How is a car's drag coefficient calculated?

Do you know what aerodynamic figure your car has?

3y ago
47.5K

In this day and age of obsessions with fuel efficiency and saving the planet, one of the largest factors found to affect consumption figures is the drag force on a car. As an object moves through air, the air being displaced by the object's movement interacts with the body, causing an opposing force to take hold.

More resistance opposing a car while in motion will mean that it takes more effort to accelerate, straining the powertrain and increasing fuel consumption. This is why car companies these days often state a value for how aerodynamically efficient their cars are, known as the drag coefficient, to show off their handywork.

Although big wings and front and rear diffusers are great for grip and cornering speeds, they can create a large aerodynamic penalty

Although big wings and front and rear diffusers are great for grip and cornering speeds, they can create a large aerodynamic penalty

Denoted in engineering terminology as 'Cd', the drag coefficient takes into account numerous factors governing the aerodynamic abilities of a car, producing a nice neat number at the end to compare with other models.

It is calculated using the following equation:

Who doesn't love a bit of mathematics on DriveTribe!

Who doesn't love a bit of mathematics on DriveTribe!

So the drag coefficient is a function of the drag force on the car (Fd), the density of the air (ρ), the frontal area of the car (A) and the speed that the car is travelling (V).

The aim is to get the value for drag coefficient as small as possible, making the front profile of the car a key design aspect on which to apply an engineer's aerodynamic knowledge.

Frontal area is everything

Frontal area is everything

Big spoilers and a large rectangular bumper will not do - the world now needs sleek, flowing lines that will cut as easy a path through the air as possible. All of this can thankfully be designed through practical testing in a wind tunnel.

The current record for the lowest drag coefficient for a 'car' was achieved by the engineers at Shell through their Shell Eco-marathon Prototype. The concept managed a tiny Cd of just 0.048, with plenty more concept cars producing values of under 0.2.

The sleekest production car comes in the shape of the Volkswagen XL1 which used a functional and futuristic body design, culminating in a drag coefficient of 0.189. Other solid performances come from the Tesla model S (0.24) and the Honda Insight (0.25).

With sportscars and supercars needing to produce large amounts of grip and downforce these days, they certainly suffer in a game of drag coefficient top trumps. Something aero-heavy like the Dodge Viper ACR comes in at a whopping 0.54, with the huge rear wing, front canards and splitter causing rather a lot of resistance as the car tries to channel its way through the air.

Formula 1 cars take downforce to another level and therefore produce coefficients of around 0.7-1.1, prioritising cornering speed over straight line performance. Their aerodynamic efficiency has been increased by the implementation of DRS (Drag Reduction System) but all-in-all they still have a fairly tough time cutting through the syrupy surrounding air during a race.

You should definitely check out exactly what the drag coefficient is for your car and see how it compares with some thoroughbred machinery - you may be surprised how your own pride and joy performs. Either that or it might reveal exactly why your fuel bill is so high every month!

Does your car manage a drag coefficient of under 0.35? Comment below with the results that you find!

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Comments (14)

  • The Audi 80 sensible-saloon was 0.29 in 1986

      3 years ago
  • 0.29 for my 987 Boxster, which is the same as a Honda accord. None of these numbers seem that meaningful when you consider that a double decker bus can have a Cd of 0.35 (www.motor1.com/news/163945/setra-s-531-dt-reveals/). For those curious this is one of the most complete lists I've found: en.wikipedia.org/wiki/Automobile_drag_coefficient

      3 years ago
  • The Cd depends on the Reynolds number. The Reynolds number is a linear function of the velocity. Since the Cd is not exactly constant is rather strange to compare Cd without stating the conditions of measurement.

      3 years ago
  • 0.32 - 1996 Ford Falcon (EF)

    0.29 - 1999 Ford Falcon (AU)

    0.31 - 1989 Daihatsu Charade

    Those are the more aero cars I've owned. The least is probably the 1990 Subaru Legacy station wagon, but I don't know it's Cd

      3 years ago
  • Should include some talk about the lift coefficient and how it affects the drag coefficient on aero-heavy cars. The resulting drag polar is enlightening to say the least

      3 years ago
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