Car Aerodynamics

April 11, 2013 By Mike Lee
Last updated on July 27, 2015

The car’s aerodynamics can have a big effect on it’s performance, but add to that, the need to create stability with downforce when accelerating, braking and cornering and it all gets a bit complicated.

1957 Vanwall VW6 Streamliner
1957 Vanwall VW6 Streamliner

A road car (especially the heavier ones) will not really need to be concerned about creating additional downforce, unless you plan on driving down the autobahn or using your car for track days, and even then you may not necessarily need to concern yourself too much about it. But those that take their cars regularly on a track or just for racing, may indeed benefit from some knowledge of your cars aerodynamics.

Believe it or not there are different types of drag on a car, some caused by obtrusions from the cars body and even dirt. Some created by the bodies design (estate, saloon, hatchback) which have an effect on the low pressure created at the back of the car and therefore the wake created, which can pull a car backwards.

Don’t under estimate the difference in drag created by even small changes, an example is the Calibre 8v which has great car aerodynamics with a coefficient of drag (Cd) of 0.26, but the 16v, 2.5 and turbo Calibras have a coefficient of drag of 0.29, just because they had to open up the front grill to allow for extra cooling. Air entering through a grill, through a radiator and out underneath the car creates a lot of drag. Some manufacturers now have much more efficient ducting of the air to the radiator and ducting away through either the wheel wells or better still strategically placed vents out of the bonnet.

Ferrari F1 2007
Front Area
The front of the car is a major factor in the Cd of a car. The point at which air separates to go under and over the car is called the stagnation point. The size of the frontal area of a car can be calculated by multiplying the height by the width, including the mirrors, less any openings. Secondly, the air flow over the car has the second influence on the Cd of a car. Air flows over the car in layers (laminar). Any separation of the air from the body causes turbulence and therefore drag. The most efficient shape for a low Cd (0.05) is a tear drop facing in this direction:

Tear Drop
The reason for this shape having such a low coefficient of drag, is because the smooth front allows the air to flow around and stay attached to the surface and to be joined back together at the back with minimal turbulence.

The way that the air moves along the body is important to reduce drag, the air must try to stay attached to the body otherwise separation will cause drag.

Car Aerodynamics – Audi R8

Audi A4 DTM 2007
Rear of Car
Just as important (if not more) as the front of the car is the rear of the car, as any unattached air will increase the wake to the rear and cause significant drag, therefore the shape/angle of the rear window and sides of the rear is important (gradual smooth is better). An increase in fuel consumption and decrease in top speed can be greatly influenced by the rear profile of a car. The vacuum at the rear of a car is used by racing drivers to gain an advantage just before they overtake (slipstream), that is because there are less air molecules for the car behind to fight against.

Rear Defuser
High and low pressure areas (both can create drag)
Low pressure areas creates lift, high pressure areas creates downforce.

When looking at downforce, it is important to ensure the cars balance is not adversely affected. For example if you create too much rear downforce on a front wheel drive car, it is possible to unload the front wheels, loosing traction – and visa versa if you do the same to the front of a car.

Active spoilers are the next step for many performance car manufacturers, with many already employing this.

Some things to improve car aerodynamics
Lower the car to a specific ride height (see suspension section)
Remove a non-functional rear wing/spoiler
Smoothing/cleaning up the underside – floor pans etc
Use a front spoiler that allows for air redirection away from under body
Clean up the car surface
Fit a smaller aerial/internal antenna
Add side skirts
Use correct tyre pressures

I’ve covered more about this subject in the body kit article, which describes the individual body kit parts and their effects.

Inside a Formula 1 Car — Aerodynamics

Race Car Take off