Sunday 24 February 2019

How Skis Turn

Ski turning is a fundamental part of alpine skiing sport. How well people ski is defined by how well they can make their skis turn in all conditions of snow surface and terrain. So the questions how do skis turn and how do you make them turn.
Ski design has evolved over many years with each year bringing in a new wrinkle to make skiing easier or to explore a different form of skiing. This blog is about how all mountain and recreational skis are designed to make skiing better.  It is a engineers perspective trying to make a very complex process easy to understand in common language. To this end, I will leave out all my coded technical engineering term.  
First, ski turning is a symbiotic relationship between three main components, the skier, boots, and skis. The boots and skis are chosen to fit the skier so all can work together to make a turn.
The turn.  The turn is made of 3 parts.  The first part is the transition or start of a turn.  This is a dynamic process, moving forward in a curve fashion.  The perfect curve is an arc of a circle that is split into 2 parts at the middle of the curve.  It is not quite perfect since the arc is used to both change direction and control your speed.  Controlling speed is the none perfect part.
The ski slides slightly sideways through the arc roughing up the snow. A “carved” turn has the minimum amount of roughing called side slipping and leave a very narrow snow path. On the other extreme where both skis are in a snowplow or wedge shape maximum of roughing or side slipping occurs. This latter form of skiing maximizes the amount of ski path width and side slipping for a beginner. The ideal path of a turn has a uniform amount of side slipping on both halves of the arc.
The ski.  Here is where it gets a little tricky. Skis are made in many different shapes and sizes for the many different uses. 

All Mountain Ski
I will stick with the all mountain ski that can be used on both hard packed and light powder snow. Newer skis are becoming wider under the foot and little longer again.  Basically, they are narrow at the boot mounting in the middle and wider at the ends. This hourglass shape makes skis unstable for moving in a straight line but helps to make turning much easier (Figure 1). Also, skis usually have flexible ends that will bend for a given load. The amount of flex will depend on what the ski is used for.  I’m assuming the boot and skis fit the skier.
When a ski starts sliding it is like turning on a switch, it becomes alive.  As soon as it starts to move it gains kinetic energy with speed. The ski must then be controlled.
To make a moving ski turn is a simple process.
Knee Tilt
You, the skier, must tilt your knee (figure 2) in the direction you want to go and at the same time  press on the ball of your foot to move the balance point forward (figure 3). 

Ski Pivot Locations

This engages the front portion of the ski tending to bend the ski (the darkened area on the ski in figure 1). The more you press on your toe pad the more the ski will bend and turn on the forward pivot point. This puts more pressure on the front of the ski and relieves some pressure on the back. You can then control the size of the arc and the amount of side slipping. Hence the shaded area showing the change in of the arc shape with the change in pressure, in the (figure 4).  As the turn progresses the pressure on the boot moves back to the heel and thus the pivot point.  The transition starts again.

What is this kinetic energy that I slipped in? Simply, it is the ability to do work. As soon as you start to move you have it.  You have to stop to get rid of it.  In skiing, you push snow out around. This will either slow you down or if you push hard enough you’ll stop. You can also use some of this energy to move your skis around by just pressing on your skis at the right place and time. This is similar to riding a bicycle where it takes very little energy to steer the bike. You use a small amount of energy, in order to move a large heavy object, in a different direction.
The boot. I use a relatively soft forward flex boot and like to keep my boots loose on hardpack snow. This allows my ankles to bend but still have the side rigidity of the boot for making the ski’s metal edge dig in. The boot has another function in that when clamped down in place it will stiffen the centre portion of the ski. This function helps when skiing on ice conditions. It also ads weight for kinetic energy.
The Person.  How you stand on skis is most important in making them turn easily and effectively. If you can shift the pressure on your boots from toe to heel equally well, then you are centred on your ski. The easiest way is to just move your boot forward and back under your upper body.  Leaning and/or moving your hands back and forth works. There are other ways to start a turn such as pushing on the middle of the ski or on the boot heel. Pushing on the middle of the ski takes more effort and time. Most people are taught to ski this way.  Pressing on the heels moves the ski balance point to behind the boot, also a way to ski. In the adaptive sports application there are some people that can’t bend forward or get their weight forward, then this is another option.
Remember this is about skis turning not skiing.  The person must add the skills and timing to make turning happen.
Basically the ski is shaped and bent to follow a chosen path.

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