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Moment of Inertia
Moment of Inertia (MOI) has become one of THE biggest - and most abused and misused - buzz terms in golf these days. These three simple words are being used as a magic talisman to sell more golf equipment than just about any other technology buzz term out there right now, often when its use is entirely inappropriate.
MOI essentially means resistance to rotation. That's all. Basically, the further a concentration of weight is from the pivot point of an object, the harder it is to rotate. Think of sitting in an office chair, and spinning yourself around in it. Stick out your legs. What happens? You slow down. Now pull your legs in. What happens? You speed up. You see it in figure skating as well. The spinning skater brings his/her arms in close, and spins faster. So how does this apply to golf? Several ways, actually. We will touch on the principal applications here.
1. MOI Around the Clubhead�s Centre of Gravity
The first, and best known, way in which MOI applies to golf is the MOI about a clubhead's Centre of Gravity. When a club hits a ball, the clubhead is actually capable of rotating around its own centre of gravity, or "sweet spot". It doesn�t actually rotate around the shaft when it hits the ball. The shaft has already thrown it forward, so it is free to twist. Naturally, the more the head twists on off-centre impact, the more energy is lost, and the shorter the ball flies. However, if you move weight out of the centre of the clubhead to the perimeter, you reduce this twisting, reduce the loss of energy on off centre impact, and reduce the lost of distance. The club hits more consistently.
The most obvious example of this, to most people, is the cavity back iron, and it has certainly been effective. If you stop and think about it, however, hollow wood and hybrid heads are also clearly perimeter-weighted by the very nature of being hollow. If the designer does his/her job right, they can be even more effective perimeter weighted clubs than cavity back irons. That's part of why hybrids are easier to hit than long irons. They are more forgiving on lost distance on off-centre hits. The most radical examples of high moment of inertia in anyone's bag are oversize titanium drivers and modern "super mallet" putters. Drivers can have MOI scores (expressed in thousands of grams per centimetres squared), over double that of cavity back irons. Some putters have MOI scores several times that of even the most forgiving irons, and much larger than the best drivers.
There are two important factors to keep in mind about MOI, though. A given increase has a much greater effect at the lower end of the scale than it does at the higher end. Take a muscle back blade with a MOI of about 1800 gms/cm2 and compare it to a cavity back of 2300 gms/cm2, or 500 gms/cm2 higher MOI. You will notice a greater difference in performance between these irons than you will between modern oversize drivers with the same 500 gms/cm2 difference. The higher the MOI of a design in the first place, the less effect an increase of a given amount will have. So when the big companies get into marketing wars over which is the most forgiving among the latest generation of ultra-high MOI drivers, (the square heads and elongated or "stretched"-looking clubs), they are really splitting hairs. The differences between these clubs can be measured in feet at most, at least as far as MOI effect is concerned. The quality of a club's face design can have more effect, but that�s another topic.
One other factor to keep in mind is MOI effect on accuracy. This is greatly exaggerated. Many people think "more forgiving" means straighter. Well, it does, but it is a very minor factor in club accuracy. Other things like length, loft, face angle, offset, swingweight/MOI of the whole club, and club total weight affect direction control much more. Mostly, "forgiveness" is about DISTANCE control, about losing less distance on off centre impact, and being more consistent in controlling distance as a consequence. It means carrying a greenside bunker by a yard or two, rather than burying in the face. It means averaging 5 yards more off the tee, because your off centre hits don�t lose as much distance. Right and left dispersion, and slice/hook control, is only slightly effected. So slightly, you probably couldn�t even measure it.
2. MOI of the Clubhead Around the Shaft
Another application of the term MOI, which is particularly filled with marketing hype, is the MOI of a clubhead around its hosel, or around the shaft. As heads get bigger, the Centre of Gravity generally moves further away from the shaft. This means that the MOI of a clubhead around the shaft gets higher. In theory, this makes such heads harder to square at impact. Some people have made a really big thing about this, but practical research has shown that you need a really big difference in MOI around the shaft before you get much of a visible difference in direction. The only real area where this has had much practical effect has been in drivers. The large increase in head size has had a small but noticeable increase in susceptibility to slicing. Increased club length has had much more effect, but some people in the golf industry have blithely ignored this, and instead tried to market certain shaft designs under the "MOI" label as being better suited to modern, oversize drivers.
