Motorcycle Aerodynamics | Aero By Numbers | Drawing The Line

The aerodynamics of racing motorcycles are determined more by the rules of the racing organizations than by the principles of efficient design. By dictating what may be covered and what must be exposed, the rules don’t give much freedom to designers.

So when a bike comes along that’s claimed to be “perhaps the most aerodynamic roadracing motorcycle ever,” we take notice. The newest version of the MotoCzysz e1pc was introduced just before this year’s Isle of Man TT Zero race, where it became the first electric bike to lap the 37.73-mile Mountain Circuit at more than 100 mph. This bike is basically the firm’s 2011 model with perform- ance upgrades in chassis, electrics and electronics, topped with new aerodynamic bodywork.

That bodywork is characterized by inlets, outlets, winglets and swoops reminiscent of an open-wheel racecar’s aero complexity. An open-wheel racecar and a motorcycle are both aerodynamic “messes,” in that in both cases the rules don’t allow the use of a “clean,” streamlined shape but require severe aerodynamic compromises. On the car, the uncovered wheels deny smooth airflow overmuch of the car, while on the motorcycle, the uncovered rider is the primary culprit.

Thinking about the car-bike aero connection, I called Swift Engineering in San Clemente, California, to get an idea of the current state of aerodynamic design in road and racing vehicles, two-wheel and four. Swift has decommissioned its wind tunnel and has instead been using Computational Fluid Dynamics (CFD), a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flow.

The wind tunnel gave rapid results. In minutes you could check different parts, different ride-heights and altered riding positions. Swift even had drag readouts on bikes in the wind tunnel, so riders participating in the tests could see in real time what the effect of changing their position on the bike could do to reduce drag. They found, for example, that the exact arch of the rider’s back was perhaps the most critical variable. Small things like whether the rider’s leathers were worn inside or outside his boots were noted. Ever wondered why Valentino Rossi wears his boots inside his pants legs? But running both a wind tunnel and CFD is an expensive process, and Swift found that long-term planning was more effective with CFD.

CFD may not give immediate answers, but it provides information that can be planned from. The analysis of fluid flow can be massively complex. Imagine the situation in which a vehicle is following another, moves out to pass, and then pulls in front. Passing wins races, so it’s important. A wind tunnel built to realistically model this situation would be prohibitively more expensive than a single-vehicle tunnel. Yet Swift has modeled this situation in great detail in CFD, working its supercomputer for many days at a time to perform the many millions of calculations necessary to get results that can help a vehicle pass another more easily.

CFD is thus scalable, working with vehicles and scenarios that would not fit in a wind tunnel—a definite plus. A minus when working on motorcycle CFD aerodynamics is the fact that, while there will be a CAD model of the bike available, an accurate model of the rider is also necessary, including leathers, boots, helmet and all, and ideally the model rider should be allowed to change position on the bike. Swift Engineering works primarily with cars, the hard surfaces of which don’t have the “soft” rider to deal with, and which may be more suited to CFD analysis. The bike-with-rider may still have to go to a wind tunnel for best results, but accurate rider models are certainly possible.

Is the MotoCzysz the most aerodynamic roadracing motorcycle ever? We don’t have numbers to compare the old bodywork to the new, or to compare this electric bike to an internal-combustion racer, but those numbers would certainly be fascinating.

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