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# A little quiz - Page 2

### CM vs pressure correction

Centripetal force is m v^2 / r, thus directly proportional to speed squared and inversely proportional to radius of curvature.
John Mason: Not much viceversa here. This isn't chicken and the egg. Your body is connected and resisting forces at your feet. By changing pressure mix between the feet you shift forces that will move your CM. If not there, you might be able to change your CM by what, waving your arms? Bending at the waist? The kenetic chain is still intact and not a theory.

You can certainly change the CM by bending at the waist. And I assure you that it will have a direct effect on the pressure your feet exert on the skis/ground. In a dynamic situation, the movement of the CM can certainly affect the pressure on the skis.

But, I agree that in most skiing situations, movement of the CM is initiated by pressure changes at the feet.

Bob, I think I better understand what Vail Snopro said about the CM dictating where pressure is needed rather than outright controling it.
Quote:
 Originally Posted by Rick Here's a little quiz to stimulate some thought: How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?FASTMAN
We use angulation to choose an edge angle (and hence turn radius), that produces centrafugal forces which are balanced by the movement of CM. If intent comes first, then we 1st choose the radius, the radius determines the centrafugal force and the CM movement required to balance against it. Finally all these things taken together determine the extent of angulation.

I know the definition of angulation is a touchy subject, What I mean by angulation is the bend at the hip or knee which allows the edge angle to be different from the overall inclination.

This leaves the matter of pressure distribution between the feet. For a given turn radius and CM movement a variety of weight distibutions would be possible (e.g. 100-0, 90-10, 50-50), but as pressure is shifted to the inside ski the effective balance point moves to the inside of the turn. Apart from physical challenges of managing this weight on the bent inside leg, balance becomes harder (smaller movements will cause more radical weight deviations - ie instability), and more extreme inclination and angulation will be required to stay in balance.

So it seems the more CM is moved to the inside of the turn, the greater the advantage of keeping pressure on the outside ski.
jpowrie-

Are you suggesting that a WC racer is initially selecting the path his skis will take around a gate, without regard to where it will direct the CM? Rather then the path his core/CM will take down the hill? I will agree with you that the radius is the first thing, but it is the radius (path) the CM will take, not the skis.

Having coached at that level for many years, I can attest to the fact that CM takes priority over all else. With extensive trial and error, and thousands of gates of practice, they have learned (each individually) what path they can best put their CM on, and still maintain clearance of the gate. This is true in both SL and GS. Watch a race- the skiers who best control their CM are the faster skiers. Sure, even Bode loses control for a turn or two, but he gets it back, and is fast. Imagine how much faster he'd be without those glitches!

How much edge/angulation is applied is determined by the required support of the path of the CM. This is where the difference lies between elite skiers, and the rest of the sport. The average skier applies force first, then adapts the CM to it. As I stated in an earlier post, this is usually an arbitrary amount, resulting in a turn controlled by the ski, not by any particular intent.

Let me throw out another analogy-

You are skiing down a hill at a fair pace. Suddenly there is an obstacle in front of you. What part of your body do you move first to avoid this obstacle? I can tell you now that if you tried to move the feet first, you just hit it! But the average person will attempt to move the CM to a new path first. And then the legs/feet will come along. If all is successful, then you will remain upright. But it still comes back to "intent".

Many will claim the "drift" that Bode and many top WC racers are using denies this principle. In fact, it further supports it! They have "chosen" to make their CM take a particular line, and then "deflect" it at a certain moment. If they were not the extremely gifted athletes they are, (or if you and I tried it) they/we would get rocked right of the snow and likely into the trees!

More great footage to support this is watching the "Kaiser"- Franz Klammer, winning the 1976 Olympic DH in Innsbruck. He skied it like a cat on a hot tin roof! But if you watch carefully, you will notice his core/CM never once varied from the path he wanted it to go.

Want more? Watch the new crop of WC bumpers skiing. You will see absolute control of the CM. It remains extremely calm, while the legs deal with what they need to, in order to allow the CM to remain calm. Tell me their goal/intent isn't to go fast, go straight, and be as smooth as possible. If they bobble- they are gone. Could this be done by using the legs/feet as the center of their focus? I think not!

For those of you who are skiing soon... Give it a try... ! Put your focus on your CM, and play with it... Make it go side to side, make it go up and down. Learn to control it! It may be the most fun you've had on skis in awhile!
Quote:
 Originally Posted by Rick Here's a little quiz to stimulate some thought: How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?FASTMAN
I'll compare truning into falline, with turning out of falline.

