Great graphics

[mister moose]

So you are both saying that when you flex gravity goes away? Really? Interesting. The point I am trying to make is that gravity still effects you, yes even when we flex. So when we remove our bos our com moves downward towards the center of the earth. Does it not? Or are we re-writing physics here on Pugski?

You ask "Does it not?"
Answer from experience: No, it does not.

I think you need to go out onto snow as soon as you can and try flexing the old outside leg/new inside leg in a completed turn, while doing nothing else. You will not fall down to the old inside, I guarantee it. And you'll be able to put a new movement pattern in your tool box.
Old dogs DO learn new tricks. I'm old and I affirm this message.

You two are talking two different things.

Loki1 is correct, gravity prevails. Of course. Remove the BOS from anything, doesn't have to be a skier, and it will move downward. Note that the rate of downward movement starts at zero and increases exponentially, but it is the starting at zero that gives us time to react. A good example of Loki1's statement is a strong retraction turn. If you are successful in achieving an honest and rapid retraction (without a cheating pre-hop), your feet will come off the ground even if ever so slightly, your BOS is gone, and your COM will lower. Otherwise Newtonian physics is myth.

LiquidFeet is talking about translational movement, flexing the old outside leg at the very end of the old turn, projecting the COM down the slope, (Down-slope is not the same as vertical) this is not the same as removing the BOS. The COM does not move vertically here. It's path only has a vertical component.

Liquidfeet, ask yourself this - where in the turn you describe is the pressure on the outside foot the greatest, and where is it the least? That leads you to the answer, and it isn't related to falling inwards or to removing the BOS. Flexed leg ≠ zero support.

 

[razie]

Asking what happens to rotational motion when you remove the centripetal force has no real application in skiing. We don't ski that way. We incrementally change forces and we incrementally blend.

You mean to say we incrementally create or remove centripetal force, which creates turns. yeah, that's skiing by describing our kinetic chain interactions and muscle activations, we can be very specific about how we do what we do...

I think great skiing can exist outside great science understanding and vice-versa - it's unlikely for someone to ski like Hirscher or Reilly and understand physics like Newton or Einstein - we're all in-between somewhere.

However, very clear and detailed mental model as to how things work are very important for top athletes and it is very likely that a misunderstanding of physics and biomechanics will hold one from skiing better (why try certain technique/movements if you think they won't achieve goal X), just like relying on some word of mouth and feelings can result in poor explanations on the science side - like "push" and "early pressure" etc.

@mister moose - you're confusing things a bit. Forces and vectors compose and they are talking about the same thing and LF has it right. Assume for a second that we're standing on the outside leg at the end of the turn - normally inclined to resist both gravity and centripetal force. The outside leg is the only thing keeping us there, in balance, com on top of bos against the resulting force, so the gravity is totally balanced out of the equation.

When we relax this outside leg and let it collapse, the centripetal and centrifugal forces disappear and the body will be subjected to gravity only, pulling it straight down to the snow (without affecting the resulting movements much, like I calculated already - air friction probably influences things more? too much math) and will also continue its movement in whichever vector it was moving, without any other acceleration, just maintaining its last constant speed. We don't fall down and then move forward, they combine - the downward movement being accelerated while the forward movement is constant speed.

The skill of the skier that wants to maintain speed going down the hill is to release at the right time, before killing all the forward momentum by resisting against that direction. You'll notice that most racers start to release right after the fall line, to keep the momentum going down the hill. If you're late on the line (because you need to turn some more), you're also slow!

It's a little obscured by the panel, but she is releasing right at the gate, letting the outside leg relax right after the fall line, starting with frame 2 - in frame 3 the ski is no longer bent and turning, the leg is already relaxed and fairly flexed. If she had kept the leg long in frame 3 she'd totally kill the forward down the hill momentum and she'd have to hop over to help transition out of the turn, while turning up the hill as well. Also visible is how she timed the pressure so well, the ski turned a lot between frames 1 and 2...

