which causes aerodynamic lift. Typically, there are air intake openings in the front of the bumper directing air over the brakes to cool them. This air gets trapped in the wheel arches and creates high pressure resulting in lift over the front wheels reducing traction. Relieving this built up high-pressure system in the wheel arches reduces front axle lift thus increasing safety.

Trapped air in the wheel arches also creates aerodynamic drag. Drag is the force of air pushing in the opposite direction of the vehicle, reducing efficiency. Some research suggests wheel arches account for up to 30% of aerodynamic drag of a vehicle. With fuel efficiency ratings becoming increasingly more strict on modern vehicles you can understand why so much attention is being directed towards the reduction of this inefficiency. Efficiently routing air into and out of the wheel arches increases fuel economy through the reduction of drag.

Another benefit of fender vents is to efficiently vent hot air generated by heavy breaking out in the air stream. Quickly relieving hot air generated from the brakes makes room for more cool air to pass over the brakes, thus increasing efficiency. Finally, it is worth mentioning that trapped air in the wheel arches also contributes to cabin noise. Reduction of cabin noise is more pleasant to drivers on long trips reduces driver fatigue.

* For another topic on aerodynamic efficiency and lift reduction, read our article on rear diffusers. It explains what they are used for and how they work, click the link HERE.

Do the fender vents vent hot air from the engine?

No, not on BMWs. Some vehicles tie in the fender vents exhausting hot air trapped under the hood and out the fender but on BMW models the fender vents are not related to venting hot air from under the hood.

Are all BMW fender vents functional?

No. Some are just for show. The E46 M3, for example, has nonfunctional fender vents. The original design was intended to use functional fender vents however the engineering team decided the vents were not needed through testing. The final vehicle design was already approved by the marketing team however and as a result, the fender vents stayed with the design of the vehicle but were made nonfunctional. Since the E46 M3, each subsequent generation M3 has had the fender vents become an iconic design element of the car. The M4, however, does have functional fender vents.

What does BMW call their solution to efficient air flow?

BMW is marketing these new aerodynamic features as “Efficient Dynamics”. "BMW EfficientDynamics is the name of the strategy from BMW to minimize fuel consumption and CO2 emissions while, at the same time, increasing dynamics and driving pleasure. It is a package of functions covering the drive system, energy management, and vehicle concept – and is a standard feature in every BMW." - BMW.com

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