# What Is MOI (Moment of Inertia) in Golf?

## The Golf Definition of MOI and Its Role in Golf Clubs' Fforgiveness'

The acronym "MOI" stands for "moment of inertia," and in golf MOI is a measurement of a club's resistance to twisting. The term is usually applied to clubheads, but can also be applied to golf balls and even shafts.

In layman's terms, a higher-MOI golf club will be more forgiving than a lower-MOI club. Why? It's that resistance to twisting.

Think of a driver impact in which the golf ball is struck off the toe of the driver. That impact creates force that pushes against the toe of the driver, causing the clubhead to twist a little bit (rotating the face open). Likewise, striking the golf ball toward the heel will cause the clubhead to twist back from the heel-side of the face. That twisting of the clubhead in response to off-center strikes leads to distance loss, and no golfer wants to lose distance.

But if the moment of inertia can be increased, the club becomes more resistant to twisting. Therefore, a higher-MOI clubhead will twist less on off-center strikes than a lower-MOI one, meaning less loss of distance.

The way that manufacturers boost a club's MOI is by playing with the weighting properties; any object will increase in MOI as more of its weight is moved outward, around its perimeter. (This is one reason perimeter weighting led to the game-improvement club category, and a reason why manufacturers today often use weight plugs around the perimeters of clubheads.)

The maximum allowable MOI rating (including tolerances) in a golf club under the Rules of Golf is 6,000.

### Getting Technical With MOI

The above is the plain-English explanation of the role of moment of inertia in golf clubs. Now, let's get technical. We turned to golf club designer and clubmaker Tom Wishon, founder of Tom Wishon Golf Technology, for that:

"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."

Wishon says we can better understand that technical definition by picturing a figure skater:

"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 skater'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 less resistance to the rotation."

There are actually multiple "moments of inertia" that can be measured on a golf club. But the one that companies tout in advertising and that golfers read about in golf magazines and websites has to do with the clubhead, its center of gravity location, and a vertical line we can imagine running through that CG location.

Or, in Wishon's terms, "the MOI of the clubhead about its vertical center of gravity axis."

Wishon continues:

"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."

We can sum that up this way:

• Higher MOI equals more resistance to the object being rotated around an axis;
• lower MOI equals less resistance to the object rotating around an axis.

Or, in plainer English:

• Higher MOI equals more forgiveness on off-center strikes with your golf club;
• lower MOI equals less forgiveness on off-center strikes.

### The Other MOIs In Golf Clubs

As mentioned earlier, there are more measurable moments of inertia on a golf club than just the one we are most familiar with (the one cited in ads and articles).

What follows was written for us by Wishon to explain those other MOIs in golf clubs:

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 that 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 center of gravity. This is the MOI golfers hear about and are most likely to know about. 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.