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Instructors: do you teach moving forward at initiation? If so, how? Or why not? - Page 4

post #91 of 195
Quote:
 

1.  Let's assume you are traveling downhill in a circular path on skis, making a single long radius turn.

In the middle of this circular path, you all of a sudden bend forward at the ankles, moving your whole body straight ahead relative to the straight length of the skis.

You do not change anything else you are doing.

How's this straight-line-forward-movement of all those protons you own going to affect the direction of travel?

 

2.  Let's assume you are side-slipping straight downhill, constant speed.

In the middle of this sideslip, you all of a sudden bend forward at the ankles, moving your whole body straight ahead relative to the straight length of the skis.

You do not change anything else you are doing.

How's this movement of your CoM going to affect the direction of travel?

 

3.  Let's assume you are traversing gently across a slope, more or less in a straight line and at a more-or-less constant speed.

In the middle of this traverse, you all of a sudden bend forward at the ankles, moving your CoM ahead over the fronts of your skis.

You don't change anything else.

How's this movement going to affect your direction of travel?

 

LiquidFeet--instead of looking for a theory of what "ought to" happen, I suggest doing some experienting to find out what does happen (yes, I know, that takes a little more snow than most of us have access to right now). And then, once you know what happens, figure out why, and seek an explanation.

 

I say this for a very practical reason. As I mentioned in an earlier post, expectations and beliefs can severely color our experience and observations. Believing that something should happen a certain way can trigger nearly imperceptible movements--especially in a highly skilled skier--that will force it to happen the way you believe it should. We have many ways of "cheating"! We've already discussed the effects of fore-aft pressure control on a sideslip.

 

There can be benefits of doing it the other way around, of course. If you know what to look for, sometimes it is easier to find it. But if your understanding is not complete, you're prone to "find" something that isn't true!

 

Either one can work, but beware these caveats.

 

---

 

That said, you described your three questions as "experiments," so perhaps that's what you intended. I just want to point out that one simple answer to each of your questions, in which you described movements of your CM (relative to your feet) is that that movement is, itself, your answer to the question ("how does this movement of your CM affect your direction of travel?"). Perhaps you mean, "how does it affect ski performance?" Or am I completely misreading your question?

 

Best regards,

Bob

post #92 of 195
Quote:
Originally Posted by Bob Barnes View Post
 

 

LiquidFeet--instead of looking for a theory of what "ought to" happen, I suggest doing some experienting to find out what does happen (yes, I know, that takes a little more snow than most of us have access to right now). And then, once you know what happens, figure out why, and seek an explanation.

 

I say this for a very practical reason. As I mentioned in an earlier post, expectations and beliefs can severely color our experience and observations. Believing that something should happen a certain way can trigger nearly imperceptible movements--especially in a highly skilled skier--that will force it to happen the way you believe it should. We have many ways of "cheating"! We've already discussed the effects of fore-aft pressure control on a sideslip.

 

There can be benefits of doing it the other way around, of course. If you know what to look for, sometimes it is easier to find it. But if your understanding is not complete, you're prone to "find" something that isn't true!

 

Either one can work, but beware these caveats.

 

---

 

That said, you described your three questions as "experiments," so perhaps that's what you intended. I just want to point out that one simple answer to each of your questions, in which you described movements of your CM (relative to your feet) is that that movement is, itself, your answer to the question ("how does this movement of your CM affect your direction of travel?"). Perhaps you mean, "how does it affect ski performance?" Or am I completely misreading your question?

 

Best regards,

Bob

I agree with Bob, and the short answer to your questions LF is that it depends on the torque resistance distribution and camber characteristics of your skis. The long version is more complicated than one might think.

post #93 of 195

Thanks, Jamt.

 

Said another way, if you're looking to create beliefs about what "should" happen, you're likely to suffer from the CGeib Principle that "Believing is Seeing." Better, if that's the case, to go in open-minded and without expectations and exploit the principle that "seeing is believing."

 

With complex issues such as this, full understanding can take a while--and may still involve the caveat of over-simplification that makes it inaccurate in the real world. I will suggest a couple variables that can greatly impact the results of your "experiment." Edge angle (particularly, "platform angle"), and upper body movements, including arms and poles, can completely change the outcome, even when they are all but imperceptible.

 

Believe nothing. Question Everything. We're back to that!

 

Best regards,

Bob

post #94 of 195
Quote:
Originally Posted by LiquidFeet View Post
 

To follow up with this line of thought, I'd like to suggest three thought experiments with help from yous guys who are physics pros.

I'd love a breakdown of how this works if anybody wants to do it, in easy English.  

 

1.  Let's assume you are traveling downhill in a circular path on skis, making a single long radius turn.

In the middle of this circular path, you all of a sudden bend forward at the ankles, moving your whole body straight ahead relative to the straight length of the skis.

You do not change anything else you are doing.

How's this straight-line-forward-movement of all those protons you own going to affect the direction of travel?

 

2.  Let's assume you are side-slipping straight downhill, constant speed.

In the middle of this sideslip, you all of a sudden bend forward at the ankles, moving your whole body straight ahead relative to the straight length of the skis.

You do not change anything else you are doing.

How's this movement of your CoM going to affect the direction of travel?

 

3.  Let's assume you are traversing gently across a slope, more or less in a straight line and at a more-or-less constant speed.

In the middle of this traverse, you all of a sudden bend forward at the ankles, moving your CoM ahead over the fronts of your skis.

You don't change anything else.

How's this movement going to affect your direction of travel?

Ok, LF....I'll play :). but before I do, I want to address an assumption you make in all 3 questions and that is, if I bend forward at the ankles that I am necessarily moving my CoM forward as well.

