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What's the Purpose of Separation in a Ski Turn?

geepers

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That is benefit #1 of separation in any plane, imho: range of motion for dynamic balance!

Razie, given this statement above (which aligns with my experience), I don't quite understand what's written in your next post. Which seems to imply there's little ability to control lateral balance.

As discussed: when bending forward at the waist, to stay in balance, the hips naturally move back. So when you turn, the hips simply move in too.

Remember, angulation does not move the average COM to the outside of the turn, it merely allows more inclination at the legs, i.e. more angles.

Seems to me there's a few factors in play.

Firstly there's the amount of mass above and below the 'original' BoS/CoM lateral balance line. Mass below the line will tend to bias lateral balance in favor of gravity (so inside of turn) and mass above that line will tend to bias balance in favor of 'centifugal' forces (to the outside of turn).

Secondly, the distance between the lateral pivot point (typically inside edge of outside ski) and the mass affects the moment of the forces that must be summed to determine whether we are in toppling inside, balanced or toppling out of the turn.

The top part of the upper body has quite a bit of heavy stuff - arms, shoulders, chest, head - and angulating moves that amount of mass above the 'original' balance line. Whilst the hips may indeed move below the balance line, the top part is further from the pivot point and hence able to exert a greater moment.

My understanding and experience is that we control the relative movement of mass wrt the balance line through the amount of separation and the amount of flex at the hips in order to control our lateral movement into and out of the turn. This relates more to longer turns (such as Batz above - blue outfit skiing in post #10).

Am I missing something here?
 

François Pugh

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Remember, angulation does not move the average COM to the outside of the turn, it merely allows more inclination at the legs, i.e. more angles. If the average COM would move outside when you angulate, there would be no point in doing it, it would be a negative result, as you would simply wash out of the turn.
As noted by @geepers ,the upper mass has more leverage, but the main issue is one of point of reference or grounding. It's like arguing the sun goes around Earth versus Earth goes around the sun. As I see it, if you start your analysis with a given turn and tipping angle with no angulation, keep the skis angled as they are and just angulate from there you will end up moving your mass to the outside and fall down fall over if you don't do something (move to the inside of the turn) about it; if you start from a well balanced not so tight turn and just angulate from there you will end up tipping your skis more - and if you don't do something about it (move to the inside) you will fall over too. Either way you end up with angulation and a centre of mass balanced over the base of support with a given tipping angle in a given turn, only with more angulation you have a safety margin on what you can do to compensate for whatever might come next.
 

Sanity

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It doesn't though, does it? As discussed: when bending forward at the waist, to stay in balance, the hips naturally move back. So when you turn, the hips simply move in too.

Remember, angulation does not move the average COM to the outside of the turn, it merely allows more inclination at the legs, i.e. more angles. If the average COM would move outside when you angulate, there would be no point in doing it, it would be a negative result, as you would simply wash out of the turn.

Similarly with your example, if counter would move more weight outside of the let's call it "original" COM - BOS axis, you wash out from the turn... that's not a benefit! Luckily, that is not what happens :hail: unless allowed on purpose at a certain point in the turn...

There are big benefits from bending forward at the waist some, but being more forward is not one of them. "Hunch forward more to be more forward" is not a thing, is it? How many here teach that?
For a given edge angle, angulation moves the center of mass outside relative to no angulation. That's why we angulate. It allows more tipping angle without having to incline the center of mass as much, so that we can increase the edge angle for lower speeds, as others have noted.

With counter, many people say to limit the rotation of the pelvis. If that's the case then the hips won't move as much as you're worried about.

I'm not suggesting to bend at the waste any more than the appropriate amount.
 

Seldomski

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In short turns, separation isolates the upper body from ski rotary forces. Meaning, your upper body does not yaw left/right in turns. Essentially separation reduces the rotational inertia that the skis must accelerate/decelerate in each turn. So the ski rotary is acting on just the legs and skis themselves, not the hunk of meat above the pelvis. You can take it a bit further by not just separating, but actively moving the upper body into to reduce the apparent inertia of the lower body.

This allows for faster short turns, or simply lowers forces/torque at ski/snow interface for similar cadence short turns with/without separation. Without separation, more force/torque is needed to rotate the entire skier. In short, the *efficiency* is greatly improved by making these turns with separation, allowing the skier to make even faster short turns, or simply use less energy skiing at the same tempo. The better your separation, the more grip is available to you since less grip is wasted in needless motions of the meat above the pelvis.

For longer turns, the benefits of separation are not as great simply because the rates of change in yaw rates of the upper body are much lower and the torque to move the entire body is less since angular accelerations are lower.

