Question

Generally most of the mass of the flywheel os placed on the rim

Answer 1

Flywheels are used to store angular momentum, which is angular velocity times the moment of inertia. Now to store the maximum amount, you need either a lot of angular velocities or a large moment of inertia. Your angular velocity is limited as with higher speeds you have to build things stronger and then there’s vibrations and such and more wear and tear. So you want to maximize the moment of inertia. For a point mass, the moment of inertia is mr^2. So for every piece you add to the flywheel, the moment increases with the mass added, but also with the square of the radius, so you want to take advantage of this and add as much mass as possible as far out as possible. Putting more mass at the hub does you no good (other than having a strong hub).