Fore/Aft Balance and Physics

[Mike King]

I thought about the feet and COM. Pressuring the tip has to do with moving COM forward. If we didn’t do that, the tip would not be engaged and pressured, relative to the rest of the ski. And, we can pressure the tip a lot. We can practically “land” on the tip. Which brings me back to “effective edge”. Just as we can pressure the tip at entry, we can jet out of a turn with the tails, pressuring those. That means pressure can continuously move from fore to middle to aft. That means, as the tip bends and is engaged, the center (thus the whole ski) is engaged, then the tails are engaged, the shape of the arc changes. And, one can also do a most dynamic leaper (more centered exit, than aft), releasing at a high tipping angle and entering a turn with high angle, towards tip or centered. All of this and more means that the arc of a carve is not just an arc of a circle. It can be elliptical, anything in between; it can be even non-symmetric, having a tight radius at entry, longer radius at apex, and medium radius at exit. None of this means skidding. All of this can be carved, or drawn like a stroke of calligraphy.

@karlo, this is clearly a belief of yours. I would encourage you to go out and experiment with your belief and document the results. Get video, climb back up the hill and examine the track of your skis, do some MA. I suspect you will find your beliefs challenged.

 

[karlo]

@karlo, this is clearly a belief of yours. I would encourage you to go out and experiment with your belief and document the results. Get video, climb back up the hill and examine the track of your skis, do some MA. I suspect you will find your beliefs challenged.

Which part? That the skis won’t carve without skidding if pressure is applied from Fore to Aft through the turn?

 

[Mike King]

Which part? That the skis won’t carve without skidding if pressure is applied from Fore to Aft through the turn?

Time for exploration.

 

[karlo]

Time for exploration.

I have been. On account of injury to left glute and rotators, earlier in the season, I was doing what I think are slow carved turns. Kept them short by pressuring front then middle then aft of ski. Really working rotators. But, maybe they aren’t carved turns, though they feel like carving. Then again, maybe I don’t know what a carved turn is. I’ll try to video it

 

[JESinstr]

I have been. On account of injury to left glute and rotators, earlier in the season, I was doing what I think are slow carved turns. Kept them short by pressuring front then middle then aft of ski. Really working rotators. But, maybe they aren’t carved turns, though they feel like carving. Then again, maybe I don’t know what a carved turn is. I’ll try to video it

Karlo, Carving is the process of using skis to convert straight line travel into circular travel. So if you are executing that process you are carving.... maybe not as well as others, but carving nonetheless.

It's interruption of the process (as many intermediates do by pushing heels and over rotating) that returns your skis and mass to straight line travel resulting in a skidding condition.

Regarding rotation, it's ok to use a rotary redirect into the carving process as an "on ramp" to meet the parameters of the skis and demands of the slope but the more you build the carving process, the less you will use rotary skills. This is how I view the concept of "You turning the skis vs the skis turn you".

 

[LiquidFeet]

...Carving is the process of using skis to convert straight line travel into circular travel.
...This is how I view the concept of "You turning the skis vs the skis turn you".

JES, you often talk about skiing as something that can best be understood if the skier thinks about how straight line travel (generated by gravity and momentum) and circular travel (generated by the ski-snow interaction) do their thing as we ski. This does not work for me. Not at all.

I have line drawings of different types of ideal turns stored in my head, and those simple models depict direction of actual travel. The lines are dark when the pressure is high and light where it's light in these ideal turns. These models take into account degree of turn completion and radius. If I want to make these mental models a little more inclusive, I add width to the tracks where the skis skid/brush across the snow surface. Manipulating those elements in my mental models - radius and shape, degree of completion, pressure, and skid width - pretty much covers all kinds of turns -- for me.

Adding some reference to the force vectors that change and interact with each other to these somewhat simple models makes them too complicated for my simple brain to make use of .... unless I have pencil and paper in front of me, which I usually don't.

"The skis turning you vs you turning the skis" works OK for me. It takes care of the width of the skidding tracks part of those models. But get this. Sometime recently I started a thread here about "you turning the skis vs the skis turning you" and wow was I surprised by the responses. People were all over the place in their understanding of what those two phrases meant. I had no idea. So now I don't use them without providing an explanation first.