Greatly simplified equation-- (1 degree of freedom spinning top)
T = J * alpha = (rotational inertia) * (time rate of change of angular velocity)
 
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razie

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As noted by @geepers ,the upper mass has more leverage, but the main issue is one of point of reference or grounding. It's like arguing the sun goes around Earth versus Earth goes around the sun. As I see it, if you start your analysis with a given turn and tipping angle with no angulation, keep the skis angled as they are and just angulate from there you will end up moving your mass to the outside and fall down fall over if you don't do something (move to the inside of the turn) about it; if you start from a well balanced not so tight turn and just angulate from there you will end up tipping your skis more - and if you don't do something about it (move to the inside) you will fall over too. Either way you end up with angulation and a centre of mass balanced over the base of support with a given tipping angle in a given turn, only with more angulation you have a safety margin on what you can do to compensate for whatever might come next.
Yes, that's what we're both saying... it doesn't change the average COM position, but provides more range of motion to maintain balance, i.e. compensate in both direction etc.

@geepers I see no contradiction, it's saying the exact same thing. Think of it this way: if you stand tall and incline with no lateral separation, you have no range of motion to absorb or compensate: if your skis slide away on a patch of ice, you have no ROM left to extend and grip so you slide out, while if you hit a bump, the shock will be sent straight to the spine, sending you again out of balance. So one of the main things that you gain by separation in the lateral plane (of the skier), i.e. counterbalancing or lateral angulation, is the range of motion to absorb and compensate, to maintain balance. Same of course and even better with counter and bending forward at the hips, as the ski goes through the turn... more range of motion to maintain dynamic balance, much better ability to react!

@Seldomski is also onto something good: I don't think angular momentum was mentioned before here and that's why you can't ski short turns or bumps without counter and separation and the blocking pole plant :geek: that's benefit #3 in my list. There are more...
 
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Mike King

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Well, you've answered a big part of your own question. You can only get so much angulation by bending sideways, pinching your hips to your ribs at the side. Rotate your upper body and you can get some more by, as it were bending forwards to the side.
@razie . Yes the typical move of bending at the waist to get more forward typically results in a compensating butt backwards move, but it doesn't have to.
On a related note ski straight and move your head to the side, notice you naturally compensate to stay in balance by moving your hips to the other side, at least most folks do.
Well, it would appear you are suggesting the purpose of rotational separation is to increase angulation. I can buy that argument, but perhaps there are other reasons?

I do think there is a difference between turning your upper body versus allowing the legs to rotate under the pelvis. Often the result of rotation the upper body over the legs is to push the inside hip back and inside, which easily can become a hip dumping position. Also, the forward tilt of the body can increase angulation, but only if the hip has already abducted to create angulation.

It isn't clear that folk got the point about leg rotation and hip angulation. Try these three positions to see what they do for your ability to move the hip laterally. Rotate the legs externally to the turn (a countered position). Now see how far laterally into the turn you can move the hip. Now try placing the legs neutral to the turn (e.g. the feet and pelvis are square to the fall line). How far laterally can you move the hip inside the turn? Less, right? Finally internally rotate the legs relative to the turn. Now how far laterally can you move? Far less, right? The leg rotation is blocking your ability to achieve hip angulation.

Still, I'm wondering if the only reason for rotational separation is to achieve greater angulation. Any other reasons?
 
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Mike King

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For a given edge angle, angulation moves the center of mass outside relative to no angulation. That's why we angulate. It allows more tipping angle without having to incline the center of mass as much, so that we can increase the edge angle for lower speeds, as others have noted.

With counter, many people say to limit the rotation of the pelvis. If that's the case then the hips won't move as much as you're worried about.

I'm not suggesting to bend at the waste any more than the appropriate amount.
Doesn't creating angles in the body on the plane of the force vector create a rotational force around the point of angulation? Doesn't this rotational force provide increased grip?
 

Sanity

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Doesn't creating angles in the body on the plane of the force vector create a rotational force around the point of angulation? Doesn't this rotational force provide increased grip?

The body angles allow higher edge angles which gives better grip. At a moment in time, the sum total of the centrifugal force balances with the sum total of gravity across your body to be balanced. That net force is met by the reaction force of the ground. There wouldn't be any other net torque (rotational forces) in that plane (which is perpendicular to the skis), otherwise the skier wouldn't be balanced. There can be torque across parts of the body that you would feel as strain in your muscles, but unless that causes you to pressure different parts of your boot, it wouldn't affect the net forces along the edges. Though, in a plane parallel to the skis, you could have torque along the length of the ski that changes the fore/aft pressure and the angle of that pressure which would affect edge grip. Torque in the perpendicular plane would not apply torque along the length of the skis, because it's orthogonal to the skis.

One way to think of it is with equivalencies. Let's say for a snap shot in time you replace the skier with a stick. You could add one bend or a thousand bends in the stick, but as long as the ski is balanced on it's edge, and all the stick variations have the same equivalent point source of mass at the same point in space, the exact distribution of the mass doesn't make a difference.
 

LiquidFeet

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....
Still, I'm wondering if the only reason for rotational separation is to achieve greater angulation. Any other reasons?
The most important function of separation is to aid in balance.

Allowing the mass of the upper body to rotate generates angular momentum. Once rotating, our torso's mass will continue to rotate until we do something to stop it. If we allow that rotation during rapid tempo short radius turns, then we will have to stop the upper body's rotation at the end of each turn in order to make the next turn in the other direction. Stopping that rotation rapidly over and over is difficult to do. The torso will want to continue turning, and we will tend to lose our balance. To see this in action, watch any intermediate skier who does not yet have separation in their toolbox as they try to ski a direct line in the bumps. Separation fixes this problem. It's is especially important in short radius turns because it eliminates the upper body's persistence in turning after each turn is done. It makes controlling balance easier.

Same thing for medium radius turns. But with medium turns, the resulting imbalance from persistent upper body turning, once a turn is over, happens less frequently and with less insistence. The need for separation to avoid this balancing problem is less in medium turns compared to short turns, but it's still there.

In long radius turns, separation is even less important. The speed of the rotation is less, and it happens less frequently. Skiing with minimal separation in long turns does not compromise balance. The minimal separation that is used does, however, aid angulation.

Separation aids angulation in any size turn. But its most important purpose is to help with maintaining balance when the upper body's angular momentum threatens to screw it up.
 
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Dakine

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The most important function of separation is to aid in balance.

Allowing the mass of the upper body to rotate generates angular momentum.ew it up.

I think there are two ways to look at this.
Separation can also be viewed as a way to keep the upper body from rotating while the lower body rotates through the turn.
A quiet upper body is one thing I look for in good skiing.
Really good skiers continue to drive the upper body down the hill while all kinds of things go on with the lower body.

Angulation coming from the femurs rotating in the hip sockets adds a degree of freedom to the system allowing for more control inputs.
 

LiquidFeet

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I've been thinking more about this thread topic from the point of view of teaching separation. When I have a client who skis without it, who stays pretty much "square" to their skis, that client is usually an intermediate making skidded turns, not a skier making carved turns where the tails follow the tips. That client is taking a lesson because of lack of control over turns in certain conditions.

That intermediate skier is more likely than not tipping and/or rotating their body as a unit to tip and/or rotate the skis. This whole body approach works when the conditions are just right, but it comes with problems when they aren't. Teaching such a skier to ski "with separation" means teaching that skier to use their legs to tip and/or rotate the skis, without involving the upper body. I may add tipping their feet at the ankles for "extra credit."

To frame this as an answer to the thread title question ... the fundamental purpose of skiing with separation is to control the skis, and thereby the turns, with the legs instead of the upper body.

The consequences of using the upper body to tip/rotate the skis are twofold: impaired balance from too much angular momentum in short turns, and too much weight on the inside ski in all turns from leaning in. Those two dysfunctions block precision control over everything the skier needs to do.

To phrase it in a positive way, skiing with separation offers the potential of precision control over two things. There can now be strong control over edge angle, because angulation is now possible. And the skier will maintain better balance by managing where the body's mass is relative to the skis, now that the upper body is no longer leaning in and rotating.
 
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Sanity

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I've been thinking more about this thread topic from the point of view of teaching separation. When I have a client who skis without it, who stays pretty much "square" to their skis, that client is usually an intermediate making skidded turns, not a skier making carved turns where the tails follow the tips. That client is taking a lesson because of lack of control over turns in certain conditions.

That intermediate skier is more likely than not tipping and/or rotating their body as a unit to tip and/or rotate the skis. This whole body approach works when the conditions are just right, but it comes with problems when they aren't. Teaching such a skier to ski "with separation" means teaching that skier to use their legs to tip and/or rotate the skis, without involving the upper body. I may add tipping their feet at the ankles for "extra credit."

To frame this as an answer to the thread title question ... the fundamental purpose of skiing with separation is to control the skis, and thereby the turns, with the legs instead of the upper body.

The consequences of using the upper body to tip/rotate the skis are twofold: impaired balance from too much angular momentum in short turns, and too much weight on the inside ski in all turns from leaning in. Those two dysfunctions block precision control over everything the skier needs to do.

To phrase it in a positive way, skiing with separation offers the potential of precision control over two things. There can now be strong control over edge angle, because angulation is now possible. And the skier will maintain better balance by managing where the body's mass is relative to the skis, now that the upper body is no longer leaning in and rotating.

When skiing a direct line in the bumps the purpose of separation is very clear. 1. For rapid turns it takes too long to move the mass of the upper body back and forth. 2. There needs to be a reaction force to rotate the skis when steering which takes place at the top of the turn of nearly every turn in a direct line in the bumps. It takes the mass of the upper body moving the opposite direction to provide an equal and opposite reactionary force to rotate the skis quickly. 3. It keeps the shoulders in a position to start the turn with forward pressure.

With longer radius turns, there's more time to rotate the upper body back and forth and build pressure slowly as the body shifts into a more forward position. There also can be less steering, so it's not essential like it is for turning quickly in a direct line.

I've seen people with poor separation ski a direct line in the bumps, and what I see happening are two things. First, their speed is capped. As they start trying to go faster, everything falls apart, because they can't move their body back and forth that fast, and I'd agree that bad balance is the first symptom. Next, in certain lines, they are always late with forward pressure and lose their speed control and have to bail out of the line.
 

Racetiger

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Geez, these technical threads are getting slow. I think I know why. As I understand it, sometimes the technical acumen can be somewhat questionable on these online forums. I only mention this because, if we are referring to skiing specifically, I believe the correct term is “delamination”. Anatomical separation is going to lead to rapid blood loss and sudden fatality so I do not recommend it, especially during a family vacation. All the average middle class parents with 2.5 children may know what I am talking about. A very messy experience to say the least. Sometimes it can take up to 10 seasons for a skier to delaminate their CoM from their BoS. The key for achieving a quicker delamination is to get a little water seeped into a manufacturing defect void in your construction, which, upon freezing, will further expand the delamination of this bond. When too much delamination occurs, spread an epoxy adhesive between the CoM and BoS and clamp them together in a vice overnight. I know exactly what you are thinking right this second and, yes, it is “OK” to sniff a little during the process. I’ve seen a skier’s entire technique become fully delaminated after a bad impromptu lesson from their suddenly less significant other, in which case a little needle and thread on the chairlift will do wonders. There is also the term, ski “decapitation”, where the binding comes off altogether. Now, “skier” decapitation, a form of anatomical separation mentioned above, involves low lying branches camouflaged with snow and stretching across the line of a skier who is not good at vertical separation resulting in cranial separation which, again, is correctly referred to as decapitation in this and other circles. When people tell you that you need to get your sh*t together, this is probably what they are talking about and, unless you are a skier, there’ll be nothing you can do about it.
 

Sanity

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Geez, these technical threads are getting slow. I think I know why. As I understand it, sometimes the technical acumen can be somewhat questionable on these online forums. I only mention this because, if we are referring to skiing specifically, I believe the correct term is “delamination”. Anatomical separation is going to lead to rapid blood loss and sudden fatality so I do not recommend it, especially during a family vacation. All the average middle class parents with 2.5 children may know what I am talking about. A very messy experience to say the least. Sometimes it can take up to 10 seasons for a skier to delaminate their CoM from their BoS. The key for achieving a quicker delamination is to get a little water seeped into a manufacturing defect void in your construction, which, upon freezing, will further expand the delamination of this bond. When too much delamination occurs, spread an epoxy adhesive between the CoM and BoS and clamp them together in a vice overnight. I know exactly what you are thinking right this second and, yes, it is “OK” to sniff a little during the process. I’ve seen a skier’s entire technique become fully delaminated after a bad impromptu lesson from their suddenly less significant other, in which case a little needle and thread on the chairlift will do wonders. There is also the term, ski “decapitation”, where the binding comes off altogether. Now, “skier” decapitation, a form of anatomical separation mentioned above, involves low lying branches camouflaged with snow and stretching across the line of a skier who is not good at vertical separation resulting in cranial separation which, again, is correctly referred to as decapitation in this and other circles. When people tell you that you need to get your sh*t together, this is probably what they are talking about and, unless you are a skier, there’ll be nothing you can do about it.
That's Rich.
 