 

Try this:

Standing still in a vertical position with your arms at your side and your fingers pointed straight down, drop as rapidly as you can so your fingers touch the floor just in front of each ankle.  Where do you find yourself?  I find myself on the balls of my feet with my ankles bent forward, my butt behind, my chest forward  and my head up. A lot of things have moved but my CoM has not. Thanks to all my vertical joints I am in balance and I have NOT moved forward in relation to the floor to which I am attached by gravity.  Likewise I am attached to my skis by boots and bindings. It is the skis that move in relationship to the snow. I am just the driver of the skis.

 

So if I understand you correctly, by asking me to bend my ankles and move my CoM in line with the direction of the skis,  you are are asking me to LEAN forward.  If that is what you mean then in each case I am applying leverage forward, bending the front of the ski more than the back destroying the circular capabilities of the ski's design. So..

 

In #1 your skis will have a good chance to wash out with your tails breaking loose thereby destroying the centripetal nature of the turn.

 

In #2 your skis will begin to move forward in a circular path of sorts because you have taken the linear inertia of your slideslip and transferred that energy to an edged, albeit improperly bent ski .

 

in #3 It is a half assed version of #2 because you if you are traveling across the hill on shaped skis in a relatively straight line then you need to be skidding. If you are on your edges then railroad tracks appear.  

 

So as an observation based on your questions, what may really be worthy of note is that just because you are bending at the ankles at initiation doesn't mean you are moving your CoMForward. Yes/No?

post #95 of 195
Thread Starter 

Hmmm.......this is a non-real-world thought experiment or armchair exercise.  I don't want to go out and try it to see what will happen, because as Bob Barnes points out, variables will creep in.  That's the whole point of asking it this way.

 

When one bends forward at the ankles without changing anything else, then the knees, hips, shoulders, arms, hands, and head will move forward.  Thus, wherever the center of mass is, I can't see any way it won't move forward, since all those other parts move forward.  

 

Clarification:  You can bend forward at the ankles, aka you can dorsiflex/close your ankles, while keeping your heel locked down on the sole of your boot.  You could bend the ankles so far forward that your heels lift inside the boot, but that's not what I'm asking about.  Just imagine yourself bending forward at the ankles somewhat, while keeping the heels locked down on the boot sole.  When the heels start to lighten and lift, you stop the bending forward, so that won't happen.  

 

Clalrification:  In these thought experiments, the CoM goes forward because of the bending of the ankle, but not enough to lift the heel.  The forwardness of the weight will affect the bending of the ski, of course.  But something else is gong to happen as well.  The forward movement will alter to the momentum, yes?  You bend forward to a certain point, then you stop bending forward.  Something happens to the momentum as a result of this forward motion that stops.  What? 
 

I must admit I don't want to go out and experiment on the grass.  I promise I will go out and do this once there's white stuff.  But when I do I'd like to compare what really happens when I try it, to what physics predicts should happen in an uncluttered world when only the ankles bend.  This way I may detect if I'm adding any English to my simple ankle-bending.  

 

If anyone wants to play, that's fine.  If not, that's OK too.  No biggie.

post #96 of 195
Quote:
Originally Posted by LiquidFeet View Post

 

...

 

I'd like to compare what really happens when I try it, to what physics predicts should happen in an uncluttered world when only the ankles bend.  

That's part of the problem, LiquidFeet--it is not an "uncluttered world." One particularly important variable is the amount of edge angle and edge engagement involved when you make your "forward movement." Are the edges gripping somewhat--skidding, as opposed to just slipping--when you sideslip? If so, the part of the edge that receives the most pressure will "drag" more than the part with less pressure. Another variable involves your starting point--where were you, fore-and-aft, when you started, before you flexed your ankles and moved more forward? And how extensive is the forward movement you're envisioning? Finally, in the first (turning) and third (traversing) scenarios, are your skis railed out carving (not really possible if they're traversing in a straight line, unless they have no sidecut), or are they skidding to some degree? How much? And what about snow conditions?

 

As far as Jesinstr's objection that flexing (bending, dorsiflexing) the ankles does not necessarily move your center of mass forward, if you are clear--as you were--that flexing the ankles is the only thing you do, then of course, your CM does move forward (relative to your feet)--and somewhat down at the same time. Interestingly, the more flexed they were before you started flexing them more, the more the resulting movement of your CM is down; the more upright (shins vertical) at the start, the more flexing your ankles moves your CM forward. Jesinstr--note that in your example of dropping lower, you are doing more than just flexing your ankles--you are also involving your knees, hips, and probably spine and arms. So I have to agree with LiquidFeet's premise that flexing your ankles--and only your ankles--moves your CM forward relative to your feet. This is a small point, though, because LF did specify that your ankle movement does move your CM forward, so even if you involve other joints, you must end up forward according to her instructions.

 

Anyway, it's still not that clear and "uncluttered" what happens to your skis when you do this, LF. Take the edges, the snow, and everything else out of the picture for a moment--in fact, go ahead out on the grass, or inside on your floor in bare feet. Starting "fore-aft neutral," with pressure along your whole foot (ball to heel), dorsiflex ("bend") your ankles, keeping your feet flat on the floor, such that you end up somewhat more "forward" and balanced over the balls of your feet. Pay close attention. You'll notice that the first thing that happens as you begin to flex your ankles is that pressure moves back onto your heels, as your forefoot gets "lighter" on the floor. This causes you to then "topple" forward, which results in the pressure moving forward on your feet, especially as you "catch" yourself and stop the forward movement. So that flexing of your ankles first shifts pressure back, momentarily, and then shifts it forward strongly, and then finally ends up somewhat forward as a result of your CM being more forward (relative to your feet) than it was before.