This is one of the reasons Loki's reasoning (and resulting technique) was flawed due to missing the tangential direction issue - the release by relaxing the outside leg must start early, so that the body keeps moving as much down the hill. That's the biggest issue with outside-ski to outside-ski and early pressure thinking and why all great skiers float in transition, avoiding late pressure like the plague (depending on course shape and offset, some late pressure is tolerable and some can be turned into forward momentum as in forward along the ski - or lateral across the hill, but that's a different level of discussion).

A good racer will keep the ski engaged and bent only as long as needed to get enough impulse across the hill to the next gate - not a millisecond longer. Also, this is not an on/off switch, as you noted. Most will relax the leg gradually, and keep the ski turning somewhat while reducing the slowing down of the body after the fall line. It's a fairly interesting bit of physics when looking at the high-performance turn, I'm guilty of overthinking and overanalyzing it often over the past decade and not having the time to put some well-documented posts with detailed diagrams on it, just these wordy answers here and there...

Some academies would also consider this kind of stuff valuable knowledge... you can see how the technique is directly affected by the understanding of physics and some will always be slower and less efficient than others...

imho - of course.

 

[LiquidFeet]

....When we relax this outside leg and let it collapse,
... the body .... will continue its movement in whichever vector it was moving...
....release at the right time,
....right after the fall line
....the release by relaxing the outside leg must start early, so that the body keeps moving as much down the hill....

@razie, I hope you forgive me for so abbreviating your post. Those words above that I extracted helped me see something in a way I haven't thought about before. Thank you!

For my benefit, I'm going to restate in different words the point I just absorbed from your post. I want to see if this makes sense when I write it out. I'd appreciate any comments from those participating in this thread.

1. The upper body travels along a path different from the skis. Skis take a longer path than the body. They travel out from the body, then back in towards the body, then they go out to the other side of the body, then back in. While they are doing this, the body is following a sinuous path that doesn't reach as far outward; it's path is shorter.

2. So there are two paths, the path of the upper body (CoM), and the path of the skis (BoS). The paths cross between turns.

3. Top tier racers and highly skilled skiers do things to make sure that the path of the CoM doesn't line up with the path of the BoS between turns.

4. Those skiers achieve this goal by beginning the release just after the fall line, not later. In this case, a flexion release (new inside leg flexes) will allow the upper body's momentum to take it across the skis, unimpeded by the old outside leg. The two lines will cross because of the flexion release.

5. Most skiers delay releasing until later. Because they hold onto the turn after the fall line, they end up allowing the path of their upper bodies to line up with the path of the skis. This happens in the bottom half of the turn.

6. If these skiers wait to flex their new inside leg to release until after the two lines match up, their bodies may tilt laterally towards the bottom of the hill but the momentum of their body will not forcefully carry them across the skis. That's because the path of the body is lined up with the path of the skis. A flexion release will be too weak to get such skiers firmly into the new turn. They may find the flexion release leaves them feeling insecure.

7. These skiers will feel far more secure if they extend the new outside leg to push themselves across the skis.

 

[mister moose]

However, it is very likely that a misunderstanding of physics and biomechanics will hold one from skiing better (why try certain technique/movements if you think they won't achieve goal X), Yes, I agree, but only when explanations are simple, concise, and not full of 12 letter words. If you can't explain it simply, it won't get much mileage.

@mister moose - you're confusing things a bit. Forces and vectors compose Don't know what you mean here by 'compose' and they are talking about the same thing and LF has it right. Assume for a second that we're standing on the outside leg at the end of the turn - normally inclined aligned to resist balance both gravity and centripetal force. The outside leg is the only thing keeping us there, in balance, com on top of bos aligned, not on top, there's a difference against the resulting force, so the gravity is totally balanced out of the equation. Gravity is not removed or 'balanced out'. It is constant.

