LIGETY’S ROLL OVER EXPLAINED: THE SIMPLE VERSION – PART 2


Part 1 left off with the inside ski flat on the surface of the snow after it had completed its rotation about its current (uphill) edge when pressure was applied to the ski by stepping on it. The current or uphill edge was the point where snow reaction force S was acting. The pressure W, applied under the heel of the inside leg and foot, on the proximate center of the ski, was offset from S resulting in a moment arm that tended to rotate the ski downhill. This rotation was opposed by a force exerted against the inner aspect of outside of the boot shaft  by the inside leg being abducted (moved outward) by the thigh as shown in the insert in the graphic of Ana Fenninger below.


Fenniger Abduct

When the ski rotates into contact with the snow surface, rotational momentum wants to maintain the rotation.

Rotational Momentum

 

If the piste is firm or icy, there will be little or no penetration into the surface as the ski moves beyond full contact with the snow surface as it changes edges.

 

 

Ski Flat SRF

One way or another, there will be a translation of the plane of the base  of the ski into a different plane as it changes edges and begins to rotate about the inside edge of what will become the outside ski of the new turn. Translation is part of the event that I call Roll Over.

 

Translation

If the pressure stays in the center of the ski as it changes edges and translation starts, there will be a problem. Even though COM will eventually pass the axis of rotation of the new edge, translation will be resisted by the Pressure applied to the center of the ski. This is the literal Moment of Truth. If the Pressure stays on the center of the ski, force exerted on the inside of the inner side of the boot shaft will cause translation to occur against the Pressure that continues to rotate the ski out of the turn. What has to happen for Pressure and Translation to be in phase, so Roll Over can occur, will be the subject of Part 3.

2 comments

  1. What happened to Ted Ligety at Val d’ Isere? (GS) He looked pretty bad compared to most of the other racers. It looked like his technique was preventing him from being versatile enough to qualify for run2.

    1. I haven’t haven’t seen video of the race yet. But I am not surprised at Hirscher’s results this WC season. While Ligety can get edge grip at very high angles on his outside ski with the application of multi-plane torques, this will not be as effective on very hard pistes and especially ice as the use of the inside ski. The inside ski allows much greater edge grip at much lower base angles with the snow. I talked about this in my post on the Austrian Move. The drawback of the Austrian Move to the inside ski in the bottom of a turn is that directional control is limited. Once a racer makes a move to the inside ski they are stuck with the line they move on to. Having said that, a few good high load turns under the right conditions using the inside ski can win a GS race. I predicted that Shiffrin would not win GS races until she started to use her inside ski in some turns. In the two recent slaloms where she destroyed her competition she used her inside ski to advantage in several turns. I tend to agree with your call.

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