 

All this is academic, probably, because I think that your objective is pretty clear--what do your skis do once you have moved the balance point forward along their length, in the three different scenarios of your questions? Right? If so, then assuming that the skis are not completely flat on the snow, which is a fair assumption at least in your first (turning) and third (traversing) scenarios, imagine a simpler situation. Instead of flexing your ankles, imagine that we just put heavy weights on the tips of your skis, as you stayed centered. Can you picture the added "drag" from those weighted tips with their edges engaged, as the skis skid sideways, causing the tails to skid more than the tips? (Jamt will probably find fault with this simplification--rightly so--because it eliminates some of the torque effects that result from keeping the "weight" attached to the skis at the bindings even when we lean forward--which actually adds to the tendency of the tails to wash out.) Just flexing your ankles would cause the same thing--adding pressure to the tips--while also lightening the tails--even bending the skis and lifting the tails right off the snow if you move far enough forward. The tips would grip more, and the tails would slip more, causing your whole body (because you specified "doing nothing else," so it could not twist in any joint) and skis to rotate. In the turning scenario, you already have some rotational ("angular") momentum of your body just due to the turn itself, so when you lighten your tails and they break loose, you could find yourself quickly "spinning out." The same thing would happen in scenario 2 (straight sideslip), unless your skis were perfectly flat on the snow, which is not very likely. Even if they were, there would still be a little more resistance--friction--from the more-weighted tips on the snow, causing torque that would cause you to rotate, tails down the hill.

 

So that's my take on "the theory," with a few assumptions added to your "do nothing else." I suspect that by "do nothing else," you are particularly implying "don't rotate or counter-rotate your body" as you flex your ankles forward--and that is, indeed, critical to your experiment. At the same time, it's pretty hard to avoid entirely. And really, I suggest that even if you could eliminate all "body english" in the experiment, it would lose relevance to actual skiing, because these movements are so integral to real skiing.

 

One final very important point: As I mentioned before, the laws of physics do not cause gravity to pull the more heavily weighted end of your skis down the hill faster, as some people may assume would happen. Yes, the gravitational force ("weight") is proportional to the amount of mass (by definition--weight is mass X the gravitational constant), so the more mass, the more force. But more mass means also more inertia (resistance to change of motion), so ultimately, a boulder and a pebble will fall from a height at the same rate (in a vacuum, eliminating air resistance--which is another variable that could have "cluttering" significance in your experiment!). 

 

You know the difference between theory and reality, of course. It's that In theory, there is no difference, but In reality, sometimes there is!

 

Best regards,

Bob

post #97 of 195

 

Quote:
 
Originally Posted by Bob Barnes View Post
 

That's part of the problem, LiquidFeet--it is not an "uncluttered world." One particularly important variable is the amount of edge angle and edge engagement involved when you make your "forward movement." Are the edges gripping somewhat--skidding, as opposed to just slipping--when you sideslip? If so, the part of the edge that receives the most pressure will "drag" more than the part with less pressure. Another variable involves your starting point--where were you, fore-and-aft, when you started, before you flexed your ankles and moved more forward? And how extensive is the forward movement you're envisioning? Finally, in the first (turning) and third (traversing) scenarios, are your skis railed out carving (not really possible if they're traversing in a straight line, unless they have no sidecut), or are they skidding to some degree? How much? And what about snow conditions?

Boy your not kidding.  Fore and aft movement can also affect stance.  Stance affects the parallel relationship of your skis.  Divergence and Convergence. Step onto a set of turn tables and play around.


Edited by Pierre - 11/5/14 at 3:42am
post #98 of 195
Quote:
Originally Posted by Bob Barnes View Post
 

That's part of the problem, LiquidFeet--it is not an "uncluttered world." One particularly important variable is the amount of edge angle and edge engagement involved when you make your "forward movement." Are the edges gripping somewhat--skidding, as opposed to just slipping--when you sideslip? If so, the part of the edge that receives the most pressure will "drag" more than the part with less pressure. Another variable involves your starting point--where were you, fore-and-aft, when you started, before you flexed your ankles and moved more forward? And how extensive is the forward movement you're envisioning? Finally, in the first (turning) and third (traversing) scenarios, are your skis railed out carving (not really possible if they're traversing in a straight line, unless they have no sidecut), or are they skidding to some degree? How much? And what about snow conditions?

 

As far as Jesinstr's objection that flexing (bending, dorsiflexing) the ankles does not necessarily move your center of mass forward, if you are clear--as you were--that flexing the ankles is the only thing you do, then of course, your CM does move forward (relative to your feet)--and somewhat down at the same time. Interestingly, the more flexed they were before you started flexing them more, the more the resulting movement of your CM is down; the more upright (shins vertical) at the start, the more flexing your ankles moves your CM forward. Jesinstr--note that in your example of dropping lower, you are doing more than just flexing your ankles--you are also involving your knees, hips, and probably spine and arms. So I have to agree with LiquidFeet's premise that flexing your ankles--and only your ankles--moves your CM forward relative to your feet. This is a small point, though, because LF did specify that your ankle movement does move your CM forward, so even if you involve other joints, you must end up forward according to her instructions.

 

Anyway, it's still not that clear and "uncluttered" what happens to your skis when you do this, LF. Take the edges, the snow, and everything else out of the picture for a moment--in fact, go ahead out on the grass, or inside on your floor in bare feet. Starting "fore-aft neutral," with pressure along your whole foot (ball to heel), dorsiflex ("bend") your ankles, keeping your feet flat on the floor, such that you end up somewhat more "forward" and balanced over the balls of your feet. Pay close attention. You'll notice that the first thing that happens as you begin to flex your ankles is that pressure moves back onto your heels, as your forefoot gets "lighter" on the floor. This causes you to then "topple" forward, which results in the pressure moving forward on your feet, especially as you "catch" yourself and stop the forward movement. So that flexing of your ankles first shifts pressure back, momentarily, and then shifts it forward strongly, and then finally ends up somewhat forward as a result of your CM being more forward (relative to your feet) than it was before.

