How does Diffusers work



Bumper Diffuser

  • A bumper diffuser is a piece of automotive equipment that most people choose to buy after they purchase their vehicle so that they can get it at lower cost. The ostensible purpose of a bumper diffuser, which is a special type of rear bumper, is to steady a car as it drives so there is less turbulence that could lead to distraction, a spin out, or even a crash. While these rear bumper diffusers are traditionally installed on race cars, and similar high-performance vehicles, they've become more and more common on street vehicles.


Downward Force

  • To understand the purpose and operation of a bumper diffuser, a person first has to understand how a car maintains its grip on the road. As a car drives, air is flowing above and beneath it. The air going over the car is moving slower than the air going underneath which creates a downward force that presses the car against the road and helps it maintain a grip. However, if the air moving beneath the car moves too fast , it can cause turbulence for the driver and passengers as it slides out from under the car and once more joins the regular air behind it.




  • This is where the bumper diffuser comes in. Since it's at the rear of the car (and is in fact the rear bumper), a bumper diffuser mediates the problems caused when the high velocity wind meets the slower moving air. According to, the bumper diffuser does this by providing an extra space, which allows the under carriage air stream to slow down slightly before it rejoins the rest of the air behind the car. This is different from a rear spoiler, which is a wing-shaped fin on the back of a car above the trunk that's supposed to provide a similar, steadying effect. Mostly a rear spoiler is only for aesthetic purposes, whereas a rear bumper diffuser has actual, steadying effects.


  So you see them under the rear bumpers on touring cars, Formula 1 cars and many high-dollar exotics--those strange vertical fins that fill the large gap between the bumper and the ground. Diffusers are a little-understood aero-enhancement that can make or break a car's handling and stability at high speed.



The Venturi Effect

  • Imagine a cardboard tube with one side bent in toward the middle. As air flows through the tube it compresses on the front of that kink and speeds up to squeeze through the gap between the kink and the rest of the tube. After it gets past the kink, the air slows down and decompresses. This decompression creates a little vacuum force on the back side of the kink in a phenomenon known as the Venturi Effect.

  Airflow calculation for racing-car rear diffusers, also known as underbody tunnels or venturis




Under-Car Aero

  • Imagine the bottom of your car as the kink in that cardboard tube. Air flowing under the car's front bumper compresses and speeds up to squeeze between the car and the ground. A diffuser is essentially a tunnel or void in the floorpan that allows air to expand and create a vacuum under the chassis. This vacuum helps suck the car to the road at high speeds to enhance stability with little to no penalty in aerodynamic drag.




The Vertical Fins

  • The vertical fins under the rear of the car help to guide air outside. Without these fins, air exiting the underside of the car would tumble and swirl, reducing vacuum under the car (ground effect) and increasing drag. You may also notice that the rear bumper on many diffuser-equipped cars is set much higher than the front bumper; this is to allow air to leave as slowly and efficiently as possible.