Golf Terminology: Golf Club Moment of Inertia (MOI)
The “Moment of Inertia” or MOI is a term thrown around very loosely in the golf industry. But when it comes down to it, MOI basically is how well-balanced a clubface. It can get a bit more complicated when you factor in how a golf ball reacts itself — and the clubface MOI (or twisting of the face at impact) then what will occur on an off-center hit as a result of the manufacturer’s engineering and your personal adjustments to the club, such as loft or swingweight.
The idea behind applying MOI to golf at its heart is just making sure that the face strikes the ball cleanly without without twisting too much, throwing off the ball’s motion so it doesn’t move efficiently — like straight and far. But throw in a round ball and it gets more complicated, yet the physics behind MOI essentially are the same.
Generally speaking, MOI is used in golf by distributing the weight of clubheads and balls outward to lessen twisting and other golf equipment and human variables (Hopefully, you’ll understand why as you read on and watch the video below.).
In physics, strictly speaking, MOI is a property that indicates the relative difference it takes to put an object in motion from a defined axis of rotation (Keeping up? See diagram below.). The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion. On the other hand, the lower the MOI, the less force is needed to make the object rotate about an axis. So let’s explain the whole axis and rotational stuff first.
(This is where golf club engineers get creative. But more on that at another time.)
Here’s a common example of MOI: The ice skater. When the skater starts a spin, she reaches out her arms and the speed of the spin is intentionally slow as it builds and she begins to pull her arms closer to her body. She is no longer resisting the speed of rotation, so her MOI is falls to a low point. It’s an inverse kind of formula since when she puts her arms out again, she slows and her MOI goes up higher as her resistance to the speed of rotation increases.
Now here is the official physics definition of MOI in golf:
“Moment of inertia is the name given to rotational inertia, the rotational analog of mass for linear motion. It appears in the relationships for the dynamics of rotational motion,” according to the Georgia State University Physics Department. “The moment of inertia must be specified with respect to a chosen axis of rotation. For a point mass, the moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, I = mr2.
“That point mass relationship becomes the basis for all other moments of inertia, since any object can be built up from a collection of point masses.”
“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.”
Not too bad to understand, right?
As far as golf goes, here’s a nice, down-to-earth explanation of MOI from Rotary Swing golf instructor Clay Ballard: