Jeff Summitt Examines Driver Size and Limitations
You might be surprised just how much you can tell just by looking at the shape and size of a golf driver head. After you read this article and with a few measurements, one should be able to approximate some information about your driver that it takes a machine costing several thousands of dollars to do. What prompted this article? I had a customer recently email me why we don’t publish more information on our golf clubheads. I politely responded that in many cases it can confuse the average golfer. He was interested in particular to the MOI of our heads. He wondered which of our non-square models had an MOI of 5900 g-cm2 (an explanation will come later).
In the Rules of Golf, there are a few specific limits outlined in Appendix II regarding clubheads, in particular a driver head, that manufacturers are required to know prior to designing a new model. Manufacturers are constantly trying to improve from one generation of clubhead to the next. Many of these changes are minute as each year passes and we get closer and closer to approaching these limits set forth by the USGA and R&A Rules Limited.
Just because there are limits, does not mean that they are necessarily achievable. One such limit is the MOI or moment of inertia about the face of the club. Many of the design variables go hand in hand. That is one change to a certain parameter will have an effect on another. This will help to explain why there are certain limitations depending upon the shape of the club.
Golf club Volume is one of the limits on size stipulated by the USGA. The maximum allowable volume is 460cc (+10cc tolerance). That is if you submerge the clubhead in a calibrated beaker of water up to the hosel (if one exists), it will displace no more than 460cc. The other method is weighing the clubhead when submerged in water using an electronic scale. Some electronic scales have a tare button which will zero out the weight on the scale. All you have to do is tare the scale, submerge the clubhead, steady it, then read the new gram weight on the scale. If the weight is 460g, then the volume will be 460cc.
There is not a rule that governs the weight of the driver directly. Driver heads on average are 200g ± 4g. What happens when you change weight? One change is it affects the swingweight of the club. Most Men’s drivers are 45” have a weight of @ 200g and this creates a normal swingweight range. Drivers that are produced longer than 45” usually have slightly lighter heads. If heads are made shorter than 45”, clubmakers typically find ways of adding weight to the head. Weight is also a factor which affects our next topic.
For a more in-depth discussion on swingweight, please click on the article Fitting For Club Club Swingweight.
The USGA does have a limit on the MOI of a golf club (5900 g-cm2) about its face. This is a measurement that helps describe the resistance of twisting about a specific plane. The higher the number, the greater resistance there is to twisting, in this case off-center shots. The MOI is a function on the weight (and weight distribution) of the head as well as its physical dimensions.
For a more in-depth discussion on the moment of inertia, read Golf Club Moment of Inertia Defined.
In addition to volume, another subject regulating size according to the Rules of Golf is the physical dimensions or proportions. The maximum length from heel to toe of a driver head can be no more than 5” (127mm). This may seem like a cut and dry measurement, but the definition of where the actual heel is can be in question if the heel is not clearly defined. One contingent rule is the distance from heel to toe must be greater than from front to back. This means if the full 5” from heel to toe is met, then the breadth (front to back measurement) can be at most just shy of 5”. Most clubs come no where near the maximum breadth. As a way to describe clubs, we can categorize them based on the percentage of the breath as compared to the heel to toe dimension.
The Traditional category had been a mainstay in golf for a very long time. From the days of the modern “pear-shaped” persimmon or wooden heads, then when metal wood first came into existence, traditional shaped clubs are still very popular today. The proportion of the traditional shaped wood has a breath @ 80% of the distance from heel to toe. At 460cc, these will possess the deepest faced models and at 200g, the MOI range will be approaching 4200 g-cm2. Even though this is far from the 5900 g-cm2 limit, these heads are still very forgiving compared to smaller heads in the past. Better golfers will gravitate toward this category of club for two reasons; it is a shape that they may familiar with and adorn at address, plus the ability to “work” the ball if need be.
Starting around 2005, an industry trend started in which clubhead manufacturers began stretching the breadth of the head to shift the center of gravity deeper or further behind the face. We will call this the Full Bodied category whose proportions to the breath are @ 85% of the distance from heel to toe. The additional 5% does not sound like that is a lot, but considering the heel to toe dimension is 5”, this equates to the breadth increasing by 0.25” (6.4mm).
In 2006, manufacturers began stretching the breadth even greater to increase the Face MOI and shift the CG deeper within the head. We will coin this category as Wide Body with the breath @ 90% of the distance from heel to toe. Another term that you may hear to describe this type of design is “bullet shaped”. In order to maintain the maximum allowable volume, then the head must become shallower in crown height. One trade off for the higher Face MOI is that now there is less room for error above and below the center of the face to make contact with the ball.
Super Wide Body
2007 marks the year we started to see a manufacturer develop a driver closer to the 5” x 4.99” limit. Probably a good term to describe these proportions would be Super Wide Body in which the breath is @ 95% of the distance from heel to toe. In order to maintain the maximum allowable volume, then the head becomes very shallow. The other alternative is to scoop out the crown to form an inverted shape so that it takes up less volume when placed in a beaker of water. The resultant is a higher pitched sound at impact due to creating the non-traditional crown shape. Manufacturers have to be extremely creative in producing a model in this category.
Comparing the Different Shapes
In the following table are some approximations of the dimensions we just described. For instance, let’s look at a 200g Full Bodied titanium driver without screws or additional internal weighting. At 460cc, the club can be a maximum of 5” (127mm) from heel to toe, have a breadth of 4.25” (108mm) and a crown height of 2.45” (62.2mm). Without even placing the clubhead on a MOI measuring device, the Face MOI is going to in the 4400 range. If you like that shape, but want a head with a significantly higher Face MOI, it will not occur as there are physical limits that no manufacturer can defy.
As mentioned previously, not all drivers weigh the same, plus you have to factor in the plus and minus weight tolerances in production. The last row in the table shows the approximate range if the weight was between 196 and 204 grams. Finding heads that have the ability to change to heavier weight screws is another way to increase the Face MOI although one must consider the consequences by increasing the head weight in certain applications.
It is not as easy as it sounds, but if a manufacturer can make head any lighter with the maximum volume by either creating thinner walls or utilizing lighter materials, then the discretionary weight can be re-distributed elsewhere in the head. Screws, internal weighting and carbon top clubs are examples of these as another way to increase the MOI slightly higher with the proportions in the previous charts. The problem is it is hard to make the wall that much thinner than they are presently or substitute any lighter materials than what are already employed. Plus the down side is incorporating some of these features into a design can be more prone to the head prematurely breaking from normal use.
Changes to Geometry
Due to limitations with a conventional shape manufacturers are pushing the threshold in producing heads with more geometric shapes. The triangular shape is the latest being tested by a few manufacturers. This allows the head to be broader from face to back without sacrificing crown height and still be in at the maximum 460cc as there is material devoid from the rear heel and toe areas.
Square heads naturally create a higher MOI as more weight is in the rear corners. If the heel-to-toe, crown height and breadth dimension are the same as a conventional head of 460cc, then the volume will be a higher. Therefore either face height or the breadth has to be sacrificed to allow it to conform. The following chart is a comparison if the head shape was “square”.
OK, I’ll admit “
Hopefully you gained a better understanding on not only the limits on MOI, breadth and crown height, but also why. For example, why you do not see deep face drivers that are broad from front to back? Or why certain design parameters cannot be achieved and still allow the head to conform to certain rules established by the USGA? Lastly, why certain parameters are not physically able to be produced with the current materials and technology available today? So when someone asks you what non-square driver model on the market has a face MOI 5900 g-cm2, you can safely say they do not exist.
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