or Connect
EpicSki › The Barking Bear Forums › Ski Training and Pro Forums › Ski Instruction & Coaching › The "Virtual" bump.... real or fiction?
New Posts  All Forums:Forum Nav:

The "Virtual" bump.... real or fiction? - Page 8

post #211 of 340
Quote:
Originally Posted by Davey View Post

Hi @Skidude:

 

Quote:
...but you are not really correct, at least in so far as describing VB as it is used in conventional ski coaching.  VB is not created from pressure...its created from the change in "effective slope" as we turn back and forth down an inclined plane.

".. not created from pressure" ?... Wouldn't you need pressure to resist the turning force that is required by Sir Issac's elegant laws of motion?  (No pressure would mean no turning).

The pressures in the left-right sinusoidal turning translate directly into the up-down virtual roller pressure that the skier would experience.

 

I described the inclined plane, and I described the change in effective slope.  Certainly, if a skier was to travel over rollers then changes in pressure would need to be dealt with.

The skier encounters a hill of increasing pressure, followed by a downslope of decreasing pressure - at which point the skier can add some of his own drive to pump the turn.
If it was a bump - the skier would pop.  On the lateral, the skier would not so much pop as shoot forwards (as in the 'accellerative turn' diagram diagram I cribbed from Brodie et. al.)


I don't think you need to contradict anything I wrote in order to add to the debate.


I don't have time to read past this post, so maybe it has already been dealt with, but the "virtual bump" exists in addition to any turning pressures required to cause you to turn.  If you can visualize your tracks on the snow as railroad tracks, then take away the snow and imagine a roller coaster going along on those rails you will see there are turns and "bumps" that are sections of horizontal track between sections of descending tracks (much like the flights of stairs and stair landings you would get if you straightened the line). 

post #212 of 340

To Skidude72:

To follow up some of your comments on VB and Effective Slope.

Quote:
Finally, you mention that VB is a just an idea in our heads...it isnt.  If you turn down an inclined plane the effective slope changes...it has to.  There is no "made up" anything.  Since the rate or amount of change in effective slope is dicated by the line we ski for a given slope...and since we can alter out line, we can alter the bump...hence the bump is said to be "virtual"...unlike a real bump or roller, which is what it is, and we cant change it...at best we can ski around it.  Where as the VB will follow us no matter what...however how big and steep it is, we can manipulate with line and choice of runs we ski.

 

 

 

"Effective Slope" Isn't Made Up, we can agree on that.  (But the 'Virtual Bump' is a concept.  Concepts exist only in the mind.)

 

I prefer to look at the effective slope in terms of what it does to motive force.  Here's the derivation I use.


F = mg.sin(alpha).sin(beta)

 

where alpha is the slope angle on our inclined plane, and beta is what you are calling the 'effective slope angle'.

 

F is force in the direction the ski is pointing at any instant.

 

However, in my analysis, the virtual bump is generated by the resisting of the turning force.  The skier acts on the ski to incline it, and a turning force is generated from the snow.

 

The force is proportional to V squared and also proportional to 1/r where r is the instantaneous radius of turn.  So the Virtual Bump can get huge.

 

Principle of the Conservation of Energy

Now, where I wanted to go with the "Virtual Bump" was to show that we can use it to pump more energy into the system so that you get more work-energy out than would be present simply from the potential energy/ Kinetic energy conservation principle.

 

You quote a Brodie video (which is presented in a rather jokey, entertaining way).  He addresses the principle of energy conservation, I was delighted to see.

The two skiers (he explains) have the same energy at the start, but one of them (orange) puts in more force above gate 6 and wins even though blue was slightly ahead at gate 6.  Brodie explains that the effect of the accelerative turn on gate 6 was delayed.
Now for me, the point above gate 6 where the early pressure was applied was orange using the virtual bump and timing a muscular input in the same phase and direction as the F force we derived earlier.  This "Pumps" the turn.  By Pump, I mean adding muscular energy to augment the conserved energy balance.

 

Downweighting and Down-Unweighting

You (Skidude77) Commented Quote:

... by dedicating a section to try and explain the significance of your theory.  And there you wrote it teaches how slow down or speed up...both by "down unweighting".

(btw, It isn't 'my' theory)

 

"both by "down unweighting"?." Actually no, what I actually wrote was: Quote:

To slow down, the skier could downweight more into the virtual roller on the crossover between the gates. 

(To speed up - pre-jump the virtual bump on the crossover by down-unweighting).

[also]

To speed up, the skier would apply pressure on the down-side of the virtual bump (i.e. before the fall-line).

 

As I think I already mentioned, I don't think you need to contradict anything I wrote in order to add to the debate.
 

Thanks again for your patience.

post #213 of 340

Ghost

Many thanks for your comments.

I hate to quote my entire original (again) so I won't.

 

So you can read the whole thing here:

http://www.skipresto.com/BloggingServices/SkiingBlogs/tabid/105/entryid/21/The-Virtual-Bump-in-a-ski-turn-why-it-exists-and-how-to-use-it.aspx

 

There I cover the straightening of the left-right line and substituting an up-down "bump" or series of rollers down the slope.

 

With your analogy of the roller coaster, we run into the principle of conservation of energy.

But skiers are dynamic - not like the static body on an undulating slope (where gravity can do no extra work into the system, even though things are moving up and down).


I've tried to develop the virtual bump into something the skier can use to kick extra muscle work-energy in to the system, and this allows accelerative turning.

So far, I'm happy with the progress.
Thanks for taking the time to read and comment.

post #214 of 340
Quote:
Originally Posted by Davey View Post

To Skidude72:

To follow up some of your comments on VB and Effective Slope.

 

 

 

"Effective Slope" Isn't Made Up, we can agree on that.  (But the 'Virtual Bump' is a concept.  Concepts exist only in the mind.)

 

I prefer to look at the effective slope in terms of what it does to motive force.  Here's the derivation I use.

F = mg.sin(alpha).sin(beta)

 

where alpha is the slope angle on our inclined plane, and beta is what you are calling the 'effective slope angle'.

 

F is force in the direction the ski is pointing at any instant.

 

However, in my analysis, the virtual bump is generated by the resisting of the turning force.  The skier acts on the ski to incline it, and a turning force is generated from the snow.

 

The force is proportional to V squared and also proportional to 1/r where r is the instantaneous radius of turn.  So the Virtual Bump can get huge.

 

 

Your analysis is understood.  Its just not VB.  Call it something else.  I dont care what.

 

Language and ski teaching systems and created to convey ideas to students, and other instructors.  If you create your own language, and concepts, thats fine...but no-one can communicate with you. 

 

Davey: "Hey come learn about skiing and to ski with me...oh btw, I have my own made up terms and concepts you will need to learn first..."

 

Its not a good way to go.  You are simply describing increasing and decreasing pressure from centrifugal forces...Newton beat you to it.

Quote:
Originally Posted by Davey View Post

 


Principle of the Conservation of Energy

Now, where I wanted to go with the "Virtual Bump" was to show that we can use it to pump more energy into the system so that you get more work-energy out than would be present simply from the potential energy/ Kinetic energy conservation principle.

