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# The "Virtual" bump.... real or fiction? - Page 4

Quote:
Originally Posted by L&AirC

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.

No you are confusing Vaulting with Virtual Bump.....while obviously related they are separate concepts.  It is possible to flex such that you reduce vaulting to effecitivley nothing....this is shown in TDk6's post 104 diagram...second one if memory serves.....but you cannot make the virtual bump disappear when turning on a slope....you can "work with it"....but you cant make it go to 0, unless you go straight, or are skiing on flat (ie not pitch) ground.....

Ok the "S" on the left is the skis path....viewed from above...pretty standard.

The view on the right shows the skis trajectory....or RATE at which they change elevation.  I think that might be an easier way to think of it.  So as you are in the transition, your skis are not losing elevation...this is the "flat" bit you see on the diagram to the right...as the skis turn into the fall-line you see the skis drop in elevation....this is shown on the right, as the steeper bit....as you ski from the transition to the fallline....that is equal to skiing from a flat to a pitch....and that of course is equal to skiing over a bump...or rollover.  That is the virtual bump concept.

Another way to think of it, or help understand using numbers:  As you skis are arcoss the fall-line they are at 0degrees.  As they are in the fall-line your skis angle will match that of the ski slope...assuming a 45 degree slope, then your skis are at 45 degrees.  As you ski from "across the fall-line" to "in the fall-line" and back again.....your skis then go from 0 to 45 then from 45 back to 0.

Hopefully that makes sense.

Edited by Skidude72 - 6/13/10 at 9:23pm
Quote:
Originally Posted by L&AirC

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.

So, think about what you just said -

"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."

So, what exactly is the skier feeling if it is not the turn forces? Are you trying to say that he is feeling some virtual turn forces?

Rebound refers to the forces, but they might not be felt????

Then you said that you can have a virtual bump without rebound

Could it be that there are turn forces, which you can label virtual bump if you'd like, and depending on how you deal with them, you either get rebound or not.

I am a firm believer in turn forces and I think we all agree on that. However, it seems to me that this virtual bump terminology does anything but help us understand how to manage the turn forces....

The reason I compared virtual bump to bumps is that is what others said previously - that both the virtual bump and real bumps have a lot in common and are handled alike.... This is not correct at all. However, you also compared the two and said how similar they are.

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.

I still have found that just about all of Ron LeMaster's disciples are quite confused in their thinking. The analogy one ski school director once used regarding Ron's thinking and teaching skills, " he couldn't teach his way out of a wet paper bag.." Just because his books have great pictures, do not make them correct. Just because some "famous" people have endorsed it, does not make it correct either. (Juris Vagners is a personal friend of ours...)

No problem discussing things like this. Very educational for sure. However, I still think that the virtual bump is not beneficial. I'd much rather hear people describe the turn forces, what they do feel and how they use them, than to hide behind some term such as the virtual bump.

Quote:
Originally Posted by Atomicman

So is rebound!!!

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

Bump = skiing over a bump will cause the ground to push the skier up
Virtual bump = turning will cause the ground to push the skier up

Turn rebound = after you have made a turn the ground is pushing you up

Ski rebound = after you bend the ski the ski unbends

Unweighting = reduced pressure under skiers feet when CoM accelleration up is stopped or CoM is dropping down

Up-unweighting = reduced pressure under skiers feet caused by skier lifting CoM up

Down-unweighting = reduced pressure under skiers feet caused by skier dropping CoM down

These deffinitions were just pulled out of the top of my head. IMO turn rebound is one component of the virtual bump. So is ski rebound. Unweighting is a natural result when the ground first pushes you up and then stops pushing. With leg movements you can even out the forces or add to them. My theory is that up-unweighting is a stand alone method of reducing pressure under your skis while down-unweighting needs linked turns. In other words the virtual bump and rebound of some sort. IMO down-unweighting is the same as up-unweighting from a force vector standpoint. In UUW the force lifting your CoM up is a man made muscle extention of the legs while in DUW the force lifting your CoM up is caused by the virtual bump or rebound of some sort. Or just a real life bump for crying out loud .

So yes, rebound is real

Quote:
Originally Posted by Skidude72

OK the "S" on the left is the skis path....viewed from above...pretty standard.

The view on the right shows the skis trajectory....or RATE at which they change elevation.  I think that might be an easier way to think of it.  So as you are in the transition, your skis are not losing elevation...this is the "flat" bit you see on the diagram to the right...as the skis turn into the fall-line you see the skis drop in elevation....this is shown on the right, as the steeper bit....as you ski from the transition to the fallline....that is equal to skiing from a flat to a pitch....and that of course is equal to skiing over a bump...or rollover.  That is the virtual bump concept.

Another way to think of it, or help understand using numbers:  As you skis are arcoss the fall-line they are at 0degrees.  As they are in the fall-line your skis angle will match that of the ski slope...assuming a 45 degree slope, then your skis are at 45 degrees.  As you ski from "across the fall-line" to "in the fall-line" and back again.....your skis then go from 0 to 45 then from 45 back to 0.

Hopefully that makes sense.

Skidude72,

I think we are both trying to get the other to understand something different.  What you describe above is (in my post #91) #1 and #2.  I understand the virtual bump effect at these points and am comfortable with it. Let's clear things up first.

1. We both agree with the virtual bump effect.

2. As in Ghost's #81 post, the virtual bump effect happens during the turn, though there isn't a real bump.  Same as what you have drawn.

3. The real bump the virtual bump (from Ghost #81) is being compared to is perpendicular to the fall line and the skier would go in a straight line over the real bump but feels the same thing without a bump during a turn.

OK.  So the third type of virtual bump I'm referring to happens only during cross under turns so the real bump it would be compared to goes parallel to the fall line.  Picture someone skiing a ridge and constantly skiing from one side of the ridge to the other staying towards the top of the ridge.  They are skiing back and forth across a very long.  When you are doing cross under turns all the way down the trail, in addition to the virtual bump effect you feel in the turns you make you will feel a third bump because you are constantly extending, flexing and extending your legs from side to side "under" you.  If your head doesn't rise up any higher than when you are at the apex of the turn while doing this, your lower body feels everything it would when skiing the ridge line.

Somebody tell me I'm not crazy!

Thanks,

Ken

Quote:
Originally Posted by L&AirC

Skidude72,

I think we are both trying to get the other to understand something different.  What you describe above is (in my post #91) #1 and #2.  I understand the virtual bump effect at these points and am comfortable with it. Let's clear things up first.

