Originally Posted by Rick
The important points, however, are:
1) In very hard snow the platform will only increase in depth a minuscule amount, certainly not enough to accommodate the new shape the ski desires to assume. And:
2) The snow is of high enough integrity that it will not sheer.
Which when taken together, means the arc of the ski will not be altered. If you are finding that you CAN alter the arc of the ski, then one or both of the above are cannot be happening at the same time.
Originally Posted by Rick
The force needed to cut can indeed be smaller than the force the resultant platform will support. Just as the force an ice climber uses to get a bite in the ice with his crampon can be smaller than the force that bite will subsequently support.
I agree with you that less force is required to cut into the snow pack, but the resistance to sheering of the snow at the given edge angle remains the key issue; one is still standing on the ski with only the resistance to sheer stopping the ski from sliding out.
There is always a sheer force applied when you are on edge. If the snow pack can support a higher force without sliding away, then you can (a) ski faster, (b) turn sharper, (c) be fatter
This is one scenario -- Sheering happens:
If you apply more edge angle, then two things are happening: You have moved the center higher above the snow pack -- there is now less snow directly underfoot, and the force is directed more along the surface of the snowpack. This certainly happens on really hard snow, as the edges won't dig in further.
Both of these attributes promote center deflection : the center is on less snow than before, and the forces are directed even more towards the direction in which the snowpack will sheer. It's not just that there is MORE force, but that it is applied on LESS snow. Sheering at the center of the ski happens.
The second scenario -- No center deflection, but change of arc anyway.
How? This snow must be softer, allowing the tip and tail to press further into the snowpack as the edge angle increase, without the movement of the center of the center of the ski up and away.
The arc will change because the change of angle made the tips of the ski bite deeper. The ski tips now push into more snow, and like a toboggan, want to ride above it. So, the tip will deflect upwards changing the arc of the ski. Clearly, the snow is soft enough for this tobbogan effect to occur.
Rick is right, the quality of the snow matters, so center deflection will happen on some snow, and not on others. And to make matters worse, both tip deflection and center displacement can occur simultaneously in some conditions!
I don't really think that there is much disagreement in this thread. I do think BOTH tip deflection and center displacement happen.
What this implies to me is that skis should be very sharp tip to tail, to maximize the change of direction by tip deflection. I can't think of skiing by control of center displacement alone as too much is happening at the tip. And if the conditions are such that center displacement really is is the dominant feature in the change of arc, then the tip and tail better be sharp!