When golf clubs are being discussed, especially drivers and fairway clubs, a lot is written about club shafts: length, weight, materials, flex—all the physical attributes that affect a shaft’s performance. But how, and how much, does the shaft’s behavior really affect the club’s performance, and ultimately, ball flight? And how important is the single most-discussed aspect of a club shaft—stiffness?
[Before I get much further, let me get something off of my chest: Most articles about club shafts misuse the term “torque”. Torque is a force input—the moment of a force acting perpendicular to an axis—but it seems as if every article I read uses the term to denote a shaft’s reaction to that force, e.g., referring to a “low-torque” shaft; instead of the force itself. In this usage a “low torque” shaft is one with high torsional stiffness; that is, a high resistance to twisting.]
One very important thing to understand about the club shaft—and something that many golfers are probably not aware of—is that the majority of its influence on ball flight happens before impact. Doesn’t sound right to you? Let me explain:
The ball and club face are in contact for such a short period—contact time is measured in milliseconds—that there simply is not enough time for the shaft to react to impact forces and significantly affect the interaction between club and the ball[i]. However, the forces acting on the club head during the swing have plenty of time to affect the shaft, which in turn affects the orientation of the club face—and that’s where the physical properties of the shaft come into play.
The downswing is initiated when the golfer accelerates the grip end of the club. The higher the acceleration—that is, the more quickly the golfer gets the club up to speed—the greater the force acting on the club head, at least in the initial portion of the swing. [If you remember anything from Freshman Physics, it should be F=MA (Force = Mass × Acceleration).]
The ball and club face are in contact for such a short period—contact time is measured in milliseconds—that there simply is not enough time for the shaft to react to impact forces and significantly affect the interaction between club and the ball[i]. However, the forces acting on the club head during the swing have plenty of time to affect the shaft, which in turn affects the orientation of the club face—and that’s where the physical properties of the shaft come into play.
The downswing is initiated when the golfer accelerates the grip end of the club. The higher the acceleration—that is, the more quickly the golfer gets the club up to speed—the greater the force acting on the club head, at least in the initial portion of the swing. [If you remember anything from Freshman Physics, it should be F=MA (Force = Mass × Acceleration).]
Thanks to inertia, the club head wants to stay where it is while you are accelerating the grip, and this inertial lag induces a backward bend in the shaft at the beginning of the downswing while the club head catches up. Studies have shown, however, that this effect is short-lived, and is quickly overcome by the forces acting on the club head as it is comes up to speed[ii].
You may have heard the term kick point in reference to club shafts and envisioned something like the shaft flicking the club head forward and adding impetus to the impact—but you would be wrong. While the club head is rotated forward in response to the forces acting on the shaft during the downswing, it is not a “flick”.
CAUTIONActual science/engineering content follows. Marketing/sales personnel may feel free to look away, or skip entirely.
This is how it works: The centrifugal force generated as the club is swung acts through the perpendicular distance between the center of mass of the club head and the line of the shaft—the moment arm—to produce a twisting force, or moment, which bends the shaft forward, increasing the loft of the club face. The point in the shaft at which it begins to flex under the influence of the swing-induced moment (frequently manipulated by changing material and/or thickness) is the kick point.
The kick point is one of four factors which affect the magnitude of this phenomenon: 1) swing speed, 2) club head center of mass location, 3) shaft stiffness, and 4) kick point location. The least intuitive of these factors is the location of the center of mass, but think of it this way: the further behind the club face the center of mass is located, the longer the moment arm is that is being acted upon by the force generated by the swing speed – in simpler terms, the force acting on the club head has more leverage. (I break this down a bit more a few paragraphs on.)
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| Centrifugal force acting on the center of mass of the club head produces a twisting moment that can increase the loft of the club face. |
You hear a lot about Trackman data showing that pros and elite amateur players have a negative attack angle with driver—they swing down on the ball—while recreational golfers are told to swing up on the ball. Well, part of the reason that elite players get away with a negative attack angle is that, even with the stiffer shafts they play, their higher swing speeds generate an increase in loft, resulting in that high, soaring flight that we all wish/hope for off the tee.
So how does this play into selecting the best shaft for your game? Delve deeply into that question and it starts to look like a real can of worms.
There are three club-related factors that affect ball flight—swing speed, swing path, and club face orientation—and since swing speed also affects the orientation of the club face by way of the deformation that it induces in the club shaft, it is doubly important when analyzing the swing.
