 Poster: A snowHead
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Not sure if this topic has been covered before, but it's an interesting one. Much of what people attribute to end do the turn "REBOUND", can actually be a result of the Virtual Bump. It's important to not only understand it exists, but to also know how to harness it's power, for purposes of use or avoidance. Have a look at the following link for an explanation, video clip and drawing of what the Virtual Bump is, then feel free to discuss. All questions or comments are welcome.
http://www.yourskicoach.com/SkiGlossary/Virtual_Bump.html
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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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kevindonkleywood, great observations. No, the air in I get in the video is not solely the result of the virtual bump. Rebound is a contributor. That is proved by the fact that turns that end parallel to the falline, as opposed to an angle to it, as in my video, can also result in spontaneous unweighting. The important point for people to understand is that while it's commonly thought rebound the reason skis launch off the snow at the end of a turn, the virtual bump is generally also a contributor.
And you make another astute observation in noticing that my skis are still on edge as the disengage from the snow. It's generally thought that a turn is concluded by tipping the skis off edge, but that's clearly not the case here, is it?
One might think that means I must have contributed to the conclusion of the turn by manually unweighting my skis is some manner, such as an up move. That's not the case either.
What I did was something called "oversteering". The term comes from something you do on a bicycle or motorcycle to facilitate a turn. Basically, it's done by turning aggressively in the opposite direction you want the new turn to go. This causes the bike or skis to cut sharply under your body, which automatically leaves you tipped into the new turn.
In the video I do it by simple adding more edge angle, via knee angulation, through the bottom portion of the turn. Look close, and you can see my outside knee tip the ski up onto a higher edge at the end of the turn. What then happens is the skis cut under my body. Eventually they cut so far under, my body is no longer able to pressure them. When that happens, they disengage and stop turning. The stoppage of turning causes the Virtual Bump effect to kick in, while the sudden loss of pressure causes my skis to snap out of reverse camber. Both contribute to my launch into the air.
So absolutely, the air you see is a combination of rebound and the Virtual Bump. The oversteer actually contributes to the influence of the Virtual Bump on the launching you see, because it sharpens the end of the turn, in essence steepening the bump. Make sense?
The above point about how sharpening the turn at the end enhances the effect of the virtual bump also supports your first contention about how a long traverse before new turn engagement would actually reduce it. In your example, the bulk of the virtual bump effect would be felt as you finish the small radius portion of the turn and enter the traverse portion. That's where the most significant gradient change will occur.
Thanks, for your great feedback, kevindonkleywood. It took a pretty good level of technical understanding and thought to come out with, so it required a response of a corresponding level. For those of you who are reading this and are thinking, "Oh great, 2 posts and my head is already hurting", please don't abandon ship. Ask your questions and we'll gladly step the depth of the conversation back a notch.
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You need to Login to know who's really who.
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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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blockhead, do you mean you don't understand the "virtual bump" or you don't understand the question and answer?
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You'll need to Register first of course.
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FastMan, is there any sound on that video? I'm not getting any. Ron Lemaster looks quite hard at the "virtual bump" too - I might go and read that again, try to understand more than I did the previous time, then come back to this thread.
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FastMan,
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| What I did was something called "oversteering". |
I think for the UK the term would be 'counter-steering' if applied to Bikes.
I'd be interested to see a longer video sequence if you have one.
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blockhead, sorry, I knew it was getting a bit deep, but to answer that question in a reasonable length post I had to make some assumptions that kevindonkleywood would understand the terminology and concepts I was talking about, or ask me specific follow up questions. Fire a question to me and I'll answer it in a more down to earth manner.
Nope, pam w, no voice on that clip. The text below is meant to fill that roll.
david@mediacopy, yes, I've seen it referred to as both, and I think counter-steering is the more universally understood term. I should have used it. Thanks.
I don't have a longer clip readily available. Give me some time and I'll see what I can dig up. I'm on the slopes over the next several days.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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Rick, I think there are a couple of statements there that require a bit more examination. (and sorry blockhead, this is going to continue racking up the geek-quotient even higher  - but no-one would expect anything else from me  ). First off though, I agree with the "virtual bump" concept as described in that link, but am not convinced by some of the follow-up stuff.
| kevindonkleywood wrote: |
| I would have thought that the 'air' seen in the video is not solely a result of the virtual bump but a combination of releasing the energy stored in the bent skis as they are carving the turn and the deliberate movement of the skier to unweight them to transition the edges? |
and
| FastMan wrote: |
| One might think that means I must have contributed to the conclusion of the turn by manually unweighting my skis is some manner, such as an up move. |
What I see towards the end of the turn is a definite compression of the body - your waist folds a bit, your shoulders come forward, and particularly your right knee rises just as you bring your left hand into your body. I guess you are absorbing the buildup of pressure at the end of the turn there, along with adding that bit of angulation for the "oversteer". You then - as your left hand moves away from your body - undo those movements, with what I would expect to be a strong ILE movement on your right leg, and your upper body rises. This then carried through to the air at the end of the bump. Essentially what we are seeing there is an underdamped spring/shock-absorber: the impulse from the ILE is enough to keep your body rising as the slope falls away - and you've only just got past the point at which the skis are flat to the snow at the point the vid reverses. So yes I do think it is "unweighting", but as a consequence of the type of transition you are using, rather than a deliberate "unweight" move. Were you to do a more "cross-through"/OLR rather than "cross-over"/ILE move, the skis would stay on the deck.
Secondly,
| FastMan wrote: |
| The stoppage of turning causes the Virtual Bump effect to kick in |
. Hmmm? As we're looking at things pretty closely here, that needs picking up on. The change in slope is due to the change in our angle of passage across it. Yes, straight down gives us the full slope angle and straight across gives 0 degrees (and for any sad enough to be bothered, if you make an angle of "traverse angle" from the fall line, and the drop rate - i.e. sin(effective slope angle) - varies as cos(traverse angle)). So the bump, i.e. variation in effective slope angle, only occurs when turning is happening, i.e. as soon as you stop turning you get back onto a constant slope gradient. The virtual bump "unweighting" only happens as we turn back down the slope and reduce that traverse angle.
So actually by that argument, getting the air requires a bit more care in considering the virtual bump. When your skis leave the ground you are still on the uphill edges, so still turning into the hill, and so still reducing your effective slope angle - i.e. you are still on the rising edge of the "bump". You can't start increasing that angle until you get back onto the snow, as you'll carry on in a straight line while in the air. But what you are doing is increasing the radius of the turn, and so reducing the rate of change of the effective slope angle - you're still on the rising face of the bump, and not even started flattening off yet, but it's no longer getting any sharper.
Another important thing is happening though, as that radius of the curve is increasing, and that is that the acceleration needed to keep you in the curve is reducing, which you experience as a lower centrifugal force. Hence you have to reduce your leg pressure to match or you end up pushing your body away from your skis, and so you get that sudden extension - which as you have started your crossover transition into the new turn, bounces you into the air - and I think that is what we see in the vid. And this is all to do with the dynamics of turning, and actually nothing to do with the "virtual bump" concept at all.
Also, could I suggest that there's a bit of back-seatedness there, as the skis very definitely come off the ground tip first and tails are quite dropped while in the air? No doubt that was entirely deliberate of course .
Now where did I put that shiny new tin hat I got for Christmas? .
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thats ok the other fella done my head in already
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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videos speak louder than words ,so that i might understand it 
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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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virtual bump concept. It was developed in the 1950s by an Austrain Professor of Physics who was commissed by the Austrain ski team to help them improve their technqiue. The result of his work became known as "Austrian Turns" or "Downunweighting" or "Aval ma" or "Bend and Stretch". All mean roughly the same thing.
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You know it makes sense.
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boy im on a roll now.give that shiny new tin back 
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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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blockhead, You are just stiring LOL,
I think a discussion on the merits of straight skis and narrow stance whilst wearing a nevica one piece in fluro pink and yellow may follow
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Poster: A snowHead
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LOL
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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GrahamN,
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sin(effective slope angle) - varies as cos(traverse angle)). So the bump, i.e. variation in effective slope angle, only occurs when turning is happening
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Does it therfore follow that:
And turning can only occur whilst the skis are in contact with the snow becaue the momentum of the skier will carry them in a straight line if there is no snow contact.
From this I would suggest that the unweighting effect of the virtual bump can never on its own be sufficent to cause the skis to leave the surface.
This is because at the point at which your skis unweight to zero the skis will tangent the curve and because they are now running a tangent the effect of the virtual bump is reduced to zero and for this reason the effect of the unweighting of the virtual bump can never give you a negative weighting on the ski (i.e a net upwards force)
The unweighting may however be sufficent to trigger the rebound of the skis to release the energy stored in the flex. It is therefore the tightness of the turn (more stored flex) and the speed of the release of this energy will determine the magnitude of the upwards force hence short fast turns will give you more flex in the skis and a faster virtual bump unweighting so a faster release of the energy of the flexed skis
(please note my physics A level was a long time ago and if i recall correctly it was not such a good grade either  )
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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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blockhead, the hop check is not what FastMan is performing, His body/legs look to stiffen slightly as he approaches the end of the carve with his mass having moved back over the tails (relatively) to maximise the kinetic store and release in the skis. He then stays in the back seat and taut muscled not letting the skis drop to follow the surface contour and uses the 'air' time and the spring from the skis to both cross-under and set the opposite edge and to rotate forward to set his mass into the front of the skis on landing. If his legs were relaxed through to complete the transition he would lose some of the exit velocity and also extend the distance to the next turn initiation. This is very similar to a boarder ollieing(sp) out of a toe carve (not so easy heelside).
It's a lot of fun to do but a booger on the Quads.
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You need to Login to know who's really who.
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GrahamN, well thought out post. But dang ya, it carries the potential get us off to the land of deep again. I'll try to KISS in this response.
First, there is no ILE in my transition. Go frame by frame in my clip, using your right arrow on your keyboard (something my clips allow), through the last several frames just before I launch. Watch the inside leg, and the outside ski. Prior to unloading of the outside ski there is almost no extension of the inside leg. It played no role in the pop into the air you see. There are other factors involved. Those factors are the effect of the virtual bump, combined with rebound. If you can't see it, you may just have to trust me on that, as I am the skier in the clip, and I know exactly what I did to execute that transition and facilitate that result. The extension you see comes immediately after I begin to loose contact with the snow, and is done in an attempt to quickly reach for the snow to reengage after the pivot I'm about to execute.
On the surface it would seem to make sense that for the virtual bump concept to work as described, the skier would have to first begin the new turn, to begin heading down the back side of the bump. In reality, the sensation of lightness at the top of the bump is most experienced as the old turn ends. Here's why. As you go through the turn, pressure is growing. Tthe forces the skier is exposed to (centrifugal and gravity) are coming into alignment and combining forces. The moment the edges disengage and the turn comes to an end, centrifugal forces go away, and the skier senses a feeling of lightness. Combine that with the rebound that occurs at the end of a dynamic turn, and the skier can actually launch right off the snow. That launching happens in my clip, and it can also be seen happening in an almost identical manner in the photo Ron LeMaster includes on page 39 of SKIER'S EDGE book, which he uses to show the effect of crossing the virtual bump.
Interestingly, and a bit contrary to the concept, once the new turn is begun the sensation of the Virtual Bump goes away, because the re-appearance of centrifugal force causes that feeling of lightness through the transition to go away.
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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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[quote="kevindonkleywood"]GrahamN,
| Quote: |
The unweighting may however be sufficent to trigger the rebound of the skis to release the energy stored in the flex. It is therefore the tightness of the turn (more stored flex) and the speed of the release of this energy will determine the magnitude of the upwards force hence short fast turns will give you more flex in the skis and a faster virtual bump unweighting so a faster release of the energy of the flexed skis
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Exactly, kevin. Though my personal theory about rebound is that there's also body mass rebound involved, beyond that which comes from the bent ski.
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You'll need to Register first of course.
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| Masque wrote: |
blockhead, the hop check is not what FastMan is performing, His body/legs look to stiffen slightly as he approaches the end of the carve with his mass having moved back over the tails (relatively) to maximise the kinetic store and release in the skis. He then stays in the back seat and taut muscled not letting the skis drop to follow the surface contour and uses the 'air' time and the spring from the skis to both cross-under and set the opposite edge and to rotate forward to set his mass into the front of the skis on landing. If his legs were relaxed through to complete the transition he would lose some of the exit velocity and also extend the distance to the next turn initiation. This is very similar to a boarder ollieing(sp) out of a toe carve (not so easy heelside).
It's a lot of fun to do but a booger on the Quads. |
Well done, Masque.
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Unweighting = reduced pressure under skiers feet when CoM accelleration up is stopped or CoM is dropping down
Maybe you don't have to loose the weight.
The ski will only bend to a maximum amount without damage or breaking loose from the snow. F=kx is the force required to depress the spring to it's maximum distance x with spring constant k. F= m(V^2)/r. At a given mass we can achieve that with higher V and smaller r. With higher M, we need less v or not so small an R, F=M(v^2)/R. If we are in the turn of constant radius, no real difference in the acceleration and forces; the centripetal forces are in balance with the spring forces. However if we suddenly reduce the radius to make use of the spring, our acceleration out of the spring is given by F/M or F/m. Clearly a skier who has his ski flexed to the max and finds a way to use the stored energy will get more POP if he has less mass.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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blockhead, Nah you've been dragged in now, you can never escape....
...your life is ruined
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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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hell yea im already getting a nervous tick 
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Try this, blockhead. The virtual bump effect will cause you to feel light at the end of a turn. The faster you go, and the sharper you turn, the lighter you'll feel. In very dynamic turns it can become something the skier must control, so they can minimize the amount of time they're disengaged from the snow. The longer they're disengaged, the longer it takes to get the next turn started.
That's the crux of what the student needs to know about the virtual bump, beyond learning the methods of controlling it, or using it. Once they're skiing at the level they're actually noticing it, they'll know it's real, you don't have to convince them of it's existence, so most of the above conversation is never needed.
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You know it makes sense.
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FastMan, you quote ( most of the above conversation is never needed).
now you tell me I think i need a drink
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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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blockhead, Neither am I but it's fun to f'wit them
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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dig that man
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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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| blockhead wrote: |
FastMan, you quote ( most of the above conversation is never needed).
now you tell me I think i need a drink |
Next time you decide to read one of my threads, perhaps have the drink first!
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You need to Login to know who's really who.
You need to Login to know who's really who.
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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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Seconds out - round two
I agree with your explanation of how the forces involved can interact etc, but having looked at that frame-by-frame, am now even less convinced that the vid demonstrates what you want - or were engineering - it to. The forward motion part of the clip is frames 1 to 29. Your ski tips leave the ground in frame 23, and the interesting part is from frames 11 to that point. In frame 11 you make a small change to the angle of attack of the inside ski, possibly abetted by the outside ski hitting a small bobble in the snow (which started in frame 9), which results in a pronounced divergence of the skis in frame 12 (now what would v8 have to say about that ). The additional work the inside ski is doing is shown by the greater plume of snow coming from under the full length of that ski in frame 13. Your outer ski doesn't actually get back parallel to it until frame 15, after that bit of extra knee angulation. The fact that you are now well into the transition is shown by your body starting to rise by frame 15 (when your left hand starts moving out from the body), which then of course continues until 23. Both skis are still engaged (throwing up plumes of snow) until at least frame 20, and possibly 21, so I'm not sure I get your point about the skis disengaging (as here they disengage because you're leaving the snow, rather than you leave the snow because they've disengaged) I certainly don't buy for a minute the assertion that taking away a centrifugal force associated with the ski turning allows gravity to launch you off the slope surface - if you were asserting that a centrifugal force around an axis defined by the ski - associated with a change in angulation - allowed that then I'd agree, as that's another way of putting my analysis of what we're seeing. OK, I'm happy to accept that there's little leg extension between 15 and 23, but there's been the exact same desired result of ILE - the tipping of the body towards the outside of the old turn - by not allowing that inside leg to shorten as you tightened the end of the turn (frames 11-15), and by frame 17 it's well on that new up-and-over vector (your body is caused to rise at the very least by that inside leg not shortening as the shin becomes more perpendicular to the snow surface). It's that up-and-over motion of the body that's causing you to leave the snow, and I'm now convinced more then ever that it's nothing to do with the change in traverse angle across the slope at all.
Masque has berated us before about treating a skier as a rigid point body, and not allowing for the relative motion of the skis and the skier, but in this case I'm darned sure this is the cause here. And I think what you are calling "body rebound" is the same as I was referring to when talking about a lightly-damped spring: when you compress the body your muscles resist that force, then when the force is released, the muscles take a bit of while to reduce their push and overdo the extension. (Actually it's probably models as a lightly-damped spring less than it does as a highpass filter with feedthrough).