 

You quote a Brodie video (which is presented in a rather jokey, entertaining way).  He addresses the principle of energy conservation, I was delighted to see.

The two skiers (he explains) have the same energy at the start, but one of them (orange) puts in more force above gate 6 and wins even though blue was slightly ahead at gate 6.  Brodie explains that the effect of the accelerative turn on gate 6 was delayed.
Now for me, the point above gate 6 where the early pressure was applied was orange using the virtual bump and timing a muscular input in the same phase and direction as the F force we derived earlier.  This "Pumps" the turn.  By Pump, I mean adding muscular energy to augment the conserved energy balance.

 

 

I only posted that video, because it was his first that he posted online, and from there you can find the others.  He doesnt really get into pumping in that video.  However he wrote a lenghty and technical paper on the subject, and showed that pumping really only works out of the start gate, etc....but not when moving through the course at "ski speeds".  Pumping is an old idea discussed to death here...it dont work excpet in certain circumstances such as shallow greens up to a low speed. 

 

FWIW - Orange skier was faster because he skied a smoother line, and as a result needed to break less then the blue, who skied a tighter line but had more breaking....its that simple.

 

 

Quote:

 

Downweighting and Down-Unweighting

You (Skidude77) Commented Quote:

(btw, It isn't 'my' theory)

 

"both by "down unweighting"?." Actually no, what I actually wrote was: Quote:

 

As I think I already mentioned, I don't think you need to contradict anything I wrote in order to add to the debate.
 

Thanks again for your patience.

 


What is the differnce between down unweight and down unweigting?  One is a verb, one is noun.....its like saying "I am going for a walk"...and "I am going walking" are different.  They arent.


Edited by Skidude72 - 1/17/13 at 3:22pm
post #215 of 340
Quote:
Originally Posted by Davey View Post

Ghost

Many thanks for your comments.

I hate to quote my entire original (again) so I won't.

 

So you can read the whole thing here:

http://www.skipresto.com/BloggingServices/SkiingBlogs/tabid/105/entryid/21/The-Virtual-Bump-in-a-ski-turn-why-it-exists-and-how-to-use-it.aspx

 

There I cover the straightening of the left-right line and substituting an up-down "bump" or series of rollers down the slope.

 

With your analogy of the roller coaster, we run into the principle of conservation of energy.

But skiers are dynamic - not like the static body on an undulating slope (where gravity can do no extra work into the system, even though things are moving up and down).


I've tried to develop the virtual bump into something the skier can use to kick extra muscle work-energy in to the system, and this allows accelerative turning.

So far, I'm happy with the progress.
Thanks for taking the time to read and comment.

 


I went into my files - here is the full Brodie paper.  3.7.1 discusses "acclerative turning".  Cant "attach"...here it is online:

 

http://www.sportnz.org.nz/Documents/Research/awarded-grants/Brodie%20%282009%29%20Optimisation%20of%20Performance%20in%20Alpine%20Skiing.pdf

 

 

It also discusses your diagram.

 

 

Full thesis here:

 

http://mro.massey.ac.nz/bitstream/handle/10179/1041/02whole.pdf?sequence=2


Edited by Skidude72 - 1/17/13 at 4:10pm
post #216 of 340
Quote:
What is the differnce between down unweight and down unweigting?  One is a verb, one is noun.....its like saying "I am going for a walk"...and "I am going walking" are different.  They arent.

Again, SkiDude77, that is not what was written.. Quote:

 

"both by "down unweighting"?." Actually no, what I actually wrote was: Quote:

To slow down, the skier could downweight more into the virtual roller on the crossover between the gates. 

(To speed up - pre-jump the virtual bump on the crossover by down-unweighting).

 

Downweight: Pressing down with your weight to increase pressure.

 

Down-Unweight: Moving your c.o.m. downwards to obtain an unweighting effect to reduce pressure.

This is Joubert's "Avalement".

post #217 of 340

Skidude77

 

Quote:

I went into my files - here is the full Brodie paper.  3.7.1 discusses "acclerative turning".  Cant "attach"...here it is online:

 

http://www.sportnz.org.nz/Documents/Research/awarded-grants/Brodie%20%282009%29%20Optimisation%20of%20Performance%20in%20Alpine%20Skiing.pdf

It also discusses [the accelerative turn] diagram.

 

Just to say thanks for this Skidude.
I've read the key para and Brodie is describing how to add muscular input to the turn to get extra reaction forces out of the snow.  Here's a short quote and it is pretty much the line I was taking...

 

Quote:
[this turn] demonstrates how the athlete might use ground reaction forces to create
positive power and increase speed through a turn (Figure 3-18). Previously David Lind
hypothesised how the athlete might theoretically do this (Lind & Sanders, 1997) in his book,
“The Physics of Skiing”. Lind compares the athlete’s motion to the motion of a child on a
swing ...
[...]
Athletes can increase speed through additional muscle work when their centre-of-mass and
ski trajectories are diverging.  (‘provided there is little snow resistance and that there is
not too much ski skidding’).

So, It's this muscle-input at the correct timing that Brodie is describing in his accelerative turn.

It's more than just "Orange got a better line and could accelerate due to gravity whilst Blue had to brake".
 

post #218 of 340

Skidude77 wrote

 

Quote:

Your analysis is understood.  Its just not VB.  Call it something else.  I dont care what.

 

Language and ski teaching systems and created to convey ideas to students, and other instructors.  If you create your own language, and concepts, thats fine...but no-one can communicate with you.

OK, how about we agree that Virtual bump is defined by Ron Lemaster in "Ultimate Skiing"?, (and not some thing that Georges Joubert may or may not have written)

(Earlier you wrote :- Quote:

"VB is very well established theory on skiing, it was created Dr. Joubert...a Uni professor and true revoulutionary in development of ski technique from the 50s, 60s and 70s and a key person behind major WC developments."

But then conceded: Quote:

I think you are right, that he never coined the term VB...but VB was the reasoning behind these transition methods.

 

)

post #219 of 340
Quote:
Originally Posted by Davey View Post

Skidude77

 

 

Just to say thanks for this Skidude.
I've read the key para and Brodie is describing how to add muscular input to the turn to get extra reaction forces out of the snow.  Here's a short quote and it is pretty much the line I was taking...

 

So, It's this muscle-input at the correct timing that Brodie is describing in his accelerative turn.

It's more than just "Orange got a better line and could accelerate due to gravity whilst Blue had to brake".
 


True, but you need to read and understand the whole thing....its alot of "might", "maybe" and its "theorectically possible".  But where is the beef?  How do you duplicate it, on every turn...or at least most turns....???????  He clearly states (paraphrasing) - that there is caveat - that it only works if you can do this without skidding etc....in his work, a WC skier only did it on 1 turn, on 1 run.....so while possible ( and we get that), its not really a golden bullet....its more a red herring, particularily for the majority of skiers.

 

Note - Brodie concludes with:

Quote:
Increasing ground reaction forces, however, also increases snow resistance forces, which
always act to reduce speed. The combination of snow resistance work and ground reaction
force work over after the first gate is always negative and there is always far more potential to
loose speed through poor technique and poor timing than to gain speed through additional
muscle work.

