A rear wing is not a spoiler

The aero device most of WRC cars have above the boot lid, over the rear window or at the rear of the roof is often named as a spoiler or rear wing indistinctly. This is a common mistake many people (including ourselves) make, as spoilers and rear wings are different in shape and they have different purposes. In this post, we will try to explain what is the role of each one and we will give some clues of how to name them correctly.

A spoiler is something that spoils. Spoiling is, according to the dictionary, to damage something so that it loses part of its value, beauty or usefulness. When thinking about air, a spoiler is used to damage (to disturb) the airflow.

H.Mikkola/A.Hertz, Ford Escort RS1800 MKII, Rally Portugal 1980, ret. – author unknown

The air coming from the car roof naturally flows over the boot lid (as with an old Ford Escort), the rear window (as with a hatchback such as the Vauxhall Chevette) or the rear of the roof (as with the Deltona). If the air does not find any barrier, it can flow faster and (by Bernoulli) the pressure there will be low, thus lift will be generated (by pressure difference with the undercar). Things change if we add a barrier (a spoiler), as airspeed is reduced, causing the pressure to increase, thus lift is reduced over the car’s rear.

A very well designed spoiler can reduce the lift to zero or even to negative values (downforce). However, the addition of a spoiler can have also downsides, as it can potentially increase drag. It all depends on the skill of the engineer who designs it.

J.Kankkunen/J.Piironen, Lancia Delta HF Integrale, Sanremo Rally 1992, 2nd – picture by fotosport

It is easy to detect a spoiler at first sight: it is usually a plane set at a certain angle at the boot lid/rear window/roof, attached to the car with no space in-between, which forces all air to flow above the spoiler. They were very common in the 70s and 80s, when many iconic rally cars included them, with different sizes: Porsche 911 Carrera RS, Ford Escort, Fiat 131 Abarth, Lancia Stratos or Lancia 037 Rally, to name only a few.

H.Toivonen/J.Piironen, Lancia 037 Rally, Rally of the 1000 lakes 1984, 3rd – picture from Mondiale Rally

In conclusion, adding a well-designed spoiler to a car means reducing lift at the rear, with minimal or zero drag increase.

A wing is the part of a plane designed to generate lift when the plane reaches high velocity. In WRC and other racecars, what we name as a wing should be really named as an inverted wing or inverted airfoil, as this is how wings are used in racecars: in an inverted position to generate negative lift (downforce).

D.Sordo/C.Del Barrio, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 3rd

In a rear wing the profile and the shape is the same as in the airfoil of a plane, but in an inverted position. It is difficult to distinguish it in current rear wings due to the presence of endplates. The picture below shows an approximation (dotted line) of the inverted wing profile in the 2019 rear wing of the Hyundai i20 Coupé WRC.

D.Sordo/C.Del Barrio, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 3rd

For the wing to be effective, the air has to flow over and under the (inverted) wing with different velocity, to generate (negative) lift. This is a distinctive characteristic of the rear wings over the spoilers, as spoilers force all air to flow over them, and no intermediate space is available. Also, this makes the rear wing much more effective in terms of downforce generation, as pressure difference is generated between both sides of the wing, allowing the designer to better maximize this difference by choosing the most appropriate wing profile for each case. Italian engineer and journalist  Enrico Benzing compiled the wing profiles most widely used in racecars in his book Ali/Wings.

S.Loeb/D.Elena, Hyundai i20 Coupé WRC, Rally Catalunya 2019, 4th (also Torstein Eriksen behind)

The size of the rear wing currently used in WRC cars is so big that the total front area of the car significantly increases with the addition of a rear wing. This means that the drag increases, as the drag force is also dependent on the frontal area of the car. The big deal for engineers is to reach an optimal balance between drag and downforce (which is known as lift/drag ratio).

In conclusion, adding an inverted wing to a car means (more effectively) generating downforce at the rear, but at the cost of increased drag.

Then, if it’s so easy to tell them apart, why do we constantly mistake them? To facilitate the task, here are some simple rules we propose, in order to put an end to this common mistake:

  • a spoiler is a barrier forcing air to flow above it, with no space separation with respect to the car, and
  • a rear wing is an inverted airfoil-shape element, separated from the car’s surface to allow air to flow above and below the wing.

Also, the rear wing can include a lower plate, that is, an element located under the upper wing, which can have the shape of an inverted wing, but with no separation space from the car’s body, thus preventing air from flowing under it. Our proposal is to name it as a plate, although Citroën named it ‘shovel’, as in the C3 WRC it had the shape of a shovel.

This is our contribution to reducing the confusion between these two elements. We hope you find it useful.

About Post Author

6 thoughts on “A rear wing is not a spoiler

  • 2020-05-03 at 10:08

    Muchas gracias LLuis
    para los aficionados a los rallies, con tus comentarios, es más fácil entender el porque de sus formas y ubicaciones.

    • 2020-05-03 at 10:14

      Me alegro que te guste. Y muy contento de tener lectores aficionados a los rallies y además grandes amigos!

  • 2020-05-07 at 02:55

    Thank you for another great article in this hard times. I’m curious about several things:
    1. Are you planning on grouping this article together with the ones you did on front spoiler and diffuser? If so, then will you move on to car internals next (suspension, engine, ECU, etc.)?
    2. What car types work best with spoilers, and vice versa with rear wings? Or is advancement in technology meant engineers can just design whatever they want these days? I think rear wings hold more performance potential; do you agree with this?
    3. Do you think current ‘performance’ versions of road cars with their aero body kits can be competitive in rallying (given enough engineering to make them durable enough)? If so, then which current car model impresses you the most?

    A always, thanks for your hard work!

    • 2020-05-13 at 15:01

      Thanks to you for your kind words. No plan but it is a series of articles devoted to technique. Next will be suspension. Engine and ECU are hardly related to aero, I can hardly give useful info on them.
      You’re right, rear wings usually allow higher performance improvement…but each car is different. Same with road cars, each case is different. If I had to choose, my heart says Subaru WRX, my head the new Yaris GR (at least until we can see the new Hyundai)?
      You’re welcome, glad you like it!

    • 2021-01-02 at 20:31

      Regarding item 2 – On sedan cars (aerodynamic classification: notchback vehicle) a spoiler is much more effective than a wing because the airflow behind the backlight is mostly detached and with lower local velocities. A spoiler will make the static pressure on the trunk to increase. This new pressure distribution on the body usually reduces the drag and rear lift.
      A rear wing will also create a similar redistribution of pressures, but less intense.
      You need to install the rear wing at a good height over the trunk on a notchback.
      Modern sedans that look like a coupé, such as the Mercedes´ and Audi´s, have a more streamlined back.
      A wing with an appropriate airfoil for the Reynolds number of operation, give the most efficient results if it is not working inside a wake.

      • 2021-01-03 at 10:58

        Thank you Alejandro, very good answer. I can only add that the solution can be different depending on the type of car, a road car or a racecar, as the goals to achieve are different.


Leave a Reply

Your email address will not be published. Required fields are marked *