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Car Spoilers

Strangely a "Spoiler" is the aerodynamic device that alters for the better the air flows underneath the car. 

Those installed at the bottom edge of front bumper are called a "Chin Spoiler" or "Air Dam", and those installed at the bottom edge of the car's sides as "Skirts". 

Their job is to stabilize/smooth the underside air flow.

Air flows underneath the car are always undesirable. There are many components, such as engine, gearbox, driving shaft, differential etc, exposed in the bottom of the car. They will obstruct the air flow, not only cause turbulence which increase drag, but also slow down the air flow increase lift and generally mess about with any work we may have put into the Aerodynamics of the body shell. 

A Spoiler is used to reduce underside air flow by encouraging air to pass either side of the car. As a result, drag and lift caused by underside air flow can be reduced. 

Generally speaking, the lower the spoiler locates, the better the results obtained. Therefore you can see endurance racing cars having spoilers nearly touching the ground. 

Of course road cars cannot go so low unless you want to be constantly repairing the bodywork, That's not to say some don't go very low .

As an aside  reducing drag means we need less power to hold any given speed, and power is produced of course by burning fuel. 

So it holds that an aerodynamic car is not only faster but also more economical.  

Its just a shame then that road car spoilers have to be set relatively high to clear such obstacles as sleeping policemen. The net result of such traffic calming measures is to limit the Aerodynamic efficiency and hence economy of our cars.