 

All this is academic, probably, because I think that your objective is pretty clear--what do your skis do once you have moved the balance point forward along their length, in the three different scenarios of your questions? Right? If so, then assuming that the skis are not completely flat on the snow, which is a fair assumption at least in your first (turning) and third (traversing) scenarios, imagine a simpler situation. Instead of flexing your ankles, imagine that we just put heavy weights on the tips of your skis, as you stayed centered. Can you picture the added "drag" from those weighted tips with their edges engaged, as the skis skid sideways, causing the tails to skid more than the tips? (Jamt will probably find fault with this simplification--rightly so--because it eliminates some of the torque effects that result from keeping the "weight" attached to the skis at the bindings even when we lean forward--which actually adds to the tendency of the tails to wash out.) Just flexing your ankles would cause the same thing--adding pressure to the tips--while also lightening the tails--even bending the skis and lifting the tails right off the snow if you move far enough forward. The tips would grip more, and the tails would slip more, causing your whole body (because you specified "doing nothing else," so it could not twist in any joint) and skis to rotate. In the turning scenario, you already have some rotational ("angular") momentum of your body just due to the turn itself, so when you lighten your tails and they break loose, you could find yourself quickly "spinning out." The same thing would happen in scenario 2 (straight sideslip), unless your skis were perfectly flat on the snow, which is not very likely. Even if they were, there would still be a little more resistance--friction--from the more-weighted tips on the snow, causing torque that would cause you to rotate, tails down the hill.

 

So that's my take on "the theory," with a few assumptions added to your "do nothing else." I suspect that by "do nothing else," you are particularly implying "don't rotate or counter-rotate your body" as you flex your ankles forward--and that is, indeed, critical to your experiment. At the same time, it's pretty hard to avoid entirely. And really, I suggest that even if you could eliminate all "body english" in the experiment, it would lose relevance to actual skiing, because these movements are so integral to real skiing.

 

One final very important point: As I mentioned before, the laws of physics do not cause gravity to pull the more heavily weighted end of your skis down the hill faster, as some people may assume would happen. Yes, the gravitational force ("weight") is proportional to the amount of mass (by definition--weight is mass X the gravitational constant), so the more mass, the more force. But more mass means also more inertia (resistance to change of motion), so ultimately, a boulder and a pebble will fall from a height at the same rate (in a vacuum, eliminating air resistance--which is another variable that could have "cluttering" significance in your experiment!). 

 

You know the difference between theory and reality, of course. It's that In theory, there is no difference, but In reality, sometimes there is!

 

Best regards,

Bob

Bob, Yes your are correct. I did not follow LF's prescribed conditions which I should have.  I jumped to what should be vs what LF was asking for. However I think the outcomes to the 3 scenarios were valid and among the many things that could happen.   

 

In reading the rest of what you wrote above, I am right on board. 

 

One question that has popped into my mind as these discussions progress is regarding Center of Mass (CoM) which is referenced a ton in this forum.  Most know what CoM is but since it is a "floating entity", constantly changing as we move, how does the brain know exactly where it is at any point in time?  My research tells me that there are multiple sensory inputs feeding the brain including sight, touch, equilibrium, spatial awareness to name a few. When I touch my index finger to my nose, my brain knows exactly where my finger is due to the inputs I just described.  But since my CoM is not a part but a place, I would appreciate your thoughts on how we develop CoM awareness.  My short answer is that it requires knowledge, education and as Paul Hornung used to say: "Practice, Practice, Practice"

post #99 of 195
Thread Starter 

I'm listening.  Two things.  

 
Originally Posted by Bob Barnes View Post
....

That's part of the problem, LiquidFeet--it is not an "uncluttered world." One particularly important variable is the amount of edge angle and edge engagement involved when you make your "forward movement." Are the edges gripping somewhat--skidding, as opposed to just slipping--when you sideslip? If so, the part of the edge that receives the most pressure will "drag" more than the part with less pressure. Another variable involves your starting point--where were you, fore-and-aft, when you started, before you flexed your ankles and moved more forward? And how extensive is the forward movement you're envisioning? Finally, in the first (turning) and third (traversing) scenarios, are your skis railed out carving (not really possible if they're traversing in a straight line, unless they have no sidecut), or are they skidding to some degree? How much? And what about snow conditions?

....

 

1. Bob, you say "One particularly important variable is the amount of edge angle and edge engagement involved when you make your "forward movement." Are the edges gripping somewhat--skidding, as opposed to just slipping--when you sideslip?"  I always thought the term "slipping" meant straight down the fall line with skis perpendicular to the direction of travel, and "skidding" meant traveling somewhat diagonal to the direction the skis are pointing.  A sideslip by definition needs to be slipping.  Given pretty hard snow, which is normal where I ski, one can sideslip with almost flat skis and not be in danger of cartwheeling down; precision is possible.  Do you mean that if you have a higher edge angle while side-slipping, the results will change when you bend forward at the ankles?  

 

LeMaster has something in his book indicating that when you compare moving weight forward with more edging compared to less edging, the results reverse themselves.  Is this what you are talking about? If so, I'd like to hear more about what makes the reversal happen when more edging is compared to less edging.

 

 

2.  How about the added momentum caused by the forward movement of the whole body above the feet, temporary though it is?   I would think this would alter the direction of travel in all three instances, separate from the effects of skis bending more in the shovel, and separate from effects of edges gripping snow as the weight goes forward.

post #100 of 195

LF. Lets discuss some physics of what could happen when you change CoM/feet relative position.

 

Assume that you are in balance in the middle of the ski and sideslipping down an ideal slope. The system is in balance, which means that the total turning moment around the ski is zero. If we assume that the ski is infinitely torsionally stiff and straight we have the same edge angle all along the ski if we compare the front to the tails.

With this assumption nothing happens when the CoM is moved, because the turning moment parallel to the slope is the same as the moment perpendicularly to the slope along the ski.