 

 

How do you achieve divergence/pumping so it works at least 50% of the time?.....If you want to achieve that...great...if you do...it will make you one of the greats.  I am happy to work with you on it here...it would be a neat exerercise.  But be prepared to be challenged....fluff, wont cut it.

 

 

I think you are jumping to conclusions that even the original authors have not made....


Edited by Skidude72 - 1/18/13 at 12:42am
post #220 of 340
Quote:
Originally Posted by Davey View Post

Skidude77 wrote

 

OK, how about we agree that Virtual bump is defined by Ron Lemaster in "Ultimate Skiing"?, (and not some thing that Georges Joubert may or may not have written)

(Earlier you wrote :- Quote:

But then conceded: Quote:

)


VB definatley comes from Joubert.  Ron himself told me that.  And besides, he first wrote of it in "The Skiers Edge" well before "Ultimate Skiing".

post #221 of 340
Skidude72 Quote:
I am happy to work with you on it here...it would be a neat exerercise.  But be prepared to be challenged....fluff, wont cut it.

Well, I thank you very much. I do appreciate the mentor factor.  I visit the woods of the Barking Bears infrequently, and when I need to develop a line of thinking. Forging new metal requires heat and a lot of hammering into shape.  The finished product is extremely tough and can be sharpened to a keen edge.  (pardon the rather laboured allegory).

I don't think there is any appreciable 'fluff' content so far from me.
 

Originally I was looking for non-resistive speed control

My original objective was to respond to an assertion by my sometime coach three weeks ago that "Speed Control can only be done by friction - doing work into the snow".

Linear

I then researched the F=mg.sin(a).sin(b) which is a quick mathematical check that the ever-changing effective traverse angle b for Beta controls the motive force. So far so good for linear acceleration.  But we can show (as every skier knows) that changing the effective slope angle on to a lesser slope will cause a slowing down.

"Everyone knows" - well what I've found is that there is a misunderstanding that gravity acts down the fall-line, when of course it acts vertically underneath the skier, and it is only when b for Beta is 90 degrees (sin 90 =1) that the actual Force down the fall-line is seen to be F=mg.Sin(a) times 1  where a is the slope angle.

Angular

Then, I moved on to understanding a way of describing what happens in the turn.

When dealing with the theory, most examples are for static bodies acting under gravity alone negotiating constant velocity curves under ideal conditions.  But you have to start somewhere.

Here's a simple animation: 

http://www.physicsclassroom.com/mmedia/kinema/avd.cfm

We can see that the curve is on the flat.  So we need to add another vector to include the gravitational force if our idealised semicircular curve is on a slope.

The vector-sum will be  at some angle between the radial and the forwards.

Conservation principle

How to gain speed.  The limitations of the conservation of energy leads us to understand that on say, a roller-coaster can't gain energy by non-external work done on it when it rises and falls.  There is no perpetual motion machine.  The work-energy equivalence states that the work done in raising the mass to the to of the slope defines the energy in the system.

This is explained nicely by Lind and Sanders on p86 of "The Physics of Skiing"( 2nd edition).

However, as skiers, we can strive to prevent outside opposing forces doing external work on our system.  (we can ski smoothly, we can crouch better etc).

Also, we can actively add energy from muscle-power (poling at the start gate, and applying accelerative forces at the right time and line).

Path of fastest descent

Lind and Sander also bring out the Brachistochrone cycloid on page 131.  This is the mathematical proof of a closed system allowing two bodies to descend a slope one on a longer (initially steeper) faster path and the other on a direct path

http://curvebank.calstatela.edu/brach/brach.htm

(The actual maths is more than we need at the moment).

But really, we want this faster indirect path to be on the outside - not the fall-line side, because on arrival at the gate post, there would be a problem of facing the wrong way.

And this got me to thinking that a steepened "effective slope" of the "virtual Bump" could be brought into play, the skier could get an advantage.

The effective slope only steepens if muscular force is applied in the "uphill quadrants".

 

Setting out

Anyhow.  I want to bring the solutions into focus and to avoid the usual fallacies that come in to block thinking and scatter the truth.

I'm looking to have a robust physics and biomechanical base from which to refer when talking with ski race coaches and aspirant ski teachers, generally a group who would rather ski a black bump field on one ski than be presented with a sine curve or a parabola, let alone the formulae.

I want to have simple principles and not consider too much at the moment how practical they are.  That's a different job.

 

Intuition and simple proofs

Intuitively, we know that the gravitational motive force is strongest in the fall-line.  If we ski with timed pressure applied in that direction down the hill, we'll go faster.
Intuitively we know that we need to ski the optimum smooth line at the optimum speed for our strength (even if that's slow) or else in 3 gates' time we'll be slamming on the brakes or skiing out.

If we can pull in the proof, then we can counter at least the wilder of the beliefs people pick up.

 

Thanks again for taking the time to read and comment.

post #222 of 340
Quote:

Anyhow.  I want to bring the solutions into focus and to avoid the usual fallacies that come in to block thinking and scatter the truth.

I'm looking to have a robust physics and biomechanical base from which to refer when talking with ski race coaches and aspirant ski teachers, generally a group who would rather ski a black bump field on one ski than be presented with a sine curve or a parabola, let alone the formulae.

I want to have simple principles and not consider too much at the moment how practical they are.  That's a different job.

Well, I think you do have a robust physics and biomechanical base.  And I fully agree that while its important for some to understand this stuff, its certainly not nesscary to present it to all...However on your last point, on "not considering too much how practical some things are"...to my way of thinking it would be worth determining the limitations of these ideas, (acclerative turns) if only to clearly define the issues that need to be overcome.

 

Anyway, it would be good to have you stick around here....this has been good.

post #223 of 340

SD, Brodie's paper is phenomenal. If only I could run similar research for my degree. Thank you for posting!

post #224 of 340
Quote:
Originally Posted by Skidude72 View Post


VB definatley comes from Joubert.  Ron himself told me that.  And besides, he first wrote of it in "The Skiers Edge" well before "Ultimate Skiing".


To Skidude72

It's Lemaster's Virtual Bump!  Official.
I have it from a very reliable source that Ron Lemaster was indeed the inventor of the term "The Virtual Bump".  But he was inspired by Joubert's work indirectly.

 

Lemaster On Joubert

Lemaster saw a mound-like shape in his mind's eye whilst reading page 197 of "How to Ski the New French Way" by Georges Joubert.  (Joubert was actually describing the average slope during a turn being less than the actual slope.  He depicted real slope, average slope as straight angled lines, with a sinusoidal graph of instantaneous slope as experienced by the skier.). 

However, that isn't the complete VB.  That was the trigger that got him thinking.  Lemaster developed the Virtual Bump by considering pressure distribution resulting from how centrifugal force experienced by the skier varies during the turn and at the end of the turn, aligns with the gravitational force down the slope.

 

Joubert also (In "Skiing - an art a Technique" - English version) In the small-print of "Documents for Specialists" makes reference to normal (right angles) reaction pressure build-up from the ground making the skier take off vertically. (Ref Page 293 para 439 "Compression and unweighting by extension").  Ron saw a diagram of the locus of the skier's c.o.m. during an "unweighting by vertical extension" (bottom of page 293) and it looked like a bump. 

Joubert also in the same book in Para 436 and 437 writes "..Often [during] an unweighting made after the platform at the beginning of a turn, as during the unweighting produced by a bump ..."

This was the sort of description of pressure that seeded Ron Lemaster's development of the VB concept.

 

Lemaster's Books "The Skier's Edge" and "Ultimate Skiing"

But Ron develops the idea into a tilted frame of reference - not just the vertical.

 

Ron Lemaster describes the "Virtual Bump" in "The Skier's Edge" (1999) Page 37 "The Virtual Bump"

"...In a sharply carved turn on a smooth slope for instance, the skier will feel light at the top of the turn and heavy at the bottom.  This is because the gradient that the skis are on changes through the course of the turn in the same way as it does when you ski off one bump and into another."

[...]

"Another way to look at the same phenomenon..." [is] "The component of gravity that is not opposed by the snow always points down the fall-line.  At the start of the turn, centrifugal force points in the opposite direction, making you feel a bit light.  At the end of the turn the two forces are pointing in the same direction, making you feel heavy. In between, the total force you feel builds up continuously.  This, again is just like skating first into the trough between two bumps, and into another bump at the end of the turn".

In "Ultimate Skiing" (2009) Ron Lemaster develops the Virtual Bump with "Adding to the effect is the fact that when going from one turn to another you go from being inclined to one side of your feet to being inclined the other way.  This action tosses you upwards just as a bump would".

 

In "Ultimate Skiing" Page 42 Ron Lemaster looks at the Virtual Bump from yet another standpoint.  That of the bump being the high-point of the racer's head as the skeleton crosses under.  The height of the skier's shins in a short-leg-short-leg crossunder making the skier's head rise in the transition, as if the skier was negotiating a bump.

 

"The Skier's Edge" has a vector diagram superimposed on a skier progressing through the turn. (Fig. 4.9) Illustrating how the Gravitational force and the Centrifugal force experienced by the skier interact to combine after the fall-line.  (This was omitted from "Ultimate Skiing", and replaced by a single vector diagram on a different photo of a racer.)

Wrapping up

What else is there to add?

The VB is Metaphorical "the skier feels a bump" where there is no real bump.

The VB is Similie "The skier experiences the forces like a bump" where there is no real bump.

The VB is apparent "The skier's head follows a path on the crossunder transition that appears to be a bump.

 

We could perhaps have a "Virtual Kicker" instead of a bump to more exactly describe the launchpad effect of the reaction forces.

In terms of adding force at the critical point in the turn, we can imagine the "downslope" of a "bump" (or the effective downslope) getting steeper as a result of an earlier impulse by a racer getting extra acceleration generated by muscular input.  The metaphor is useful to show the delayed action - and here we get into Brodie territory where his strain gauges and pressure sensors record accelerations in real time.

So - we can't so much use the downside of the bump.  It only exists after a delay.  It steepens as a consequence of an earlier action by the skier.

 

That's about all I have at the moment on "The Virtual Bump".

D

post #225 of 340
Quote:
Originally Posted by Davey View Post


To Skidude72

It's Lemaster's Virtual Bump!  Official.
I have it from a very reliable source that Ron Lemaster was indeed the inventor of the term "The Virtual Bump".  But he was inspired by Joubert's work indirectly.

 

 

Yeah, like I wrote before...I think you are right on that.  But the concept was from Joubert...not "inspired" or "indirect"...it was Joubert's work...its why he said WC skiers should flex in transitions and extend to fall-lines....its why he developed "avalment"....I am not sure who your source is...but mine is Ron himself, told me that in a personal conversation.  I heard it else where also.  Ron might have given it, its own name...but that is it.

 

Quote:
Originally Posted by Davey View Post


To Skidude72

 

Lemaster On Joubert

Lemaster saw a mound-like shape in his mind's eye whilst reading page 197 of "How to Ski the New French Way" by Georges Joubert.  (Joubert was actually describing the average slope during a turn being less than the actual slope.  He depicted real slope, average slope as straight angled lines, with a sinusoidal graph of instantaneous slope as experienced by the skier.). 

However, that isn't the complete VB.  That was the trigger that got him thinking.  Lemaster developed the Virtual Bump by considering pressure distribution resulting from how centrifugal force experienced by the skier varies during the turn and at the end of the turn, aligns with the gravitational force down the slope.

 

Joubert also (In "Skiing - an art a Technique" - English version) In the small-print of "Documents for Specialists" makes reference to normal (right angles) reaction pressure build-up from the ground making the skier take off vertically. (Ref Page 293 para 439 "Compression and unweighting by extension").  Ron saw a diagram of the locus of the skier's c.o.m. during an "unweighting by vertical extension" (bottom of page 293) and it looked like a bump. 

Joubert also in the same book in Para 436 and 437 writes "..Often [during] an unweighting made after the platform at the beginning of a turn, as during the unweighting produced by a bump ..."

This was the sort of description of pressure that seeded Ron Lemaster's development of the VB concept.

 

Where did you get this from?  It seems you are stretching, and combining basic concepts to fit your world view.  Aligning centrifugal forces with gravity is not new, and not part of the VB concept...these are separate ideas.  Yes they combine...but VB also explains fore/aft issues...aligning gravity and turning forces does not.

 

Quote:
Originally Posted by Davey View Post

 

Lemaster's Books "The Skier's Edge" and "Ultimate Skiing"

But Ron develops the idea into a tilted frame of reference - not just the vertical.

 

Ron Lemaster describes the "Virtual Bump" in "The Skier's Edge" (1999) Page 37 "The Virtual Bump"

"...In a sharply carved turn on a smooth slope for instance, the skier will feel light at the top of the turn and heavy at the bottom.  This is because the gradient that the skis are on changes through the course of the turn in the same way as it does when you ski off one bump and into another."

[...]

Yup..that is the VB.  Clean, neat, simple.

 

Quote:
Originally Posted by Davey View Post

 


"Another way to look at the same phenomenon..." [is] "The component of gravity that is not opposed by the snow always points down the fall-line.  At the start of the turn, centrifugal force points in the opposite direction, making you feel a bit light.  At the end of the turn the two forces are pointing in the same direction, making you feel heavy. In between, the total force you feel builds up continuously.  This, again is just like skating first into the trough between two bumps, and into another bump at the end of the turn".


 

Sure...but does he call this "virtual bump"?  I dont have my books with me to check at the moment...but I would be surprised if he did.  Aligning forces again, does not explain the fore/aft issues that comes from bump...only the VB concept explains this...this seems to be a critical part you are missing.

 

Quote:
Originally Posted by Davey View Post

 


In "Ultimate Skiing" (2009) Ron Lemaster develops the Virtual Bump with "Adding to the effect is the fact that when going from one turn to another you go from being inclined to one side of your feet to being inclined the other way.  This action tosses you upwards just as a bump would".

 

In "Ultimate Skiing" Page 42 Ron Lemaster looks at the Virtual Bump from yet another standpoint.  That of the bump being the high-point of the racer's head as the skeleton crosses under.  The height of the skier's shins in a short-leg-short-leg crossunder making the skier's head rise in the transition, as if the skier was negotiating a bump.

