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.
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 accordmania.com, 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.
