Now, lighten the assembly by three pounds by hollowing out its center (Figure 4). The MoI drops to 9.75 lb ft2, hardly any savings in inertia at all from the 10 lb ft2 calculated above. However, taking the three pounds off the outside of the rim (by dropping the diameter), will lower the inertia to 7.1 lb ft2-a 30 percent savings for only a 15 percent drop in weight (Figure 5). Now, only 3.08 foot-pounds of torque are required for a similar acceleration, unused torque that can be put to better use for linear acceleration of the whole bike.
The same theory applies to any part of a motorcycle that spins-sprockets, transmission gears, clutch plates, crankshaft. Not only is the weight important, but also how that weight is distributed about the axis of rotation. Now for the quick test: Which saves more inertia, a two-pound lighter set of brake discs (12-inch diameter), or a one-pound lighter tire (24-inch diameter)?
Not only are objects with a high MoI harder to spin, but also they are more affected by gyroscopic reactions, which can in turn adversely affect handling. The gyroscopic moment is a reaction felt when a spinning, body is rotated along the axis of spin (we've all done the bicycle wheel trick), and, essentially it redirects your input into a different orientation. The reaction is proportional to the inertia of the body, its spin velocity and how quickly the axis is rotated. In short, lowering the MoI of any spinning parts will lessen the gyroscopic reactions introduced, making your bike easier to turn from side-to-side.
Considering the entire motorcycle as an object now (Figure 6), there are three axes around which it can rotate: roll (side-to-side), pitch (front-to-back) and yaw (left-to-right). While pitch and yaw certainly affect handling to an extent, what we are most interested in is the roll MoI, because it determines how quickly a motorcycle can be leaned from one side to another, a measure of its "flickablilty." One part of the inertia equation, the mass, is addressed easily-less mass means easier turning. But the motorcycle's roll axis, the line about which it rotates when turning from side-to-side, is defined less easily. In a simplified analysis, the motorcycle rotates about the tires' contact patches, so it would make sense to centralize as much mass as possible close to that axis (low to the ground).
We will discuss gyroscopic effects as they pertain to cornering a motorcycle in a future issue, but consider for a moment that the front wheel is steered from under the motorcycle-in the opposite direction of the corner-entering a turn. This would mean that the axis of rotation (the roll axis) lies above the ground and closer to the bike's center of gravity (CoG) during at least a portion of the turn. But having the CoG too low affects a bike's behavior in the middle of a corner, once the bike is leaned in. Honda's 1984 NSR500, with its fuel tank underneath the engine and pipes over top, is an extreme example of lowering the CoG-and Honda reverted to a more conventional layout the following year. In this light, the current trend toward mass centralization (compacting a bike's heavier parts nearer to the CoG with less emphasis on simply placing everything as low as possible) makes sense.
Oh, the answer to the test. The lighter tire drops twice as much rotational inertia as the lighter discs.