 

Again, I dont have my books with me, atm...but I am pretty sure what you are describing is "vaulting" and is a separate concept from VB.

 

Quote:
Originally Posted by Davey View Post

 


"The Skier's Edge" has a vector diagram superimposed on a skier progressing through the turn. (Fig. 4.9) Illustrating how the Gravitational force and the Centrifugal force experienced by the skier interact to combine after the fall-line.  (This was omitted from "Ultimate Skiing", and replaced by a single vector diagram on a different photo of a racer.)

Wrapping up

What else is there to add?

The VB is Metaphorical "the skier feels a bump" where there is no real bump.

The VB is Similie "The skier experiences the forces like a bump" where there is no real bump.

The VB is apparent "The skier's head follows a path on the crossunder transition that appears to be a bump.

 

We could perhaps have a "Virtual Kicker" instead of a bump to more exactly describe the launchpad effect of the reaction forces.

In terms of adding force at the critical point in the turn, we can imagine the "downslope" of a "bump" (or the effective downslope) getting steeper as a result of an earlier impulse by a racer getting extra acceleration generated by muscular input.  The metaphor is useful to show the delayed action - and here we get into Brodie territory where his strain gauges and pressure sensors record accelerations in real time.

So - we can't so much use the downside of the bump.  It only exists after a delay.  It steepens as a consequence of an earlier action by the skier.

 

 

If any of these conclusions where in fact correct, how would you be able to apply the "acclerative turn" ideas of pumping (swing) and pushing off at the top of the turn...these ideas only work, if the bump does in fact exist.....particularily the swing pump....???  Can you find any quotes from Ron or Joubert backing this conclusion??????  Seems you are making a huge leap here.

 

Quote:

Originally Posted by Davey View Post

 

That's about all I have at the moment on "The Virtual Bump".

D

 

I am just getting started.biggrin.gif


Edited by Skidude72 - 1/19/13 at 6:37pm
post #226 of 340

Hi SkiDude77

 

Quote:
Again, I dont have my books with me, atm...

What.. virtual books? smile.gif Not a bad idea - you can get everything on a kindle nowadays...

 

The reference point for all my "virtual bump" considerations is Ron Lemaster.  It's his term and his development. Lemaster is our reference for this subject.
I've quoted chapter and verse directly from my own (real, not virtual) copies of his books, which are right in front of me. 

 

My references to Joubert's book is similarly a real book.

 

My understanding of the word "Virtual" is

 

Virtual:
"Existing or resulting in essence or effect though not in actual fact", "Existing in the mind, especially as a product of the imagination". 

 

There is no actual bump in the terrain. We call it virtual, because it only exists in essence or effect and not in actual fact.

However, the skier experiences an effect as if there was an actual bump.  However, the experiment is being done on a totally flat inclined plane.  No real bump.

 

The Accelerative turn requires impulse input early in the turn, and the effect is that, compared with another skier (Blue), Orange would seem to have found a steeper path, because later, his velocity increased.
But that's only in retrospect.  The actual slope was identical for both skiers. There was no real bump or steeper bit.

 

Skidude 77 wrote Quote:

Where did you get this from? It seems you are stretching, and combining basic concepts to fit your world view. Aligning centrifugal forces with gravity is not new, and not part of the VB concept...these are separate ideas. Yes they combine...but VB also explains fore/aft issues...aligning gravity and turning forces does not.

Where from? Ron Lemaster.

The vector alignments? See The Skier's Edge" page 38 Under illustration of figure 4.9 the caption reads

"Gravity and centrifugal force interact through the course of the turn to make the skier feel lighter at the start, and heavier at the end: the same as if he were skiing in bumps"

and as re-laid out "Ultimate Skiing" Page 41 The same caption but the photo is Benni Reich, and the superimposed vector diagram is a single diagram.

 

 I'm sure if you check my copious page references, you will see that we can agree on at least what Ron Lemaster describes as "The Virtual Bump".

 

Virtual Vaulting:

If you are into the surreal: here's a famous image of a virtual Voltaire.

http://www.google.co.uk/search?q=dali+disappearing&hl=en&tbo=u&rls=com.microsoft:en-GB:IE-Address&tbm=isch&source=univ&sa=X&ei=XOf7UIb3Daal0AWS24GQDQ&ved=0CDYQsAQ&biw=1366&bih=587#imgrc=2g21GM7c_sVeDM%3A%3BVrsNIPcpoR8MrM%3Bhttp%253A%252F%252Femilyowenartist.files.wordpress.com%252F2011%252F04%252Fdali2.jpg%3Bhttp%253A%252F%252Femilyowenartist.wordpress.com%252F2011%252F04%252F01%252Fst-petersburg-fl-chihuly-and-dali%252F%3B1072%3B745

 

Here's a joke about vaulting:

Two Olympic athletes from Europe talking in the cafe in London last year.

The Brit says "Hello. Are you a famous pole vaulter?"

The other replies "No, actually I am German - but how did you know my name was Walter?"

post #227 of 340

Skidude77 Wrote

 

Quote:

..particularily the swing pump..

By this we mean the addition of energy into a system that resembles an "Inverted Pendulum" or a metronome swinging-arm.
References : Lind and Sanders "The Physics of Skiing" Technote 11 (P234 -241) "The Skier as an Inverted Pendulum".  In particular half way down P 237 where the forces in equilibrium and those generated at the ski are discussed.

post #228 of 340

More on swings and pumps

 

Pump up off the bump?

More on using a notion of a virtual bump to understand how a skier can pump energy into the system.   This is what I've been searching for from the start.  The VB was incidental.

 

Ref: "The Physics of Skiing" (Lind and Sanders) Page 231 Technote 10 "Pumping to increase velocity"

Child's Swing

A pendulum idea as in a child's swing can be used to imagine skiing down the fall-line over real bumps, anticipating the bumps. 

The child can pump positive feedback energy in by extending at the lowest point. There are centripetal forces being resisted by the child's legs.

If the child pumps negative feedback by compressing with (rather than extending against)  the centripetal force and moving the c.o.m towards the seat of the swing as the swing passes the lowest point, the energy in the system is reduced.

Looking at how a real bump works

Similar to the negative feedback of the child's swing above - The skier in a bump-field taking the high-line/low-line can pump the legs up into a retraction on the peak (moving c.o.m. relatively downwards) and pump the legs down in the troughs (Moving the c.o.m. upwards as the extended legs raise the skier up vertically).  This controls the displacement of the centre of mass to a manageable average by taking kinetic energy out of the system.

Looking at the same forces, but on a smooth (no real bumps) slope

By translating this into the race course, but using positive feedback the expert skier can displace the centre of mass towards the instantaneous centre of the turn and generate energy.  (Lind and Sanders explain that this is a gain in kinetic energy of h times the Centripetal force, where h is the distance moved in).

The gain is achieved by doing work against the centripetal forces generated in the turn.

 

Here's a worked example:  A skier of mass= 70kg negotiates a GS gate at a velocity v=10m/s with a tightest part of the turn r-4m.  The skier moves his com inside the turn by a value h= 0.5m, and therefore pumps energy into the turn.