Would you find it interesting to start a thread asking the community to tell you what they think these factors mean for a ski turn (straight line and circular travel), and how useful it has been for them to think about them to better understand their skiing? Maybe a large number of people like thinking about that particular interaction and find it illuminating.

 

[karlo]

I think of brush strokes as it relates to ski snow interaction

Dancing as it relates to movement

So, I probably shouldnt have jumped into a physics thread

 

[razie]

This is getting out of hand... so much assumption and pseudo equations and so little skiing... I'm honestly having a hard time to figure out who's arguing what... I've missed again a lot of posts with all this racing!!

When standing on a ski, both tail and tip are pressured because of the stiffness of the ski, just like sitting on a chair sends 1/4 of the weight to each leg if you're centered. For race skis, more of the weight is directed forward and back, simply because they are stiffer. More can be directed fore or back, depending on mounting point.

When you stand on ice, the same is true, up to the point where the ski camber flattens out. As you tip the ski more, the basics become true again and more is sent to the tip/tail, as the ski bends.

Even when you are "back" there is pressure on the tips, unless you're leveraging the boots and lifting the tips.

More can be sent to the tips than tails, over and above the ski flex pattern, by moving the hips fore or aft on the skis aka moving forward.

That is just simple static analysis, from behind a keyboard. Skiing is much more interesting when we consider how the skis move versus how the hips move (for those of us that displace the COM laterally, so that excludes @karlo .

I think acceleration and deceleration of our COM is relevant. However, as to the skis in the context of a turn, as the COM moves in a constant velocity downhill, and as the ski carves a turn, I’d say it’s always accelerating in the arc.

https://physics.stackexchange.com/questions/122284/why-doesnt-an-orbiting-body-keep-accelerating

My skis are not orbitals, they merely slingshot around the space-time continuum distortions produced by the atoms that got friendly enough to hang out together, forming the earth.

Look it up, maybe you need to ride my model, you don't have to push it around, it's really smart and uses rides Einstein's tensors: https://en.wikipedia.org/wiki/Gravity_assist

Nobody integrates the forces on the ski to arrive at just the correct vector acceleration to make the best time down the mountain

Speak for yourself. Do you know how much math it takes to surf those gravitational waves, looking for slingshot acceleration?

Wait!! By over-thinking simple 3rd degree acceleration equations, you're saying that my mind over-heats enough to slow me down ??? Now I understand why I'm slow!!!

PPS. Something to think about. Almost all skis have a shorter turn radius in the front of the ski than in the tails, so if you pressure the ski on hard-packed snow so that it is firmly on the snow what radius will it carve? Pressuring it more, or more fore or more aft will not change its shape, until it slides forward into the new freshly made groove.

I bet you 5 8/19th credits that pressuring it more on the front thus bending the longer and softer part of the skis more will reduce the radius! I will only bet you 1/19th of a credit if the angles we're talking about are never bigger than 10 degrees. I will bet you 100 credits if these edge angles reach close to 90 !! I will bet you 10,000 credits if you also let me get so forward as to leverage the tails out of the snow!

Wait... what was the bet about?

Here's Robby Kelley over the weekend, bending clearly the front part of the ski more than the tail:

Compared to one of my guys, in the same gate:

The latter looks a lot more forward, but bending a different part of the ski!!!

How come? Maybe you can help me make him faster, with geometry and COM/BOS equations!

p.s. does someone here think R.K. is "central" on the skis? Or is he... I don't know... more forward?

 

[karlo]

But, nevertheless, here I am.

This acceleration deceleration thing. Setting aside the lateral movement of com, has anyone carved with no lateral movement? When I do, I do not perceive acceleration or deceleration of com. Com just goes down at constant velocity along a straight line, so long as pitch of slope remains the same.

That means, from transition to transition, the skis need to travel a longer distance, in an arc. And, I say that the arc can be, but need not be, an arc of a circle. Since the skis must keep up with the traveling COM, think of COM as the Sun, and if arc is that of a circle, then doesnt the skis’, think of skis as the planet, angular velocity need to be constant? Then constant g’s. I think that’s right. Another thing I’ve been playing around with is park and ride carves with as quick an edge change and tipping as possible at transition I think I do, relatively speaking, feel constant g’s through the arc.

Just train of thought. Don’t know where this leads

 

[Josh Matta]

your guys has way more upper body rotation. upper body rotation put you on your tail no matter how hard you try stay forward .

 

[karlo]

How come? Maybe you can help me make him faster, with geometry

I would need a CPU with a much higher clock rate and a huge heat sink and fan.

 

[razie]

your guys has way more upper body rotation. upper body rotation put you on your tail no matter how hard you try stay forward .

Wait, what?? Are you saying it's actually not as simple as "get forward" ??

So, as @Mike King is saying, let's say we ski more central and square early. What then?

I would need a CPU with a much higher clock rate and a huge heat sink and fan.

Nah... just a better understanding of the fundamentals - see @Josh Matta post above, for instance

 

[karlo]

So, as @Mike King is saying, let's say we ski more central and square early. What then?

I think we lose propulsion. I can’t explain the physics. I just feel that would happen

 

[Seldomski]

Trying to follow this thread, but I got confused, perhaps on the first post.

My understanding is that the feet move aft relative to the body at turn start and opposite at turn end to allow the body to move down the fall line more directly. If you keep the body in a static position relative to the skis, the body takes a longer path (that of the skis). If you move feet fore/aft relative to the body, the skis can take a longer route around the skier. If they stay lock step with the skier's body, it's more work for the skier since a larger mass is constantly accelerating in multiple directions instead of just down the fall line.

If you don't move forward (pull feet back) at turn start, the skis (especially on steep terrain) will accelerate away from you and you land on your butt, or they generally are very hard to direct from the aft position. So this is more of a turn survival thing, not so much being efficient.

If you don't allow the skis to come forward (you move aft, feet go forward) at turn completion, your entire body will be redirected away from the fall line, traveling with the skis. This redirection will result in traversing a bit before starting the next turn, since your body is accelerated perpendicular to the fall line at each turn completion. You will not be able to link turns as tightly. You also are doing more work since your entire body is moving side to side instead of down the fall line. I think I tend to ski this way (i.e not letting the skis come forward at turn end). This is not 'correct' and it becomes a problem in ungroomed snow for me.

For longer radius turns, the degree of fore/aft is smaller (though if you are moving fast enough, like GS, you need to move more once again). This is why you can do pure carved RR tracks on a flattish slope while staying mostly forward. More pronounced fore aft at lower speeds can be found in short radius carved turns, or anywhere it gets much steeper.

Now skis may be designed to exploit this most efficient way to ski. But I don't really think of pressuring the ski where it wants to be pressured. I think more about getting my body/feet where they should be and then the ski supports me.

My 2c, please help me understand.

EDIT to add -- TLDR version see also Bob's infinity move.

 

[Mike King]

This is getting out of hand... so much assumption and pseudo equations and so little skiing... I'm honestly having a hard time to figure out who's arguing what... I've missed again a lot of posts with all this racing!!

When standing on a ski, both tail and tip are pressured because of the stiffness of the ski, just like sitting on a chair sends 1/4 of the weight to each leg if you're centered. For race skis, more of the weight is directed forward and back, simply because they are stiffer. More can be directed fore or back, depending on mounting point.

When you stand on ice, the same is true, up to the point where the ski camber flattens out. As you tip the ski more, the basics become true again and more is sent to the tip/tail, as the ski bends.

Even when you are "back" there is pressure on the tips, unless you're leveraging the boots and lifting the tips.

More can be sent to the tips than tails, over and above the ski flex pattern, by moving the hips fore or aft on the skis aka moving forward.

That is just simple static analysis, from behind a keyboard. Skiing is much more interesting when we consider how the skis move versus how the hips move (for those of us that displace the COM laterally, so that excludes @karlo .




My skis are not orbitals, they merely slingshot around the space-time continuum distortions produced by the atoms that got friendly enough to hang out together, forming the earth.

Look it up, maybe you need to ride my model, you don't have to push it around, it's really smart and uses rides Einstein's tensors: https://en.wikipedia.org/wiki/Gravity_assist



Speak for yourself. Do you know how much math it takes to surf those gravitational waves, looking for slingshot acceleration?

Wait!! By over-thinking simple 3rd degree acceleration equations, you're saying that my mind over-heats enough to slow me down ??? Now I understand why I'm slow!!!



I bet you 5 8/19th credits that pressuring it more on the front thus bending the longer and softer part of the skis more will reduce the radius! I will only bet you 1/19th of a credit if the angles we're talking about are never bigger than 10 degrees. I will bet you 100 credits if these edge angles reach close to 90 !! I will bet you 10,000 credits if you also let me get so forward as to leverage the tails out of the snow!

Wait... what was the bet about?

Here's Robby Kelley over the weekend, bending clearly the front part of the ski more than the tail:

View attachment 65585

Compared to one of my guys, in the same gate:

View attachment 65586

The latter looks a lot more forward, but bending a different part of the ski!!!

How come? Maybe you can help me make him faster, with geometry and COM/BOS equations!

p.s. does someone here think R.K. is "central" on the skis? Or is he... I don't know... more forward?

This is getting out of hand... so much assumption and pseudo equations and so little skiing... I'm honestly having a hard time to figure out who's arguing what... I've missed again a lot of posts with all this racing!!

When standing on a ski, both tail and tip are pressured because of the stiffness of the ski, just like sitting on a chair sends 1/4 of the weight to each leg if you're centered. For race skis, more of the weight is directed forward and back, simply because they are stiffer. More can be directed fore or back, depending on mounting point.

When you stand on ice, the same is true, up to the point where the ski camber flattens out. As you tip the ski more, the basics become true again and more is sent to the tip/tail, as the ski bends.

Even when you are "back" there is pressure on the tips, unless you're leveraging the boots and lifting the tips.

More can be sent to the tips than tails, over and above the ski flex pattern, by moving the hips fore or aft on the skis aka moving forward.

That is just simple static analysis, from behind a keyboard. Skiing is much more interesting when we consider how the skis move versus how the hips move (for those of us that displace the COM laterally, so that excludes @karlo .



My skis are not orbitals, they merely slingshot around the space-time continuum distortions produced by the atoms that got friendly enough to hang out together, forming the earth.

Look it up, maybe you need to ride my model, you don't have to push it around, it's really smart and uses rides Einstein's tensors: https://en.wikipedia.org/wiki/Gravity_assist



Speak for yourself. Do you know how much math it takes to surf those gravitational waves, looking for slingshot acceleration?

Wait!! By over-thinking simple 3rd degree acceleration equations, you're saying that my mind over-heats enough to slow me down ??? Now I understand why I'm slow!!!



I bet you 5 8/19th credits that pressuring it more on the front thus bending the longer and softer part of the skis more will reduce the radius! I will only bet you 1/19th of a credit if the angles we're talking about are never bigger than 10 degrees. I will bet you 100 credits if these edge angles reach close to 90 !! I will bet you 10,000 credits if you also let me get so forward as to leverage the tails out of the snow!

Wait... what was the bet about?

Here's Robby Kelley over the weekend, bending clearly the front part of the ski more than the tail:

View attachment 65585

Compared to one of my guys, in the same gate:

View attachment 65586

The latter looks a lot more forward, but bending a different part of the ski!!!

How come? Maybe you can help me make him faster, with geometry and COM/BOS equations!

p.s. does someone here think R.K. is "central" on the skis? Or is he... I don't know... more forward?

@razie, you continue to think of this as a static exercise. The very first post in this thread points out the importance of acceleration. The photos you post tell us nothing about fore/aft as they convey no information about the relative motion of the skis versus the CoM.

R.K. might be forward, he might be in the center of the ski. He most probably isn't back. We do need to know something about acceleration to form a conclusion.

Mike

 

[Mike King]

Wait, what?? Are you saying it's actually not as simple as "get forward" ??

