What Is Drag Coefficient (Cd) – Aerodynamics 101 And The Most Aerodynamic Cars

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What Is Drag Coefficient (Cd) – Aerodynamics 101 And The Most Aerodynamic Cars

In its simplest form – aerodynamics is the study of how air moves around things. The rules of aerodynamics explain how an aeroplane is able to fly or a Formula 1 car is able to stick to the ground at staggering speeds. Even a kite reacts to aerodynamics.


Road cars are no different, as they move on the roads, they are actually pushing air molecules present in the atmosphere over, under, and around the car. At low speeds, the effects or aerodynamics are pretty minimal, however, once you speed up, these aerodynamic forces become greater and can actually affect how the car moves, and the rate at which it moves.

Aerodynamics 101: Honda City

Firstly, a little bit of physics.

At any one time, there are always four forces acting on a car in motion. They are called weight, lift, drag, and thrust.

Aerodynamics 101: BMW G80 M3

  • Weight: is the force acting downwards towards the road caused by the weight of the car
  • Thrust: is provided by the engine to push the car forward
  • Drag: is a force that tries to slow something down. It makes it hard for an object to move.
  • Lift: Lift is the push that lets something move up. The force that acts in the opposite of weight.

By looking at the image above, it makes sense that if a car encounters less drag as it moves, the easier it would be to move the car forward, thereby requiring less thrust to move it, or if we add more thrust, the car would move faster. An easy way to understand this is to ask yourself if it would be easier to run through water or run through air? Of course air, this is because water causes more drag on your body than air.

Aerodynamics 101: W222 S-Class

Therefore it’s easy to understand why car manufacturers go to great lengths to reduce the effects drag has on a car’s body. This is because the more aerodynamically efficient (i.e.: less drag) a car body is, the better its fuel economy will be, and the faster it can go.

Consider the shapes of a sports car versus a lorry, of course, the two are made for very different reasons, but you can start to understand why sports cars and race cars through the ages, have always had a beautiful yet small front end, a smooth and streamlined body, and clean proportions front to back. As if to cut through the air like a knife through butter.

Aerodynamics 101: Jaguar XJ220

In all these cases, manufacturers use a concept called Drag Coefficient or Cd

The aerodynamic drag coefficient is a measure of the effectiveness of a body shape in reducing the air resistance when moving in forward motion.

In its simplest form, the drag coefficient refers to a formula that allows one to calculate the forces acting on the car’s body at different speeds.

  • Fd = cd 1/2 ρ v2 A

Now, we are not going to whip out our scientific calculators and notepads. However, here is a relative range of Cd figures depending on the vehicle type:

  • Passenger car: 0.22 – 0.40
  • Sports car: 0.28 – 0.40
  • Small/Light van and MPV: 0.35 – 0.50
  • Bus: 0.40 – 0.80

Aerodynamics 101: Maxus G10

Note that sports cars actually have a higher Cd value than that of passenger cars such as sedans or hatchbacks.

In this day and age, aerodynamics play a huge part in the construction and design of any car. In all cases, manufacturers will use Computational Fluid Dynamics (CFD) modelling and extensive wind tunnel testing to perfect any car's aerodynamic efficiency. 

Aerodynamics 101: Ferrari SF90 Stradale

Supercars have complex surfaces and downforce elements to balance lift and downforce (given their higher speeds), and this, unfortunately, has a negative impact on a sports car’s overall Cd values.

Passenger cars, hybrids, and EVs on the other hand actually have far lower Cd values, as they use clever aerodynamic design, to push air around the car to reduce drag so the vehicle can travel further.

Here are some examples of the most streamlined and aerodynamic cars on sale today:

Tesla Model S, Mercedes-Benz (W205) C-Class, and Toyota Prius – Cd: 0.24

Aerodynamics 101: Tesla, Mercedes-Benz, Toyota

Both the Tesla Model S, (W205) Mercedes-Benz C-Class have a Cd factor of 0.24 thanks to superb proportions, smooth body surfaces and intelligent design to channel air around the car.

Audi A4 – Cd: 0.23

Aerodynamics 101: Audi A4

The Audi A4 just pips the 0.24 Cd club and is one of a very few cars with this low of a Cd value. Despite its slightly boxy shape – the A4's very intelligent design at the front and smooth flowing surfaces along the flanks and rear of the car make it very efficient. 

Mercedes-Benz A-Class Sedan and (G30) BMW 5 Series – Cd: 0.22

Aerodynamics 101: BMW G39 5 Series Mercedes-Benz A-Class Sedan V177

Yes, you read that right, the (G30) 5 Series is one of the most aerodynamic cars on sale today. Again, great design, extensive wind tunnel testing, and CFD modelling help this German Luxo-barge to an extremely low Cd value belying its size. The current record holder for the lowest drag value of any production vehicle in the world is the new (V177) A-Class sedan, which features a Cd value of 0.22. The new A-Class sedan follows in the footsteps of its older sibling the (C117) CLA sedan, which previously held the world record.



Arvind

Arvind

Writer

Arvind describes a car in the same way he would describe a woman; this is not very healthy. Unlike the eternal sunshine of a spotless mind, soulful naturally-aspirated soundtracks and trigger quick (self-applied) gearshifts are all that fill the darkest recesses of his mind. Arvind is still trying to understand women...


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