Understanding Shaft Mechanics
You will find discussions on the following topics:
1. What role does the shaft play in the performance of the golf club?
2. How does the shaft work for you during the golf swing?
3. How does the golf shaft's natural frequency improve our play and why is Frequency the only
measurement of shaft stiffness we should use? See Proof
4. How do we fit the golf shaft today?
5. How to use FitChip for shaft fitting?
The Shaft is the Key component in a Golf Club
The shaft in a golf club is basically a spring, that like any spring when we pull on it, or load it, it stores energy or a reactionary force. When we let go of that spring or release it, that reactionary force, pulls it back to its equilibrium position at a velocity and in a time period dependent on its own natural frequency. The natural frequency is strictly a function of its stiffness of the spring or in this case the shaft. In the case of the golf shaft we think of it as the kick of the shaft. This shaft kick can add as much as 10 to 12% to the club head speed if timed properly. When you accelerate the club in your down swing you are loading the shaft storing that energy or reactionary force in the shaft. The release of the shaft or spring occurs when the acceleration being applied to the club starts to be reduced.
Now is when the mechanics of the shaft is introduced into the equation. Any spring including the golf shaft will vibrate or cycle in the form of a sign wave. From the curve of Figure 1 we can get an idea of how the shaft will react when loaded and released. The shaft is pull or loaded to the peak load position on the
curve. The continuous oscillating line in Figure 1 represents a shaft with the grip end of the shaft rigidly held in a clamping device pulled to the marked peak load position and released to freely vibrate or cycle through its natural frequency reaction. The shaft will follow this oscillating curve slowly loosing amplitude because of damping properties in the material it is made from. In the case of a player swinging a golf club the grip end of the club will be held ridged thru the first quarter of a cycle (the downward part of the curve to The Peak Speed location) by the hands because the shaft is loaded hard against the fore finger of the right hand and the heal of the left hand to the point where the shaft is back to it straight/equilibrium/Peak Speed location. Beyond this point the grip is no longer held tight in the hand and the vibration or cycling is quickly damped by the softness of the hands.
We can also use this sine wave illustration to show how different shaft stiffness's will react to the same loading. The stiffer the shaft the shorter the period is from peak to peak on its sine wave curve. The continuous wave of Figure 1 represents the ideal shaft that comes to straight/equilibrium/peak speed at the desired time of ball impact. Because of the longer period time for a softer shaft it is short of reaching the ideal ball impact location and the stiffer shaft has reached the ideal location prior to ball impact. Figure 2 shows how the
shaft would appear in these situations. This can also be seen from Videos where the same golfer using the same swing, swings three different stiffness shafts in Figure 3, 4 and 5. The light streaks in the video are the results of reflection from aluminum foil that is rapped around the shaft to identify the shafts position. the length of the streak indicates the time a single frame of the video was open recording the picture. The leading edge of each streak indicates the position of the shaft when the shutter closed. In Figure 3 we see
Figure 3 Figure 4 Figure 5
a shaft that is flexed back or loaded early in the swing but prior to reaching ball impact has kicked through so that the head is ahead of the shaft prior to impact. This indicates the shaft is too stiff for this players swing timing. In Figure 4 we see the shaft is back to straight at ball impact which should be the objective of shaft fitting. In Figure 5 we can see the club head is still lagging the shaft at impact, indicating it is too soft for this players swing timing.
The shaft kick shown in these pictures as straight contributes added club head speed up to 10 to 12% The shaft that is too soft has not come up to full speed yet when it gets to impact. The shaft that is too stiff has reached peak speed prior to ball impact and is losing speed by the time it gets to the impact position. Another important thing we can see in these pictures is that the shaft is loaded into a flexed position and unloads though 1/4 cycle of its natural frequency to ball impact when the shaft is properly timed with the players swing. Then we can also see the the shaft is loaded and reacts only in its first frequency mode, Butt Frequency, as a cantilevered beam through the first 1/4 cycle. Meaning it is held firm in the players grip and has a single bow in the shaft which is free at the club head end of the shaft. Since the ball is gone after this first 1/4 cycle any shaft reactions have no affect on the ball. Therefore our primary concern in shaft fitting is only to the shafts reaction during loading and that first 1/4 cycle to ball impact.