What is

1) The ratio of in kinetic energy KE to the Change in KE after the move inside?

And

2) The change in the skier's weight?

 

Here's the working principle:

The amount of work done against the Centripetal force Fc is (by principle of conservation of energy) exactly equal to the change in kinetic energy (KE).

 

The centripetal force increases with velocity and so the work done against Fc, and hence the increase in KE, increases with velocity.

Fc = Mv(squared)/r

 

The work done against Fc is exactly equal to the change in Kinetic energy which is worked out by (h/r ) * m.v(squared)

The Ratio of (Change in KE caused by the pumping) to the (KE of the skier)

is:

2h/r, because  KE is 1/2 mv(squared)

That works out an answer for question 1) as The (Change in KE) divided by the original KE is 1/4 = 0.25

 

Moving on to the weight force:

The acceleration a  = v squared/r

Putting in the values for v=10, r=4

a= 100/4 = 25 metres per second squared

 

Centripetal force F=ma so for a mass of 70kg times this acceleration;  F works out at 70*25 = 1750 Newtons.

Compare this to the skier's normal weight:

The skier's rest weight would be mg where g is the acceleration due to gravity which we'll approximate roughly to 10 m/(second squared) i.e. 70*10 =700Newtons.

Depending on the slope and trajectory, there will be added on a component F= mgSin(alpha)*Sin(beta) where alpha is the slope angle and beta is the instantaneous traverse angle.

Is there more?

We have worked out the percentage change of KE as having a value of 0.25 * 100 = 25%

The skier would need to be skilled enough to direct this extra energy in the correct direction.  Realistically, to get a 0.5m movement inside would be difficult at the start of the turn.

This, though is the way the GS skiers appear to accelerate out of the turn and past the gate-post.

Summary, and Vertical/ Horizontal equivalence:-

That's for a lateral movement calculation of a skier on a flat inclined plane.

The same calculation works vertically over a 4m trough, but the skier's weight is even more, as the acceleration into the trough a is 25 which is approx 2.5 times gravity, but the skier's own full weight of mg (not a vector component of it) needs added on as it is in the vertical in the middle of the trough.

 

This is a discussion - not to be quoted as "Gospel".  If anyone can improve it - please do so.

 

Concluding today:

Here's a better link to the virtual Voltaire:-

http://www.planetperplex.com/en/item/slave-market-with-the-disappearing-bust-of-voltaire/

post #229 of 340

Davet,

 

I think you are confusing things.  The "pump" in the Lind example does not appear to be the same "pump" as a swing...they are more describing the two footed push off or skate or "divergence" (as Brodie called it)...different idea.

 

Go back and find that diagram you first posted in the Brodie paper I gave you....study it, read Brodie's commentary.  He refers to the two footed skate (on the left) and the pump (on the right).  Different moves, at different times.

 

A childs swing works by:

 

Sitting on a stationary swing, our COM is at the lowest point in the swings arc.  It has to be.  We can generate movement, but moving our body such that the COM is now up a little bit from the bottom of the arc (we increased its potential energy)...since all things want to return to their lowest energy state, the swing will move to once again return the COM to the lowest part of the arc (potential energy is converted to kinetic)...if we dont move our COM anymore, the swing will stop once again at the lowest energy state (bottom of the arc)...if we keep moving our COM to a higher energy position at the right times...the swing will keep moving, more and more and more as it converts the potential energy into kinetic.  I think we agree here.

 

Key point - we use our muscular effort to move our COM into a state of higher "potential energy", which is immediatley translated into more and more "kinectic energy".  Again I think we agree here.

 

Skiing: 

 

We need to do the same thing...ie move our COM into a state of higher "potential energy"....where/when can we do this?  Can we just stand taller?  NO!  Why not?  Becuase we will still retain the same height at the bottom of the hill so no extra potential energy was created (the differential remained the same).  Ok, what about if we started taller...but flexed as we went down the hill...will this make us faster?  Believe it or not, yes it should.  How much?  Well if you consider even a short ski run, with only a 100 vertical feet...and we flex as we make our way down, so our COM dropped (say you could flex 1 ft), then our COM dropped 101 feet.  Meaning we had more potential energy converted into kinetic...so it should be faster.  How much more energy?  in this example 1%.  In a real example, where say you drop 1000ft?  Well, then 0.1%.  Not alot, but hey, if it wins you a race....go for it.

 

But...reality now.

 

We know to ski well, we need to flex and extend etc....so wont we just lose all the positive extra energy created by flexing when we extend again?  Not really, because we are using our muscles to raise our COM again, not our kinteic energy...but we would likely slow some due to wind drag etc, this wind drag would exceed the benefit of the pump...otherwise speed skiers would do this.  

 

So how do we get the best from this?

 

Well we can manipulate things a little bit.  For example, coming off a pitch onto a flat...we can flex at the transition to the flat into a tuck...converting more potential energy into kinetic energy which means more speed, which we can carry across the flat...which is faster.

 

Where else?  VBs...since VBs are change in effective slope, same as going from pitch to flat...we can get the same boost as we leave the fall line and enter the transition.  This would not work, if VBs were just a made up way of describing what we feel....this only works, if VB is actually happening...so in effect we can gain a little boost, by flexing at the steepest part of the effective slope as it hits the shallowest part of the effective slope (ie lowered our COM more, thus translated more potential energy into kinectic)....

 

But, we know we flex anyway at this part of the turn, for a variety of reasons...is there benefit to flexing more and really getting our COM low and even back up the hill to get more potential energy converted to kinetic?  Well according to Brodie, and common sense....not really as the potentail to scrub speed with a skid (negative Ground Reaction Force) etc, far outweights the gains...but, in cases where there is the ability to do this, without creating excess drag...it could work....but as Brodie wrote, this is fairly limited in a real race scenario.

 

 

What else?

 

Well to use Daveys "real bump" example....actually Id argue the exact opposite...we flex over the bump, and extend into the trough...so we put kinetic energy INTO the system...to REDUCE the potential energy that is released...this helps us control our speed a little  (as much as we were able to absorb/extend).  This would be doing the opposite of my idea above...instead of starting tall and flexing to the bottom, we could start low, and rise to the bottom...so our potential energy only was from 99feet, as opposed to 101feet from the guy who started tall and flexed to the finish.


Edited by Skidude72 - 1/21/13 at 10:54am
post #230 of 340
Skidude77 Quote:
The "pump" in the Lind example does not appear to be the same "pump" as a swing.

It's the same as the child's swing, in that in the swing example, the pump is done by moving the c.o.m in towards the radial centre of the angular motion (The fixing hook).

This is achieved by the child standing up and doing work against the centripetal force (coming from the seat), as the child makes a standing-up movement using muscular force.  (This isn't an attempt to increase potential energy, it's a direct input of kinetic work-energy against the turning forces).

The child can increase the net energy - and therefore the amplitude, or reduce the net energy, and therefore tend to stop the swing in order to dismount.  The child learns to do this by timing the muscular input.  As the swing comes past the lowest point, then using avalement will absorb energy, using extension at that point will increase the energy.

This is a different dynamic system to that where a seated child swings the legs to add energy.

 

Instead of a swing, you may prefer to  think of the system as a pirouetting ballet dancer who, to speed-up a spin, brings the outstretched arms in closer to the body (The axis of rotation) in order to increase angular velocity, and hence, the kinetic energy of the system.  The dancer must do work against the centrifugal force felt by the arms.

 

In the GS turning example, there must be work done against the turning forces in order to pump kinetic energy in to the system.  The skier achieves this by "getting inside more" - i.e. moving the c.o.m. in towards the radial centre (the instantaneous position of a changing radius) of the turn by doing work against the centripetal force that is creating the turn.  This is most simply done by whole-body  inclination and leg-extension.  This is different from divergence or lateral projection by stepping-in, although the skier may be doing these things too, and they'd probably have the same effect.

 

In the straight-line descent over a bump and trough example, (this analysis of the dynamics of the trough is almost identical to the child on the swing) the skier indeed adds work energy in to the system on the trough, working against the vertical normal reaction force, but absorbs the energy from the facing-forces on the upslope as work is done on the muscles during an avalement movement.  So the vertical movement of the c.o.m is averaged out to much more of a straight line, and the speed down the hill is controlled.  I termed it negative feedback - in other words, it's energy, but when added to oppose the input, reduces the output.  This is fully explained on page 237 of "The Physics of Skiing" by Lind and Sanders with a classic engineering Control System diagram showing input, output and negative feedback path.

 

In terms of Virtual Bump, this (as I explained) is generated by the work input by the skier during a turn.  The turn is created by the skier resisting the turning forces that are required by Newton's laws.  The average slope through the turn can be seen to be less than the actual slope.

The effect of a muscular input of KE against the turning forces is a steepening of the instantaneous effective slope to make it steeper than the average slope - or maybe even steeper than the actual slope.

It isn't possible to have a circular argument over this as if the change in effective slope is something that can be then used to generate the extra energy.  That would be to design a perpetual motion machine.

However, on the next turn, the extra velocity of the skier (as if there had just been a steeper bit in the track) can be detected when the skier is seen to gain a distance advantage. 

Brodie makes a point about the delayed effect.


Edited by Davey - 1/22/13 at 8:09am
post #231 of 340

Keep reading Davey,

 

You are getting there.  I am seeing your explainations shift in the right direction, this is good.  Keep in mind:  a childs swing doesnt work the same, as an ice skater....as I wrote above these are two different ideas...you are still talking about the "swing" then move to the skater example when talking skiing.....the swing and ice skater both have applications for skiing thou...but you are forcing an explanation onto a swing to make it fit skiing....dont work like that.  Think of your "pump" as a two footed push on ice skates....the other "pump" (Brodies) is like a swing...

 

Your understanding of the VB is getting there...keep going with increasing your understanding of effective slope, and change of.

 

This has been really good so far.

post #232 of 340

I hope we are not

  • Getting into arguing over word definitions
  • Arguing over the number of virtual angels that can simultaneously dance upon a pinhead

 

SkiDude77Quote:

 

Quote:
Go back and find that diagram you first posted, in the Brodie paper I gave you....study it, read Brodie's commentary. He refers to the two footed skate (on the left) and the pump (on the right). Different moves, at different times.

 

 

Terms

We need to define our terms. I always try to do this as loose terms like "Pumping" are interpreted differently by different authors. Lind and Sanders were much earlier (2002) - so, in my view, on this occasion, we should use their definition as it's more inclusive, whilst recognising that other authors like Brodie separate the meainings into angular and linear contexts.

The definition : Meaning of the word "Pumping"

Yes, it's clear that Brodie uses the term "Pumping" differently and more limitingly to Lind and Sanders. 

It is a bit of a slang colloquial term, so it's natural to see variations in usage.

Brodie separates the terms - Lind and Sanders combine the terms

Brodie separates "Lateral Projection" and "Pumping".  But they are both just different ways of "Pumping" to Lind and Sanders, who define it (as I do Simply: an energy pump.)  as "Adding KE into the system by doing work against the turning or reaction forces". 

(You can get a reaction force without turning). 

 

For Brodie there's energy gain by lateral projection of the c.o.m away from the skis off the edges.  But the energy gain is in doing work by moving the c.o.m into a position closer to the centre of the turn against the centripetal forces.

 

Lind and Sanders call this "Pumping by 'a technique similar to what we have called Lateral Projection'"

But they also define a different way of pumping as doing work against a linear velocity reaction force, as they go on to include "..pumping action may occur in a direction perpendicular to the snow surface, which adds to the kinetic energy and thus to velocity"

 

In Brodie's example he uses Lateral Projection in the same way as I described as Pumping, and he also uses what he calls "Pumping" (a pre-jump and/or a gain in height / doing work against a normal surface reaction force like a rider in a half-pipe can do on .).

 

I restricted my child's swing example to just the radial projection work-energy, but of course, the result is a gain in elevation for the next swing cycle.  So there are both mechanisms working there.

 

Definition of "Potential Energy"

(Muscle power is PE too.)

Lind and Sanders refer to a "Reservoir of Potential Energy" in the human body; meaning that the muscle energy in itself is potential energy that can make kinetic energy directly by doing work against the reaction forces.  Potential Energy is defined as "Energy stored for future use".


I notice SkiDude77 in his example has a slightly different concept of PE. His example uses muscular energy to gain height and only then does he consider it potential energy. (?)


In my example, I used the muscle potential energy to do work directly against the turning forces, and don't use SkiDude's definition which (I think) sees PE as only something that is associated with work done against gravity.  i.e. the skilift adds potential energy by doing work against gravity.  This PE is the reservoir for most of the work-energy exchange into KE as height is lost during the descent.  In this closed case, PE = mgh (skier mass x acceleration due to gravity x vertical height above the finish)
Conservation of energy principle states PE + KE = constant.


But for a real skier rather than some static block of matter, we need to be aware of the skier's chemical energy - glycogen stored in the muscles and fat and other reserves.

This is where the pump comes from.  Muscular action directly from the skier's chemical energy adding direct KE in to the dynamics of the system.

post #233 of 340

Davey,

 

My handle is Skidude72...not 77. 

 

 

I dont care what you call this or that..but be clear and consistent, particularily when using well known and defined terms such as VB.  I agree "pump" is more generic so I think it is fine to use it your own way...BUT childs swing, was not what you were describing...in your "pump"...I am glad you corrected that.  As for PE, I use proper physics definitions, I dont make up my own.   

 

Where is Jamt when you need him???

 

 

Well since you have gotten lots of new info now, and have shifted your views (which I applaud btw), it might be worth you starting at the beginning and stating your latest postion on all this.  Perhaps starting at the beginning wth VB.

post #234 of 340

Hi SkiDude72 Sorry about the "77"

What does the number signify?

post #235 of 340

   Wow Davey...I had NO idea skiing was so complicated....I'll have to remember my abacus next time for the chairlift ridesrolleyes.gif. Having said that, and having personally experienced "VB" in carved cross-unders, this is a good conversation...though like Skidude asked, where's Jamt?? Guys' a genius when explaining the physics of skiing!  

