Frank Talk

I‘ve heard a lot about big-headed drivers and their high moment of inertia (MOI). A high MOI makes them more forgiving, but doesn't that also make it hard to square the face? --T.S., Muncie, Ind.

Warning: This answer is harder to explain than the question was to ask. But the answer is, "Yes." If you put a chunk of lead tape on the toe of a club, it makes the toe lag behind the lighter heel, and vice versa. This is because the MOI is different depending on where the weight is positioned. In layman's terms, MOI is the measure of the resistance to twisting. MOI increases by shifting weight as far as possible away from an axis of rotation. The problem is that the typical driver has more than one axis (see A, B, C and D below), and each axis has a different MOI. Many super-oversize drivers have a high MOI around the vertical axis through the center of the clubhead (see A), and that's why they have forgiveness on misses toward the heel and toe. However, a high MOI head design means there will be resistance to twisting around a closely parallel axis (like the axis through the hosel, D), which can make it harder to square the face.

That's why making a good large driver head is more complicated than just making the head bigger. To reduce the shaft MOI without reducing the MOI of the head, you have to figure out a way to move the center of gravity closer to the shaft. What's hard about that? Well, as the head gets bigger the center of gravity tends to move away from the shaft. So physics is the reason you might find big drivers hard to square at first. But it's not an insurmountable task. MOI is only meaningful in relation to a specific axis, and a club can have a high MOI and be difficult to hit. I think MOI is becoming an overused acronym. It's important, but only for designers to worry about, not golfers.

Moment of inertia is a complicated bit of engineering, especially as driver heads get extra large. Understanding it is important—if you're a designer, not a golfer.
Illustration: Chris O'Riley

 

The ideal shaft would be absolutely stiff and extremely light. True?
-- Serge Matovic, Ontario, Canada

Not necessarily so. A shaft is most efficient when it is able to bend and then unbend (and release all that stored energy) just before impact. An extremely stiff shaft is just like an extremely stiff fly-fishing rod: No feel and difficult to manipulate. Hard-swinging pros can get an X-flex shaft bending like most of us get an R-flex shaft bending. Without that bending, we aren't able to take advantage of the shaft's design and our potential.

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I've heard about increasing the weight or swingweight of a set of irons. What about how to make them feel lighter?
-- Gene Galantini, West Orange, N.J.

To make the club feel lighter, you have to reduce the weight of the component parts (grip, shaft and head). A lighter shaft (graphite vs. steel) would certainly be a good first step, but that changes swingweight, feel and balance. Understand that making the club feel lighter and reducing the swingweight might not be the same thing. In fact, changing only to a lighter grip will decrease the total weight but will increase the swingweight, because the head will now be heavier in relation to the grip than it was before. You could add weight to the grip end, and though you would raise the club's total weight, the swingweight would decrease. For example, wearing a golf glove reduces swingweight by three points. The ultimate question is whether a lighter club or a lighter-feeling club performs better for you.

As featured in Frank Talk, Golf Digest September 2004.