# Shaft Flex

## What Is It, and Why Do We Care?

Shaft flex is a rating of how easily the shaft bends when a force is applied to it.

That's an obvious definition, and not very useful to a club designer. Before we go to a more precise and useful definition, let's first examine what happens to the shaft in the swing. First, consider a swing where the flex is matched perfectly to the golfer.

• During the slow backswing, there isn't much bending force on the shaft.
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• At the top of the backswing and beginning of the downswing, the change of direction and downward acceleration "loads" the shaft; that is, it causes the clubhead to lag the hands. The club bends backwards in response to this acceleration force.
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• During the downswing, there is some degree of accelerating force at all times. But at some point, the stored up "springiness" of the shaft takes over and starts to straighten the shaft out. This is similar to the scenario in the chapter on Physics, section on vibrational frequency; however, instead of plucking the end of the club, we're turning the butt of the club to supply the initial force.
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• At the moment of impact, the shaft will ideally have "unloaded". At this point, potential energy of the shaft flexed shaft has been converted to kinetic energy in the form of tip velocity in the same direction as the swing. The shaft's energy is adding to clubhead speed. This is not much: no more than 3% of clubhead speed for most golfers, and very seldom more than 6%. Still, that is significant enough to be worth designing for. And there are a few golfers that researchers have tested with up to 10% kick velocity.
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• In practice, this "unloaded" position involves some forward bending of the shaft. We will discuss later how much, and what the performace result is.
This is the scenario for an ideally matched shaft flex and swing. But things can go wrong if they're not ideally matched. Here's the spectrum of possibilities:
• The shaft is too stiff: In this situation, the shaft never fully loads, so there's not much to unload. The shaft is straight at impact, not because it's in the middle of its vibration, but because it's been straight for the whole downswing. The contact feels "dead" to the golfer rather than "crisp", and the shaft contributes nothing to the shot.
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• The shaft is exactly right, as in the scenario above: In this situation, the shaft is adding to the clubhead speed in a "neutral" way. Because it is straight (as at address), there is no loss of accuracy due to the shaft's vibration.
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• The shaft is a little too flexible: At impact, the shaft is bent forward; it has passed through its straight position. It may well be contributing even more clubhead speed, but the clubface won't be exactly as it was at address and the ball may go somewhere other than where you aimed. Because the shaft is bent forward, the clubface is pointing higher and more to the left than it was at address, so the most likely inaccuracy is too high or hooked. Wishon and Summitt call this a "high risk, high yield" shaft, where you choose to trade accuracy for distance.
• Note: Many find it counterintuitive that the shaft would be bent forward, and that it would contribute more to clubhead speed. But there is substantial experimental evidence, and I believe it.  I also think I have an explanation.  The "swinger" (as opposed to the "hitter") uses centrifugal force to get clubhead speed; thus the clubhead is pulling the shaft, rather than the hands pushing it.  This would also explain why some very small minority of golfers slice with a too-flexible shaft rather than hook; they must be rather extreme "hitters", to the extent that the shaft bends backwards.

• The shaft is a lot too flexible: As the shaft gets more flexible relative to the golfer's swing, the forward bend becomes more pronounced. The loss of accuracy increases, but is no longer compensated by increased clubhead speed. In fact, now both accuracy and clubhead speed become rather erratic and unpredictable.
•  Summitt and Wishon report this happens when the clubhead speed is more than 13% above the speed for an ideal straight shaft.

So our goal is to estimate the straight-shaft flex for the golfer, then choose a shaft with the flex to put the club where the golfer wants it in the distance-accuracy tradeoff space.