However if the ski is like most skis it will be more torsionally stiff towards the middle of the ski, and many skis are also wider in the front.  This means that if you move the weight forward from the center location the torsion twist will be larger in the front of the ski and thus the edge angle will be lower in the front. The ski will slip more in the front.

 

In reality many other things can happen e.g.

The uphill foot may be further forward and thus the edge angle and relative weight distribution of the upper vs downhill ski will play an important role. 

the fore-aft state between the two skis may be different.

momentum

post #101 of 195
Thread Starter 
Quote:
Originally Posted by Jamt View Post
 

LF. Lets discuss some physics of what could happen when you change CoM/feet relative position.

 

Assume that you are in balance in the middle of the ski and sideslipping down an ideal slope. The system is in balance, which means that the total turning moment around the ski is zero. If we assume that the ski is infinitely torsionally stiff and straight we have the same edge angle all along the ski if we compare the front to the tails.

With this assumption nothing happens when the CoM is moved, because the turning moment parallel to the slope is the same as the moment perpendicularly to the slope along the ski.

However if the ski is like most skis it will be more torsionally stiff towards the middle of the ski, and many skis are also wider in the front.  This means that if you move the weight forward from the center location the torsion twist will be larger in the front of the ski and thus the edge angle will be lower in the front. The ski will slip more in the front.

 

In reality many other things can happen e.g.

The uphill foot may be further forward and thus the edge angle and relative weight distribution of the upper vs downhill ski will play an important role

the fore-aft state between the two skis may be different.

momentum

Interesting....

 

1.  I think you are saying that on an" ideal" slope with no permutations (hey I get these here in NE!!! but I doubt most folks would call them ideal), if one has skis which are absolutely torsionally stiff and absolutely straight in shape, then nothing happens if you bend forward at the ankles.  Even though you softly "lunge" forward and then stop.  

 

But if you have forgiving recreational skis that are not totally torsionally stiff, or any ski that is wider in the front (all skis today), the tip will bend away more than the middle, the edging along the shovel will therefore be reduced, its grip will diminish, and that shovel will therefore slip downhill more than the middle of the ski.  This action will turn the whole ski effectively downhill.  The rate of turning will depend on its torsional stiffness and width dimensions.  Am I getting that right?

 

2.  What are the implications for side-slipping down in a countered position instead of facing the trees in a "square" stance?  Assume your weight is originally more over the downhill ski, centered over your arch on the downhill ski, and assume you don't change your weight distribution left-right as you bend the ankles forward at both ankles....

post #102 of 195

Jamt, Bob,

 

It seems that one of you is saying in a side slip scenario (assume only one ski for simplicity), moving forward would increase the pressure on the front of the ski causing greater drag so that the tail of the ski would slip down the hill ahead of the tips. The other is saying that for reasons of variable torsional rigidity and longer moment arms the front of the ski will slip more due to lessened edge angle and the ski tips will move down the hill. Both effects seem to make sense to me so which would have the greater affect on the ski.

 

In other words, if you are both right, who wins

 

fom

post #103 of 195
Quote:
Originally Posted by LiquidFeet View Post
 

Interesting....

 

1.  I think you are saying that on an" ideal" slope with no permutations (hey I get these here in NE!!! but I doubt most folks would call them ideal), if one has skis which are absolutely torsionally stiff and absolutely straight in shape, then nothing happens if you bend forward at the ankles.  Even though you softly "lunge" forward and then stop.  

 

But if you have forgiving recreational skis that are not totally torsionally stiff, or any ski that is wider in the front (all skis today), the tip will bend away more than the middle, the edging along the shovel will therefore be reduced, its grip will diminish, and that shovel will therefore slip downhill more than the middle of the ski.  This action will turn the whole ski effectively downhill.  The rate of turning will depend on its torsional stiffness and width dimensions.  Am I getting that right?

 

2.  What are the implications for side-slipping down in a countered position instead of facing the trees in a "square" stance?  Assume your weight is originally more over the downhill ski, centered over your arch on the downhill ski, and assume you don't change your weight distribution left-right as you bend the ankles forward at both ankles....

1 Yes

 

2. Depends on the edge angles. When you are countered the uphill ski will be more forward. Thus, if you edge the uphill/front ski a bit less and have equal distribution the tips will seek the fall line. If you edge the uphill ski more the tails will seek the fall line. 

 

There is a lot to play with. Two ways of letting the tips seek the fall line are. 1. more fore and less edge on the downhill ski. 2 more fore and less edge on the uphill ski. (think one ski pressured flat in the front, the other gripping in the back)

 

I love playing with these fine motor skills.

post #104 of 195
Quote:
Originally Posted by fatoldman View Post
 

Jamt, Bob,

 

It seems that one of you is saying in a side slip scenario (assume only one ski for simplicity), moving forward would increase the pressure on the front of the ski causing greater drag so that the tail of the ski would slip down the hill ahead of the tips. The other is saying that for reasons of variable torsional rigidity and longer moment arms the front of the ski will slip more due to lessened edge angle and the ski tips will move down the hill. Both effects seem to make sense to me so which would have the greater affect on the ski.

 

In other words, if you are both right, who wins

 

fom

Hehe, good question. I'm sure we could demo it both ways to prove out points :-)

 

If you believe a ski will slide a certain way it will :-D 

 

Another factor is; -It is quite difficult to tip a ski without also transferring some rotation force to it and if you start to rotate in a certain direction there is a tendency that it will continue.. 

post #105 of 195
Thread Starter 
Originally Posted by Jamt View Post
....

Another factor is; -It is quite difficult to tip a ski without also transferring some rotation force to it and if you start to rotate in a certain direction there is a tendency that it will continue.. 

 Red:  this is exactly why I'm asking about isolating forward movement without any tipping changes.  I want to know its effect all by itself.  Especially since my experience differs from what some folks claim should be happening.  I want to know if I'm cheating to get the results I've been told I should get.  I'll admit that's entirely possible.  

post #106 of 195
Thread Starter 
Originally Posted by Jamt View Post
Originally Posted by LiquidFeet View Post
....

