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# Lightening - Page 3

With respect to lightening, BobB has pointed out the desirability of minimal interference with the path of the CM as it crosses over the path of the skis as viewed from above.

Bob, is it equally desirable for the path of the CM through space, when viewed from the side, at ground level, also be minimally perturbed. In other words, the path of the CM through space should not have a "glitch" in it in any direction at the transition. If this is so, then, assuming smooth terrain, the height of the CM above the ground won't change through the transition in efficient skiing, and that immediately tells us:

a) If we make our legs too rigid, the CM will momentarily rise as we go through the transition;

b) If we actively retract, the CM will start to momentarily dip towards the ground as we go through the transition; and,

c) If we control our legs so that we "let" the skis pass under us naturally, the CM strands the best chance of flying through space in a straight line at the transition.

Thoughts?

Tom / PM

PS - I must confess that I haven't had time to read each line of this thread in detail, so if this concept has already been discussed, just ignore this msg.

[ November 09, 2003, 09:30 PM: Message edited by: PhysicsMan ]
I'm zooming out to present a broader view of pressure management (which includes lightening) which is where I feel this is, or should go, anyway if understanding is the goal.

As presented the physics are there, and available. But what we choose to "DO" affects the extent the energy available from those physics enters into any given turns and effects it's dynamics.

As for what we "DO":
The legs thru either extending, flexing proactively affect pressure on the skis. (Static legs passively allow pressure to increase or decrease just due to external forces). In high-performance skiing these movements are usually directed lateral/diagonally vs. vertically. But one of the most important aspects of skilled pressure management is that the "rate" of flexion/extension greatly alters the movement's effect on the pressure the skis experience.

Thru bottom portion of turns, as paths of Cm and skis converge, ones options are:
Static legs, allowing pressure build-up from turn dynamics only.
Extending to compound pressure build up on skis.
Flexing slowly so as to manage rate of pressure build up on skis.
Flexing rapidly so as to reduce or completely release pressure on skis.

Thru upper portion of turns, as paths of Cm and skis diverge, ones options are:
Static legs, pressure buildup may be delayed.
Extending rapidly to create quick pressure build up on skis.
Extending as needed to allow skis to move away without pressuring skis.
Flexing to absorb terrain induced pressure build-up on skis.

Think of pressure management as something we do to facilitate achieving our "intended outcome" with our skis/turns. This facilitation can be engaging, releasing, absorbing, un-weighting, decreasing radius, increasing radius, etc. That semantically abused lightening is simply a result of pressure management usually associated with reducing or releasing pressure for whatever reason desired. It comes down to knowing what you want to do, and what your options are to best get it done (for your pleasure).

This all come back around to learning not to try and cause that which we can allow. But also recognizing that part of the learning may be that we will need to first cause it a little, so that we can first experience it, enabling us to then learn to allow it a lot.

[img]smile.gif[/img]

[ November 09, 2003, 04:08 PM: Message edited by: Arcmeister ]
Quote:
 lightening is simply a result of pressure management usually associated with reducing or releasing pressure for whatever reason desired
Finally! [img]graemlins/thumbsup.gif[/img]
Quote:
 Finally!
Were you going to elaborate on that, Nolo?

Quote:
 I'm zooming out to present a broader view of pressure management (which includes lightening) which is where I feel this is, or should go, anyway if understanding is the goal.
Arc--certainly, pressure management as a whole is a worthy topic of discussion, and "lightening" could easily take on a broader meaning than the way it has been used in this particular thread. I agree that many of the specific issues addressed in this thread fall into the broad topic of pressure management. But the way it came about in this discussion, "lightening" refers only to the active lifting/retracting of one leg, ostensibly to effect a "weight transfer"/change of support foot. As used in this thread, "lightening" has not encompassed any other aspects of pressure management. I won't argue that there hasn't been some "semantic abuse" of the term "lightening" here, but that is the title of the thread, and active weight transfer has been the topic.

This thread has remained pretty tightly focused so far. Are you sure you want to "zoom out" and change the topic?

Best regards,
Bob
I thought Arc explained "lightening" in a way that even I could understand.
Quote:
 Originally posted by nolo:I thought Arc explained "lightening" in a way that even I could understand.
But, did he explain it in a way that matched the definition that began this thread? I, for one, don't think so.

ssh
ssh,

What Ydnar said:

Quote:
 Seems to me that what we are talking about is the managing of pressure through the last section of the arc leading to the starting of a new arc.
What Arc said:

Quote:
 Think of pressure management as something we do to facilitate achieving our "intended outcome" with our skis/turns. This facilitation can be engaging, releasing, absorbing, un-weighting, decreasing radius, increasing radius, etc. That semantically abused lightening is simply a result of pressure management usually associated with reducing or releasing pressure for whatever reason desired. It comes down to knowing what you want to do, and what your options are to best get it done (for your pleasure).
I thought Arc's response was a match, no?
Quote:
 Originally posted by nolo:I thought Arc's response was a match, no?
Oh.

Nevermind... (Think Emily Latella!!!)

ssh [img]graemlins/angel.gif[/img]
Hey the midwest is up and skiing!!!!!!!!!!!
My home area Sunburst opened and I made my first arcs of the season yesterday.

I even noticed that was engaging, releasing, absorbing, un-weighting, decreasing radius, increasing radius, even did som lightening. Sometimes I even did those things on purpose.
[img]smile.gif[/img] [img]smile.gif[/img] [img]graemlins/thumbsup.gif[/img]

[ November 10, 2003, 04:25 PM: Message edited by: Arcmeister ]
Okay Bob, you asked:

But consider this question: how do bicyclists, unicyclists, monoskiers, "skwallers," snowboarders, and one-legged skiers (without outriggers or poles), all of whom lack the ability to "transfer weight" (change support foot), accomplish this "crossover"?

I'd been contemplating this same sort of question for "one support" skiers in the context of this lightening/transfer balance model of the pure carved turn and don't know the answer. So, how do they?

I will hazard a guess tho. I'd guess they tilt the ski/monoski/snowboard (rather than rotate it?).
I can only relate it to sking linked turns on one leg.
Thru the falline, I am progressivly rolling the foot to engage the sidecut and strengthing the leg as forces build up. Arc'n around into the transition, I soften the leg enough to release the Cm from being re-directed as I start to roll the foot over to release the edge, and keep it rolling to re-engage on the new edge. After the edge change I extend the leg a the ski arcs out around into the falline. If I haven't fallen down by now, I repeat the process.
:
Arc,

Do you need to soften the leg to make the transition? Standing here on one leg rolling my foot back an forth, I'm wondering: Would just rolling the foot do it eventually? - and softening the leg simply accelerates it?

I kinda relate this to slalom water skiing (after getting a good zing across the wake and letting go of the rope). Seems like I can slide along for quite awhile turning back and forth, simply by rolling my feet, and not have to flex at all to release the edge. Of course, maybe you'll tell me I'm all wet here since I'm on the water : and it will give way being softer - although I've never noticed this softness when I catch a toe :
Tom/PM:

Thanks for the illustrations and explanations!

I'll take a shot with some thoughts. [Caveat: Just an average skier speaking here [img]smile.gif[/img] ]

Quote:
 Originally posted by PhysicsMan:With respect to lightening, BobB has pointed out the desirability of minimal interference with the path of the CM as it crosses over the path of the skis as viewed from above. Bob, is it equally desirable for the path of the CM through space, when viewed from the side, at ground level, also be minimally perturbed. In other words, the path of the CM through space should not have a "glitch" in it in any direction at the transition. If this is so, then, assuming smooth terrain, the height of the CM above the ground won't change through the transition in efficient skiing, and that immediately tells us: a) If we make our legs too rigid, the CM will momentarily rise as we go through the transition; b) If we actively retract, the CM will start to momentarily dip towards the ground as we go through the transition; and, c) If we control our legs so that we "let" the skis pass under us naturally, the CM strands the best chance of flying through space in a straight line at the transition. Thoughts? ...
I would have to go with "c)". I would agree that minimizing the vertical rise and fall of the CM is one of the goals. However, wont this change with the velocity and amplitude? At some combinations of these, wont the skier end up extremely low at the apex of the turn and physically need to rise in order to pass thru transition (assuming a transition in balance)? Seems to me that vertical movement of the CM would be acceptable to the extent that it results from the pressure management and geometry of the skier.

For "a)" & "b)", I don't see how they can exist in the sine wave turns you presented. Wouldn't you get spikes and dips, before and after the transition, in the forces on your illustration, instead of the smooth flow shown, if this were to happen?

