car aerodynamics

The automobile industry is one of the most ever-growing industries in the world. We can see the Car manufacturing companies competing with each other introducing faster and better Car Models every year. What do they really upgrade in their cars? How are they making cars with better fuel efficiency and better performance? One of the answers to these questions is Aerodynamics. The companies always compete in making more aerodynamically sound cars. This post is about the basics of Car Aerodynamics .

Why is Car Aerodynamics so important?

As the price of petrol keeps on increasing, there is high demand of higher fuel efficient cars. Aerodynamic efficiency is the key aspect that boosts fuel efficiency. Making the car highly aerodynamic means reducing the resistance from the air as it travels. The car with higher aerodynamic efficiency will consume little fuel at given speed.

In this post, we are going to learn about basic aerodynamic forces on the car. There are two aerodynamic forces acting on a car: drag and downforce.

Drag

Drag is simply the resistive force that opposes the motion of car. To understand drag properly, one can imagine a car moving through a wall of air. There is not much resistance from air at low speed but when car starts to move at higher speed the resistance is quite considerable. The wall of air begins to feel like the wall of brick. Hence, drag is the major factor that limits the maximum speed a car can travel.

Expression of Drag force

As you can see above, velocity and drag coefficient are two important factors affecting drag force. Higher the speed, higher is the drag. This is the reason why aerodynamic design plays a vital role for making high speed cars. The value of coefficient of drag is less for more aerodynamic design. Thus, drag coefficient measures the aerodynamic efficiency of the car or any object. For example, a flat plate held perpendicular to the flow has drag coefficient of about 1.25. The most aerodynamic shape is the tear drop and it has the drag coefficient(Cd) of about 0.04. Talking about car, the lowest drag coefficient is achieved by Mercedes A-Class Sedan and the Cd is 0.22.

Mercedes A-Class Sedan- the car with lowest drag coefficient
Mercedes A-Class Sedan

Can we compare drag of two cars on the basis of value of cd?

 The answer is No. Suppose you have two cars with same drag coefficient, however they are not similar in size. In this scenario the bigger car certainly has higher drag than the smaller one. Thus, frontal area should also be taken into account while calculating drag. Therefore, we use another important figure called CdA (coefficient of drag multiplied by frontal area) while comparing drags of two cars and then find out which one is more aerodynamic.

Downforce

Have you ever wondered why cars don’t lift up while travelling at high speed? Downforce is what keeps cars on the ground. Downforce is actually the negative lift. In other words, the same law of physics that helps aircraft fly upward prevents the car from flying.

Here is the shape of airfoil of an aircraft that produces lift by creating low pressure area in the upper side and high pressure downward. This pressure difference creates lift.

Lift

If you want to learn about how lift is produced in an aircraft, do check out this post: https://www.geniuserc.com/how-aircraft-fly-and-aerodynamics-forces/

What if the shape is reversed? This will create higher pressure upward and low pressure downward and hence produces negative lift. This negative lift is called downforce. Therefore, car’s wing is shaped the other way around, so the air deflected underneath it travels at higher speeds and therefore create a lower pressure area than on top. The force that pushes the car down is called ‘’Downforce’’ and it results in better grip.

how downforce is created

Now let’s talk about Formula One cars. What do you think is the drag coefficient of F1 cars? If you think they should have very low drag coefficient, then you are totally wrong. Believe me, the cd of F1 cars are between 0.7 and 1.1. But we mentioned above that low cd is more aerodynamic. Why is it so?

Formula one racing Car
F1 Racing Car

For F1 Cars, the primary aim is to increase the downforce for better grip. Better grip produces better cornering speeds, harder acceleration and braking – and quicker lap times. Higher drag also contributes to the downforce. If there is not enough downforce, the car will end up flying and crashing. In general terms a Formula 1 car travelling at 150 mph (240-ish km/h) will generate around 1600kgs of downforce. Enough technically to drive upside down in a long tunnel! This shows how significant the downforce is.

F1 Car crashing due to insufficient downforce
F1 Car crashing due to insufficient downforce

To sum up, lift and downforce are two major aerodynamic forces on the car. We have only learnt the basic concept of car aerodynamics in this post. There’s still so much to learn about car aerodynamics like contribution of different car parts and shapes to overall aerodynamic efficiency of the car. There’s going to be more posts about Car aerodynamics soon. So stay tuned and feel free to give your thoughts in the comment.

Also check out this wikipedia link for more insight on car aerodynamics https://en.wikipedia.org/wiki/Automotive_aerodynamics

By Nishchal Poudel

Nishchal is currently studying Bachelor in Aerospace Engineering at IOE Pulchowk Campus, Nepal.

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