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 9

Hi

I think it got complicated because there is a lot more under discussion here than the relatively simple change in effective slope angle that some call "Virtual Bump"

Skidude72 has been kind enough to act in the role of grizzly mentor in a kind of game to get some amateur research done.

I got some key info from Ron Lemaster about how he coined the term  "Virtual Bump" - albeit by reading previous work by Georges Joubert.

Jamt said Quote:

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

Yes, that's the starting point - but where does that lead?  We need to understand the forces, the energy and conservation of energy and momentum, both linear and angular - and this thread has explored how the theory works to gain energy by moving the c.o.m. well inside on the turn like all racers do, but a simple worked example is so instructive - perhaps not to others, but it was to me.

For me as I explained - the "Virtual Bump" is simply a metaphor.  A useful metaphor to explain things to skiers.

The forces are real enough, but there is no real bump.  That's why we use the word "Virtual" in the expression.

The work explored in the current thread goes behind the folk-tale method of explanation and attempts to see how some dynamic work-energy analysis can help us.

JustAnotherSkiPro said Quote:

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).

Yes, that's a very well-made and practical point.  Quite specific to teaching skiing.  A splash of practicality in this rather arcane study.

He goes on to say :

Quote:
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.

Yes - we showed that the force increases with the square of velocity and inversely with the tightness.
It's this square-law that kills the keen amateur in GS gates who quickly runs out of strength when they turn too late, have to hang on to the turn until the speed drops enough to achieve release.  Either that - or they turn far too early and have to do a vertical charge to the next gate..  We can use the VB metaphor to coach ski teachers attempting to learn to race for their Eurotest (Pass time is 0 FIS time + 18%) by showing how to use a GS ski to lead up to a progressively edged turn that gets pretty tight and high-pressure, but if the skier keeps control of the line, doesn't progress too far round the turn that the VB gets you, but manages to get an early release then the pressure doesn't need to be sustained too long.  We can prove that "getting inside" in the critical part of the turn - on a good gripping ski, having set it up correctly gives the kind of pinging acceleration we all try to achieve out of the apex and past the gate.

The key thing in getting acceleration in a GS turn in particular is the move of the c.o.m inside in order to generate energy over and above the Conserved gravitational energy.
Obviously one knows this, but proving it simply, and understanding the mechanism is very mind-clearing, I find.

Yes, pumping.  "Pumping is like ..." {and you do this}

The show-tell method is great, but can it be proved?  We explored what it really means to different authors, and three different ways of making it work, one of which was explained with a worked example.

I've rather enjoyed the study.  Thanks to all on this forum for indulging me.

hey there davey...so are you writing a paper or book? just curious because your penetrating analysis is so strong. are you a race coach, as you mentioned gs tactics...
?

regarding accelerating out of a turn (aft) theres been some contention here on epic (which i was a part of...)

respectfully

zentune

Davey,

You are still including the turning forces with the virtual bump forces.  The usual descriptions do not include the turning force with the bump force.  That is because most folk don't see the turn in 3-D with an inclined radius of curvature.  They see it as a turn in the horizontal plane.

It is true that vectorially, F= ma, and there is only one  force vector and it and the acceleration vector are collinear.  However the forces attributable to the virtual bump include only the vertical forces, and arise solely because the skier is no longer descending at the slope angle he was descending at before he turned.  Yes, in a turn, when describing the turn in absolute terms (including vertical and horizontal components), there are total acceleration forces which vary as velocity squared and inversely as turn radius.  In the vertical bump description, the horizontal turn is defined and the horizontal turning forces are in addition to the "virtual bump" forces.  The vector sum of these forces add up to the total force "felt" by the skier.

Go back to the roller coaster example.  If the roller coaster traveled straight down an incline then briefly traveled horizontally then down again you would have no trouble noticing and calling that horizontal bit a bump.  If the roller coaster happened to turn before traveling horizontally and turned back into the original direction you would still call it a bump.  Just because the path is being traced by a skier and not a roller coaster, does not make it any less of a bump.

The rider in the roller coaster feels the turning force as being pressed against the sidewall of the roller coaster, and feels the bump force as being pressed into the seat.

Sorry guys, I'm really curious but don't think I can successfully read through 9 pages of conflicting commentary...

Is the concept of the "virtual bump" that in an advanced parallel turn, the transition is like the peak/crest of the bump, and as you extend laterally, it's like the trough or side of the bump?

