Moment of Inertia: What Is It?

How Does It Relate to Clubhead Playability?

There is no question that the buzzword today for all golf clubs, metals, irons and putters is moment of inertia (MOI). Problem is most golfers do not understand what it is and probably a number really don’t care. Well, you need to care because an understanding of MOI can really help you play better and also help you to select equipment where increased MOI can be a big help.

MOI is a clubhead’s resistance to turning or rotating when it impacts a ball.

Moment of inertia or in our case more correctly referred to as rotational inertia is simply an object’s (clubhead) resistance to turning or rotating when acted on by some outside force (ball). If you strike a drive, a putt or an iron shot off center, the clubhead has a tendency to rotate. Its face rotates open on a toe shot and it rotates closed on a heel shot. This off center impact causes the clubhead to rotate around its own center of gravity and not around the golf shaft as most golfers think. The center of gravity of the clubhead is usually located on or very close to the clubfaces horizontal geometric center. This is obviously where we all try and hit the ball because it produces the most solid hit, solid feel and best distance.

Let’s take two different clubhead designs. One is a traditional shape muscle back iron with much of its weight placed near the middle of the clubhead. The other is a modern longer blade length cavity back iron with more of its weight distributed in the toe and heel areas. The MOI’s of these two irons will be different. The muscle back would probably have an MOI reading some 25% or more lower than the cavity back which causes the muscle back to rotate open or closed more easily on off center hits. More specifically, what this means to the golfer is that a shot hit the same amount off center on both irons would produce two different results. Using the cavity back as the comparative example, here is what the golfer would feel and see at impact; the cavity back would feel more solid, the ball would go farther, have more backspin, fly higher and it would go straighter. This would be a lot to give up for any golfer who simply must play with older design traditional blade type irons for whatever reason.

Higher MOIs are important in all golf club designs. This however is especially true for putters. Putting requires preciseness in both distance and direction control of the putt. Where the ball impact occurs relative to the center of the putters face is generally a function of how long the putt is and the golfers’ ability. The longer the putt and the lesser the golfers’ ability the more offset the ball is hit on either side of face center. Tests show impacts as much as 1” on either side of face center can occur. Touring professionals strike their putts in a ¼” circle on the putter face, but even a plus one handicap player will use a 5/16” to 3/8” circle for impacts. Touring professionals do not need higher MOI putter heads nearly as bad as all the rest of us. Remember, when I talk about face center on a putter, I am assuming that this is the putter heads center of gravity location. More simply stated, this would be the horizontal location where the putters face would balance while placed on a pointed object such as a pencil held vertically in a vise.

Look at the MOI demonstration dumbbells in the graphic built to show golfers visually what MOI is. Assume for a moment that each of these dumbbells shown are putters. They are each made up of three different parts and each part in both examples is exactly the same weight. This also means that each dumbbell is exactly the same weight. However one dumbbell has its two discs placed on the ends of the bar and the other dumbbell has its two discs placed on the inside of the bar. I put both dumbbells on my MOI machine. The result was that the dumbbell with the discs on the ends of the bar had an MOI that was over 6 times greater than the one with the discs toward the center. If these were really putters, which one would you like to putt with?

I have four different putting robots in my design studio and I have done and still do extensive putter testing. I also use a high speed camera to capture data which has the capability to shoot at 1/40,000th second shutter speed and 2000 frames per second. When I hit 24 foot putts with a very high MOI putter using toe, center and heel hits, all three balls sometimes go into the hole. The center hit always goes into the middle of the cup and the toe and heel shots will sometimes fall in on the right of left edge of the cup. If any ball misses only a tap-in is required for a two putt. With the exact same putter changed to a very low (50% less) MOI putter, the results are very different. The center hit still goes into the center of the cup, but the toe and heel hits are far short of the hole and considerably off to the right (toe hit) or left (heel hit). These are not tap-ins but 3 and 4 footers that can easily be missed.

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