EDGE MECHANICS/SKI CONTROL


The cornerstone of an effective ski technique is the ability of a skier to apply a force to the outside ski that is perpendicular to the transverse aspect of the base and aligned in opposition to the Snow Reaction Force acting at the inside edge. This configuration of forces is essential to make the edge grip and act as a pivot for rotation of the sidecut of a ski in terms of penetration into the snow surface. The ability of a skier to apply a vertical force to the inside edge must co-exist with the ability to apply and control forces acting across the inside edge. This is fundamental to balance and control of the skis.

The post SKI LEVERS describes how the sidecut of a ski creates what amounts to a dual pivot, offset lever system with the edges underfoot acting as pivots for each of the two lever systems as shown in the graphic below.

Screen Shot 2014-11-18 at 11.10.41 AM

But the sidecut of a ski results in much more than a simple offset lever. As the sidecut increases ski width beyond the minimum width underfoot, the amount of lever offset progressively increases as sidecut approaches the tips and tails of a ski. As sidecut increases the width of a ski, the magnitude of torque acting on ski increases until the maximum offset and torque load is reached at the maximum width.

The graphic below shows how the load from the weight W of COM is transferred from the proximal femur to the distal  tibia by the central load-bearing axis. The default position of the centres of the loads WL and WR is on the proximate anatomical center axis of the foot (the proximate transverse centre of the calcaneus).

Central Axis 3

Unless it can be conclusively demonstrated that the load W transferred to the outside foot of a turn has been transferred to the proximate centre of the head of the first metatarsal and is applying a force aligned with, and in opposition to, the Snow Reaction Force at the inside edge of the outside ski, the load W should be assumed to have been transferred to the anatomical center axis of the foot and from there to the transverse centre of the base of the outside ski.

The graphic below is a schematic representation of a cross section of the ski as a  dual pivot, offset lever system. The inside edge underfoot P is acting in the capacity of a pivot for rotation of the side cut on the left hand side of the schematic into the snow surface. The ability to apply a force aligned in opposition to the Snow Reaction Force acting on the inside edge is literally the pivotable issue. In the schematic below, the load W is acting on the default load transfer axis and is offset from the Snow Reaction Force P acting at the inside edge which is serving as the pivot for rotation of the ski. The force Fs is the reaction force of the snow that opposes rotation of the sidecut of the ski.

Ski Lever A

The graphic below depicts the consequences of the absence of a force acting in to opposition to W. An unbalanced moment of inversion force will result from the offset of W and P that will rotate the ski until it is in either in compliance with the snow surface or inversion reaches the physiologic limits of the subtalar joint. As the ski rotates in inversion, shear forces will be set up that cause the ski to lose its edge and slip out of the turn.

Ski Lever B

The first consideration should always be to ensure that that on groomed and especially hard pistes, the foot can apply a force (Fe) in opposition to the Snow Reaction Force acting at P. This is essential to edge grip and the role of the inside ski as a pivot.

Ski Lever Fe onlyEnsuring that a skier is able to apply a force to the inside edge of the outside ski that is perpendicular to the transverse aspect of the base and aligned in opposition to the Snow Reaction Force acting at the inside edge in concert with the ability to apply and control forces acting across the inside edge should be the highest priority. An inability of a skier to effectively apply force to the inside edge of the outside ski and especially an inability to control forces acting across the edge, will create an unbalanced inversion moment of force about long axis of the ski and foot that can create a state of inversion stress in the affected lower limb. In skis with a Width Profile under foot of 100 mm or greater, a serious condition called Fat Ski Syndrome can result if Fats are used on groomed and especially hard pistes.