THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 7


On January 12 of this year I started a new direction for The Skier’s Manifesto with a critical examination of the mechanics of platform angle after concluding that this issue and its effect on dynamic stability is the single most important factor in skiing. The platform is the portion of the stack of equipment between the sole of a skiers foot and the base of a ski. I started my discussion with a review of some of the typical technical terms associated with platform angle mechanics.

In my last post, I examined rotational force applied to a ski. I noted that in the technical terminology of skiing this is referred to as steering. I identified a number of inconsistencies, ommissions and errors pertaining to steering that I will expand on in this post.

Platform Paradoxes

Technical discussions on steering typically show a ski rotating like a propeller about the center of its long axis. In my last post I demonstrated that the source of the rotational force or steering is the femur rotating in its joint with the pelvis and applying rotational force to the foot its lower (distal) end at the tibia.

The graphic below shows the axes of rotational force (steering) applied to a ski through the foot/ski boot interface by the leg. I’ve used a large ski boot and a short ski to illustrate the effect of the location of the axis of rotation.

Technical discussions of steering don’t always mention the source of steering force let alone show its location. In addition, no explanation is offered that would explain how a ski can rotate about its center like a propeller.

The graphic below shows a ski with the running center of the long axis with approximate location of the axis of rotation indicated. In this example the axis of rotation is approximately 11.5 cm behind the running center (C). On my own skis, the axis of rotation is approximately 13.5 cm behind the running center for my 335 mm ski boot.

When the ball of the foot is located on or close to the transverse center of the long axis of the running surface of a ski the axis of rotation will move progressively towards the shovel as a foot gets shorter and move progressively towards the tail as a foot gets longer. No one seems to mention this even though it raises a number of signficant issues, among them the effect on the edge hold and carving characteristics associated with platform dynamics.

Where is the Force Applied?

Technical discussions of platform mechanics typically don’t show or even mention the location of the force applied to a ski by the weight of a skier. Since the weight of the body is transferred to the foot from the lower end of the tibia the weight tends to be transferred to the foot close to the heel.

Some discussions of platform and steering mechanics even suggest that a skier should feel their weight under their heel when steering the skis. This would place the applied force on the transverse center of a ski, behind the center of the long axis and offset from the inside edge where it will create a torque or moment arm that will degrade platform mechanics.An analogy of the mechanics of rotational force applied to a ski by rotation of the leg is a vertical shaft (leg) rotated by a force with an arm (ski) projecting outward from the shaft.

As the arm gets longer the distance the end of the arm travels for every degree of rotation of the shaft will increase.

  1. How will increasing the length of the arm effect the application of force applied to an object by the end of the arm distant from the shaft given a rotational force (torque) of a fixed magnitude applied to the shaft?
  2. How would reducing the effective length of the arm acting on a ski affect platform mechanics, in particular edge hold and carving characteristics?

There is a way to reduce the effective length of the arm acting on the ski. Elite skiers can do it. This will be the subject of my next post.

2 comments

  1. I think that the C.S.I.A. (Canadian Ski Instructors Alliance) perhaps comes closest to describing in layman’s terms your description of the rotational process originating in the hip socket and using the femur to ultimately turn the foot (as opposed to the very, very early descriptions of steering that focussed attention on turning the foot). Perhaps this is as a result of the advancement in most coaching systems over the last 40 years to better align the scientific basis for human movement with teaching systems. Regardless, describing to a student the idea that “The turning effort is led by the lower body” and then demonstrating while standing and lifting up one leg and turning (rotating) while swinging the foot through the air in a curved path that the leg starting in the hip socket is what is meant by steering now I think is a vast improvement. I started from the “squish a bug or cigarette days with your foot” era of teaching which was purely a foot turning focus. Thanks for your persistence in all these matters.

    1. “The turning effort is led by the lower body” and then demonstrating while standing and lifting up one leg and turning (rotating) while swinging the foot through the air in a curved path that the leg starting in the hip socket is what is meant by steering now I think is a vast improvement.”

      Actually this is just another wrong way of trying to explain the mechanics. Turning a leg in the air bears no resemblance to the biomechanics of turning a the leg with the foot weighted. The human lower limbs are one of the most complex anatomic structures know. Guessing is not a viable option. At the very least those who attempt to demonstrate the function of the lower lombs should invest some time educating themselves.

      One analogy that works is to instruct a skier to point their big toe in the direction they want to turn by turning their leg. This is easily demonstrated with the foot weighted on a hard floor especially when wearing socks.

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