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Golf Moment of Inertia MOI

Golf Moment of Inertia MOI

Golf Moment of Inertia MOI

What is MOI and what does it mean for golf clubs? Here’s two kinds of golf moment of inertia MOI that really matter in the game. Plus, a little bit on a third.

Golf Moment of Inertia (MOI)

Golf Moment of Inertia (MOI) can be found on the shaft’s axis line, such as in this PING G30 Driver diagram.

As part of 2nd Swing Golf’s continuing series to educate golfers and potential players about clubs and the game — like how to just start understanding golf’s myriad jargon and, hopefully, buy something with more confidence — we’ll examine golf moment of inertia MOI, which manufactures increasingly use as a golf club spec nowadays.

But first off, what is MOI (Well, you can read more HERE.)? We have some definitions and explanations below as to how MOI applies to the sport and its equipment:

MOI in golf most often applies to how well-balanced a clubface is and how a golf ball reacts at impact. All those club techy-sounding modern-day golf industry buzz words go into play here, such as perimeter weighting, counter-balancing, loft and lie on-course adjustments…

The idea behind applying MOI to golf — at its heart — is just making sure that the clubface strikes the ball cleanly without without twisting too much. You see, MOI is a physics term that measures the resistance any object set in motion while rotating on an axis. The easiest way to remember is that the greater the golf moment of inertia MOI, the more force it takes to rotate or move the object, which in this case is a golf clubhead when it strikes the ball. If the clubhead even moves back a tiny bit on a hit off the heel, it can create an even uglier shot than usual with a lower MOI or a wobbly head, pushed back.

That’s it for golf moment of inertia MOI. Pretty much. Kinda.

Golf Moment of Inertia (MOI)

Golf Moment of Inertia MOI in a clubhead.

We only wish it were that easy to achieve the right MOI, though. Golf equipment manufacturers use great resources to engineer clubs and balls — and provide you with personalized club options and encourage professional fittings (such as with 2nd Swing Golfs professional and certified clubfitters) so that any off-center hit won’t throw off the ball’s motion too much.

If we are successful, the club and ball work together to continue to move efficiently, far and straight.

Two of these golf moment of inertia MOI concepts are especially important in the design of any club: clubhead’s actual center of gravity (CG) on a vertical axis and then club rotation around the shaft’s center line. A swinging club is fighting two separate forces of physics (or just the clubhead at the very least) to smack that ball where you want it to go. 

1. When you strike the ball off the middle of the clubface — even though the clubhead and shaft are attached at the ferrule — the head will try to rotate on the vertical axis in the clubhead’s CG (Illustrated below.).

Golf Moment of Inertia (MOI)

Golf Moment of Inertia a MOI.

 2. At the same time, when the golfer downswings, the clubhead rotates around the axis through the shaft’s center (Illustrated below.), which opens or closes the clubface.

Golf Moment of Inertia (MOI)

The second example of MOI in a club takes place during the downswing on the shaft’s center axis where it connects to the clubhead.

The first example refers to the MOI of the clubhead. In marketing terms you’ve probably heard or read about this from major golf manufacturers. This is the head design property that figures in the clubhead’s “amount” of forgiveness (See full explanation of forgiveness HERE.) for off-center strikes. Often clubmakers equate more MOI to a bigger “sweet spot.” 

In general terms, the larger the clubhead, way it’s shaped or the greater the outer distribution of weight, more commonly referred to as perimeter weighting, the higher the golf moment of inertia MOI of the clubhead and its CG, which are interdependent in many ways. The first clubhead feature is why the USGA limits their size to 5,900 grams/cm2 in order to maintain fair club boundaries and set standards. And also so drivers don’t get any ridiculously larger than some are already.

Golf Moment of Inertia (MOI)

The USGA uses this machine to measure a clubhead’s MOI.

Basically, getting a higher MOI is another method to keep the clubhead from twisting too much, and then an off-center hit doesn’t come off the clubface too badly.

With a smaller clubhead, typically, more weight is positioned close to the clubhead’s center. Conversely to a large clubhead, like with an iron’s clubhead, both the vertical CG axis and golf moment of inertia MOI are lower. That produces less distance and wilder shots on off-center strikes.

Once again, the higher the MOI, the more resistance to the object being rotated around an axis, which produces more solid hits. And a lower MOI equals less resistance to the object rotating around an axis, making shots more likely to be errant.

The second example refers to the clubhead MOI at the shaft’s axis point. This has more to do with shot accuracy than lost distance, so it’s not as popular a topic.

When a clubhead is bigger or more weight is put far out on the toe, the result is a higher shaft axis MOI. The result of a higher shaft axis MOI is a more open clubface at impact, which, of course, makes for better, more forgiving shots. Lower shaft axis MOI tends to close the clubface at impact. And, as we all know probably too well, it’s tough to hit a clean shot with a closed clubface, even slightly.

Finally, the whole golf club also has an MOI. That’s usually determined by club length. The longer the club and the heavier the clubhead, shaft and grip, the higher the total MOI. A club’s total MOI is most often used when matching swing feel among all your clubs in the bag. The long-held clubfitter belief is that when the clubs’ MOIs closely match, players won’t need to change up their swings as much from club to club.

This concept is called swingweight. (And it’s covered HERE in much greater detail.)

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