Why a Motorcycle Is a Harder Aerodynamic Problem Than a Car
A racing car generates most of its downforce from surfaces that stay in a fixed relationship to the road. The floor, the wings and the diffuser all remain more or less horizontal, so the force they produce points down through the tyres regardless of what the car is doing. A motorcycle has no such luxury. Its aerodynamic surfaces are bolted to a body that rotates through a large roll angle every time it turns. Downforce generated by a wing on an upright bike points at the ground. On a bike at full lean the same wing points its force largely sideways, pushing the machine toward the inside of the corner rather than pressing it into the tarmac. The benefit does not simply reduce with lean. It changes direction.
The rest of the problem is equally unhelpful. A bike has no flat underbody and no way to seal an underfloor against the road, so ground effect is unavailable. Its frontal area is small but extremely bluff, a narrow assembly of wheels, forks, engine and rider rather than a smooth shape. The rider is a large and variable part of that shape, tucking in and hanging off the side, so the aerodynamic configuration is never quite the same twice. Designers work with a handful of external surfaces on a body whose orientation and outline keep changing.
