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WHY SKIS HOLD AND SLIP, AND HOW WE CANT OUR BOOTS


Text and images by Ron LeMaster T


here are a few fundamental things that skis must be able to do. They’ve got to slide forward easily and not stick into things while they do. That’s why they’re slick on the bottom and turned up in front. Another thing skis must be able to do is


hold against the snow so they don’t slip side- ways when you don’t want them to. That’s why you tip them up so that they’re at an angle to the snow, right? Not really. It is not the angle of the skis’


edges to the surface of the snow – what we commonly call the “edge angle” – that deter- mines if they hold or slip. As the accompany- ing photos of Matts Olsson (fig. 1) and Marcel Hirshcher (fig. 2) show, skis can hold at a low edge angle or a high edge angle. They can slip at the same angles, too. It’s the angle of the ski’s edge to something else… What that something else is, and how a skier controls it, are two of the most fundamental things about skiing. Because it has to do with holding and slipping, it has great bearing on knee and hip angulation, and on boot setup, especially lateral canting, the aspect of boot setup illustrated in figure 3.


HOW A SKI HOLDS


Your ski cuts a step in the snow when you stand on it. The surface of that step is the platform that supports you, and it’s the angle between that platform and the force that you apply to it that determines if the ski holds or slips. I call it the platform angle. That force is what creates the pressure you feel against the ski.


Think about how sideslipping works on


slopes of differing pitches. Before you start to slip, you are held in place by the platforms your skis cut in the snow. They hold you because regardless of the pitch they’re perpendicular to the force of grav- ity. When you initiate the sideslip, you do so by making that platform’s angle to gravity greater than 90 degrees. When you stop, you do so by making the platform’s angle 90 degrees again. While it’s true that the skis’ edge angle changes too, that’s not the reason the skis slip or hold: it’s the platform angle. What about holding or slipping in a turn? Gravity isn’t the only force working on you. In addition, you experience centrifu- gal force. It and gravity act in combina- tion to produce a total force that pushes on your ski at an angle, like the one shown in figure 4. That’s the angle at which you have to incline to be in balance, and that total force is what the bottom of your ski must be. If the bottom of the ski, and the plat- form it cuts in the snow, is at a 90-degree or smaller angle to that force, it holds. When that platform angle is greater than 90 degrees, the ski slips.


Marcel Hirscher holds a turn at a low edge angle. 2


Centerline of ERRW FXff


Line perpendicular to boot sole


3


A boot’s lateral cant is the angle between the FHQWHUOLQH RI WKH FXff DQG D OLQH SHUSHQGLFXODU to its sole.


1


Matts Olsson holds a turn at a very high edge angle.


38 | 32 DEGREES • SPRING 2019


The platform angle, not the edge angle, controls if the ski holds or slips. Skier: Mikaela Shiffrin. 4


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