Steve

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razie

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@Sanity that's still backwards to me: any way you look at it, the COM is the same or lower with angulation vs non-angulation, not higher as you say. If the edge angle is higher, which you agree to, then the ski bend is more and the radius is less, thus the forces are higher - thus a lower COM overall compared to the same turn, no angulation. It is also why it is more tiring, because you get more performance... unless your timing is perfect, you release early and softly and counter as needed to stack at the right time ;) pretty high-level tricks to get right turn after turn without a ton of technique and practice :ogcool:

skiing is not a zero-sum game: refined technique stands between getting more performance and getting more tired... although regular visits at the gym are part of the key to success :rolleyes:

@Mike King also has a good point on torque. you have to leave behind the half a mile radius "look ma, I am balanced" type turns and think in the real terms of a non-semicircle performance turn, comma or brachistogram turn shape (i am sure i misspelled that, as i am too lazy to look it up). Nothing is static in skiing and an entire chapter on toppling is usually associated to the end of the turn, you'll figure it out.

p.s. the Rich-ness detected in the last few posts threw me off - I figured you hang out together...? I guess it goes to show how wrong some of us can be in these technical threads...
 

Sanity

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@Sanity that's still backwards to me: any way you look at it, the COM is the same or lower with angulation vs non-angulation, not higher as you say. If the edge angle is higher, which you agree to, then the ski bend is more and the radius is less, thus the forces are higher - thus a lower COM overall compared to the same turn, no angulation. It is also why it is more tiring, because you get more performance... unless your timing is perfect, you release early and softly and counter as needed to stack at the right time ;) pretty high-level tricks to get right turn after turn without a ton of technique and practice :ogcool:

skiing is not a zero-sum game: refined technique stands between getting more performance and getting more tired... although regular visits at the gym are part of the key to success :rolleyes:

@Mike King also has a good point on torque. you have to leave behind the half a mile radius "look ma, I am balanced" type turns and think in the real terms of a non-semicircle performance turn, comma or brachistogram turn shape (i am sure i misspelled that, as i am too lazy to look it up). Nothing is static in skiing and an entire chapter on toppling is usually associated to the end of the turn, you'll figure it out.

p.s. the Rich-ness detected in the last few posts threw me off - I figured you hang out together...? I guess it goes to show how wrong some of us can be in these technical threads...

You're not wrong. It's just compared to what you're holding constant vs. what you allow to vary in the analysis. For a given edge angle, angulation allows the mass to be higher. That means that you compare different scenarios for a moment in the turn where a skier has for example, a 45 degree edge angle. In one case the skier uses no angulation. In the other case, the skier uses lots of angulation. For that split second the skier with angulation has the center of mass further outside the circle than the skier without angulation where both skiers have the same edge angle. It's just a way of looking at things that helps with the physics.

Dynamic vs. static does complicate things, but the edges can give out in a split second. So, I think it's fair to say a balanced skier will have approximately balanced forces in regards to the forces acting on the edges affecting edge grip.
 

razie

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You're not wrong. It's just compared to what you're holding constant vs. what you allow to vary in the analysis. For a given edge angle, angulation allows the mass to be higher. That means that you compare different scenarios for a moment in the turn where a skier has for example, a 45 degree edge angle. In one case the skier uses no angulation. In the other case, the skier uses lots of angulation. For that split second the skier with angulation has the center of mass further outside the circle than the skier without angulation where both skiers have the same edge angle. It's just a way of looking at things that helps with the physics.

Dynamic vs. static does complicate things, but the edges can give out in a split second. So, I think it's fair to say a balanced skier will have approximately balanced forces in regards to the forces acting on the edges affecting edge grip.

You are holding constant something that cannot be... it cannot be the same turn, since the radius and timing changes... so I don't think we can't really compare those apples to those oranges ;) that's the kind of trouble we can get into when we talk more physics than skiing :ogcool:

... what I think you are referring to is, let's take the same radius inclined and angulated, the force vector is pointing more down when angulated, it is an interesting equation, between ski bend at an angle vs force vector and snow hold.

but, since you see closer on that point now, the next interesting part is the torque. even without bringing in the dynamics, the presence of force with an arm and a direction introduces torque. Whether there is rotation or not, is irrelevant:


1625521684439.png




and this raises a lot of interesting points about why angulation is better, even from a static physics standpoint... there you have the COM to boot vector, the ski edge angle and the width of the ski ! Or course, we're missing the Normal :geek:

food for thought. I don't think this point was touched much on before, @Mike King brought it up first... of course, this assumes the idealized case of carving edge locked, but we could easily add a fudge factor for some skidding... or worse, Thor forbid, pivoting :P
 

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