Megamum, the "lift" you get by skiing up the virtual bump actually makes the transition into the next turn easier (up to a point - hence the "use or avoid" comment in the OP).
edit - ah I see rob got there first
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Yoda, Well I think Rob wins the prize for goggledegook explained. With his and your help I now understand the concept - I think a thread that is accessible to all of us (at all levels of understanding is more interesting) - nothing wrong with the KISS principle IMV. Whether my skiing is good enough to experiment with it yet I'm not so certain of - so far I have avoided anything that takes both skis of the ground at the same time! It looks like the sort of turn you sometimes see the ski racers pop-in on ski-Sunday.
Last edited by Obviously A snowHead isn't a real person on Mon 3-01-11 20:16; edited 1 time in total
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?
Megamum, I think that Rob's point about going
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fairly quick
is important here because unless you have either a steep slope, some decent speed or both then the magnitude of what you feel will be fairly small compared to the other forces you are balancing against.
I do think that people should have more stamina its only just page 2 but if we feed it and care for it properly it will grow and flourish
It looks like the sort of turn you sometimes see the ski racers pop-in on ski-Sunday.
In general they will try to avoid it (by absorbing the virtual bump) because if your skis are off the snow you are not able to steer them towards the next gate.
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
kevindonkleywood wrote:
I do think that people should have more stamina its only just page 2 but if we feed it and care for it properly it will grow and flourish
rob@rar, Megamum, I wonder. Given FastMan suggests that the air happens at the fall line, this is at odds with 'finishing' the turn.
FastMan,
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That is proved by the fact that turns that end parallel to the falline
That said, this sounds reasonable and echo's the teachings of my first Coach.
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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.
kevindonkleywood, my contribution of more 'fertilizer' to ensure future growth !
Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
FastMan wrote:
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.
I did acknowledge that you were adding knee angulation (which of course cases a cut under), but that is in addition to the move on the inside ski which occurs before you make the change in the radius of the outside ski path. Both contribute to the pedulum effect - what I referred to in a more "physicsy" context (as the physics is the basis of this discussion) as "a centrifugal force around an axis defined by the ski - associated with a change in angulation". I should have said that I agree that "The CM is penduluming over the downhill leg, not the uphill leg". But for the purposes of this discussion which leg is initiating the pendulum/up-and-over-rotation motion and which leg it's centred on is completely irrelevant; the important thing is that you're keeping a pretty constant lower body position as that rotation happens, so causing the hips to rise. Having now reread LeMaster on the subject (p75), he lists both this (which he calls the "pole valut" effect) and the effects I've been listing as a "lightly/under-damped spring" as components of "rebound". Note that that spring effect does not require the compressing force to reduce before it kicks in, it's just our muscles reacting to the increased pressure at the end of the turn. I'm now absolutely certain it's this rebound - and particularly the "pole valut" component of it, which has nothing to do with the virtual bump - that causes the ski to leave the snow.
kevindonkleywoof, miaouw!. Actually centrifugal force does exist, both in a intertial frame of reference (possibly considered a mathematical construct, but highly relevant in many cases as that percieved by a skier) and also as the reaction force by the ski on the snow, in response to the centripetal force of the snow on the ski (Newton's Third Law) - but yes there are occasions where it would have been more correct to use the term centripetal. (Oh, and - miaouw....prrrrrr - it's centripetal not centripedal daaaaahling; the only person who could possibly exert a centripedal force is Jake the Peg...diddle-iddle-iddle-um ). No argument at all with the bit you "made up" (well, the 2nd half of the para anyway), as that's exactly what I've been saying from the beginning of this thread.
Having done the maths now on accelerations around a curved path, I (shock horror) now agree with your conclusion that the "virtual bump" in itself cannot possibly cause the ski (our mythical rigid point body) to leave the snow. There are two components to an accelerating curved path - the change in the velocity tangential to the curve (i.e. along the direction the skier is travelling at any given moment), and the radial component (v^2/r) perpendicular to its path. If the surface is flat (albeit inclined) that radial acceleration is necessarily along the surface, irrespective of whether the ski is turning or not and whatever the angle of slope or traverse. It is provided by the forces associated with the ski edges gripping the snow. The tangential acceleration (as the skier speeds up or slows down through the curves) is also necessarily along the snow surface - and this is provided by gravity and the changing traverse angle. There is never any component of acceleration perpendicular to the surface, and so the ski never leaves the snow - by this mechanism without the assistance of anything else. Where the "virtual bump" does come in is to generate the "spring" component of that rebound effect - heavily loading the skier's legs and then getting a pop as the muscles rebound (and note, Masque et. al., that LeMaster - p76 - also specifically dismisses the rebound of an unweighting ski as being a significant contributor to the effect, and I understand others who have looked into it closely do so also).
My problem with the virtual bump concept is that, like most analogies, it is nearly always pushed further than its validity. In this case it's the assertion that the bump itself can get the skis to leave the snow - as the vid is attempting to demonstrate and that LeMaster also sort of asserts. I'm now 100% sure that this is not the case. What is happening is that we are getting rebound comprising muscular compression and reaction as the skis meet the "front face" of the "virtual bump - which is all about the skier's respose to pressure changes - and the "pole vault" effect of the body rotating about the axis of the skis - which has nothing whatever to do with the "virtual bump". What I'm now certain of though, is that the lightening on the "rear face" does nothing to cause the skis to leave the ground, as it may do in a real bump. LeMaster (p65) talks about having to flex to absorb the front side of the bump and extend to keep in contact with the snow on the "downside". Yes, you may wish to flex/extend to minimise the "pole vault" effect (which remember has noting to do with the "bump") by keeping the hips a constant distance from the snow. The consequences actually attributable to the virtual bump are really just a requirement on muscle reaction control to minimise (or harness) the muscular rebound, and pressure management to minimise the chances of the ski breaking away at the end of the turn (and to provide an active drive at the start of the next turn) - which are probably more easily understood in terms of resisting the hill and pushing the body down the hill rather than any bump analogy.
If this is what FastMan is now saying, as in his responses to me he's given explanations using the same concepts of rebound, and force vector sums as I and many other have used, then we have no disagreement. The OP and his webpage though make very different arguments. He even points to the failing in his thesis regarding the "downhill" part of the bump in his 2nd Jan 21:01 post. I'm afraid that's because the physics simply does not support the analogy he's trying to make.
After all it is free
After all it is free
Note to self: Run spell check
GrahamN, I sadly do not live in a matematical construct or inertial reference frame Tis an Inaginary force I say
Being as skillfull with maths as I am with spelling I instead clipped in and spent a couple of hours on dendex playing with the idea on the plastic slope here.
Its arguable that the slope was not sufficently steep and the speed was too low, however the one benifit of the plastic was it is totally flat and removed any component of 'terrain shape' from the excercise.
It was possible to 'feel' this imaginary 'bump' (for this i needed fairly fast medium radius turns) but any 'pop' of the skis I experienced whilst playing I felt was due to my clumsiness of movement into the transition. Which I think is in line iwth your
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a requirement on muscle reaction control to minimise (or harness) the muscular rebound, and pressure management to minimise the chances of the ski breaking away
(It is entirely possible that I was able to 'feel' the 'bump' in the same way people feel a 'ghostly chill' when put in a room and told its haunted but after a few runs I had convinced myself that there was a discernable effect.)
I am surprised that the rebound of the ski is dismissed so universally as it is fairly widely taught as something that assists in gaining 'air' off a kicker. You specifically look to load the ski to get the highest degree of flex prior to the lip.
It is certianally something that is used as a marketing 'feature on frestyle skis, and of course it must be true if its being used to market ski gear
but thinking about that in light of these discussions I can see that the biggest component is actually muscle rebound and correct body management to maximise the momentum of the skiiers mass.
Now what is the feeling about the accepatble degree of inner tip lead for this?
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.
GrahamN, Very nice and completely plausible except for one thing. I'm a heavy old hector and I cannot jump vertically from standing and get my hips more than 2.5 feet off the ground yet I can ollie my board to near 3.5 (at standstill). You cannot dismiss to insignificance the stored energy in a pair of bent skis. Nor can you ignore the lack of extension in FastMan's legs as he rises over the "virtual" bump. He is not jumping, he is simply using his body mass to retain compression in the tails of the skis, holding them in the carve track and riding the bounce. Why do you all think this is something that can't be done without writing a new chapter for the BASI manual. It's not rocket science it's JUST an 'ollie' with a bit of body 'english' to land in the right place and position. There is a difference between letting a ski unweight and utilising the stored energy to a purpose.
Ski the Net with snowHeads
Ski the Net with snowHeads
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It's not rocket science
I have to correct you there, it is newtonian (classical) mechanics, so it is Rocket science
runs and hides behind wall of sandbags
EDIT P.S. I blame Rick he started this....
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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I cannot jump vertically from standing and get my hips more than 2.5 feet off the ground yet I can ollie my board to near 3.5 (at standstill).
Masque, that's comparing apples and pears, isn't it? Hard to believe that, from a standing start, you couldn't do a boardless "ollie" of similar impressive dimensions.
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.
That's another fine mess you've gotten us into Ollie!
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
Yoda wrote:
Megamum, the "lift" you get by skiing up the virtual bump actually makes the transition into the next turn easier (up to a point - hence the "use or avoid" comment in the OP).
When the virtual bump happens to co-incide with a real bump, the results can be bad:
GrahamN, I sadly do not live in a matematical construct or inertial reference frame
Ironically, the transformation to turn a set of turns into a bump run is exactly that - transformation from an external into an internal/intertial frame of reference (or at least partially). :lightbulb: Ooh - that may be exactly the missing bit to resolve this "unweighting" issue! The transofrmation so far has only involved considering the path of the skis down the mountain, changing the effective slope angle by changing the traverse angle. The "rebound" effects have been considered as something external that happens, and just requires management. If the "pole-vault" effect arising from changes in angulation (and I really think it's a misnomer to call that "rebound" in the case of skiing - this arises when there's no/limited extension/flexion in the legs so not much to rebound from) are also taken into that intertial frame (so we include inclincation angle as well as traverse angle), it may well be that there's a much closer correspondence between the turn dynamics and the "virtual bump" idea. This would also cover the examples I gave earlier about getting pop when making dynamic turns across a flat (no external slope to even out, but it rocks all over the place once you start referencing to an angulating body - just look at headcam footage from someone with poor upper-lower body separation). Food for thought, but even more seriously geeky ( surely not ) and the maths would get signficantly more confusing than it has so far, so I doubt it's going to be particularly useful going too far down that route.
Regarding your experiments Kevin, just to be clear, I'm not arguing against the idea that changing the traverse angle changes the effective slope angle, and so you feel heaviness/lightness when making dynamic turns - which is the virtual bump idea at its most basic level - just that the analogy runs if you take it any further than that most wishy-washy, hand-waving level. As a result of thinking about the accelerations though, it turns out that the one area in which there is a pretty good analogy is that of tangential acceleration - movement along the direction the skis are travelling. You get faster if you point directly down the slope than if across it, and you get faster down the back of a roller than up its face. Wow.....who'd a thunk it!
Masque, we all have our little hobby-horses, and from what I've seen you post before, energy storage and "ollies" are one of yours. I'm agnostic on the subject, and (you may want to re-read what I've written) was merely reporting that others, including LeMaster, do dismiss it as a significant contributor. I might say though that, beyond a bit of hand waving and the odd anecdote, you've not done anything to prove your point. My own view is that actually that, if what you assert is true, I wouldn't be surprised if the major effect is that you are using the board as a lever allowing you to get an extended duration of push, and so a greater impulse and height in the jump, i.e. it allows a better transfer of energy from your legs - so yes, there is a bit of energy storage as evidenced by the springiness of the board but that's still not hugely significant and certainly not enough to lift you another 1' off the ground....but that's all just off the top of my head. It's also a completely different thread. Your reading is also a bit defective if you think I "ignore the lack of extension of FastMan's legs", as I've already accepted that there's little of that going on...twice. Finally, in my first post I also commented on the back-seated position when the skis rise - and so, yes, your "ollie" similarity is a valid point, in that the fronts of the skis are also possibly being levered off the snow as the balance point moves backwards, but again that's not primarily energy storage/release - and certainly nothing to do with the virtual bump concept as initially proposed.
Poster: A snowHead
Poster: A snowHead
[no longer available]
Last edited by Poster: A snowHead on Sun 3-02-13 12:51; edited 1 time in total
It's only apparently real if you straighten out a skier's sinusoidal track. First let's look at the track as alternating pressure.
Imagine the skier resisting left and right at angles of inclination to the surface of the slope. Draw out the pressure wave graph of that and you get an alternating pressure that looks like a cross-section of a series of rollers down the slope.
What
I think Masque must have written it!
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?
If it doesn't sound complicated how are they supposed to appear knowledgable? Basically as you turn towards the fall line you go down the bump and as you turn back away you go up it.
A more complicated explanation is that as you turn down the slope you are increasing the downhill angle you are skiing down until it matches the angle of the slope. As you turn back uphill you are decreasing that angle.
The feeling of the bump is essentially created by how the skier resists gravity.
FAQ
Poster: A snowHead
its only just page 2 
I instead clipped in and spent a couple of hours on dendex playing with the idea on the plastic slope here.
but after a few runs I had convinced myself that there was a discernable effect.)
surely not 
