Poster: A snowHead
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Mathematically, I am certainly out of my depth! Fortunately skiing is done on/in snow and not on paper
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Sideshow_Bob wrote: |
If you set two skis parallel to each other two feet apart at the same angle with the same pressure applied and apply no rotational/turning forces, they'll turn the same radius of curve, which will result in the skis' paths converging. |
Feck. That's why my tips cross constantly in a RR carve then.
Hint: That's why this topic of managing inside ski lead is so important.
Last edited by Obviously A snowHead isn't a real person on Wed 16-04-08 20:15; edited 1 time in total
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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Martin Bell wrote: |
Would I recommend the mathematical approach to passing instructor exams? No, just regurgitate whatever technical buzz-words are in fashion that season in that country, and try to copy the skiing style of your examiner, based on whichever national association you're taking the exam with... |
a fine nugget of wisdom right there
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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veeeight wrote: |
Feck. That's why my tips cross constantly in a RR carve then.
Hint: That's why this topic of managing inside ski lead is so important. |
Well, we all know parallel railroad turns are possible in the real world. Here are some:
http://easylink.playstream.com/daignault/carving2arailroad.wmv
So what is actually happening?
The radius of the inside ski is probably being made tighter than that of the outside ski, through added shovel flex, caused by extra shin/cuff pressure and snow compression. (On an infinitely hard surface, like in PM's model, this would not be possible). I think this is what david@mediacopy and V8 have been alluding to. Correct me if I'm wrong guys
There is also probably some minute diverging and converging of the skis, as alluded to by SS Bob.
There is occasionally slightly more edge angle on the inside shin, causing a fractionally "bow-legged" stance. (Mentioned by david@mediacopy although he doesn't "feel" it in his skiing.) This is so small as to be almost imperceptible, but remember these are very shallow arcs, tiny fractions of the full circles, so the difference does not need to be very large.
All in all, good reminders that railroad turns are an artificial drill that has little to do with the way skis actually work on the snow. IMO it's more a way of getting pupils to show body awareness and learn to focus more on the inside leg.
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Martin Bell wrote: |
In the end, these little "thought experiments" are diverting but often bear little relation to reality. I was chuckling, over lunch, imagining a young BASI trainee being told to do railroad turns with parallel shins and parallel skis, and saying "but sir, that's mathematically impossible". I can imagine the answer he would get from some wizened old Scottish BASI trainer - it would probably contain plenty of the kind of words that you could hear late at night in the "Winkie"! |
Even I would shut up and try to do what he asks, and I'm sure through a small amount of turning/steering the inside ski and trying to keep the inside edge as close as damnit as angulated as the outside, and with an almost imperceptible diverging of the skis and rotating, I'd be able to ski what would be considered a railroad turn. I've probably got some video of me skiing 'railroad' turns.
However when we get to a biomechanical level we have to look at the forces involved and acknowledge that it's more subtle than just parking and riding using the sidecut and angle and all parallel shins/feet same width apart throughout the turn and no steering/rotating. There is some steering/rotating of the inside ski. There has to be to get it to track a tighter radius curve. The only argument I have with v8 is that this inside ski is tracking a tighter turn, hence tighter radius than the outer ski.
david@mediacopy: for wide radius turns, the ski is probably being steered or pressured by loading certain points into making a tighter radius curve, along with some small amount of slipping and diverging. As the turn gets tighter and tighter relative to the length of the ski and width of the feet, there has to be more steering/slipping.
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veeeight, might I suggest that if you ever discover you are teaching people with a very high level of mathematical/scientific education, you find some means of improving their skiing without recourse to geometry or mechanics? I'm absolutely sure there's loads you could teach me about skiing, but it bugs people like me (and obviously GrahamN and sideshow bob) when you keep banging a drum in support a proposition that is provably wrong. This is not the first time.
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veeeight wrote: |
Sideshow_Bob wrote: |
If you set two skis parallel to each other two feet apart at the same angle with the same pressure applied and apply no rotational/turning forces, they'll turn the same radius of curve, which will result in the skis' paths converging. |
Feck. That's why my tips cross constantly in a RR carve then.
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They don't cross because they're doing different radius of turns because you'll be applying turning/rotating forces on the inside ski! You seem to have a problem with the concept of where the centre of the turn actually is.
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Wow, this has gotten a wee bit technical....