Is it fair to say the path down the virtual bump is limited to one turn shape allowing only variations in radius?  (IE you can't "granny ski" it, or traverse it)

Someone should write up an article on the VB for epicski!

metaphor...sure sounds good. as the bos and com crosspaths, there's an absoption that takes place (or not!), in an instant, then the bos moves out laterally...though peak and trough don't quite do this much lateral displacement justice. still, there are forces ( think ski pressuring and release) that need to be managed if one is traveling arc to arc..the skier experiences this as a sensation similar to that of absorbing a bump, tho no such bump exists

zentune
metaphor...as a continuation, one can pressure the ski more during the turn, prior to transition, effectively making the vb "bigger" and therby increase lateral movements of the skis...
imo

zentune
Edited by zentune - 1/23/13 at 6:38pm

L.V., experiencing a V.B.  Look at ,3&4. Absorption (of the forces) at 4...extension (tho not fully) at 5 &6. Her knees are nearly in her chest during maximum retraction--much like a skier encountering a mogul. Difference being...she has (virtually) no bump! I bet she felt one, though. Hence the concept...

zentune

Edited by zentune - 1/23/13 at 8:08pm
Quote:
Originally Posted by Davey

The key thing in getting acceleration in a GS turn in particular is the move of the c.o.m inside in order to generate energy over and above the Conserved gravitational energy.
Obviously one knows this, but proving it simply, and understanding the mechanism is very mind-clearing, I find.

I'm not sure what you mean with acceleration Davey. To generate acceleration you have to push with you legs before the fall-line. This pushing means that you are also pushing your CoM upwards. This means that you will not reach high edge angles, and you will loose pressure late in the turn. If the course is flat and easy this may be beneficial, but if not then your are set up for failure.

Lower edge angles means a rounder line, loosing pressure late in the turn increases risk of skidding out.

The feeling of acceleration out of the turn is quite false, What you are feeling is vertical acceleration, and that is really your kinetic energy beeing converted to potential energy. Quite the opposite of speed acceleration.

Also my view of moving inside the turn is that it is simply a necessary counter balance (and not in the hh definition) against the horizontal turn forces, which are proportional to v^2/r. Obviously then if you are going fast you have to incline a lot to be in balance. In reality balance is dynamic but lets not go there for now.

If you are not inside the turn enough you will not counter the forces and you will either rise up or skid.

Quote:
Originally Posted by Jamt

I'm not sure what you mean with acceleration Davey. To generate acceleration you have to push with you legs before the fall-line. This pushing means that you are also pushing your CoM upwards. This means that you will not reach high edge angles, and you will loose pressure late in the turn. If the course is flat and easy this may be beneficial, but if not then your are set up for failure.

Lower edge angles means a rounder line, loosing pressure late in the turn increases risk of skidding out.

The feeling of acceleration out of the turn is quite false, What you are feeling is vertical acceleration, and that is really your kinetic energy beeing converted to potential energy. Quite the opposite of speed acceleration.

Also my view of moving inside the turn is that it is simply a necessary counter balance (and not in the hh definition) against the horizontal turn forces, which are proportional to v^2/r. Obviously then if you are going fast you have to incline a lot to be in balance. In reality balance is dynamic but lets not go there for now.

If you are not inside the turn enough you will not counter the forces and you will either rise up or skid.

I'm not sure what you mean with acceleration Davey.

Acceleration is rate of change of speed (linear acceleration) and/or rate of change of angular displacement (Angular acceleration).  Acceleration is rate of change of the velocity vector which means that if the linear speed changes or the rate of sweep of angle changes, then that's acceleration.

Quote:
To generate acceleration you have to push with you[r] legs before the fall-line.

What you have explained there is one way of accelerating.  As you hint at, it's a bit crude and difficult to do.

In "getting inside" the skier uses the conservation of angular momentum to pump energy in to the system.  See for reference page 231 of "The Physics of Skiing" Technote 10 "Pumping to increase velocity.  The skier gets set up with good grip, not pushing, but chiselling a fine groove from which to go long-leg-short-leg and get the c.o.m. inside towards the instantaneous centre.

The work done in moving the c.o.m. inside is done against the centripetal forces coming at right angles up from the ski.  This work adds KE and increases velocity.  That means acceleration happens.
I used the standard similies of the spinning dancer and the child standing up on the swing and the pendulum. We also use the metronome analogy or Inverted Pendulum.
There are hundreds of references to this under "conservation of angular momentum"

Quote:
The feeling of acceleration out of the turn is quite false, What you are feeling is vertical acceleration, and that is really your kinetic energy beeing converted to potential energy. Quite the opposite of speed acceleration.

I can see you are being careful here because this runs into the principle of the conservation of energy.  The skier only increases energy in the system by doing work against the Turning forces.

No extra work is done in what you are calling "vertical acceleration".  A vertical projection would not be positive acceleration would it? It's deceleration and ballistic motion.  This appears to be what you are saying when you say "Quite the opposite of speed acceleration".

To All Can we keep this focused?

In general, can people refer to sources and not personal opinions (Mine or otherwise)?

Hi Ghost!

Ghost wrote: Quote:

The rider in the roller coaster feels the turning force as being pressed against the sidewall of the roller coaster, and feels the bump force as being pressed into the seat.

Hi Ghost, if the roller-coaster is on a banked track the same force is felt through the seat during turning as is felt down a dip.  The skier on an edge is riding a tiny banked track.

Quote:
You are still including the turning forces with the virtual bump forces.  The usual descriptions do not include the turning force with the bump force.

In my analysis, you can't have a virtual bump without a turn and you can't have a turn without any turning forces.

"Ye canna cheinge the laws o' Physics Capt'n... a turn withoot a force wad be like mixing matter and antimatter cauld!)

(O.K. Mr Scott, that's enough thermodynamics...)

In the way I've read it from Lemaster, VB is nothing if not forces, because it does not take the profile of a bump unless you plot the effect of the turn on to paper, and you obtain the same pressure effect as a bump would have caused.

You can plot the instantaneous slope angle of the skier's path during the turn or you can plot force and you must get the same graph.

As we mentioned F=mg.sin(slope of the inclined plane).sin(traverse angle).

Quote:

The usual descriptions do not include

Can we please not use "The usual descriptions"? Please will you refer to Lemaster "Ultimate Skiing" as a reference, because I already established that Lemaster coined the term "virtual Bump", and it's his published notion of a VB that we can anchor to.  If you've got other VBs that have no pressure, well please will you quote the sources so we can look them up and refer to them.

I know what acceleration is Davey, not just sure how you propose it happens at the end of the turn.

FYI, in this thread you can find my opinion about limitations of physics and ski technique: http://www.epicski.com/t/114527/physics-and-ski-technique

I don't know why you bring the principle of conservation of angular momentum into pumping. Pumping is simple pushing on the skis when the angle between the velocity vectors of CoM and skis are beneficial, and retracting when they are not. You don't need any angular momentum to do that. Sure there can be some but in downhill skiing rotation is usually a bad thing, not something that increases speed.

In fact I have a very difficult time seeing how you would store a lot och energy by rotating (angular momentum) and then convert it to speed. IMO it is much more common to rotate too much than too little.

Just to be clear, are you claiming that the rotation you have towards the end of the turn is converted into speed when you release? What movements do you do to accomplish that?

I think I know what you are getting at when you compare conservation of angular momentum to swings, metronomes etc. Your idea is that the velocities change when you move closer or further away from the rotational center. However, everything you do with your skis changes the turning radius and thus the center of rotation.  Pressure, edge angles, grip, fore-aft all have a big impact on it. This means that the center of rotation is not fixed like your example. The situation becomes a much more complex system and IMO you can easily come to the wrong conclusions.

For example, with your reasoning it shouldn't matter when you do the work to get inside? But how can you possibly increase the speed to the bottom of the hill if you push towards the end of the turn, when you are basically pushing yourself up the hill? Wouldn't it make more sense to have a simpler view that if you push when you are pushing the COM down the hill, and retract when the skis are pushing you up the hill it will bring you faster to the bottom? Or the skating analogy. I'm sure you can explain the details of skating by bringing the the angular momentum into the picture, but why complicate things if it does not help to understand?

Edit: Another example for you to think about. If you have a swing and do work to get closer to the center. If you get close enough, the velocity is zero

Quote:
Originally Posted by Davey

Hi Ghost!

Ghost wrote: Quote:

Hi Ghost, if the roller-coaster is on a banked track the same force is felt through the seat during turning as is felt down a dip.  The skier on an edge is riding a tiny banked track.

In my analysis, you can't have a virtual bump without a turn and you can't have a turn without any turning forces.

"Ye canna cheinge the laws o' Physics Capt'n... a turn withoot a force wad be like mixing matter and antimatter cauld!)

(O.K. Mr Scott, that's enough thermodynamics...)

In the way I've read it from Lemaster, VB is nothing if not forces, because it does not take the profile of a bump unless you plot the effect of the turn on to paper, and you obtain the same pressure effect as a bump would have caused.

You can plot the instantaneous slope angle of the skier's path during the turn or you can plot force and you must get the same graph.

As we mentioned F=mg.sin(slope of the inclined plane).sin(traverse angle).

Can we please not use "The usual descriptions"? Please will you refer to Lemaster "Ultimate Skiing" as a reference, because I already established that Lemaster coined the term "virtual Bump", and it's his published notion of a VB that we can anchor to.  If you've got other VBs that have no pressure, well please will you quote the sources so we can look them up and refer to them.

Davey,

You claim you want to avoid input from our "opinions", yet constantly offer your own.  Your quotes are nothing more then snipits, with your analysis....not good.

Red - this is simply not true.  You can ski a line on a given slope at 4mph, or 40mph...the VB will be the same, but the forces will be vastly different.  We can alter force throught flexion/extension also...VB is just a function of line, and the slope we are skiing on.  Force is a fucntion of line, slope, speed, as well as technqiue (flexion/extension)....As many have pointed out to you, you are confusing ideas, and combining concepts incorrectley.

Blue  - acutally I pointed out that Lemaster "coined the term" but it was Jouberts concept used to support his avalment technique.  Further, you are still not using Lemaster's description...please do so.  Why dont you cut and past his entire section on VB here for us....?  Its not that long.

Quote:
Originally Posted by Jamt

For example, with your reasoning it shouldn't matter when you do the work to get inside? But how can you possibly increase the speed to the bottom of the hill if you push towards the end of the turn, when you are basically pushing yourself up the hill? Wouldn't it make more sense to have a simpler view that if you push when you are pushing the COM down the hill, and retract when the skis are pushing you up the hill it will bring you faster to the bottom? Or the skating analogy. I'm sure you can explain the details of skating by bringing the the angular momentum into the picture, but why complicate things if it does not help to understand?

Edit: Another example for you to think about. If you have a swing and do work to get closer to the center. If you get close enough, the velocity is zero

Just to be clear, Jamt...you're referring to (forgive me here Jamt--I know you're not a fan of these terms) Cross-unders as opposed to 'overs? Wherein in the 'over, legs stay longer near completion, com comes over the top (further inside) and then down the hill...covering more distance across the fall-line in the process? As opposed to 'unders, wherein outside is long at the top of the turn (more so then the other turn type)  com staying down, so as to avoid too much across the hill travel...effectively implying that in an cross-over you are closer to this rotational center near the bottom of a turn in an over, and closer to it in a retraction-style turn, and that it continuously changes throughout, right? it makes total sense to me...

Davey...you're in an open forum, here...you'll get some "opinions", from time to time ...

zenny

Quote:
Originally Posted by zentune

hey there davey...so are you writing a paper or book? just curious because your penetrating analysis is so strong. are you a race coach, as you mentioned gs tactics...
?

regarding accelerating out of a turn (aft) theres been some contention here on epic (which i was a part of...)

respectfully

zentune

Hi Zentune.

theres been some contention here on epic

Oh goodness gracious.  Contention? on Epic, you say? Shurely shome mishtake!    (I'll endeavour to follow that subject).  I would appreciate any info you can offer.

I'm currently working on a tech written paper for my ISTD.  (International Ski Teacher Diploma).

I'm not sure whether this VB subject will be in it though.

I'm testing some well established facts as published in well known skiing books.

The Barking Bears are doing a fine job in winnowing out the wheat from the chaff in the arguments.
The books I've chosen to refer to are the usual classic ski physics books - rather than being books on hefty calculus by the maths department.

The facts need to be publicly available.

Anyhow, thanks for your encouragement, it's much appreciated.

To Skidude72 Quote:

You claim you want to avoid input from our "opinions", yet constantly offer your own.

Here's what was written.Quote:

To All Can we keep this focused?

In general, can people refer to sources and not personal opinions (Mine or otherwise)?

Meaning

• Opinions shouldn't be offered as a reference source.
• Individual opinions are perfectly valid and need to be informed by quoted sources.
• A discussion on the interpretation of the source principles can then be usefully pursued.
• Parties in a discussion should avoid fallacy, and otherwise be respectful to the individual.
• Lists of fallacies and similar tactics are readily available from Wikipedia.

If I may suggest: leave out personal address (e.g. "you want our") and focus on the facts as independently established.

Edited by Davey - 1/24/13 at 7:38am
Quote:
Originally Posted by Jamt

I know what acceleration is Davey, not just sure how you propose it happens at the end of the turn.

FYI, in this thread you can find my opinion about limitations of physics and ski technique: http://www.epicski.com/t/114527/physics-and-ski-technique

I don't know why you bring the principle of conservation of angular momentum into pumping. Pumping is simple pushing on the skis when the angle between the velocity vectors of CoM and skis are beneficial, and retracting when they are not. You don't need any angular momentum to do that. Sure there can be some but in downhill skiing rotation is usually a bad thing, not something that increases speed.

My reference was quoted as "The Physics of Skiing" Lind & Sanders Technote 10 "Pumping to increase velocity".  Skiers increase KE by generating it through doing work against reaction forces.  Their treatment of Pumping is as an energy pump.  It includes what your more restrictive definition is limited to.

In fact I have a very difficult time seeing how you would store a lot och energy by rotating (angular momentum) and then convert it to speed. IMO it is much more common to rotate too much than too little.

I don't see any potential energy store being involved.  The KE increase is available in real-time.

Just to be clear, are you claiming that the rotation you have towards the end of the turn is converted into speed when you release? What movements do you do to accomplish that?

No, it's the rotation of the skier's path in the critical part of the turn.  The skier needs to be reducing edge angle and increasing forward velocity as the gate panel is passed.

The turn in this GS turn is between 45Degree angles of "traverse", so it's not a big-old semicircle.  The skier needs to accelerate towards the next aiming point which is offset laterally (usually) from the gate being negotiated.

The "getting Inside" is done in the critilal area of the turn - which I consider to start at the riser line aiming point.  The skier's movements would be long-leg-short-leg inclination to move the c.o.m. to the inside for a very short period during the critical part of the turn just before the apex.

You will know that this is a line drawn up the slope from the gate.  The skier doesn't aim directly at the gate, but at a point above the gate.

The release is done having achieved the required aiming direction for the next rise-line.  I am featuring one of the main movements to make is to move the c.o.m. towards the instantaneous centre of the turn (whatever that radius may be between Rt = RSidecut and Rt = a few metres.)
The shape of the turn is part of an oval shape, with an approximately round apex.  The apex is where the skier goes out and back through the fall-line.

I think I know what you are getting at when you compare conservation of angular momentum to swings, metronomes etc. Your idea is that the velocities change when you move closer or further away from the rotational center. However, everything you do with your skis changes the turning radius and thus the center of rotation.  Pressure, edge angles, grip, fore-aft all have a big impact on it. This means that the center of rotation is not fixed like your example. The situation becomes a much more complex system and IMO you can easily come to the wrong conclusions.

This means that the center of rotation is not fixed like your example.

My example uses the instantaneous centre of rotation, and that's not fixed.

For example, with your reasoning it shouldn't matter when you do the work to get inside? But how can you possibly increase the speed to the bottom of the hill if you push towards the end of the turn, when you are basically pushing yourself up the hill?

The work needs to be done in the critical part of the turn and the turn needs to release as soon as the new direction is established which is about 45 degrees from the fall-line.

Wouldn't it make more sense to have a simpler view that if you push when you are pushing the COM down the hill, and retract when the skis are pushing you up the hill it will bring you faster to the bottom? Or the skating analogy. I'm sure you can explain the details of skating by bringing the the angular momentum into the picture, but why complicate things if it does not help to understand?

This is my point.  The importance of the moving of c.o.m. is missed unless the physics is understood with a simple worked example.  Usually, the coach keeps it simple and avoids explaining about KE gains from angular momentum.

The skier can attempt to push downhill at the top of the turn, but that's not very easy and grip is likely to be lost.  But in addition, the move of the c.o.m. inside has the effect of pumping the KE.

Edit: Another example for you to think about. If you have a swing and do work to get closer to the center. If you get close enough, the velocity is zero

Quite correct, but I think you'd get some relativistic effects too, because the user would have dissapeared up his own fundament.

Edited by Davey - 1/24/13 at 8:10am
the acceleration thread i mentioned was (i believe) "reasons for finishing aft", in which skidude and i had some good discourse...belabored tho it was. as for me, i'm just a washed up racer that still has an interest in both improving as a skier and improving my ability to describe ski technique Jamt, if i'm not mistaken, has a phd in physics...we can ALL learn a lot from him.

zentune

Davey, We agree that getting inside is a crucial part, the question is whether you push to do that. Theoretically you increase the speed by pushing youself inside the turn because of the angular momentum/skating/whatever that was discussed above. From this viewpoint it is not enough to just get inside. A simple way to get inside is to have different direction of the skis and CoM, however since you have not pushed yourself inside you have not performed any work and thus you have not increased the speed.

This is the defacto approach of racers, unless you are on a really flat part of the slope. They don't push themselves into the turn. Pushing and rise lines are old concepts that are not used any more.

Part of the reason is that in a high end race turn you would be pushing against 2-4 g.

There are other factors as well. When you push you also push the CoM upwards. On average the downward push against the slope is proportional to your weight times cos(slopeangle). This means that if you are artificially creating downward push early in the turn you have automatically decreased the grip later in the turn, when you really need it. If you skid just a little bit you will loose a lot more speed than what you may have gained by pushing early.

Another factor is that you cannot achieve as high edge angles since you need CoM drop for that. We all now how important high edge angles are in todays races.

This is largely a timing issue, I see all the time that my racers mistime the transition so that they have to push themselves into the turn. It is not the fastest runs.

Hi Jamt

Scope of my interest
My interest is in coaching Ski Teachers who want to learn GS in order to pass the Eurotest.
I see that as a different job to teaching World Cup racers.  Or even racers in general.  The objective of racing is winning.

My objectives with the Eurotest is to ski better, and to develop this in my colleagues and people I teach and coach.  Every one of them that gets above the level is a winner.

Purpose and Environment

The purpose of the Eurotest in my worldview is greater understanding rather than it being just a test of youthful athleticism.  Older, stiffer entrants need to use tactics and technique rather than just relying on skiing flat out and dealing with the issues on the way down with flexibility and agility.

Base of knowledge

So - we need a set of templates and tactics and rules of thumb - but we also need a usually unspoken deeper understanding that has a provable biomechanical basis.

If a gifted racer "just does it" then that's a win.  If a ski teacher can "just do it" but has no understanding, or can't teach this then that's a loss.

We need a language capable of describing these things and sometimes a brief worked example gives great enlightenment not just of the concept, but a rating scale of potential usefulness.

It's this I am attempting to compile.

We don't have bodies to burn-up

In teaching adults, we have to work smarter not harder as they have limited time, and we are not trying to select a mutually competing group, we're trying to raise everyone beyond the pass level of 0FIS + 18% and that's a high level

Race teams are pretty cut-throat sometimes, and if people drop out there's a queue of people to get their team place.

The Eurotest is just a time trial all aspirant ski teachers must pass to reach the full qualification.  Everyone helps one another.  It isn't a competition.  Language is a mutually co-operative tool.

The Eurotest courses are FIS spec but not WC steep or icy.  They don't water the course.

Language:

Please have patience with my use of outmoded terms!  I'll replace them on a pragmatic basis when a better term needs used.
I use the term "Fall-Line" - so the converse term "Rise Line" is still pretty useful to me.

I find that having a point to aim for is very useful, and my current thinking is that the business-end of the GS turn can be seen as commencing on crossing a notional rise-line drawn up from the next panel.  It's up to the skier to pick a point on that imaginary line - or move the line if necessary.  Newcomer good skiers who never raced need this.  A skier who has raced since young probably doesn't.

Quote:
Davey, We agree that getting inside is a crucial part, the question is whether you push to do that.

Pushing. I can see that actively forcing the pressure too early is dangerous as it can lose the edge.  I usually consider pushing to be undesirable for that reason.

Grip pressure in the uphill quadrants yes, but gentle resistance pressure. As the turn develops pressure builds passively as the edge rolls on -    Not "pushing" coarsely.

What I try to do is: as the forces build up then I try to extend to the inside - off the outside ski platform by leg extension whilst applying edge angle.

Quote:
From this viewpoint it is not enough to just get inside. A simple way to get inside is to have different direction of the skis and CoM, however since you have not pushed yourself inside you have not performed any work and thus you have not increased the speed.

No? I agree that there will be no KE work-energy increase if no work was done.  But speed may nevertheless get a boost by something else:-

Won't the angular velocity will have increased? In the same way as a pendulum swings faster if you place a pennyweight on top of the bob, which is the same as moving the bob towards the centre.

(I haven't yest worked this through but it certainly works for a pendulum - or an inverted pendulum (see Physics of Skiing P234 Technote 11).

(Big Ben pendulum is compensated to gain 0.25 of a second per day by the raising of the mass of the pendulum with the application of a penny).

(Can you add anything to this?  I'm not 100% on it as to how it fits with conserved energy).

[EDIT] Yes, I forgot about the clock spring (duh).

Thanks again for your thought provoking reply.  Lots of info in there.

Quote:
Originally Posted by Jamt

Davey, We agree that getting inside is a crucial part, the question is whether you push to do that. Theoretically you increase the speed by pushing youself inside the turn because of the angular momentum/skating/whatever that was discussed above. From this viewpoint it is not enough to just get inside. A simple way to get inside is to have different direction of the skis and CoM, however since you have not pushed yourself inside you have not performed any work and thus you have not increased the speed.

This is the defacto approach of racers, unless you are on a really flat part of the slope. They don't push themselves into the turn. Pushing and rise lines are old concepts that are not used any more.

Part of the reason is that in a high end race turn you would be pushing against 2-4 g.

There are other factors as well. When you push you also push the CoM upwards. On average the downward push against the slope is proportional to your weight times cos(slopeangle). This means that if you are artificially creating downward push early in the turn you have automatically decreased the grip later in the turn, when you really need it. If you skid just a little bit you will loose a lot more speed than what you may have gained by pushing early.

Another factor is that you cannot achieve as high edge angles since you need CoM drop for that. We all now how important high edge angles are in todays races.

This is largely a timing issue, I see all the time that my racers mistime the transition so that they have to push themselves into the turn. It is not the fastest runs.

Good stuff....this is a good conversation.

Summary:

• A two foot pump in the high "C" can add energy into a turn
• but it has to be at the right time
• but it has to be in the right direction
• potential of lose grip and skid is far far greater then potential for speed gain.

Anything else?

I would be interested to hear more thou, on what exactley is that "right time" and what exactley is that "right direction", how is determined, factors, etc

lets just focus on this "high C skate pump" we can discuss the "second swing pump" later.

Hi Jamt

You Say Quote:

On average the downward push against the slope is proportional to your weight times cos(slopeangle). This means that if you are artificially creating downward push early in the turn you have automatically decreased the grip later in the turn, when you really need it. If you skid just a little bit you will loose a lot more speed than what you may have gained by pushing early.

I  think this is correct.  It's the vertical reaction you would expect on an inclined plane.

Now here's the bit I want to check with you:-

That reaction force isn't the sole force preventing a skid - unless the surface is completely impenetrable (and since we are describing a turning ski, there must be edge penetration).

In my view, the ski is not on a simple inclined plane - The ski's metal edge is carving a tiny banked track cut into the inclined plane, at the angle of the edge-tilt , and this surely will provide a platform to extend the legs from and do work against the turning forces.

OK, I can see that there will be a component that acts normal to the slope, and this is the force that generates the "Virtual Bump" effect that can project the skier at right-angles up from the plane of the slope.  But we can anticipate this and by the time it occurs, the skier is on flats, short-leg-short-leg swallowing up the energy.

I have three pieces of information.

1 Doing work against the turning forces (by Long-leg-short-leg extending the c.o.m. inside the turn) generates KE

2 Skiers can and do manage to achieve this

3 It isn't easy to do because of risk of skidding and generating a normal reaction force that projects the skier.

Does this fit in with your description?

Quote:
Originally Posted by Davey

I have three pieces of information.

1 Doing work against the turning forces (by Long-leg-short-leg extending the c.o.m. inside the turn) generates KE

2 Skiers can and do manage to achieve this

3 It isn't easy to do because of risk of skidding and generating a normal reaction force that projects the skier.

Does this fit in with your description?

Yes, that would be pretty accurate. Just to be clear, the risk of skidding happens after the extension, not during.

Latest on Joubert, his Avalement movements, and "Virtual Bump"

I have studied in obsessive detail, Joubert's book "Skiing- An Art a Technique (Translated and published Poudre Press 1978).  This is a book rather like the Holy Bible in that there is no index and the "Documents for Specialists" have numbered "chapters and verses".  The book is very difficult to read and follow a structure, or use as a reference without reading the whole thing, but it was written in a pre Desktop-Publishing era.  It's a great resource nevertheless.

