The function of a rear spoiler is to control the flow of air streaming over a vehicle and properly manage it when it reaches the rear where turbulence is most likely to form. A correctly designed spoiler will counteract the lifting forces that most cars experience when speeds get high, and it will enhance rear grip by keeping a vehicle's rear end planted firmly against the road - without causing excess drag.
While aerodynamics do not play as huge a role at U.S.-legal speeds, they can make the difference between life and death on racetracks or high-speed German autobahns. In fact, the absence of a spoiler or the presence of one with a faulty design can actually cause a car to lift off the road surface when velocities reach extra-legal road speeds. All control is lost at that point, and a vehicle can end up sailing off in an untold direction - with disastrous results. So one important question to ask is which type of spoiler is right for you and your driving style.
Don't Let A Spoiler Spoil Performance
If spoilers are not properly designed, they can actually cause drag that slows down the car or makes it unstable. A good spoiler is shaped and angled to create a controlled air vortex behind you as you go. Managing how the air leaves the surface of the vehicle minimizes drag and increases stability.
Mounting an adjustable spoiler (for example, we’ve got a range of carbon fiber adjustable wings from APR Performance) allows you to test the angle at successively higher speeds to determine what the optimal wing setting is. Since the majority of spoilers are fixed and non-adjustable, we offer those that are vehicle-specific and tested by the product manufacturers to provide optimum results on your make and model.
Yet for many of us, we care less about high-speed stability, and are more interested in the sporty look that we get, even at a standstill. We caution if you do buy a larger, universal-fitting spoiler purely for looks, it's essential to pay close attention how it may change the behavior of your car before reaching higher speeds.
Spoilers Don't Have To Be Big To Be Effective
Spoilers don't have to be large, high-mounted wings to create a desirable effect. A perfect example of this is Ford's 1960s LeMans-winning GT40 racecar. When first tested on the Mulsanne track straightaway, driver Ken Miles discovered the GT40 suffered from "tail wagging" at higher speeds in spite of extensive wind tunnel testing and engineering development. Because a wagging tail can prove lethal at 200 mph, project manager Carroll Shelby ordered the master tinsmith of the team to add an aluminum spoiler to the rear of the trunk. Several inches of aluminum was shaped into a spoiler, riveted to the rear of the car, and the problem was solved.
Many spoiler designs are inspired by the large, high wing featured on the classic 1970 Plymouth Road Runner "Superbird" edition sold in showrooms with racetrack-spec body parts to give it an advantage when qualifying for production-class track competition. While the large rear wing is distinctive and has inspired countless spoiler designs since, it's not really practical. The real reason for the wing's immense height was because of difficulties finding good mounting points for the spoiler - a fact that wasn't much publicized at the time.
Engineers found if the wing was mounted directly on the trunk lid, it would bang into the roof before the trunk could be opened very far. The solution to that problem was to affix it to rear quarter panels. But because trunk lids needed to actually open on production versions, the wing needed to be pretty high up to ensure proper clearance.
What Looks Fast Sometimes Is Not
There's nothing wrong with high pods and big wings - if they're done right. They can completely change a vehicle visually, making a clear statement that an owner appreciates speed. But as we mentioned earlier, the wing must be properly angled to create the optimum amount of downforce that keeps tires planted firmly against the ground without causing extra drag that acts like an air brake.
Aerodynamics truly is a field where the devil lurks in the details. One well-respected German auto manufacturer discovered this fact after it was too late. After their design department vetoed engineers' demand for a full body kit package with spoiler on a new performance model, a number of fatal accidents resulted on German autobahns because of deadly lift created by the original body shape. After finally accepting what Carroll Shelby and Dan Miles realized decades earlier, a discreet 2" high spoiler was added.
Roof spoilers (also known as "window spoilers") are most effective on hatchbacks, station wagons, and sport utilities where the rear of the vehicle drops off sharply and there's no other trunk edge to speak of. In such cases, these roof spoilers effectively create the extra down force needed to enhance stability. However, they can also be useful on notchbacks that are equipped with larger wings on their trunk edge, because more airflow is redirected over the wing surface. On notchback vehicles without large trunk wings, they generate turbulence where there should be smooth airflow over the rear glass area. This creates a low-pressure pocket that only serves to increase drag - making them more for show than go.
For most practical purposes on the street, a properly designed lip spoiler is all you need to optimize aerodynamics for stability and performance. Don't be dissuaded by their more discreet looks, because they do add a performance touch to the design without being ostentatious. It's important to remember spoilers that look fastest and most aerodynamic often are not, and real car people know a discreet-looking spoiler can deliver results just as effectively as one that shouts loudly.
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