Moment of Inertia MOI (click)
Moment of Inertia. MOI. What is it?
Actually, it’s “mass moment of inertia”… another type is “section moment of inertia”, but that is a useful measure for things like shaft cross sectional properties… Mass Moment of Inertia describes the stability of an object when struck or a force acting on it.
MOI is measured by a function of mass x distance from Center of Gravity (CG) squared. In other words, if you see a typical putter advertised with a “Extreme MOI” of 8700 grams-centimeters squared, it means that the individual average mass x the distance that mass is from the center of gravity, squared. One gram of mass one centimeter from the CG = 1 gm-cm2. One gram of mass ten centimeters from the CG = 100 gm-cm2. So it’s obvious, that the farther from the CG the mass is, the more “stable” the object would be. Flywheels use this principle to retain energy. Golf clubheads use this principle to better tranfer energy (momentum).
The effective “root mean square distance of the mass from the CG” is known as the “radius of gyration”. It’s basically the co-efficient of the MOI possible in a design. In other words, there is an optimum, or maximum, radius of gyration possible in the Rules of Golf as to Putter head design. We know this number… but it’s unattainable. Why? Materials science & technology limits what you can do.
Typical putters have Vertical Axis MOI values between 3500 and 6000 gm-cm2. (3.5Kg-cm2, to 6kg-cm2). High MOI putters are typically 6Kg-cm2, to 9kg-cm2. It’s silly that manufacturers were bragging that their 8575 g-cm2 MOI bested the other guys 8497 g-cm2 MOI… you couldn’t tell the difference at 1000 g-cm2 differential, and they’re bragging on a few g-cm2 increment.
Our Putters have an MOI about the Vertical Axis (heel and toe stability measure) of 21Kg-cm2, and 23Kg-cm2. Roughly Triple the so called High MOI putters. These Putters exhibit extremely stable putter heads during the stroke, and through ball contact.
For more on this issue, follow the posts in the category “MOI”.