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Pivot point under foot - Page 4

post #91 of 104
Thread Starter 

If I have a camera mounted directly above a foot and always pointing down the slope. Do you think it can see the rotation difference if I hold the skis in the tip and swing them or if I hold them in the tails? You are not allowed to look at the snow.

 

Obviously there is a difference between a "proper pivot" and a heel push, but the way of measuring that is not given by the pivot point under foot definition.

I think that pivot point under foot draws a picture in peoples minds, but if you analyse it in more detail it is not as simple as that.

post #92 of 104
Quote:
Originally Posted by Jamt View Post
 

If I have a camera mounted directly above a foot and always pointing down the slope. Do you think it can see the rotation difference if I hold the skis in the tip and swing them or if I hold them in the tails? You are not allowed to look at the snow.

 

 

I dont know what you are on about.  Of course that wont work.  I told JASP and you that earlier.  I pointed out in every single post, the frame of refrence needs to include the entire skier.  Ie we are effectivley looking at the rotation of the leg/foot relative to the body. 

 

 

 

Quote:
Originally Posted by Jamt View Post

 

Obviously there is a difference between a "proper pivot" and a heel push, but the way of measuring that is not given by the pivot point under foot definition.

I think that pivot point under foot draws a picture in peoples minds, but if you analyse it in more detail it is not as simple as that.

 

Sure, we can make things very complicated no doubt.  But I'll ask again....where is the beef? ie what value is there to be gained from your approach? Why in the world would you measure it???? With what?  A protractor?  What value is there in that?

 

Its just pivoting.:dunno 

post #93 of 104
Thread Starter 
Quote:
Originally Posted by Skidude72 View Post
 

 

I dont know what you are on about.  Of course that wont work.  I told JASP and you that earlier.  I pointed out in every single post, the frame of refrence needs to be the entire skier.  Ie we are effectivley looking at the rotation of the leg/foot relative to the body. 

 

 

Then why do you keep using "relative to the feet". And for the third time, the entire skier is not a FoR.

Quote:

 Originally Posted by Skidude72 View Post
 

 

Sure, we can make things very complicated no doubt.  But I'll ask again....where is the beef? ie what value is there to be gained from your approach? Why in the world would you measure it???? With what?  A protractor?  What value is there in that?

 

Its just pivoting.:dunno 

As I said I don't have an approach, I am just trying to understand the value or the "pivot point under foot" approach.

If you say the skier is not pivoting under foot you have measured.

 

We are going in circles and I don't think you are trying to understand.

 

If you cannot even define what pivot point under foot is why would you even use it?

post #94 of 104
Quote:

Originally Posted by Jamt View Post
 

Then why do you keep using "relative to the feet". And for the third time, the entire skier is not a FoR.

 

Dont recall writing "relative to the feet", where did I ever write that? Once?  Let along "keep using"????

 

Ok, the skier is not the FoR..bad phraseology....the FoR needs to include the entire skier...better?  Same difference, I think everyone, including you, know what I meant.

 

 

 

 

 

Quote:

Originally Posted by Jamt View Post
 

 

 

As I said I don't have an approach, I am just trying to understand the value or the "pivot point under foot" approach.

If you say the skier is not pivoting under foot you have measured.

 

We are going in circles and I don't think you are trying to understand.

 

If you cannot even define what pivot point under foot is why would you even use it?

If you dont pivot under foot...you have a heel push...which you seem to understand is not desireable. 

 

Defining it is easy...I can stand there and show people...turn the leg in the hip socket, and the ski tip moves "in" across the skiers body...and the tail goes "out" away from the skiers body.  The same move happens when we do a pivot slip, or a race style stivot...its easy, simple, and clear.

 

 

I have taught thousands of people this, taught hundreds of instructors on sessions this - and frankly, you are first person, who didnt get it. 

post #95 of 104

Play nice, Skidude.  

Your points will carry more weight when not gummed up with disrespect for other people.

post #96 of 104
SD, I think jamt is not disagreeing with practical pedagogy related to pivoting and how to teach it. Pretty much everyone on this forum is a deep thinker that thinks about this stuff way more then most of our peers. I think he is just trying to dig a little deeper into the details of what is actually going on in ski turns with regards to pivoting, and the truth is it may not be so black and white as the common pedagogy would make it seem. I think there is value in exploring this deeper.

Related to a few points, I guess it was JASP who said the FoR is the foot. Maybe? The OP certainly posed that question. As JAMT tried to point out if the FoR is the foot then all ski turns pivot around the foot even grossly heel pushed ones. If you mounted a camera on top a boot, that is what you would see, you would only see varrying degrees of it.

Using the whole skier as a frame of reference is not valid because the whole skier flexes and extends and twists and bends and basically has many possible frames of reference at once. You have to be more specific. JASP tried to narrow it down to the foot but that is not really very interesting because it does not distinguish any difference between heel pushes and other types of pivots that feel to us more like a real pivot. Clearly we can feel the difference when we do them, so can't we try to define it and explain it more precisely? There can be learning in the nuances too!

Using the skier's head as a FoR is one option or perhaps the skier's CoM but the exact location of the CoM at any given instant is not the same, it can be outside the body even. ItI can be rather complex to think about how the skis would be pivoting relative to the CoM and almost certainly the skis are not pivoting around that FoR like a helicopter except under very isolated circumstances like perhaps Ric's pivot slips.

How about the hip socket? That seems to be the joint that many are using to drive or at least a mobilize pivoting. The hip socket is hard to analyze because it provides movement in 360 degrees. Perhaps you can say that what you feel in the hip socket may tell you a bit, but I think also the complete sensation we have is a mixture of things related to what you feel there, leg extension that may be combined with femur rotary to create a heel push, etc....

