THE EFFECT OF LIFT PLATES ON SKI MANEUVERS


In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn.

When it comes to discussions of the forces involved in ski maneuvers, most of the force diagrams I have been able to find by others are those that show components of gravity (G) and centrifugal forces (C) with a resultant force (R) acting at the inside edge of the outside ski. If a force diagram is really sophisticated, it might show a ground reaction force (GRF) acting in opposition to the resultant force (R) similar to what is shown in the annotated photo below.

Forces on skier

The inference of such simplistic explanations is that, far from being complicated, the forces involved in skiing are really quite simple.

Gravity (G) and centrifugal force (C) are components of a resultant force (R). The resultant force merely has to be shown aligned in opposition to a ground reaction force (GRF) at the inside edge of the outside ski in order to satisfy an explanation of the mechanics of  edge hold. Forces applied by the foot? No need to complicate things. Keep it simple. Ignore them. If other forces are ignored they aren’t important.

Riser plates? Torques? Ignore them too. It would be nice if things were that simple. But when things like centre of pressure (CoP) and the torsional effects of lift plates are added to the discussion, it quickly begins to become obvious that the forces in skiing are anything but simple. Significant forces other than gravity and centrifugal force are present. And they do affect the skier.

In THE EFFECTS OF LIFT PLATES – CONTINUED, I used a simple model to show how the presence of joints with rotational capability below the lower end of the mechanical line at the tibia affect the initial length of the torque arm acting on the outside foot stabilized in a neutral position and the force vector of the mechanical line as the foot rotates about its long axis about a pivot under the inner aspect of the foot. In this post, I am going to set this basic model in the context of the forces acting on a skier in the arc of a turn as shown in the above sketch.

The sketch below shows the forces applied to the outside foot and ski of a turn by the foot through the mechanical line in conjunction with a resultant force acting at the inside edge before a load is applied. This situation would exist if a skier were to get caught inside and lose contact of the outside ski with the snow. The tendency of a limb that unloads from a force applied to it is to release muscle tension and unwind into a supinated position. In the situation described in the sketches the foot has been stabilized in a neutral configuration with arch supports or custom insoles and/or a form fitted liner or possibly a form fitted shell. When the foot is in a neutral position the force applied to the foot will act on the proximate centre of a line that runs through the ball of the 2nd toe and the heel.

1

The sketch below shows force applied to the outside foot causing the rotation described in THE EFFECTS OF LIFT PLATES – CONTINUED. There is way more going on than shown in the sketch. But I will get to the other issues in future posts.

2

The sketch below shows and overlay of the first and second sketches to show the changes. As rotation occurs in the subtalar joint the force vector of the mechanical line shifts towards the outside of the turn. As it does the transverse angle of the ski base flattens. These changes will tend to cause the ski to slip out of the turn forcing the skier to increase the angle of the resultant force R by increasing the angle of inclination.

3

Increasing the angle of inclination makes a bad situation worse because the forces become more aligned with the slope of the hill thus increasing the magnitude of the forces that tend to make the ski slip out of the turn.

20 degreesThe NY Times video – Ligety on GS commented, “The trace of his (Ligety’s) path is smoother than that of his foes, who ski in somewhat violent fits and starts, making adjustments that spray snow.” What are Ligety’s foes doing that is different from what Ligety is doing? The forces on Ligety’s outside ski are consistently rotating into the turn. The forces on the outside ski of his foes are rotating out of the turn. But changes in the consistency of the snow texture and the forces acting on the outside ski cause changes in the edge angle. The animated video clip below show this effect.

Changes in edge angle cause the outside ski to oscillate into and out of the turn. Ligety’s foes, which includes the majority of World Cup racers, are unable to develop a dynamically tensioned base of support on their outside foot and ski because the forces they are applying are on the wrong side on the inside edge and they are unable to apply a countering torque with internal rotation of the outside leg from the pelvis. The result is the outside ski is unstable. It makes small oscillations in response to perturbations in ground reaction force necessitating a corresponding series of small adjustments by the racer. Edge angle oscillation can also cause the ski to suddenly hook into turn without warning causing a fall. In a future post I will include some video clips showing edge angle oscillation.

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