Technology: Wind tunnel comparison

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Technology: Wind tunnel comparison

King of the skies

Which Superbike offers the least resistance to the wind? Which sports tourer has the most sophisticated aerodynamics and offers the best wind protection? MOTORRAD examined five potential candidates for the king of the skies in the wind tunnel.

A click of the mouse on the PC in the control room is all it takes and the two 860 kilowatts (1170 hp) electric motors in the BMW aerodynamic wind tunnel howl, set the two huge fans in motion. Only a few seconds later a storm rages through the test section with wind speeds of up to 180 km / h. The system on the Aschheim measurement site draws electricity for tens of thousands of marks a month from the grid in order to provide a total of 3000 kilowatts for operating the two fans and the air cooler.
For two days, MOTORRAD settled in with the Munich-based company to test a new generation of superbikes and sports tourers for the aerodynamic tooth. As a reminder: the last time in 1993 were the sports tourers Kawasaki ZZ-R 1100 and Suzuki GSX-R 1100 in the wind tunnel (MOTORRAD 7/1993), the last superbike was the 1991 facelifted Suzuki GSX-R 750 (MOTORRAD 2/1991). The new generation of motorcycles is more chubby, rounder, their fairing has fewer corners and edges – an aerodynamic advantage that the current taste brought with it. So one could look forward to a new king of the skies.
Also this time a GSX-R 750 was involved, albeit from the 1996 vintage, and as the second representative of the superbike species a Ducati 916 Biposto. The aerodynamic marvels Honda CBR 1100 XX and Yamaha YZF 1000 R Thunderace as well as the new BMW K1200 RS competed as sports tourers – at least according to their manufacturers. In order to be able to better assess the wind protection, the fully clad touring steamer BMW R 1100 RT and the naked bike Suzuki GSF 1200 Bandit also went into the wind tunnel as reference vehicles, so to speak (see box on page 20).
The measurements of the air resistance and the lift on the front wheel as well as the assessment of the wind protection were carried out with the driver lying down and seated at a wind speed of 160 km / h. A pair of scales on which the motorcycle is fixed measures the air resistance that acts on man and machine and pushes them both backwards. This force increases with the square of the wind speed, that is, it is four times higher at 200 km / h than at 100 km / h, at 300 km / h it is nine times as high. The second factor besides the speed that affects the air resistance is the air resistance area (cw x A). It is therefore the job of every aerodynamicist to keep this factor as low as possible. This is the only way to convert the existing engine power into the highest possible top speed. The mostly unfulfilled dream of every aerodynamicist is that these relationships are clear to the responsible designer.
The size of the air resistance area – known from wind speed and air resistance force after the measurement in the wind tunnel – depends on two parameters: the frontal area A (the frontal projection area, it is determined with the help of a laser-optical measuring method) and the air resistance coefficient, the cw- Value. It is the measure of the aerodynamic quality of a vehicle.
The second force measured in the wind tunnel, also measured with a scale, is the lift on the front wheel, or in other words: how much the front wheel load decreases with increasing wind speed. This value depends crucially on the driver’s posture: At high speeds, a seated driver relieves the load on the front wheel more than a recumbent one, as he no longer supports himself on the handlebars, but rather pulls on it to hold on. The “lighter” the front wheel becomes at high speeds, the more unstable the motorcycle becomes on bumps and longitudinal grooves, and the more likely it is to oscillate.
With the Ducati there is no need to worry, with the rider lying down, she is the only one – and also well – below 100 Newtons (about ten kilograms) at a speed of 160 km / h: Her front wheel is relieved of only 68 Newtons. As expected, the slender Italian offers the airflow with 0.61 m² by far the smallest area to attack – with both a lying and a seated driver. It owes this not only to its slim Vau engine. The deep indentations on the tank, which perfectly integrate the driver’s thighs, or the silencers stowed under the rear fairing also make a decisive contribution to this small frontal area. A real surprise, on the other hand, is its second overall place in the “Cd value with the driver lying down”: Despite its tiny front panel, it effectively dissipates the airflow through the driver’s helmet and shoulders. Overall, it offers the least amount of resistance to the head wind, with the 916 calculating 50 hp enough for 200 km / h.
