In order to make it easy to understand the basic mechanism of Roll Over, I am going to use an overly simplistic model for illustrative purposes. The actual mechanics and biomechanics of Roll Over are much more complex. But I’ll start with simple principles and build on them.
The starting point to understand the mechanism of Roll Over is knowing in general terms the forces acting on the skis, especially across the edges. I’m using a graphic of Anna Fenninger because the head-on camera angle lends itself to showing the forces acting on her skis. The fact that Fenningers’ outside ski is holding and carving at a high base angle with the snow is typically explained as shown in the graphic below where R is the Resultant Force acting on COM and S is the Snow Reaction Force. According to the Critical Edge Angle theory, the angle of the base of the ski with the snow is irrelevant. So long as R aligns with S at 90 degrees to the base of the ski, the edge will hold.
(Click on graphics to enlarge them)
The fact of the matter is that no one has any way of determining exactly where Fenningers’ or any skiers’ Centre of Mass is in relation to the inside edge of the outside ski. That R is an attractive force, not a physical force, and sidecut is not even considered, further invalidates the Critical Edge Angle theory which really doesn’t explain anything. What we need to know is the forces acting across the edges of the ski. At this point in the left hand turn, Fenninger is about to start her transition by stepping on her inside (left) ski while it is still on its current (uphill) edge.
In the graphic below, the portion of the weight of her body W, that is acting on the inside ski, is under her heel, on the proximate center of her foot and ski. In this location W is offset to the outside or downhill aspect of the uphill edge. Unless opposed, W will cause the ski to rotate and flatten on the slope of the hill. The insert in the upper left hand portion of the graphic shows the moment of force that acts to flatten the ski.
What is stopping Fenninger’s inside ski from flattening? An opposing moment of force created by Fenninger abducting her left leg. The closed kinetic chain created by the inside ski locked on its uphill edge causes abduction of her left leg to move her thigh into the hill. As it abducts, it exerts a force against the outer aspect of the shank of her boot as seen in the graphic below. The large angle at her knee, makes it appear as if the movement is originating at her knee instead of at the pelvis.
Stepping on the inside ski while it is still on its current edge and extending, primarily at the knee, releases the force applied to the shaft of the boot and causes the ski to start flattening on the slope of the hill. As the ski rotates, the body rotates in space with the ski about the pivot created by the current (uphill) edge. In the New York Times video, Ted Ligety on GS, Ligety talks about ‘creating pressure’. By extending his knee as he rotates in space, Ligety uses momentum and gravity to exert a force against the snow at ski flat that mimics the force of gravity by applying a force that is normal or perpendicular to the slope of the hill. It is like the trick of running against a vertical surface with sufficient speed so that if you throw yourself sideways at the surface, you can momentarily defy gravity and run horizontally along it.
The graphic below shows the rotation in space of COM stacked over the ski as it rotates about its current edge. The small inset to the right shows this occurring on the slope of a hill. However, it is easier to view this movement horizontally. When the ski flattens on the snow, the rotational momentum will tend to maintain its rotation. In order for this to happen, the aspect of the ski opposite the pivot created by the current edge has to penetrate into the snow surface. On hard pistes, any penetration will be minimal. What needs to happen for the ski and skier to keep rotating? The pivot has to move to the opposite (inside) edge of the new turn and the aspect where the current edge is will have to …… rise up in relation to the pivot.
There’s a problem. If the Pressure on the ski is in the center it will act to flatten the ski on the snow surface no matter which edge the ski is pivoting about.
The Pressure exerted on the ski will oppose the ski from Rolling Over. Even the New York Times video shows the Pressure in the proximate center of the ski. What has to happen to enable the ski to Roll Over? That is the subject of my next post.