Affects of Centrifugal Force
The following Figures 6 and 7 show the affects of centrifugal force on the golf club. Centrifugal force is that force that you experience when you put a weight on the end of a string and whirl it around. When the weight gets to a certain speed it will pull the string straight and stay in a circle about your hand. the faster the speed gets the higher the force trying to pull the string from your hand. In the case of the golf club the head is the weight on the end of the shaft/string. In Figure 6 the centrifugal force is causing the shaft to deflect into a toe down or a shaft droop direction. Also shown in figure 6 are two clubs the top one with a high flex point and the lower one with a lower flex point. The maximum deflection from the centrifugal
Figure 6 Figure 7
force will align the grip end of the club with the Center of Gravity (C.G.) of the head. When you turn the club around, from back swing to down swing, you will load the shaft in a toe up direction. This toe up shaft deflection will snap back during the down swing and will add positively or negatively to the toe down position of the head at impact dependent on when in the down swing that toe up load is released. Both of these deflections depend highly on the stiffness of the shaft. Therefore when being fitted for the lie angle of your clubs the testing must be done with the club you will play with. This toe up toe down affect will affect all of your club but will have less affect as the shaft gets stiffer.
In Figure 7 we show a face up club head rotation caused by the centrifugal force. Again the maximum deflection induced by the centrifugal force will not be greater then when the head's C.G. is aligned with the grip end of the club. The flex point of the shaft as well as the stiffness of the shaft will have an affect on this deflection. This deflection is much more predominate in wood heads then in iron heads because of the G.G. location with respect to the shaft. This affect is why the club manufacturers have place more weight back in the head to help the player get more effective loft. The fallacy with this is that the average player that can use the greater loft does not have the club head speed to produce the affect. If we closely look at
Figure 8 Figure 9
many pictures of players swinging a driver we can see that the forward shaft deflection has the head well ahead of the position where the grip end of the shaft is in line with the head's C.G. as in Figure 8 and 9. This condition is caused by the shaft being too stiff and being released too early for the player's swing timing. This not only leads to inconsistencies in the players drives but a loss in efficiency of ball impact. Not only has the club loss some effective club head speed by being so far ahead of the shaft it has also built up a load in the shaft that wants to pull the head away from the ball if it should lose club head speed. At Ball impact the club head does lose some speed as it compresses the ball and transfers its energy into the ball. Because the shaft is trying to pull the head away from the ball the efficiency of this energy transfer is reduced and results in the loss of distance.
Spine Affects in the Golf Shaft
Spine alignment problems can be caused by two different shaft properties. The first which can be found in any material that shafts are made from is shaft bow or straightness. The second which is predominate in the plastic reinforced shafts such as graphite is a hard side along the shaft caused in most cases by the overlap of material along one side of the shaft when it is produced. If we analysis what the affect these two shaft defects have on the performance of the golf club we can see that the primary fault comes in the stability of the shaft as it is loaded. When we test a shaft for a spine we find that the shaft will always seek a stable position when the shaft is loaded/bent away from the hard side or with the built-in bow of the shaft. This then means that the hard side of the shaft should be at the target or the built-in bow should be at the target. This insures the shaft will load or flex on a straight back path and return on the same path for the first 1/4 cycle of the shafts natural frequency to ball impact and being stable during this loading and unloading period. What happens after The first 1/4 cycle period has no affect on the results of the shot. If the shaft is not spine aligned in the club the club head will tend to track inside or outside of the straight or stable loading path and there fore will return to impact on some inconsistent path.
What Role Does Your Swing Timing Play in Selecting Your Golf Clubs
How many times have you heard the saying "The Golf Shaft is The Engine"? What is being said is that we depend on the Golf Shaft to provide our peak performance. In our automobiles we tune our engines for peak efficiency so why not with the engine of our golf club? So what are we looking for in a golf club to provide peak efficiency? First of all we are looking for distance and secondly for accuracy. Many club fitters will offer you these to attributes as separate objectives but an understanding of shaft mechanics will lead you to understand they do occur at the same time. That time is when the shaft is back to straight at ball impact. That is because peak club head speed will occur when the shaft is straight or at the equilibrium point on it natural frequency cycle. When the shaft is back to straight and the spine has been aligned the head will be square with your swing path/plain.
