January 1 2013

# What is MOI?

“Moment of inertia,” or MOI, is a property of physics that indicates the relative difference in how easy or difficult it will be to set any object in motion about a defined axis of rotation. The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion. Conversely, the lower the MOI, the less force needed to make the object rotate about an axis.

To understand MOI, think of a spinning ice skater. At the beginning of the spin, the skater extends her arms and the rotation speed is slow. As the skater pulls her arms in closer to her body, the speed of the spin greatly increases. Thus when the arms are extended, the skaterâ€™s Moment of Inertia is very high, and the result is a slower spin because the high MOI of the skater is resisting the speed of rotation. Conversely, the reason the spin speed increases when the skater pulls in her arms is that as the arms get closer to her body, the skater’s MOI falls lower and lower, creating little resistance to the rotation.

There are several different moments of inertia that are factors in the performance of a golf club. Remember, MOI has to first be defined by identifying what axis the object is rotating around. There is an MOI for the whole golf club which, when swung, is “rotated” around the golfer during the swing.

There are also three different MOIs which can be measured for the clubhead itself. Two of these MOIs are important in the design of any clubhead.

First, when you hit a shot off the center of the face, even though the head is secured to a shaft, the head will try to rotate around the vertical axis going through the clubhead’s centre of gravity. Second, and at the same time, when the golfer swings the club on the downswing, the clubhead is rotating around the axis through the center of the shaft.

The first example refers to the MOI of the clubhead about its center of gravity. In marketing terms, this is the head design property that has a bearing on the amount of “forgiveness” a clubhead offers for off-center strikes. The larger the clubhead, and/or the more the designer incorporates perimeter weighting, the higher the MOI of the clubhead about its center of gravity vertical axis will be. The higher the MOI of the head about its vertical CG axis, the less the head will twist in response to an off-center hit, and the less distance will be lost from that off-center hit.

The smaller the head and the more head weight is positioned close to the center of the head, the lower the MOI of the head will be around its vertical CG axis, and the more distance will be lost when the ball is hit off center.

Again, higher MOI = more resistance to the object being rotated around an axis; lower MOI = less resistance to the object rotating around an axis.