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Car Aerodynamics Guide

Car Aerodynamics Guide

Posted by Glenn Rowswell on 5th March 2012

Car Aerodynamics Guide

How air flows under or over your car is critical for both its performance and your safety. So we thought we’d look at the key principles of aerodynamics.

Aerodynamics refers to the study of moving air, in particular when it comes into contact with an object. The movement and behaviour of air over an object (in this case, a car) is a technical and highly mathematical topic that people can spend their lifetimes studying.

Thankfully, we can pull out the key points that affect you and your performance car, and leave the equations to one side. Over the next few pages, we investigate some of the basics, and the key areas to focus on when considering your car’s aerodynamics.

How your car moves through the air is a critical combination of events that have a direct impact on its performance, stability and safety. At the simplest level, if you imagine the front of your car when you are driving, the air you are hitting can either go under or over it.

The point on your front bumper where the airflow divides to go either over or under is called the stagnation point. From here, the air underneath the car has to deal with unevenly shaped objects that create a lot of drag: the wheels, exhaust, driveshafts and suspension components.

Car Aerodynamics Guide

Aerodynamic drag is the force that directly opposes the motion of a car. To be specific, about a third of your car’s overall drag comes from the underside. On top of this, another third comes from the air going over the car (the whole exterior) and the remaining third comes from the airflow through your grille, radiator, intercooler and through your engine bay.

Every production car goes through testing to assign it a number, known as a drag coefficient, or Cd. The lower the number, the lower the car’s drag, and when you consider that at motorway speeds over half of the power required to drag, you start to see how important this figure is, especially for fuel economy.

Typical figures for Cd are in the region of 0.3–0.4 for production road cars. Some cars are particularly aerodynamic, such as Nissan’s R35 GT-R, which has a Cd of 0.27. On the other end of the scale, open-wheel racecars have a much higher drag coefficient. This is thanks mainly to the rotation of the wheels, which creates a lot of drag in itself. A modern F1 car, for example, has a Cd number in the region of 1.0.

Car Aerodynamics Guide

Bolting on a large front splitter to your car is a fairly easy way to add downforce, as it effectively pushes the front of the car into the tarmac at speed. This is great for cornering and braking at speed, where a high level of front-end grip is required to help eliminate understeer.

Another plus is that it reduces the proportion of lift-inducing airflow that is directed underneath the car, and instead directs it over the top of the car. A smaller, knock-on downside of a front splitter is that it increases the frontal area of the car, which has a direct relationship with drag, but the advantages outweigh the disadvantages when grip is critical.

f1 splitter

On racecars, a low, protruding front splitter is a common sight, but this is best suited to smooth, flat circuit tarmac and stiff suspension. On a road car, it’s a fine balance as to how low you can go without ripping it off pulling into the drive or tackling a speed bump.

Another method of reducing the proportion of airflow going underneath the car is a little simpler. If you lower your car on coilovers or stiffened lowering springs, less airflow will be directed under the car, which reduces both lift and drag at the same time. So now you know that lowering your car has more benefits than you might have originally thought!

what do canards do

Bumper-mounted canards (also referred to as dive planes) are becoming increasingly popular in the tuning scene and, like a front splitter, they increase downforce at the front of the car. Usually constructed from carbon fibre, they are bolted to the sides of the front bumper, most commonly in sets of four. They also direct airflow away from underneath the car, which is highly desirable for reducing lift, but they do increase drag as a result.

They only have a noticeable impact at higher speeds and are best used to fine-tune the aerodynamic characteristics of the car. For road car use, it’s questionable as to whether canards can be justified on a performance level, but for top-end Time Attack machines and very fast track cars in general, they are worth looking at.

Car Aerodynamics Guide

Just as splitters increase downforce at the front of the car, a rear wing provides downforce at the rear. They’re often referred to as spoilers, which is technically incorrect – a spoiler is there to ‘spoil’ any undesirable flow of air over the rear of a car (the clue is in the name), in order to reduce lift or increase fuel efficiency.

A wing is a device that proactively creates downforce at the rear, in order to increase rear-wheel grip and add stability. The angle of attack of the blade of the wing is critical, and also has a large impact on aerodynamic drag. The shape and positioning of the wing are also key and many rear wings offer adjustability.