This is a bit of a gimmick. Yes, the big driver heads can be a bit more difficult to square up, and yes, a shaft with more resistance to torque and a stiffer tip can help compensate for this. So, however, can the square to slightly closed faces of modern drivers (old persimmon heads were usually 1-2 degrees open to compensate for wood shafts tendancy to whip clubheads forward just before impact). In fact, slightly closed faces are generally more effective than shaft differences in battling the "fades". It is by no means necessary to match such a shaft with a modern driver head. The choice of shaft depends much more upon a player's swing speed, swing tempo, aggressiveness of transition from backswing to downswing, and how late the player releases his/her wrist cock. A tip stiff, low torque shaft could do a lot more damage to a player whose swing moves don't suit that kind of shaft profile that it could ever compensate for with help in controlling a driver head. The term MOI on a shaft, usually accompanied by a hefty price increase over other, comparable shafts, is often just a grab for your wallet.
While we're at it, let's bury another myth. Many companies have tried to market "draw biased weighting" or adding weight to the heel of a driver, as a shot shape control mechanism, or worse, an anti-slice tool. The former can be true to a limited extent with better players, if enough weight is used. The latter is just flat out not true. You have to move a lot of weight heelward to get a noticeable difference in shot shape. Even then, the difference is small, and a minor swing error can and will wipe it out every time. That's why it only works with better, more consistent players who just need a little weighting help with their draw/fade technique.
Slices are usually caused by major swing flaws that small adjustments won�t affect. If you tried to move enough weight to change a slice, a) you�d move so much weight that you'd make the club almost unplayable, and b) it wouldn�t work anyway. Slices can be controlled by changes in things like face angle, offset, and club length, balance and total weight. Forget doing it by MOI around the shaft, or draw bias weighting.
3. MOI of the Whole Club
Earlier, I mentioned MOI of the whole club as a much more important factor in accuracy. Here's why:
As mentioned previously, any object has a resistance to twisting or rotation around a pivot point. Obviously, this applies to a golf club as well. The whole business of how to measure this, and precisely where the pivot point is, gets pretty complicated. For most calculations, it's the butt end of the club, although the axis of your swing through your body comes into it later. The main thing golfers need to understand is: the higher the MOI of the club, the harder it is to swing, and in particular, the harder it is to square up the face at impact. Everyone's swing works best with a particular MOI. For some players, it's higher. For others, it's lower. A lot depends on how fast you swing, what tempo you have, what your swing path and swing plane are, and other factors. Clubfitters can test for ideal MOI in the same way they test for ideal swingweight. In fact, MOI replaces swingweight as the best way to balance clubs.
The biggest factor affecting MOI of the whole club is length. The longer the club, the higher the MOI. Head weight is next in importance, followed more distantly by shaft weight and balance point. Grips and butt weights have little effect. So if you need a lower MOI, the easiest way to get it is by shortening the club, followed by reducing the headweight. In fact, the old system of matching swingweights ensures that MOI in a set gets progressively higher with each club, which helps explain why long irons and drivers are traditionally harder to hit.
What we're finding is that most new ultra long drivers have club MOIs way above what most normal players can handle. This is especially true of players with upright swing planes, and players, usually slicers, who tend to "cast" or "come over the top" in a move releasing their wrist cock early in the swing. Higher MOI clubs resist this move, forcing the clubface to lag more open. Also, longer drivers force up the MOI of a golfer's swing around his/her body pivot point, especially if the wrist cock is released early, in effect, forcing the clubhead even further away from the body. Any clubhead speed advantage from the longer shaft is lost, face angle lag gets worse, and the result is huge slice after huge slice. This, along with poorer centre face contact, explains the Trackman test findings. These show that players hitting 45" drivers often lose a bit of distance and clubhead speed relative to shorter drivers; and see their inaccuracy get dramatically worse. So it is worth getting fit for proper MOI in your clubs. Not only will your sense of consistency from iron to iron likely be a bit better, you�ll probably end up with a driver you can hit the fairway with more of the time.
Based in Richmond Hill, Ontario, Sean Baines is a PCS-certified "Class A" clubmaker and the owner of Clubs That Fit. He is committed to providing quality, game-enhancing clubs and highly personalized service to golfers of all ages and skill levels.