Directing Pressure Along the Length of the Skis

[LiquidFeet]

But Josh, you are unusual.

 

[James]

OK people, of course the whole ski supports a skier. And a skier needs to feel the whole ski through the whole skiing process, and know what's happening with pressure and grip beneath its whole length at all times.

A skier can at the same time conceptualize the base of support (BoS) as a point under the foot. These are not mutually exclusive.

The whole ski may not support the skier.

So, we're saying that^ is the same as if the ski were on the ground? How do you equate that?

If the BoS is a point, then there's no difference skiing a 180cm long ski vs a 90cm long ski. Just lean back skiing on both and the same thing will happen, because the BoS is a point under the foot. -Makes no practical sense let alone the physics of it.

The CoM is conceptual but an actual, able to be derived point. (So, maybe it's not conceptual and there's a better term). I don't see why BoS shouldn't receive the same treatment. All of a sudden it's some etheral, not really there, doesn't function in a force diagram, but just believe it concept? Surely this thought process is headed in the wrong direction.

Are we really saying that with this guy buttering here, his BoS is a point under his foot?

How does that make sense? Even conceptually? How can a term, BoS, only work for the special case of the ski completely on the ground? It may be the most common, but it's still a specific instance. How is BoS different with ski on edge vs flat? In the point theory, it's not.

If BoS is a point, then how is it that a 115mm wide powder ski supplies more support from sinking in powder than a 70mm carving ski?

Nothing holds up with this concept of BoS as a point. What am I missing?

 

[LiquidFeet]

The whole ski may not support the skier.....
So, we're saying that^ is the same as if the ski were on the ground? How do you equate that?....
If BoS is a point, then how is it that a 115mm wide powder ski supplies more support from sinking in powder than a 70mm carving ski?....
Nothing holds up with this concept of BoS as a point. What am I missing?

"CoM to BoS" is a conceptual MODEL expressed verbally. The structure of the phrase is intentionally simplified. This does not erase the details of the real world experience. Of course the skier sometimes gets launched. Of course the skier pays attention to the pressure under the whole ski and where it's highest.

I think this disagreement has to do with not seeing the forest for the trees. It's totally a semantic disagreement. Call the whole ski the BoS if you like. Where is the CoM? Is it the whole torso, head and arms included, pelvis up?

Where are the engineers when we need them?

 

[karlo]

Now that we know what the BoS is , what we are supposed to do is control the “relationship” of the CoM to it, in order to “direct pressure along the length of the ski.”

Exploring the boundaries is a good exercise. So, in this example,

View attachment 86270

It seems to me the relationship of COM to BOS is off with respect to the ability to direct pressure along the length of the ski, making it exceedingly difficult to accomplish, not a pirouette, a turn.

 

[James]

CoM to BoS is a conceptual MODEL. It's intentionally simplified. This does not erase the details of the real world experience. Where are the engineers when we need them?

Well the point model as presented makes no practical sense. It is at odds with the real world. Models have to match the world, not contradict it.
Just go through and try to use your model to match the conditions I pointed out.

Just the fact that ski on edge vs flat is so different should cause you to question the model. Or the flotation issue, or the length.
I’d be shocked if any engineers supported this model even as a concept.
Seems anti-engineering, but they can answer.

You could try to convince me...
Why would “Base of Support” be a point and not an area? Why not call it “Point of Support”?

 

[James]

It seems to me the relationship of COM to BOS is off with respect to the ability to direct pressure along the length of the ski, making it exceedingly difficult to accomplish, not a pirouette, a turn.

How is anything “off”?
He’s directing pressure to the very front of the ski at this moment. The goal is to pivot around the very front of the ski.

Nothing says pressure has to be along the length of the ski at all times.

 

[LiquidFeet]

Well the point model as presented makes no practical sense. It is at odds with the real world. Models have to match the world, not contradict it.
Just go through and try to use your model to match the conditions I pointed out.

Just the fact that ski on edge vs flat is so different should cause you to question the model. Or the flotation issue, or the length.
I’d be shocked if any engineers supported this model even as a concept.
Seems anti-engineering, but they can answer.

You could try to convince me...
Why would “Base of Support” be a point and not an area? Why not call it “Point of Support”?

So James, are you saying that a skier should not pay attention to and control where the feet are relative to the torso? OR conversely, where the torso is relative to the feet? This spatial relationship, both fore-aft and lateral, is a major factor in controlling how the pressure is distributed under the whole ski.

I don't think you mean that. I think you are disagreeing on the basis of semantics, how something is being verbalized. Is this disagreement moving this discussion forward in an informative direction?

 

[Corgski]

Well the point model as presented makes no practical sense. It is at odds with the real world. Models have to match the world, not contradict it.
Just go through and try to use your model to match the conditions I pointed out.

Point is fine in this context if the model allows for a torque at that point.

 

[karlo]

Point is fine in this context if the model allows for a torque at that point.

You mean like a pivot slip?

 

[cantunamunch]

Where are the engineers when we need them?

Careful what you ask for.

skimoves.me

https://skimoves.me/2017/11/22/edge-change-inertia-rocker-rotation-inertia/

You mean like a pivot slip?

That's one - yaw plane/vertical axis. You need torque around all three axes and force along them to explain the tip roll.

 

[karlo]

The goal is to pivot around the very front of the ski.

So, that’s a good point. No pun intended. In this extreme example, pressure is directed to a point to enable a pivot. So, the progression is directing pressure, not on a point, but along some length of the ski, making possible something other than a pivot. And, in the context of PSIA’s fundamental, the objective being instructing learners how to make turns, that would be directing pressure along the length of the ski I suppose. Using rotary to turn, we want the rotation to be underfoot, with tip to tail pressured (along length of ski) such that the entire ski smears the snow. When edging, we want the entire edge pressured (along length of ski) and engaged, to make use of the full curvature of the ski; or, at the least, apply pressure progressively from tip to tail (along length of ski) to accomplish the same.

 

[James]

So James, are you saying that a skier should not pay attention to and control where the feet are relative to the torso? OR conversely, where the torso is relative to the feet? This spatial relationship is an important factor in controlling how the pressure is distributed under the whole ski.

I don't think you mean that.

Don’t know what these questions have to do with it. They’re red herrings. There’s also a logical fallacy in the construct you’ve made, but it’s beyond me. At the least it’s a false equivalence.

I’m trying to get you to acknowledge “Base of Support” is not a point. If you have references to that term being referred to as a point then bring it out. Sure, in general we ignore the skis when talking about it. That quickly becomes problematic if you isolate the term BoS or start taking about being very aft or fore.

I think the discussion highlights the problematic nature of the wording of the fundamental. Or not. In the Petra tail skiing example, she’s going to dramatically alter the base of support by pivoting the skis, getting them fully on the snow, and moving her com more towards the center of BoS or forward of it sagitally and inside frontally.

I’m saying one should know where the tail of the ski is. And the tip. Skis can’t be reduced to a point under the foot if you’re referencing a base of support.

We used to just refer to balance. Not BoM to BoS. The new thing appears to become messy once you start getting specific.

 

[AchtungSki]

I'm a mechanical engineer but only a humble novice skier so I won't/can't make any comment as to the validity of the PSIA terminology/skiing technique discussion.

Strictly on the physics though, technically speaking both a "point" concept and what is called a "distributed load" concept are both valid ways of analyzing forces. Intuitively, you can feel that a ski on the snow has ground reaction/frictional forces acting along its length, the distribution is given by some equation as a function of length when viewing the ski from the side and analyzing the loading along the length of the ski, visually represented by the arrows below the skis in this picture:

Such distributed loads however can analytically be resolved into a single discrete point load at some geometrical point along an object, the ski in this case. Here is a link to the math if you're really interested. This is a "Center of Force" of sorts which can change and move around in space as the loading function/profile along the ski changes, semi-similar to how your CoM location can change depending on the position of your body (That is your actual CoM, I'm assuming that the ski terminology is referring to the same thing as physics class). The location and magnitude of this force can then be used in a free body diagram to do analysis, especially in this case to find moment arms/force couples.

So just as the CoM does not describe the location of all the mass in your body being actually located in a single point in space, neither does a resolved distributed load describe all of its integral force being actually located at a single point. They are however still useful concepts in understanding where/how forces are acting.

Now obviously this is a simplification of reality, and the math of a skier moving dynamically in 3-D space down a hill with variable surface interactions and skiing techniques gets very complicated very quickly. Reality is reality and it's always good to take models with a grain of salt. Buttering is certainly a bit of a boundary case with numerous other complicating factors given that the skis are no longer moving in (comparatively) 2-D arcs across the hill. In the instant of that photo though there is still likely a distributed load on the ski tips, albeit over a small area, which can be resolved into a point load that factors into the overall summation of forces which describes the skier's dynamic balance on their tips.

This force is what allows you to stand on your skis as you turn without them sliding endlessly down/across the hill. Whether this resolved point load or a distributed load along the skis is more synonymous with the PSIA BoS concept you're all speaking of I can't say of course, but hopefully that helps.

 

[mdf]

It depends on whether you want the base of support of the skier-plus-skis system, or of the skier alone.
In your buttering photo, the BOS of the full system is where the tips of the ski are pressing against the snow.
The BOS of the skier is probably on his shins or the top of his instep -- about where the blue lines are drawn.

 

[Skisailor]

I'm a mechanical engineer but only a humble novice skier so I won't/can't make any comment as to the validity of the PSIA terminology/skiing technique discussion.

Strictly on the physics though, technically speaking both a "point" concept and what is called a "distributed load" concept are both valid ways of analyzing forces. Intuitively, you can feel that a ski on the snow has ground reaction/frictional forces acting along its length, the distribution is given by some equation as a function of length when viewing the ski from the side and analyzing the loading along the length of the ski, visually represented by the arrows below the skis in this picture:

Such distributed loads however can analytically be resolved into a single discrete point load at some geometrical point along an object, the ski in this case. Here is a link to the math if you're really interested. This is a "Center of Force" of sorts which can change and move around in space as the loading function/profile along the ski changes, semi-similar to how your CoM location can change depending on the position of your body (That is your actual CoM, I'm assuming that the ski terminology is referring to the same thing as physics class). The location and magnitude of this force can then be used in a free body diagram to do analysis, especially in this case to find moment arms/force couples.

So just as the CoM does not describe the location of all the mass in your body being actually located in a single point in space, neither does a resolved distributed load describe all of its integral force being actually located at a single point. They are however still useful concepts in understanding where/how forces are acting.

Now obviously this is a simplification of reality, and the math of a skier moving dynamically in 3-D space down a hill with variable surface interactions and skiing techniques gets very complicated very quickly. Reality is reality and it's always good to take models with a grain of salt. Buttering is certainly a bit of a boundary case with numerous other complicating factors given that the skis are no longer moving in (comparatively) 2-D arcs across the hill. In the instant of that photo though there is still likely a distributed load on the ski tips, albeit over a small area, which can be resolved into a point load that factors into the overall summation of forces which describes the skier's dynamic balance on their tips.

This force is what allows you to stand on your skis as you turn without them sliding endlessly down/across the hill. Whether this resolved point load or a distributed load along the skis is more synonymous with the PSIA BoS concept you're all speaking of I can't say of course, but hopefully that helps.

Thank you @AchtungSki !!! This is a wonderful description of something which encapsulates my own thinking about the issue of the "COM" and the "BOS", but which I would never have had the ability to explain as you have! But in my approach to issues of balance on skis, I have always thought it important to understand that these PSIA constructs can (and perhaps should?) be resolved to theoretical points which can change and move as a result of a myriad of factors. It has been important for me personally to have a more precise way of thinking of these concepts rather than the kind of training talk we often hear about the COM being "somewhere in the belly", or that at any given moment we are "balanced over the whole foot". Anyway - thanks!

 

[LiquidFeet]

....I’m saying one should know where the tail of the ski is. And the tip. Skis can’t be reduced to a point under the foot if you’re referencing a base of support.....

James, I think you think that I mean something I don't mean. Of course one should know where the tail of the ski is.

If a skier pays attention to where the feet are relative to the torso, and moves the feet forward of the torso to load the tail, or moves them back behind the torso to load the shovel, or keeps them under the body to load both equally, or lifts them in the air to relieve the ski of pressure, then that skier is paying attention to the spatial relationship of the CoM (vague, yes, so let's call it torso if you like) to the BoS (in this description, the feet).

The skier is messing with the relationship between those two in order to move pressure fore-aft along the ski, from tip to tail. The skier feels the pressure move as a result of moving the feet relative to the torso. Hopefully the skier feels the pressure move under the whole ski, not just under the sole of the foot from ball to heel and back again.

The skier could move the torso relative to the feet as well, for the same reason. Or do that in addition to moving the feet. We're still talking about fore-aft here. Two other fundamentals address the skier manipulating the spatial relationship between the CoM and the feet laterally.

Adjusting the spatial relationship between the feet (under the back of the arch) and the torso is a cause. It's what the skier does to manage pressure along the length of the ski. The result is pressure moving along the ski, under the shovel to waist to tail or in the other direction or wherever. Cause and effect are not mutually exclusive. They are locked together.

James, I still don't think we are in real disagreement about how turns work. But then I have no problem with the fundamental as stated, while you do.

• Control the relationship of the Center of Mass to the base of support to direct pressure along the length of the skis

 

[HardDaysNight]

To me, it’s odd that this so-called “fundamental” describes controlling "the relationship of the center of mass to the base of support" only as it relates to fore-aft pressure regulation (along the length of the ski). In fact, that relationship is the basis for regulation of fore-aft pressure, lateral pressure and overall magnitude of pressure - in other words, that simple relationship is the basis for all pressure control.

Also stating that we "direct" pressure toward the outside ski seems to imply that we need to make some kind of active move towards the outside ski, whereas in fact we move inside the turn (incline) as much as is required for balance on the outside ski against the forces of the turn (which are related to edge angle and speed).

Finally, and related to this discussion, "direct pressure along the length of the ski" doesn’t tell anyone what to do and is pretty much useless as a teaching aide. It doesn't say anything about where, or when, or what part of the "length of the ski" should get the pressure and it fails to tie the specific technique to any particular purpose or intent. Because of that, it makes things like inside ski turns and two-footed powder turns seem like mistakes.

Other than these observations it’s very useful!

 

[James]

Yes, we’re in agreement about how it works. They just had to throw in “base of support”. “CoM” for that matter too.

Adjusting the spatial relationship between the feet (under the back of the arch) and the torso is a cause. It's what the skier does to manage pressure along the length of the ski. The result is pressure moving along the ski, under the shovel to waist to tail or in the other direction or wherever. Cause and effect are not mutually exclusive. They are locked together.

Yeah that’s the problem of reducing the resultant force to a point vector, if that’s the term. It’s the result.
You can’t control something, CoM, to a calculated result. So BoS needs to be an entity. Though a shifting one front or rear off snow shortens it, etc
I guess.

 

[LiquidFeet]

....Finally, and related to this discussion, "direct pressure along the length of the ski" doesn’t tell anyone what to do and is pretty much useless as a teaching aide. It doesn't say anything about where, or when, or what part of the "length of the ski" should get the pressure and it fails to tie the specific technique to any particular purpose or intent. ....

Totally agree.

The fact that the fundamentals are pretty much useless as teaching aids is my complaint about them. It doesn't mean they are worthless, though. They get a lot of promotion, and as an instructor I'm expected to have them memorized.

Because those five fundamentals are promoted so strongly, trainers use them as training aids. PSIA leaves it up to the trainers, for instance, to decide where, when, and what part of the length of the ski should get the pressure, given some purpose or intent, and to communicate that to instructors so they can bring it to the public. Thus the problem. The details are missing.

The fundamentals are categories of skill-building. They are like chapter headings. The content that fills in the chapters is weakly developed in PSIA's literature. That content lies in the heads of national team memers, examiners and those they have adequately trained.

 

[karlo]

I find all of @HardDaysNight comments extremely useful.

we "direct" pressure toward the outside ski seems to imply that we need to make some kind of active move towards the outside ski, whereas in fact we move inside the turn (incline) as much as is required for balance on the outside ski against the forces of the turn

Thinking about that, I now realize we also control the relationship of COM to the BOS when we direct pressure to the outside ski. The amount of inclination and angulation changes the relationship.

it’s odd that this so-called “fundamental” describes controlling "the relationship of the center of mass to the base of support" only as it relates to fore-aft pressure regulation (along the length of the ski). In fact, that relationship is the basis for regulation of fore-aft pressure, lateral pressure and overall magnitude of pressure - in other words, that simple relationship is the basis for all pressure control.

So, maybe this fundamental is just that, a fundamental, with the key point being, (what?), control relationship of COM to BOS, (to do what?), to direct pressure to whatever part of the ski you wish, to accomplish your intent.

Finally, and related to this discussion, "direct pressure along the length of the ski" doesn’t tell anyone what to do and is pretty much useless as a teaching aide. It doesn't say anything about where, or when, or what part of the "length of the ski" should get the pressure and it fails to tie the specific technique to any particular purpose or intent. Because of that, it makes things like inside ski turns and two-footed powder turns seem like mistakes.

And, perhaps that’s the point it is so vague. Where we direct pressure, and when, depends on what we want to do.

So,the key point is to play with where COM is, relative to BOS. The part about directing it to somewhere along the length of ski is obvious to us. But, that’s why it’s fundamental. Make sense? Starting to make sense to me.