# THE MECHANICS OF PLATFORM ANGLE: PART 4

In Part 3 of the mechanics of platform angle I suggested that some unidentified force or forces are at work that enable elite skiers to alter the angle of attack of the applied force R so that it is more aggressive in terms of cutting (carving) a step into the surface of the snow. I asked the reader what the components of the applied and reaction forces would look like.

One reader correctly identified two separate forces acting on the transverse plane of the platform of the outside ski; one oriented vertically at 90 degrees to the plane and a second force oriented parallel or 180 degrees to the transverse plane with the vector aligned into the snow.

The right hand graphic below shows the 90 and 180 degree components of the angular force acting on the platform in the left hand graphic.

The right hand graphic below is the same as the graphic above but with the angular force superimposed over the 90 and 180 degree components.

I am taking the discussion of platform mechanics in small steps in order to provide the reader with a chance to assimilate the issues and ask questions if my discussion is not clear.

### THE SHOCKING TRUTH ABOUT POWER STRAPS AS A REFERENCE

Most of the views of the series on the Mechanics of Platform Angle are accompanied by views of The Shocking Truth About Power Straps which contains quotes from the medical textbook The Shoe in Sport (published in German in 1977 as Der Schu im Sport). This medical textbook has been invaluable to my efforts.

Here are some pertinent quotes by Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions.

These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.

The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).

It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

Biomechanical Considerations of the Ski Boot (Alpine)

The question for this post is what is the source of the 180 degree force? Please consider Dr. E. Stussi’s comments above when contemplating an answer to this question.

1. Agnew Drake says:

The 180 degree force applied to the transverse plane of the platform comes from at least 2 sources:

1] centrifugal force from the turning of the ski and skiers mass/inertia

2] gravity effect dependent upon the steepness of the hill

The flatter the ski, the closer the vector will be to the 180 degree transverse plane force resulting in slippage. The force opposing the flattening of the ski is the skier’s ability to apply an opposing ‘angular’ force to the inside edge of the platform by his/her stance [body position: COM forward/ SR stance engaged ] and his/her ability to control the location to which that force is applied to the ski [boot design: allowing not only engagement of the SR stance with proper ankle flexion movement, but also the forward application of force to the metatarsal-phalangeal joint 1]. This resultant vector is an effective centripetal force applied at the discretion of the skier.

Bob Drake

1. 1] centrifugal force from the turning of the ski and skiers mass/inertia
QUESTION: What forces are causing the ski to turn?

The force opposing the flattening of the ski is the skier’s ability to apply an opposing ‘angular’ force to the inside edge of the platform by his/her stance [body position: COM forward/ SR stance engaged ] and his/her ability to control the location to which that force is applied to the ski..

COMMENT: It’s more complicated than that. But the key is the ability of the skier to control both components of the angular force especially the force(s) substantiallty parallel to the transverse plane of the platform.

boot design: allowing not only engagement of the SR stance with proper ankle flexion movement, but also the forward application of force to the metatarsal-phalangeal joint

COMMENT: You are very much in the right direction – boot design is the critical factor determining the ability of the skier to apply and/or control the component forces.