And skis also pivot on their own due to self steering effects as they interact with the snow. When they do that what is the FoR? If you say the foot, then all turns including those with 100% femur rotation, those with 0% femur rotation and those heel pushes and all hybrid forms all would always show to be pivoting under the foot.

There is more going on, and even though that simple mental model may work most of the time from a pedagogical perspective, the devil is in the details. It can't be taken too literally to the point of losing site of details which might matter.

Perhaps the FoR could be the turn shape on the snow, how the tip and tail pivot away from the theoretical arc'd line that would only exist if they were arcing, which they aren't if they are pivoting. Or some other complex point on the snow related to momentum that would be damn near impossible to explain in a lesson, but nonetheless quite relevant while creating an overall comprehensive pedagogy.

I think the concept of pivot around the foot is bogus. That always happens and tells you nothing. Pivoting around the whole skier is also bogus, that simply is not specific enough.
post #97 of 104
Actually a camera on top of the boot would always show zero pivoting, so the hip socket is where you would have to mount a Camera to show how the ski pivots relative to the body.

But the more I think about it, what matters the most is what the ski is doing on the snow, and there are a variety of ways to get the skis to turn and pivot on the snow, with and without femur rotary even happening, so really the bottom line is what is happening on the snow.
post #98 of 104
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Edited by justanotherskipro - 11/22/13 at 9:28am
post #99 of 104

I agree. The obsession with frames of reference and staying true to them seems to just make things complicated. It is extremely difficult to understand, and so far seems 95% pointless if the point is to make someone understand.

It's like the force discussions, they quickly dissolve into overcomplicated messes that only a handful understand. I'd love to see someone make it understandable, but so far not so much, since it definitely requires diagrams.

post #100 of 104
Well I think the implicit question put forth in the thread is whether perhaps there is an obsession about pivoting around the foot. You think it's obsessive to bring out facts of physics and try to discuss them, yet others may feel its obsessive to keep harping about pivoting under the foot when in fact the physics of good ski turns may not actually dictate that, though you seem to prefer to remain ignorant of the details, but to each their own.

JASP, if you try to use a point along the ski, including under the foot or ahead/behind the foot, that is a bit like the camera mounted on your boot. Zero pivoting occurs from that FoR. You have to include an external thing that is not rotating, like the hip if you are using femur rotation, or the snow if not. Snow works to femur rotation too and I now contend that is the bottom line.

I really think you would define the pivoting as what the tip an tail do relative to the line you are traveling on. If the tip moves inside of that line by an equal amount of the tail fanning out, then perhaps it would on that axis you are talking about. If your tails are windshield wipering on the snow, then that axis is more forward. Perhaps carving is closer to the tail. There are so many different things that can contribute to these outcomes and it is interesting to discuss.

But I suspect we will never reach agreement about what movements cause what outcomes. Some of you feel that twisting your legs like a pivot slip will ensure some kind of even steered turn where you sense an axis of rotation under your foot. Others of us contend with equal passion that twisting the legs in a turn will likely cause windshield wipers as an outcome. For me personally that is why I don't even like to talk about pivoting the skis at the feet because it inspires the twisting movements. Yes we can say that tails fanning out is not great. Carving is perhaps an axis near the tail and some kind of steered turn would be with the axis as an outcome forward of there, but obsessing over trying to force the skis to pivot around the foot with twisting will lead to windshield wiper outcome. I realize some of you don't see it that way and some of us do and there is no more to say about it really. Is there?
post #101 of 104

mod hat on

Thanks BTS. I'd like to see JamT satisfied that he has gotten what he came for out of this thread. I'm ready to close the thread if we can't play nice, but it looks like you folks have done a fair job of self recovery.

mod hat off

post #102 of 104
Thread Starter 
Excellent posts bts. I think i have gotten enough out of it now.
post #103 of 104

Good lord.

I've seen hundreds of threads but this now takes the cake. Bts, I  applaud your willingness to explore this, but it's gotten into the theater of the absurd with this frame of reference business. I'm not sure Einstein dropping balls off a moving train could understand this. Worse, I fail to see the relevance to what people actually do on snow by wondering where the frame of reference is. Hip, ski, body, com, foot, head, blimp.

 

I guess i'd go with the moving blimp, uav, above the skier.

 

Skidude, mellow out.

Could be passive heel slide though! Pivoting around shovel.

What's the issue with the below? Other than it's actively steering? It's pretty simple.

Quote:

Originally Posted by Skidude72 View Post

If you dont pivot under foot...you have a heel push...which you seem to understand is not desireable. 

 

Defining it is easy...I can stand there and show people...turn the leg in the hip socket, and the ski tip moves "in" across the skiers body...and the tail goes "out" away from the skiers body.  The same move happens when we do a pivot slip, or a race style stivot...its easy, simple, and clear.

 

I have taught thousands of people this, taught hundreds of instructors on sessions this ...

 

The Blimp View:

These guys, Cloudgate, have been using UAV's (unmanned aerial vehicles), quad copters, to film stuff. They went to Loveland recently. I think we shall see this on the World Cup possibly this year or the Olympics. Just a guess. With the glacial pace which the IOC works it could be 5 years.

(their vimeo feed seems to have issues playing. Can't get some of the other ones to work.)

 

 

Video by Cloudgate        http://vimeo.com/cloudgate/loveland-slalom

post #104 of 104

Tog, if you can't follow or understand it exactly maybe take some time to ponder and contribute before ridiculing those of us who can.  There is plenty of space on other threads if this one doesn't interest you.  :)

 

There is nothing absurd about frames of reference, that has been discussed hundreds of times on this forum, including by your buddy BB.  Come on Tog, join the fun.

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