or Connect
New Posts  All Forums:Forum Nav:

# rotary bad? - Page 3

I'd have to think about it for a while, but it's still hard to see how a yaw rotation can induce a roll force. The snowboarder example doesn't seem applicable since they can obviously introduce twist with differential lever action of the feet --- and those forces are all in the roll axis.

But I will agree that torsion surely changes the ski's contact path with the snow, making it a non-circular section. When that path is no longer circular a skid will follow. And, from a theoretical blue-sky point of view, that makes my argument. A pure, non-skidded path must be circular, and a shaped ski need only change it's angle of attack (roll) and bend to create an infinite number of circular sections. Rotation is not required nor benefical.

What's the bottom line? Unless someone can show me that rotation somehow actually translates to ski navigation, I'm going to mark it up as likely something the human body must do to comply with the ski's path --- not the other way around.
noclevername said:
Quote:
 What's the bottom line? Unless someone can show me that rotation somehow actually translates to ski navigation, I'm going to mark it up as likely something the human body must do to comply with the ski's path --- not the other way around.
You're knees will surely like you. There is nothing wrong with keeping it simple. I am unlikely to convice you over the internet.
NoCleverName

I think this is the point I was trying to make in the Rotation Impossible thread. A twisting of the foot once the ski is edged is blocked by the snow itself. Some more complex rotations may be possible but not the simple foot twist learnt on a flat ski and it is the attempt to do this that causes all the problems.
OK, Pierre, if you can't convince me, then I'll try another stab at you!

The ski is powered by largely longitudal thrust supplied by the kinetic energy of your CM moving downhill. That thrust should remain longitudal since other vectors obviously must move the ski out of its track. As much as possible, you like that CM to move smoothly else there are all sorts of balance problems to correct on the fly (are you listening, Bode?)

Therefore, over the space of, say, 2 or 3 turns, what we have is a largely linear motion (the CM) that must be translated to curvilinear motion (turning ski). Pretty much the same as an old steam locomotive where a moving piston had its motion changed to circular via a few straight parts and pivots. Pretty much the same as straight body parts and rotations/angles.

Therefore, a primary purpose of rotation has to be to supply smooth power to the ski over a curved path. The CM vector is more or less straight, but the thrust vector on the ski has to be rotating to keep it longitudal. A motion translation mechanism is needed. Hence, rotation of body parts.

That being said, perhaps rotation can also modulate the power transfer, thus changing the forces on the ski and its resulting shape: new shapes allow different turning radii. In any event, it would seem that inefficient rotation actions would harm power transfer.

So I pretty much stand on what I said before, a z-axis rotation (yaw) of the ski is pretty much out of the question because motion about z is constrained by the snow surface (assuming edge engagement). Body rotation is used for linear-to-circular motion translation and power transfer.

So yes, rotation isn't bad, it's necessary. But there are clearly good and bad rotations. Bad rotations set up unwanted angular momemtum that must be reversed; good rotations (maybe its better to call them articulations) facilitate smooth power transfer to the ski, not help turn it.

Of course there are situations where you can impart z-axis rotation to the ski, but the edges can't be engaged at the time.
Nope Noclevername, yah weren't successful. By the way, using the word "Curvilinear" shows you is solidly from the east because the rest of the civilized skiing world doesn't use that term.

Twisting the foot to rotate the ski into the shovel sets up a chain reaction. It minimally increases the drag more under the shovel than under the tail. The shovel starts to slow down. When this happens our CM moves forward and starts to increase the pressure on the edges of the shovel. A chain reaction results as the shovel further slows down, bends in the direction of the turn and hooks the turn tighter.

You are mistaking the chain reaction for what started and continues to increase the chain reaction. Without some braking action under the shovel there is little energy to start the forward chain reaction and decamber the fronts of the skis.
Quote:
 Originally posted by Pierre:By the way, using the word "Curvilinear" shows you is solidly from the east ...
... pretty close, because using the term shows that I am solidly from MIT ...

I dunno, Pierre, that twisting the leg to set the shovel, etc., seems unnecessarily complicated when simply rolling the ski over on edge will accomplish the same thing. You can see this even without the ski on your leg: just lay it on edge, push down a bit to give it some bend, and then kind'a shove it thru the snow along the path of least resistance; it pretty much makes a turn and there's no twisting involved.

Anyway, it looks like the bar is closing and its just you and me here; let's just have another, talk about some good runs, and call it a night.
Noclevername I did not say twisting the leg, I said twisting the foot. I will bet that you move you're body and legs into a position that applies some torque to the skis throught the foot. Twisting the legs would rotate you out of counter.
New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: Ski Instruction & Coaching