In the beginning we will look at the very principle of the car aerodynamics. We will show that sometimes even development sections of car companies do tuning. In the beginning we are showing You two pictures, which very logically demonstrate the difference between the aerodynamics of Formula or opened roadster and race car with roof. And then, try to imagine, how would be these schemes look like on the usual sedan.
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Maybe, You are asking what have cars in common with dangerous tuning, but this will be discussed later. Let us assume that so-called aerodynamic envelopes drawn around and above the cars remind us of a wing profile. For those who managed the ABC of physics let us recall how does a wing function. The air at the leading edge flows at some definite speed (the speed of a plane) which is actually the same as the speed of air flow behind the trailing edge. We are interested in what is going on between these edges. The path of the air flow over the wing (over the curved side) is longer than the path of the air flow under the wing. The more is the wing curved on the top, the longer distance must the air travel and the air is forced to expand into a higher speed. So, according to the Bernoulli Effect we can say that the greater aerodynamic envelope produces greater net upward force or lift (car is lifted off the road). This is possible only if some angles remain unchanged etc. But for this very special example our simplified projection will be enough to understand this process. To understand the dependence of the lift on the height and the shape of the car, we are introducing You the net upward force (lift) of some series cars:
- VW Beetle model 2000 – lift 370 kg at 190kph
- Audi TT model 2000 – lift 195 kg at 230kph
- Porsche 911 model 2000 – lift 300 kg at 240kph
…they could be easy in trouble.
A remark – we are still talking about the car body and the question of the wings remains open.
An approach angle is also very important. The ideal is the 2°-5° approach angle of the front part. As a result, a vacuum is produced under the car. But the trouble will appear in the moment, when we do tuning, when we recline the front part of the car body. If the car hits some hole on the way, the front part of the car will lift sooner than the rear part, and at that time a vacuum under the car will turn into pressure above atmospheric and the pressure of the car body will turn into the lift.
So, what will happen at the speed above 300km/h, if the aerodynamic tuning complements are installed? Roughly speaking, maybe this:
Mercedes CLR flight at Le Mans circuit 1999
So, as you see, also car companies can sometimes do a good tuning. As for the illustration – the lift estimated at the time when the car left the ground was above 2000kg in all cases. This is approximately twice as heavy as the weight of the car and heavy enough to throw a typical sedan off as a tennis-ball. We can confirm that at the speed above 280 kph the lift in the car is considerably notable. So as You see, even the huge wing had no influence on it – more possibly seems the fact that the wing contributed to it, as it is shown on the following picture for those initiated:
So, as soon as the front part of the car begins to lift, it lifts more and more and finally the car flips over backwards. Today we have thought a little about the car body itself and about the reasons, why Renault Scenic can't be modified into super beast neither by installing MHC on the fifth door, nor by emplacing the wings on the roof.
And now we have come to another tuning specialty – as low as possible rear bumper. At first we will explain the principle of the diffuser and then, maybe, everybody will understand that rear bumper put close to the ground is another tuning mistake.
The air flows through the gap between the car and the road, and we never will be able to stop it. Even the super cool tuning-bumper will be helpless. The air under the car increases its flow, creating a low pressure. If we don’t want to accumulate the air under the rear part of the car, we should allow the air to get out. And this cannot be done by putting there a bumper which is too close to the ground. On the contrary, we should open it!!!
The vertical-flat skirts are necessary to avoid mixing air flows with different relative speed rates and pressures (the speed of air alters from the front part to the rear part and from the centre to the sides).
As soon as a diffuser allows the air traveling underneath the car a place to expand, this free place works as a pump, drawing up the air from under the car. At the same time the diffuser reduces the air’s speed to the speed rate of the air behind the car. This helps to reduce the aerodynamic resistance of the car and to increase its downforce. So, if you spread the rear bumper close to the ground – it is a big mistake, it is the same as if you covered the radiator grille, failing to cool the engine. The fact that this is really a good thing proves the rear part of Enza on the picture.
Some examples of circuit cars aerodynamics:
What's your opinion on this article?
TUNING FORUM - Car aerodynamics
