The ultimate guide to car aerodynamics
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Aerodynamics is one of the most important but least understood subjects in the whole of motoring. Although it’s not obvious to the naked eye, a car that can guide air round itself effectively will (all other things being equal) be faster and quieter, will handle better, will have superior cooling and will use less fuel and emit less CO2 than one that can’t. All cars nowadays are designed with this in mind, to a greater or lesser extent.
For most people, aerodynamics is an overwhelmingly complicated science. Fortunately, many aspects of it are reasonably simple, and those are the ones we’re going to concentrate on in this article. By the time you’ve finished reading it, you should have a better understanding of why cars are shaped the way they are.
Before aerodynamics
The very earliest cars were designed with no thought of aerodynamics at all. This is partly because they were designed by people who didn’t know much about it, but also because it wasn’t necessary.
The motoring pioneers had more fundamental things to think about, like making their cars go and stop. Air resistance wasn’t an issue when the driver and passengers were out in the open, and nobody was getting anywhere quickly. The UK speed limit was raised from 4mph to 12mph in 1896 and then again to 20mph in 1903. Aerodynamic design becomes important at quite low speeds, but not as low as that.
Unaerodynamic glamour
By the 1920s, cars no longer looked as much like horseless carriages as they once did, but they also mostly had vertical windscreens and radiator grilles, along with separate headlights.
Many of them were very beautiful, and you’ll find people who believe car design has never been the same since this basic shape went out of fashion. Aerodynamically, though, they were a disaster, not so much guiding air round themselves as bludgeoning it out of the way. Designers around the world started to wonder if things could be organised better.
Early experiments
As powered flight turned from science fiction to science fact, worldwide understanding of aerodynamics snowballed, and in some cases former aircraft designers turned their attention to cars. One of these was former Zeppelin employee Paul Jaray, who created some astonishing prototypes in the 1920s.
Faster and more economical than the production cars they were based on, they also looked so unusual that the chances of anyone buying them were effectively zero. Nevertheless, they provided a good insight into how car design was going to develop in the near future.
Aerodynamics becomes mainstream
Very aerodynamic cars became available to the public in the 1930s. 1934 was an especially notable year, since it saw the introduction of the British Crossley Streamline (launched at the 1933 London Motor Show), the Czech Tatra T77 (designed partly by Paul Jaray), and, most famously, the American Chrysler Airflow, also marketed in short-wheelbase form as a DeSoto.
The Crossley looked the most conventional for its time at the front, but like the Tatra and the earlier Jaray prototypes it had a long curve extending from the roof to the rear end. This created at least part of the “teardrop” shape which was already accepted as creating the least aerodynamic drag because the currents of air split apart by the front of the car reform most smoothly once it has gone past.
The Chrysler, which had a less dramatic rear but a very smooth front with enclosed headlights, was far more successful than the others, remaining in production until 1937 (DeSoto production was abandoned the previous year). In absolute terms, though, it was a commercial failure. The world still wasn’t ready for cars like this.
Low drag for higher performance
As cars became faster, aerodynamic efficiency took on a new significance. An important factor is a car’s frontal area, which is essentially how much of it you can see when you’re looking at it head-on.
The smaller the frontal area, the less you disturb the air which the car encounters as it speeds along. Low cars (such as the 1960s Ford GT40, so named because it was only 40 inches tall) are inherently more aerodynamic than tall ones, so more of their engines’ power is used to drive them along rather than to shift the air out of the way.
Low drag for better fuel economy
Fuel economy became a serious issue during the 1973 oil crisis. Very aerodynamic cars such as the NSU Ro80 were already in production by this time, but they weren’t common.
They soon became so. Before the decade was out, people were familiar with the swoopy Citroen CX (pictured) and Rover SD1 which would have seemed quite alien ten years earlier.
These and other streamlined models treated the air far more gently than their boxy predecessors. As a result, they could gain speed using far less power, and therefore burned far less fuel. The oil crisis is now more than four decades behind us, but its effect on car design can still be seen.
Low drag for reduced CO2 emissions
Nobody cared much about CO2 emissions until they started to affect how much tax should be paid on a car. At this point, buyers really sat up and paid attention. Manufacturers in turn had to respond by making their cars emit as little CO2 as possible, at least on the official test.
CO2 emissions are related to fuel economy, and although advances in electronic engine control have been responsible for incredible advances on both counts this century, aerodynamic design has also played a large part.
Low drag for quieter running
One of the first things you notice if you drive a car from the 1980s or earlier is that you can hear a lot of wind noise. This is partly because sound insulation has improved greatly since those days, but partly also because air is now directed around the car in such a way that the amount of sound is reduced.
