MOI is an acronym for Moment of Inertia.
The Moment of Inertia (MOI) of a putter is a measurement of its resistance to twisting on off-center hits. The greater the MOI, the more stable your putter will be. However, it has only a small effect on most putts.
It is the techno-talk manufacturers use to promote their putters. In simple terms the moment of inertia is the measure of a putter's resistance to twisting on off-centre hits. The greater the moment of inertia, the less the putter blade rotation and loss of energy.
The way to increase the MOI of a putter is to move weight away from the centre of gravity (CG). In other words you redistribute the weight away from its balance point.
Dave Tutelman gives the example of two identically weighted coat hangers suspended by a piece of string. If you tap the centre of each hanger it will swing back and forth, but not twist.
However, tap the end of each hanger and the hanger with the weights close to the centre of gravity will rotate faster than the hanger with the weights spread further apart.
Although both hangers have the same mass, their weight distribution is different, and this affects their moment of inertia.
In 1966 Karsten Solheim used this principle of physics to create what became known as the Ping Anser putter. By taking away the mass at the centre of the putter through a cavity back and redistributing it at the heel and toe of the putter, he improved the playability of his putter. This innovation can now be found in almost every putter on the market.
Bobby Grace Putter Design
Other manufacturers have joined the MOI bandwagon either by copying Solheim's design or with big-headed mallet putters with extreme weighting away from the centre of gravity.
MacGregor Golf produced an elongated putter, the Response ZT 615, which was used by Jack Nicklaus to win the 1986 Masters.
Increasing the heel to toe length helps to stabilise a putter at impact in much the same way as a tightrope walker uses a long balancing pole.
The Moment of Inertia of a putter would be irrelevant if golfers were capable of consistently contacting the ball on the putter head's sweetspot. Because they can't, the effect of off-centre hits can be minimised through design.
However, whatever the manufacturers say, you can't enlarge the sweetspot - you can only enlarge the area of forgiveness. How significant this is for distance and directional control is a matter of opinion.
Scatter of Hits
Frank Werner and Richard Greig in their book How Golf Clubs Really Work and How to Optimise Their Designs concluded that the scatter of hits on the face of the putter for a typical golfer, in other words hit pattern errors, have a surprisingly small effect on most putts.
Far more important were errors in alignment angle and squaring the putterface to the target.
So what does this all mean to you? In all probability you are more likely to miss putts for reasons other than the moment of inertia of your putter. Besides the average golfer wouldn't have a clue what the MOI of his or her putter is, or even what moment of inertia means.
If you putt with a recognised make and model of putter that has been custom-fitted for you, your focus for improvement is best directed to working on the different skills of putting. A change to another putter based on its comparative MOI is probably not warranted.
1 = www.bobbygraceputters.com
2 = Adapted Drawing from How Golf Clubs Really Work and How to Optimise Their Designs