 

  If the skier in question wants to employ a more direct-down the fall line tactic (as opposed to an across the hill type of movement--more of a float phase)  as often needed in slalom, for instance, he/she is better off employing more of a cross-under type of movement. Head/com stay "down" bos passes underneath, side to side. Extension, retraction, extension, etc...Most of the forces generated during the turn get released during transition. The ONLY way for the bos and com to cross paths with the com staying down is to actively (in some cases) suck "it" up...this is where the ephemeral bump in question exists. Gotta say though, I haven't read all of Lemaster's works, and don't know them by heart...

 

 

  P.s. also didn't read this entire thread, just this last page....Guilty as chargedwink.gif!

    respectfully

 

      zenny

post #236 of 340

SkiDude72

 

Quote:
be clear and consistent, particularily when using well known and defined terms such as VB

Yes, thanks, we've agreed on the Virtual Bump definition as found in "The Skier's Edge" (1999) and "Ultimate Skiing"(2010) both by Ron Lemaster.  Everything I write is as clear and consistent as I can make it.

 

Quote:
I agree "pump" is more generic so I think it is fine to use it your own way...BUT childs swing, was not what you were describing...in your "pump"...I am glad you corrected that.

Energy Pump: Kinetic Pump

Yes, I (and Lind and Sanders) define "Pumping" as adding Kinetic Energy directly into a system by means of working against the reaction forces either in a turn or off the flat. (This is described in "The Physics of Skiing" by Lind and Sanders. (2nd Edition 2003). 

Child's Swing model

(at the risk of repeating myself)

The child's swing has many dynamic movements, and there are lots of ways of pumping energy in - but the simplest one I used was the single suspension pendulum, with the child standing up and working radially against centripetal force to directly generate Kinetic Energy by moving up against that force that comes up through the feet whilst standing on the seat.  The force is strictly radial. There is no secondary bend of the suspension rope or chain.  It can be treated as a solid bar.  It's basically a parametric oscillatory system.

See http://www.grinnell.edu/files/video/clip2.mov (except it's a lecturer - not a child).. 

This extension against the radial force uses the principle of conservation of angular momentum to increase the velocity as the centre of mass is moved up to make the distance to the fixing point smaller.  (This is the same principle as the pirouetting ballet dancer moving arms in to speed up the rotation.)

For large oscillations, this has a secondary result in that as KE increases, the swinging amplitude is increased therefore the downswing has more starting elevation which converts to yet more KE.

This is described in "The Physics of skiing" P135.  There is more to this if you go on to consider secondary pendulum model and fore-aft rocking.  I intentionally omitted these second order effects at this stage, although I plan to re-visit later, as there are direct parallels in skiing.

No correction was necessary, as far as I am aware.

Potential Energy considerations

I put question marks against a couple of items relating to understanding what you wrote on PE - If you like, you could clarify what your intended meaning.

As I said PE includes the stored muscular energy of the skier and this can be released to create an input of Kinetic Energy that adds to the conserved energy exchange that takes place during the descent where the PE of the stored work done by the skilift against gravity can be converted back into KE during the descent.

 

Quote:

 

Well since you have gotten lots of new info now, and have shifted your views (which I applaud btw), it might be worth you starting at the beginning and stating your latest postion on all this.  Perhaps starting at the beginning wth VB.

Thanks for applauding :-) but I have not done much shifting of views - if at all. I've simply worked all along with the published work by Lemaster and Lind and Sanders.

I'm not planning to start again, though.  (Well not on this forum).

VB

The "Virtual Bump (as I see it, and as described in several different ways) exists only as a measureable effect of making a turn.  It is a concept described by Ron Lemaster, and it is the "Ultimate Skiing" definition that is used here.

VB is an effect that we can use to explain the action of turning velocities, especially for speed control.  VB is an effect generated as a turn progresses.   As this happens, there is a change of velocity (magnitude and direction).  The pressure curve of the F=mg.sin(a).Sin(b) and with it the apparent slope profile changes smoothly throughout the turn.  If the average slope through the turn is plotted, it is seen as less than the actual slope.

(This was discussed by Joubert in "How to Ski The New French Way" on page 197 and he observed that the average slope encountered by the skier was less than the actual slope).  The graph of slope against progress around the turn resembles a sinusoidal distribution.  It was Lemaster who  - upon reading that page, coined the term "The Virtual Bump", because it was an apparent bump according to the slope graph, but obviously not a real bump except that the skier would have felt the same pressures had he not turned but skied straight down and over a real trough and bump instead.

The same pressure and slope variations experienced in doing turns can be seen in skiing (without turning) over real bumps and troughs.

The skier needs to cope with this virtual bump in the same way as he would if it was a real bump.  However, as VB is a product of a turn - it can't be used to influence that turn.

I can't find a practical use for a virtual bump - just to explain the forces in such a way as a skier can understand them.

post #237 of 340
  • 50x50px-LS-18038c77_avatar.jpeg

@ zentune Abacus? Take log tables.  More fun for jumping on.

post #238 of 340
I'll try it wink.gif

zenny
post #239 of 340

Wow! It sure seems like a long and difficult way to get to the task of expressing the idea that the tips drop (relative to the feet and tails) as we turn downhill and they return to level (again relative to the feet and tails) as we turn across the hill. That is in a nutshell what Jolbert and LeMaster expressed. So Davey while skiing over a trough and bump is similar, traversing through some very small bumps may be a bit closer to what we would feel. Mostly because the traverse would include both feet at different elevations (leg lengths are different).

 

An important element when we start to talk about finishing turns and transitioning to the next turn is how the legs progressively flex to produce the different leg lengths. Additionally the Flex to Absorb idea doesn't occur in isolation outside of a few contrived drills. Edge and Rotary elements are also present and contributing to the overall "feel" of a turn, or more specifically the finish and the transition into the next turn. That is why multiple transition options exist and no one way is always superior to the others but what they all have in common is the peception of increasing pressure being felt by a skier as their skis turn across the hill. To expand on this a bit, the faster the skis turn across the hill, the more prevalent the virtual bump feels. In that way the virtual bump has value since it expresses one of the reasons we feel more pressure as the skis turn across the hill. Even on groomed terrain where no bump is actually there.

 

Obviously in real bumps the undulating terrain and the incidental slope angles exceed the range of what we would, or should attribute to the virtual bump concept but again, the feeling is quite similar. Like Ron, my opinion is dealing with the pressure attributed to the virtual bump becomes the bottom line. As pressure increases we must either flex to absorb some of that pressure, or drive the edges much harder into the snow to prevent snow shear. Either way the perceived pressure increase is not imaginary.


Edited by justanotherskipro - 1/23/13 at 12:43pm
post #240 of 340

Wow, I agree with JASP, thats a lot of words for relatively simple concepts.

 

Like others I have viewed the VB to be due the the changing instantaneous slope angle under the ski.

 

Regarding the pumping, yes of course you can increase speed with muscular effort. Skating is probably the simplest example. Pumping is just skating with the legs together.

 

In flat parts of courses it may be useful, but as soon as it is steep enough it is more about managing speed, edging and forces than creating new ones.

New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Ski Instruction & Coaching
EpicSki › The Barking Bear Forums › Ski Training and Pro Forums › Ski Instruction & Coaching › The "Virtual" bump.... real or fiction?