Nissan paid a lot of attention to this when it was designing the Leaf. Like all electric cars, the Leaf has a potential problem with wind noise, not because there’s necessarily a lot of it but because there isn’t any engine noise to drown it out.
Door mirrors are a major source of wind noise because they can collide with air that the nose of the car has not already pushed out of the way. The Leaf’s bulbous headlights look the way they do because they have been shaped to divert air round the mirrors. This is not the only reason why the Leaf is very quiet, but it’s an important one.
Good and bad shapes
Broadly speaking, conventional cars (as opposed to SUVs and MPVs) come in three shapes: saloon, hatchback and estate. Saloons have been unpopular in the UK for a long time because of their lack of practicality, but as far as aerodynamics are concerned they’re the best bet.
This is because most hatchback and estates have relatively flat roofs which end sharply at the tailgate. Aerodynamically, this is very bad news because a lot of turbulence is created immediately behind the car, which in turn increases the amount of drag. (Cars which look like hatchbacks but have gently falling roof lines have been designed that way specifically to avoid this problem.)
Saloons are better because the next surface after the roof is the bootlid, not the road. The drop from the bootlid of a saloon is much smaller than that from the roof of a hatchback or estate, so there’s less turbulence and therefore less drag.
The curious case of the Kamm tail
As mentioned earlier, the body shape which creates the least aerodynamic drag is the teardrop. Its biggest practical problem is that it makes the car very long, because to minimise turbulence it has to extend from the highest point of the roof to just above ground level. Fortunately, there’s a way round this.
In the 1930s, it was discovered that you could get nearly the minimum amount of turbulence, and therefore drag, if you cut off the tail sharply, leaving a flat vertical surface, as long as the area of that surface was a small proportion – ideally no more than half – the area of the whole car as seen from the back. Several people developed this idea, but the resulting body shape has become known as the Kamm tail (or sometimes Kammback), after the German aerodynamicist Wunibald Kamm.
Cars with Kamm tails went into production shortly after World War II, and you can still see them now. The shape is particularly popular in hybrid cars such as the Toyota Prius, whose manufacturers want to enhance the low fuel economy and CO2 emissions of these vehicles as much as possible.
There is a lot of confusion on this subject. Some people think that any car with a sharply cut-off rear end, regardless of the shape of the roof, has a Kamm tail, but this is not true.
The problem of lift
Drag is a very important consideration in aerodynamic design, but it’s not the only one. A streamlined car may be able to go very quickly in a straight line but handle terribly in corners because its shape tends to make it rise from the ground. Even if it doesn’t leave the road surface completely (which it probably won’t), the lift will still make the tyres lose grip.
This is a big problem in cars with long, flat bonnets. There is a low-pressure area above the bonnet because most of the air that has been pushed upwards by the front of the car doesn’t come down again until it hits the windscreen. The front end will tend to be pulled into the low-pressure zone. The longer the bonnet, the greater the effect.
Nowadays, cars have shorter and more sculpted bonnets than they used to. If they have long ones they generally also have large, heavy engines which reduce the effect. Earlier cars such as the Ford Capri had more of a problem because their bonnets were enormous and in many cases they had small, light engines.
Downforce from wings
The opposite of lift is downforce, which pushes the car into the ground and increases grip. The most popular way of achieving downforce is to use a wing.
An aeroplane wing is designed so that air moves more quickly across the top part than the bottom part. The faster-moving air is thinner, so its pressure is lower. The wing rises into the low-pressure area and takes the rest of the plane with it. This works so well that it can hold a 400-tonne aircraft 30,000 feet above the ground if it’s going fast enough.
This is no use at all in cars, so instead their wings (if fitted) are designed so that the air moves more quickly across the bottom part. As a result, the car is pushed downwards, greatly increasing the grip of the tyres.
Downforce and drag
Wings also create a lot of aerodynamic drag, which has a terrible effect on straightline performance. If you took the wings off an F1 car it would be much faster along the straights. However, its grip would also be severely reduced, so much so that lap times would suffer very badly, and lap times are much more important in racing than top speed.
The ideal balance between downforce and drag varies from circuit to circuit. An F1 car will be set up to have relatively little drag at Monza, where the emphasis is on getting down the straights as quickly as possible while still having enough grip through the corners. At Monaco, where there’s a much higher proportion of corners, it’s all about the downforce.
Wings on road cars
Most road cars don’t have wings because they never go fast enough to need much downforce and would use too much fuel because of the extra drag.
If they’re very powerful, they will have wings so they remain stable at extremely high speeds. Often they are raised and lowered automatically so they are reasonably safe at 150mph or more but not too uneconomical at 60mph or less.
Hot hatches are often fitted with wings (usually at the rear) and other, smaller aerodynamic devices. They are used mostly for cosmetic effect and don’t do much at normal road speeds, though they should provide some benefit on a trackday.