1. We both agree with the virtual bump effect.

2. As in Ghost's #81 post, the virtual bump effect happens during the turn, though there isn't a real bump.  Same as what you have drawn.

3. The real bump the virtual bump (from Ghost #81) is being compared to is perpendicular to the fall line and the skier would go in a straight line over the real bump but feels the same thing without a bump during a turn.

OK.  So the third type of virtual bump I'm referring to happens only during cross under turns so the real bump it would be compared to goes parallel to the fall line.  Picture someone skiing a ridge and constantly skiing from one side of the ridge to the other staying towards the top of the ridge.  They are skiing back and forth across a very long.  When you are doing cross under turns all the way down the trail, in addition to the virtual bump effect you feel in the turns you make you will feel a third bump because you are constantly extending, flexing and extending your legs from side to side "under" you.  If your head doesn't rise up any higher than when you are at the apex of the turn while doing this, your lower body feels everything it would when skiing the ridge line.

Somebody tell me I'm not crazy!

Thanks,

Ken

You are crazy....there is only 1 virtual bump concept.  The other concept you seem to be trying to describe...as far as I can tell....is called Vaulting....which is totally valid by the way, it is just separate to Virtual Bump...the two are not exclusive in any way.

Edited by Skidude72 - 6/14/10 at 3:33am

Let continue to color code like geranimals.

Originally Posted by L&AirC

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.

So, think about what you just said -

Thought about it.  Still thinking about it.  My thinking hasn't been convinced to think differently because you are asking me to think about what I believe without giving me the slightest hint of why I should think differently.

"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."

So, what exactly is the skier feeling if it is not the turn forces? Are you trying to say that he is feeling some virtual turn forces?

In a real bump the skier feels the ground pushing back.  In a turn, or virtual bump, they feel the same thing.

Rebound refers to the forces, but they might not be felt????

I'm only trying to point out that rebound and virtual bump are not the same thing.  You can "feel" a virtual bump but there might not be any rebound to feel.  Do you believe that each and every turn has rebound?

Then you said that you can have a virtual bump without rebound

Ditto to what I just wrote.

Could it be that there are turn forces, which you can label virtual bump if you'd like, and depending on how you deal with them, you either get rebound or not.

Yes and in in a turn you will feel the same forces you would going over "A" bump.

I am a firm believer in turn forces and I think we all agree on that. However, it seems to me that this virtual bump terminology does anything but help us me understand how to manage the turn forces....

Fixed it for you.  It helps me and others quite a bit.  There are things in life I don't get.  That doesn't mean they are wrong.  Some things I heard in my early 20's, I didn't really get until me late forties.  I'm hoping I will "get" more things now I've reached the big 5 0.

The reason I compared virtual bump to bumps is that is what others said previously - that both the virtual bump and real bumps have a lot in common and are handled alike.... This is not correct at all. However, you also compared the two and said how similar they are.

I, and I believe everyone else, compared it to "A bump" (singular) not bumps with an "S" (plural) as in a bump run or moguls.  If you make a run and there is one single bump on the entire trail and you decide to go straight over it (no turning) you will need to flex and then extend your legs if you are going to keep your skis on the snow.  It will feel as though the ground is pushing up at you from your skis up towards your head.

During this same run, each time you turn 9no bump in sight) your legs will feel the same way because you are flexing and extending them and you will feel the ground pushing back at you from your skis towards your head.

The only one that keeps bringing up "bumps" in this thread is you.  Please show me where others are comparing it to bumps. I think you'll find everyone else comparing it to a bump.

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.

I still have found that just about all of Ron LeMaster's disciples are quite confused in their thinking. The analogy one ski school director once used regarding Ron's thinking and teaching skills, " he couldn't teach his way out of a wet paper bag.." Just because his books have great pictures, do not make them correct. Just because some "famous" people have endorsed it, does not make it correct either. (Juris Vagners is a personal friend of ours...)

I agree with questioning everything.  I do.  That doesn't mean what I'm questioning is wrong.  You seem to not like Ron LeMaster much.  What does understanding the science of skiing have to do with instructing?  Two different skill sets.  Makes me think if someone else wrote about virtual bump it would be OK.

No problem discussing things like this. Very educational for sure. However, I still think that the virtual bump is not beneficial. I'd much rather hear people describe the turn forces, what they do feel and how they use them, than to hide behind some term such as the virtual bump.

Quote:
Originally Posted by Skidude72

You are crazy....there is only 1 virtual bump concept.  The other concept you seem to be trying to describe...as far as I can tell....is called Vaulting.

Then I have not described it well enough and will try tonight.  I agree there is only one virtual bump effect but you feel it in a turn (initiation to finish) and during a cross under turn, during the transition (apex to apex).

I'm not trying to describe vaulting.  I'[m actually thinking about taking pictures in my basement tonight to try to illustrate this better.

Quote:
Originally Posted by L&AirC

Then I have not described it well enough and will try tonight.  I agree there is only one virtual bump effect but you feel it in a turn (initiation to finish) and during a cross under turn, during the transition (apex to apex).

I'm not trying to describe vaulting.  I'[m actually thinking about taking pictures in my basement tonight to try to illustrate this better.

Well...ok, just for fun then....as this might be easier then taking pictures.....can you describe vaulting for me, as you understand it....and then explain how what you are saying is different to vaulting?

The thing to understand is that if you are skiing down a slope, following the fall line and you turn right  out of the fall line and left back into it, you are not only turning right, you are turning up and you are not only turning left, you are turning down.  This is like driving through a chicane with a bump in the middle of it.

L&AirC,

You have described Vaulting perfectly.  Your body will go up and over, if you go from inclined left to inclined right without flexing.  This may be like a bump, but it is not what we are talking about when we talk about the virtual bump.

There are centripetal forces in turns, they exist on the flats, on the slopes, and are additive to other forces. They reach a maximum at the apex.  These forces and there increase/decrease are important, but are not what we are talking about when we talk about the virtual bump.

It is possible to just look at all the forces and directions and teach skiing without the virtual bump concept.  The laws of Newtonian Physics certainly apply, and if you understand them well enough, that's all you need.  A lot of people didn't do so well in physics, but can drive a car around corners with bumps in them, and have done so.

BTW,

I have found a some of LeMaster's desciples have difficulty explaining their own theories.  They usually resort to proof by professorial intimidation.  Telling me I have to read his book, and when I read the part in question and explain it to them, they tell me I don't understand and tell me I have to read another one of his books, instead of providing valid arguments.  Instead of playing the let's descredit the authority, I prefer to lay the facts and arguments out in black and white RIGHT HERE AND NOW, not say this expert says that, and he's an authority.