So, as @Mike King is saying, let's say we ski more central and square early. What then?



Nah... just a better understanding of the fundamentals - see @Josh Matta post above, for instance

I never said anything about being square.

Mike

 

[James]

has anyone carved with no lateral movement? When I do, I do not perceive acceleration or deceleration of com. Com just goes down at constant velocity along a straight line, so long as pitch of slope remains the same.

How is it that you're carving in a straight line? Even on "straight skis", which I'm pretty certain you're not on, it would be an arc. An arc is not a straight line.

 

[razie]

@razie, you continue to think of this as a static exercise.

Oh, what gave me up? This?

That is just simple static analysis, from behind a keyboard. Skiing is much more interesting when we consider how the skis move versus how the hips move

...about the two guys above - well, there's the gate, the skis are both engaged and in the fall line and I'm filming from the side. That's a steep double black like-injected run Not sure what more you would need for your dynamic analysis? They clearly turn on the same foot...

Your hypothesis in the OP was that as the ski moves into the fall line, it accelerates and they both need to move forward to keep applying pressure to the middle of the ski.

Yet both have clearly moved forward along the ski but one applies pressure forward of middle and the other back of middle.

 

[karlo]

@razie, you continue to think of this as a static exercise. The very first post in this thread points out the importance of acceleration

“Static exercise”. Actually, that’s a good term, and, Mike, I think the concepts in the OP might be what are static.

If the pressure distribution is in the center of the ski, the resulting shape of the ski, when edged, will be to bend the ski into an arc of a circle. Why is this circular shape important? Because a semi-circle is the only shape where as the ski moves forward, the tail can follow the same path as the tip. Any other shape (parabola, ellipse, etc.) results in a path where the tail will be displaced from the tip and that path will result in drag, or friction. So, the most efficient bend pattern to obtain in the ski is a semi-circle, and that pattern results from having the pressure point of the skis be at their center.

The tail doesn’t drag. It doesn’t drag because drag requires both snow contact and pressure. If the pressure is forward, it’s not back at the tail.

Then, there’s the question of tail falling in line with the track initiated forward of it. The tail doesn’t follow the tip. The piece of ski right behind the tip, still forward of center follows. As the tip leaves the track it created, that following piece continues the track, not by going over the same track, but by cutting new track in front of what’s been cut. That’s the dynamic view. That new piece of track can be a different radius while the arc remains smooth, though not circular. Now make each following piece infinitesmillay small, which they are, the final track is smooth, the track cut by the last piece, by the tail, is an arc that tangentially meets the infinitesimally small penultimate segment of the arc.

Which gets to my “effective edge”. It’s the piece of edge that, infinitesimally small increments that is in contact AND has pressure.

If one really moves the CoM forward of the center of the ski, then the ski will be bending more in the front than the tail and that's an inefficient bend pattern -- the tail will be displaced relative to the tip and create drag

No drag per above.

as the skis turn down the hill and enter the fall line, they are accelerating. In order to keep up with the skis and to keep the pressure point under the center of the skis, the CoM must move forward along the length of the ski.

“Accelerate”. I still don’t understand what your meaning is. Do you mean the centerline of the ski begins traveling downhill at a higher velocity? If so, I’m not sure I agree. Certainly it does Relative to one’s COM, to catch up to the COM. Certainly it slows down after apex, when COM catches up to the ski’s centerline. But, I prefer to think of acceleration as it relates to forces, specifically g’s, like the g’s we experience as a car turns. And g’s is what affects pressure, the important thing in skiing. In that context, the ski’s, throughout their arc is always accelerating, except when released in transition.

And, similarly, as the skis decelerate as the the skis exit the fall line and traverse across, or even up the hill, the CoM must move aft to keep the pressure distribution centered on the ski.

Per above, in the arc, they are still accelerating, simply at a lower rate, and lower g’s

 

[karlo]

How is it that you're carving in a straight line? Even on "straight skis", which I'm pretty certain you're not on, it would be an arc. An arc is not a straight line.

I meant COM going in a straight line. The skis arc