Why Do Cars Have Spoilers?

I’ve noticed that many passenger cars these days are equipped with ‘spoilers’ on their deck lids. But they don’t look like the kind you would see on actual racecars. They are much smaller, and they appear to be flat instead of inclined. I don’t see how a spoiler that is not on an incline could serve as an aerodynamic aid. Do these spoilers actually do anything to improve traction, or are they just there for looks?

And if they did improve traction, wouldn’t they also reduce the gas mileage?


On cars like that, they’re there for looks.
The purpose of spoilers is to make the car stick to the ground. I read this somewhere, but I think once a car goes 200 mph (with both front and back spoilers), there is enough downforce on the car that it could drive upside down (on the roof of a tunnel). The spoilers on cars work like the reverse of airplane wings. Wings provide lift, spoilers provide downforce.

Something that always makes me laugh is seeing those large spoilers on little Hondas. At high speeds spoilers keep the car on the gound, but at normal highway speeds and racing off the line, they do nothing.

As far as gas mileage goes, I doubt there is a change, but it’s possible

They’re there to “spoil” the looks, hence the name. :slight_smile:

Seriously, they are almost always put there for looks. A car at speed will tend to have lift reducing the weight on the rear wheels. As you noted, race cars use spoilers to increase the downforce on the car giving better adhesion in high speed turns.

In passenger cars, however, the spoilers are designed for looks and usaully end up actually adding to the lift – opposite the effect that is desired. And whether properly desinged for downforce or for cosmetic purposes they usually reduce gas milage as you suspect. There may be a few cars that benefit either aerodynamically for gas mileage or downforce out there, but they are few and far between.

One thing I remember about spoilers is this:
My friend had an Audi TT, it was recalled and Audi gave the customer the option of A) getting a rear spoiler or B) upgrading the suspension. Evidently the TT was losing contact with the road far too frequently, so in sports cars they’re needed, but evidently can be replaced by a better suspension.

For most passenger cars, they’re for looks. Saturn used to say as much in their brochures – “Buy the spoiler; it doesn’t do anything, but it looks cook.”

Also - since most cars are front-wheel drive, surely the spoiler pushes down the rear of the car and thereby causes the front (driving) wheels to lift off the road?! Most race cars are rear-wheel drive I believe so having a spoiler on those makes sense - you want the driving wheels to remain in firm contact with the road/track.

Spoilers serve two distinct purposes on cars.

One is to provide downforce for the driving wheels and to improve high-speed handling. These spoilers are generally shaped as reverse air-foils, made of carbon fibre, and have a significant surface area. Formula 1 (F1) cars have this type of spoiler, front, rear, and (in 2000) mid (x-wings).

Generally, these spoilers produce extreme amounts of downforce. As an example, a Williams FW17 F1 car generates more than 2000lbs of downforce at 260kph. This, being that the car weighs less than 1400lbs, would render it capable of driving on a ceiling. This is however, not the intended use.

Generally, it is assumed that in high speed cornering (above 180kph), the downforce is sufficient to allow F1 cars to corner at 2.5G, as opposed to approximately 1.5G for an unfaired/unspoilered vehicle.

The coefficient of drag of these cars is subsequently greatly increased. In fact, simply lifting off the gas in an F1 car at speed will produce braking force in excess of max braking of a street car, due solely to the downforce.

On a street car, spoilers are often added by the manufacturer to decrease the coefficient of drag. How is this possible?

The coefficient of drag of a car is based on the frontal area, but also on the shape of the car. A car with a higher tail has a lower Cd than a car with a low tail. For instance, a Ford Focus station wagon has a lower Cd than the Focus sedan. A small lip spoiler, that does not produce much downforce at all, but effectively raises the rear of the car, thus lowering the Cd. This will also have a positive effect on fuel economy.

Most street racers who have big wings on their cars are misguided. A properly engineered wing will produce positive downforce at freeway speeds, but most of the big wings you see on Civics are simply styled, not engineered for downforce, and have a negative effect on both handling and Cd.

An example of functional spoilers for fuel economy are the lip on the Honda Insight. An example of a functional spoiler for downforce on a production car is the spoiler on the 911 GT2.

Hope this helps.

  • Bjorn240

Not forgetting these guys…

(I believe this page was even a link in Weird Earls)

I know that with the New Volkswagen Beetles they tend to have Spoilers to help keep them on the ground at 90 MPH+ so as to help keep them on the ground, also seeing as how vehicles that are front wheel drive (such as the New beetles) have the engine over the drive train you don’t have as much of a porblem with lifet on the front as you do on the rear since the weight of the engine is pressing down on the front wheels.

I forget which Porsche possessed this but one of their models had an active rear spoiler. That is, when standing still, the spoiler was completely retratced into the body of the car. As the car increased speed a computer would decided how to best deploy the spoiler (both how high off the car and the angle of attack of the wing). Theoretically this provided optimum aerodynamics at whatever speed (don’t know if it really did…might have just been a gimmick).

