Technology: aerodynamics and acoustics comparison

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Technology: aerodynamics and acoustics comparison
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Technology: aerodynamics and acoustics comparison

Technology: aerodynamics and acoustics comparison
Close to the wind

Cycling is generally a windy affair. How hard the driver sails on the wind, however, depends on the machine concept. MOTORRAD examined it using the example of a super tourer, a super sports car and a naked bike.

Waldemar Schwarz

05/19/2004

An eerie silence surrounds the tester, who crouches on the motorcycle in the soundproofed room of the BMW acoustic wind tunnel. Suddenly a mild breeze announces that the 3000 HP blower has started its work. The gentle breeze gradually grows into a thundering hurricane. With different types of motorcycles and two riders of different sizes, the scenario repeats itself over and over again. The opulent Superttourer from BMW, the K 1200 LT, has practically a home game
their shell was developed on site. For the guild of super athletes, the Kawasaki ZX-10R fight the elements while the Yamaha XJR 1300 represents the category of naked bikes.
It is clear that a large tourer offers more resistance to the wind than a fully disguised super sports car. Also, that the Supertourer offers the rider much more protection from the wind than an undisguised motorcycle. But the wind
has a host of other influences on the motorcycle and its rider. What about the acoustics or driving stability of the various motorcycle categories? Do small drivers really always have an advantage over big drivers when it comes to aerodynamics? Is a disguised motorcycle always quieter than an undisguised one? Question after question, which MOTORRAD examined on the aerodynamic test bench and in practice.
In order to describe the aerodynamic qualities of the motorcycles by measurement, scales in the three spatial levels determined the forces that arise at a wind speed of 160 km / h. They can be used to determine the air resistance and the lift on the front and rear wheels. And from the air resistance and the frontal area, the
Calculate the drag coefficient, which indicates how streamlined a body is. The biggest differences between the three test persons can already be seen in the frontal area. With a driver who is 1.72 meters tall and adjusts the window in the middle position for comfort, the BMW continues K 1200 LT an area of ​​more than one square meter against the airstream. The Kawasaki punches a hole of just 0.62 m2 in the air. In both cases, the values ​​change only minimally when the driver crouches. Not so with the Yamaha XJR 1300. Depending on the pilot’s seating position, the frontal area varies between 0.72 and 0.81 m².
The determination of the
cw values. With a value of 0.57, the thick K 1200 LT with the steep disc is aerodynamically just five percent behind the slim, aggressively crouching Kawasaki ZX-10R. The aerodynamicists at BMW have obviously done a great job. As expected, the Yamaha XJR 1300 falls behind. With the driver folded, it is surprisingly on par with the BMW, but when sitting upright it falls far behind with a drag coefficient of 0.8.
What does that mean in practice? For a speed of 200 km / h, the BMW has to mobilize 82 HP to overcome the pure air resistance without any other driving resistance, the Kawasaki only 47. With the Yamaha, the airflow from the engine requires between 65 and 80 HP for 200 km / h, depending on the position of the driver. H. Serious differences that not only affect driving behavior, but also fuel consumption.
And what influence do drivers of different sizes have in this game? After the first series of measurements with a 1.72 meter pilot, the 1.88 meter comparison driver braced himself against the hurricane. He also raises the windshield of the BMW until optimal wind protection is achieved? with him it is extended about two thirds. Nevertheless, the air resistance only increases by one percent. This minimal difference between the two drivers also results from the Kawasaki in the folded position. And that although the tall driver does not sort his elbows in front of his knees, but rather next to him, so the flow around his body seems to be more favorable. In contrast, the difference is six percent with a normal sitting position? the frontal area then increases visibly. With the Yamaha, the differences lie in both seated and lying riders
at two percent each. Conclusion: While driver size does not play a major role at BMW, the tall one has to be at Kawasaki
and Yamaha open the throttle further to keep up with the short.
Which is of course much less important than the influence of wind on driving stability. The wind pressure acts as a force on the entire front surface. It causes a torque around the transverse axis of the motorcycle, which unloads the front wheel and loads the rear wheel. Driving stability suffers from the changed wheel load distribution, especially at higher speeds.
On the huge BMW, the front wheel is already relieved of 17 kilograms at 160 km / h, which is not particularly significant in terms of driving dynamics given a total wheel load of 196 kilograms. The driver of the K 1200 literally enjoys the calm in the eye of the hurricane while the elements rage around him. At higher speeds, the current rolling in behind the driver merely pushes the upper body gently forward.
A completely different impression emerges on the Yamaha. With it, the driver’s upper body absorbs a large part of the approaching wind. The front wheel of the XJR 1300 is relieved of around 25 kilograms at 160 km / h with one
Wheel load of just 138 kilos. And another phenomenon reduces the driving stability of the Yamaha. In order to defy the storm, the driver has to absorb the force acting on his upper body with his arms on the handlebars, whereby he himself continuously introduces unwanted steering impulses into the chassis.
The front wheel of the Kawasaki (wheel load 126 kilograms) is already relieved of 18 kilograms at 160 km / h. As the driving tests showed, however, it remains largely unaffected by these airstream influences up to the highest speed regions, especially since the driver’s wrists tend to be relieved rather than strained at higher speeds due to the forward-leaning sitting position.
The impressions of the wind tunnel meeting were also confirmed in practice by BMW. The driver travels on it as if in Abraham’s lap, and only beyond 160 is he gently pushed forward by the pressure of the wind. It goes without saying that the Kawasaki pulls a lot harder in a normal sitting position. Although the upper body is in the slipstream of the panel, there is strong turbulence on the head. Crouching behind the fairing, the wind protection increases, but so does the turbulence in the helmet area. The strong relief of the front wheel and the uneven holding forces introduced into the handlebars by the driver lead to a permanent tendency to oscillate from around 180 km / h when driving on the motorway with the Yamaha. In addition, such a speed can only be achieved for a short time due to the extreme fatigue of the pilot.
What about the wind noise? In addition to the two test drivers, the colleague with the artificial head now makes his appearance. With this special dummy, microphones in the ear area of ​​the helmet record the driving noise. Although there is a lot on the ears with all three motorcycles, the differences are enormous. On the BMW, the sound pressure level rises between 100 and 180 km / h from 81 to 97 decibels (dB) ?? a huge one
Level that is subjectively perceived to be far more than twice as loud (ten decibels are considered to be doubling the volume).
Compared to the Yamaha and Kawasaki, however, this is almost a background noise, on the XJR 1300 alone it is almost as loud at 100 km / h as on the K 1200 LT at 180. To everyone’s surprise, the ZX-10R continues despite the fairing 102 dB at 100 km / h one more. At 180 km / h, the levels on the Yamaha and Kawasaki swell to over 110 dB (see diagram on page 46). It behaves in a similar way to the wind protection: While the flow of the XJR hits the rider’s head in a laminar manner, the cladding of the ZX-10R creates strong and loud turbulence in the helmet area. Cladding therefore requires considerable detailed work in the wind tunnel in order to optimize not only the wind protection but also the noise level. The test procedure with the artificial head provides practical measurement values ​​for this; In any case, the human testers were able to confirm the results of their artificial colleagues. In the area of ​​maximum speed with the Kawasaki and the Yamaha, downright infernal sound pressure levels occur, which sooner or later lead to hearing damage. Nobody should do without earplugs, not even when driving a comparatively quiet BMW.
At least there is still a lot to do for the helmet manufacturers, because whether someone wants to travel comfortably in the relative silence of a K 1200 LT or sweep the highway at a brisk pace on the ZX-10R should be left to the personal taste of each individual in the future.

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