The second superbike measured, the Suzuki GSX-R 750, needs five more horsepower. Although it has improved compared to its 1991 predecessor (cw x A lying 0.359 m²), it still falls significantly behind the Ducati. The aerodynamically favorably styled rear fairing cannot change this either, which would have to be at least ten centimeters higher at the transition to the bench for a truly turbulent airflow – but this was probably not clear to the designer. Both MOTORRAD testers rated the GSX-R as worse than the 916 in terms of wind protection.
While the five horsepower that the Suzuki engine has to provide for 200 km / h more still sounds relatively harmless, for 300 km / h – because of the square and so – it’s a whole 17 hp more. Mathematically, the Ducati is enough for 300 km / h 159 hp, the Suzuki would have to lift 176 hp. Of course, neither of the two machines in the standard trim is capable of this, but at the World Championship run in Hockenheim the superbikes reach these speeds on the long straight forest straights, albeit under more favorable conditions: significantly slimmed down and without mirrors, indicators and other protruding components. So it’s no wonder that the Ducati, which are somewhat weaker in terms of top performance, but are slimmer, can keep up with the Japanese four-cylinder squadron even on high-speed courses. On tight courses, the Italians then take advantage of the strong start of their Vau-Zwo.
For a long time, the strong Kawasaki ZZ-R 1100 held the top speed record among sports tourers. Your aerodynamic key data with the driver lying down from the wind tunnel comparison 1993: Cd value 0.510, frontal area 0.666 m², results in a drag area of ​​0.340 m². In contrast, the weaker Suzuki GSX-R 1100 with its seven percent greater aerodynamic drag was lost. But in 1996 a dark shadow popped up in the rearview mirror of the fast Kawasaki, blinked and passed: the Honda Double-X.
The nominally 164 HP strong CBR 1100 XX has since carried the blue ribbon for the fastest production machine in the world. MOTORRAD determined 284 km / h (issue 22/1996) with real 162 hp. These two data already indicated that Honda has not only set new standards in engine performance. The confirmation then in the wind tunnel: With the driver lying down, the Doppel-X achieves a sensationally low drag coefficient of only 0.439. There are more than six hundredths of a second between it and the rest of the world of sports touring – a world for an aerodynamicist. Even with the driver seated, its drag coefficient remains just above that of the aerodynamically very good Ducati with the driver lying down. The decisive factor for the overall higher aerodynamic drag of the Honda is its significantly larger frontal area, with it having at least 53 hp engine power for the 200 km / h.
In Nardo’s high-speed oval, it is said to have been measured at 300 km / h, but mathematically it requires 170 hp – a value that, even taking into account the dynamic pressure charging in the airbox, can hardly be achieved through the two tiny slots in the fairing shell (The effects of a Ram-Air system are not taken into account on a performance test bench). Even removing mirrors and indicators is probably not enough to make the Doppel-X the first production motorcycle to crack 300 km / h.
The wind protection that the Honda offers is less intoxicating than the drag coefficient – at least compared to the BMW. Despite the absolutely identical frontal area with a lying and a seated driver, the K 1200 RS keeps the wind away better from the driver, especially from the seated one. When the windshield is up, the forces that pull the head, neck and upper body of the body are only nuances stronger than on the fully faired BMW R 1100 RT tourer.
With the smoke trail clearly visible, the K indicator tubes effectively divert the airstream in front of the fingers. From an aerodynamic point of view, however, it would have made sense to also integrate the mirrors in these ears – Honda shows how, even if only in a double function, because the flow-optimized mirror sockets with integrated indicators are of no use for the protection of the driver.
Another indication of the good wind protection of the K is that the drag coefficient with the rider lying down and the windshield folded down (the worst in the field of superbikes and sports tourers) is almost identical to that with a seated rider and the windshield folded up (the second best in the whole field ). It was obviously not the bare top speed that was as high as possible – at least a measured 254 km / h in the open version (issue 9/1997), but a high top speed that the driver could endure for as long as possible the development goal.
With the v-max of 261 km / h determined by MOTORRAD (issue 9/1996), the Thunderace is not exactly one of the slower ones. Your manufacturer always refers to the sophisticated aerodynamics that make this speed possible. In the wind tunnel, the Yamaha had to uncover its ace of thunder and realize that at least it didn’t make a trick against the Honda and the Ducati. After all, despite the comfortable two-seater bench, it beats the Suzuki in the cw value and only needs three horsepower more for the 200 km / h because of its larger frontal area. That the YZF 1000 R offers the worst wind protection among the sports tourers, is no surprise, as it emphasizes – more than the Honda and BMW – on sport.

