Automotive aerodynamics explained
Car makers spend a fortune on making their cars slipperier, whether it’s to make them more fuel efficient or faster, or both… here’s how to be an instant expert on automotive aerodynamics.
ACCORDING TO THE Macquarie Dictionary of Motoring, aerodynamics is “the dynamics of atmospheric interactions with moving objects or – more colloquially – the science of reducing wind drag”.
And by reducing wind drag, vehicle designers, in this instance, can make vehicles faster (slipperier through the air because there’s reduced drag) and thus more fuel efficient. This article isn’t intended to be an in-depth explanation of aerodynamics and/or the ultimate science behind it, rather it’s designed to give you a better understanding of aerodynamics and how it relates to vehicle design, and some of the history behind it.
The history of automotive aerodynamics
The first vehicle to break the 100km/h barrier was the Jamais Contente (Never Satisfied) and it was all-electric. The year was 1899. The driver of that ‘car’ was Camille Jenatzy, a Belgian, who determined that a streamlined shape was likely to be slipperier than a brick-like design – he was bang on, and we’ll explain why shortly. Look at the picture below, it looks like he’s riding a torpedo.
Borrowing from Jenatzy’s design, the 1910 Vauxhall became the first car to exceed 160km/h (with a three-litre engine) and you can see why in the picture below. But it was the 1921 Rumpler Tropfenwagen (main picture) that was the first determined attempt to use aerodynamics in the design of a vehicle. And it worked… the company didn’t have a wind tunnel like car designers do today, but recent wind tunnel testing of the Tropfenwagen revealed it had a coefficient drag of 0.28. That’s about the same as a Jaguar F-Type.
The measurement of aerodynamics
Motoring journalists, quite often reading from the car company’s script, include a number they probably know little about. This number is the coefficient of drag (Cd) and engineers spend millions of dollars and endless amounts of times in wind tunnels to make that number as small as possible.
The pioneering boffins of aerodynamics determined that a teardrop was the most streamlined shape you could get and gave it a coefficient of drag of 0.00 (or zero drag). Since those days, however, tests on teardrops have revealed they’re not quite as slippery as originally thought and should actually be assigned a rating of 0.05Cd, while a plate stood on its end into the wind realises a rating of 1.17Cd. To make this more brain-hurting, there are other qualities to consider, like front lift (Cif); rear lift (Clr) and the moment of yaw (Cmy), which means the change in shape when the shape is on an angle, like a cornering car, or a car hit by a side wind.
Typically, cars fall into the 0.25Cd to 0.35Cd range while SUVs tend to start off in the high 30s and into the 40s although even these are becoming slipperier. The formula to determine you own car’s drag at a given speed requires a lot of knowledge the average motorist isn’t likely to have or be easily able to get their hands on, but to determine, roughly, the drag on your car at a given speed you’ll need to use this formula: Cd x frontal area x density of air x speed squared.
Why do car companies bother?
The idea is that, and here’s a typical scientific disclaimer, all things being equal, the car with the least amount of wind resistance will thus require less fuel and power to maintain a given speed.
Indeed, with car companies wanting to produce ever more fuel efficient cars, the science of aerodynamics is becoming more and more important. And making a car slippery is tricky and that’s down to the shape and protuberances, like wing mirrors, door handles, and even the front grille (hence the development of active grilles that can close and force air over the bonnet.
Because cars are generally shaped like a box, they start at with about 50 times as much drag as, say, something shaped like a cigar (of the equivalent size and weight and travelling at the same speed), or the Jamais Contente.
You want the vehicle to travel through the air causing the least amount of disturbance as possible, which is why car designers together with engineering colleagues work on shapes and surfaces to create something visually appealing but also slippery. The air should flow evenly around the front of the vehicle and then come together at the rear; when you force something brick-shaped through the air the air becomes turbulent causing drag.
Most car companies will admit that around a 10% improvement in a vehicle’s aerodynamics will result in a fuel efficiency gain of around 4%. This fuel efficiency gain can be quickly overcome, though, when one fits a roof rack to their car and adds a ‘box’ to it… it can often increase fuel consumption by up to 20% as the vehicle needs to work harder to overcome the wind resistance.
The effect of drag, or driving an aerodynamically bad car
At highway speeds most vehicles use around five times as much power to overcome the effects of drag as they do overcoming things like rolling resistance (tyres) and weight. This means, the higher your vehicle’s Cd number, the harder it’s having to work to push through the air and keep up with traffic, thus it’ll use more fuel.
At very low speeds (like 1-2km/h) wind resistance or drag will be virtually nil and so there’ll be little difference between a sports car and a boxy SUV like the Toyota LandCruiser 200 Series, but as the speed doubles the wind resistance quadruples meaning its effect will be greater on the less aerodynamic vehicle (when travelling at the same speed).
Some tricks to make a slippery car?
Don’t fit a roof rack is the first and most important one as a loaded roof rack can increase fuel consumption because the vehicle is having to work harder to overcome the drag. Other little things like ribs on tail-lights or creases in the body all help to make the airflow around the car force dirt and water away from it while helping it slip through the air.
Other little tricks like the shape and size of the wing mirrors can help, as can adding side skirts to cars. A diffuser at the rear of the car can help to smooth air coming out from under the car to reduce turbulence, the shape of the wheel arches is also important.
So, the shape of your car and what you add onto it will have a net effect on the contents of your wallet, all things being equal. Upset the slipperiness of your car and the thing will have to work harder to overcome the additional drag and thus use more fuel.