Basically, there are two main driving resistances that prevent us from going infinitely fast: rolling resistance (FRoll) and air resistance ( FAir). These are two forces that act against the vehicle's driving force on a flat road and literally slow the car down.
Source: Wikipedia (https://bit.ly/2EVmSyR)
The way these resistors work depends heavily on the vehicle's speed. If you're traveling at approximately 30 km/h, rolling resistance accounts for about 90% of the total resistance, while air resistance accounts for only about 10%. So you could even drive with a wall unit mounted on the roof of your car without significantly increasing your fuel consumption—provided the wall unit is lightweight.
At a speed of approximately 100 km/h, the effect of air resistance is already a remarkable 63%; at 160 km/h, it is already 81%. At even higher speeds, the vehicle's driving force is practically spent solely on overcoming air resistance.
In order to develop a roof box that does not negatively affect the vehicle's performance even at high speeds, an aerodynamic shape is essential.
Contact pressure
Sporty vehicles, in particular, generate contact pressure due to their design. Although this increases fuel consumption, the force acting perpendicular to the road enhances performance in curves and improves safety. This is a factor that should not be underestimated, especially for vehicles in the high-performance segment.
Fun Fact: A Formula 1 car generates so much downforce at high speeds that if you turned the track upside down, it would still “stick” to the asphalt.
So far, so good. We have taken Mother Nature as our model for implementation, because nature is the best guide when it comes to efficiency. Over millions of years of evolution, nature has adapted perfectly to its environment. This insight has now also found its way into the world of engineering. So it is no coincidence that a penguin, shaped like a teardrop, has the perfect aerodynamic shape.
Result
The result is impressive: an optimized roof box, made in Germany, with no compromises. Constructed from CO2-neutral, high-end natural fibers. Designed to create a perfect symbiosis between the car and the roof box.
Source: CFD support (https://bit.ly/3lKLpro)
The drag coefficient (Cw) of the ASPHALTKIND roof box is approximately 0.11, which is about 30% lower than that of conventional roof boxes. That might not sound like much at first. However, it’s important to note that the geometric design generates downforce rather than lift, as is the case with conventional roof boxes. In numerical terms, the roof box exerts a vertical downward force of approximately 110 N (approx. 10 kg) on the vehicle at a speed of 160 km/h. By comparison, a conventional roof box pulls the vehicle slightly upward. This reduces traction when driving on the highway and therefore increases the risk of accidents, especially on wet roads. The result can be illustrated as follows:
Conventional roof box with a large dead water area (black spot)
Conventional roof boxes exhibit high turbulence intensity (turbulent = bad) in the rear section. This results in a large so-called dead water area, which occupies approximately one-third of the roof box’s length, before the streamlines converge again into a laminar flow (laminar = good). Such turbulent effects have a negative impact on the roof box’s performance and lead to increased fuel/energy consumption due to the negative pressure that is created.
ASPHALTKIND roof box with a significantly smaller dead water area
With the ASPHALTKIND roof box, the aerodynamic teardrop shape results in significantly less flow separation at the rear and, consequently, significantly lower fuel and energy consumption. The result is a much more uniform flow pattern with a pronounced laminar flow. Just like a penguin.