Make sense?

As an aside, while I was meandering through this thread today, reading about the movements of the "ideal" turn and those movements that are needed/accepted/required by experts when their turn falls shy of being ideal, I recalled a simple expression (of unknown origin), that was shared with me at some point, that I think applies: "In theory there is no difference between practice and theory, but in practice there is!"
You certainly do not "need" to soften the legs, but it sure helps if you want to agressivly ski arc2arc. If you do not soften, or retract, the legs the Cm must gets pole vaulted up and over into the next turn. This not only usually produces some unweighting, which then delays re-ingaging, but the high trajectory of the Cm limits how far the path of the feet can laterally move away from it and achieve any early edge angle. Again, it depends on what you are trying to achieve.

As for water skiing, I do the same thing coming off the wake. Relax the legs and allow the ski to keep going wide as the Cm is pulled down lake by the artifical horizontal gravity provided by the boat. If you are running a slalom course, this is key to achieving a wide arc with the ski out and around the ball at short line lengths.

H2O make for good skiing, melted or frozen. [img]graemlins/thumbsup.gif[/img]
As for the b) above, if skiing terrain with any pitch, and the legs angularly extended, retraction at the end of the turn will unlikely be extreem enough to release the CM parallel to the slope and most likely produce a more horizontal release trajectory, which actually moves the Cm away from a slope with any pitch at all. In a high energy situation, without relaxation or retraction you get the proverbial huckover launching the Cm into the on-no zone and disconnected from any pressure management options for the start of the next turn while the legs desperately reach for the ground as the skis skitter ineffectivly across the snow.
I think we may finally have this issue of "lightening" nailed down.

In response to a suggestion to consider the motion of the CM towards and away from the surface of the snow, Cgeib and Arcmeister responded:

CGeib:
“I would have to go with "c)". I would agree that minimizing the vertical rise and fall of the CM is one of the goals. … ”

Arcmeister:
“… You certainly do not "need" to soften the legs, but it sure helps if you want to aggressively ski arc2arc. If you do not soften, or retract, the legs the Cm must gets pole vaulted up and over into the next turn. This not only usually produces some unweighting, which then delays re-engaging, but the high trajectory of the Cm limits how far the path of the feet can laterally move away from it and achieve any early edge angle. …

As for the b) above, if skiing terrain with any pitch, and the legs angularly extended, retraction at the end of the turn will unlikely be extreme enough to release the CM parallel to the slope and most likely produce a more horizontal release trajectory, which actually moves the Cm away from a slope with any pitch at all. In a high energy situation, without relaxation or retraction you get the proverbial huckover launching the Cm into the on-no zone and disconnected from any pressure management options for the start of the next turn while the legs desperately reach for the ground ”

Other than a couple of technical questions (which I’ll address below), both responses seem to prefer my option (c), i.e., minimizing unnecessary vertical motion of the CM. Too stiff (ie, option “a”) , and you do a huckover, while if you do too energetic of an active retraction (ie, option “b”), and you will either wind up too low or, in some situations, possibly have the problems that Arc suggested.

In light of the above, I think that the whole question of lightening is essentially solved. Basically, as the skier goes through the transition, he (she) has to adjust the extension of each leg independently so that:

(a) The total combined force from both legs provides an appropriate amount of force acting on the skier’s torso to get his CM to "fly" through the transition smoothly, with as few vertical motion glitches as possible (consistent with terrain undulations, more strategic levels of “intent”, etc.); and,

(b) The difference in force between his L and R legs provides an appropriate amount of torque on the skier’s torso to get it to fly through the transition with as few glitches in its smooth horizontal motion as possible, as well as minimal glitching in the slower angular changes of the torso during the turn (again, consistent with terrain, intent, desired degree of counter, … blah, blah).

Over the past couple of days, I have been thinking about other ways to express the above "maximum smoothness" concept, perhaps working up some computer simulations of the force on each leg over time, or the glitch-like effects on the path of the CM caused by non-optimal individual force histories for each leg. Unfortunately, because all of the different possible combinations of speed, turn radius, slope angle, simultaneously occurring terrain undulations, etc., I don’t think that showing the results of a simulation or two would assist understanding any more than simply stating the above “control policy” and simply giving a couple of text examples of what happens if the skier doesn’t adhere to the above “policy”.

