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# "Re-directing" skis in the air? - Page 2

### air carving

It seems the thread has take a turn to aerials. And while this is not an aerial video, it does talk about redirecting in the air.
http://mysnowpro.com/jonathanlawson/2007/03/post_4.php
Jon
I am sure Bob Barnes or Arcmeister or Vailsnopro, etc. could say this better than me but.... Once you leave the ground you will not be able to change your trajectory significantly. You can certainly change the way your skis are pointing but the path of the cm is not going to change from any skier effort. It may arc due to gravity's pull but that is basically all. I am not an expert in physics but those who are would likely agree with me on this.

b
Quote:
 Originally Posted by bud heishman You can certainly change the way your skis are pointing ...
And how would one do that?
Lonnie, by tipping them.
Then use air resistance to cause your legs to twist round.
It's the only way.
Quote:
 Originally Posted by Wear The Fox Hat Lonnie, by tipping them. Then use air resistance to cause your legs to twist round. It's the only way.
You're saying that the tipping causes them to act as an airfoil and they the come around? That's what I thought. Thanks for clearing it up for me!
So what is the answer you are looking for Lonnie?.......
Quote:
 Originally Posted by bud heishman So what is the answer you are looking for Lonnie?.......
I'm not 100% sure, but I THINK that the ONLY way you can change the direction your skis are pointing once they leave the ground is by rotating them. Now the forces one uses to rotate them once they are in the air can be debated, but my guess is the easiest way is by muscular tension.

Now if we go back to the HH bump skiing video, the only way that HH can make his skis move on the turns I pointed out is by rotating the feet and legs. As fox eluded to, no amount of tipping is going to make the skis do what they do in those frames.

Quote:
 How is this skier doing it? (see from 1:25-1:35 and 2:39-2:41 for good examples...)http://www.youtube.com/watch?v=tW2cT9DArUo
Lonnie,

I don't see much air in that series of turns, however, I do see some anticipation release where his "counter movements" tension the muscles and once edge hold is released and his pole plant blocks his torso, the skis redirect from the untwisting of the muscles. This could be passive or active depending on his relaxation or tightening of the muscles upon edge release.
IMO

b
But is it not the "dreaded" rotation?
Lonnie, it's only dreaded if you're scared of it!
Quote:
 Originally Posted by Lonnie I'm not 100% sure, but I THINK that the ONLY way you can change the direction your skis are pointing once they leave the ground is by rotating them. Now the forces one uses to rotate them once they are in the air can be debated, but my guess is the easiest way is by muscular tension. Now if we go back to the HH bump skiing video, the only way that HH can make his skis move on the turns I pointed out is by rotating the feet and legs. As fox eluded to, no amount of tipping is going to make the skis do what they do in those frames.
If you think Harald Harb is a fraud, why not just start a thread with that title?
I am no physicist, but once you're in the air (ie. in a "closed system"), I would think the "law of conservation of angular momentum" takes over.
I found an explanation on the internet here, which talks a little bit about its application in skating:
http://www.sparknotes.com/testprep/b...section6.rhtml

It would be great to have Physicsman or Bob Barnes come on and explain how this law relates to skiing.

WTFH, I think you'd have to be going quite fast before air resistance played a significant role.
Quote:
 Originally Posted by MilesB If you think Harald Harb is a fraud, why not just start a thread with that title?
I'm not saying he's a fraud, but I'm also not the one claiming I never roatate my feet. You'll have to decide where the truth is. Can you explain how he does it otherwise?
Where does PMTS say rotation doesn't happen?
Quote:
 Originally Posted by Lonnie I'm not 100% sure, but I THINK that the ONLY way you can change the direction your skis are pointing once they leave the ground is by rotating them. Now the forces one uses to rotate them once they are in the air can be debated, but my guess is the easiest way is by muscular tension. Now if we go back to the HH bump skiing video, the only way that HH can make his skis move on the turns I pointed out is by rotating the feet and legs. As fox eluded to, no amount of tipping is going to make the skis do what they do in those frames.
Great vid.

Rotation, why no thats smoke and mirrors anything but rotation.

