EDGING PLATFORM MECHANICS

I like to think of edging/platform mechanics in terms of the forces that are acting to cause the ski to rotate into the hill (positive edging forces) or away from the hill (negative edging forces). Some of these forces will be active forces while others will be resistive or reactive forces. In edge mechanics on hard pack snow and ice I consider the ski portion of the uphill edge under foot (the waist of the ski) as neutral since it is the proximate pivot point for rotation of the ski about its long axis. Depending on the perspective you choose you could look down the ski and see forces that tend to rotate it clockwise vs. forces that tend to rotate the ski counter clockwise.

It doesn’t really matter what terms one chooses to use to describe the forces so long as they end up being arranged so we know which ones are positive edging forces and which ones are negative edging forces.

We know that the forces acting on a skier are trying to pull them down in line with the slope and down towards the center of the earth. There are other forces as well. But we are really interested in the result of all the forces that pull the skier towards the snow. This force is not surprisingly called ‘the resultant force’.

In order to know what result the resultant force is going to produce in terms of edging/platform mechanics we need to know what other forces are acting on the ski. For the sake of simplicity let’s say that the force acting on the uphill edge underfoot is ‘neutral’ since this is the pivot point of the teeter-tooter.

On the uphill side of the pivot point the force from the edge of the sidecut of the ski will resist rotation of the sidecut edge into the hill by pushing back. So it is a negative force.

On the downhill side of the pivot some snow will resist rotation of the ski away from the hill depending on how soft the snow is. So this is a positive force. Since it is usually small compared to other forces we will ignore it.

The foot applies a force to the ski as a result of the external forces acting on the skier. Whether it is a negative or positive force depends on what side of the pivot point it is on. So the force applied by the weight of the skier on the ski can either be a positive or negative force.

A force applied under the ball of the foot could be a positive force or a negative force depending on foot position in the boot in relation to the ski and ski geometry. But a force applied under the heel will always be a negative force because it acts on the proximate center of the ski.

[ = limit of uphill sidecut, ] = the limit of downhill sidecut, (T = pivot point - uphill waist edge, ) = downhill waist edge, F = center of force applied by foot, S = center of force applied by snow

The examples below all relate to the outside ski of a turn. The software for posts prevents me from using spaces to show a scale so you will have to use your imagination until I can import some drawings.

Let’s assume the mechanics necessary to establish and maintain a platform exist. Here is what the forces would look like on the ski.

Example 1

[S- F+ (T ) ]Downhill >

If F is greater than S then the ski will rotate into the hill until F = S. If either force changes, then the other force must also change to maintain balance. Otherwise the unbalanced forces will cause the ski to rotate.

Now let’s assume force F is acting under the heel.

Example 2

[S- (T F- ) ]Downhill >

Now S and F are both negative so the ski will rotate away from the hill.

What if we increased the width of the waist of the ski as shown below.

Example 3

[S- (T F- ) ]Downhill >

Now F is on the downhill side of the pivot. So it is a negative force. The ski will rotate away from the hill.

At this point some will protest that I have overlooked the edging force that can be applied by the cuff of the boot. Actually I haven’t. I will address this in my next post.

I like to think of edging/platform mechanics in terms of the forces that are acting to cause the ski to rotate into the hill (positive edging forces) or away from the hill (negative edging forces). Some of these forces will be active forces while others will be resistive or reactive forces. In edge mechanics on hard pack snow and ice I consider the ski portion of the uphill edge under foot (the waist of the ski) as neutral since it is the proximate pivot point for rotation of the ski about its long axis. Depending on the perspective you choose you could look down the ski and see forces that tend to rotate it clockwise vs. forces that tend to rotate the ski counter clockwise.

It doesn’t really matter what terms one chooses to use to describe the forces so long as they end up being arranged so we know which ones are positive edging forces and which ones are negative edging forces.

We know that the forces acting on a skier are trying to pull them down in line with the slope and down towards the center of the earth. There are other forces as well. But we are really interested in the result of all the forces that pull the skier towards the snow. This force is not surprisingly called ‘the resultant force’.

In order to know what result the resultant force is going to produce in terms of edging/platform mechanics we need to know what other forces are acting on the ski. For the sake of simplicity let’s say that the force acting on the uphill edge underfoot is ‘neutral’ since this is the pivot point of the teeter-tooter.

On the uphill side of the pivot point the force from the edge of the sidecut of the ski will resist rotation of the sidecut edge into the hill by pushing back. So it is a negative force.

On the downhill side of the pivot some snow will resist rotation of the ski away from the hill depending on how soft the snow is. So this is a positive force. Since it is usually small compared to other forces we will ignore it.

The foot applies a force to the ski as a result of the external forces acting on the skier. Whether it is a negative or positive force depends on what side of the pivot point it is on. So the force applied by the weight of the skier on the ski can either be a positive or negative force.

A force applied under the ball of the foot could be a positive force or a negative force depending on foot position in the boot in relation to the ski and ski geometry. But a force applied under the heel will always be a negative force because it acts on the proximate center of the ski.

[ = limit of uphill sidecut, ] = the limit of downhill sidecut, (T = pivot point - uphill waist edge, ) = downhill waist edge, F = center of force applied by foot, S = center of force applied by snow

The examples below all relate to the outside ski of a turn. The software for posts prevents me from using spaces to show a scale so you will have to use your imagination until I can import some drawings.

Let’s assume the mechanics necessary to establish and maintain a platform exist. Here is what the forces would look like on the ski.

Example 1

[S- F+ (T ) ]Downhill >

If F is greater than S then the ski will rotate into the hill until F = S. If either force changes, then the other force must also change to maintain balance. Otherwise the unbalanced forces will cause the ski to rotate.

Now let’s assume force F is acting under the heel.

Example 2

[S- (T F- ) ]Downhill >

Now S and F are both negative so the ski will rotate away from the hill.

What if we increased the width of the waist of the ski as shown below.

Example 3

[S- (T F- ) ]Downhill >

Now F is on the downhill side of the pivot. So it is a negative force. The ski will rotate away from the hill.

At this point some will protest that I have overlooked the edging force that can be applied by the cuff of the boot. Actually I haven’t. I will address this in my next post.