What is aerodynamic drag and does it matter?

Car designers and engineers are always working to create vehicles that blend appealing aesthetics with functional elements such as aerodynamics to help improve economy and dynamics.

A lower aerodynamic drag is often touted as an important feature of a new vehicle, such as the 0.21Cd of the Hyundai IONIQ 6 or the 0.20Cd found on the new Mercedes-Benz EQS.

The Lucid Air also has a wind-cheating 0.21Cd drag coefficient, compared to the Tesla Model S at 0.24Cd.

But what does that mean? What effect do those figures have on economy? Do car companies have to surrender aesthetics to ensure the vehicle slips through the air as efficiently as possible?

And is there anything you can do with the car in your driveway to make it more aerodynamic and therefore more efficient?

Yes, there is…

CD is the common abbreviation for compact disc, however nowadays – with CDs fast disappearing from our cars – the term ‘Cd’ is perhaps more relevant as the short form for a unitless numerical value known as the coefficient of drag.

Cd is a measure of how much resistance an object will move through the air, with a lower value representing less impedance.

Outside of powertrain efficiency, your car’s fuel consumption is affected by rolling resistance – think vehicle mass and how sticky your tyres are – and aerodynamic losses.

The amount of aerodynamic drag is dependent on the frontal area of your car – imagine it punching a hole through a banner at the start of a footy game – as well as the efficiency of your vehicle shape (that is, the Cd) and, most critically, vehicle speed.

At 100km/h the fuel penalty for aero drag is close to three times more than that of rolling resistance.

Whilst a Formula 1 car is very ‘aerodynamic’, its intent is very different from that of a passenger car. F1 race cars utilise wings, vents and underbody parts to push the vehicle to the ground, allowing high cornering speeds.

This is at the cost of efficiency, with a typical modern F1 car having a Cd value between 0.7 and 1.1, depending on their configuration.

In the aviation industry, running costs are dominated by fuel consumption and so modern jet airliners achieve very low Cd values. The Boeing 787 is just 0.024Cd, for example.

Common passenger cars tend to have a Cd from 0.25 to 0.35, with the upper end of that spectrum being SUVs.

The attention on Cd values has become increasingly relevant with the rise in prominence of electric cars.

Manufacturers chasing efficiency above all else tend to mimic aerofoil-type profiles with a general disregard for conventional styling. The Hyundai IONIQ 6 is a case in point.

This is partly due to the architecture restrictions of EVs chasing a long driving range.

Electric vehicles already have high powertrain thermal efficiencies (roughly twice that of an ICE car), and less powertrain components to develop.

Adding batteries is restricted by packaging volume and at the detriment of vehicle weight.

Therefore, it’s no surprise to find that most car-makers are investing big development dollars on aerodynamics and spending time in wind tunnels learning how to cheat the wind.

Fortunately, engineers and designers have found various solutions to preserve vehicle styling whilst improving aerodynamics.

Functional air curtains help redirect flow into the wheel housing, reducing turbulence and therefore drag.

Large grille openings are required to cool a conventional engine (unlike an EV), which create Cd losses.

Unbeknownst to many, modern cars have grille shutters, which at certain conditions relating to speed and engine temperature can close off portions of the grille and recover some of these losses.

Another active aero aid becoming common in the US full-size pick-up segment is front spoilers that deploy at highway speeds.

Put into practice, we have seen a significant decrease in production car Cd values even just over the past few decades.

The Land Rover Defender of the 1990s had a relatively poor Cd value of 0.59, but the new-generation body shape available from 2020 saw this improve to 0.39 whilst still maintaining the same general silhouette.

You don’t want your engine to overheat, so we don’t recommend covering up your grille…

But there are a few things you can do (or not do) to help improve your car’s Cd and hence fuel economy.

Checklist for improving aerodynamics: