Aerodynamics of cars

It is certain that in us, everyone has the opportunity to see cars of all kinds and colors, crowded in the ' car river ' when the road is blocked or seeing them sticking to each other on the highway. But perhaps few people understand, when moving, especially at high speeds, what forces are affected by cars.

In theory, when moving, cars must overcome many types of drag: rolling resistance, inertial force, friction force and especially the resistance of the wind as the vehicle rushes forward.

Rolling resistance relates to pavement quality, tire quality. Inertial force relates to the vehicle's mass and acceleration. Friction force related to materials, fabrication technology and lubricant. And wind resistance is related to the aerodynamic shape and speed of the vehicle. This is also the most complex type of resistance we want to cover in this article.

Aerodynamic resistance

The aerodynamic effect of a vehicle is determined by its drag coefficient (Cd). Simply put, the drag coefficient is the effect of the vehicle's shape on air resistance when the vehicle runs. In theory, a metal sphere with Cd equals 1.0, but if taking into account the turbulence effect of air behind it, that value is approximately 1.2. The lowest aerodynamic coefficient is for objects with teardrop-shaped shapes. The drag coefficient is 0.05. However, we can hardly build a car like that. Modern cars often have a Cd coefficient of around 0.30.

The resistance is proportional to the drag coefficient, the area of ​​the vehicle's nose and the vehicle's square. This means that a car traveling at 193 km / hour must win a four-fold drag on the vehicle's resistance when traveling at 97km / hr. And so, the maximum speed of the vehicle produces the maximum resistance. If we want to raise the maximum speed of the Ferrari Testarossa from 290km / hour to 322km / hour like the Lamborghini Diablo, without changing the shape of the car, we have to raise its capacity from 390 hp to 535 codes. force. If we spend a lot of time and money on studying the car's aerodynamic shape, it can reduce its Cd coefficient from 0.36 to 0.29.

Fastback (surfing tail form: since the 1960s, race car engineers really valued the car's aerodynamic form. They discovered that, if the rear slope of the car was reduced to 20 degrees or lower, the airflow will go down the hood smoothly and significantly reduce the resistance.They call this design ' fastback ', so many racing cars like the Porsche 935/78 ' Moby Dick 'lasted and lowered the overly imaginary tail.

With a sedan or hatchback, the airflow will curl up at the end of the hood due to the sudden drop of the rear of the hood creating a low pressure area, thus creating side air turbulence. behind the hood. This disturbance always has a negative effect on the drag coefficient.

If the corner of the rear hood is even more erect, the airflow will change dramatically, greatly affecting the stability at high speed. In a new era of development, carmakers did not understand this issue deeply, so they built some of them.

Lift force: according to aerodynamic theory, when the car runs, the air flow above the hood moves with longer distances than the air flow below the undercarriage, in front of faster: more behind, should follow Bernoulli's principle, Different velocities of the gas flow will generate pressure difference, which helps lift the vehicle to reduce the road's traction.

As well as drag, lift force is proportional to the floor area of ​​the vehicle, with the square of the speed and the lift coefficient (Cl) - this coefficient depends on the shape of the vehicle. At high speeds, the lift force can increase excessively and negatively affect the movement of the vehicle. The lifting force is concentrated mainly in the back, if the lift is too large, the rear wheels will slip, and so it is very dangerous, especially when the car runs at a speed of more than 200km / hour.

Therefore, it is very difficult to achieve drag coefficient and optimal lifting factor at the same time. However, people have studied very elaborately and found some optimal solutions to reduce drag and lift force to the lowest .

Measures to improve aerodynamic features

In order to improve aerodynamic features, minimize Cd coefficient, people often use the following measures:

Tail tail : In the early 60s, Ferrari engineers discovered that, by attaching a barrier (we simply call WIND) to the rear tail, the lift force could be significantly reduced or even Releases generate compressive forces. Meanwhile, the drag force only increases a very small amount.

The wing acts to guide most of the air flow on the hood to escape straight back without turning back, thus reducing lift. If you increase the angle of the wing, you can increase the compressive force even to 100kg. At that time, there was only a very small flow of air running behind and under the wing. Thus, the wing significantly reduced the air turbulence that appeared in a non-fastback vehicle, and eliminated the lift, the vehicle was only subject to resistance.

Underwing wings: The underwing wing is the general name of the windward wing mounted below the front bumper and the windward wing mounted along the side of the vehicle. The undercarriage under the nose of the vehicle has the effect of transforming the air flow under the vehicle. We often call the mounted wings at the bottom edge of the front bumper as 'front wind bumper'. And the side shields are 'horizontal wind screens'. To understand their effects, let us first analyze the air flow on the underfloor.

The airflow at the bottom of the floor is always undesirable. There are many parts such as engines, gearboxes, steering axles and some other parts exposed to the bottom of the car. They will prevent airflow, which is not only the cause of turbulence that increases the resistance, but it also slows the flow of air and boosts the force of Bernoulli's principle.

The wing and horizontal wings are used to reduce the airflow below by directing air through the sides of the vehicle. As a result, they reduce drag and lift due to the lower air flow generated. In general, the lower the wing, the higher the efficiency. That's why you see racing cars with wings and a wing close to the road. Of course with common cars it is impossible to do so.

Smooth car boom : We can also reduce the impact of the lower airflow by making the undercarriage smooth to avoid turbulence and lift.

Road effects : For racing engineers, tail wings can be a good solution to reduce lift, but they are not what they really want. A Formula 1 racing car went off for about 4 seconds after accelerating, which required compression to keep the wheels clinging to the road. Installing a wing with a large angle can meet this requirement, but it increases the drag coefficient.

In the 1970s, Collin Chapman found a completely new way to create compressive forces without affecting drag. That is the pavement effect. He created an air path at the bottom of his Lotus 72 racing car. This air path is quite narrow in the front and extends gradually behind. Because the chassis is close to the road surface, the combination of the air path and the road surface creates a tunnel that is almost closed. When the vehicle is running, the air entering the tunnel from the side of the nose and then escaping straight to the rear causes the air pressure to gradually decrease towards the rear and thus generate compressive force.

The pavement effect has a better effect than the tail wing and it was soon applied to Formula 1 cars. In 1978, Brabham (the famous F1 McLaren builder) applied to his Gordon Murray vehicle by the way. On the other hand, instead of the expanded airway, he used a large-capacity blower to create low pressure near the rear. Of course the FIA ​​(International Automobile Federation) has recognized that. However, the pavement effect is not suitable for common cars. Because this type of car needs to have a high chassis to suit common roads and so the pavement effect is almost ineffective. In general, testers do not consider this a good solution to create compression even though there are cars like the German Duner 962 that can adjust the height of the underbody to take advantage of the pavement effect and reach compressive force up to 40%.

Some world records on the coefficient of blocking Cd

Model CdFord Probe V (1986)0.137GM EV1 (1996)0.19Mercedes-Benz C111-III (1978)0.195OpelCalibra(1989)0.26

0.27 VW Passat (1997) 0.27 Lexus LS400 (1997) 0.27 BMW 318i (1998) 0.27