What is MOI?

A Measure Of Forgiveness
MOI (Moment of Inertia)

It will be easier to understand MOI if you think of it as an MOF (Measure of Forgiveness). Moment of Inertia is a measure of a body’s resistance to angular acceleration (twisting).

In golf, MOI comes most into play on imperfect contact, when the ball and the clubface meet someplace other than the sweet spot. But the simplest way to illustrate the concept is to put golf aside for a moment, and consider a weightlifter like Stick Man, below.

In the sketch on the left below, Stick Man has the weights on the bar close to his body. He will find it relatively easy to twist the bar on his shoulders, because the MOI of the bar is relatively low.

Similarly, in designing a driver, if we move weight from the center of the club head to the toe and heel it will increase the MOI and resist twisting at impact. Mishits will fly straighter, and the club will be more forgiving when ball contact is made away from the center toward the toe or heel. It will feel like it has a BIGGER SWEET SPOT.

In hollow metal woods, the weight is all in the shell and as far away from the center of gravity of the head as possible. This gives it a high MOI and makes it very forgiving of mishits, something most of us can relate to. The forgiveness in these big heads enlarges the effective sweet spot in all directions on the face – up and down as well as side to side. This is why manufacturers have been making driver heads obscenely big and as forgiving as possible.

Because it is so light, titanium has allowed the club head to grow without adding so much weight that it becomes uncontrollable. As an added benefit, its strength allows the face to be made thin enough that it actually deforms and springs back during impact, acting like a spring or trampoline.

The USGA, because of its concern about how far elite golfers -- the best male amateurs and tour professionals -- are hitting the ball, set a limit of 470 cc on the size of a driver’s head. It had no real evidence to support its concern, only anecdotal evidence. (While I was Technical Director, I often had to respond to anecdotal claims of a magic club or ball that added 20 yards to a golfer’s game. These claims – almost always “20 yards” – never held up to scientific scrutiny.) To avoid criticism, the USGA hid behind the “traditional and customary” clause: ‘It was just not traditional to have heads any bigger.’ (I sometimes think the real reason was that the USGA didn’t want to offend or get sued by the manufacturers who couldn’t make head covers any bigger than that size.)

When William Tell took aim at the apple on his son’s head, he was preparing to fire a bow and arrow. Today, the vast majority of golfers stand in the place of Tell’s son, and the elite minority are the apple sitting atop our head. The USGA is aiming at the apple, but unfortunately for us it has a double-barreled shotgun in its hands. I don’t have to tell you what the results are going to look like.

Ninety-nine percent of the golfing population doesn’t hit the ball far enough. Research shows that the average male golfer (a 90-to-95 shooter) drives the ball 192 yards, though we think we hit our average drive 30 to 40 yards farther than that. A rule that is trying to reduce distance will hurt the vast majority of golfers more than it does the elite.

But even more than distance, we need all the forgiveness we can get because, unlike the elite 0.001%, we don’t always or usually hit the sweet spot. Bigger does not mean longer, though it generally means more forgiving. Distance is not size dependent, or dependent on MOI. COR (Coefficient of Restitution), or spring like effect, is to some degree, but most clubs are already at the COR limit, and this is most effective when impact occurs exactly on the sweet spot.

In explaining the reasoning for this proposal, the USGA has admitted the truth about its head size limitation: The distance that elite golfers are hitting the ball must be harnessed. ‘A high MOI will encourage the superstar to swing harder without concern’.

So the rest of us are being penalized for what the pros may be tempted to do. We, who don’t often hit the center of the face and thus need maximum forgiveness, must accept restrictions on the amount of help our club designers will be able to provide.

Make no mistake – this is what restricting MOI means. The forgiveness built into a club means much less to the pros than it does to us; if this were not true, pros would use perimeter-weighted irons instead of blades. They don’t need the measure of forgiveness the way we do. Yet this is what the MOI issue is really about, or should be. Does it make much sense for the USGA to limit this forgiveness property?