If you’re serious about going fast on track, then buy a wing that comes with some CFD (Computational Fluid Dynamics) data – this can link the amount of downforce, drag and power consumption with speed. That way you know that at 100mph, you have the equivalent weight of three fully grown male badgers sitting on the back of the car, or whatever it may be.

Car Aerodynamics Guide

It would be even better to invest in some wind tunnel time, but that costs serious bucks and for most road and track cars, simply buying a decent, lightweight rear wing is sufficient. A Gurney flap is sometimes fitted to rear wings. This is a piece of technology that has been borrowed from aeronautics. A small strip of material, usually at a right angle to the surface of the wing, helps a clean separation of air from the wing and maximizes downforce.

Car Aerodynamics Guide

Many performance cars come fitted with spoilers as standard, and this isn’t just for aesthetic reasons – safety plays a large part here, too. Take the Audi TT for example. When it was launched, there were various reports of high-speed crashes due to a lack of stability at high speed. These were all linked to Audi’s decision not to fit a rear spoiler, which resulted in the car being recalled.

Likewise, Top Gear’s Stig had an incident in a Koenigsegg CCX with no rear spoiler and therefore a lack of stability. Spoilers are used to reduce the turbulent airflow at the back of the car by creating a clean separation of flow, which reduces unwanted lift.

This might be worth considering before you go unbolting your car’s standard spoiler! Active rear spoilers, such as the one found on the Bugatti Veyron, double up as air brakes and not only offer stability when on the move, but also offer huge amounts of drag under heavy braking to help slow the car, similar to when a plane lands.

Aerodynamics Guide

There are big gains to be had by optimising the airflow underneath your car, and there have been various techniques used over the years, particularly on racecars, to find advantages in this area. The main goal is to create an area of low pressure underneath the car, so that there’s not an opposing upwards force fighting the downforce being created by the splitters and spoilers you’ve just bolted on.

The main way to create an area of low pressure is to accelerate the airflow underneath the car. This is linked to Bernoulli’s principle, which states that an increase in air entirely flat floor would be the best solution, so as to completely bypass all the components underneath that create drag. This technique is only really adopted in full-on racecars, but a more common solution for track cars is to use an extended splitter or undertray that extends underneath the engine, so as to minimize drag created by engine bay components.

This technology has even found its way onto fuel-efficient road cars, such as the VW Polo BlueMotion, which uses an extended front lip and part undertray to reduce drag.

carbon fibre rear diffuser

A rear diffuser is used to help the high- speed airflow underneath the car rejoin the slower-moving air behind the car. This mixing of different airspeeds often gives rise to unwanted drag, so a diffuser is used to expand and slow the air at the back of the car, before it joins the wake behind the car.  The use of longitudinal fins in the diffuser is common and promotes rear end high-speed stability, too.

Diffusers are often fitted to Time Attack cars, but not always at the correct angle – there is window a of effectiveness for a rear diffuser, which, although subjective, is said to be between seven and ten degrees.

what does ducting do

This is vital in any car, but particularly you offer the air no choice but to go through. Above Lots of ducting needed on this big- power turbo setup in turbocharged cars where heat management is more of an issue. Ducting is important for any item that needs force-fed cooling. Be it an intercooler, radiator, oil cooler or brakes, having proper ducting is the only way to ensure that airflow gets to the object you are trying to cool.

For example, if you’ve got a front-mount intercooler fitted with big gaps around the sides between your bumper, then the air will always take the path of least resistance as opposed to flowing through the core and maximising cooling. If the item you want cooled, then it’ll do its job properly.

Aerodynamics Guide

Ducting also applies to cars going for top-speed runs, such as those that run at the Bonneville Salt Flats, where it’s commonplace to see duct tape over the bonnet shuts to stop drag being created. In the UK, these techniques have been used by the likes of Reyland Motorsport with their Escort Cosworth, which succeeded in hitting 200mph at Bruntingthorpe. Careful attention needs to be paid so that the car still receives the necessary levels of cooling to the engine, though.