When we relax this outside leg and let it collapse, it does not collapse, it is only reduced, and in a controlled, not free fall manner the centripetal and centrifugal forces no such thing as centrifugal force, it is an imaginary force in a rotating reference frame disappear This means all turning has ceased and the body will be subjected to gravity only not correct, pulling it straight down to the snow you're still sliding down the hill, forces are not just straight down (without affecting the turn much, like I calculated already) and will also continue its movement in whichever vector it was moving, without any other acceleration, just maintaining its last constant speed. Agree, this is basic inertia We don't fall down and then move forward, they combine - the downward movement being accelerated downward in what direction, down the slope or vertical, you aren't clear here while the forward movement is constant speed only if you are at terminal velocity for that slope and snow condition.

The skill of the skier that wants to maintain speed going down the hill is to release at the right time, before killing all the forward momentum by resisting against that direction. Forward momentum does NOT have to be directly downhill You'll notice that most racers start to release right after the fall line, to keep the momentum going down the hill. How then do they make the next gate that is not directly downhill of them

A good racer will keep the ski engaged and bent only as long as needed to get enough impulse across the hill to the next gate - not a millisecond longer. Yes... I agree, but this does not square with your comment above Also, this is not an on/off switch, as you noted. Most will relax the leg gradually, and keep the ski turning somewhat while reducing the slowing down of the body after the fall line. It's a fairly interesting bit of physics when looking at the high-performance turn, I'm guilty of overthinking and overanalyzing it often over the past 6-7 years and not having the time to put some well-documented posts with detailed diagrams on it, just these wordy answers here and there...

Some academies would also consider this kind of stuff valuable knowledge... you can see how the technique is directly affected by the understanding of physics and some will always be slower and less efficient than others...

I get (mostly) what you're saying, but it's imprecise language and physical relationships both of which that cause confusion in readers minds, and keeps the conversation from being productive.

Again, LiquidFeet is talking about translational movement, moving the COM across the snow instead of singularly moving it closer (downward) to the snow. That is if I understand his point. To sum up, removing the BOS results in a motion towards the snow, which is also generally towards your feet, where as flexing the old outside leg 1) doesn't happen all at once, 2) still provides support to the torso as it is flexing, 3) results in moving the torso towards the base lodge, not your feet. LF is talking about adjusting, not removing the BOS.

 

[mister moose]

5. Most skiers delay releasing until later. Because they hold onto the turn after the fall line, they end up allowing the path of their upper bodies to line up with the path of the skis. This happens in the bottom half of the turn.

Everyone holds onto the turn past the fall line. The turn continues until the green transition line, or where one turn ends and the next begins. The crux of LF's discussion (I think) is the COM passes over the skis slightly before or at the edge change.

We are used to talking about separation in the upper body not facing the same way as the skis. We also separate the COM from being laterally over the BOS, most often referred to as dynamic skiing. And then there is also the separation of positioning ourselves uphill of the feet in second half of the turn and then downhill of the feet until they reach pointing down the fall line, and then they begin to move below us again.

I'm going to refer to this as skiing's 3 degrees of separation™.

I adapted LF's first diagram. I drew in where I think the feet are in purple, and divided it in half. Each purple arrow shows the relationship of feet and body. On the downhill half, I drew dots. Look how the red dot is downhill of the black dot (shown by the light yellow green lines)

I don't think the timing of this is rigid, there likely is subtle variations in skiers.

 

[LiquidFeet]

Everyone holds onto the turn past the fall line. The turn continues until the green transition line, or where one turn ends and the next begins. The crux of LF's discussion (I think) is the COM passes over the skis slightly before or at the edge change...

@mister moose, I switched images because the first one had too many words in it. I must have been doing that as you were working on your post.

The crux of my discussion is that when the skier releases determines two things.
--The timing of the release determines where in a diagram the cross of the two lines will happen. In an idealized turn, that cross happens nicely right between the two curves and each line maintains its nice sinuous curviness. But in real life, for many skiers, that cross happens much later. A late release causes the two lines to flatten out and line up with each other. I have never seen the diagram of the two paths drawn this way. Someone want to do it?
--The timing of the release determines the effectiveness of a flexion release. When the two lines have time to line up with each other, the delayed timing of the release makes a flexion release less effective.

