Everybody has at least some understanding of aerodynamic drag, so I’ll keep this brief. Wind resistance tries to slow a vehicle as it moves through the atmosphere, which means you need more energy to stay in motion. Energy comes with costs, so we try to reduce the amount we use. Engineers therefore design cars to move through the air with minimal wind resistance, which reduces fuel consumption and improves range, making everyone happy.
It’s pretty basic stuff. Anybody who has held their hand out of a car window and felt the air flow through their fingers will have a broad-brush understanding of what’s going on here. It’s why track cyclists wear those funny helmets and why a donkey looks different to a parakeet; aerodynamic drag quietly shapes everything around us, especially things that move quickly.
Automotive designers have known about it for over a century. One of the earliest land speed records was held by a car named La Jamais Contente, or The Never Content, a Belgian electric car that in 1899 attained the then-remarkable velocity of 65mph. Shaped like a rocket, it could accommodate its driver Camille Jenatzy and little else beyond its crude 50kW dual-motor set-up. Later, in the early 20th century, grand machinery like the Delahaye 135M Torpédo Roadster or the Tatra T77 were among the highly desirable luxury vehicles showcasing the industry’s understanding of drag. Soon, the idea of aerodynamic efficiency found its way from vehicle design to the wider styles and art movements of the time.
The Rumpler Tropfenwagen – which featured in 1927’s Metropolis – was arguably the first series-produced aerodynamic vehicle. It might not have been a very big series (barely 100 were sold, mostly to be used as cabs) but when launched in 1921 it was revolutionary, and many thousands of vehicles designed since have followed in its tyre tracks.
The aerodynamic lie
All the most aerodynamic vehicles, from the 19th century to the present, share the same approximate characteristics. They’re sleek, they’re pointy and they faintly resemble what a kid would draw if prompted to design something “fast”. From the Tropfenwagen to modern cars such as the Porsche Taycan or the Xiaomi SU7, they all follow the same recipe – except for SUVs, which do not.
A drag coefficient (Cd) is what we use to describe the resistance a car will encounter when it moves through the air; the smaller the number, the sleeker the machine. The Porsche Taycan has a Cd of 0.22, for example, while your common-or-garden Volkswagen Golf is around 0.31 – a relatively aerodynamic machine but nowhere near as sleek as the Porsche. At the less impressive end of the scale are things like the Jeep Wrangler (0.54) and the brick-like Mercedes G-Class (0.51) and the old-fashioned Land Rover Defender, which compares unfavourably against a cube. At first glance, this seems like a valid way to assess and compare different cars’ exposure to drag.
But the Cd value is a simplification. Or rather, it doesn’t mean a lot on its own, despite what car manufacturers would have you believe – you often read press releases or hear adverts touting a car’s drag coefficient, without acknowledging that this only tells half the story. A low-slung sports car with a Cd value of 0.25 is going to be exposed to far less drag than an SUV claiming the same number, simply by virtue of its size – or rather its frontal area. The larger the frontal area, the more the drag coefficient will be multiplied; the CdA value (where A is the frontal area) takes this factor into account and is a far better way to understand how efficient a vehicle is.
Doing a very quick fag-packet calculation of CdA is relatively easy. Find the frontal area of a car by multiplying its height by its width, then multiply that number by the Cd value. A Smart Fortwo has a drag coefficient of 0.35, which is higher than the Range Rover’s 0.30, making it seem like the latter is “more aerodynamic”. But this crude arithmetic shows that the Smart has a frontal area of about 2m2, while the Range Rover’s is around 3.8m2, giving them CdA values of 0.7m2 and 1.14m2 respectively – more accurately reflecting the two cars’ relative aerodynamic performance.
That’s because larger, taller, wider SUVs are more susceptible to wind resistance across the board, regardless of their streamlined designs. This has always been quite bad news for fuel economy but becomes particularly troublesome if you’re in the business of selling EVs – batteries are heavy and expensive, miles-per-kWh is a key selling point and electric cars are already at a disadvantage at higher speeds.
The amount of energy required to maintain and increase velocity is quite badly affected by aerodynamics; SUVs might be lucrative, but once on a motorway you quickly come up against physics.
Could the SUV era be over?
It’s true that while there are several factors incentivising manufacturers to build only large, cumbersome cars, the electric era presents more reasons to prioritise efficiency over heft. The market will still probably favour upright designs and tall driving positions, but these are in near-direct conflict with other prevailing priorities – eking the most miles out of every charge and ensuring that the anticipated range doesn’t plummet on a dual carriageway.
It’s worth noting that many of China’s more recent launches have been of saloon and coupé models. The Xiaomi SU7, for example, decisively bucks the SUV trend; with a 515-mile claimed range and a top speed of 130mph, this debut vehicle from what was until recently a mobile phone manufacturer is an antidote to the lacklustre e-crossovers that have been produced over the past few years. I recently drove the BYD Seal, a sporty saloon, which I thought was very good. It’s possible that the tide is turning against gratuitously large vehicles and that China’s obsession with efficiency will fuel that change.
Obviously there are other reasons for manufacturers to embrace smaller and more efficient cars, just as there are other factors that contribute to an EV’s energy consumption. And while it would be lovely for the likes of the Delahaye 135 or the Stout Scarab to return to our roads, the next generation of aerodynamically-optimised machinery will probably be normcore Chinese saloons and slow A-segment runabouts – none of which have quite the same charm.
But, as automotive visionaries in the late 19th and early 20th centuries already knew, reducing drag is one of the most effective ways to improve the efficiency of road vehicles – something we need today more than we did then. Perhaps this burgeoning era of electrification will inspire new forms of beauty, just as it did 100 years ago.
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