Another excellent slo-mo verifies my theory?

2016 April 19
by Daniel Lakeland

On the way down at 0:49 see how the club is bent back away from the direction of travel? This is due to a torque applied by the arms to get the club rotating as well as translating. Since the club is going around a circle with a constant angle relative to his arms the rotation and the translation of the club are kinematically linked.

At about 0:53 as the wrists unlock, the club is straight and at 0:56 or so it's actually bending the other way (towards the ball). At about 0:56 the hands are also at their slowest. My assertion is that the left hand is applying a backwards force on the grip of the club while the hands hinge. That backwards force is what's responsible for the *forward* club bend that is very visible at 0:57. That backwards force is a necessary part of getting the club into almost pure rotation around the hands, because it's stopping or at least slowing the translation of the club grip end. Tuttleman's point from the previous post is that you shouldn't apply a pure torque with your hands or at least should do so very late. But a single backwards force applied at the grip end of the club together with the inertial force of the center of mass form a torque (force couple) in the accelerating frame of reference of the club center of mass, and that's the torque that is responsible for the forward bend of the club.

No, that above bit doesn't make sense. Sure a backwards force at the grip and the inertial force form a couple, but they wouldn't cause shaft bend, they'd cause change in rotational speed. Only with a torque applied to the club by the hands would we get a shaft bend. But the whole thing happens quickly, a torque that is helpful for getting the club head going by locking the lag angle is probably still persisting because it takes a while for humans to respond.

3 Responses leave one →
  1. Richard Kennaway permalink
    April 20, 2016

    That backwards bend is just the reverberation of the club after impact. The hands play no role except to provide a passive mass on the grip end that affects the dynamics of the reverb.

    Angular momentum is a red herring here. It can be measured about any point, not just the centre of rotation, and what point it is measured about has no physical significance, any more than the reference point for measuring positions. Change the point of reference and the value changes, without anything physical happening. Changing the centre of rotation is a physical thing, but has no effect on angular momentum referred to a fixed point.

    Consider a pendulum, swinging towards the vertical, at which point the cord meets an obstruction half way along its length. The bob continues with the same velocity as before, and its angular momentum referred to the new centre of rotation is halved.

    I once saw a life-size mechanical model of a golf swing. At "full cock", the wrists were locked to hold the club at right angles to the arms (modelled without elbows). When the shoulders, driven by an elastic cord, were released, the wrists were automatically unlocked at a certain point in the swing. The wrists were otherwise unactuated, making it equivalent to an upside-down trebuchet. The club was vertical at the point of impact.

    So applying the wrist muscles during the release phase is not necessary to make a golf shot. Whether it is useful and what the optimal force curve is can only be decided by a mathematical analysis. I'll be interested to see what results you get.

    • Daniel Lakeland
      April 20, 2016

      The bend I'm referring to is that immediately prior to impact the club bends so that the shaft bends the head *towards* the ball. You can see this effect more dramatically in some of the photos linked on Tutleman's website:

      scroll to the bottom and you can see a frame from the "dark background" Tiger woods swing immediately prior to impact, the golf club bends so that the head is bent towards the ball prior to impact.

  2. Daniel Lakeland
    April 20, 2016

    Tutelman points out that this means there's a clockwise torque being applied by the wrist. That seems counterintuitive because it should slow the club head, but I suspect it's an inevitable part of good form. Humans can't "turn off" their muscles instantaneously, and they are linked together. So as there's a transition from loading up energy into the club using a stiff wrist to the point where the upper body and arms slow to put the hands right into the correct place to form the pivot point, they also are still applying their backwards torque that's a part of loading the club.

    You're right that you can't invoke conservation of angular momentum unless you're referencing everything to a constant pivot point. But I still think that you can't analyze the golf swing without understanding how the transition occurs between locked wrist upper body turn to unlocked wrist with a pivot point around the wrists and how it does or doesn't put energy into the club. It wouldn't surprise me to hear that the stiffness or softness of the club shaft is important for storing and releasing energy, considering how much effort has been put into fancy club shaft materials.

    The mathematical analysis will be interesting, but it'll take some time to set up.

Leave a Reply

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS