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Home  /  Moment of Inertia–Part One

Moment of Inertia–Part One

High Moment of Inertia or MOI is the what’s HOT in driver head design. It’s also what’s been increased in putter head design, wide soled irons and any other item where more mass has been added to the back, side or bottom of the clubhead. So what is MOI?If you are thinking about getting one of the new square drivers you might be interested in what the hype is about. A good place to start is Tom Wishon’s article about MOI vs Head Shape in his October, 2006 eTECH Report. enewsletter

Here is an overview of the physics pertaining to MOI. For those of you who did not study mechanical physics a quick review of some basic principles is useful for understanding MOI.

1. A body at rest stays at rest and a body in motion stays in motion unless an outside force intervenes. In a golf swing the clubhead is the force that acts on the ball (which is at rest) and the ball is the force that acts on the club head (which is in motion).

2. If a net force acts on an object it will cause an acceleration of that object. This is Newton’s 2nd Law of Motion. Newton’s 3rd law states that if one object exerts a force on a second object–e.g., a club head hitting a golf ball–the second object (golf ball) exerts an equal and oppositely directed force on the first object (club head).

3. Moment of inertia (MOI) is defined as a point mass about the center of rotation and is calculated to determine an object’s resistance to rotation. Moment of inertia is also called rotational inertia.

The greater the mass is away from the center of rotation the more resistant it is to rotating. For example, when a figure skater pulls in her extended arms her moment of inertia decreases and she spins or rotates faster. (You might read the term moment arms” in conjunction to MOI. The “arms” allude to the figure skater, which is an easy visual for understanding this concept).

The club head’s interaction with the golf ball produces a whole set of physical challenges. First, the force of the club head causes the ball to go into motion and accelerate. The force has to be applied so that the ball is launched at an angle that causes the acceleration to keep the ball going forward.

Second, the force of the ball on the club head causes it to rotate. When the clubface rotates the ball accelerates right or left depending on the rotation. We can’t control the force of the ball so the club makers want to control the effects of that force, by controlling the flex and torque of the shaft, as well as the MOI. Ways to increase MOI, which increases the resistance to rotating, include adding mass to the club head; distributing the mass over a larger area; adding weight to the perimeter, and concentrating the weight away from the face.

The center of gravity (COG) of the club head is positioned to minimize rotation during the swing and optimize the launch angle when the club face hits the ball. The COG of the current generation of 460cc drivers tends to be low and deep or toward the back of the head.

So, why do all of the drivers claim to have a high MOI and “optimal” COG and yet few players can hit equally well with all the drivers?

The physical differences between one person’s swing and someone else’s are the wild cards in the equation. This is why a launch monitor is probably the only way a golfer can really determine the best driver”head/shaft/grip combination”for his swing.

References: http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html; Titleist, Cleveland Golf and Nike Golf; /Cliffs Quick Review Physics, copyright 1993

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