 Poster: A snowHead
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Why is the skier assumed to be at the bottom of the bowl instead of on the back side, say?
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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 . Please elaborate.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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If the ski is flexed into a smooth curve with no breaks, why is the assumption made that gravity pulls through the middle between toepiece and heel?
Instead of forward through the toepiece, say?
Maybe should be in BzK. Maybe should PM Physicsman.
Last edited by Well, the person's real but it's just a made up name, see? on Tue 4-12-07 20:37; edited 1 time in total
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You need to Login to know who's really who.
You need to Login to know who's really who.
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I understand. There's more ski in front of the bindings that behind.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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comprex Ah, I understand now. Don't know the answer though. I think Tom's definately ya man.
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You'll need to Register first of course.
You'll need to Register first of course.
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SMALLZOOKEEPER, fair cop, I was playing with the Shamans at the w/e.
But, that's one answer and some of the questions I haven't even asked yet. 
spyderjon, I think I've had part of a meaningful answer already, here:
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What makes a ski turn in powder (without specific user input) is a slight differential between the force of the snow pushing upward on the front of an edged ski compared to the force pushing upwards on the rear of the ski. If the ski isn’t edged, this force differential simply makes the tip rise vertically upwards. If the ski is edged, the force differential pushes the tip in the direction of edging. When the ski is moving forward through the snow, snow is coming directly at the upwardly curved front section of the ski, so the fore-aft force differential is larger for a ski that flexes, compared to one that doesn’t.
When you think about it, a fore-aft force differential is what causes skis to turn both in powder and on hardpack. The difference is simply whether the force that the snow exerts on the ski is all concentrated at the ski’s edge, or is spread out over the entire width of the base. So, “pressuring” certainly happens on both types of snow), but a force differential is the key to any turn. |
Another interesting thing happens when the sidecut gets involved. (Related to the discussion we had here: http://snowheads.com/ski-forum/viewtopic.php?p=764236 )
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comprex, What makes a ski turn in powder without meaningful user control input is the bizarre sensation of skis rotating over head, @rse rotating over/around t1t and snow everywhere, including all those places that sand doesn't really get to (if you know what I mean)...or not?
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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David Murdoch, was that supposed to be meaningful?
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comprex, I think so - if I don't make meaningful control input, I get the described result. And if I have skis on and make no specific user input, I get the same results... 
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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David Murdoch, I make a meaningful control input to the lift button which makes the lift arrive, but one still hasn't explained how it happens. Physicsman's note had more to do with the other side of the ski: you make meaningful control input which does -what?- at the ski level. Answer: causes a force differential between tip and tail. Notice that the explanation is valid for both powder and hardpack.
My question about *flexed* ski boot ramp calculations has more to do with the post several down after the one quoted:
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The net force that the snow exerts perpendicular to the base of the ski pushes the ski upwards when the ski is flat, and pushes the ski to the side when it is edged. This is necessary, but not sufficient for a turn to continue. In addition to pushing sideways, the tip of the ski must continue to rotate in the direction of the turn as the turn progresses.
One way to accomplish this is for the skier to provide an adequate rotatary torque. This will work even on a pair of 2x4's (ie, straight skis with essentially infinite sidecut radius). If the skier provides too much torque, the tails will skid out. If the skier provides not enough, the turn will stop, even though the ski might still be edged (note: still talking about powder conditions).
A second way to accomplish this is for the ski itself to provide this rotary torque. This is what the force differential that I was talking about in my previous post provides. The extra force on the tip compared to the tail is (as you point out) a torque, and it keeps the turn going automatically, even with no torque provided by the skier's leg.
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I am looking to reverse the argument and wonder what happens to the flexed ski if a binding ramp is set up based on a flat-ski assumption. The force providing the rotary torque is derived from the skier and follows the self-righting instinct of the skier. The body, the legs want to be "UP" no matter what surface you put them on. This is why SZK's answer was an answer that overcame most of the possible quibbles I could come up with, the boot sole flex of the boots mentioned adapts to the skier wanting to be "UP" and would negate small changes in binding ramp. Without passing that adaptation up further to the ankle, knee and hips. It could possibly even adapt to changes in effective binding ramp caused by edging of a tapered ski.
Yes, this thread continues my long-term curiosity about hidden or masked fore-aft pressures we put on the skis. Previously I've disguised this curiosity with questions like
"In a 90/10 weighted turn, do you keep the ski tips even by flexing muscles inside the 10% weighted boot, or by yanking back with the hamstrings so that the 10% weighted ski has a hidden (pitch-plane or sagittal-plane) torque on it sufficient to flex the 10% weighted boot"
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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comprex, "That's right Boo-Boo i'm smarter than the average bear." 
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SMALLZOOKEEPER, have you been raiding the pic-a-nik baskets again
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You know it makes sense.
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comprex, hmmm. But seriously.
My intuition (and I will be the first to admit that applied physics is often not intuitive) is that:
Whatever resultant force of any turn will pass from the skiers centre of mass through the bit of ski that's perpendicular to it. Which could be anywhere along the ski really.
A competent skier will have that tangent move from towards-tip to towards-tail as the turn progresses.
There are two ways to turn short radius turns in powder, compression and retraction, and the difference is just in the timing. Long radius turns are more like a piste carved turn and the turn shape is dictated by the degree of decamber. Short radius turns are more forced as you create a platform under the ski to trigger or complete a turn. Any forced rotation is sort of between turns.
I don't think ramp angle makes a difference as the skier will compensate in the quasi-fluid medium that is powder as opposed to having to ride on the hard surface that is piste.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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| David Murdoch wrote: |
Whatever resultant force of any turn will pass from the skiers centre of mass through the bit of ski that's perpendicular to it. Which could be anywhere along the ski really.
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I think I understand what you mean here and I have no issue with it. The problem I pose might be restated: how does the skier generate clean resultant turning force, and what binding ramp will help make sure there are no
- hidden forces acting to propel the ski or put the skier in the back seat
- hidden pressures generated by skier body tension that are balanced at the ski by Ground Reaction Force that might act to slow a ski/flex a ski beyond what is intended.
(you know as well as I do that flat-running skis can be slow or fast depending on the skier's balance upon them, what about turning skis? Another easily visualised extreme example is if I deliberately flex my toes up and run the skis like water skis.)
I posed the original question as I did because a skier standing at the bottom of the bowl and a skier standing on the back of it could take a different ramp depending on their boot fit allowance and strength of righting instinct.
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A competent skier will have that tangent move from towards-tip to towards-tail as the turn progresses.
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I find this a self-fulfilling description and not directly useful.
Just so it's clear, for the purposes of this discussion I see no difference between powder and hardpack turns.
No immediate response needed, I'm away from all comps for a few days
Last edited by Otherwise you'll just go on seeing the one name: on Wed 5-12-07 21:56; edited 1 time in total
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Poster: A snowHead
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comprex,
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A competent skier will have that tangent move from towards-tip to towards-tail as the turn progresses. |
I find this a self-fulfilling description and not directly useful. |
Actually, I don't. I see many skiers who don't make that progression...in fact, the opposite may be one description of "parking and riding" - as opposed to "parking and flying" which is entirely different. And generallly more painful.
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| Just so it's clear, for the purposes of this discussion I see no difference between powder and hardpack turns. |
hmmm, in general, I agree, in detail, I don't think I do, at least not on real, skinny skis... 
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