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The "Virtual" bump.... real or fiction? - Page 3

post #61 of 340
Quote:
Originally Posted by Skidude72 View Post


and I would suggest the quickest turners are WC racers.....and WC racers dont rely on this rebound idea to win races....they do however know, understand, and work with the virtual bump


My understanding of the 'virtual bump' is popping. I do not see any popping in downhill racing. Racers are carving, riding their edges and whipping their feet around to get better angles thru gates. I see that as a different movement than what we are discussing... No?

post #62 of 340
Quote:
Originally Posted by kgbudz View Post




My understanding of the 'virtual bump' is popping. I do not see any popping in downhill racing. Racers are carving, riding their edges and whipping their feet around to get better angles thru gates. I see that as a different movement than what we are discussing... No?



No.   Virtual bump is not popping.....Virtual bump IS what you see in WC SL, GS, SG, and DH.  Virtual bump is about retraction in transition and extension into the fallline....well at the WC level anyway....generally speaking.

 

Edit:  Sorry, I really shouldnt do this after a bottle of red.  The Virtual Bump is actually a concept that explains via physics, which forces we feel and why....it also explains what forces we DONT feel...and why.   It really does nothing more then that.  It does not dictate how we should deal with these forces, or lack of them....it just provides understanding as to what is happening.  Technique is about managing those forces....WC technique for doing it, is broadly speaking described above.


Edited by Skidude72 - 6/12/10 at 3:29am
post #63 of 340
Quote:
Originally Posted by kgbudz View Post




My understanding of the 'virtual bump' is popping. I do not see any popping in downhill racing. Racers are carving, riding their edges and whipping their feet around to get better angles thru gates. I see that as a different movement than what we are discussing... No?


Definition of "Virtual" from Wikipedia:

 

"The term has been defined in philosophy as "that which is not real" but may display the salient qualities of the real."

 

My understanding is that when folks are talking about the virtual bump (vb), they are saying that you feel the same things as when going over a real bump (rb).  This happens in a turn.  The apex in vb feels the same as cresting a rb (its apex) unless you catch air.  .  The only significant difference is that in a virtual bump, I'll have inclination, angulation, and my legs will be extended/flexed differently from each other.

post #64 of 340


 

Quote:
Originally Posted by michaelA View Post

 

Now that's an interesting thought. I'm not really sure how to model it that way. 

 

If I have a heavy puck and light puck sliding around the same icy curve, neither would go faster than the other.  Add a spring under each puck (which compresses due to CF) and ... I'd see the same results, wouldn't I?

 

That said, on its own the spring's rebound would add nothing regardless the weight of the puck... er, right?  (Am I missing something here?)

 

 

'Stability' is an interesting term in itself.  Provide a good definition in relation to actual skiing and that might be an interesting topic to explore.


.ma

You could be right; maybe you don't have to loose the weight.

Let me think about this.

 

The ski will only bend to a maximum amount without damage or breaking loose from the snow.  F=kx is the force required to depress the spring to it's maximum distance x with spring constant k.   F= m(V^2)/r.  At a given mass we can achieve that with higher V and smaller r.  With higher M, we need less v or not so small an R,  F=M(v^2)/R.  If we are in the turn of constant radius, no real difference in the acceleration and forces; the centripetal forces are in balance with the spring forces.  However if we suddenly reduce the radius to make use of the spring, our acceleration out of the spring is given by F/M or F/m.  Clearly a skier who has his ski flexed to the max and finds a way to use the stored energy will get more pop if he has less mass.

 

Sorry, you gotta loose the weight.  However, don't be too quick to loose it; heavier skiers can ski faster

post #65 of 340
Quote:
Originally Posted by Ghost View Post

L&AirC,

You are right on both counts.  The lighter would have difficulty bending the stiff ski, unless he were skiing 4 times as fast.  However once he has the ski bent (due to great speed and dynamic loading) his acceleration out of the turn is greater due to his lower mass.


Understand the concept but there is no real world application unless you go to the extremes - give a 250# novice racing skis and then give the same skis to a 150# racer.

 

In order to get the rebound affect, you need to be able to carve so if we take two carvers, how can the one that is significantly lighter go 4 times faster?  Or even twice as fast for that matter?  It is usually the larger person that goes faster provided they have the same skill set.