From transition to falline as the path of CM diverges from path of skis (as they ski over the virtual drop-off from flatter transition into the steeper falline), pressure has greatest potential to decrease. Skiers with highly developed pressure control skills offset this effect with rapid enough leg extension so as to manage pressure (maintain or increase) for desired intent.

From falline to transition as the path of CM converges with the path of the skis (as they ski into the virtual compression from steep falline to flatter transition) pressure has greatest potential to increase. Skiers with highly developed pressure control skills offset this effect with relaxing and/or flexing of legs so as to to manage excess pressure (absorb or reduce) for desired intent.

Wax on, Wax off
Pressure on, Pressure off
Arcmeister:

This is exactly true, and is one of the best explanations of pressure control I`ve read.

cdnguy
yeah they tell me - keep skis on snow & then suck it up
This is great article.
Creativity of the rasers reminded me one quote:
"Technical skill is mastery of complexity,
creativity is mastery of simplicity"
Well, well,,,, looks like I'm going to be handing out a lot of A's!!

Gone for the weekend, and I come back to find an admirably extensive collection of high quality comments. Well done all, I'm quite impressed!!

While the question I asked specifically questioned the relationship between pressure and lateral CM movement, the responses where wide ranging in theme. This is as it should be because skiing at its root is founded in the relationship between foot and CM, and all other technical elements revolve around that foundation.

Skiing is very much about the management of forces. As soon as we start to slide down a hill forces of momentum spontaneously emerge that combine with the constant force of gravity to influence the nature of our decent. We can use those forces to enhance the efficiency of that decent, or we can butt heads with those forces and make control of our decent an unnecessarily difficult struggle.

Where we choose to position our CM ultimately dictates where those forces we are attempting to manage will be directed. Through these lateral (left to right) CM location options we have complete control over what percentage of the forces of momentum and gravity are assigned to each of our feet. How we choose to make those assignments affects how our skis will respond and perform for us.

At this point I'd like to refer you to Jaspain's post. This is an extremely impressive piece of work. It very precisely explains the nature of the forces at work as we ski, and explains how they combine to act upon us. If you can work through this rather complicated explanation, and come to comprehend it, it would make this whole concept of the importance of CM location very clear, and would provide you with a very comprehensive understanding of what constitutes balance on skis. I would encourage you all to give it a go, and ask questions if it's unclear: your questions may help clarify it for others too.

Quote:
 Here's my take from a physical perspective. Consider the inertial reference frame of the slope. The turning skier is moving in a circle. Assume for now constant speed and radius of curvature. For this circular motion to occur the net force on the skier must be parallel to the slope and directed towards the center of the circle (centripetal force). The magnitude of the centripetal force is proportional to the skier's speed and inversely proportional to the square of the radius of curvature. The centripetal force is provided by the force of the snow on the skis, primarily on the outside ski. This force of the snow must also counteract gravity, which points straight down towards the center of the earth. Thus the snow force must be directed towards the inside of the turn and upwards from the snow. The skier's center of mass must be aligned with the snow force vector. Otherwise the snow force exerts a torque on the center of mass causing the skier to roll towards the inside or outside of the turn. This rolling torque, which the skier senses as being out of balance, will cause the skier to fall if it is sufficiently large. To align with the snow force, the skier's center of mass must move laterally towards the inside of the turn. The extent of this lateral move depends on the direction of the snow force vector. The pressure that the skier senses in the ski is proportional to the magnitude of the snow force vector. Both the magnitude and direction depend on the skier's speed and turn radius, and these ultimately determine the relationship between lateral motion of the center of mass and pressure on the skis. We could also consider other complicating factors such as changing speed, changing radius of curvature, and weighting/unweighting motion. Each of these would change the magnitude and direction of the snow force vector. Thus they would affect the position of the CM required for balance (both inside/outside and fore/aft) and pressure on the skis.
Quote:

Great Job Jaspain! Hope you'll chime in again to address any questions that may emerge.

I plan on soon addressing many of this threads post individually. Stay tuned.

FASTMAN
Yukon and Rotofury:
Sorry for the vagueness and buzz words. Typically I try to minimize that in my posts, but in this case I was hoping clarity would emerge out of the responses.

Pierre,
Thanks for kicking off the discussion, but no reason to have your hands over your butt, you're safe here.

Quote:
 Originally Posted by Pierre the anatomy of the human body would suggest that changing the pressure distribution foot to foot directly controls the lateral movement of the CM no matter how efficiently its done.
Clarification needed here Pierre. I would change the word "controlls" to "demands". Pressure can be changed momentarily without moving the CM laterally by simply extending the leg above the foot you want pressured and relaxing the other. But this in itself does not create balance, and a fall may follow.