In truth, it is all obviously unconscious, so Mr Harb may claim to his disciples that you do not actively twist/rotate in turning ones skis etc etc .
Quote:
 Originally Posted by Martin Bell I am no physicist, but once you're in the air (ie. in a "closed system"), I would think the "law of conservation of angular momentum" takes over.
Martin you are right, but conservation of mometum talks about conserving the angular momentum we already have, but doesn't explain how to get the skis turning in the first place (it also explains why "Counter-rotation" was such a popular answer before....)

And lets be clear, we are talking about TWO things here. The path of our body through space, AND the path of the skis through space.
Perhaps there is a clue here:
http://en.wikipedia.org/wiki/Angular_momentum
"a system's angular momentum stays constant unless an external torque acts on it"
Any external torque would presumably have to be applied before leaving the ground.
Quote:
 Originally Posted by Martin Bell Perhaps there is a clue here:http://en.wikipedia.org/wiki/Angular_momentum "a system's angular momentum stays constant unless an external torque acts on it" Any external torque would presumably have to be applied before leaving the ground.
Let's say the "system" we are talking about is just the foot/boot/ski. Could you not apply a torque to the foot through the legs while it's still in the air? Yes. And that has to be done by rotating the leg (through a variety of methods). Simply tipping the ski while it's in the air does virtually nothing to make it change it's direction along that axis.
Quote:
 Originally Posted by Lonnie Let's say the "system" we are talking about is just the foot/boot/ski. Could you not apply a torque to the foot through the legs while it's still in the air? Yes. And that has to be done by rotating the leg (through a variety of methods). Simply tipping the ski while it's in the air does virtually nothing to make it change it's direction along that axis.
I don't think you could view the foot/boot/ski as an independent "system", because it is attached to the rest of the body. (Any physicists out there please feel free to correct me on this.)

My understanding of the law of conservation of angular momentum is that if you twisted the foot/boot/ski in one direction, some other part of the body would have to twist in the opposite direction to compensate. (Again, I am NOT a qualified physicist; if any physicists out there would like to correct me on this, please go ahead.)

I don't see how tipping the ski on to its edge would change its direction in the horizontal plane.
Yup, and there we have the gist of the semantics differences. It's not the tipping of the ski that changes the horizontal direction of the ski. It's the tipping of the foot that causes action up the "kinetic chain" that causes the foot and leg to rotate that causes the ski to change direction.
A lot of interesting stuff in this thread, including the aerials stuff which I know nothing about and is always quite fascinating to hear about.

I think the original intention of Lonnie's post was to ask about a few frames in the HH bump video where HH is clearly airborne while pivoting in the air. Particularly towards the bottom of the run. Lonnie would like to either understand or debate this as it relates to PMTS. Yes?

First, I don't think it relates to PMTS at all. I think that is just HH skiing by instinct when his skis are in the air. From what I know about PMTS, it deals primarily with making the skis turn while they are on the snow. What happens in the air is something else. I do not think this invalidates PMTS in any way at all, but it is not representative of PMTS either.

Clearly, as Bud pointed out, our CM can't change direction when we're airborne. As some others have said, we can change the direction our skis our pointing in some limited fashion. Its difficult to do from a dead-static position. In other words, if you went airborne with zero rotational forces acting on you and no anticapatory tension in your body...floating gracefully through the air and then decided while airborne you want to turn the skis in the air. That takes a LOT of effort and as others have pointed out, would require counter-rotating your upper body to allow it to happen.

I suspect that the air-borne pivoting you see done by HH in that video is, as someone else also pointed out, based on anticipation. HH is using anticipation in every one of those BPST turns, even though he does not like to use that word. He is using that skill. Anticipation is particularly effective when your skis are going airborne. The Anticipation has to be setup before going airborne, once airborne, your body will unwind with almost no effort at all and provide the airborne pivoting.

There was some LeMaster photos of Liggety I saw a while back showing something like this. I don't have time to find it now.