OK.  Tonight me, the camera and a bosu ball are going virtual skiing.

If I described vaulting, I did a poor job of getting my point across and will address this when I return from work.

Ghost, I'd add that when we're in the fall line the tips are downhill of the feet and the feet are downhill of the tails. Sort of like an airplane turning and diving at the same time. When we are traversing across the hill (pependicular to the fall line) the tip, tail feet and tail are at the same elevation..Like the plane in level flight. IMO this is how I would explain the virtual bump idea, a diving turn where you level off by end of the 180* turn. I'm not sure that means you are actually going uphill though. You could but getting most skiers to be patient enough to finsh their turns past 45* can be a challenge.

Poor Ron!  Probably true that some (maybe many) do not understand his material - is that necessarily his fault/problem?

Could part of the confusion here be the distinction being made between Vaulting and Gradient changes, where Ron combines both as parts of the Virtual Bump?

Skidude72:

I'm just sitting here shaking my head in disbelief about how confused and convoluted this whole issue has become. I guess since the discussion is basically about turn forces and how to manage and use them, it's a pretty broad topic.

I still do not see why you want to have so many terms though. You have the turn forces and you can explain them using physics and body mechanics. For years, we have talked about them and have come up with words such as rebound, check. So, add to that vocabulary vaulting and virtual bump, but why? Is it necessary to have new words? Sure, modern skis have changed the dynamics a bit, but physics is still physics and the body is still the body.

I guess that since we are all ski instructors and since this top is about the virtual bump, wouldn't you also want to tie this discussion and resulting increase in understanding also into how to communicate and teach? Different stroke for different folks; Student centered instruction - however, even though I use quite a variety of methods and words to communicate when I teach, I still try to avoid words, images, analogies that also confuse. That is how I feel about the usefulness about the turn "virtual bump". Personal opinion and preference.

Now understand the turn forces is indeed worthy of consideration!

Quote:
Originally Posted by Skidude72

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....

So, it seems part of the discussion as well as a large part of the confusion is that there seems  to be quite a bit of nit-picking about definitions and precious terms instead of trying for understanding and communication. Granted, we need to understand each other by precise definitions, so then why not use simpler, unequivocal words, or as close it you can get. I know I won't get far with this bias here on this forum, but I just thought I'd throw out the concept that KISS is pretty good and communicating reality if you really want to understand each other. It also helps keep our thinking straight.

This said to ask why you don't like it if to use the word "spring" or "springy" when describing the "UP sensation", as you call it. Obviously we do not have suspension like cars, we have muscles and bones and skis, boots and snow. We don't have springs, but we do have a springy sensation. Is that not pretty descriptive terminology that people can understand.

I agree with your description about why it's springy - flex absorbs, more rigid springs.

However, "UP" is not a good word here because in reality it doesn't go up - the direction of the force depends on the direction of the reaction combined with the velocity and momentum of the objects. That is not up, especially in a high speed turn.

The "UP" part of understanding is critical for someone to be able to deal with the turn forces. What you put into your head will effect to a great deal about what movements you will allow your body to make.

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

Call it what you will - vaulting, jumping/landing, springing, rebound, virtual bumping, floating, zero-geeeing it, diving into danger, pushing-off with a hop...... Is it well understood? I think not. Listen to the discussion here. Are all modern ski books accurate, even though they may be popular? I think not. Are modern ski books, even though they may have nice vectors and drawings, science? Nope. Many are basically more akin to wolves in sheep's clothing, albeit well meaning and not vicious wolves. Sadly, we are not that professional yet in ski instruction...

Quote:

Originally Posted by Cookie Bewley Hale

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.

Again, call it what you want. Vaulting, springing, bouncing etc. I don't care. However, you are looking at an effect, not a cause when you start talking about the greater angles that WC racers have today compared to 20-30 years ago. The modern equipment is "stronger" and allows us to be able to handle more force than we previously could. The result of greater forces is greater speed. The result of all of this is that we need greater angles to line up with those forces to be able to manage them and stay in balance. So, even though as you say, "The greater the inclination, the greater the potential for a Vaulting effect. ", the inclination is not what gives the greater potential, it's the greater forces that we can now successfully handle.

Again, I think the issue with so many ski instructors is that we get see without understanding what is behind it all. We look at positions of the body without "feeling" the forces and understanding why the body positions.  We end up teaching body positions and get results that don't work so well all the time. Instead we need to teach people to feel the snow, feel their skis, feel the forces. They are enjoying all those sensations, so why not teach them to pay attention to them even more! A rabbit trail and pet peeve, sorry.

Even though you see the inclination and refer to it, I think you do understand the forces and that is what I was talking about.

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.

Regarding snow science; even young children learn how to intuitively judge the quality of the snow. How many different words do the Inuit have for naming snow. I don't think it's necessary to go into a detailed chemical/physical description of how moisture, integrity, cohesion, air content temperature all effect the snow and which factor is the most critical. Your mentioning temperature at least indicates that you are realizing that snow is not all the same all the time. Since it is snow that we are skiing on, the differing reactive properties of the snow obviously influence what is possible in our managing our movements. So you may say that stating it over doesn't make it true, but to me, it's such an obvious truth - that not all snow is the same... some snow is more springy than other snow. You know this to be true too in your own skiing, so why do you want to try to claim that the snow has no effect on rebound, vaulting, virtual bumps or whatever you want to call things?

Skidude72:

Here is something we agree in on - at least in concept.

Quote:
Originally Posted by Skidude72

No you are confusing Vaulting with Virtual Bump.....while obviously related they are separate concepts.  It is possible to flex such that you reduce vaulting to effecitivley nothing....this is shown in TDk6's post 104 diagram...second one if memory serves.....but you cannot make the virtual bump disappear when turning on a slope....you can "work with it"....but you cant make it go to 0, unless you go straight, or are skiing on flat (ie not pitch) ground.....

The part of the above that we agree on is this and here is the logic:

Quote by Skidude72: "but you cant make it go to 0, unless you go straight, or are skiing on flat (ie not pitch) ground"

The turn forces are always there and you have to do something with them. You can't get rid of them unless you go straight and don't turn!!!

I love it - but I have to get back to work - way to go TDK6!!! I see your logic!!! The only problem is the direction of the movement is not up!!!

Quote:
Originally Posted by tdk6

Bump = skiing over a bump will cause the ground to push the skier up

Only if the skier does not absorb with his legs. However, bump is really there and not dependent on turn forces.
Virtual bump = turning will cause the ground to push the skier up

Actually the ground/snow is reactive and not active. Feels like "push", but really isn't.

