HotCars explains: the difference between fenders and spoilers and how they work | Taza Khabre

Key findings

  • Wings and spoilers work in different ways to reduce lift and create downforce. Spoilers slow down and redirect airflow, while wings need smooth airflow to work effectively.
  • Spoilers can be of any shape and only need to slow the airflow, while wings must be carefully designed, positioned and angled for optimal performance.
  • Spoilers are more commonly used by manufacturers on road cars because their shape does not matter much and they can reduce both lift and drag. Wings can provide more downforce, but at the expense of increased drag.


I pulled up at someone’s house the other day and the owner looked at my car and said, “Hey, nice rear spoiler!”

“No,” I said, “it’s a wing.”

“Oh, whatever,” came the reply.

Although wings and spoilers are often confused with each other, they are different types of aerodynamic elements. wings these are devices of a special form which require a smooth flow of air from both sides before they will work well. On the other hand, spoilers deflect and slow down the air, and as long as they do, their shape doesn’t matter much. Both wings and spoilers are used to reduce lift (or provide downforce), but spoilers can also perform another function – they can reduce aerodynamic drag.

So which is best for your car?

The author of three books on automotive aerodynamics, Julian Edgar has spent more than 25 years engaged in aerodynamic testing and modification of automobiles.


Wings create different pressure above and below

Julian Edgar / HotCars / Valnet

First, rear fenders on stock cars—even high-performance ones—are rare indeed. (The Plymouth Superbird pictured above is one of those rare exceptions. Yes, it has a real wing.) But what most people call wings are actually spoilers – so what are wing?

2023 Ford GT MKIV rear wing
via Speed ​​Phenom (YT)

A wing is an aerodynamic shape that has faster airflow on its lower surface than on its upper surface. The faster the flow, the lower the air pressure, so the wing creates low pressure on its underside. This pulls the wing down – downforce. High pressure is created on the upper surface. In turn, this pushes the wing down—again downforce. Thus the two surfaces combine in their action to provide a downward thrust.

In addition to the special shape, two other things are needed for the wing to work.

The wing must be in fresh air

wing should be in the fresh air— that is, air that is not turbulent. (Turbulent air has many random directions and velocities. A good example is the turbulent air directly behind the car in what is called a “wake.”) If the wing is located in turbulent air, the smooth flows over the upper and lower surfaces of the wing necessary to his work

The wing should not stop

Details of car wings
Julian Edgar / HotCars / Valnet

Secondly, the wing should not skid. As the wing’s angle of attack to the airflow increases (that is, the rear of the wing gets higher and higher than the front), the downforce will increase until the wing reaches a critical angle called the stall angle. At this point, the airflow no longer follows it wing shape—especially the shape of the lower surface. This is bad, because the downforce drops sharply and the resistance increases greatly. Note that the angle of attack is measured relative to the actual direction of the air flow, No compared to horizontal (i.e. parallel to the ground).

I see a lot of ineffective fenders on cars either because they are not in the open air or (more often) because their angle of attack to actual airflow direction so cool that they’ll probably stop.

Rear fender on Honda Insight Gen 1
Julian Edgar / HotCars / Valnet

For example, a wing on a car with a sloping back works on air that moves down at the same angle as the rear of the body. Sometimes, as in this photo, this means that the wing actually has to be angled so that the front of it is higher than the back to get the right angle to the airflow.

The rear spoilers are slow and redirect the airflow

The rear spoiler works very differently than the wing. The spoiler slows down and redirects the air flow. A key part of this is slowing down the airflow on the car’s body in front of the spoiler. Since the speed of the air flow over the car body is directly related to the pressure (push or pull on the panels) that creates the air, slowing down the flow increases the pressure.

Porsche 981 rear wing up
Julian Edgar / HotCars / Valnet

Imagine a car with a sloping rear window (like this 981 Porsche Cayman) or a car with a sloping trunk lid. If the airflow velocity over the roof, rear window and trunk lid is high (as it is), the air will create low pressure trying to lift the car. If we can slow that air down by placing a spoiler in its path, the velocity of the airflow in front of the spoiler will be reduced and therefore the pressure will be higher. This means less lift (or even downforce). So not only is the air pushing down on the spoiler itself (due to the airflow being deflected upwards), but more importantly, it’s also increasing the downward pressure on the panel in front of it. In fact, it’s easy to measure the pressure change of a spoiler 3 feet in front of it!

