Skiing Principles

                                                                                (more links at the bottom of this page)

                                                             

Turning

Skis have an hour glass shape called side cut (narrow in the waist). If the ski is put on edge and pressured, it bends into an arc that makes a turn when in motion. Adding pressure to the front of the ski will engage the tip and tighten the radius at the start of a turn. Skiers can also pivot or steer the skis. So the ski turns you or you turn the skis or both in a blend of carving and skidding.

   

Forces
 

Motion is normally in a straight line. When turning, forces build and can push skiers back and inside the turn. Progressively flexing the ankles and creating angulation keeps the skier’s momentum moving forward and downhill relative to the feet. The force increases with speed, steepness, conditions, and type of turn.

 

Think of the upper body separate from the feet. This is also referred to as the center of mass (COM) and the base of support; the COM can be in front of the upper body. Think of the body in two parts, upper and lower; they can work in opposite directions.         

 

There are two types of forces in skiing, internal and external. The external forces are gravity and resistance from the snow and air. Internal forces are created by the body. Things move in a straight line unless some force changes their direction (inertia). When the skis turn, pressure increases as the body tries to go straight. Pressure is greater moving past the fall line when gravity and inertia begin to work in the same direction.

  

Skiers can maintain their downhill momentum in a turn, and then it will help move them toward the next turn. We can absorb some of the pressure while edging to maintain momentum. Turns can end with retraction which allows turn forces to push the skis under the COM as it moves downhill (cross under). The closer we finish to the fall line in GS turns, the more the momentum is heading downhill. 

 

Or we can disengage the edge more gradually with extension, so momentum moves the skier over the skis (cross over) as the turn forces are reduced more slowly. How and when we release pressure affects our path and speed into the next turn (momentum). 

 

Parts of turns

 

Turns can be divided into at least two parts. Looking at the skis, the first half is from the crossover point, where the skis are flat and across the hill, to the fall line. Then from the there to the next crossover point is the second half. This describes a turn relative to the skis.

 

Another way to think about a turn is in terms of the COM. When linking turns, the new turn starts when the pressure is released, either while edging or after. If edging ends near the fall line, then turns start and end at the fall line rather than the crossover point. When edging ends near the fall line, then momentum works in the direction of gravity, and reduces when turning across the hill more. 

Size and shape

 

Turns have a range of sizes from small to large. Speed is controlled by the shape of the turn, which means turning more across the hill, or the skis can be skidded. The further across the hill a turn is made the more momentum is redirected across the hill, which slows the skier. These are the two main characteristics of a turn: size and shape.

 

In small turns, skiers travel straight downhill while the skis turn beneath them. Small turns require quicker movements and more knee angulation. When making large turns, the body turns to start following the skis more, requiring more hip angulation which is why GS tends to be more difficult and challenging than slalom turns. Small turns can be made at slow and fast speeds, just like larger turns.

  

                                                    4 types of movement

 

These four moves below affect the edging, rotation, and pressuring of the skis (fore/aft, ski to ski, and overall.) Think of the body in two parts, the upper and lower.

 

Fore/aft- moves maintain balance in the center of the skis, adjust pressure on the front and back of the skis, and can keep momentum moving forward relative to the feet. The ankles, knees, hips, waist, and back play a part. Starting in a skiing stance centered over the middle of the skis, progressive ankle flex moves skiers forward while edging; then they move back when the skis are released. 

 

Ankle flex can be active to pressure the tip or passive in response to the turning. At first, ankle flex applies pressure to the tip of the ski to help it start the turn, and then it absorbs building pressure to keep skiers moving forward while the skis are edged.

Most skiers lack ankle flex, or they flex their knees too early or quickly which moves them back on their skis. This is done intentionally in some cases, such as in Asian countries that use quick knee flexing. 

If skiers have their ankles fully flexed early in the the turn, they may flex so low with the knees they get square and skid or stem the tail of their downhill ski. Other skiers don't flex their knees and lean back at the ankles.

Side to Side- moves are also called lateral movements and include weight transfer from foot to foot, inclination (the body moving inside to start the new turn), and angulation which is tipping the upper body to the outside of the turn. Many skiers will naturally lean inside the turn and don’t add the angulation, which is called banking. Some will create hip angulation all at once and hold it through the turn, which is called park and ride.   

 

Inclination and angulation control edging. Inclination occurs in the first part of a larger turns. Then progressive angulation continues to: increase edging, maintain lateral balance so you don’t fall inside the turn, direct more force into the slope, and maintain the upper body momentum to the outside of the turn. Angulation can be created from the feet to the neck, but more knee angulation is used for small turns, and hip angulation is used more in larger turns. When angulation begins, it should be progressive to the end of the turn to maintain momentum.