Turn rebound = after you have made a turn the ground is pushing you up

Same as above about the ground.

Ski rebound = after you bend the ski the ski unbends

Pop goes the weasel...

Unweighting = reduced pressure under skiers feet when CoM accelleration up is stopped or CoM is dropping down

Don't get the first part, but CoM dropping down is right - retraction of feet also gives unweighting.

Up-unweighting = reduced pressure under skiers feet caused by skier lifting CoM up

Yup, Extension, push off and then go with it

Down-unweighting = reduced pressure under skiers feet caused by skier dropping CoM down

Yup. Classic bend-stretch and also modern flexing in lower section of turn to manage the pressure.

These deffinitions were just pulled out of the top of my head. IMO turn rebound is one component of the virtual bump.

So is ski rebound.

Yup

Unweighting is a natural result when the ground first pushes you up and then stops pushing.

Kind of - ground can't push, but you can react to the turn forces and make it "feel" like that.

With leg movements you can even out the forces or add to them.

Totally agree.

My theory is that up-unweighting is a stand alone method of reducing pressure under your skis while down-unweighting needs linked turns.

Not exactly, you can down-unweight by simply dropping your bum while standing still. Both are easier while moving.

In other words the virtual bump and rebound of some sort. IMO down-unweighting is the same as up-unweighting from a force vector standpoint.

Unweighting is unweighting... Agree.

In UUW the force lifting your CoM up is a man made muscle extention of the legs while in DUW the force lifting your CoM up is caused by the virtual bump or rebound of some sort.

Ummm, not really. Yes to UUW. However, the DUW is not lifting up your CoM, it is removing or reducing the support of the CoM to the snow. It has to be active with muscles too and you don't need any bumps, virtual or not to do it.

TDK6 - (A little side note here - considering your modern racing scenario - if you manage the pressure at the bottom of the turn by flexing, that is unweighting in the sense that you are reducing the pressure, however that is not unweighting in the sense that there still is a helluva lot of pressure on yourself and your skis at that point in the turn. You are simply managing it to keep it from increasing. However, in moving by flexing there, you will be able to more effectively control the direction of the rebound effect when you begin to release from that turn into the new turn so that you will not go "UP", but rather go more foragonnally into the turn. This is greatly to be desired in the race course and you will smooth out the vertical component of the path your body takes and help you go faster. Rebound never speeds you up, it may feel like that. However, if you control it, you can use it in the intended direction of travel. Gravity and skating/pumping at the top of the turn are your engines that can actually speed you up. Properly managing the lower part of the turn results in preventing you from slowing down any more than you need to at that point.)

Or just a real life bump for crying out loud .

So yes, rebound is real

I agree, crying out loud! Are we all simply discussing turn forces while worshiping our very own sacred cows?

Quote:
Originally Posted by justanotherskipro

Ghost, I'd add that when we're in the fall line the tips are downhill of the feet and the feet are downhill of the tails. Sort of like an airplane turning and diving at the same time. When we are traversing across the hill (pependicular to the fall line) the tip, tail feet and tail are at the same elevation..Like the plane in level flight. IMO this is how I would explain the virtual bump idea, a diving turn where you level off by end of the 180* turn. I'm not sure that means you are actually going uphill though. You could but getting most skiers to be patient enough to finsh their turns past 45* can be a challenge.

Yes.  By turning up, you are not necessarily turning ALL the way up, just in the the up direction, just like when you turn left, it doesn't have to be 90 degrees.

Okay, I'll try only one bit at a time.

Quote:
Originally Posted by L&AirC
..........

So, think about what you just said -

Thought about it.  Still thinking about it.  My thinking hasn't been convinced to think differently because you are asking me to think about what I believe without giving me the slightest hint of why I should think differently.

I'll try to explain better then. Here is what you said:

"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."

I am trying to point out that both the "virtual bump" and rebound are the result of the same turn forces and how the skier manages them. You can't separate them based solely on the concept that you"feel' them. You said that the virtual refers to what the skier feels and the rebound refers to the turn forces regardless if they are felt or not.

Don't you find that somewhat illogical? Trying to say it another way:

1. Virtual bump is what the skier feels - by your definition

2. What the skier feels is how he is managing the turn forces

3. Therefore, virtual bump is related to the turn forces

4. Rebound refers to the turn forces - by your definition

5. Rebound can be minimized or maximized by the skier to "feel" it more

6. Rebound results from how the skier managing the turn forces

7. This is going in circles - if both rebound and virtual bump are the result of turn forces and the skier, how can the difference by what is felt by the skier? What are you feeling in the virtual bump, were it not for turn forces?

So, what exactly is the skier feeling if it is not the turn forces? Are you trying to say that he is feeling some virtual turn forces?

In a real bump the skier feels the ground pushing back.  In a turn, or virtual bump, they feel the same thing.

Real bumps don't push. They are inate objects with no muscles...

Rebound refers to the forces, but they might not be felt????

I'm only trying to point out that rebound and virtual bump are not the same thing.  You can "feel" a virtual bump but there might not be any rebound to feel.  Do you believe that each and every turn has rebound?

On one turn, you can choose to make it either a big or a little "bump" depending on how you manage the turn forces. Same with rebound - you can choose to make it big or little. As TDK6 says, it only goes away completely if you don't turn.... go straight. Then the only bump you can feel is a real bump. There will not be any turn forces to create either rebound or a virtual bump. (I guess you could argue that if you tried to jump up and down while going straight.. ..but that is sort of idiotic to argue

that.

So trying to differentiate between virtual bumps and rebound by what is "felt" does not make sense.

Then you said that you can have a virtual bump without rebound

Ditto to what I just wrote.

Could it be that there are turn forces, which you can label virtual bump if you'd like, and depending on how you deal with them, you either get rebound or not.

For clarity sake, I'll try to repeat it once again in slightly different way:

Turn forces are a reality.

Rebound must have turn forces to exist.

Virtual bump must have turn forces to exist.

What you "feel" in either one has only to do with how big or little you made each by your managing of the turn forces.

What you "feel" is not some way to distinguish between rebound or virtual bump.

Regarding bumps and bump. One bump is even quite different from a virtual bump - but all the more reason why the word "bump" is a poor choice for skiers. The word bump for skiers refers to bumps, moguls which seldom occur singularly.

Personally, I don't like to make things more complicated and difficult. If I want to understand something complex, I break it down to the parts and simplify it to make it manageable.

