Hi Jay,
Let me see if I can put swingweighting in perspective for you. There are two parameters that define how a rigid body responds to being thrown around. The first is it's mass, which defines it's linear inertia and acts at the 'center of mass', or 'center of gravity', of a rigid body, and the second is the moment of inertia, which defines its rotational inertia. Mass is fairly easy to understand, it is the property of a rigid body that resists attempts to accelerate it, or slow it down, or change it's direction if it's moving. Moment of inertia (MOI) is a property determined by the shape of a body and how its mass is distributed and is the property of a rigid body that resists attempts to rotate it about some axis, or slow down it's rotation if it is rotating about some axis. Where mass can be treated as simply acting at the center of mass, moment of inertia is referenced to the axis about which rotation is occurring. Because a body (such as a golf club) can be rotated about a great many axes (an infinite number, in fact) it has an infinite number of values for the MOI, one for each axis about which the body can be forced to rotate.
There are two axes of rotation that are commonly discussed with reference to a golf club. One is an axis basically down the club shaft about which the club face is rotated open and closed and the second is an axis roughly through the butt of the club (perpendicular to the shaft) about which the club is rotating near impact with the ball (the wrist 'hinge'). Matching the MOI of a set of clubs about this axis through the butt of the club is what the Lorythmic Swingweight Scale was designed to do. At the time it was developed measuring MOI was only done in test labs. The swingweight scale was developed as an approximation of the MOI that could be done easily with simple equipment, but it is a static measurement of a dynamic property. For a set of standard weight iron heads and the same shaft and grip, it's a pretty good approximation. A MOI variation of only a few percent exists with a set of irons swingweight matched. If you include standard weight wood heads, with the same shafts and grips as the irons, the variation extends to maybe 6 percent, or so. The difficulty came when we started putting shafts in the drivers and fairway woods that were half the weight of the shafts in the irons. Now the variation in MOI between a pitching wedge and a driver, that are swingweight matched to each other, is up to 10 percent, or more. The reason for this is that MOI is determined in simple terms by the product of the mass of an object times it's distance from the axis of rotation squared. Swingweight is the mass times the distance to the pivot (not squared) and so as you make something longer the MOI goes up faster than the swingweight. If you put a lightweight shaft in a driver, you make it longer to keep the swingweight the same, but the MOI has gone up significantly. This is also the basis for Dave Tutelman's comments that a weight added to the butt of the club won't affect MOI significantly - it simply isn't that far from the axis of rotation.
When a golfer starts their downswing the movement of the club is mostly translational, it moves linearly and that motion is resisted by the mass of the club. It quickly starts rotating, however, and near impact the movement of the club is both translational and rotational, so it's mass is important, as is it's MOI. But MOI about what axis? The axis through the butt of the club (the wrist hinge) is sort of pretty good right at impact (the hands would have to be stationary at impact for it to be good), but between the start of the downswing and impact the axis about which the club is rotating moves all over the place and the MOI of the club about the rotation axis is varying continuously. Herein lies the crux of the problem. If we want all of the clubs in our bag to swing the same, they all have to have the same mass, and the same mass distribution - they have to be the same; same head mass, shaft mass, grip mass, and mass distribution so that their MOI about all these varying axes of rotation will be the same. The loft of the heads could be different because that doesn't change the mass distribution of the heads very much. This is the basis for the 'constant length clubs' discussed in another thread. But there are other problems with the constant length concept.
So if we are going to have different weight heads, different shaft weights and lengths, and (possibly) different grips we are faced with trying to find a few (at most) parameters that we can measure that we can use to get clubs whose dynamics behavior is close enough that we can swing them effectively and repeatedly. That is what swingweighting was developed for and has been in use for something like a hundred years. Modern electronics has made measuring MOI much more practical and there are those (and I am one of them) that think that is the way we should match clubs in the future. The current belief is that MOI should be measured and matched about an axis through the butt of the club. But that could easily change as we move forward. Keep in mind, though, that this is still only an approximation of what the club is really going through. One data point out of a sea of them.
There is no single parameter that we can measure (or calculate) that will match the dynamic behavior of a set of clubs (short of making them all the same). This is true of swingweight matching and with MOI matching. Becuase MOI is at least a dynamic property of a club I believe it has more potential than swingweight matching. Regardless, there is always going to be room for the experienced clubmaker to find the best match between an individual golfer and his equipment.
I started my engineering career with a slide rule, and cheered when hand calculators became available. I quickly learned that even with hand calculators one had to be wary of obvious mistakes. But it was a better tool than the slide rule it replaced. I believe that over the next ten years I am going to find my swingweight scale in the same dust bin as my slide rule (actually, in my case it pretty much already is). There are many out here that have been using swingweight for years to find that mythical match between golfer and gear, and it works for them and some of them won't ever change. But if you are young and starting out in this business, I would suggest that you become familiar with what MOI is and how it can be measured and used to match equipment to golfers. I really do believe that this is going to be the 'hand calculator' of your generation.
Regards,
Alan Brooks
At 03:51 PM 1/17/2005 -0500, you wrote:
Hi all,
I'm just getting started with clubmaking. I've made a few clubs and was pretty happy with them. I'm now moving on to swingweighting. I've read all the stuff from Jeff Jackson and Dave Tutelman. I can find lots of stuff on how to increase swingweight. I can't seem to find anything on how to reduce swingweight. Did I miss something in all these online sources? What do I do if I need to reduce? (and so far this is only hypothetical since I haven't gotten a swingweight scale yet or tried to any of this) Dave T. says that adding weight to the butt of the club doesn't work, it only modifies the feel, but doesn't do anything to the MOI. If I sound confused, it's because I am. Any help from you veterans out there is much appreciated.