Lonnie, I think you're probably trying to find a new angle to argue that rotation debate, but I think you're barking up the wrong tree with this one. The PMTS BPST turn does have anticipation in it and the skis do pivot on the snow at slow speeds. The thing is that skiers are not taught to forcefully make the pivot happen through rotational muscle-driven movements(here we go again). But clearly there are times when PMTS skiers have non-arced turns with some amount of pivoting happening. Anticipation is a strong contributor to this IMHO. Not Muscle driven rotatary skills. That is just not the PMTS way of doing it. I'm making no attempt right now to say one way is better than the other. Just saying PMTS, does not use muscle driven, conscious, rotary. For some odd reason they don't like to use the word anticipation either, but its clearly visible in all of the BPST demos, so I believe they are using this skill as part of the BPST.
Quote:
 Originally Posted by borntoski683 A lot of interesting stuff in this thread, including the aerials stuff which I know nothing about and is always quite fascinating to hear about. I think the original intention of Lonnie's post was to ask about a few frames in the HH bump video where HH is clearly airborne while pivoting in the air. Particularly towards the bottom of the run. Lonnie would like to either understand or debate this as it relates to PMTS. Yes? First, I don't think it relates to PMTS at all. I think that is just HH skiing by instinct when his skis are in the air. From what I know about PMTS, it deals primarily with making the skis turn while they are on the snow. What happens in the air is something else. I do not think this invalidates PMTS in any way at all, but it is not representative of PMTS either. Clearly, as Bud pointed out, our CM can't change direction when we're airborne. As some others have said, we can change the direction our skis our pointing in some limited fashion. Its difficult to do from a dead-static position. In other words, if you went airborne with zero rotational forces acting on you and no anticapatory tension in your body...floating gracefully through the air and then decided while airborne you want to turn the skis in the air. That takes a LOT of effort and as others have pointed out, would require counter-rotating your upper body to allow it to happen. I suspect that the air-borne pivoting you see done by HH in that video is, as someone else also pointed out, based on anticipation. HH is using anticipation in every one of those BPST turns, even though he does not like to use that word. He is using that skill. Anticipation is particularly effective when your skis are going airborne. The Anticipation has to be setup before going airborne, once airborne, your body will unwind with almost no effort at all and provide the airborne pivoting. There was some LeMaster photos of Liggety I saw a while back showing something like this. I don't have time to find it now. Lonnie, I think you're probably trying to find a new angle to argue that rotation debate, but I think you're barking up the wrong tree with this one. The PMTS BPST turn does have anticipation in it and the skis do pivot on the snow at slow speeds. The thing is that skiers are not taught to forcefully make the pivot happen through rotational muscle-driven movements(here we go again). But clearly there are times when PMTS skiers have non-arced turns with some amount of pivoting happening. Anticipation is a strong contributor to this IMHO. Not Muscle driven rotatary skills. That is just not the PMTS way of doing it. I'm making no attempt right now to say one way is better than the other. Just saying PMTS, does not use muscle driven, conscious, rotary. For some odd reason they don't like to use the word anticipation either, but its clearly visible in all of the BPST demos, so I believe they are using this skill as part of the BPST.
I am sorry but I just have to comment on this one

So Mr Harb when he skis by instinct he rotates actively etc ad nauseam.

But he teaches not to actively rotate etc.

Don't be offended but ...............ROFLMFAO
Quote:
 Originally Posted by therusty Yup, and there we have the gist of the semantics differences. It's not the tipping of the ski that changes the horizontal direction of the ski. It's the tipping of the foot that causes action up the "kinetic chain" that causes the foot and leg to rotate that causes the ski to change direction.
So can you explain how the "kinetic chain" works if there is nothing for the skis to push against (ie they are in the air)?
Quote:
 Originally Posted by borntoski683 I suspect that the air-borne pivoting you see done by HH in that video is, as someone else also pointed out, based on anticipation. HH is using anticipation in every one of those BPST turns, even though he does not like to use that word. He is using that skill. Anticipation is particularly effective when your skis are going airborne. The Anticipation has to be setup before going airborne, once airborne, your body will unwind with almost no effort at all and provide the airborne pivoting.
So, if anticipation will unwind (rotate) the skis via muscular tension while they are in the air, won't it do the same thing while they are on the snow. How is that NOT a rotary turn? The legs are (un)rotating to shape the turn. Does how we build that tension matter? It might. Convince me.