Quote:
Originally Posted by cgeib

Poor Ron!  Probably true that some (maybe many) do not understand his material - is that necessarily his fault/problem?

Could part of the confusion here be the distinction being made between Vaulting and Gradient changes, where Ron combines both as parts of the Virtual Bump?

Sometimes the illogical and contradictory is hard to understand. Sometimes what is not physically accurate and is also hard to understand. Sometimes deductions that are made that are invalid and negate some of the facts are hard to understand.

I wouldn't necessarily say it's Ron's fault. It just is another fact. Ron isn't always correct.

I think some of the confusion here is just illustrating the fact that the concept of explaining turn forces by labeling a "virtual bump" is indeed not a good concept to promote because it creates confusion and does not do much to help understanding - other than the fact, that in the process of wading through the confusion, people will begin to think more deeply and finally learn something by that process..... Is this a good teaching tactic?

Quote:
Originally Posted by Cookie Bewley Hale

Skidude72:

I still do not see why you want to have so many terms though. You have the turn forces and you can explain them using physics and body mechanics. For years, we have talked about them and have come up with words such as rebound, check. So, add to that vocabulary vaulting and virtual bump, but why? Is it necessary to have new words? Sure, modern skis have changed the dynamics a bit, but physics is still physics and the body is still the body.

Virtual bump and Vaulting are not simply new words for old concepts such as rebound.  They are entirely different, far more accurate, concepts which explain what happens in a transition and why.  Understanding this is important for top level coaching....that does not mean you need to teach these concepts to students....but I would expect any examiner or high level coach to know this stuff, as it helps diagnose skiers and most importantly its helps ski pros understand skiing and ski technique better....which of course in turn helps us explain things better and simplier to all students.

Quote:
Originally Posted by Cookie Bewley Hale

I guess that since we are all ski instructors and since this top is about the virtual bump, wouldn't you also want to tie this discussion and resulting increase in understanding also into how to communicate and teach? Different stroke for different folks; Student centered instruction - however, even though I use quite a variety of methods and words to communicate when I teach, I still try to avoid words, images, analogies that also confuse. That is how I feel about the usefulness about the turn "virtual bump". Personal opinion and preference.

Yes talking about how to apply this with regular students is of course a worthy discussion.  But it in itself is a whole other topic.  If you want to discuss that, you can always start a new thread.  This thread is clearly an instructor to instructor discussion about the physics of skiing to help understand what is really happening.

Quote:
Originally Posted by Cookie Bewley Hale

Now understand the turn forces is indeed worthy of consideration!

I am glad you agree on that.

Quote:
Originally Posted by Cookie Bewley Hale

Regarding snow science; even young children learn how to intuitively judge the quality of the snow. How many different words do the Inuit have for naming snow. I don't think it's necessary to go into a detailed chemical/physical description of how moisture, integrity, cohesion, air content temperature all effect the snow and which factor is the most critical. Your mentioning temperature at least indicates that you are realizing that snow is not all the same all the time. Since it is snow that we are skiing on, the differing reactive properties of the snow obviously influence what is possible in our managing our movements. So you may say that stating it over doesn't make it true, but to me, it's such an obvious truth - that not all snow is the same... some snow is more springy than other snow. You know this to be true too in your own skiing, so why do you want to try to claim that the snow has no effect on rebound, vaulting, virtual bumps or whatever you want to call things?

I have no question snow can exhibit lots of different properties.  Any kid who grew up with snow, whether a skier or not, knows that....as do snow scientists such as Avalanche Forecasters.  However degree of "springyness" is not a quality snow exhibits...regardless of moisture content, age, temperature etc.  I think you know that too....probably no point in continuing with this "springy snow idea" unless you can offer somthing more then your "insistance"...it wont fly.

Quote:
Originally Posted by Cookie Bewley Hale

Virtual bump must have turn forces to exist.

Well....not really.  Virtual Bump (VB) is a function of line and the pitch of the ski slope.

Now of course, as we agree, if you go straight we wont have a VB.....so we must be turning, thus turning forces must be there....but I think that is causing the confusion.  Think cause/effect......does the VB cause the forces? or do the forces cause the VB?

Well the VB causes the forces.  The line we ski, determines the forces we feel/generate.  I am sure you will agree with that.  However we tend to think of the line we ski in a 2-D plane parallel to the ski slope....this is an oversimplification of reality.  We actually ski in and our line is actually 3D.  We go left/right and down (with maybe some up in certain instances , such as mogul, or big knoll...).

The VB concept explains this, and shows the forces that our line through a 3D world generates.

I think that is the exact opposite to Rebound, which suggests the forces cause the rebound....plus the forces come from a 2D view of line...not reality.

Quote:
Originally Posted by Cookie Bewley Hale

Personally, I don't like to make things more complicated and difficult. If I want to understand something complex, I break it down to the parts and simplify it to make it manageable.

I think this is a sound approach....always makes things as simple as possible....but no simpler!

There are three terms, rebound, virtual bump, and vaulting, because there are three distinct things to talk about.

The virtual bump is a geometric feature of the slope and line. It is the bump in the elevation profile of a given line down an inclined slope.  Sure if everybody understood Newtonian physics in three dimensions we wouldn't need to talk about the virtual bump, but many people didn't even take physics in high school.

Vaulting is using one's  legs like a pole vaulter's pole  to increase elevation of cm as the body vaults over the skis.

Rebound is what happens when energy stored in a strained ski is released as the ski returns to it's normal position.

Nothing confusing about it.... once you understand it

“Look!  Everyone’s out of step but my son.”  It’s a caption from a cartoon of Boy Scouts marching in a parade with one Scout out of step.  That’s how I feel right now.  Either I’m out of step or all of you guys are.

Again, I’ve only been skiing a few years and only seriously for the last two.  Just got L1 in March this year so I’m posting this with the understanding that I believe it to be true based on my studies and experiences.  I’m perfectly willing to accept I completely missed the entire “Virtual Bump” bus.  In one of my previous posts, I stated that I thought I knew what it was right away and then started believing it was something else because so many people, in unison, were stating it.

So this is what’s in Ken’s head:

Vaulting = Going into a turn and building up the energy so that when you come out of the turn, you are vaulted through the transition and into the initiation of the next turn.  During the vault, your skis will probably leave the snow.  Similar to when a gymnast runs and jumps, vaulting themselves into the air.

Cross Under = Your upper body is square to the fall line and your skis travel from one side to the other “under” you.  When on each side of you, your legs will extend away from you and when under you, they will flex so that when your feet are directly under you, more than likely, anyone looking straight at you from the front, will see very little, if any of your upper leg.  Your head will stay at approximately the same height (see video link below).

Rebound = This one hurts my head even more than the rest but it is the energy being released coming out of a turn by the skis.  Similar to when a gymnast uses a spring board to vault themselves into the air.

Virtual Bump = Picture this.  I’m on a 15 degree groomed trail going down the fall line without turning.  Half way down I go across a cross over trail that is perpendicular to my direction of travel.  When I ski across the cross over trail my skis will bend, my legs will flex and I’ll need to extend my legs as I finishing crossing the cross over into the fall line of the 15 degree trail or you will catch air.

On my second run I go down a similar trail but it doesn’t have a cross over trail on it.  Flat groomed 15 degree pitch all the way down.  Again I’m not turning and then half way down I see Cookie, Skidude, and Ghost directly in front of me (represented by the smiley face above) and they are wondering why I’m not turning.  Just before I get to them, I turn to the right, to miss them, and then head back directly down the fall line.  When I turned to miss them, my legs felt the same pressures on them and my skis bent the same way as on the first run because I turned across the fall line but there wasn’t a cross over trail there on the second run.  I just experienced a “virtual bump” as described by several folks here.

Here's the front view of the second run

Provided both runs were the same length, I would have traveled the same distance even though one run had a real bump and the other trail didn’t but did have a turn.

Cross Under Virtual Bump = if I made the same two runs as described above but on the second run, instead of just dropping into the fall line when I passed the smiley face, I continued the cross under cycle until I was directly under  the happy trio, my skis felt the virtual bump when I went right and then left, but my legs felt an additional “bump” because of the way they cross under.

Look at the first seven seconds of this video.

http://www.vimeo.com/3371564

The skier, Hisayo Sato, demonstrates beautiful cross under technique and if you watch his skis and legs, it’s as if he’s skiing over a bump directly under him and he absorbs it by flexing his legs.  So his legs extend to each side and they are flexed under him.  Which gives you long leg, short leg, and long leg.  The same thing you would have if you were skiing over a bump from left to right or right to left.

This would be similar to someone standing next to a telephone pole lying on the ground.  They are at one end and for a workout decide to hop from side to side, always facing the other end of the pole.  As they start their jump their legs first flex then extend.  When the telephone pole is directly under them, their legs are flexed and when the land on the other side, their legs extend to meet the ground and then flex to absorb the jump and extend into the next jump.  That is the virtual bump I see when people do cross under turns and it is in addition to the virtual bump of turning across the fall line.

Thank you,

Ken

Quote:
Originally Posted by Ghost

There are three terms, rebound, virtual bump, and vaulting, because there are three distinct things to talk about.

The virtual bump is a geometric feature of the slope and line. It is the bump in the elevation profile of a given line down an inclined slope.  Sure if everybody understood Newtonian physics in three dimensions we wouldn't need to talk about the virtual bump, but many people didn't even take physics in high school.

Vaulting is using one's  legs like a pole vaulter's pole  to increase elevation of cm as the body vaults over the skis.

Rebound is what happens when energy stored in a strained ski is released as the ski returns to it's normal position.

Nothing confusing about it.... once you understand it

Good summation

Well, you are sorta partially right in various things....but....I will try to give you the True/False defintive answers you asked for:

Quote:
Originally Posted by L&AirC

“Look!  Everyone’s out of step but my son.”  It’s a caption from a cartoon of Boy Scouts marching in a parade with one Scout out of step.  That’s how I feel right now.  Either I’m out of step or all of you guys are.

Again, I’ve only been skiing a few years and only seriously for the last two.  Just got L1 in March this year so I’m posting this with the understanding that I believe it to be true based on my studies and experiences.  I’m perfectly willing to accept I completely missed the entire “Virtual Bump” bus.  In one of my previous posts, I stated that I thought I knew what it was right away and then started believing it was something else because so many people, in unison, were stating it.

So this is what’s in Ken’s head:

Vaulting = Going into a turn and building up the energy so that when you come out of the turn, you are vaulted through the transition and into the initiation of the next turn.  During the vault, your skis will probably leave the snow.  Similar to when a gymnast runs and jumps, vaulting themselves into the air. False

Cross Under = Your upper body is square to the fall line and your skis travel from one side to the other “under” you.  When on each side of you, your legs will extend away from you and when under you, they will flex so that when your feet are directly under you, more than likely, anyone looking straight at you from the front, will see very little, if any of your upper leg.  Your head will stay at approximately the same height (see video link below).  True-ish?

Rebound = This one hurts my head even more than the rest but it is the energy being released coming out of a turn by the skis.  Similar to when a gymnast uses a spring board to vault themselves into the air. True

Virtual Bump = Picture this.  I’m on a 15 degree groomed trail going down the fall line without turning.  Half way down I go across a cross over trail that is perpendicular to my direction of travel.  When I ski across the cross over trail my skis will bend, my legs will flex and I’ll need to extend my legs as I finishing crossing the cross over into the fall line of the 15 degree trail or you will catch air.

On my second run I go down a similar trail but it doesn’t have a cross over trail on it.  Flat groomed 15 degree pitch all the way down.  Again I’m not turning and then half way down I see Cookie, Skidude, and Ghost directly in front of me (represented by the smiley face above) and they are wondering why I’m not turning.  Just before I get to them, I turn to the right, to miss them, and then head back directly down the fall line.  When I turned to miss them, my legs felt the same pressures on them and my skis bent the same way as on the first run because I turned across the fall line but there wasn’t a cross over trail there on the second run.  I just experienced a “virtual bump” as described by several folks here. True

Here's the front view of the second run

Provided both runs were the same length, I would have traveled the same distance even though one run had a real bump and the other trail didn’t but did have a turn.  Well if by "distance" you mean elevation...then yes I think this is True.

Cross Under Virtual Bump = if I made the same two runs as described above but on the second run, instead of just dropping into the fall line when I passed the smiley face, I continued the cross under cycle until I was directly under  the happy trio, my skis felt the virtual bump when I went right and then left, but my legs felt an additional “bump” because of the way they cross under. Ah....I think you are describing just experiencing Vaulting with the Virtual Bump together...which of course is entriley possible...True?

Look at the first seven seconds of this video.

http://www.vimeo.com/3371564

The skier, Hisayo Sato, demonstrates beautiful cross under technique and if you watch his skis and legs, it’s as if he’s skiing over a bump directly under him and he absorbs it by flexing his legs.  So his legs extend to each side and they are flexed under him.  Which gives you long leg, short leg, and long leg.  The same thing you would have if you were skiing over a bump from left to right or right to left.