The interesting bit to me is that we all know that we can ski RR tracks, but "on paper" we argue against that possibility. For me I think it is possible due to the constant dynamic changes that are happening to the skis edge angle, camber and inner shovel flex due to pressure that Martin Bell, refers to along with angulation and the skis relationship to the terrain and the curves they transcribe in the snow. I certainly feel the skis "load up" when i am carving short radius turns and i think this helps keeps the two skis parallel.
interesting to think deeply about this though...
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laundryman, I frequently do, and have the advantage that on snow I can disprove their paper calculations. But I take your point, but conversely, none of you have proven yet how it is that you can overlay L&R ski tracks and they are identical in RR turns?
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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skimottaret wrote: |
interesting to think deeply about this though... |
Yes, although this thread should come with free aspirin to anyone who makes it through to the end
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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Quote: |
You seem to have a problem with the concept of where the centre of the turn actually is.
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That's because there is no static single position of a centre, and this is throwing you off!
The "centre" is constantly moving with the arc and skis, therefore you will NEVER get the situation where alleged two circles with a fixed centre point path crosses!
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veeeight wrote: |
laundryman, I frequently do, and have the advantage that on snow I can disprove their paper calculations. But I take your point, but conversely, none of you have proven yet how it is that you can overlay L&R ski tracks and they are identical in RR turns? |
You most definitely cannot overlay left and right (or inside and outside) ski tracks in a railroad turn and have them identical. What on earth gives you that idea?
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You know it makes sense.
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$100. I'm sure someone at the EoSB will comply.
Last edited by You know it makes sense. on Wed 16-04-08 21:06; edited 1 time in total
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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Poster: A snowHead
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Err, V8, just so I'm sure what you're saying, you think you can overlay inside and outside tracks, where the tracks are always a fixed width apart throughout the turn? Are you sure this is what you mean?
Ok, here's a test for you. Run down a nursery slope and do a nice easy 90 degree railroad turn to the right.
Now, go back up the slope. You should be able to see your tracks, right? Ski down the same set of tracks, but where your inside ski was the first run, put your outside ski. After all, they're the same turn, you can overlay them, right?
Now, go back up again and repeat. Make sure each time you're doing that same 90 degree turn, and each time, the outside ski tracks exactly the same line as the inside ski from the previous run. How many times can you do this? Think about it!
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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Fascinating stuff, no really it is!
We've heard the mathematician, the instructor and the racer's view. So now it's my turn, the engineer
Firstly I agree with those who say the inside ski must be following a tighter radius than the outside ski, however small that difference might be over a 15-20 m radius. I think the mathematics of that have been proved beyond reasonable doubt.
I would also put strong money on the resultant tracks having a moving centre, rather than a fixed point. I can't draw a perfect circle freehand and I know I can't ski a perfect 15 m circle either!
It's also fair to say that the vertical load/pressure acting on each ski varies through the turn due to the load transfer from lateral acceleration. I don't really buy into the 50/50 weight thing, it's got to be more like 70/30 or 60/40 at best when you're generating significant lateral acceleration. I know you can adjust your centre of mass to affect the exact load transfer, but I suspect less than you really think you can.
Skis have a sidecut radius which also varies with the amount of camber flex. The more heavily loaded outside ski will flex more into reverse camber in the middle of the turn, effectively reducing its turning radius relative to the inside ski. But this is couterproductive for carving our perfect circle, because the outside ski is now following a tighter arc than the inside ski.
So I'm guessing in the real world some compensation must be being made to "fudge" the inside ski into following a tighter arc than it naturally wants to. This may come from a combination of power steering from your inside leg (Warren Smith style) and minimising the lateral load transfer i.e. aiming for as close to 50/50 as you can get, or better still slightly more weight on the inside ski.
I've no idea how this relates to inside tip lead, other than from the inside ski following a shorter path through the turn. That's what it feels like to me anyway and I try to minimise it to prevent my outside ski dragging behind at the end of the turn. A good instructor made me aware of this and it seemed to help my skiing, which is what counts in the end.
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Sideshow_Bob wrote: |
Ok, here's a test for you. Run down a nursery slope and do a nice easy 90 degree railroad turn to the right.
Now, go back up the slope. You should be able to see your tracks, right? Ski down the same set of tracks, but where your inside ski was the first run, put your outside ski. After all, they're the same turn, you can overlay them, right?