 

The rate of edging movements controls the size of a turn; quicker moves make smaller turns. How long edging movements are made determines how far across the hill a turn is made. Keep clear the difference between rate and duration; they are not the same.

 

Up and Down- moves are also called vertical movements; they adjust pressure and affect the lateral range of movement and upper/lower body separation. People can have a hard time standing in a skiing stance and coordinating their joints so they can move down and up while staying over the center of their skis.

 

1)- When linking turns, extension on the new outside leg begins to apply pressure which increases as the skis are edged. Extension in moguls can apply pressure on the downhill side of the mogul. Extension to release the ski disengages the edge more gradually than retraction.  

 

2)- Moving down while edging can absorb the increasing pressure faster, slower, or at the same rate it is building, so it will reduce, increase, or maintain the pressure created from turning. It can also maintain the non-stop movement of the body downhill especially when pressure builds quickly from moguls, deep snow, or turns that are made well past the fall line. 

 

3)- Moving down after edging by retracting the legs allows the skier to move toward the next turn while turn forces push the skis under the skier. The legs can just relax and allow the body to move toward the feet, or the feet can actively be pulled up to the body.

 

In racing turns where turns are completed near the fall line, there is usually not much down motion until retraction after the edging is completed. Or down motion is used during edging to enhance knee angulation when a quicker turn is needed. When turns are completed more across the hill, the pressure can get very high and create a skid or chattering; moving down can reduce the pressure.

 

So vertical moves affect how we load and unload the skis to make them turn. But vertical movements can also be used to unweight skis so rotary movements can be used to turn them. Down unweighting is moving down quickly which reduces pressure at first when skis can be turned and then increases pressure as the down motion slows. 

 

Rebound unweighting relies on the fact that for every action, there is an equal and opposite reaction. When down motion stops abruptly in a turn, the energy rebounds and can unweight the skis which allows for an unwinding or pivoting of the skis.

 

Up unweighting occurs with a strong up extension that first increases the pressure on the skis, but then unweights both skis near the top of the up motion so they can be pivoted or steered. It can be useful in very narrow situations.

Some skiers have a tall stance and not much vertical movement, but others have low stance and can move down too far. A low stance feels stable and allows more lateral motion for knee angulation, but it also is harder to resist the force of the turn the lower skiers go. They may rotate their shoulders or hips and brush the outside ski downhill to find the edge (called an abstem). These skiers have a hard time making quick small turns because they cannot stop the down motion fast.

  

Rotary- moves produce an internal turning force to pivot or steer the skis. Pivoting is turning around the vertical axis of the body, such as hopping up and turning the skis. Steering is done by turning the feet over a longer period with some weight on the skis, as when making a turn. Rotary moves can be active or passive, and they can occur in the upper or lower body.

 

Skiers often rotate their upper body in the direction they want to turn, especially if they not flexing their ankles and using the tips of their skis to start a turn. Turning the upper body first is how people move when they are walking, so it is very natural to rotate on skis. At the end of the rotation, the turning force is transmitted to the skis. But rotation makes it hard to progressively angulate and directs momentum toward the old turn rather than the new one. Edge angle is reduced as banking replaces angulation which can result in skidding.

 

In small turns, the lower body turns under a stable upper body that is already facing downhill. It produces a wound up position with the lower body facing across the hill and the more massive upper body facing downhill. When the skis are unweighted, they automatically unwind or pivot to face downhill like the upper body, and the feet can actively be turned. For every action, there is an equal and opposite reaction, so the upper body will start to rotate at a slower rate in the direction the skis were facing. A pole touch will block this rotation of the upper body.

In larger turns, inside lead puts the hip in a position so hip angulation can be created. Inside lead is also called skiing into counter, parallel position, active anticipation, strong inside half, or upper lower body separation. People can create too much lead too soon which moves the hip too far back and inside the turn which prevents the smooth flow into the new turn. 

 

Here are links for all our engineer instructors who want to look at more details, but the knowledge you need to be a great engineer is different than what you need to be a great skier. Just prior to the 2018 Olympics, Bode Miller gave this advice to young racers; "Just make sure that your mind is focused on going fast." Franz Klammer was asked about technique and he said, push on one ski then the other. 

 

Skiing Mechanics

Science and skiing conference books

Finite element analysis of a carving snow ski 

 

 

                                                                                                                    

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