Quote:
 Originally Posted by borntoski683 Lonnie, I think you're probably trying to find a new angle to argue that rotation debate, but I think you're barking up the wrong tree with this one. The PMTS BPST turn does have anticipation in it and the skis do pivot on the snow at slow speeds. The thing is that skiers are not taught to forcefully make the pivot happen through rotational muscle-driven movements(here we go again). But clearly there are times when PMTS skiers have non-arced turns with some amount of pivoting happening. Anticipation is a strong contributor to this IMHO. Not Muscle driven rotatary skills. That is just not the PMTS way of doing it. I'm making no attempt right now to say one way is better than the other. Just saying PMTS, does not use muscle driven, conscious, rotary. For some odd reason they don't like to use the word anticipation either, but its clearly visible in all of the BPST demos, so I believe they are using this skill as part of the BPST.
Again see above. I think this gets back to Unsean's arguement. Does it matter if they are thinking "Turn your feet" or not if that's what happening? Can ANYONE give any other way for the skis to re-direct as shown from 2:39-2:41 without a muscluar (un)twisting of the legs?
I'm not a physicist either, but Martin Bell is right.

When you're on the ground, you can change your momentum (including your angular momentum) by pushing off of the ground. The various movements required to produce a particular change in momentum are a bit complex, and are often referred to as "skiing." The fact we have to push off the ground in order to do various things is a big part of the reason skis have edges.

When you're in the air the only thing (other than gravity, of course) which can change your momentum, including your angular momentum, is pushing against the air, i.e. air resistance and aerodynamics. Aerodynamic effects really only come into play at quite high speeds, though -- and, with the exception of Nordic ski jumpers, I don't think very many skiers have really developed the skills to control them. With, again, the exception of Nordic jumpers, the people who deal with aerodynamics in the air the most are downhill racers, whose general goal is to minimize aerodynamic effects, as they're more likely to be destructive rather than helpful (e.g. they tend to make the skier pitch up so he lands on his tails). You do see skydivers doing various aerodynamic tricks -- sticking out a hand to start a spin, for example. They, however, are moving at something in the neighborhood of 200 mph. Generally not something recommended for people to do while airborne (or on the ground, for that matter), unless they have special equipment (like a parachute, for example).

Gravity, obviously, changes your momentum ... which is a good thing, since otherwise when someone went off a jump he'd never come back down. It does it in a highly predictable way, the result of which is that (putting aerodynamics and air resistance to the side) the center of mass of everything that's thrown into the air unequipped with a jet or a rocket* travels along a parabola until it runs into something (be it the ground or a tree).

Angular momentum also does not change, absent air resistance. A clever athlete who's already spinning (divers and skaters do this) can increase the rate of revolution by pulling mass in toward his center of mass. Angular momentum still doesn't change. You cannot stop a spin that exists when you take off, or create a new one. You can, however, rotate part of your body, so long as you rotate another part the other way.

*Even then, you're just fiddling with the ordinary results by pushing air (a jet) or some of your own original mass (a rocket) away from you in a really emphatic fashion.
Quote:
 Originally Posted by Lonnie Can ANYONE give any other way for the skis to re-direct as shown from 2:39-2:41
I don't see any "redirection" in that video. His rotation -- and there's not very much of it, actually -- was established when he went into the air.
Again this is all about the intent, intent on the original posters question and intent on the skier in the video.

The mass of the skier can be influenced in the air with judicious use of limbs to alter the initial trajectory if that is what is needed, admittedly not by large amounts.

The angle of the skis and the direction of the skis can significantly be altered in the air.

If the mass of the object requiring change/alteration is lower than the main structures mass, then it can be moved with little effect on the main mass or the overall trajectory. i.e. moving arms or legs including skis can effectively be manipulated, using the overall mass of the body as a lever, the main mass will also maintain it's original trajectory as long as the changes are smooth.