 

Either this only works on paper or I'm missing something?

post #66 of 340


 

Quote:
Originally Posted by kgbudz View Post

Leaning back on your tails may give you some springing rebound... However, after a few turns your tails will more than likely cross, hook up, and force you to bail out of whatever you were doing. Wouldn't this be textbook back seat skiing? Stiff tails are important because they help you maintain an aggressive forward position and also are key in initiating quick turns. In my opinion, pop comes from edges, not tails. You ever push an object over a friction surface and the object starts bouncing up and down as it moves forward? Its kind of like a stuttering effect. Loading your frontside edge and then releasing will 'pop' you into the air (assuming the force is great enough to lift your feet, boots and skis). Most other movement requires work on the skiers part to lift limbs and gear into the air. In moguls, im pretty sure if you flex your skis between 2 bumps you will get a bit of pop. I dont think this is a very sustainable strategy when moving forward thru the fall line.

Yes back seat, but not clasic back seat skiing.  Carefully timed backseat skiing.  You have slight forward bias at initiation, centre weight bias at apex and  keep enough weight forward through out the turn to keep the tip engaged until you want to access the pop.  When, and only when you want to play with the rebound, you shift the main contact point, or more precisely the point at which your force vector goes through the ski to the tails.

 

If you bend a ski into a curve, like the bow of a bow and arrow, and cut the bow string when the middle of the ski is on the ground, the tip and tail sping down and the only rise you get is the middle coming up to compensate for the tip and tail going down - centre of mass of the bow stays in the same place.  Your ski might jump six inches off the ground.  Talk about minimal forces.  However if the tail of the ski is on the ground, the tail will push the ski up.  Of course, you don't get your ski vertical; there is only so much you can do to get close to this, even if you're doing ski-ballet, but you can get a noticeable force out of the ski, directed forwards and up by being on the tails when you release the energy.  

 

However it is not a big force in the grand scheme of things. It's only a fun thing to play with; not a technique to better turns.  If you see any WC footage of same, it's because the skier made a big mistake and got way too far in the back seat, and most likely will crash, or at least lose lots of time trying to make the next gate.  There is an element of it in play in all turns, tips slow - tails fast, but only an element, not a game changer.

 


 

post #67 of 340


 

Quote:
Originally Posted by L&AirC View Post




Understand the concept but there is no real world application unless you go to the extremes - give a 250# novice racing skis and then give the same skis to a 150# racer.

 

In order to get the rebound affect, you need to be able to carve so if we take two carvers, how can the one that is significantly lighter go 4 times faster?  Or even twice as fast for that matter?  It is usually the larger person that goes faster provided they have the same skill set.

 

Either this only works on paper or I'm missing something?

I've discovered on this form that there are a lot of folks who choose not to ski faster, even though I suspect Michael isn't one of them; in order to ski faster he would probably have to do his carving in icy chutes.

 

However you could be missing post 64.  The lighter skier can get maximum bend out of the ski by skiing a smaller radius. 
 

post #68 of 340
Quote:

Originally Posted by Ghost View Post

 

...there are a lot of folks who choose not to ski faster...

 

However you could be missing post 64.  The lighter skier can get maximum bend out of the ski by skiing a smaller radius. 

 

We aren't talking about them.  We need to compare apples to apples so the only difference should be weight.  The larger person choosing not to ski fast would be matched with a smaller person that chooses to not ski fast.  Otherwise we are comparing skill sets and not weight differences.


Nope.  Saw #64.  So could the heavier skier.  OK.  I didn't check the math and I won't pretend to understand the formulas but what it means is that given the same rebound force, a lighter skier will receive more of a rebound than the heavier one.  The lighter skier can achieve this by either entering the turn faster or turning tighter, right?

 

The only advantage the lighter skier has is coming out of the turn.  The heavier skier has the same turn shape advantages as the lighter skier going into the turn.  The lighter skier is at a disadvantage going into the turn.  Even if we compares oranges to clementines, and say the orange doesn't want to go too fast, that person is probably shaving speed by turn shape so they aren't entering the turn that fast but lets say they are going 20 mph when they enter the turn.  20mph seems rather slow to me but it is a # to start with.

 

Your explanation requires the smaller person to get to 80 mph on the same run going into the same turn and I don't think that is close to realistic assuming both are carving albeit one is carving slower.

 

Again, I agree with the math; just not the application.

post #69 of 340

The 4 times as fast is hyperbole, but faster will produce more force (twice as fast = 4 times as much force), as will turning tighter (1/2 the radius = twice the force).

 

There are many ways to ski slower, including starting the steep pitch at a lower speed, choosing a line that meanders a great deal instead of following the fall line, etc.