This would be similar to someone standing next to a telephone pole lying on the ground.  They are at one end and for a workout decide to hop from side to side, always facing the other end of the pole.  As they start their jump their legs first flex then extend.  When the telephone pole is directly under them, their legs are flexed and when the land on the other side, their legs extend to meet the ground and then flex to absorb the jump and extend into the next jump.  That is the virtual bump I see when people do cross under turns and it is in addition to the virtual bump of turning across the fall line.  False...that would be the combined effect of Vaulting and the Virtual Bump....I think you described it fine....you just seem to be muddling the two concepts into 1.

Thank you,

Ken

Quote:
Originally Posted by Skidude72

Virtual bump and Vaulting are not simply new words for old concepts such as rebound.  They are entirely different, far more accurate, concepts which explain what happens in a transition and why.  Understanding this is important for top level coaching....that does not mean you need to teach these concepts to students....but I would expect any examiner or high level coach to know this stuff, as it helps diagnose skiers and most importantly its helps ski pros understand skiing and ski technique better....which of course in turn helps us explain things better and simplier to all students.

That is your opinion. In our opinion - including from the high level examiner - vaulting and virtual bump are not as you say. We disagree. They are in your model and may seem good to you, but just because you believe them, does not make them any more real and valuable to other people.

Yes talking about how to apply this with regular students is of course a worthy discussion.  But it in itself is a whole other topic.  If you want to discuss that, you can always start a new thread.  This thread is clearly an instructor to instructor discussion about the physics of skiing to help understand what is really happening.

I am glad you agree on that.

Being instructors, are we not all always looking for better ways to also teach? That is why I mentioned that.

I have no question snow can exhibit lots of different properties.  Any kid who grew up with snow, whether a skier or not, knows that....as do snow scientists such as Avalanche Forecasters.  However degree of "springyness" is not a quality snow exhibits...regardless of moisture content, age, temperature etc.  I think you know that too....probably no point in continuing with this "springy snow idea" unless you can offer somthing more then your "insistance"...it wont fly.

So why is it okay for you to go with invented words like vaulting and virtual bump, but my calling snow springy not okay? Don't call it springy then if you don't like to, but you even admit, as will just about all skiers, that different snow has different characteristics making some snow more conducive to rebounding or virtual bumping or vaulting than other snow - THAT was the whole point I was making originally - that snow is a factor that goes into a discussion about managing the turn forces. It was you, who did not like me calling snow springy. I do call it springy because that is a very descriptive way to describe how it feels. You have a problem with that and that's okay. But don't try to say it's not a quality that snow exhibits, not a scientific term, but very descriptive.

Quote:
Originally Posted by Skidude72

Well....not really.  Virtual Bump (VB) is a function of line and the pitch of the ski slope.

Now of course, as we agree, if you go straight we wont have a VB.....so we must be turning, thus turning forces must be there....but I think that is causing the confusion.  Think cause/effect......does the VB cause the forces? or do the forces cause the VB?

I said that the VB must have turn forces to exist. So, you want to rename turn with "line" is that it now? You need to make a turn, or you need to take a line that is not straight. Simple. That is what I said and you said no...

Well the VB causes the forces.  The line we ski, determines the forces we feel/generate.  I am sure you will agree with that.  However we tend to think of the line we ski in a 2-D plane parallel to the ski slope....this is an oversimplification of reality.  We actually ski in and our line is actually 3D.  We go left/right and down (with maybe some up in certain instances , such as mogul, or big knoll...).

The VB concept explains this, and shows the forces that our line through a 3D world generates.

Give me another break. Your VB is virtual, remember. The VB does not and cannot cause anything. Without the turn forces of gravity and momentum, you would not have any VB. It does not cause the forces. The line or turn you ski does determine what forces you will feel/generate. The VB does not create or cause the forces. Your decision to ski a certain line or turn and manage the forces in a certain way cause you to feel the VB in a certain amount - same with rebound, vaulting and whatever.

Of course we ski a 3D world - up/down/side/side/around/inside/outside kliens bottles and what have you. Why on earth do you think that before you had a VB, nobody thought in 3D terms? You think rebound is a 2D antiquated term?

I think that is the exact opposite to Rebound, which suggests the forces cause the rebound....plus the forces come from a 2D view of line...not reality.

I guess you do think rebound is a 2D view. I'm sorry if your view of rebound is 2D. Nobody ever said that. It's not only 3D, but it's actually 4D when you consider it's time element - momentum through space. What causes rebound, I'll repeat again, maybe in terms that you will allow -  You have a skier moving through space, on snow choosing a line or turn to make. Because of the laws of our physical world, there will be gravity and momentum mainly and the centripetal/centrifugal forces because of how he uses his skis and his body on the snow. These are turn forces. How he decides to manage these turn forces will determine exactly how much rebound and VB he will experience - and then have to manage that too... What is so 2D about that? Nothing.

Quote:
Originally Posted by Cookie Bewley Hale

Well....not really.  Virtual Bump (VB) is a function of line and the pitch of the ski slope.

Now of course, as we agree, if you go straight we wont have a VB.....so we must be turning, thus turning forces must be there....but I think that is causing the confusion.  Think cause/effect......does the VB cause the forces? or do the forces cause the VB?

I said that the VB must have turn forces to exist. So, you want to rename turn with "line" is that it now? You need to make a turn, or you need to take a line that is not straight. Simple. That is what I said and you said no...

Forces and line are not the same thing.  Related, yes.  But not one in the same.

Well the VB causes the forces.  The line we ski, determines the forces we feel/generate.  I am sure you will agree with that.  However we tend to think of the line we ski in a 2-D plane parallel to the ski slope....this is an oversimplification of reality.  We actually ski in and our line is actually 3D.  We go left/right and down (with maybe some up in certain instances , such as mogul, or big knoll...).

The VB concept explains this, and shows the forces that our line through a 3D world generates.

Give me another break. Your VB is virtual, remember. The VB does not and cannot cause anything. Without the turn forces of gravity and momentum, you would not have any VB. Well that is not true....see below.

It does not cause the forces. The line or turn you ski does determine what forces you will feel/generate. The VB does not create or cause the forces. Can you explain what you mean here further....seems very contradictory.  You seem to agree that line determines the forces....VB is just a map of our line in 3D.....but that line...when named VB,  doesn't create the forces?