This allows the skis, in the air, to be rotated as and when required, to make adjustments using, primarily rotation as the steering mechanism.
Quote:
 Originally Posted by Lonnie So, if anticipation will unwind (rotate) the skis via muscular tension while they are in the air, won't it do the same thing while they are on the snow.
Absolutely!!

And if you watch the PMTS BPST demos, the skis are in fact pivoting on the snow when the speed is slow enough and the radius is small enough. Though there have been a few enthusiastic PMTS followers that try to claim PMTS sking does not EVER pivot on the snow, that is not an absolute rule. I don't believe the PMTS books make this claim either. Certain kinds of turns yes, other kinds of turns no.

PMTS skiers do not ALWAYS have to pivot on the snow. In fact most of what PMTS skiers strive for all the time is to reduce or eliminate the pivoting to an absolute minimum, mainly by not trying to do it, but rather trying to avoid it. Anticipation is present in an extremely short radius turn for anyone that is keeping their upper body facing down the falline (including PMTS skiers). In a longer radius turn, anticipation is less present and will have less of an effect on pivoting the skis. Medium to long radius PMTS turns do not maintain the upper body down the fallline and hence do not use Anticipation and instead rely on arc-to-arc skiing fundamentals, which do NOT include skis pivoting on the snow in any form at all. That is probably what you are thinking of. However, they also teach a short radius method, called the BPST and it very well may include pivoting on the snow.

Quote:
 How is that NOT a rotary turn? The legs are (un)rotating to shape the turn. Does how we build that tension matter? It might. Convince me.
Well its easy to get lost in translation on this topic. I prefer to use the word "pivot" when referring to what the ski is doing on the snow, and the word "rotary" to what you're doing with your legs to make it happen.

PMTS skiers are not ever taught to actively rotate their legs. They are not taught to use muscle activated movements to achieve ski pivoting as an end result. They are taught other ways to do it, if and when they do it at all.

Now do their femurs rotate in the hip sockets as they are trying to avoid active rotary? Of course! But they think about rotating their legs about as much as I think about rotating my legs when I'm walking down the street.

The BPST is their approach to making short radius turns, including the possibility of ski pivoting. Anticipation is present. Active leg rotary is not. Anticipation is definitely different then using your muscles to twist the legs/skis. Anticipation allows a passive pivot to happen. Hence you often hear people trying to use the words active rotary or passive rotary, etc..and then the confusion begins as everyone debates what active and passive means to them and whether there is even a difference.

You will never ever ever hear a PMTS instructor teach a PMTS student to twist their legs to pivot the skis. NEVER. Perhaps you may claim that all PMTS skiers everywhere are doing it anyway on their own even though their PMTS teacher not only avoided talking about it, but went to great lengths to demonize it and gave them drills to block it from happening, but that is a pretty wild speculation. The reality is that if and when PMTS skiers obtain ski pivoting, it is a more passive form that is derived from anticipation and the effects of gravity and inertia. There is definitely a huge difference between anticipation-caused pivoting and muscle-leg-twisting-based pivoting.

Quote:
 Again see above. I think this gets back to Unsean's arguement. Does it matter if they are thinking "Turn your feet" or not if that's what happening? Can ANYONE give any other way for the skis to re-direct as shown from 2:39-2:41 without a muscluar (un)twisting of the legs?
Yes it can matter a whole lot. If for no other reason, one is less tiring then another. There are actually more advantages than that, but I'd never be able to prove it to you in writing.
Quote:
 Originally Posted by sjjohnston You cannot stop a spin that exists when you take off, or create a new one. You can, however, rotate part of your body, so long as you rotate another part the other way.
Jump straight up off the floor. Spin 180 in the air. Land facing the other direction. How did you do it?
Quote:
 Originally Posted by Lonnie Jump straight up off the floor. Spin 180 in the air. Land facing the other direction. How did you do it?
Better experiment is to jump straight up with the explicit direction that you can't start spinning until after your feet are completely in the air.

The reason you can do a 180 that way that you are describing is because before your feet leave the ground they are pushing you in a rotary fashion, or if you setup some anticipation before your feet leave the ground, that can make it happen to. perhaps a combination.
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