INVERSION STRESS – 20TH ANIVERSARY


Today marks an anniversary for me, albeit a bitter sweet one. It was 20 years ago to the day that a patent application I filed on November 29, 1993  was published world wide. The material in the application represents what I believe to be the first disclosure of the mechanism that Kim Hewson and I describe as a state of inversion stress. The application resulted in the awarding of US Patent No. 5,459,949 on October 24, 1995 and subsequent international patents. US Patent No. 5,459,949 formalized and made public the validation of the findings of my hypothetical model of the mechanics, biomechanics and physics of skiing as confirmed with data from the Birdcage research vehicle and my first 2 turns on rock hard piste on a steep slope of Whistler’s summer glacier.

Cover

The reason this achievement is bittersweet for me is that it confirmed my perception that much of what is held sacred in skiing is based on eminence, not evidence and with little or no supporting principles in science. The concern that immediately arose out of the Birdcage studies  was the emerging trend away from conventional ski geometries with their standardized width underfoot in the order of 7o mm. Marketing is about statements or stories that resonate with the consumer. Light (aka Lite) resonates. Long (as in ski length) doesn’t. Width as in wider does. It was obvious to me that the odds were great that marketing would influence a trend towards wider skis, especially wider skis underfoot and that this direction would find enthusiastic, albeit unquestioning, support. In recent years, width underfoot has exceeded 100 mm and has now passed 150 mm.

Wide skis or Fats as they are now affectionately referred to, make perfect sense on untracked snows which are closer to the physical qualities of liquids or even gases. But on groomed conditions, and especially hard pistes, skis that are wide under foot will unavoidably and unequivocally cause Fat Ski Syndrome because of the long unbalanced inversion lever. Unfortunately, fats are being widely promoted as all mountain or big mountain skis implying that they are suitable for all conditions. This is not the case. Fats have a place in skiing. But that place is not on groomed conditions and especially not on hard pistes. When I included a description of the mechanism of inversion stress as a source of trauma to the fragile knee in the patent application, I did so with the fervent hope that it would raise a red flag in what should have been an obvious issue, one that if ignored, would eventually place the design of ski equipment under a microscope. It appears as if my hopes were in vain. Fat Ski Syndrome has drawn attention to width of the ski underfoot as an obvious and undeniable source of lower limb  imbalance and, with it, the erosion of stable edging technique leading to compromised skier performance and  knee trauma.

Cols 59-60

Inversion stress in 549

The patent figures below represent a best case scenario, one in which W is transferred to the ball of the foot and from there to the ski. Over a certain width threshold this is no longer possible. When skiing on fats, W will be transferred to the proximate transverse centre of the ski. This will result in an unbalanced inversion moment on the outside ski and foot in the order of 50 mm for a ski of 100 mm width underfoot and 75 mm for a ski of 150 mm width underfoot: almost 5 times the length of the potential central axis load transfer moment arm for a typical GS ski.

 

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Figs 70-72

 

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2 comments

  1. How has your research, and resulting patents, changed ski boot design? Which ski boot companies are paying you royalties for use of your patents

    1. All of my ski boot and ski equipment related patents have expired long ago. In accordance with the terms and conditions of patent law, the teachings of the art contained in my patents are now the property of the world to use for their enjoyment and to further the art in the field of the invention. As far as I have been able to ascertain, my patents and the teachings disclosed in them have had minimal, if any, impact on any aspect of the design of ski equipment or ski teaching methodologies, ski coaching or ski technique. I have no financial arrangements, connections or affiliations with any party in the ski industry. Aside from the current patents for a new footwear technology incorporated in the Kinex cycling shoe line I receive no financial compensation from the sporting goods industry. Should the cycling shoe gain traction, as a major shareholder in Equipower Sports Inc (the parent company of Kinex), I may realize financial gain at some point in the future. But as with any new technology, this is far from certain.

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