Spoilers: Good for go or just for show?

On ordinary cars in ordinary driving conditions, spoilers are about as useful as a bumper sticker saying "My car is powerful" or "I drive fast".

Photo credit: Shutterstock

What do “spoilers” at the back end of cars do? They seem to be universal in all forms of motorsport and, if they bring significant benefits, why don’t all cars have them?  Tam.

In a nutshell, spoilers help racing cars go round corners faster. Nothing else of any benefit, and… they are effective only at very high speeds and are designed and placed in precise accordance with much other high-tech aerodynamics, scientifically tailored to each specific vehicle.

For readers comfortable with basic scientific principles, they deflect the air passing over the top of the vehicle (which at 150 km/h is a gale!) to induce a downforce over the rear wheels.  Downforce acts like a heavy weight, which makes the tyres grip the road surface more firmly (but does not change the actual weight the engine has to haul).

A spoiler does, however, create wind resistance (which does sap power) on an otherwise streamlined body. So spoilers are a trade-off between the advantages they bring and the drawbacks they cause.

On ordinary cars in ordinary motoring circumstances, they do about as much good as a bumper sticker saying “My car is powerful” or “I drive fast”.

They are symbolic image cosmetics.In less pejorative terms, they are “aesthetic”. Other than that all they do is add cost, add weight, cause extra wind resistance/drag, reduce top speed, and increase fuel consumption. Slightly.

In Formula One they make a huge difference – any car that entered without one would come last, by a record margin. If they tried to keep up with their spoiler rivals on fast bends they would spin off into the crash barriers every time.

But then, F1 cars are super light, super powerful, and race for about 200 km involving more than 2,000 corners at an average (!) speed of 150 km/h or more, peaking down long straights at more than 300 km/h.

Their spoilers are as precisely tailored to those needs as the latest technology can devise (at any price) and in close and interactive conjunction with the vehicle’s overall aerodynamic design including splitters and ducts and ground-effect floorpan designs elsewhere on the car. 

Such precision is so crucial that their spoilers are adjustable, so their already “perfect” settings can be fine-tuned for the different corner patterns of different circuits (the balance between sweeping bends and tighter turns) and even different weather patterns and track temperatures!

The basic science behind all this is that any moving vehicle is punching a hole through the air, which rushes underneath, down the sides and over the top of the vehicle at the speed the vehicle is travelling at.

The wind force is the same as something standing still…in a headwind of that speed. 20 km/h is a breeze. 100 km/h is a storm, which a human would have difficulty standing up in. 200 km/h is a tornado, strong enough to rip the roof off a house or knock down a tree.

The point is that although it is only “air”, at high speed it exerts a huge force, which is why swift cars have sleek shapes to help the air slip over them with minimum resistance.  And the faster they go the sleeker they have to be. 

If you add a spoiler (a broad flat slab across the direction of travel) it will obstruct the force of passing air.  That’s why it is called a spoiler!

Of course, the principles apply to ordinary cars, but the forces (good or bad) increase geometrically with speed. And at highway speeds the only good thing about spoilers (downforce) is not enough to make any difference. Nor is it needed. 

On ordinary cars in ordinary driving conditions, spoilers are about as useful as a bumper sticker saying "My car is powerful" or "I drive fast".

Photo credit: Shutterstock

The penalties of wind resistance/noise/power reduction, and higher fuel consumption are equally modest, but why pay them at all?

Some high-performance road-going cars (Porches, Ferraris and the like) are designed in the factory with spoilers on their boot lids, but they are shaped and placed in a way that creates little resistance and little downforce – primarily to make the car more stable at very high speeds…  in a straight line.  

They don’t have F1’s extreme ground-effect floors, so the flow of air under mass-production sports cars tends to lift the vehicle upwards rather than suck it down.

After all, the quickest can get close to the take-off speed for an aircraft! When a plane reaches that speed on a runway, if it angles its wing flaps up (like a spoiler) it will stay on the ground. If it puts the flaps down it will harness the same wind force for lift instead of downforce, and take off.

Sportscar boot lid spoilers reduce so-called “drag”. That is a principle that will be very familiar to sailors. Simplistically, when the wind is behind them, they hang the sail out sideways as far as possible so the wind force “pushes” them.

But the wind captured for that creates a low-pressure area on the other side of the sail cloth, and that low-pressure area “sucks” them forward, too.

Ditto with cars that punch a hole in the air. They face and create high-pressure areas in front of them and over their bodies, but immediately behind them, that creates a low pressure zone, which “sucks” the back of the car. The car wins, but is using some of its power to fight against that “drag”.

In racing this low-pressure area is known as a “slipstream” which following cars can use to gain speed. At close quarters, the air they are punching a hole in just got thinner (less wind resistance) and the suction is also pulling them forward. So they now have the power to go faster and overtake the car that is doing more wind-moving work.

F1 well illustrates this process with its DRS (Drag Reduction System). The cars can tilt/open their spoilers like wing flaps horizontally, greatly reducing their wind resistance.

Using this and the slipstream of the car ahead, on long straights they can go between 10 and 20 km/h faster than the car ahead of them to catch up and possibly overtake before the next corner… which demands (by the laws of physics, the logic of competitive strategy and the rules of the FIA) they have to flip their spoilers back to full downforce.

That rule is so strict that the drivers don’t have to obey it. The car does it, using automatic computer controls.

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