Diffusers: how do they work?

Diffusers, you may have heard this word in the world of motorsport and aerodynamics. But what actually are they and how do they work? To start off, the obvious first question is: what is a diffuser? Well, essentially a diffuser is a device on the back of a car made with the intention of almost sucking the car to the ground. This is effectively increases downforce. Now, why would we want to do this? Well, on principle, a car is able to convert it’s chemical energy to mechanical energy in the engine, however, this only turns the wheels. Now, for the wheels to convert this rotational energy to actual motion, it needs to have enough friction to move the car.

F=\mu_{d} \times N 
\mu_{d}: \text{Coefficient of dynamic friction} \\
F:\text{Resultant force} \\
N:\text{Resistive Force}

We want that resistive force to be as much as possible. How can we do this? Increase the force it is resisting against i.e. that is to create a greater downforce. Now how will this actually help our wheels? Well it’ll push the rubber into the gaps in the asphalt creating more mechanical grip, the more effective this seal the better the transfer of energy from the drive chain to the ground.

Now to answer what a diffuser actually is. It’s a curved area at the bottom end of a car that rapidly accelerates air to effectively make the entire vehicle a wing. We can see this below (with my crudely drawn attempt at an F1 car diagram):

What a diffuser is

As you can see, the air goes from an area of high compression to an area of low compression (air close together under the car and very spread out behind the car). What this effectively does is create a pressure differential between the bottom and the top of the car. What this means is that the entire car acts lack a wing – or in more official language: an airfoil. To illustrate this, I’ve made another diagram below:

In my defense, I was really tired when making these

What we see is a low pressure zone above the car as the air is moving at slow speed. However, the air underneath the car has been compressed, resulting in an increase in air speed. Due to the Venturi Effect .The Venturi Effect can more easily be understood through Bernoulli’s equation:

Bernoulli’s Principle and the Vn

What we see here is that as a fluid is compressed, the velocity increases and the pressure decreases. While this may seem contrary to what one may believe (would result in greater pressure) in practice that intuition is completely false. Therefore, if we go back to the context of compressing air underneath our diffuser, we have a very low pressure area undeath the car. In a system, an area of high pressure always tries to move to an area of low pressure. This means that our force is created downward as the system tries to make itself in equilibrium.

Therefore, we have created a lot of downforce without adding any weight or overcomplicating the aerodynamics (which would add drag or make the car more fragile). As for how an airfoil works, it is a bit beyond the scope of this article – hopefully I’ll post a new one explaining that in the future.

Does Physics and Computer Science stuff


Aditya Rao


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