If you caught our discussion on load transfer you’ll remember that downward pressure on a tyre helps it grip the road a bit better, and when a vehicle is moving swiftly enough, aerodynamic downforce can be generated to help push the tyres down.
The first thing we need to keep in mind with aerodynamics is that it’s not actually the air that’s flowing past the vehicle. In calm conditions at least, it is the vehicle carving its way through the air. That’s important to remember, because wind tunnel tests do it the other way around, which can make us think about it the wrong way around.
The second thing of importance is that the majority of downforce is actually occurring underneath the vehicle or its aero device. Just as the very low pressure above an aeroplane’s wings pulls it up into the sky, a low pressure below a vehicle or any of its aero bits will pull it down (it’s the tyres that experience a downwards push after it has been transferred to them through the suspension).
So, when looking at any flow data, pay more attention to the underside because that’s where the magic happens. Meanwhile, the flow on the top should be looked at mostly in the context of its disruption to the airflow (something we do and don’t want).
In terms of how downforce is generated, there are loads of different devices a vehicle could have depending on various factors, especially the motorsport regulations under which it competes.
The basic principle for all of them is getting the air underneath to escape behind the vehicle or device faster than the air on top. Or maybe we should word that, having the air on top reach the rear of the device or vehicle more slowly than that which is underneath.
Overall though, the whole vehicle should initially be thought of as a wing. Having just punched a hole in the air, behind a vehicle is a low pressure pulling the air back in towards that region. So, if the flow above the vehicle is hindered, and the flow underneath is encouraged to move up, you get downforce.
Therefore, the aforementioned disruption on top can be thought of as beneficial to hinder the flow (slowing it in wind tunnel terms, so actually pushing it forwards), lowering that low pressure region immediately behind, and thereby pulling more air up from underneath. That’s also the explanation as to why a Gurney flap (a near-vertical lip at the top rear edge of a car’s wing) works. It hinders the flow on the upper side (also at the expense of more drag), and simultaneously encourages the flow on the underside to fill the low pressure area instead, enhancing the wing’s downforce.
Tintops however, do have the disadvantage of rear windows. These not only create drag and interfere with airflow for any rear wing, the low pressure behind the window is angled upwards, creating some lift.
Anyway, designers of ground-effects race cars of the ’70s and ’80s thought of the whole vehicle as a wing, and even today there are categories that allow devices and designs which achieve a similar effect.
The main such device is a diffuser, which is basically an upward-channeled rear undertray. You may also notice some race cars are more obviously raked up at the back.
The interesting thing about race car wings, diffusers, ground effects, or any other type of upwardly-ramped tray or channel, is the centre of pressure (the actual downforce) is usually found in the region where the upwards trajectory starts. The first ground effect cars discovered this when the centre of pressure was way too far forwards, so far so that many ditched the front wings entirely or even angled them to create a bit of lift just to improve the aero balance front-to-rear.
There are many other devices like skirts, barge boards, cannards (dive planes) and more that add to this overall effect, but we can discuss those later.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.