What a spoiler needs to work

Nissan Z rear spoiler
Nissan

For the spoiler to work, it should not have a special shape. Unlike a wing (where shape is critical), a spoiler can be flat, curved, or whatever, as long as it sticks out into the airflow and slows the air ahead. However, like a wing, a spoiler should not be in a turbulent air flow: the air in front of it should be “attached” to the body, that is, flow along the body panels.

Manufacturers mainly use spoilers

Aston Martin V12 Zagato
Aston Martin

On road cars, almost all manufacturers use spoilers rather than wings. (And if they use what looks a bit like a wing, it often stalls because its angle of attack is so high. So then it acts as a spoiler…like the Aston Martin above.) Because their shape isn’t what matters spoilers are much easier to work with and for stock cars it allows stylists to do whatever they want and still get a decent aerodynamic result. For modders, this means you can choose any spoiler shape that suits your style.

Everrati 964 Rear three quarters
Michael Theo Van Runkle / HotCars / Valnet

Reduction of lift and drag

Another reason manufacturers prefer spoilers is that they can reduce both lifts and to drag If pressure builds up on the panel in front of the spoiler and if that body leans down toward the rear, some of that pressure build-up will help push the car forward as well as down. This, in turn, reduces aerodynamic drag. However, it’s a balancing act because too big a spoiler will increase the size of the wake (disturbed air behind the car) and therefore total drag can increase.

Tanner Foust VW Jetta Drift Taxi
David Trevor Adolph / HotCars / Valnet

Wings can create more downforce than a spoiler, but always at the expense of drag. However, a really well-designed wing will provide very little drag for its downforce. But often aftermarket fenders go a bit crazy in shape, chasing downforce (or just style) at the expense of high drag. For example, it is not difficult to get a wing that has twice the downforce of a less radical one, but at the expense of eight times more drag.

Remember that F1 cars and the like have huge aerodynamic drag, so don’t look at these wing shapes if you’re considering a wing for your road car. (For amateur racing, where downforce is usually more important than drag, be as radical as you like.)

The main differences between wings and spoilers

  • Wings and spoilers work differently, reducing lift or creating downforce.
  • If spoilers are placed in the attached airflow, they can reduce rear lift (and sometimes drag).
  • The shape of the spoiler is not critical.
  • Wings need to be very carefully designed, positioned and angled to get the best results.
  • Wings can provide more downforce than spoilers, but always at the expense of drag.

The location of the wings can cause other effects

Rear view of the 2024 Ferrari SF90 XX Spider.
Ferrari

Another thing to consider about fenders is their placement. I already said that they should be in the fresh air, but there is something else to consider. I said the pressure on the bottom of the wing is low and that is normal. But what if the wing is not too high above the car body? In this case, the low pressure on the underside of the wing will contaminate the pressure on the car panel below, i.e. lower that pressure as well. So the wing pushes down, but the body (with its lower pressure) pulls up! The result is that the wing, when measured on a real car and not just in space, is less efficient than it should be.

But what if the wing is installed slightly behind the car? In this case, the low pressure caused by the wing cannot contaminate the top surfaces of the car, but it can reduce the pressure behind the car. This will increase drag (badly) but also cause better airflow under the car and if you have the right downforce under the car it can be very beneficial for downforce. In short, the performance of the wing depends on the car and how it is installed.

Starting to get a little complicated? The good news is that fenders and spoilers are very easy to check, which I’ll cover in detail in the next article (or, if you can’t wait, see my main book).

A wing or spoiler can be much more than just an appearance

2015 Ford Mustang Mach 1 spoiler-1
David Trevor Adolph / HotCars / Valnet

Finally, should you install a wing or a spoiler? for aerodynamics, and not only for appearance? The answer is yes, but with a major caveat: If you place a wing or spoiler in a turbulent airflow, or if you have something that looks like a wing but doesn’t have a true airfoil, or you use a proper wing. but it stopped, you won’t get many positive results. But if your wing or spoiler is selected and configured correctly, or better yet, combined with some front end aero modifications, you’ll get results that you can easily feel on the road at legal speeds. These results include improved straight-line stability and better cornering grip. It’s all for you, but only if you do it right.

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