How to Comply With the USGA Ruling on moment of Inertia of Golf Clubheads and Optimize Clubhead Design

Since March 1, 2006, the United States Golf Association (USGA) has imposed a limit on maximum moment of inertia (MOI) of golf clubheads. The limit is on moment of inertia around the vertical axis through the clubhead center of gravity (Izz). On April 11th 2006, the USGA announced that the maximum allowed MOI has been revised to 5,900 g-cm2 with a tolerance of +/- 100 g-cm2.

1. Why impose a limit on moment of inertia?

Moment of Inertia (MOI) is a measurement of a clubhead’s resistance to twisting. It is a strong indication of the “forgiveness” of a clubhead – that is, the extent to which a good result can be achieved from a less than ideal contact with the ball. Further increases to MOI could reduce the challenge of the game by reducing the skill required to hit the ball straight. In addition, that could also result in an increase in average driving distance by reducing the likelihood that swinging faster will produce a poor result.

As stated by the United States Golf Association in a communication to golf manufacturers in March 2005, “moment of inertia of driver heads has approximately tripled over the past 15 years. The USGA is concerned, however, that any further increases in clubhead moment of inertia may reduce the challenge of the game. Future materials with greater strength and lower weight than materials currently used in clubheads could potentially enable significant further increases in moment of inertia. There may be other means of further increasing moment of inertia as well.”

Research conducted by the USGA has shown that the clubhead size limitations already in place will not effectively prevent increases in clubhead moment of inertia beyond the levels achieved by clubs submitted to the USGA in 2005. The USGA has allowed substantial increases in MOI, but it now believes that a limit is appropriate.

2. How is the rule enforced?

Every new club needs to be submitted to the USGA to test conformance with the Rules of Golf. Every club that has been submitted to the USGA and already been ruled conforming to the Rules of Golf by the USGA remains conforming with the limit on moment of inertia.

Every new club as submitted by the club manufacturer will be tested for MOI around the vertical axis through the clubhead CG. Clubs with movable weight designs need to meet MOI limits in all intended configurations.

The USGA will use its own test rig to measure a clubhead. If the measured MOI is under 6,000 g-cm2 (5,900 + 100 for tolerance), the clubhead passes the test. If it is over, it fails the test. Please note that the USGA will only pass or fail a clubhead, without giving any justification.

3. How can I make sure that my clubhead will pass the USGA test?

Before we can answer this question, you need to understand how to measure moment of inertia through the center of gravity and how accurate the results are.

Moment of inertia is measured by making the clubhead oscillate around the axis you need to measure. MOI is proportional to the period of oscillation. In our case, the USGA needs to measure the moment of inertia around the vertical axis passing through the center of gravity of the clubhead. As the center of gravity is unknown, the USGA makes 6 measurements around 6 axes parallel to the axis they want to measure. Through a least squares regression analysis, they calculate the moment of inertia around the vertical axis passing through the center of gravity.

Any measurement has an uncertainty. The USGA uses an instrument that has 0.5% uncertainty on each measurement. However, the method used to recalculate MOI through CG gives different accuracies depending on several factors, including the size of the clubhead and the CG offset of the clubhead from the MOI instrument centerline. In reality the measured MOI can have an uncertainty of up to 92 g-cm2.

What does it mean for your clubhead? It means that if the real MOI of your clubhead is 6,000 – 92 = 5,908 g-cm2 then you can be sure that it will pass the USGA test.

4. How can Raptor Scientific help me?

Raptor Scientific has been working with golf club manufacturers for more than 40 years to measure moment of inertia and center of gravity of golf clubheads and balls.

Raptor Scientific offers a wide range of products and services to meet all needs and all budgets:

• Moment of inertia measurement instruments
• Center of gravity measurement instruments
• Measurement services (moment of inertia and center of gravity)
• Fixture design and manufacturing to reduce measurement uncertainty
• Consulting in mass properties
• Automated least squares regression analysis as defined by the USGA to calculate the moment of inertia through the center of gravity (contact us for more information)

5. How to optimize a golf clubhead design to maximize moment of inertia while staying within the USGA limitation

So how can we make sure that a particular clubhead complies with this requirement? It needs to be measured. And here comes into play your own ability to measure moment of inertia. If you measure MOI with a 0.5% accuracy instrument, you have to leave the same margin for your own measurement. It means that you can only design clubheads with a moment of inertia of 5,908 – 92 = 5,816 g-cm².

Raptor Scientific can help you achieve better designs. Our different models of moment of inertia instruments can measure moment of inertia with an uncertainty of 0.01% to 0.25%. Here are the results achieved with these instruments.

At this point we believe that the moment of inertia of clubheads is far enough from the limitation imposed by the USGA that an XR10 gives results that are accurate enough. However, getting closer to the limit will become a concern with the advances in technology.