Your decision to ski a certain line or turn and manage the forces in a certain way cause you to feel the VB in a certain amount - same with rebound, vaulting and whatever.  I think it will help if you just stick to the forces, and what causes them....and not worry about how we manage them until later.

I think that is the exact opposite to Rebound, which suggests the forces cause the rebound....plus the forces come from a 2D view of line...not reality.

I guess you do think rebound is a 2D view. I'm sorry if your view of rebound is 2D. Nobody ever said that. It's not only 3D, but it's actually 4D when you consider it's time element - momentum through space. What causes rebound, I'll repeat again, maybe in terms that you will allow -  You have a skier moving through space, on snow choosing a line or turn to make. Because of the laws of our physical world, there will be gravity and momentum mainly and the centripetal/centrifugal forces because of how he uses his skis and his body on the snow. These are turn forces. How he decides to manage these turn forces will determine exactly how much rebound and VB he will experience - and then have to manage that too... What is so 2D about that? Nothing.

What I wrote was is that Rebound has a 2D view of LINE.  If I got that wrong...fine....please explain how Rebound or what Rebound teaches us about 3D line, and how it is used, or what insights Rebound offers us in this regard.

Originally Posted by tdk6

Bump = skiing over a bump will cause the ground to push the skier up

Only if the skier does not absorb with his legs. However, bump is really there and not dependent on turn forces.
Virtual bump = turning will cause the ground to push the skier up

Actually the ground/snow is reactive and not active. Feels like "push", but really isn't.

You are right, but it feels like the ground is pushing you up

Turn rebound = after you have made a turn the ground is pushing you up

Same as above about the ground.

Ski rebound = after you bend the ski the ski unbends

Pop goes the weasel...

Unweighting = reduced pressure under skiers feet when CoM accelleration up is stopped or CoM is dropping down

Don't get the first part, but CoM dropping down is right - retraction of feet also gives unweighting.

There is a resistance to you CoM change of motion. Its called inerta. When your CoM gets pushed up for any reason this resistance will cause your virtual CoM (to your mass is added the force pushing you up) to lagg behind. Note that gravity is accelleration. Its not speed alone. Thats why the effect is momentary. Same applies when you de-accellerate. That is how we feel more and less pressure. If you are in an elevator going down you feel it in your stomach when the elevator sets in motion but after the initial accelleration downwards your inertia catches up with you. The difference in up-unweighting and down-unweighting can easily be experianced in a roller coster. The effect of your stomach going woooouuuuiiii is longer when you go up on a ridge and down again at speed in one motion than simply dropping down from up high after the initial hoist up in the sky.

Up-unweighting = reduced pressure under skiers feet caused by skier lifting CoM up

Yup, Extension, push off and then go with it

Yes, the term up-unweighting is somewhat confusing because its not the motion up that unweights you, its the deacceleration and the dropping down. Moving your CoM up is increasing pressure.

Down-unweighting = reduced pressure under skiers feet caused by skier dropping CoM down

Yup. Classic bend-stretch and also modern flexing in lower section of turn to manage the pressure.

Good point coupling it to modern skis and skiing since the new skis allows for a style where more turnforces are generated at lower speed and the effect of the virtual bump and rebound is stronger. This is what is fueling our transitions. The float.

These deffinitions were just pulled out of the top of my head. IMO turn rebound is one component of the virtual bump.

So is ski rebound.

Yup

Unweighting is a natural result when the ground first pushes you up and then stops pushing.

Kind of - ground can't push, but you can react to the turn forces and make it "feel" like that.

Yes, the ground cant push but I wanted to pickture it from the skiers standpoint feeling the pressure. This way I dont have to make any difference between a real bump or the virtual bump.

With leg movements you can even out the forces or add to them.

Totally agree.

My theory is that up-unweighting is a stand alone method of reducing pressure under your skis while down-unweighting needs linked turns.

Not exactly, you can down-unweight by simply dropping your bum while standing still. Both are easier while moving.

My point is that its not functional. The moment of unweighting is so short. Gravity pull is very quick. Up-unweighting gives you much more time. But dont expect to read this in any book. As far as I know Im the only one thinking like this

So I will not be offended if you dont agree

In other words the virtual bump and rebound of some sort. IMO down-unweighting is the same as up-unweighting from a force vector standpoint.

Unweighting is unweighting... Agree.

In UUW the force lifting your CoM up is a man made muscle extention of the legs while in DUW the force lifting your CoM up is caused by the virtual bump or rebound of some sort.

Ummm, not really. Yes to UUW. However, the DUW is not lifting up your CoM, it is removing or reducing the support of the CoM to the snow. It has to be active with muscles too and you don't need any bumps, virtual or not to do it.

Well, I kind of turned the UUW and the DUW terms to consepts. Brodening the pickture a bit. A pure DUW turn initiation does not exist. Or lets say its not functional due to its short time eclips. What Im kind of saying is that UUW is a leg extention to push your CoM up while DUW is absorbing the virtual bump and or rebound or othere force pushing you up such as a real bump.

TDK6 - (A little side note here - considering your modern racing scenario - if you manage the pressure at the bottom of the turn by flexing, that is unweighting in the sense that you are reducing the pressure, however that is not unweighting in the sense that there still is a helluva lot of pressure on yourself and your skis at that point in the turn. You are simply managing it to keep it from increasing. However, in moving by flexing there, you will be able to more effectively control the direction of the rebound effect when you begin to release from that turn into the new turn so that you will not go "UP", but rather go more foragonnally into the turn. This is greatly to be desired in the race course and you will smooth out the vertical component of the path your body takes and help you go faster. Rebound never speeds you up, it may feel like that. However, if you control it, you can use it in the intended direction of travel. Gravity and skating/pumping at the top of the turn are your engines that can actually speed you up. Properly managing the lower part of the turn results in preventing you from slowing down any more than you need to at that point.)

I totally agree with what you are saying here. I have all these coaches telling me that you need to pump and increase your speed allways but that is not the real question IMO. Its managing what you have and moving your CoM as fluedly as possible down the hill. Positioning your self favorably. The good skiers always seem to do less.

The rebound does indeed help you fuel your transition but its a force working in slightly the wrong direction. Its like heat generated at a powerplant that you use for heating homes in the nearby area

Or just a real life bump for crying out loud .

So yes, rebound is real

Skidude & Ghost,

Thank you both for you comments on my posts and helping with the definitions across the last couple of days.  I'm going to start a new thread on Vaulting so as not to muddy the Virtual Bump thread any more than it already is since I'm still confused over that one.

Thanks,

Ken

Edited by L&AirC - 6/15/10 at 3:44am
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