2.  What are the implications for side-slipping down in a countered position instead of facing the trees in a "square" stance?  Assume your weight is originally more over the downhill ski, centered over your arch on the downhill ski, and assume you don't change your weight distribution left-right as you bend the ankles forward at both ankles....

....

 

2. Depends on the edge angles. When you are countered the uphill ski will be more forward. Thus, if you edge the uphill/front ski a bit less and have equal distribution the tips will seek the fall line. If you edge the uphill ski more the tails will seek the fall line. 

 

There is a lot to play with. Two ways of letting the tips seek the fall line are. 1. more fore and less edge on the downhill ski. 2 more fore and less edge on the uphill ski. (think one ski pressured flat in the front, the other gripping in the back)

 

I love playing with these fine motor skills.

In this countered scenario, what if you don't change the edging; both skis have the same edge angle with the snow for the entire experiment, and your body stays aligned over those two skis in the saggital plane without any change.  You have more weight over the outside ski, and you don't change this.  There is some weight on the inside ski, but not as much.  You only flex forward at the ankles.  What happens then in this surreal ideal impossible world?

post #107 of 195
Quote:
Originally Posted by LiquidFeet View Post
 

 Red:  this is exactly why I'm asking about isolating forward movement without any tipping changes.  I want to know its effect all by itself.  Especially since my experience differs from what some folks claim should be happening.  I want to know if I'm cheating to get the results I've been told I should get.  I'll admit that's entirely possible.  

 

Quote:
Originally Posted by LiquidFeet View Post
 

 What if you don't change the edging; both skis have the same edge angle with the snow for the entire experiment, and your body stays aligned over those two skis in the saggital plane without any change.  You have more weight over the outside ski, and you don't change this.  You only flex forward at the ankles.  What happens then in this surreal ideal impossible world?

 

 Its a hypothetical surreal situation as you put it.  hehe.  But what is driving you to ask about this?  what point or question are you trying to get to?

post #108 of 195
Quote:
Originally Posted by LiquidFeet View Post
 

In this countered scenario, what if you don't change the edging; both skis have the same edge angle with the snow for the entire experiment, and your body stays aligned over those two skis in the saggital plane without any change.  You have more weight over the outside ski, and you don't change this.  There is some weight on the inside ski, but not as much.  You only flex forward at the ankles.  What happens then in this surreal ideal impossible world?

I'm a bit hesitant to answer without trying it on snow. I don't think I have concentrated on isolating that movement. 

 

Anyway based on what I said before about the COM location the tips should seek the fall line if the CoM is fore. However, what actually happen when you move the CoM forward by dorsiflexing? The first thing that happens is that you pressure the heels, and then the body starts to move forward. Eventually you will stop the forward motion with the cuffs and/or calf muscles and then you will settle in the more fore position. So, first we pressure center, then aft, then a lot more fore, than a bit more fore than we started. The three latter stages could have different effects and it is difficult to say what would happen. Also depends on the DIRT.

 

What is your experience of what happens if you have tried this LF?


Edited by Jamt - 11/5/14 at 11:23am
post #109 of 195
Quote:
Originally Posted by Jamt View Post

LF. Lets discuss some physics of what could happen when you change CoM/feet relative position.

Assume that you are in balance in the middle of the ski and sideslipping down an ideal slope. The system is in balance, which means that the total turning moment around the ski is zero. If we assume that the ski is infinitely torsionally stiff and straight we have the same edge angle all along the ski if we compare the front to the tails.
With this assumption nothing happens when the CoM is moved, because the turning moment parallel to the slope is the same as the moment perpendicularly to the slope along the ski.
However if the ski is like most skis it will be more torsionally stiff towards the middle of the ski, and many skis are also wider in the front.  This means that if you move the weight forward from the center location the torsion twist will be larger in the front of the ski and thus the edge angle will be lower in the front. The ski will slip more in the front.

In reality many other things can happen e.g.
The uphill foot may be further forward and thus the edge angle and relative weight distribution of the upper vs downhill ski will play an important role. 
the fore-aft state between the two skis may be different.
momentum

Thanks for providing some food for thought on my flight, Jamt!

I'm not sure about the piece I bolded above. In theory it sounds compelling, but I have to wonder: Is it only the increased pressure from moving forward that causes the torsion?

Seems there has to be resistance under the shovel of the ski for that torsion to take place and exist. Otherwise, would it not just slip rather than twist?

So how can there be resistance to cause the twist and the twist also causes the slip?

Sounds like having your cake and eating it too!
post #110 of 195
Quote:
Originally Posted by cgeib View Post


Thanks for providing some food for thought on my flight, Jamt!

I'm not sure about the piece I bolded above. In theory it sounds compelling, but I have to wonder: Is it only the increased pressure from moving forward that causes the torsion?

Seems there has to be resistance under the shovel of the ski for that torsion to take place and exist. Otherwise, would it not just slip rather than twist?

So how can there be resistance to cause the twist and the twist also causes the slip?

Sounds like having your cake and eating it too!

Good point, but consider the case that the edge is locked, i.e. you are standing still. Then the tip is more twisted. If you then gradually de-edge the ski it will start to slip first at the tip. 

post #111 of 195
Thread Starter 
Quote:

Originally Posted by Jamt View Post
....

This means that if you move the weight forward from the center location the torsion twist will be larger in the front of the ski and thus the edge angle will be lower in the front. The ski will slip more in the front.

 
Originally Posted by LiquidFeet View Post
 

In this countered scenario, what if you don't change the edging; both skis have the same edge angle with the snow for the entire experiment, and your body stays aligned over those two skis in the saggital plane without any change.  You have more weight over the outside ski, and you don't change this.  There is some weight on the inside ski, but not as much.  You only flex forward at the ankles.  What happens then in this surreal ideal impossible world?