 

Apples to apples, high skill set, both skiers can max out the ski. The lighter skier will not get more rebound, but will notice the rebound more because it will affect him more; the same rebound force will cause a greater change in his motion.

post #70 of 340

Rebound is small and hard to access.  The virtual bump, and vaulting are large and easy to access.  Many people encounter forces from the virtual bump and vaulting and wrongly attribute them to rebound.  Many believe it is better to deny the existence of the smaller less significant forces so as not to complicate things.  I would rather explore it all.

post #71 of 340
Quote:
Originally Posted by Ghost View Post

Rebound is small and hard to access.  The virtual bump, and vaulting are large and easy to access.  Many people encounter forces from the virtual bump and vaulting and wrongly attribute them to rebound.  Many believe it is better to deny the existence of the smaller less significant forces so as not to complicate things.  I would rather explore it all.


 

One of my favorites sayings is:

 

"Knowledge, like most things in nature, seeks the path of least resistance, so you have to keep an open mind."

post #72 of 340
Quote:
Originally Posted by Ghost View Post

The lighter skier will not get more rebound, but will notice the rebound more because it will affect him more; the same rebound force will cause a greater change in his motion.


What of the force required to bend the ski, Ghost.  Wont it have a greater effect on the lighter skier as well?  Is there a net gain or loss at the end of the turn?

post #73 of 340

Correct, there is no free lunch.  To get energy out of the ski, you have to put it into the ski first.

post #74 of 340

Is there not also tax on that lunch too?  Compression of the surface to bend the ski further that is not returned, losses/friction/whatever from the interaction of the ski with the surface as it bends, additional slip/drift/scrubbing, whatever...

 

Will the ski return 100% of what is put in?

 

I don't know, but seems intuitively like a net loss to me to try to expend additional effort for this effect.  Not to say it isn't fun, though:)  

post #75 of 340

^^^

Absolutely right, but hey, if you've got it, spend it.

 

There are two things not to get confused: one is playing with rebound, like a mad kangaroo, the other is getting the most energy back from the ski that you can without compromising the turn.  They use the same physics, but are different ends of the spectrum.

post #76 of 340
Quote:
Originally Posted by L&AirC View Post




Definition of "Virtual" from Wikipedia:

 

"The term has been defined in philosophy as "that which is not real" but may display the salient qualities of the real."

 

My understanding is that when folks are talking about the virtual bump (vb), they are saying that you feel the same things as when going over a real bump (rb).  This happens in a turn.  The apex in vb feels the same as cresting a rb (its apex) unless you catch air.  .  The only significant difference is that in a virtual bump, I'll have inclination, angulation, and my legs will be extended/flexed differently from each other.





Quote:
Originally Posted by L&AirC View Post




 

One of my favorites sayings is:

 

"Knowledge, like most things in nature, seeks the path of least resistance, so you have to keep an open mind."



Well.....just a tip.  If you are sincere about exploring skiing and and all it has to offer, dont rob yourself of discovery and knowledge by simply looking at the title ascribed to a concept....some, perhaps many, of which were originally named in another language.....and then think you know and understand the entirety of the concept.

 

You cant judge a book by a cover, nor can you glean the information inside by knowing the title.....nor can you understand, deduce or apply a ski concept by taking literal, or perhaps even Webster's dictionary definitiions of the words used in a concept's title.

 

I think it would be fair critisim to say the ski teaching/coaching industry uses terms that dont quiet sink with everyday usage, and are thus confusing.  But, right or wrong....those are the terms used. 

 

In practice, what most pros do, is learn and understand the ski terms...not so much so they can chat on "Epicski" ...but to learn what the concepts mean, and how they fit together.  Of course by the time they have done that, the terms kinda make sense, and hence they adopt them as their own.  However, many (most?) pros, while they have the ski terms as their own, they are also cognicent that their students likely do not, hence they reword things using everyday language for the benefit of their cleints.  

post #77 of 340

Okay. Have a wee bit of time this evening. So, after reading all the thoughts to date in this thread, thought it would be "quicker" to try and simply describe our model of rebound, or the virtual bump if you want to call it that.... 

 

First REBOUND

 

First history of rebound.

 

In the 1920's thru about 1940's, the Arlberg method was THE way to ski. The skis were long, the edges sometimes were not there at all, the boots were soft and the bindings were quite variable.

