The Dynamic Flexing of a Golf Club Shaft During a Typical Swing

8/12/2019 The Dynamic Flexing of a Golf Club Shaft During a Typical Swing

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THE DYNAMIC FLEXING OF A GOLF CLUB SHAFTDURING A TYPICAL SWING

Simon Newman Lecturer n Helicopter Engineering, Department o AeronauticsAstronautics, University o SouthamptonStephen Clay P. G.A. Golf Professional, B lackmoor Golf C lub, HampshirePaul Strickland Senior Lecturer in Sports Technology, U niversity o PortsmouthThe Big Bouncer had a head o it the size of an astronaut s helmet, and the sound itmade when it struck the ball was similar to that offive hubcaps being dropped on amarble porch fro m a second storey window.

The Perfec t Dr iver - Dan Jenk ins

IntroductionThe normal golfer, if that is not acontradiction, without exception getsembroiled in the purchase of numerous sets ofgolf clubs during their playing life. They arevery often enthusiastic about their game andwill, in many cases, succumb to thetemptations of high technology advertising.The golf swing must be a very precisemovement of the human body and the golf clubitself to enable a small area of club head tostrike the ball in exactly the correct direction ifthe successful outcome of a high quality golfshot is to be obtained. The sear ch for length ofshot has been like a holy grail and itsachievement a desperate wish, if not longing.To this end club and ball manufacturers havefor many years been devdoping the latest inmaterials and manufacturing processes toenable the golfer to deliver the correct strike tothe ball with the maximum club head speed.The achievement of precision at high speed isthe goal and it usually falls on the golf clubprofessional to supply his or her customerswith the clubs necessary to enable eachindividual swing to be optimised. It soundseasy but the dynamic behaviour of the golfclub, in particular the shaft, during the fewseconds of a full golf swing is a very complexprocess whereby kinetic energy and strain

energy are being constantly exchanged. Tofurther complicate the problem, the wrists ofthe golfer are in constant rotationalrealignment. This means that the shaft is beingsubjected to bending moment distributionswhilst also having the axes of the shaftrotating. It is because of the complicated natureof the golf swing that a piece of research workwas initiated as a final year student project [ ]to use existing techniques to measure the clubshaft deflections as a time history.

Strain Pattern AnalysisApart from an interest in playing golf, thereason for a helicopter engineer becominginvolved in club fitting might seem strange.The case for the defence is as follows. Thehelicopter rotor blade is subjected to a rapidlychanging aerodynamic and dynamicenvironment during the rotation of the rotor.The measurement of any blade deflectionsduring flight is a necessary requirement for theintroduction of new blade designs or the use ofnew materials. A number of years ago theRoyal Aircraft Establishment at Farnborough[2] developed a method of obtaining suchmeasurements in the very difficult flightregimes of high speed. A number of straingauges were fixed to the blade along its lengthand the responses of these gauges were then

2650-8186-8025-319710.00 1997 EEE


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performing this exercise a number of times thelink between the way strain gauge responsesvary along the blade length can be made to theblade deflection profiles. In other words, thepattern of strain along the blade can be

Figure Location of Strain Gauges

shaft and placed at E.The gauge positions are summarised i n Table

bendin g and torsional deflections.


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Modal Excitation backwards two things happen. Firstly theIn order to provide a set of modes for the clubshaft the club w as fixed to a shaker which wasdriven by a variable frequency generator. Afrequency sweep was initiated and the clubbehaviour was monitored by the illuminationof a variable frequency stroboscope. Acalibrated back drop was used to measure theclub deflections. The aim was to establish atleast the first two modes with any others beinga bonus. In similar helicopter bladeexperiments this technique has shownconsidera ble success, however, in the case of agolf club, the influence of the relativelymassive club head very rapidly dominates thevibratory characteristics of the club. Thefundamental mode was easily established butthe overtone modes proved particularlydifficult to isolate because of the inertia of theclub head. In o rder to complete the experiment,a set of vibration shapes and frequencies wasselected and used as the basis for the clu b shaftdeflection experiments.

inertia of the clubhead causes it to followbehind causing the shaft to bend in lead, andsecond ly as the rotation of th e swing, abo ut theshoulders, begins, the clubhead is thrownoutwards by the centrifugal forces. Since thecentre of gravity of the clubhead lies in front ofthe shaft axis a toe down bending will result,point B.Second part of the backswingAs the club passes the horizontal and risesabove shoulder height, the wrists begin theirturn which complicates the results since theflexing of the shaft in IeadAag and toe upldownwill exchange roles. Consequently, the clubshaft will start to recover, point C and theinitial lead deflection will diminish and, apartfrom some oscillations, will not vary greatlyfor much of the remaining backswing.However, the toe down deflection caused bythe clubhead being thrown outwards will nowbe influenced bv motion of the club in theswing plane since the shaft has now beenrealigned by the wrist turn.Swing Experiments

The instrumented club was then tested onvarious pe ople with varying degrees of ability.For o bvious reaso ns, the test was concentratedon the swings of a professional. The typicalclub shaft behavior is shown in Figure 4 norder to explain the important features of theresults, Figures 5 6 are simplifications of thestrain gaug e outputs s shown in Figure 4. As aguide to magnitudes, the maximum club headdeflection corresponds to about 3 inches. Theclub used had a particularly stiff shaft sogreater club head deflections can be expectedfor softer shafts.

ResultsLead/Lag Toe Up/Down

A typical variation in leadflag and toe up/downflexing of the club shaft is shown in Figure 5.(For clarity, the toe upldown trace has thegreatest m agnitud e at point D.)

First part of the backswingAt the beginning of the traces, point A a slightforward press of the hands is indicated by theshaft flexing to e dow n and in the lag direction.The downswing proper commences at the firstvertical line. As the hands accelerate

Top of the backswingAs the top of the backswing is reached thehands will slow to a halt at the top and themomentum of the clubhead will causes theshaf t to flex in a toe up sense.

Commencementof the downswingAs the downswing begins, s shown by thesecond vertical line, the high acceleration ofthe hands downwards towards the ball causesthe inertia effects of the clubhea d to be felt bythe shaft, only this time in the toe up direction.The heavy acceleration gives the greatestdeflection of the shaft, poinr D particularly asthe hitting area is encountered.

Hitting areaThe situation is again complicated by rotationof the wrists and the energy stored in the clubshaft by its bending will begin to release s theshaft recovers but the direction of this varies asthe shaft rotates. The IeadAag trace shows anincrease in lag as the downswing progressesbut as impact is approached, the turning of theshaft causes the energy release to be felt in theleadflag direction even though it s created bythe shaft bending in the toe u p direction. It is arule of golf that club shafts must be equallyflexible in all planes containing the centerline

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Impact

understood if

cond part of the backswing

e massive

ionsesearch experiments haveted manner of golf club

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Time

lead/lag

toe upbe down

Toe DownL

Figure Le ad na g Toe Upmow n Variation of Golf Club Head

I torsion

Figure 6 TorsionalBehaviour of Golf Club Head

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Acknowledgmentswould like to thank their contributions toPaul Thomson wouldfor his individual prcolm Scott the P.G.A.Golf Club, Hampshir

former Assistantrd G olf Club, Hampshired whereby the

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The Dynamic Flexing of a Golf Club Shaft During a Typical Swing