[ThinkingPlus]

Let me take a stab at giving everyone a feel for moment of inertia. Let us take two wheels, one that is lightweight aluminum and the other is steel. However, we are going to manufacture our wheels different geometrically while keeping the overall weight the same.

The aluminum wheel will be solid. The steel wheel will be a ring. If you were to mount these wheels such that you could rotate them with your hand (like when they are trying to close the valves when the submarine is sinking) and then try to rotate them (as if the wheel were rolling), the following would be what you would feel.

The steel wheel would be harder (2X as a matter of fact) to get started rotating than the aluminum one even though they weigh the same and are the same diameter. Once rotating, the steel wheel will be harder to stop than the aluminum wheel. This is the concept of moment of inertia.

Now the concept of MOI matching with golf clubs is too make each club feel the same as you rotate the club along your swing plane, i.e., have the same moment of inertia. Since we have different geometry in play (clubs of different length vs. our wheels of identical size) we must modify something else to match MOI. In the case of golf clubs you redistribute and change mass (weight) to match MOI (typically adding tip or grip weights) just like we did with our wheels by concentrating all the mass in a ring (steel) vs. having it uniformly distributed as in the aluminum wheel.

The same concept applies with clubheads. A high MOI driver has the mass concentrated in the shell as far away from the center of mass as possible. Also, the mass can be concentrated on either side of a preferential axis of rotation to increase the MOI about that axis. This is an important thought to keep in mind: MOI is a quantity that is only defined about an axis of rotation. With putters or drivers this axis is typically about the perpendicular to the face of the club meaning it will be resistant to motion (twisting) about that axis.

The upside is that high MOI clubheads want to be square when you swing them (they resist opening or closing). The downside is if you do manage to get them open or closed during your swing, they will resist yours efforts to get them back to square more so than a club of lessor MOI. Food for thought.

Oh, well enough physics for today. I'm heading to the golf course.