Conversely, moving the CM will move the ground intersection point of the resultant force vector of momentum and gravity acting on it. CM movement is the controller of pressure location. If you change the contact point, such as by extending a leg, you must purposely relocate the CM to establish balance over that new contact point to remain upright. It doesn't happen automatically.

FASTMAN
Quote:
 Originally Posted by cdnguy it is essential when making a parallel turn-that is it is essential to incline. BUT it is also essential to do something else and that is keep most of our weight on the outside ski. It`s a tricky thing when you think about it-you must move to the INSIDE but keep most of your weight on the OUSIDE. Hmmmmmm.
Exactly right. This is where coming to understand Jaspain's post makes what can seem conter-intuitive suddenly very clear and logical.

Quote:
 Fortunately our bodies can bend. We can incline to the inside with our lower bodies (from the hips down) and bend our upper bodies slightly to the outside. This will keep weight on that outside ski. This bending is called agulation.
Yep. Specifically what angulation does is fine tune CM location. If we're carving a particular turn shape there's a single edge angle we must employ to create that shape. This dictates a specific and necessary angle of inclination from the knee down. If having our CM lie on that same plane (that same angle of inclination) does not provide balance, then we must move our CM laterally while maintaining the needed edge angle. This is what angulation does for us. The type of angulation we use dictates how far we can move our CM off the edge angle/inclination angle. With extreme angulation our CM can even be moved completely off our physical body, and into a place in space.

Quote:
 So in response to your question, I think that one implication of pressure and the lateral movement of the CM is the need to be able to angulate effectively.
Quote:
 Originally Posted by HeluvaSkier Most have related the turn to where the CM should be, but not where the CM should be in relation to the pressure on the ski (by this im assuming youre talking in terms of outside ski pressure). As the angle increases i think it is safe to say that outside ski pressure (in theory) should also increase. Thus, the center of mass should move further inside of the turn to counter act the motion that cdnguy talked about above
That's right Greg.

As the edge angle increases, and the turn sharpens, the momentum aspect of the momentum/gravity force combo increases. Momentum is a horizontal force, so as momentum side of the force picture increases it tips the resultant force vector (combined effect of momentum and gravity) more toward horizontal.

To achieve balance the CM must therefor be relocated so that the more horizontal force vector acting on it, and emerging out of it, still intersects the ground under the inside edge of the outside foot. To do that the CM must be moved down and in, leaving the more upright position it occupied during the lower momentum turn that produced a more vertical resultant force vector.

FASTMAN
Quote:
 Originally Posted by TomB So in other words, the goal is to use the CM to control the pressure. While it makes perfect sense, it does (kind of) kick the kinetic chain theory in the nuts. .
Yes, Tom; move the CM to maintain the desired distribution of pressure as turn forces change. And, I suppose from a purist sense it may seem like it disputes the kinetic chain therory, but I would suggest it does not.

The determination of the desired turn shape is always our first step. That is a mental activity.

From there two things happen simultaneously: the appropriate edge angle needed to produce the desired turn shape is employed, and the CM is relocated to a position that creates efficient balance on that new edge angle. Those are physical activities that I would suggest do in fact have their origin in muscular activities that produce the desired result because of the existing contact of the feet to the ground.

The ground is the base of support that responds to our bodies muscular input, our push, and pushes back on us. Our feet are the connection to that energy source. That ground/foot connection is needed to make the CM location modifications we desire.

FASTMAN
Quote:
 Originally Posted by Bob Barnes/Colorado Tom--you've brought up what may be at the heart of one of the deeper-rooted philosophical differences that cause much disagreement here at EpicSki, regarding technique. Is technique an end in itself, such that our movements are measured in terms of their technical correctness against a standard of some preferred technique? Or is technique merely the tool we use to accomplish a task, and movements are measured in reference to their effectiveness toward that end? I strongly lean toward the second answer, personally. Which means that, to me, we most definitely do not use the CM to control pressure--we use pressure to control our Center of Mass!
Hi Bob, glad you brought up this little conflict.

I whole heartedly agree with your belief that technical efficiency is best measured by a particular techniques effectiveness in accomplishing a chosen task, as opposed to a conformity to some arbitrarily preference selection. But I'd suggest that in regard to the conflict (in the way you describe it) there should be none because both sides are right.