 

One of the techniques for turning in higher speed skiing was to first check your speed with a down stem. This not only checked your speed, but it also created a rebound, an up un-weighting, effect, which, with whole body swing enabled the skier to turn. The pivot point was up front and the tails were displaced creating the turn.

 

A lot of people called that rebound unweighting. 

 

Around the late 1940's, this developed to displacing both skis downhill, using a tail thrust, in a check turn. This still used today. This is what I learned to ski with, an old friend check turns are.

 

As ski design evolved, the pivot point worked its way from the tip of the skis back to under the feet.

 

Rebound still remains as a widely used method of unweighting to turn.

 

(Note: Today you will also still see many skiers using an ab-stem to help them make a parallel turn. They push off of their outside ski to start the new turn. This is actually quite similar to creating rebound, but only with a lesser amount of energy. However, this ab-stem move is totally not necessary, it's considered "bad" skiing these days.)

 

Rebound with our "modern" skis, is still quite fun to use. Various factors contribute to the amount of energy you can produce with a rebound: 

(Note: these are very concise, so each word counts and I'm not repeating them to explain right now for the sake of time)

 

1. The speed, shape and size of the turn

 

2. The amount of pressure transferred to the skis by resisting the turn forces. Resisting means not flexing the legs or body to yield to the turn forces.

 

3. The amount of pressure added to the skis via muscular energy by either/and/or:

      a. Pushing down on the skis by extending the legs, or 

      b. Dropping the body through flexing and then suddenly stopping the flexing to transfer more weight to the skis

4. The timing of your response to the turn forces (late weight transfer creates more rebound)

5. The snow texture and consistency - how much cohesion or inherent integrity it has, this is a function of moisture content, air content and temperature

6. The shape of the terrain

7. The stiffness and design of the boots and skis

 

So, making round turns, with strong lively boots and skis, on strong snow, "cutting off" the turn with high edge angles, "loading" the skis up by resisting or even adding to the turn pressure forces, and then quickly releasing the energy will give you a big "pop" or rebound.

 

How you manage rebound really tells you how efficient you are as a skier.

 

Pressure management is the hardest skill to master.

 

Rebound is only one way of managing the turn forces. It only exists if you, the skier, chooses to react to the turn forces in such a way to create the rebound.

 

By using progressive weight shift, progressive flexion/extension, progressive edging, you can absorb and manage the turn forces to create a smooth finishiaton with no minimal rebound. 

 

The choice is yours.

 

 

 

 

 

 

 

 

 

 

 

 

 

post #78 of 340

A brief bit on my take of the virtual bump. Obviously, as we can see from so many interpretations of the term, it is a very confusing term. For some it's rebound and for others it's not. Some are Ron LeMaster's fans and some interpret his virtual bump a bit differently, with a different WC twist on it all... but say it's the same.

 

As far as I can tell, both rebound and the virtual bump are different words that are used to describe the same turn forces and how to manage them. So basically, they refer to them "stuff" (how is that for a technical term) in the turn.

 

I find that term confusing because of the word bump. In skiing we have real bumps. Rebound is not necessarily similar to skiing real bumps. 

 

There is quite a variety of techniques to ski the bumps. You can use rebound in the bumps, but normally, the turn forces in the bumps are not consistent with creating a lot of rebound. However, the terrain in the bumps can be use. Of course, on a groomed slope, you don't have the use of the real bump.

 

It seems to me that the term the virtual bump was created mainly for sales - something new, updated, unique, to sell - a new fad. 

 

So, I personally will never refer to the virtual bump when I teach or clinic. I prefer to talk about the turn forces and how we manage them. I will use the word rebound because it is a very historical and descriptive term.

 

 

 

 

 

post #79 of 340

So I think ive jumped way too far forward in some of my statements. Skidude72 states: Virtual bump IS what you see in WC SL, GS, SG, and DH. Then there are references to fall lines which racers never enter since gates are spread out in an anti fall line matter. So maybe I need to understand how racing thru gates is riding a fall line? Ghost describes the virtual bump as back seat carving. If I understand both statements, it seems like you guys are talking about carving forces. Sure, its fun to do and I do it all the time on the flats. Attempt this on any sort of challenging/steep terrain and you will be on your ass or maybe even your face faster than you can say "OH SH*T". So when I described 'popping' as my idea of the virtual bump, I threw something out there so unknown that I would say 98% of skiers out there have no clue what im talking about and therefor cannot understand. I dont mean this as a diss. If anything, ive misunderstood the concept being discussed here.