For example, we have already talked about the effects of too much or too little total combined force from both legs. This essentially was the (a), (b), and (c) possibilities of my previous post.

The effects of too much force differential between the L and R legs at the transition were talked about earlier in this thread. For example, too large of a force differential could be caused by too active of a retraction of the old outside leg, and would lead to passive “falling into the next turn” (assuming no compensatory L-R balance adjustments). With compensatory balance adjustments it could lead to “stepping up”. Too large of a force differential between the L and R legs at the transition could also result from skiing the way a veteran, narrow-stance, legs-locked powder skier might ski if put onto hardpack for the first time. He might enter the turn banked, retract both legs simultaneously to get his CM and skis to cross paths and then bank into the next turn. This wouldn’t be disastrous in a narrow stance (ie, like mono-boarding), but in a wider stance, if he persisted in simultaneous L-R retraction and extension, he could start generating some very odd L-R weight distributions like temporarily putting all of his weight on the uphill edges of his (old) uphill skis.

I don’t think that too little of a weight differential between the skier’s L and R skis at the transition (ie, no relaxation of the DH leg at the transition in a wide stance) was explicitly discussed, but would almost certainly hinder the cross over of the CM and skis, and thereby put a glitch in the path of the CM as viewed from above.

Finally, let me address CGeib's technical concerns:

CGeib:
“…However, wont this change with the velocity and amplitude? At some combinations of these, wont the skier end up extremely low at the apex of the turn and physically need to rise in order to pass thru transition (assuming a transition in balance)? Seems to me that vertical movement of the CM would be acceptable to the extent that it results from the pressure management and geometry of the skier. For "a)" & "b)", I don't see how they can exist in the sine wave turns you presented. Wouldn't you get spikes and dips, before and after the transition, in the forces on your illustration, instead of the smooth flow shown, if this were to happen? …”

You are absolutely right that as a result of some particular combination of skiing and terrain variables, you don’t want to wind up too low at the apex. My option (c) should have said to "let the CM fly through the transition in as straight a line as possible, while moving generally parallel to the surface of the snow" (ie, adding the phrase after the word, "while"). Basically, I am thinking that the issue of removing glitches from the transition itself is dealt with by the “fast” control policy that we’ve been talking about, while the issue of not winding up over extended or over flexed through the rest of the turn is dealt with by a slower control policy.

You are also absolutely right that (a) and (b) could not exist in the sine wave turns I presented, certainly not when viewed from the side. I felt this was true, but wanted people either to find a flaw in my argument, or else be forced to the same conclusion that I came to, thereby leaving only one viable option, (c).

Tom / PM

--------------
PS (in edit) - You might notice that appears to be a possible omission in the above argument - I never once mentioned edge angles. There is a reason for this. In exact analogy to my post timestamped November 08, 2003 11:38 PM in this thread, if you know exactly the course that the object is taking through space (for example, "the skier's CM is going in a straight line"), then you know the total force acting on that object. There are no "if's, and's, or but's" about this because it is just a restatement of F = ma. So, for example, in the case of the skier's CM, this knowledge immediately leads me to my conclusions about the total combined force of both legs.

What I haven't made any statement about is what the skis are doing, only what the CM is doing. We know that at the transition, we want the skis to cross under (cross-over / cross-under symantics not withstanding) the skier, also going in a straight line, but at an angle to the line that the skier's CM is taking.

So, with the downward force on each ski essentially established by previous arguments, and neglecting fore-aft motion, the only real option left for manipulating the skis is edge angle. The angle from the skis up to the CM will be dictated by the convergence of these two lines, so this is not a variable. Fortunately, however, a skier can contort himself into varying degrees of the classical "bananna" shaped position of angulation, so that he (she) can adjust this variable (within limits) as the skis come under him. These motions are almost totally independent of the downward forces on the skis that were discussed above, so that changes in edging at the transition can be discussed quite separately from the forces, and that's what I decided to do in the previous material. It doesn't mean edging is unimportant, just that it can be separated out from a discussion of (say) vertical forces.

PS #2 - Geeze, this got long. Sorry folks. Thank God none of us actually ski thinking about all this stuff.

