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If I pay extra to get a spoiler added to my car, will it actually affect how it drives? – Tim, Ottawa

Spoiler alert: chances are, the spoiler on your sedan isn't doing much, other than looking sporty.

"The vast majority of spoilers out there don't do anything – you don't get any bang for your money," says Dr. Martin Agelin-Chaab, assistant professor in automotive engineering at the University of Ontario Institute of Technology (UOIT) in Oshawa, Ont. "They only work if they're properly installed, and even then, they only work at speeds of at least 100 km/h or more."

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Spoilers are supposed to spoil aerodynamic lift: the force that wants to pull your car off the ground. They're supposed to push your car onto the road.

"Usually, spoilers are intended to increase downforce – they deflect air upward, which creates a downward force on the car," says Dr. Ron Miller, an engineering professor at Carleton University who has worked on race-car design. "This helps stick the tires to the road to give the car better grip and therefore better handling in cornering."

Spoilers can also reduce drag, Agelin-Chaab says. And the less air resistance your car has, the less gas you'll have to burn. But you probably won't notice the difference driving the kids to soccer, unless you're taking the highway.

"At 120 km/h or more, 50 per cent of fuel goes to combat drag," he says. "So, at high speeds, reducing the drag even slightly can have a huge impact on fuel economy."

But a spoiler only works if it's cutting through the air at the correct angle, Agelin-Chaab adds.

"Factory-installed spoilers on higher-end sports cars are very effective. Many others out there probably aren't so effective – and some car manufacturers even say their spoilers are for looks only."

Properly installing a spoiler requires at least three hours in a wind tunnel (UOIT's costs $700 an hour), Agelin-Chaab says. If you're installing one yourself, you can measure the drag on your vehicle doing a coastdown test.

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While a spoiler might not help your car's handling or fuel economy, it can't hurt, right? Sorry to be a spoilsport, but a badly installed spoiler could mess with the airflow around your car. "Since it was not part of the aerodynamic design of the car, a spoiler may or may not interact correctly with the flow around the vehicle to improve things," Miller says. "It can even make things worse."

A badly installed spoiler can give the rear wheels too much grip, causing understeer.

"It can make the car more reluctant to turn as the straight rear wheels tend to overrule the turned, less gripping, front ones," he says.

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