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Unweighting

post #1 of 8
Thread Starter 
I have noticed a paucity of agreement in the area of unweighting, hence here are my feelings on the matter to aid in discussion and perhaps eventual understanding.

Not everybody is or has been taught by a ski instructor.  To those of us who follow the way of the edge, aka the path of the carve, i.e. general speed freaks, ex DH racers, "taught by father"s, etcetera, this term is no-doubt strange and unseemly.  We typically ski by maintaining some weight force between our edges (and bases) and the snow, dictating our path down the hill by manipulating forces to shape the ski edges (and bases) while maintaining our skis' direction of travel as closely parallel as possible to those carefully controlled edges (and bases).

Yet, there is another way to ski wherein the skier deliberately breaks the bond between the edge and the snow or ice surface so that they may more easily position and shape their skis edge (and base) in order to take advantage of the forces they may so generate with a skidding edge (and base)!  (There are also a few occasions when there is cause to break that bond as doing so will in these cases allow one to more-quickly reposition the skis.)

In order to accomplish their goal of breaking the bond between their edges and the snow, these skiers temporarily reduce the weight force holding the two together. 

As we are all aware, gravity acts on our mass, pulling it into the snow, and when we resist by pushing against the snow with an equally strong force using our legs, our centre of mass (cm) will maintain its current state of motion relative to the snow.  If we relax our legs momentarily, the net force acting on our cm (gravity combined with our pushing) accelerates our cm towards the ground, i.e. if our mass was moving towards the ground before we relaxed, it moves towards the ground faster; if we were maintaining a steady position of our cm above the ground, our cm begins to approach the ground; and if our cm was moving away from the ground its upward velocity slows.  If we force down harder with our legs, then our cm will accelerate upwards, i.e. if it was moving towards the ground it will slow its approach to the ground; if we were maintaining a steady position of our cm, it will begin to move in an upward direction; and if our cm was moving away from the ground it will increase it's upward velocity.

Unweighting is accomplished by a simple reduction of force in the legs, allowing gravity to accelerate our cm towards the ground whatever its (our cm's) previous state of motion was.

Up unweighting is the term applied to an action that causes the cm to first accelerate away from the earth, usually to the extent that it is actually moving away from the earth so that when we reduce the force in our legs the resulting acceleration of the cm causes it to slow its upward motion, sometimes even to the extent that its upward velocity becomes zero and it starts to move down.  In all cases of up unweighting an upwards net force is applied first so that the downwards acceleration during the unweighted phase accelerates a cm that has just been accelerated upwards.

Down unweighting is the term applied to an action that causes the cm to accelerate towards the earth without regard for it's current state of motion.  It does not require us to first ensure an upward velocity of the cm before hand. 

Jump, and you have just experienced up unweighting; the unweighting occurred while you were in the air.  Suddenly pull your feet up off the ground and you have just experienced down unweighting.

Obviously slowing down the upwards motion to zero takes some time, so up unweighting gives us more time to do things in an unweighted state.

It is not necessary to reduce the weight force acting between skis and snow to zero.  Any reduction in the amount of force can be considered unweighting, no matter how slight.

Please note, carving desciples, before you throw out this unweighting as unneeded actions for modern carving, please consider how much more fluid and easily controlled your legs are when untaxed by the combined weight and forces of acceleration often encountered in high level skiing.
post #2 of 8
Ghost,  great post, clear even to a real unweighted guy like me.  thanks
post #3 of 8
Pretty accurate, Ghost. 

Just remember, by simply manipulating the balance equation, via ILE or OLR, the forces being resisted by the body become reduced, to the point that managing how the legs tip and angulate an the CM crosses is relatively easy.  It's really not necessary to execute further unweighting to make it easier.  Doing so interupts the flow of the CM from one side of the skis to the other, and represents wasted effort.   It takes significant muscular input to move the CM up, and also to catch it from hitting the snow when moving down.  It doesn't serve efficiency. 
post #4 of 8
Quote:
Originally Posted by Ghost View Post

Unweighting is accomplished by a simple reduction of force in the legs, allowing gravity to accelerate our cm towards the ground whatever its (our cm's) previous state of motion was.

 
Unweighting is also accomplished by reducing the arc of the turn, thereby reducing the centripetal force acting upon our skis. Gravity  plus centrifugal force combines to form the resultant force acting on the skis.  This is certainly a factor at the end of a turn when the turn straightens as the skis flatten through the transition. One could question whether getting tall at the beginning of the turn reduces pressure by up unweighting or by flattening the skis and reducing the arc. 
post #5 of 8
Thread Starter 
I tend to think of weighting and unweighting in terms of loads and forces in the direction of gravity, loading and unloading being the somewhat equivalent terms, without that particular directional restriction.  This nuance is important when considering that it is gravity that effects the bond between ski and slope, and the direction has certain implications when considering direction change in three dimensions along with slope angle.
post #6 of 8
 Rick,

yea!

How much? Just enough.  Only mileage will tell you that.
post #7 of 8
Quote:
Originally Posted by BigE View Post

 Rick,

yea!

How much? Just enough.  Only mileage will tell you that.
 

And what type of skier you are...
post #8 of 8
Great posting Ghost

Rick, unweighting is not needed to help initiate a turn when you carve but when you are not its very helpful. In particular if you are not linking turns that will give you rebound and fuel your transition. Also, its true that moving the CoM up and down when you carve at high speed can interupt the flow but note that if you use unweighting for better flow of the CoM its very different to unweighing in order to initiate a slow skidded turn.

The transition type OLF will not push your CoM up by muscle effort while ILE will. However a critical issue here is how fast you move. If you are on GS skis and making big arcs then ILE will not be as harmfull as if you are making very quick reacing turns on SL skis.
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