[ November 12, 2003, 01:19 PM: Message edited by: PhysicsMan ]
Quote:
 Originally posted by Arcmeister:You certainly do not "need" to soften the legs, but it sure helps if you want to agressivly ski arc2arc. If you do not soften, or retract, the legs the Cm must gets pole vaulted up and over into the next turn. This not only usually produces some unweighting, which then delays re-ingaging, but the high trajectory of the Cm limits how far the path of the feet can laterally move away from it and achieve any early edge angle. Again, it depends on what you are trying to achieve. As for water skiing, I do the same thing coming off the wake. Relax the legs and allow the ski to keep going wide as the Cm is pulled down lake by the artifical horizontal gravity provided by the boat. If you are running a slalom course, this is key to achieving a wide arc with the ski out and around the ball at short line lengths. H2O make for good skiing, melted or frozen. [img]graemlins/thumbsup.gif[/img]
Arc,

As you probably gathered from my prior response to PhysicsMan, I agree with what you're are saying in practice - as it relates to either form of H2O [img]smile.gif[/img] When I responded concerning the "need" to soften the legs, I was pretty much thinking along the lines of the bicycle, unicyclist etc. examples that I thought you were relating to with the one legged skiing. In the cases of the bicycle and unicycle there is no ability to relax the frame through transition from one turn to another (assuming rigid frame and seated!). I don't ride a unicycle, but I am able to make turns on my bicycle without being launched or having the bike leave the road (although I'll readily acknowledge that it is easily possible to do). So while I certainly understand the benefit of managing pressure in dynamic turns as you've describe. I wonder if it is possible to make the sine-wave turns PM illustrated with minimal flexing .... instead of relaxing the legs thru the transition, letting CM rise (assuming theoretically that this turn is possible). I guess the real question is, in the "ideal" sine-wave turn, would the CM necessarily be pole vaulted or should you be returning to the point of transition perfectly balanced with equal weight on both skis, both skis flat, moving in the direction the skis are pointed, because you've perfectly managed edge angle and balance throughout the entire turn - only having flexed the legs as necessary to achieve the required edge angles, then returning to the neutral athletic stance at transition? Again, theoretically.

Thoughts?
Quote:
 Originally posted by cgeib:...When I responded concerning the "need" to soften the legs, I was pretty much thinking along the lines of the bicycle, unicyclist etc. examples ... In the cases of the bicycle and unicycle there is no ability to relax the frame through transition from one turn to another ... I guess the real question is, in the "ideal" sine-wave turn, would the CM necessarily be pole vaulted or should you be returning to the point of transition perfectly balanced with equal weight on both skis, both skis flat, moving in the direction the skis are pointed, because you've perfectly managed edge angle and balance throughout the entire turn - only having flexed the legs as necessary to achieve the required edge angles ...
CGeib – Excellent question. (Sorry I didn’t notice it before this.) On a bicycle, another way of stating that you don't get launched is that you don’t generate enough upward G’s at the transition to overcome gravity. Although I have never timed this, I think that the most important factor is that the time through the transition in most bicycle turns is longer than the time through the transition in an aggressive ski turn. (Maybe someone has real numbers they can contribute.)

For example, lets assume that in both sports, the athlete goes from leaning 45 degrees to the left to 45 degrees to the right and his CM is 1 meter off the ground. If the bicyclist makes this change in 1 second (and I suspect 2+ sec is more usual), the “huck-over” acceleration will only be about one quarter of a G ( 0.252 G) . On the other hand, if the skier makes the same change in lean angle in 0.5 second (and also keeps the distance between his CM and skis constant at 1 meter), his huck-over acceleration will be almost exactly 1 G, i.e., his CM will want to rise, and he will get into the exact groping-for-the-ground situation that Arc described earlier.

Put differently, a less aggressive skier (i.e., over 1 sec through the transition) will never have a problem, and, even a more aggressive skier (ie, under 0.5 sec) won’t have a problem if he retracts enough to keep his distance off the snow constant instead of having his CM go upwards for a part of a circle.

> ... with equal weight on both skis, both skis flat, moving in the direction the skis are pointed ...

You probably meant this, but to be perfectly precise, at the transition, "you" (ie, your CM) won't be going exactly in the direction your skis are going, but at some small angle to it as your CM and your skis cross paths.

Cheers,

Tom / PM

PS – BTW, the formula I used is just acceleration = radius * omega^2, so you can easily plug in numbers to see the effects of other angles and times in the above examples.

[ November 14, 2003, 11:53 AM: Message edited by: PhysicsMan ]
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