I just bought an Infiniti G35 with a rear spoiler. The only reason I have it is because the model I wanted with other miscellaneous options forced it on me. Still, I like the looks and not least because the third brake light is incorporated into the spoiler instead of behing the rear windscreen. Reading the Infiniti literature their spoiler does in fact produced downforce although looking at it you wouldn’t think so. Nevertheless the difference between un-spoilered car and the spoilered car was minimal (.27 vs. .26 or some such).

As for front lift some cars incorporate an air-dam on the front under the bumper. IIRC this helps prevent air from going under the car thus also reducing lift.

FWIW I have felt cars start to lift under me at around 85-90 mph. I could feel the steering getting spongy and the wheels would skitter a bit if they hit bumps in the road. As said, however, for 99.9% of city driving the spoiler will do little to nothing.

The spoiler on my Sentra is almost certainly for looks. Usually, I don’t like superfluous spoilers, but I like mine…it is actually designed well in the spirit of the car. However, my car is not a sports car (it has good pickup for a compact, but still)…I doubt the spoiler does much of anything as far as lift or downforce is concerned.


I asked basically the same question in this thread almost exactly two years ago. Some good information there, although the discussion spirals off without concrete resolution when people start bandying about anecdotes about their own high-powered muscle cars. :wink:

My ride is no Formula racer - my spoiler is strictly for looks. I can accept that. It’s tasteful, as most factory spoilers are, with a design that accents the rear of the car. It doesn’t significantly impede my sightline out the rear window, and it gets the third brakelight out of the way. I think this holds true for most if not all passenger cars, the only exceptions being high-performance sports cars. It all boils down to personal taste - I’ve seen my car without the factory spoiler, and the rear end looks weird. OTOH, I’ve seen cars on which the factory spoiler looks absolutely horrid.

And of course, nearly all aftermarket wings, unless they are made to mimic the (typically subdued and tasteful) factory spoiler, look utterly stupid.

Audi not also had to equip the TT with spoilers but also the A2 to increase highspeed stability because both cars had big problems with this. Both was in the same year, so people had good laugh about Audi’s engineers that time. In case of the A2 it really spoiled the looks badly.

As Bjorn240 says, the rear deck spoiler can have a large effect on the coefficient of drag. I have seen a number of flow visualizations that make this clear, but can’t lay my links on any right now. Basically, the spoiler serves as a dam which prevents the wake behind the car from feeding forward and separating the flow on the rear window. By allowing the flow to remain attached until it reaches the spoiler, the size of the wake is dramatically reduced and the drag is consequently smaller.

As an aside about the active spoiler Whack-a-Mole mentions, wasn’t that spoiler deployed as a cooling element? If I recall correctly the 911(?) had its radiator in the rear spoiler, and it extended as needed to affect cooling. Given the shape of the 911 (especially top-down convertibles) I suspect the downforce affect was minimal.

One other thing spoilers do is direct airflow. The air coming off the tail of a square backed car is a tangled mess of vacuums and vortices which, for vans, station wagons and SUV’s, can result in a lot of nasty stuff being slammed back towards the car. This “stuff” can be dust which makes the rear window dirty or exhaust fumes which make the driver so so sleepy. Cite.

According to this site ( Porsche 911 Evolution ) it seems the 911 wing was indeed a spoiler. (Bolding mine)

As noted by Whack-a-Mole the Porsche spoiler actually does something. I don’t know about the 996-911, which has a liquid-cooled engine, but the 993-911s (which featured the retractable spoilers) and earlier were air-cooled. That is, no radiator. However the rear decks and, if installed, spoilers had a grille to allow cooling air into the engine bay. The 911 Turbo model had, IIRC a turbocharger that stuck out of the engine compartment and the spoiler covered the part that stuck out (the turbo’s air filter?). If you look at a Turbo spoiler, you will see that the sides are more or less vertical – making a “box”. Compare that to the Carrera’s optional tail. They did not have the “box” underneath. (Although they did of course have an air passage inside to the deck opening for cooling.)

FWIW, my 911SC handled quite well at over 100 mph without a rear spoiler.

I believe that in many circles, spoilers and wings are not the same thing.

Wings are actual airfoils, which provide downforce (and, inevitably, lift-induced-drag as well). Open-wheel racers have them front and rear.

Spoilers don’t need an airfoil-like shape to be effective. I believe micco explained how they work fairly well - particularly the part about drag reduction.

They are similar in appearance, but they perform different functions. Sam Stone speaks quite eloquently about this in the thread that Cervaise linked.

A friend of mine has a porshe with the active spoiler. It deploys(ed) at 50mph.
He, and many other owners, installed a kill. They don’t like having a beacon saying “I’m going 50” I guess. :wink: