06 Motion/Constants

Four

Constants of motion which get in your way

 

Skidding: Why and how

The apex of a turn/traverse is a problem spot for many beginners.  It is at this point that many lose control and spiral into confidence-destroying oblivion.  Splattered upon the snow, frost clinging to your brows and lashes, you seek an answer from no one in particular with imploring eyes.

 

The apex of a turn is the point where gravity shifts from a position of aid in movement, to a hindrance.  Newton’s first law of motion states that “An object in motion will continue in motion (in a linear manner) unless acted upon by an outside force.” In other words, once you begin to move, you have inertia, or momentum.  This first law of motion assumes a relatively frictionless surface, which, coincidentally, you are now sitting on.  Once you have pointed the board downhill, gravity goes about it’s business of setting the object (you) in motion, accelerating that object at a consistent rate dependent upon the pitch of the slope and the consistency of the snow.

 

Once you have pressured an edge, the frictional coefficient along that edge will increase, inducing a ‘negative acceleration’ along that edge13.  (For the sake of example, whenever you apply the brakes in a car, you accelerate negatively).  This negative acceleration is the “outside force” mentioned above.  The slowing of the pressured edge, in combination with the sidecut of the board, causes you to deviate from your original line of motion.  The farther you try to deviate from this line (initially), the more you must work against your own momentum, and the acceleration due to gravity.

The idea is to change the direction of the momentum from downhill, to crosshill.  The ‘easiest’ method would be to roll the board up to a very high edge angle relative to the snow, and  ‘rail’ the board, letting its decambered sidecut do the work.  The problem here is balance.  For the newcomer to the sport, this maneuver is not be feasible, given the lack of coordination, foot strength, and balancing tactic.  The solution then, is to pressure/angle the edge to the maximum degree which will allow you to remain upright.  A board on it’s edge will not spin, unless the speed coming into the turn is exceptionally high, or if the snow gives way, or both.  Experiment with greater edge angle/pressure until you have converted the direction of your momentum without uncontrolled skidding.  Once again, stand in the center of the board with equal weight on both feet.

 

Slope, gravity, entropy A little knowledge is a dangerous thing

More often than not, when you use additional edge angle  as a means of lending direction to your descent/traverse, you will either not use much more than previously, or you will use too much.  Generally, when too much edge is applied, you will tip over quickly to the inside of the turn.  Additionally, the board may shoot out from under your body, dumping you on their butt14.  This is an opportune time to graphically demonstrate a few points about stance and movement of body mass.

 

A board pointed down the fall line will accelerate at a consistent rate dependent upon the slope, represented as a ratio of the rise/run. A horizontal line would be represented as 0/1, and would be referred to as having zero slope.  A vertical line would be 1/0, and would be referred to as having no slope.  As the gradient of a hill approaches no slope, the acceleration due to gravity will approach the free-fall value of 9.8 meters/second2.

Gravity’s pull is exerted upon the center of mass of an object.  Since the center of mass of the snowboard is exceptionally close to the surface over which it slides, it will proceed down the hill by itself in a manner relatively free of mishap.  You, however, have a relatively high center of mass relative to the frictionless surface.  This surface, having slope, is the only thing standing between the center of mass of the snowboard and free-fall.  You,  on the other hand, have, on average, about two and a half feet of air between your center of mass and the frictionless surface.  Assuming that the center of mass is located somewhere around the hip area, this means that your center of mass has more than two feet of free-fall potential.  The only thing restraining your center of mass is the stability of your legs, which is specious, at best.  Therefore, it is necessary that your legs be put in their strongest position underneath your center of mass15. (Thus the triangulated relationship between the knees and feet).  If, for some reason, this position should shift, so that your legs move out from under your center of mass, the extremities of the body will tend to rotate around this center.  This is why your feet will shoot out from underneath you once you lose your balance.  The bottom line is this: you must make certain to move your center of mass along with the board in a stable, balanced position.

And don’t think that you can improve your chances of remaining shiny side up by lowering your center of mass.  If you crouch down, you will only make it harder to balance and apply pressure to the necessary parts of the board.

(13) Once you…  I think that this statement holds when the board is hardly on edge, such as in the context of the first few days on snow.  Once the rider has developed a little skill, it is better to think of the board as a means of creating a curved embankment underfoot (think bicycle velodrome).  The amount of board tilt determines the steepness of the banking.  The banked turn serves as the surface that accelerates the mass of the rider towards the center of the arc.  The outward pull commonly attributed to centrifugal (hah) force is simply the result of the rider’s momentum.

As far as skid is concerned, skidding is the result of bending the front of the board excessively. If the front of the board creates an arc that the back of the board (not bent) cannot follow, a skid is the result.  Most riders mount their bindings too far forward.  Chatter, on the other hand, is the result of too much edge…

 

(14) Obvious grammatical error.

 

(15) Therefore…  Ok; knees together is not a ‘strong’ position.  Oh, I am so embarrassed.  Back when he was designing for Liquid, I overheard Mike Kildevaeld describe it like this “…If you were going to brace yourself against a heavy load, would you intentionally stand knock-kneed?”  Probably not his exact words, but that is the gist of it.

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