 

edited to be nice :]


Edited by kgbudz - 6/13/10 at 1:10am
post #80 of 340
Quote:
Originally Posted by kgbudz View Post

I threw something out there so unknown that I would say 98% of skiers out there have no clue what im talking about and therefor cannot understand.

Here, denizens have adopted "97%" as our universal precision for opinions.

 

 

Quote:
Originally Posted by kgbudz View Post
I dont mean this as a diss. If anything, ive misunderstood the concept being discussed here.

 

edited to be nice :]

 

Trying to describe anything experiential in a text based medium is always a challenge and we certainly appreciate your niceness..!   

 

In the posts above people have described some uses and effects of the Virtual Bump but I'd agree things could be written more clearly and definitively about "what it is".  (If we've posted poor explanations, that's not the fault of the concept itself.)   I'll try to post an alternative description that might work better when I get time.

 

.ma
 

post #81 of 340


 

Quote:
Originally Posted by kgbudz View Post

So I think ive jumped way too far forward in some of my statements. Skidude72 states: Virtual bump IS what you see in WC SL, GS, SG, and DH. Then there are references to fall lines which racers never enter since gates are spread out in an anti fall line matter. So maybe I need to understand how racing thru gates is riding a fall line? Ghost describes the virtual bump as back seat carving. If I understand both statements, it seems like you guys are talking about carving forces. Sure, its fun to do and I do it all the time on the flats. Attempt this on any sort of challenging/steep terrain and you will be on your ass or maybe even your face faster than you can say "OH SH*T". So when I described 'popping' as my idea of the virtual bump, I threw something out there so unknown that I would say 98% of skiers out there have no clue what im talking about and therefor cannot understand. I dont mean this as a diss. If anything, ive misunderstood the concept being discussed here.

 

edited to be nice :]

I've been misquoted.  The "virtual bump" is not backseat carving; carving is not even required.  Despite what Cookie says, and I'm glad he is not using the term in his this teaching, it is not rebound; if you were skiing on rigid skis with zero flexibility, you would still have a virtual bump.  The virtual bump only occurs on a slope that is not horizontal, and it occurs on every turn on a slope regardless of backseat, front seat, or even mode of travel.  It is a change in the vertical profile of your path.

 

Let me try again. 

 

The virtual bump is the bump in the ELEVATION profile of your skiing skiing run.  Imagine skiing a perfectly smooth uniform slope of say 20 degrees.  If you were to attach a little wheel and counter to your ski's tail (or stick a gps with a 0.01 second recording rate (not your average GPS; mine only goes down to 1 per second) in your pocket), and then plot on an X-Y the elevation as Y and the distance traveled as X, you would see a virtual bump when you turned out of the fall line.

 

Imagine skiing straight down a slope that has a road through the middle, with a vertical profile like so.

\

..\

....\

......\

.......---------

...................\

.....................\

.......................\

You would feel a bump, because you were skiing down at a big angle until you hit the road and the road made you ski horizontally.

 

Now imagine skiing straight down a slope with no road.

\

..\

....\

......\

........\

..........\

............\

You would feel no bump.

 

Now imagine skiing straight down a slope with no road, but you make one turn out of the fall line and back into it in the middle of the run.  You feel a bump because you were skiing down at a big angle and your turn made you ski horizontally.  There is no road on the hill so the bump is virtual.

 

Rebound is using the spring of the ski, no matter how that energy got into the ski.  97% of skiers can't make use of rebound, and 97% of those who can, don't understand it.

 


 

post #82 of 340
Quote:
Originally Posted by Cookie Bewley Hale View Post

A brief bit on my take of the virtual bump. Obviously, as we can see from so many interpretations of the term, it is a very confusing term. For some it's rebound and for others it's not. Some are Ron LeMaster's fans and some interpret his virtual bump a bit differently, with a different WC twist on it all... but say it's the same.

 

As far as I can tell, both rebound and the virtual bump are different words that are used to describe the same turn forces and how to manage them. So basically, they refer to them "stuff" (how is that for a technical term) in the turn.

 

I find that term confusing because of the word bump. In skiing we have real bumps. Rebound is not necessarily similar to skiing real bumps. 

 

There is quite a variety of techniques to ski the bumps. You can use rebound in the bumps, but normally, the turn forces in the bumps are not consistent with creating a lot of rebound. However, the terrain in the bumps can be use. Of course, on a groomed slope, you don't have the use of the real bump.