I'm a bit hesitant to answer without trying it on snow. I don't think I have concentrated on isolating that movement. 

 

Anyway based on what I said before about the COM location the tips should seek the fall line if the CoM is fore. However, what actually happen when you move the CoM forward by dorsiflexing? The first thing that happens is that you pressure the heels, and then the body starts to move forward. Eventually you will stop the forward motion with the cuffs and/or calf muscles and then you will settle in the more fore position. So, first we pressure center, then aft, then a lot more fore, than a bit more fore than we started. The three latter stages could have different effects and it is difficult to say what would happen. Also depends on the DIRT.

 

What is your experience of what happens if you have tried this LF?

My experience is that the tips seek the fall line.  I don't think I'm cheating and turning the skis with upper-body wishful-thinking rotation.  But maybe....

The part above in italics is the best explanation I've heard for why this will happen.  

post #112 of 195
Thread Starter 

I keep thinking about an added momentum thing going on as a result of the forward movement.


Let's imagine that I'm in a nice smooth sideslip on that ideal slope, and I lunge forward fast, dramatically, on my skis by bending forward with my ankles quick, and let's also imagine I punch my arms straight forward along my skis to add drama to this lunge, parallel to the slope surface.

 

Will the forward movement of my body and my arms, that stops suddenly when my shins hit the boot cuff and when my arms reach their full extension, be transferred to the skis, sending them more forward?  I think so.  So since they were heading straight downhill before, they will have the old momentum still pointing downhill and moving them along, but the skis will feel the new momentum pointing to the side of the trail too.  Combine these two and you get a downward point of something.  Not sure it's the tips.  It could be the feet in the middle of the skis.

 

I assume I'm using the word momentum wrong, but I hope you engineer types get the idea anyway.

post #113 of 195

my opinion is that whatever happens during falling leaf is more related to the END of turns, not the start of turns.  You start a turn when you engage the downhill edges.  As long as you're on the uphill edges you're still at the end of the old turn.

post #114 of 195

We need to be careful here or we may end up complicating things.

 

fom

post #115 of 195
Thread Starter 

Hey, this is fun for some people.  Candy?

.  

post #116 of 195
Quote:
Originally Posted by LiquidFeet View Post
 

I keep thinking about an added momentum thing going on as a result of the forward movement.


Let's imagine that I'm in a nice smooth sideslip on that ideal slope, and I lunge forward fast, dramatically, on my skis by bending forward with my ankles quick, and let's also imagine I punch my arms straight forward along my skis to add drama to this lunge, parallel to the slope surface.

 

Will the forward movement of my body and my arms, that stops suddenly when my shins hit the boot cuff and when my arms reach their full extension, be transferred to the skis, sending them more forward?  I think so.  So since they were heading straight downhill before, they will have the old momentum still pointing downhill and moving them along, but the skis will feel the new momentum pointing to the side of the trail too.  Combine these two and you get a downward point of something.  Not sure it's the tips.  It could be the feet in the middle of the skis.

 

I assume I'm using the word momentum wrong, but I hope you engineer types get the idea anyway.

LF, consider the frame of reference. As @Bob Barnes has pointed out the forward movement of the CoM is in relation to the feet. There is very little friction between the skis and the snow in the longitudinal direction to give you any significant forward movement relative to the slope. If you stand on a frictionless surface and dorsiflex the feet will move backwards and the head forward. The CoM will not move.

post #117 of 195
Thread Starter 
Quote:
Originally Posted by Jamt View Post
 
Quote:
Originally Posted by LiquidFeet View Post
 

I keep thinking about an added momentum thing going on as a result of the forward movement.


Let's imagine that I'm in a nice smooth sideslip on that ideal slope, and I lunge forward fast, dramatically, on my skis by bending forward with my ankles quick, and let's also imagine I punch my arms straight forward along my skis to add drama to this lunge, parallel to the slope surface.

 

Will the forward movement of my body and my arms, that stops suddenly when my shins hit the boot cuff and when my arms reach their full extension, be transferred to the skis, sending them more forward?  I think so.  So since they were heading straight downhill before, they will have the old momentum still pointing downhill and moving them along, but the skis will feel the new momentum pointing to the side of the trail too.  Combine these two and you get a downward point of something.  Not sure it's the tips.  It could be the feet in the middle of the skis.

 

I assume I'm using the word momentum wrong, but I hope you engineer types get the idea anyway.

LF, consider the frame of reference. As @Bob Barnes has pointed out the forward movement of the CoM is in relation to the feet. There is very little friction between the skis and the snow in the longitudinal direction to give you any significant forward movement relative to the slope. If you stand on a frictionless surface and dorsiflex the feet will move backwards and the head forward. The CoM will not move.

Ahhh.  What you say makes perfect sense.  I'll drop the forward momentum thing.  

post #118 of 195

I would like to address a concept regarding human movement.  It ties into this discussion as well as most discussions regarding skiing movements.   Muscle function as it applies to moving joints is described as such.   Muscles have what are called insertions and origins.   Basically the two ends of the muscle.  Take for example the tibialis anterior muscle which is one of the muscles which dorsi flexes the ankle.  It's origin is essentially at the knee ( not to be too specific).  It's insertion is at the medial and planter aspect  of the foot.  Generally the insertions and origins are listed by how the muscle typically functions.  Insertions typically move toward origins when the muscle contracts.  So if we hold the leg "fixed" and contract the Tibialis anterior muscle the ankle dorsi flexes.  However, if we hold the foot fixed and contract the tibialis anterior muscle the shin moves towards the foot. In both cases the ankle is dorsi flexing.   Another example might be the biceps muscle of the upper arm.  It can flex the elbow and bring the wrist to the shoulder or if you are doing pull ups it can bring the entire body  to the hand.  It depends  upon which end of the muscle you fix.    The reason I bring this up is because human movement is very complex and trying to get too technical in describing muscle and joint function can lead to error.   I can bend my leg and bring my foot to my butt or I can bend my leg and let my butt sink onto my foot.   It all depends upon what the rest of my body is doing.   YM

post #119 of 195
Quote:
Originally Posted by JESINSTR View Post
 
...
 