Shaft weight and the overall balance of the club (“swing weight”) have some effect on swing speed, but the overall stiffness of the shaft and the location of the transition, or kick point (if there is one), are the most important characteristics of the shaft when it comes to affecting club head position at impact.
As mentioned above, the shaft’s response to swing speed—that is, the amount of “kick”—is dependent upon the overall stiffness of the shaft, the location of the kick point, and the magnitude of the force acting on the shaft. The magnitude of the centrifugal force is determined by swing speed and the length of the moment arm, M, which is determined by the location of the club head’s CG, or center of mass. Now do you get what I said about a can of worms?
A look back through that shopping list of factors which determine how the shaft affects club face orientation at impact can set your head spinning when you consider the variability of those factors— and perhaps the most variable of them all is the swing.
Variability in swing speed can be a matter of inconsistency or design. We don’t always swing at the same speed, even if we are trying to. Sometimes we may take a real big whack at it, trying to out-drive a playing partner, or carry a hazard, and sometimes we throttle back—and as swing speed varies, so does the amount of change in loft that is caused by the forces acting on the club head.
A good club-fitter, with the right data-gathering technology and a wide variety of shaft/club head combinations at their disposal, will be able to sort through the variables and put any golfer into the optimum combination of shaft and club head for their swing. But which swing are you being fit for? Your usual 9/10ths driver swing? The little poke you take to keep the ball in play on a narrow fairway? Or your all-out, swing-for-the-fences lash for those long carries or ego-tempting drivable par-4s? Is one shaft going to be satisfactory for the full range?
Because swing speed is so variable, and because the ripple-down effect of swing speed on flex-induced loft increase varies with swing speed, I think that reducing one area of variability in the swing—shaft flex—can go a long way toward producing consistent ball flight, and the best way to do that is to go with a stiff shaft.
But Your Swing Speed Determines Your Shaft Flex, Doesn’t It?
So why are more flexible shafts usually recommended for slower-swinging players? The popular wisdom is that they are generally lighter in weight, and therefore easier for players with less strength or less-efficient mechanics to swing at their best speed. The problem with that hypothesis is that studies[iii] have shown that within the normal range of shaft weights, a reduction in weight has little effect on swing speed.
I think that the real effect here is actually a sort of back-formation; that is, since lower swing speeds produce less loft increase, these players can use a more flexible shaft without inducing a large increase in loft which would cause the ball to launch at a too-high angle. Also, shafts that are more flexible generally also have a softer “feel”, a purely subjective aspect of club performance that many players associate with improved touch and control.
Launch angle is the aspect of ball flight that is probably most affected, and most directly affected, by the kick point loft increase that we have been discussing—but launch angle can be controlled more consistently by ball placement relative to stance, and by teeing height. Set up with the ball more forward in the stance and/or teed up higher, and a higher launch angle results; moving the ball back in the stance and teeing it lower will do the reverse.
So why, when launch angle can be managed directly and more consistently by these two very controllable factors, would a player rely on the highly variable factor of kick-point loft increase to achieve the desired launch angle? Looking at the problem from that point of view, I have come to the conclusion that the best shaft for any given swing is the lightest shaft which is stiff enough to minimize shaft flex before impact.
I feel that, in general, the golf industry makes things a lot more complicated than they need to be when it comes to designing and selecting club shafts. Fine-tuning flex and kick point for swing speed in order to optimize the increase in loft that arises from the centrifugal force acting on the club head is a complex task. As a design engineer I always look for the simplest solution to a problem, and to my way of thinking the lightest, stiffest shaft possible is the best way to go.
Depending upon shaft flex to provide loft-up to help get the ball into the air brings into play cumulative effects from two factors that change when the swing is dialed back or pushed up—reduced/increased kinetic energy and less/more loft increase—due to the change in swing speed. Going with a stiff shaft eliminates, or at least reduces, a highly variable factor that has a great effect on club/ball interaction at impact.
Use a stiff shaft and let ball flight be affected by factors that are more directly controllable, like swing speed, and teeing height. The physical loft of the club face at impact will vary less with a stiff shaft, simplifying the job of consistently delivering the club face to the ball in the best orientation.
[i] Dewhurst, P. The Science of thePerfect Swing Oxford University Press 2015: 41-42
[ii] Milne, R.D. and Davis, J.P. “The Role of the Shaft in the Golf Swing.” Journal of Biomechanics 25, no. 9 (1992):
975–983
[iii] Cross, R. and Bower, R, “Effects of Swing Weight on Swing Speed and Racket Power” Journal of Sports Sciences 24
(2006): 7–15.