 

[Mike King]

Two things.

1. Flexing the outside leg is NOT removing the base of support. It is transferring it, at least initially, to the inside foot. And since the inside leg has a less acute angle to the center of mass, it creates a lever to cause the CoM to move up and over the skis. It's leverage. If you don't want the CoM to move up, then you will need to not only flex the outside leg, but the inside leg as well. But how do you get the CoM to diverge from the path of the feet?
2. You can tighten the arc of the feet without increasing inclination (the balancing of the CoM against centripetal force). How do you do that? Angulate. The angulation movement increases edge angle and if you do not further incline, the result is that the CoM is released from it's arc while the feet continue on theirs. Coupled with flexion AND actively pulling the feet through in front of you, you will fall into the top of the next turn.

Watch Tom Gellie's turn transitions webinar. It will clarify all of this for you.

Big Picture Skiing

bigpictureskiing.com

 

[LiquidFeet]

@Mike King, any chance you'd write out your thoughts about what I posted? I'm curious.

 

[geepers]

Two things.

1. Flexing the outside leg is NOT removing the base of support. It is transferring it, at least initially, to the inside foot. And since the inside leg has a less acute angle to the center of mass, it creates a lever to cause the CoM to move up and over the skis. It's leverage. If you don't want the CoM to move up, then you will need to not only flex the outside leg, but the inside leg as well. But how do you get the CoM to diverge from the path of the feet?
2. You can tighten the arc of the feet without increasing inclination (the balancing of the CoM against centripetal force). How do you do that? Angulate. The angulation movement increases edge angle and if you do not further incline, the result is that the CoM is released from it's arc while the feet continue on theirs. Coupled with flexion AND actively pulling the feet through in front of you, you will fall into the top of the next turn.

Watch Tom Gellie's turn transitions webinar. It will clarify all of this for you.

Big Picture Skiing

bigpictureskiing.com

This aligns with my current understanding. Not surprising as watching same videos by TG.

That's the extra bit I see over and above the commentary provided by razie on the Vonn graphic. Angulation increasing through frames 1 to 3 (ish) so that her upper body is already moving out of the turn and starting across and down the hill whilst her feet are still completing the turn. That is being combined with a softening of the outside leg so when both legs flex (frame 4 with both tips off the snow) her upper body is well inclined for the new turn. At frame 4 her hips are still inside the old outside ski edge but with the edge change by frame and onto the new outside edge by frame 5 enough of her her mass would be on the downhill side of that new edge to send her CoM inside the the turn. Some angulation from foot tipping she mostly stays low (little bit of rise - frame 5) with outside leg extending outward on the path of the new turn. in subsequent frames. Building great angles frames 5 through 8 and progressively adding hip angulation from 6 through the rest of the turn.

Her new outside arm is also moving up between frames 3 and 6.


@razie..... motor cycles. It's a thing about humans that some of us are so intent on seeing the differences between things that we disregard all the things that make them similar. The mechanics of getting a motorcycle to incline are different to skiing but the CoM toppling over the BoS (and the need to topple the other way when the time is right) are common. If you want to focus on differences I'll give you another. Motor cycles have 3 gyroscopes (front/rear wheel and crankshaft) and the precession induced torque must be overcome to get a fast moving bike from footrest scraping one side to footrest scraping the other side in an S bend. That's why handlebars and counter-steering are a lot more popular than leaning off the side of the bike when planning to make a given bend. If you've spent any time herding 1970's japanese motor cycles around racetracks (and herding was the right word for it) then you'd appreciate that modern bikes do not unduly flex.

 

[razie]

Looks like @mister moose managed to render the entire discussion useless - we do not "incline" but instead we "align". Darn... 7 pages wasted... unless we can say that we align by inclining? Ther's actually a book out there based on a technique based on "aligning", I think Align, Balance, Coil were the "thing" - I thought an interesting way to simplify things .



Btw, yes - vectors compose (or add) - they also decompose into "components" ... etc

Anyways...

@geepers you brought up motorcycles initially for the purpose of showing something that doesn't flex and it seems we agree you have to hop over them - but now that our lack of knowledge about the existance of 1970's motor hearding techniques is rectified some, I feel better.

@Mike King we don't necessarily get on the inside ski if we relax the outside. Only if we keep it engaged. If it's also relaxed or rather retracted like it was supposed to be, we do a normal release and transition, like the one Vonn's doing above. If you keep it a bit engaged and start un-relaxing it, you get what Ted likes to do, to get on the new outside ski early, when it's still the inside ski. It's a good thing... at the right time!

Watch the deep flexing of the skiers here - the one on the right has both feet in the air often while the one on the left has a very massive and clear collapsing of the outside leg, super-bending it and also ends up with both off snow sometimes (not a desirable thing - just a result of the forces they generate) - but neither ends up unbalanced on the inside ski:


@LiquidFeet sounds right to me. That's a good diagram - it shows when the ski stopped bending and turning, i.e. the pressure was removed

 

[mister moose]

@mister moose, I switched images because the first one had too many words in it. I must have been doing that as you were working on your post.

Yes, it was hard keeping up with you! I like the first wordy diagram far better though. The less wordy one has (IMO) large errors of skier path. More later.

The crux of my discussion is that when the skier releases determines two things.
--The timing of the release determines where in a diagram the cross of the two lines will happen. In an idealized turn, that cross happens nicely right between the two curves and each line maintains its nice sinuous curviness. But in real life, for many skiers, that cross happens much later. A late release causes the two lines to flatten out and line up with each other.

In your description of a late release the cross happens later because it doesn't happen at all for a while. The direction of travel of the COM is the same as the direction of travel of the feet, in your wordy diagram those arrows become and remain aligned. I'm expanding what you are saying by changing 'delayed' to 'paused'. I think it's more than a timing issue in many skiers, it is a continuity issue.

but the CoM toppling over the BoS (and the need to topple the other way when the time is right) are common.

(Not picking on Geepers, but he was there with the usage I wanted to highlight)

I'm not sure we topple. I'm also not sure that projects a good learning image. I think we transition. I think we move. Furthermore, I think the movement of 'toppling' is a continuation of something we are doing all of the time; which is pressing our feet outward, away from our core in a turn. Just prior to transition, and just after, we change the direction of that pressure because we change the direction of our feet under us. When that direction to our feet goes from below us to above us, some describe that (and the associated translational movements) as toppling. We really just continue to apply pressure to the feet. Once you see the direction of pressure to the feet as a constant, the change of toppling isn't really toppling. And the change isn't really a change, it's following.

 

[Mike King]

@Mike King, any chance you'd write out your thoughts about what I posted? I'm curious.

In general, I agree with most of what you've written. The diagram is a bit more of a problem for two reasons: first, phase 1 and 3 overlap (your phase 3 contains a lot of what you drew as phase 1) and second, the edge angle in the beginning of phase 1 may still increase if one is using increasing the edge angle to loosen (release) the CoM and begin the transition to the new turn.

...
@Mike King we don't necessarily get on the inside ski if we relax the outside. Only if we keep it engaged. If it's also relaxed or rather retracted like it was supposed to be, we do a normal release and transition, like the one Vonn's doing above. If you keep it a bit engaged and start un-relaxing it, you get what Ted likes to do, to get on the new outside ski early, when it's still the inside ski. It's a good thing... at the right time!

Watch the deep flexing of the skiers here - the one on the right has both feet in the air often while the one on the left has a very massive and clear collapsing of the outside leg, super-bending it and also ends up with both off snow sometimes (not a desirable thing - just a result of the forces they generate) - but neither ends up unbalanced on the inside ski:

Well, if all you do is flex the outside leg, the inside ski is going to be engaged unless it is off of the snow. And if that's the case, then flexing the outside leg is going to do anything to the CoM as you've not disturbed the ground reaction force working on the CoM by shortening the outside leg. In order to get the CoM moving across the skis, you've got to change the relationship (e.g. the vector) between the base of support and the CoM.

As Gellie identifies, there's three ways to do that:

1. Tighten the arc of the feet
2. Loosen the arc of the CoM
3. Some combination of both.

Transferring pressure to the inside foot (it doesn't have to be all, nor all at once) is about number 2. Increasing edge angle by angulating is about 1. Doing both is 3.

Following on @LiquidFeet's description of being late, Gellie would put this movement into the second category. But instead of moving against the force in the turn, one waits and moves against gravity stepping down on the old inside ski -- in fact, that inside foot may look more like an inrigger displaced toward the center of the earth earlier in the turn. Perhaps banking in @razie's taxonomy.

Mike

 

[razie]

Let's agree to disagree with that, @Mike King - when I'm at high edge angles, the inside leg is mostly relaxed except for keeping it flexed and not pushing in the snow - i.e. retracted and tipped.

When relaxing and flexing the outside, the inside keeps being retracted and largely off the pressure. I don't really want much pressure on a bent leg anyways - I'd get too tired... so my old inside leg stays largely retracted through transition and then extends on the other side until I let pressure appear.

cheers

 

[LiquidFeet]

@Mike King, I didn't create that graphic I posted, just letting you know. It's from some CSIA document.

 

[Mike King]

Let's agree to disagree with that, @Mike King - when I'm at high edge angles, the inside leg is mostly relaxed except for keeping it flexed and not pushing in the snow - i.e. retracted and tipped.

When relaxing and flexing the outside, the inside keeps being retracted and largely off the pressure. I don't really want much pressure on a bent leg anyways - I'd get too tired... so my old inside leg stays largely retracted through transition and then extends on the other side until I let pressure appear.

cheers

What happens to pressure on the inside foot as you flex the outside leg? How are you disturbing the force vector to move your CoM into the next turn? If there's no pressure on the inside foot and you are flexing the outside leg, aren't you simply moving down to the outside foot with no release of the center of mass across the skis?

 

[Uke]

Mike,

Flexing the outside leg will change the lateral component of the force vector allowing the com to take a less curved path setting it up to cross the path of the skis. This works really well if you have used an angulation movement to already start the com on a path to cross the skis.

uke

 

[Mike King]

Mike,

Flexing the outside leg will change the lateral component of the force vector allowing the com to take a less curved path setting it up to cross the path of the skis. This works really well if you have used an angulation movement to already start the com on a path to cross the skis.

uke

Thanks. I've thought that this was simply as a result of leverage on the old inside leg. I will have to experiment when I get back on snow...

 

[razie]

yeah, I think that's how it works, physics wise. You got the impulse to the hips at the apex and it will continue moving down the hill and in as you release the outside leg.

I don't want weight on flexed legs - I tire quickly if so. That's why many don't flex well imho because the first time you try it, you'll get the timing wrong and you'll get tired and it feels wrong/bad.

 

[Mike King]

Mike,

Flexing the outside leg will change the lateral component of the force vector allowing the com to take a less curved path setting it up to cross the path of the skis. This works really well if you have used an angulation movement to already start the com on a path to cross the skis.

uke

Thinking about this more, does it not depend on the angle of the outside leg relative to the angle of the centripetal force vector? If the outside leg is parallel to the force vector, shortening the leg would not change the relationship of the lateral versus vertical vectors, right? So the result you speak of holds if there is angulation of the outside leg with respect to that force vector. So, it would seem that flex to release is manipulating angulation to cause a loosening of the CoM...

 

[James]

If she had kept the leg long in frame 3 she'd totally kill the forward down the hill momentum and she'd have to hop over to help transition out of the turn, while turning up the hill as well.

The transition just would have happened a little further on. The retraction is for quickness of transition. No ‘hop’. Sure, maybe if you keep the leg the exact length of the apex, but where do we do anything stiff legged?