One question that has popped into my mind as these discussions progress is regarding Center of Mass (CoM) which is referenced a ton in this forum.  Most know what CoM is but since it is a "floating entity", constantly changing as we move, how does the brain know exactly where it is at any point in time?  My research tells me that there are multiple sensory inputs feeding the brain including sight, touch, equilibrium, spatial awareness to name a few. When I touch my index finger to my nose, my brain knows exactly where my finger is due to the inputs I just described.  But since my CoM is not a part but a place, I would appreciate your thoughts on how we develop CoM awareness.  My short answer is that it requires knowledge, education and as Paul Hornung used to say: "Practice, Practice, Practice"

 

A very good point, JESINSTR. The Center of Mass (CM) is the basis of balance and pressure management in skiing. Because it is so fundamental, misunderstanding--and even worse, assuming or believing the CM to be something it is not  (it is not "the hips," for example)--can escalate into grossly incorrect conclusions about many movements--what is, what ought to be, and what causes what.

 

Center of Mass is the point about which all of a body's mass is equally distributed, such that in many ways the entire body acts as if it were located at that point. It is the point about which an object will rotate--in any axis--when spinning through the air. As JESINSTR points out, the CM is not a body part, like the liver, or the navel or something. It need not even be a point inside the body--think of the CM of a donut (it's in the hole) or a boomerang (a good image, since as skiers, we often bend or angulate into a shape resembling that boomerang). While it is convenient (and not always inaccurate) to think of it as "approximately where your belt buckle would be," its actual location depends on each person's body proportions and--of critical importance--the relative positions of the various body parts. In other words, it moves around within--and often outside of--our body, depending on what position we're in. One of the biggest and most problematic misconceptions is that the CM is where the hips are. If true, this would mean that the hips would need to be directly over the feet for "static" balance, and that the fore-aft position of the hips entirely determines the fore-aft pressure point on the skis. These are not true!

 

In any case, one thing that defines all great skiers is an uncommon awareness at all times of the location and movements of their CM and their base of support (balance point), as well as the paths or trajectories of these two points--the movements and paths shown on my simplified animations of the X-Move. They know exactly where these two things are, and where they're going, and where they'll be in the near future, relative to each other and relative to the slope--in four dimensions (fore-aft, lateral, vertical, and over time). They know what they need to do to change either or both of those trajectories as needed--and they know when not to interfere and to simply let their momentum continue undisturbed. It is not necessarily a conscious awareness, but it is always there, underlying and informing all of their movements, as simple and fundamental as the "sense of balance."

 

Many of us might take this "sixth sense" awareness for granted, but the more I work with skiers, the more I realize that it is something learned through experience, and sadly lacking in many skiers--especially in the "fourth dimension." It is the key to "flow" and "smoothness," as well as speed, dynamic balance, and the effortlessness and accuracy of the X-Move.

 

I don't know of anything that can hasten the development of this sense, other than lots of practice and mileage. Can't argue with your closing statement: 

Quote:
 My short answer is that it requires knowledge, education and as Paul Hornung used to say: "Practice, Practice, Practice"

 

Best regards,

Bob

post #120 of 195
Quote:
Originally Posted by borntoski683 View Post
 

my opinion is that whatever happens during falling leaf is more related to the END of turns, not the start of turns.  You start a turn when you engage the downhill edges.  As long as you're on the uphill edges you're still at the end of the old turn.

 

BornToSki--I'd have to disagree with that statement--and I'll bet you will too, when you think about it. I'd argue that turns begin the moment the edge(s) release their grip on the snow such that they can no longer resist the pull of gravity down the slope. Since the edges release before the skis are flat on the snow (think of a sideslip), many turns actually begin on the uphilll edges--the outside edges of the new turn. Visualize a straight traverse, in which you slowly relax your edge angles until the skis let go and your path begins to curve smoothly down the hill. As that curve begins, you are still standing on your uphill edges--even though they have released their grip. Look closely at this illustration of a "basic parallel turn," especially the edge angle representations on the right side:

 

 

 

 Simplified representation of fundamental movements of basic, linked, offensive turns. These turns begin on the uphill edges! 

 

The turns in this illustration begin (and end) in frames 4, 12, and 20. These are the points at which the critical two paths (CM and Base of Support) cross, the points I call "turn neutral." As the edge angle illustration in the green box shows, the skis are not flat on the snow here (although they are at "critical edge angle"--a "platform angle" of 90 degrees to the force the skier applies to them). In this illustration,I arbitrarily drew the skis rolling through "flat on the snow" in the next frames, after the turns have begun (frames 5, 13, and 21). The actual timing in real turns will depend upon speed, steepness of the hill, and turn radius--as well as the "perfection" and accuracy of the skier's movements through the turns.

 

Certainly, however, the shaping phase (carving phase, pressure phase) cannot begin until after the skis have rolled onto their inside edges (right edges in a right turn)--or at least, the inside edge of the outside ski. I'm guessing that that is what you were thinking about. Indeed, we can think of that change of edges, along with movements to establish balance in the shaping phase, as the primary purpose of the transition--and the core topic of this thread!

 

Best regards,

Bob

 

(PS--Before Jamt jumps on it, notice that this illustration represents a fairly low-speed basic parallel turn. As such, it does not portray the "straight line" transition phases of the feet and the CM that will occur in highly dynamic turns--especially when the skier becomes "weightless" or airborne in the transition. I have a similar diagram for "Dynamic Parallel Turns" that also leaves out the "straight" phases--I should probably rework it!)

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