At the speeds that today's performance motorcycles are capable of, aerodynamic drag is the greatest force that must be overcome to push that limit. Rolling resistance and weight do play a part, but at higher speeds those forces are an order of magnitude less. John Bradley, in his book "The Racing Motorcycle: A technical guide for constructors," goes into considerable detail on the subject and shows that while power required to overcome aerodynamic drag increases proportionally to the coefficient of drag and frontal area, it increases with the cube of velocity. This means that making your bike smaller or more slippery by 10 percent will increase top speed by approximately 10 percent, but increasing horsepower by 10 percent will only increase top speed by 3 percent.
I have done hundreds of top-speed runs as part of performance testing for Sport Rider and Motorcyclist magazines, and know firsthand just how strong aerodynamic drag can be at speeds upwards of 250 km/h. The tiniest change in frontal area by tucking a knee or elbow in a few centimetres from the wind can make a difference of more than a few kilometres per hour. And yet project bikes with 10 or even 20 horsepower more than stock are lucky to show that kind of improvement in top speed.
Modern production sportbikes are usually incredibly stable at high speed. In the right conditions, little input is required during a top-speed run other than holding the throttle to the stop and experimenting with body position to find the last bit of speed. During a comparison test with multiple bikes, it can almost become routine and a bit boring. Almost. If conditions are not perfect - a slight cross wind or a bumpy surface, for example - top-speed runs can easily become the most stressful and dangerous part of performance testing. Things can go very wrong, very quickly at those speeds.
Top speed for production motorcycles has been almost a moot point for many years now, as most bikes are limited to a maximum of 300 km/h. When the Suzuki Hayabusa and Kawasaki ZX-12R were first introduced 15 years ago, there was enough of an outcry over spiraling performance levels that a gentleman's agreement was put into place capping top speed. Without the pressure of increasing top-speed capability with each model release, manufacturers have developed bikes accordingly; simply derestricting a particular bike is not necessarily a true indication of its top-speed potential.
All that in mind, it's a safe bet that the Ninja H2, the road-going version of the H2R that is set to be introduced next month, will be limited to a top speed of 300 km/h. The H2R, however, is not intended for street use and is not bound by agreements or other concessions to street use, and there have been indications that its top speed is well over 320 km/h. Just how fast will it go?
Current litre bikes, unrestricted or otherwise, are typically capable of close to 300 km/h with 200 horsepower. It's unlikely that the H2R - with elaborate wings for downforce and only a three-quarter fairing - will have an aerodynamic profile superior to a litre bike, leaving any increase in top speed dependent on its higher power output. With horsepower requirements increasing with the cube of speed, this gives a top speed for the H2R in the region of 340 km/h. That is a significant increase from what we have seen in the past from a production motorcycle; when more details on the H2R and H2 are available, it will be interesting to see what Kawasaki has done in addition to the elaborate wings to accommodate such an increase in top speed.