Now lets take a look at swing timing and the sequence of the golf swing. In figure 10 we have broken the swing down into four
separate sections (not to any specific scale). The light blue area represents the start of the payers down swing during which the player puts or releases energy into accelerating the club head up to speed. This energy input can be accomplished in various ways. 1. pulling the club down with hip turn, shoulder rotation, arm movement and then wrist action by allowing the to freely hing or forcing/powering the release of the hands. Powering the release of the hands in many cases will cause the shaft to be double loaded which I will discuss later. The red area of the diagram represent the are in which the the start shaft of the shaft kick or release begins. The criteria for this shaft release is that the loading phase of the swing is completed and the load on the shaft has started to fall off faster than the natural frequency cycle of the shaft its self will release its load. The time in which this shaft release will occur is only .05 seconds in duration. In the case of a driver if it has occurred at the beginning of this .05 second period it would call for a 170 CPM shaft in the club to get back to straight at ball impact which occurs at the end of the light green area. If the it occurs at the end of the .05 second period it will require a 300 CPM frequency shaft to get back to straight at ball impact. If we look at the frequency range of the OEM shafts the red area becomes only .025 seconds wide. To select a shaft that best responds to a players swing timing (+ or - 4 CPM) our time for accurately selecting the proper release point is .003 seconds. The green area represents the time the shaft is returning to straight though its 1/4 cycle time of its natural frequency cycle which is in the range of .1 to .045 seconds
How Are We Fitting Golf Clubs Today
Fitting With a Launch Monitor
Fitting with the Launch Monitor and that measures the Launch Angle off the tee optimizes the carry of the ball for the shaft and club head combination you are hitting and your club head speed. You must remember however that optimum carry does not mean optimum distance. Again, if you are not using the shaft that best fits your swing you are not going to get the best possible results. You must select the shaft prior to using a Launch Monitor for determining optimum launch angle and club head loft. What the launch monitor is best used for then, is selecting the proper face loft of the club head.
Fitting With Club Head Speed
Club Head Speed has been used by the industry as a simple approach to selecting a shaft flex range. However, there is no scientific relationship between club head speed and shaft reaction. It is only some ones opinion to satisfy the better player's ego, in the way the industry uses it. It has been experienced many times, that different players with the same club head speed, who fit themselves by trial and error, do not pick the same stiffness shaft or stiffness change from club to club, as the industry builds sets, to achieve their "best performance".
Fitting With Tempo
Tempo is measured as the time between club take away and return to ball impact. This technique has a better scientific relationship to the clubs shaft reaction than fitting with club head speed but still lacks the refinement to accurately recommend a shaft stiffness. Within tempo there can be to many variations. The length of the swing and the time spent in each segment of the swing can vary greatly. For further explanation see our fitting technology page.
Fitting with the Shaft Lab
The Shaft Lab was a follow on to the Determinator, using a dedicated club instrumented with strain gages to record shaft loading versus time. Even though this is a good approach the Shaft Lab only uses the magnitude of the load as a fitting parameter and no timing parameters are used to relate the club reaction to the swing. This system is really a modification of club head speed in that the more area under the load curve the higher the club head speed. If two players have the same area under their curves, but different peak loads, the one with the higher peak load will get a stiffer club than the other. This is correct in many cases but with out the timing parameter being taken into account the accuracy and reliability is questionable. This is because the higher peak load could release the club earlier in the swing then the player with the same area under the curve or same swing speed that would release the club later. Any player that generates a lot of area under the curve without a high peak will be a late releaser where a player generating club head speed with a high peak load can have a release that occurs early, late or any time in between.
Fitting With the Mizuno Shaft Optimizer
The Mizuno Company has developed an on the club system similar to the FitChip shaft fitting system. However there are significant differences in their approach to the final results. First of all they use a dedicated iron of single length which does not allow for finding the slope that the set of irons should be built on to fit the players swing progression through a set of irons. Therefore if their recommendations are accurate for that club length it does not mean it is accurate for the full set of clubs. With our experience from successfull, full set, FitChip fittings we have found that less the 5% of players fall on the standard industry slope of about 4.3 cpm per club. However, obviously Mizuno has chosen to make their "custom clubs" on the standard industry slope. Secondly they are doing irons only. The third discrepancy in their method of finding what they call a "release factor" is based on a club turn around time at the top of the swing. From this turn around time they estimate a "release factor" from 1 to 9 and relate that number to available off the shelf shafts. Again from our findings with FitChip fitting this limits them to accurately fitting only 40% of the players if their approach to selecting the "release factor" is accurate. We have had a chance to fit three players that were dissatisfied with the Mizuno fitting results and in all cases found that our successful fitting recommendation was drastically different then theirs and proved to provide better and in all cases found that our successful fitting recommendation was drastically different then theirs and proved to provide better results then theirs. Thus, the accuracy of their approach is at the best
questionable. If we study the Mizuno fitting chart in Figure 11 the Release factor is not considered in the chart unless they consider it in a secondary manner with Tempo. From a shaft mechanics stand point the release factor should be a primary consideration and have a much greater affect on shaft selection than shown here for Tempo.