On Page 301 Chapter 540 "Initiating Turns by Avalement", verse 542 (d) there is a tiny mention (just in passing) of "The Imaginary Bump".

This (SkiDude - let joy and rejoicing be unbounded :-) ) reads as follows:

Quote from "Skiing- An Art a Technique"
"Avalement during the compression of the short traverses between round, short-radius turns down the fall-line, corresponds to an avalement of an imaginary bump which simulates the slope followed by the skier's feet. (see 218)

(Verse 218 is earlier in the book (page 273 describes the fore-aft balance adjustments required by a skier making a turn as the "slopes followed by the skier" being sinusoidal in profile).

So:- A simulated slope makes an imaginary bump? Apparently - that's a virtual certainty!  Joubert has spoken to us from back through nearly half a century.

Are we closer to an answer on "The "Virtual Bump" - Real or Fiction"?  Clearly a lot of words have been written so far.  There are romantic associations back to the days of yore - vissage, avalement, Un Art - Une Technique, foot-thrusting, platforms, wedln (Joubert likes the Austrian Technique).  Romantic - yes, but not fiction.

Options and logical test

"Fact or Fiction?", "Real or Imaginary?" would have been better options - if only that we could have achieved a logical solution in Joubert's own words:

"Real or Imaginary?" The answer has to be found on page 301 - "Imaginary"!

(But still a useful metaphor).

Footnote:

I even have the French original (Le Ski – Un Art, Une Technique) and the wording is subtly different.

The concept of the “Imaginary” bump is not in the original French as written by Joubert.

On the contrary, he uses the phrase "...en fait,..." (in fact).

Joubert writes: “En virages boucles a courts rayons enchaines de long de la ligne de la pente .  L'avalement qui permet d'escamoter les reprises d'appui dans le brèves traversees qui separent les virages correspond a l'avalement a l'espece bosse que dessine en fait, a cet endroit, la pente suivi par les pieds du skieur au cours de la deplacement.”

“Imaginary” is a word that appears only in the English translation done by James Major, Sim Thomas and Doug Smith.

This they use to translate with interpretation the literal French:-

Avalement that allows the retraction of the support applied in the brief traversings that separate curves, corresponds to the
avalement [of a] species of hump which draws, in fact, here [at each place], the slope followed by the skier's feet in the displacement [turn].

So did Joubert know about the Imaginary Bump - or is it down to interpretation by the translation team?

Pretty good translation Davey, you forgot avalement = swallowing!

Seems like he thought of it as a factual bump that the feet trace during their displacement across the hill between turns.

Quote:
Originally Posted by Ghost

Pretty good translation Davey, you forgot avalement = swallowing!

Seems like he thought of it as a factual bump that the feet trace during their displacement across the hill between turns.

Swallowing. Nudge, nudge. (Beavis sniggered) ...
Yes, we all know that "avalemant" can be translated as "swallowing", but we use the the verbatim French word in English, in the same way as we use other French words unchanged :"braquage", "angulation", "apres-ski", "discotheque", "couloir", "piste" "salon","avalanche", "Salopettes". The American-English translation uses the French word "avalement" because "swallowing" loses meaning/ has multiple meaning in the translation.  Obviously it really means absorption, rather than anything that involves une salope!
Joubert is clearly more of a source of the 'bump' concept than others.  He calls it a "species of bump" in that the [factual] profile of the slope traced by the feet would be a topological bump if projected in a different plane.

In my race coach 2 course, the conductor said to me at one point, "You look like you're skiing the virtual bump. There is no bump--get over it!" His point was to create speed I'd need to maintain the skis' pressure against the snow, and stop absorbing through the transition.

So while we can ski a virtual bump in recreational skiing, and while it's totally fine to do so, it's not the way to generate power. I've also seen the virtual bump used as a "cheat" for people who aren't otherwise capable of steering their skis through the transition. So it's ideal to be able to ski both ways.

Quote:
Originally Posted by Metaphor_

In my race coach 2 course, the conductor said to me at one point, "You look like you're skiing the virtual bump. There is no bump--get over it!" His point was to create speed I'd need to maintain the skis' pressure against the snow, and stop absorbing through the transition.

So while we can ski a virtual bump in recreational skiing, and while it's totally fine to do so, it's not the way to generate power. I've also seen the virtual bump used as a "cheat" for people who aren't otherwise capable of steering their skis through the transition. So it's ideal to be able to ski both ways.

Quote:
Originally Posted by Skidude72