Compare clothing – jacket like pants or the romper?

Every winter, MOTORRAD test driver Helmut Faidt is faced with the same tricky question: I’ll take the thin leather and get decent top speed values, but then I’m in bed with the flu, or do I take the textile suit and then I can’t save myself from reader inquiries want to know if my pants were full and why wasn’t there anymore? Measurement driver Faidt usually decides in favor of the reader and against the bed, and that’s a good thing. How many km / h would actually fall by the wayside with a textile suit up to now, nobody really knew. A test in the wind tunnel provided the answer: In the case of the Ducati 916, the drag coefficient with a textile suit decreased by almost six percent (assuming the same Face). This means that the Biposto no longer races through the light barrier at 261 km / h measured in a one-piece sports driver’s suit (MOTORRAD 5/1997), but purely arithmetically at 256, five km / h slower – at least for motorcycles with a fairing that is as tightly cut as the 916, the loss of top speed is likely to be roughly identical in percentage terms. For the next winter, measurement driver Faidt has a pocket calculator and a table with the air density depending on the temperature next to his phone, which has been improved with an aero combination specially tailored by Dainese for the wind tunnel comparison including the helmet spoiler made of transparent plastic (small picture) Cd value compared to a normal sports driver’s suit of almost two percent. The mathematically achievable speed would then be 262.5 km / h. A further optimization of the helmet spoiler with cardboard and tape (as shown in the large picture) did not result in any further improvement.

Disguise vs. Naked – That must be a pain in the ears

Motorcycles with no fairing are in, inhuman and aerdynamically considered complete nonsense: An invernal storm rages a few millimeters next to the ears, the airstream presses the helmet visor against your nose, and you grip the handlebars with your fingers in order not to back off the box The frontal area that you punch into the atmosphere on a naked bike, for example, is only about the same size as on a disguised motorcycle of the same displacement, but the drag coefficient is significantly worse due to the turbulence at the many corners and edges. At 160 km / h and the driver is seated, the front wheel becomes “lighter” by almost 25 kilograms. The upright Bandit driver theoretically needs 88 hp for 200 km / h, in practice it will probably bend the handlebars backwards at 199 km / h. When lying down, the air resistance drops enormously, so that 20 hp less is sufficient for this brand. On a fully faired touring steamer like the BMW R 1100 RT, the driver’s posture has little effect on the top speed. The boxer even beats the two superbikes and the Yamaha YZF – including the case and topcase – in the drag coefficient with the driver seated. Without a case and topcase, its drag coefficient even drops to 0.520. Only the massive cladding slows it down: With a frontal area of ​​just under one square meter, it has to compete against the wind, which demands 84 hp from it at 200 km / h. The idea is quite clever to keep the fingers of the BMW driver, but it doesn’t work effectively. A plume of smoke introduced at the cooling air inlet confirms the experience already made by MOTORRAD testers with the R 1100 RT: The distance between the hand and the air outlet is too large, the heat is lost beforehand.

Ducati 916 – The slim one

A V2 installed lengthways offers very good conditions for a slim motorcycle. In addition, the Ducati integrates the driver perfectly thanks to the tight knee joint, so that it has the smallest frontal area against the wind. In combination with its very good drag coefficient, it can shine with the lowest air resistance and also the lowest lift on the front wheel. The king of the skies is therefore called the Ducati 916.

Honda CBR 1100 XX – The sleek one

The Honda engineers had to make compromises in favor of a touring-compatible windshield – as far as the frontal area of ​​the CBR 1100 XX is concerned. In contrast, their striving for the best drag coefficient among all series-produced motorcycles, in order to convert the enormous engine power into a high top speed, was both uncompromising and successful at the same time. For the promised 300 km / h, however, a few horsepower are still missing both in practice and in purely mathematical terms.

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