Now, go back up again and repeat. Make sure each time you're doing that same 90 degree turn, and each time, the outside ski tracks exactly the same line as the inside ski from the previous run. How many times can you do this? Think about it! |
Infinite times. I just keep starting the turn higher up the slope. This diagram is produced by drawing one arc, then copy and pasting each subsequent inner arc. These arcs all are identical.
Last edited by You need to Login to know who's really who. on Wed 16-04-08 21:28; edited 1 time in total
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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Martin Bell wrote: |
The radius of the inside ski is probably being made tighter than that of the outside ski, through added shovel flex, caused by extra shin/cuff pressure and snow compression. |
Quote: |
There is occasionally slightly more edge angle on the inside shin, causing a fractionally "bow-legged" stance. |
I'm of the view that the diameter is probably the same.
I don't think that skiers have to edge\tilt the inside ski more than the outside ski in order to leave 2 clean tracks. This is sort of confirmed by the calls for 'parallel' shins in race training (not that I'm any sort of racer.... )
I also don't think that there is more pressure\bend of the inside ski than the outside ski, certainly underfoot.
Sideshow_Bob mentions
Quote: |
for wide radius turns, the ski is probably being steered or pressured by loading certain points into making a tighter radius curve, along with some small amount of slipping and diverging. |
but if there is steering, slipping or diverging, the track would not be clean / carved.
Quote: |
As the turn gets tighter and tighter relative to the length of the ski and width of the feet, there has to be more steering/slipping. |
For sure, you get to a point where it is impossible to 'carve' and steering/pivoting/turning movements take over.
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uktrailmonster, david@mediacopy, I am avoiding the mention of the premis that since the dominant outside ski has a higher loading on it, it therefore bends more (which is a good assumption to start with) and the inner ski has to be fudged. You're almost there, except that I know you can ski two clean tracks with a dominant outside ski, and not have to fudge or pivot or slip the inside ski to get it to match, this is a whole new can of worms that I don't want to open (yet).
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veeeight wrote: |
except that I know you can ski two clean tracks with a dominant outside ski, and not have to fudge or pivot or slip the inside ski to get it to match, this is a whole new can of worms that I don't want to open (yet). |
From an engineering standpoint, it's extremely hard to propose another alternative to a good old bit of "fudging". You have to remember that the required fudge is very small and perhaps not measurable in the real world. I mean what is the radius difference likely to be in absolute terms? A tenth of booger all!
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The "fudging" difference in radius also gets less and less as you make the turn radius larger, to the point where you are straightlining and don't have to "fudge" at all
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veeeight wrote: |
Sideshow_Bob wrote: |
Ok, here's a test for you. Run down a nursery slope and do a nice easy 90 degree railroad turn to the right.
Now, go back up the slope. You should be able to see your tracks, right? Ski down the same set of tracks, but where your inside ski was the first run, put your outside ski. After all, they're the same turn, you can overlay them, right?
Now, go back up again and repeat. Make sure each time you're doing that same 90 degree turn, and each time, the outside ski tracks exactly the same line as the inside ski from the previous run. How many times can you do this? Think about it! |
Infinite times. I just keep starting the turn higher up the slope. This diagram is produced by drawing one arc, then copy and pasting each subsequent inner arc. These arcs all are identical.
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But your feet are not parallel, and you've definitely got a lot of tip-lead, outside tip lead!, which then becomes inside tip lead at the end of the turn. Most curious. Let's also look at the start of the turn, if you're skiing with your feet parallel and level, how come the inside ski starts turning a lot earlier than the outside ski?
Now do the same with a 180 degree turn. Can you? The fact your feet are so far apart (front/back) at beginning and end of the turn suggests to me you can't.
Last edited by You'll get to see more forums and be part of the best ski club on the net. on Wed 16-04-08 22:04; edited 3 times in total
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double-post, sorry
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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I've lost the will to live.
That was a freehand sketch that too all of 8 seconds in total.
I will carve a J turn or bananna turn back up the hill. And repeat inside ad infinitum. With parallel clean edged tracks. This is a standard drill for racers.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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veeeight wrote: |
That was a freehand sketch that too all of 8 seconds in total. |
And you so nearly got it right! All you needed to do was to start and end the turn with your feet parallel and no tip lead. That is after all what you've been preaching, right?
If we add lines in to show the feet starting and ending parallel with no tip lean either direction, it suddenly becomes clear that this is a diagram of scissoring! Let's look at the lower-most turn. The inside foot (towards the bottom) starts to turn immediately. As we go further up, we find that the outer foot progressively starts to turn after the inside foot, and finishes the turn before the inside foot.
