The minimum weight of a four-cylinder MotoGP racebike is capped at 148 kilograms, or 327 pounds. In World Superbike a four can weigh no less than 162 kg. (357 lbs.), and a twin can't tip the scales below 168 kg. (371 lbs.). The main reason behind these regulations is to limit costs, as lightweight parts often carry heavyweight prices. As it is, there's plenty of titanium and carbon-fiber on these bikes. Weight limits have changed the game when it comes to mass. Instead of reducing mass, the designers of these bikes work to place mass in the best location. The game is now mass centralization, which sounds like a simple program. It's not.
A motorcycle in motion moves in roll, yaw and pitch. Bank into a corner and the bike rolls around a roughly horizontal axis oriented from front to rear. In the corner the bike yaws, rotating left or right around a vertical axis. As the bike accelerates or decelerates there's movement in pitch around a horizontal, lateral axis. Pitch doesn't directly affect the direction of travel, but it's got to be controlled-think of the downsides of an uncontrolled wheelie.
The closer the weight is concentrated around these axes, the easier it is to rotate the motorcycle around them. In a counter-example, a man walking a high wire between two skyscrapers doesn't want to rotate around his roll axis-he doesn't want to lean too far left or right. To help slow his rotation, he carries a horizontal pole that provides a mass (on each side) that's relatively distant from his roll axis. The pole makes it easier for him to balance because it makes it harder for him to rotate around his roll axis.
As we move the mass of the motorcycle along these axes, we are moving toward the center of mass in each direction. CAD makes it relatively easy for designers to know the effect of their efforts at mass centralization, as the weight and weight distribution of each part and each assembly can be calculated by the computer.
Single-shock rear suspensions, with the damper mounted just aft of the swingarm pivot, have been an important element in mass centralization. The old dual-shock suspensions not only had the dampers and springs mounted far to the rear, but the weight of those components was far from the motorcycle's centerline. Another significant improvement in mass centralization results from putting the fuel tank behind the airbox, moving a heavy object rearward and a light object forward. The vertical stacking of transmission shafts in new engine designs is an effective mass-centralizer, moving the crankshaft rearward relative to the transmission.
The fuel tank itself is an important mass element, and one that can present special problems. A sportbike with a 5-gallon tank weighs 30 pounds more with its tank full than empty, and that's a big percentage of the total weight of a 400-pound machine. Put the tank too far fore or aft and the front-rear weight distribution may change more than you want as the tank empties. Make the tank tall and slim and the height of the center of mass may change too much.
One aspect of mass centralization that's not dealt with much is the centralization of steered mass. In this case the rotational axis is the steering head, and if steered mass can be reduced, steering effort will decrease and the chance of unwanted steering oscillations (headshake, tankslappers) can be lessened. With a telescopic fork, the entire weight of the front suspension is steered, making for relatively high steered mass.
Much of this mass centralization involves choosing a desired center of mass and moving any weight possible closer to that point. The latest Husaberg engine represents a radical approach. Its heavy crankshaft has been moved considerably closer to the center of mass, but has also been moved up by about the same distance. In this case the center of mass is no longer a fixed target, because moving the crank closer necessarily also raised the center of mass. Here we have a case in which the benefit of mass centralization must be weighed against the possible detriment of a higher mass center. I've heard both praise and criticism of the Husaberg's handling, and I think we don't yet know the verdict.
Kawasaki has been thinking along the same lines, as its sportbike, dirtbike and ATV versions of crank-over-transmission designs from a 2006 patent application attest. Will Kawasaki be as innovative as Husaberg and start putting its mass where no one has put it before?