So yes, we agree there's an unweighting effect due to the transition. But I don't believe your vid shows it having anything to do with the slope/traverse angles, which was your original premise. Yes I do think there is an effect due to the slope angle (and I have experienced it plenty of times myself. If we consider a series of turns as clock faces stacked such that 5 on one face touches 11 on the next (and 7 onto 1 for the next pair), keeping the pressure on at about 10 and 2 can be tough (and the key to a good carve), which is where the slope dependent virtual bump effect will apply. I just don't think that vid demonstrates it, as it's showing the effect at about half-past-four. And looking at the LeMaster picture, I think he shows exactly the same as you do - and the caption under the picture just does not agree with what the picture shows. "Despite this skier's flexing through the first three frames" - well no, you can clearly see the knees and ankles are straighter in frame 3 than 2, and 2 than 1. His head also rises pretty much as much from 1 to 2 as 2 to 3. Yes he's leaving the ground, but because he's allowing his hips and body to rise as he crosses over.
If the virtual bump were due to the change in traverse angle across the slope, then there would be no unweighting effect if you were making dynamic turns on a flat slope. And you must know even better than do I that there's plenty of that - and it's only too easy to demonstrate. The effect is just not as pronounced as on a steeper slope as you have none of the gravitational enhancement you talk about above.
kevindonkleywood (post of 20:33). Para 1, yes (except for the fact that "straight line" is actually a parabola pointing at the centre of the earth (ignoring, for the pedants among us, wind resistance, coriolis forces....). Para 3 - don't think so. By tangent, I assume do you mean tangent at the point of inflection in the skier's path. I think you are forgetting momentum here. All that's required for the skis to leave the ground (in a rigid point body ) is for the curvature of the path to require an acceleration greater than that due to gravity. With a bit of maths it's too late to do now it's probably fairly simple to work out what radius of turn would cause such to happen for a given slope and traverse angle (which equation would probably be something like r = v*v/g * f(slope angle, traverse angle) ).
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You'll need to Register first of course.
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GrahamN, the missing element in your attempted evaluation of what you see in my clip is the lateral imbalance I purposely create by the knee angulation I add through the end of the turn. It causes the skis to cut under my body, and my body to release down the hill. The CM is penduluming over the downhill leg, not the uphill leg. There comes a point when the force vector has moved so far outside the base of support that the load my body mass is imposing on my downhill ski begins to drop, eventually being eliminated totally. The disengagement process follows in unison. As the load drops, my downhill ski starts to rise off the snow, evidence of the immediately proceeding start of the disengagement process.
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GrahamN, Well yes, in this case there is a momentary point where 'rigidity' occurs. but that is transitory at the end of rearward mass transfer and used to complete the tail loading and to provide an inception point for the cross-under. The waist compression you see after that allows the torso momentum to continue in a steeper path than the ski's tracks and combined with a taut core gives the air clearance for the skis to move laterally into a new track and to land directly into a balanced carve.
You guys are trying over complicate a relatively simple input/reaction/response manoeuvre. Abeit one that does require the participant to recognise and control both their body and their equipment.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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I think this thread qualifies for the geekiest thread of the year award, and will still be in the running on 31/12/11!! It may even rival 'inner ski tip lead'. I think it just wooshes over heads of folks like me, but my sole comment on the basis of the viideo footage posted above is that, if nothing else, it looks rather 'energetic' and I wouldn't think sustained skiing doing it could be carried out for any length of time unless you were very fit. I think I'm glad it won't be bothering me just yet
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Megamum, at the end of a turn there is a build up of pressure (you feel heavy). At the start of a turn there is considerably less pressure (you feel light). If you link your turns well the release of the high pressure at the end of a turn linked to low pressure at the start of the next turn can lead to a little pop, in much the same way as you feel when you go over a bump. If you are travelling reasonably quickly, link your turns well and have good timing that little pop can make you airborne. Simples, as meerkats everywhere would say. I've never thought of it as a complicated thing, but after reading this thread I'm beginning to loose the will to live.
If you want to play with it you must (a) link your turns well, (b) allow the pressure to build up at the end of the turn (ie 'complete the turn') and (c) have a well timed transition from one turn to the next with no hesitation. It helps if you can keep on your skis' edges, as a skidded turn makes it difficult to get the level of 'pop' that you're looking for.
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FAQ
's argument 