Fitting by Trial and Error
The trial and error method is the best of these fitting systems if you can afford the time and cost to do a thorough test. Many of the playing professionals have successfully used this process to find the shafts they need for the best performance. These professionals have the time and the access to the wide variety of shafts necessary to successfully complete the trial and error process. The other requirement that the professional provides this process is feel.
How the Golf Shaft Fittings Should be Done
The FitChip Shaft Fitting System was developed to accurately address the mechanics of the shafts response to an individuals golf swing. This is accomplished by recording data at every .002 seconds during the down swing and identifying the shaft loading and unloading pattern for that individual player and matching the shaft to the players timing.
I would like to redefine “shaft loading” as stated by Jeff Jackson. Even though it can occur at the transition point between back swing and down swing, for most players the peak load occurs at some place during the downswing. The timing of this loading and most importantly the start of unloading is the key factor in club fitting. The key parameter that FitChip uses in selecting a shaft is the time between the start of unloading or shaft release and ball impact. The golf shaft is a spring. Depending on the stiffness of that spring it takes a certain amount of time for the spring to recover from the deflected position to the neutral position (for the golf shaft, straight). It is at this neutral position that the golf shaft reaches its greatest effectiveness (maximum speed and club face square). As some may have suggested, if ball impact occurs at peak loading, the shaft will still be flexed and be much less than effective in aiding the golfer. Based on spring/shaft mechanics the stiffer the spring the shorter the time of recovery. The natural frequency of the golf club describes this recovery time and the mechanism that drives the shaft back to straight upon release. Therefore the earlier the release is in the swing the softer the shaft and the later the release the stiffer the shaft. You will quickly find out using this system that shaft selection has nothing to do with club head speed as the industry has used it in the past. In fact I can show you, that if two players have the same time between release and ball impact, the one with the higher speed needs a softer shaft. This occurs because there are two mechanisms acting on the shaft to return it to straight and square. The first one, which we all understand is the spring action of the shaft that is described by natural frequency. The second one is the centrifugal force pulling down on the weight of the head to straighten the shaft and is a direct function of club head speed. Since this Club head speed induced force is helping the spring action the spring action needs to be reduced to get the correct timing to be back to straight and square at the time of impact. Then since the high club head speed player gets more help from club head speed he would need the shaft with a lower frequency (softer shaft). Both of these timing mechanisms are accounted for in the FitChip Shaft Fitting System.
The FitChip analyzes this timing and club release problem and selects the clubs natural frequency that will return it to straight and square at ball impact. No other system available today for club fitting can identify the point of club release accurately during the swing. This timing is what best creates the feel and timing between the player and his clubs. The data collected by the FitChip (up to 84 full swings) can be downloaded to any computer to view the pertinent individual swing data. You will find that many players have double loads and releases that make the process even more difficult.
The Test and Analysis Process With the FitChip
For your swing testing you may use the players current clubs or a set of test clubs. We recommend testing be done using the 9 Iron, the 5 Iron, 3 Iron, Hybrid or 5 Wood and the Driver. This insures a sufficient number of data points to establish a slope or curve on which you would build the set of clubs. The player will swing each club 4 to 10 times to establish sufficient data for a reliable average. This data (up to 40 swings per player and a total of 84 swings) will then be downloaded to a larger computer for further analysis of the players swing pattern.
The larger computer basically makes the same analysis as the FitChip. However, on the larger computer we can actually view the Shaft Acceleration or Load pattern Figure 12 and possibly refine the selected point of release for better accuracy. The following Chart is representative of the type of acceleration data provided by the FitChip. The blue line represents the player's club head acceleration or shaft load curve. The red line represents the release unloading of the shaft (as a spring) in the swing. The time from the start of club release to ball impact determines the clubs shaft stiffness. The Raw Data and Club Head Speed curve Figure 13 is used to select ball impact. The abrupt drop in club head speed is caused by Impact. Once each
Figure 12 Shaft Acceleration or Load Figure 13 Club Head Speed
Figure 14 Frequency Chart
of the (up to 40 swings per player) swings have been analyzed and marked "USE" the data can be plotted (small dots) on the following Club Length VS Frequency Curve (Frequency Chart) Figure 14. The larger red dots represent the average of the several selected swings for each club.
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