There is a particular distribution of pressure between the feet that provides a more efficient management of turn forces, and a more biomechanically efficient edge engagement, so in that sense it is important to manage the location of our CM to provide that most optimal pressure distribution. Or,,, we may not be concerned about optimal efficiency, and for a series of turns chose to distribute pressure in a less than efficient manner, just because we desire to. Again, even in executing this intent, we use CM location to achieve our desired outcome. In this sense, CM placement to control pressure placement is right.

But how do we make the desired modifications to CM location to achieve the outcomes we desire, be them efficient or otherwise? A force is required. Two are at our disposal: 1) a muscularly created force, and 2) the existing forces of momentum and gravity.

We use these forces in combination to produce the desired CM relocations. The recipe for that combination is; the less the dependence on muscular input and the more the utilization of momentum and gravity, the greater the efficiency of the action.

In this sense forces control CM, but the 2 types of forces (existing and self generated) must be combined to achieve the desired result. The existing forces of momentum and gravity can't alone completely control CM movement. There must be well timed/controlled elements of muscular resistance and release present. Left to its own accord, momentum would result in an immediate CM cross-over upon its creation. As soon as a turn started momentum would carry the CM to the outside of the arc and cross-over would occur. Only by resisting that momentum through muscular application until the desired direction change has been accomplished can we avoid a premature turn completion. Once the desired amount of turn has been completed we can turn off the muscular resistance and let gravity and momentum do their thing.

Both sides are right.

What we don't typically do, though, is use CM location to control pressure magnitude. If we do this we are allocating turn shape and direction of travel a subordinate role to the turn G's we desire to experience. Sure you can do that, if that's your desired focus, but in normal skiing, where direction of travel is the primary focus, it won't accomplish the objective.

FASTMAN
Quote:
 Originally Posted by jpowrie **If intent comes first, then we 1st choose the radius, the radius determines the centrafugal force and the CM movement required to balance against it. Finally all these things taken together determine the extent of angulation. **What I mean by angulation is the bend at the hip or knee which allows the edge angle to be different from the overall inclination. **This leaves the matter of pressure distribution between the feet. For a given turn radius and CM movement a variety of weight distibutions would be possible (e.g. 100-0, 90-10, 50-50), but as pressure is shifted to the inside ski the effective balance point moves to the inside of the turn. Apart from physical challenges of managing this weight on the bent inside leg, balance becomes harder (smaller movements will cause more radical weight deviations - ie instability), and more extreme inclination and angulation will be required to stay in balance.
Great stuff jpowrie!

Your order of events in the top paragraph is right on.

So too with your description in the second paragraph of the relationship between the edge angle and the CM to foot inclination angle. They're almost never the same.

The magnitude of the forces attempting to drive our CM (gravity which attempts to drive it down into the snow, and momentum which attempts to drive it laterally into the trees) determine what direction their combined effect will act on our CM. That combined effect is called the angle of resultant force, or the resultant force vector.

Only rarely does the edge angle match the angle of the resultant force vector. To get them both to intersect the ground at the same point, and thus create balance, we must do just as you say: angulate so that our CM is moved to such a location as to make that common ground intersection point happen.

That point is ideally under the inside edge of the outside ski. If you imagine a picture of this, what you see would be two lines emerging out of the ground at the same point (inside edge of outside ski). One of those lines would leave the snow and travel in a direction perpendicular to the ski base: this is called the edge angle. The other line would leave the snow and travel to the CM: that is called the resultant force vector, the angle of which is dictated by gravity and momentum.

If a particular turn shape is our intent we can't change the angle of the resultant force vector; that angle is completely force dependant. Therefore, to make that vector intersect the ground under the outside ski we must move our CM to a location that makes that happen. Angulation is how we do that, just as you've described.

And finally, your third paragraph does a fine job of explaining some of the cons of over pressuring the inside ski.

Nice work.

FASTMAN

### Keep Your CM Moving With The Turn

Quote:
 Originally Posted by Rick Here's a little quiz to stimulate some thought: How does pressure distribution during a turn relate to lateral CM movement, and what are the implications?FASTMAN
If you move your CM lateral your outside ski is in the air and you need to move your CM forward to redistribute your weight over both skis. Prsessure distribution has little to do with the lean inside until you bring both skis back underneath you.

A question with a pre-determined answer I would believe. Don't lean to the inside of your turn but angulate so the CM is moving through your turn with both skis underneath you.

Sorry, I am not much for force vectors, they tend to loose everyone in the velocity section!
This CM vs feet talk reminds me of something I heard in a tennis lesson. "You can't fire a cannon from a canoe".
The other thing I think of is something the local Schwietzers do for the tourists. They roll a coin around the lip of a bowl. The coin being the CM and the bowl being the snow pushing back against the bent ski.
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