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You know it makes sense.
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Sideshow_Bob, that's only half a turn, do the other half
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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Poster: A snowHead
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Actually if you look at Cuche and Grandi very carefully, there is a miniscule amount of ski divergence at the beginning of some of their turns.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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comprex wrote: |
Sideshow_Bob, that's only half a turn, do the other half |
What, a 180 degree turn? Has to be a different radius of curve. You can't copy-paste a curve that goes over 180 degrees and move it relative to the first so the start point of the curves are parallel without it overlapping.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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Martin Bell wrote: |
Actually if you look at Cuche and Grandi very carefully, there is a miniscule amount of ski divergence at the beginning of some of their turns. |
But not inside ski divergence.
You are absolutely right, there is divergence, only on some of their turns, freeze frame it and you'll see that it's the outside ski running wide as a result of of them moving to the inside a little too quickly
Don't you start throwing red herrings in, Martin (and forgive me for appearing to take you to task on this)
Here's one such turn from Grandi in that video that shows an apparent ski divergence (in this case it's the outside ski running wide)
Grandi starts his turn totally on his inside ski (ala White Pass). Outside ski diverging, pointing to the outside.
Grandi gets more inclined, and is pretty well rotated into the turn. Still balancing predominantly on his inside ski.
Starting to engage the outside ski and build pressure in the fall line. So at this stage if you take this snapshot alone, it looks like the inside ski is diverging into the turn, but as we have the pictures above, we know that actually, it was the outside ski running wide.
At last! Both skis hooked up and carving well.
Maybe I should find some pretty instructor skiing with just the lower joints
Last edited by Well, the person's real but it's just a made up name, see? on Wed 16-04-08 23:51; edited 3 times in total
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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I am reluctant to dip my toe in this water, but FWIW but think its right and helpful that we should have these discussions.
I have been trying this out on snow today.
I was wearing my "big" skis which are quite stiff- when carving long rads if I want to get sufficient pressure on the outer ski to vary the radius through the turn I have apply way more pressure to it than the inner one say 80/20. So given that my feet were not broken (I do tend to have a lead of about 10cm in practice) , and I had equal edge set on both skis, then the radius of the outer ski would be significantly smaller than the inner one due to the extra pressure. If the engineers and math guys were right it would have resulted in the inner ski tending to go straight on and the outer ski crossing over it and me ending up in hospital, I am pleased to say that this did not happen. I was not consciously at least, applying any pivot to the inner ski, they just run parallel.
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jbob, Good on your for trying it out for real.
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veeeight wrote: |
Don't you start throwing red herrings in, Martin |
Is it a red herring or something more significant?
We already have equations from a physics professor that seem to disprove that a "pure" carved turn (parallel skis, constant separation, parallel shins) can be done in theory. Yet we have seen it done in practice.
Therefore, something somewhere must, indeed, be getting "fudged". The question is what. Ski divergence and convergence (i.e. non-parallel tracks or "scissoring") may be one of those "fudges". Although no longer a standard feature of turns like it was in the 1980s, divergence is still seen, quite frequently:
http://www.ronlemaster.com/images/2006-2007-B/slides/kelley-aspen-2006-gs.html (frame eight)
http://www.ronlemaster.com/images/2006-2007-B/slides/ligety-aare-2006-gs-1.html (frames 2 and 13)
http://www.ronlemaster.com/images/2006-2007-B/slides/matt-bc-2006-sl-1.html (frame 3)
http://www.ronlemaster.com/images/2006-2007-B/slides/paerson-aare-2006-sl-2.html (frame 5)
http://www.ronlemaster.com/images/2006-2007-B/slides/raich-aare-2006-gs-2.html (frame 7)
http://www.ronlemaster.com/images/2006-2007-B/slides/zettel-aspen-2006-gs-2A.html (frames 5 and eight)
http://www.ronlemaster.com/images/2006-2007/slides/hosp-aspen-2006-sl-2.html (frame 1)
http://www.ronlemaster.com/images/2006-2007/slides/poutiainen-aspen-2006-gs-1A.html (frame 3)
http://www.ronlemaster.com/images/2006-2007/slides/schld-aspen-2006-sl-2.html (frame 5)
http://www.ronlemaster.com/images/2006-2007/slides/zettel-aspen-2006-gs-1.html (frames 5 and 11)
http://www.ronlemaster.com/images/2006-2007/slides/schlopy-bc-2006-gs-1.html (frames 8 and 9)
http://www.ronlemaster.com/images/2005-2006/slides/ligety-bc-2005-sl-1a-flat.html (frame 4)
http://www.ronlemaster.com/images/2005-2006/slides/paerson-aare-2006-sl-2-web.html (frame 5)
http://www.ronlemaster.com/images/2005-2006/slides/rahlves-bc-2005-gs-2-web.html (frame 9)
http://www.ronlemaster.com/images/2005-2006/slides/zettel-aspen-2005-sl-2-web.html (frame 4)
Not to mention converging skis:
http://www.ronlemaster.com/images/2006-2007-B/slides/stiegler-aare-2006-sl-2.html (frame 5)
And how about both:
http://www.ronlemaster.com/images/2006-2007/slides/schild-aare-2006-sl-2.html (frames 5 and 6)
I realise that "you can use World Cup sequences to show anything" but when you see something occur as frequently as this, you start to wonder. Is every one of those instances down to a mistake by the racer, every time? I'm not so sure...
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I received a severe . . . and to be honest, a well deserved bollocking(sic) for being lead footed earlier in this thread. OK I let a bad experience outside the forum influence my mood in posting, but underneath my mood there was content. I'm sitting here reading people describing a turn as a fixed geometric path. that would be fine if we skied on a white billiard table at a constant incline with skis that had fixed turn radii.
Approaching this from an un-tutored and quite ignorant viewpoint I know that skis (or a board for that matter) have a side-cut that has a radius measured in metres . . . however that side-cut also enables the tip and tail of the ski when inclined to engage with the surface allowing the centre of the ski under the foot/centre of pressure to bend and form an arc where the entire effective edge of the ski is in contact with the surface. Depending on the torsional strength, flexation stiffness, speed, incline, weight and skill of the rider, the turn radius of the ski is shortened*
*here's a question for you. A ski's stated turn radius, is it the side-cut dimension with the ski at rest or when the ski is dynamically stressed to maximum deflection?
Since the human body is homogeneous and unable to split into separate halves over skis to have two centres of rotation . . . which the diagrams above clearly imply, then for two tracks to be truly parallel through a carved turn they have to have slightly different radii . How that is achieved is clearly only achievable through differing loadings being placed into the skis through the turn.
The outside ski (in a speedy carved turn) seems to be the preferred dominant ski with the greatest force put into it then the inside ski must be used differently to keep it in a parallel track. The inside ski is less weighted than the outer it must be at a more acute angle to the surface or to be forward of the downhill ski so that the tip is subject to greater deflection enabling the ski to bend in a tighter arc without needing or supporting the skier's weight.
So is "Inner Ski Tip Lead" an essential part of carving parallel 'tram tracks' and all the angst above is just about tip-lead produced by other factors of body position and skiing confidence and in reality is NOT a problem in its own right but just a symptom of other skiing issues that when corrected will eliminate undesired poor foot position?
I have ski boots with virtually no flex, somewhere about the 150 mark on the SMALLRODENTMOLESTER's scale . . and that's before I lock them off . Because of that, when I'm cranked over in a short radius carve being driven on my downhill big-toe, my inside leg has no option but to move significantly forward, advancing the tip and geometrically increasing its angle to the surface keeping the skis in a parallel track . . . otherwise I'll be turning on my inside leg and far less stable with my outer ski having nothing but my leg strength or ligaments to stop it flapping in the breeze as it cannot physically reach the snow surface.
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You'll get to see more forums and be part of the best ski club on the net.
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MB, I would dispute that scissoring, convergence, and divergence is intent in modern technique or tactics, if trying to ski arc to arc turns. None of us here coach it, we do everything possible to eliminate it, we avoid patterning it. (and I think you're just jerking my chain here )
I will agree though, that at speeds and performance going beyond recreational arc to arc skiing, my inside ski plays less of a role in the overall steering effort, and in most cases is going along for the ride, matching my dominant outside ski.
But as you yourself know, outcome when you're going balls to the wall is a different matter. Comeon, do you really want me to MA all those sequences to explain the outcomes?
If people really want to use WC Racers as a model for technique we probably should be looking primarily at their practice runs rather than when they're going for broke in a real race. There's efficient skiing and then there's winning. Sometimes the 2 meet.
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