 

It seems to me that the term the virtual bump was created mainly for sales - something new, updated, unique, to sell - a new fad. 

 

So, I personally will never refer to the virtual bump when I teach or clinic. I prefer to talk about the turn forces and how we manage them. I will use the word rebound because it is a very historical and descriptive term.

 

 

 

 

 

 

Cookie,

 

Virtual Bump and Rebound cross paths but they aren't the same thing. 

 

The Virtual Bump refers to a skiers body position and what the skier feels in a turn.  It is not about skiing bumps and isn't about bump skiing techniques.

 

Rebound refers to the forces in that same turn, whether the skier feel them or not.

 

The two aren't interchangeable.

 

They happen at approximately the same place (the turn) but the discussion on each it about two completely different things.  One is the discussion of our bodies position and the other is a discussion on the science of the energy of forces at play.

 

You can have a virtual bump without rebound.


Because some people get them confused is all the more reason to continue discussing them.  The first step is denial.

 

I think this thread proves that I have had a hard time understanding rebound (vaulting I'm OK with).  Several here where patient enough to walk me through it.  I will probably never have a discussion on it with anyone other than a few race buddies, but when I do, I will be able to discuss it and probably prove to them that what they think is rebound from the ski is probably vaulting.

 

No one has to use the virtual bump in their classes or their thoughts.  Ron LeMaster (whose books I love) describes it and I believe for no other reason than as an analogy to help people understand. It is only a tool that you can choose to add to your tool box. It helped me but it may confuse others.  What book/definition/analogy on any subject hasn't done that?

 

Should virtual bump ever come up while teaching a class or for some reason I think it will contribute to a students understanding, I'll find a "whale back" have them ski over it without catching air (see post #29 or #81), then have them do a turn at the same speed and ask them the differences and similarities between the two with relation to their bodies position.

 

Should the same thing happen with rebound, I'll tell them to go see the Snow Sports Director

 

Ken


Edited by L&AirC - 6/13/10 at 7:08am
post #83 of 340

I agree with Ghost that the 'virtual bump' and 'rebound' are not the same thing.

 

Rebound is potential energy in the system being turned into kinetic energy. Your ski decambers thus increasing the ski's potential energy. If you release that energy quickly you get significant rebound. This is usually perceived as the ski shooting under your body from one turn into the next. You have to consciously permit that energy to 'trampoline' your ski. If you interfere with it, that energy is transferred elsewhere. That elsewhere is often your body. Consider that rebound's effect is typically at an angle square to the waist of the ski. Rebound at the end of the turn, when the ski is pointing across the hill, will direct most of the energy up the hill. Rebound at the apex of the turn (when the ski is pointing down the fall line) will result in the energy being directed across the hill. That is why you don't typically see SL skiers hanging on to their edges past the apex of their turn. They want the energy directed advantageously across the hill, not up the hill.

 

The virtual bump is the result of our CoM crossing from one side of our skis to the other as we go from one turn to another. Flexing 'over' the 'virtual bump' is a pressure management tactic, just as flexing over a real bump is pressure management. (That is the only similarity I see to merit the use of bump in both terms.) Failure to flex over a real bump will often cause you to either fly off the bump or have your knees forced to your chest or double heel eject. Failure to flex over the 'virtual bump' will cause forces to build at the end of the turn. These forces can result in undesired edge hold and the failure of the ski to finish or release from a turn sending you over the high side or worse. This type of fall looks a lot like getting tossed in the bumps. Coincidence? Or similar forces acting on the skier?

 

The greater the angulation and/or inclination used in a turn, as well as the quickness of the turns (SL being quicker than GS), the more significant the 'virtual bump' to be dealt with.

post #84 of 340

Im with Ghost. The virtual bump is real .

post #85 of 340
Quote:
Originally Posted by tdk6 View Post

Im with Ghost. The virtual bump is real .


So is rebound!!!

 

I relate them both to a crossunder/retraction turn but not a crossover turn.

post #86 of 340



 

Quote:
Originally Posted by Cookie Bewley Hale View Post

Okay. Have a wee bit of time this evening. So, after reading all the thoughts to date in this thread, thought it would be "quicker" to try and simply describe our model of rebound, or the virtual bump if you want to call it that.... 

 

First REBOUND

 

First history of rebound.

 

In the 1920's thru about 1940's, the Arlberg method was THE way to ski. The skis were long, the edges sometimes were not there at all, the boots were soft and the bindings were quite variable.

