Moment of inertia MOI

As part of an ongoing series on clubhead characteristics, welcome to the joys of MOI… 

In golf, the MOI or Moment of Inertia is essentially a measure of the ability of the club to resist twisting when it hits the ball.

Imagine a metal bar hanging from a piece of string. If you tap that bar absolute dead center, it will swing forwards and back, but it won’t rotate. 

If you tap it just to the left or right of that center point, it will swing forwards and back, too, but it will also start rotating.

It’s a similar principle in the golf club. If the ball hits the “dead center” of the clubhead, then the clubhead won’t twist at all. 

If the ball hits the clubhead off center, then this clubhead will twist.

How much it twists depends on the MOI - a club with a low moment of inertia will tend to twist more than one with a high moment of inertia.

The actual degree of twist isn’t as bad as the diagram would indicate. In most cases, it’s imperceptible.

Is twisting bad? Well, it’s not ideal, because it means the clubface is no longer facing the way intended - meaning the shots are not straight. Even a small degree of twist reduces the accuracy of your shots.

This is why you’ll see manufacturers praising the high MOI of their latest clubs - especially in putters, where accuracy is so crucial. The higher the MOI, the more likely you are to be able to hit the ball off-center and still get it to fly or roll in the intended direction.

However, MOI is just one of numerous factors that determine how a club reacts when it hits the golf ball. Swapping your putter for one with a higher MOI is not going to solve all your putting problems. Higher moments of inertia are an incremental improvement, but - unfortunately - not a miracle cure.