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Originally Posted by visionary
What’s it all about?
Motorcycles with extensive or “full” bodywork are rare beasts indeed. Informed opinion, let alone genuine academic research into their aerodynamic performance is hard to come by. I’ve started this thread in order to open the debate and deepen the pool of knowledge, as well as supporting my own project, which could benefit from better sources of information.
If you have knowledge of good sources please post links. Personally I have found very little real scientific knowledge to be available. I do however come across quite a lot of misinformation and misapplication of aero principles from other disciplines.
My “pet” hate is the oft cited “centre of pressure must be behind the centre of gravity” which applies to bodies in free flight and is not directly applicable to road vehicles. Yet this phrase is routinely applied by “experts” in motorcycle dynamics. Even if true, it must be seen through the complex relationship between front and rear tyre slip angles, which I have never seen mentioned.
There seems to be a lot of conflicting theories about sidewind stability and how a low drag motorcycle would be affected. Is it more, or less prone to disturbance than a normal rider seated on a naked bike, and if so – why? My personal opinion is that the critical factor is body height, since a lower body also has a smaller moment around the contact patches.
I have been looking at some youtube videos of small-scale wind tunnels, with a view to building a simple version solely to investigate these issues. Making provision for a balance instrument that measures overturning forces at various angles of lean and yaw. I have never seen a motorcycle in a wind tunnel at anything other than upright and “zero yaw”.
Since the accuracy of numbers is not critical, we only need to understand the relationships, would a small-scale tunnel purpose-made for this task bring any benefit? For those with windtunnel knowledge, what would you imagine a suitable design to be – just to get a feel for the principles.
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*There is a very good SAE Paper that was published for the HONDA Hawk MC streamliner project which has heavy emphasis on directional stability.
*The Easy Riders' dual-engine Harley-Davidson remains the fastest bike I believe.It's body was tested at Texas Tech in 1990 and returned Cd 0.11.
*The E-Z HOOK MC streamliner is the fastest single-engine bike and its body was developed in the Cal Tech wind tunnel and was reported as the lowest drag of any wheeled vehicle ever tested there,at Cd 0.103 If memory serves me.
*Early 1950s (NSU,Moto-Guzzi,etc.) streamliners from Germany,Russia,and Italy in most cases had provisions for rider body re-positioning/weight shifting as an aid in crosswind conditions by way of body openings.
*The IHPV competitions and USFRA racing at Bonneville have limits for crosswind conditions on the course.I've seen cyclists in the weeds at Battle Mountain.They have no provision for weight-shifting inside the body shells and are at the mercy of gusts.