 

One of the techniques for turning in higher speed skiing was to first check your speed with a down stem. This not only checked your speed, but it also created a rebound, an up un-weighting, effect, which, with whole body swing enabled the skier to turn. The pivot point was up front and the tails were displaced creating the turn.

 

A lot of people called that rebound unweighting. 

 

Around the late 1940's, this developed to displacing both skis downhill, using a tail thrust, in a check turn. This still used today. This is what I learned to ski with, an old friend check turns are.

 

As ski design evolved, the pivot point worked its way from the tip of the skis back to under the feet.

 

Rebound still remains as a widely used method of unweighting to turn.

 

(Note: Today you will also still see many skiers using an ab-stem to help them make a parallel turn. They push off of their outside ski to start the new turn. This is actually quite similar to creating rebound, but only with a lesser amount of energy. However, this ab-stem move is totally not necessary, it's considered "bad" skiing these days.)

 

 

 

 


Well.....the effect you are referring to is known as "Vaulting".  If you now want to call that rebound, that is fine I guess....but the effect is caused by vaulting.  Not any spring like effect. 

 

It works like this:  As you turn you will have some degree of inclination....due to the inclination, your COM is lower to the snow then it would be if you were just standing....now as you complete the turn with an edge set as you describe above...whether on old skis, or new, you effectivley have just increased the rate at which your COM and BOS are converging.  Your COM wants to keep travel in its direction, but your skis wont allow it....so either your legs can flex to abosorb the COMs energy or you can keep them rigid, or at least semi rigid....if you do this the legs act like a pole vaulters pole....this pushes our COM UP....that is what cuases the UP sensation....not springs....

 

Again, the concept of Vaulting is well known, understood and written about in modern ski books. 

 

Quote:

Originally Posted by Cookie Bewley Hale View Post

 

Rebound with our "modern" skis, is still quite fun to use. Various factors contribute to the amount of energy you can produce with a rebound: 

(Note: these are very concise, so each word counts and I'm not repeating them to explain right now for the sake of time)

 

1. The speed, shape and size of the turn

 

2. The amount of pressure transferred to the skis by resisting the turn forces. Resisting means not flexing the legs or body to yield to the turn forces.

 

3. The amount of pressure added to the skis via muscular energy by either/and/or:

      a. Pushing down on the skis by extending the legs, or 

      b. Dropping the body through flexing and then suddenly stopping the flexing to transfer more weight to the skis

4. The timing of your response to the turn forces (late weight transfer creates more rebound)

5. The snow texture and consistency - how much cohesion or inherent integrity it has, this is a function of moisture content, air content and temperature

6. The shape of the terrain

7. The stiffness and design of the boots and skis

 

So, making round turns, with strong lively boots and skis, on strong snow, "cutting off" the turn with high edge angles, "loading" the skis up by resisting or even adding to the turn pressure forces, and then quickly releasing the energy will give you a big "pop" or rebound.

 

How you manage rebound really tells you how efficient you are as a skier.

 

Pressure management is the hardest skill to master.

 

Rebound is only one way of managing the turn forces. It only exists if you, the skier, chooses to react to the turn forces in such a way to create the rebound.

 

By using progressive weight shift, progressive flexion/extension, progressive edging, you can absorb and manage the turn forces to create a smooth finishiaton with no minimal rebound. 

 

The choice is yours.

  

 

 

Yes the Vaulting effect is greater with modern skis.  Why?  Well the greater amont of ski performance means we incline more.  In the WC we see WC racers make some turns with their hips literally inches from the snow.  While the average person will never got over this far, most are capable of getting over to say 45degrees...that is certainly further then we could do on wood skis, with leather boots!.  The greater the inclination, the greater the potential for a Vaulting effect.  It works like above....just greater on shape skis because we started from a lower position, are likely skiing faster...so we get vaulted higher, quicker (vectors! still)....thus a greater feelling of weightlessness....but still, it is valuting....no springs or bouncing going on.

 

Also, you still need to explain #5....just stating it over and over doesnt make it true.  To help thou, you will also need to consider the snows structure, it changes with age...plus snow layers etc...also I know you mentioned temperature...but it is really the temperature gradient that is most critical...well in avalanche forecastinng it is anyway....not that it will make your case thou, but these are critical factors in snow science. 


Edited by Skidude72 - 6/13/10 at 7:18pm
post #87 of 340
Quote:
Originally Posted by Atomicman View Post




So is rebound!!!

