MOI – I have heard of it, but what is it?

The world’s leading manufacturers are constantly talking about MOI in relation to the golf equipment that they manufacture. Whether it is high MOI, increased MOI or in some cases low MOI! So what exactly is this mysterious stuff?

MOI stands for Moment of Inertia and it means:

A measure of an objects resistance to any change in its state of rotation. Moment of inertia is the inertia of a rotating body with respect to its rotation.

Ok so that is not so simple, what is Inertia then?:

In-er-tia

The property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force.

This in effect means that MOI is an objects resistance to twist when acted upon by another force.

So how does this relate to the physics of swinging a golf club? Well, there are three different ways MOI relates to golf:

1. The first MOI is an object being rotated around a central point such as a golf club being swung around a player. When a club is heavier and longer it has a high MOI as it takes more energy to rotate that object around a point. A low MOI is created by a lighter, shorter club as it takes less effort to rotate around a player.
   
   Many manufactures are now producing low MOI drivers; this directly refers to the weight of the club and not the club head. Drivers that are being marketed as low MOI will almost always have very lightweight shafts, grips and the weight within the head will have also been reduced so that takes less effort to rotate/swing around a player.

2. The second MOI is the act of the head rotating around the point at which it is attached to the shaft. It also an important consideration for golf club designers and engineers as it will affect the stability of the club when swung. If the head is liable to rotate around the shaft axis it will lead to incosistinent ball striking and flight. This MOI is little talked about by golf club manufacturers, but is still very important to the mechanics of a golf club.

3. The third MOI in golf is when a shot is hit off centre on the face. The head will then try and rotate around the centre of gravity of the head. This rotation of the head affects the stability and ‘forgiveness’ of the club. Manufacturers will manipulate and develop the distribution of weight in order to create a higher MOI or in other terms resist the club head twisting on impact. This twisting leads to inconsistent shots.
   
   This second MOI is the most commonly referred to by manufacturers and is a constant consideration for golf club designers and engineers looking to develop new golf equipment. An example of engineering to create a high MOI is perimeter weighting, this allows the club to become more stable and resist twisting upon impact.
   
   Another major innovation in relation to this MOI is the introduction of square drivers into the market. The shape of the driver allows for weight to be distributed to four points of the head, much like a car has four wheels and as a result is more stable that a 3 wheel or two wheel vehicle. Although initially popular on tour square headed drivers have slowly become less and less used as players felt they were ‘too stable’ and could not create the shot shapes that they required.

MOI is often referred to in one other way and that is by advanced golf club makers who may choose to build a MOI matched set. This means that they take the most consistent and often favourite club in a player’s bag, measure the MOI in terms of the effort it takes to swing and then match all the other clubs in the set to this MOI.  This in theory makes every club require the same effort to swing and in turn create more consistency within a players game.

MOI matching differs from the traditional method of club making in which clubs will be ‘swing-weighted’. This means that clubs are matched in by the weight ratio of the shaft butt to the club head.

Many of today’s golf clubs are being constantly improved by golf companies R&D departments to maximise MOI in relation to the stability of the head and minimise MOI in relation to the effort it takes to swing. So the next time you go to purchase a new driver or set of clubs, look out for the improvements that the manufacturer has made to the MOI!