Car aerodynamics represent an area of vehicle design that not many drivers understand, yet are impacted by it on a regular basis. How your car handles air resistance - also known as drag - determines how much power is required to push through, limiting how much energy goes directly into actual movement.
As such, just like car tyres with low rolling resistance, the aerodynamics of your vehicle have a direct impact on your fuel efficiency.
Air resistance basics
So how does air resistance work? As a car moves, it collides with the air particles in front of it. However, the faster an object moves, the more particles it has to deal with at once, making further movement more difficult.
As particles hit the front of the car, pressure builds up, making it more difficult to accelerate. These particles, however, try to get out of this high pressure zone, typically by moving over, under or around the sides of the car. How this occurs is influenced by the design of the car body.
Air resistance and car designs
When cars were first made, no one focused on aerodynamics. This was in part due to a limited understanding at the time, combined with a limit in technology. However, the average early car speed was low (the national speed limit in the UK didn’t reach 20 mph until 1903), so it wasn’t as important as it is today.
Today, cars on the road travel up to 70 mph. To keep fuel efficiency as strong as possible, cars adopt a sleeker shape. During the design process, wind tunnels are used to determine how and where the air flows, eliminating spots where pressure might build up.
Cars can be designed in many ways to reduce air resistance.
Race car aerodynamics follow totally different rules. Of course, for super cars, hyper cars and motorsport vehicles, these designs can focus purely on aerodynamics, rather than other practical features. This is why these cars sit so low to the ground. It is better to send air particles around and over the vehicle, rather than underneath where it can push the vehicle up and interfere with its ability to stick to the road.
This also gives them a smaller frontal area, reducing the amount of resistance generated.
Lift and downforce
Two key forces that occur are lift and downforce. Lift occurs when there is light pressure passing over the vehicle - typically in smooth areas, such as the roof or bonnet. Here, the reduced particles actively ‘lift’ the vehicle up, rather than pushing it down. It is this force that is used to help airplanes take flight, but it is not so welcome in vehicles.
However, in areas where there is high pressure, such as a windscreen with a sharp incline, there is downforce. This is the opposite to lift and represents pressure and airflow pushing the car down, rather than up.
Furthermore, the underside of the vehicle can also generate lift or downforce. Many manufacturers, especially in sports cars, design cars to be lower at the front (whether through chassis design or different wheel and tyre sizes). This way, a small amount of air enters into a larger space (due to a higher rear), creating low pressure and less lift as a result. Designers can use this pressure, combined with forces throughout the car, to generate downforce.
How do car designers work with aerodynamics?
Naturally, car designers are aware of aerodynamics and try to improve the ratio of uplift and downforce to their benefit. Most often, this favours downforce, as this keeps the car down on the road, making it easier to turn and handle corners. Fortunately, there are many ways a vehicle can be designed - or later modified - with this in mind.
There are many ways to design cars with air resistance in mind.
Splitters and skirting
Car splitters are usually aftermarket parts which extend the bumper down close to the ground. This reduces the air that enters underneath, decreasing any potential lift, creating a net gain in downforce. Of course, these features aren’t practical for street cars - just try driving such a vehicle over a speed bump!
Side skirts do the same thing on the side of the vehicle. When turning, the air flow comes at a different angle and side skirts ensure air doesn’t swoop in from the sides.
Dive planes are also designed to generate extra downforce, this time through channels at the front of the vehicle (usually close to the sides) designed to push air up, rather than down. Again, this limits airflow underneath the car and improves the downforce above.
Vents and air intakes
You may have noticed that many luxury or high-end vehicles feature additional vents, grills and other air intakes. By making the front of the vehicle semi-permeable, rather than a solid wall, the high stagnation of pressure at the start can be reduced. Of course, this air has to go somewhere, typically through the engine bay and radiator.
Vents and intakes can also improve car aerodynamics.
Side vents can also be found near the car wheels, allowing air to pass through this area, over the tyres and back out of the vehicle.
A diffuser is a shaped part of a car’s undercarriage, designed to open up space near the rear. This allows air to expand into a larger cavity, reducing the pressure. By angling it upwards, air leaves the car at an improved direction, improving the uplift/downforce ratio.
Spoilers and wings
Both spoilers and rear wings improve the uplift/downforce ratio but through different means. Spoilers are shaped much like airplane wings, angled to reduce lift by obstructing life-generating airflow, causing it to move horizontally or sharply up. A rear wing, on the other hand, generates additional downforce by pushing air upwards.