 

I relate them both to a crossunder/retraction turn but not a crossover turn.


That means there are two three spots to bring see the virtual bump on a relatively flat trail; left turn, right turn and dead center of a cross under turn.

 

The first time I heard the term I immediately thought of a cross under turn.  Specifically at the point when your feet are directly under you.  You're then in the same position you would be in going over a bump and absorbing the energy by flexing, though there isn't actually a bump.

 

At some point later I heard of it as it's been discussed in most of this thread and it is located in the turn when your feet are extended (though flexing) at your side and thought that was the spot because more were talking about it there.

 

tdk6 put some drawings in post #104 in the "Blending" thread that help you see the virtual bump.

post #88 of 340
Quote:
Originally Posted by MastersRacer View Post

I agree with Ghost that the 'virtual bump' and 'rebound' are not the same thing.

 

Rebound at the apex of the turn (when the ski is pointing down the fall line) will result in the energy being directed across the hill. That is why you don't typically see SL skiers hanging on to their edges past the apex of their turn. They want the energy directed advantageously across the hill, not up the hill.

 

 

 

Well this interesting.  I know this was conventional wisdom about 15 years ago....but I thought this idea has been bunked.  The tactic you describe is still obviously what is done in SL and GS, but the reasoning behind why it works is more about maximising glide to maintain maximum speed through transition and to smooth out the transition enabling less dramatic disruption of the COM allowing for earlier and more effective edge engagment of the new turn

 

 

 

 

Quote:

Originally Posted by MastersRacer View Post

 

The virtual bump is the result of our CoM crossing from one side of our skis to the other as we go from one turn to another. Flexing 'over' the 'virtual bump' is a pressure management tactic, just as flexing over a real bump is pressure management. (That is the only similarity I see to merit the use of bump in both terms.) Failure to flex over a real bump will often cause you to either fly off the bump or have your knees forced to your chest or double heel eject. Failure to flex over the 'virtual bump' will cause forces to build at the end of the turn. These forces can result in undesired edge hold and the failure of the ski to finish or release from a turn sending you over the high side or worse. This type of fall looks a lot like getting tossed in the bumps. Coincidence? Or similar forces acting on the skier?

 

The greater the angulation and/or inclination used in a turn, as well as the quickness of the turns (SL being quicker than GS), the more significant the 'virtual bump' to be dealt with.


 

I think you are confusing Virtual Bump with Vaulting.  Two entirley different concepts.....for clarity thou, they are not mutually exclusive, and work together to explain effects experienced in transition.

 

post #89 of 340
Quote:
Originally Posted by L&AirC View Post




That means there are two three spots to bring see the virtual bump on a relatively flat trail; left turn, right turn and dead center of a cross under turn.

 

The first time I heard the term I immediately thought of a cross under turn.  Specifically at the point when your feet are directly under you.  You're then in the same position you would be in going over a bump and absorbing the energy by flexing, though there isn't actually a bump.

 

At some point later I heard of it as it's been discussed in most of this thread and it is located in the turn when your feet are extended (though flexing) at your side and thought that was the spot because more were talking about it there.

 

tdk6 put some drawings in post #104 in the "Blending" thread that help you see the virtual bump.



Um....well, like a bump...it is not an instaneous point....it is a curve....so what is more important?  The top of the bump (curve) or the bottom?  Well, trick question: obviously it is all important....you are flexing as you are going "up" the bump....and extending as you go "down" the back side......it is all critical to the concept and indeed.....real skiing.

 

 

Also....just checked those drawings....those show Vaulting....not the Virtual Bump.


Edited by Skidude72 - 6/13/10 at 8:27pm
post #90 of 340

Skidude72,

 

I think you missed my point.  I wasn't pointing out different parts of the bump.  I was pointing out that depending on your view, there are three places to see it.  In my crude drawings below, #1 is a turn viewed from above, #2 is the same view turning in the opposite direction.  #3 is a cross under virtual bump and the view is from down the mountain looking up at the skier.  A crossover turn wouldn't have this virtual bump but it would have 1 and 2. 

 

In a cross under your upper body stays at the same(ish) height but your legs extend and flex from side to side.  So in a cross under you have the vb when you turn right, when you turn left and you have an additional vb because your skis have traveled under you from left to right in between the two turns.  #3 starts at the apex of one turn and ends at the apex of the next.

 

The OLF drawing could be used to illustrate the cross under virtual bump.

 

 

virtual bumps.png

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