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A simple self-assessment test - Page 25

post #721 of 736
Quote:
Originally Posted by fatoldman View Post
 

 

Are you aware that this is the statement that a paranoid person would make?

 

I'm aware that it's a factual statement.  If the moderators have never simply blocked a poster when the poster's posts weren't offensive and couldn't be deleted (and where in some cases the posts were not only factual, but totally in agreement with established outside authorities) -- well, I'd be real interested to hear them try to make that claim.

 

It's the web, it's not a big deal.  Passive readers just need to remember that what they get is among others things not wholly representative of what the real world view might be.

 

I'm also aware that I started the day off with a substantive post about real-world dirt jumping over the weekend.  FOM, who wants the thread locked, who thinks I am making paranoid statements, did not.

post #722 of 736
Quote:
Originally Posted by CTKook View Post
 

 

I'm aware that it's a factual statement.  If the moderators have never simply blocked a poster when the poster's posts weren't offensive and couldn't be deleted (and where in some cases the posts were not only factual, but totally in agreement with established outside authorities) -- well, I'd be real interested to hear them try to make that claim.

 

It's the web, it's not a big deal.  Passive readers just need to remember that what they get is among others things not wholly representative of what the real world view might be.

 

I'm also aware that I started the day off with a substantive post about real-world dirt jumping over the weekend.  FOM, who wants the thread locked, who thinks I am making paranoid statements, did not.

 

Too bad that you started that post by lamenting that you are so picked on around here.

post #723 of 736
Quote:
Originally Posted by jack97 View Post
 

 

If you haven't found it already, chap 5 "Skiing in Non Planar Surface. 

 

Yes have read it.  Read it again just now.  It only substantiates everything we have been trying to say so far, and does not in any way support the theory of anti-pumping put forth by some of you, nor does it make any mention to angular physics as put forth earlier in the thread.  Its all about pressure management, nothing more.

 

Just to be clear, the section makes no mention at all about slowing down through absorption, rather the section is about normalizing pressure under the feet, by lifting the feet on the crests of bumps and extending into the troughs.  Pretty basic concept overall....

 

But I think you have tried to extrapolate more out of it then what Lind actually presented, so let's get into the details and see if that holds water.

 

This is where you're going wrong:

 

Quote:
Originally Posted by jack97 View Post
 

The later implies that the com can be moved to make external force non constant.

 

Let me explain Lind's presentation; for why you have it backwards.

 

 

First an example is given where a skier does not flex and extend through the bumps, rather skis through statically posed, showing that a skier feels heavier in the troughs and lighter on the crests.  More pressure in the troughs, less pressure on the crests.  More and less reactionary forces.  Note this it not how any of us have been taught to ski bumps, the example is to make a point.  That increase and decrease in pressure creates accelerating forces on the skier up and down.  

 

Lind then gives an example of a skier who is flexing to absorb the crests and extending into the troughs.  The point is made that the reactional force remains constant this way, ie, no additional accelerations besides gravity.

 

I believe that alone is opposite of the point you made above, that moving the CoM makes reactionary force non constant.  Quite the opposite, flexing and extending maintains constant reactionary force, while being statically posed would allow non-constant reactionary forces (ie, pressures) to change while going through 3D terrain.

 

More detail......

 

Quote:
Originally Posted by jack97 View Post
 

Key point is that the external force (snow's reaction force) accelerate the skier down the hill. Skier's inertia is changed by extending or retracting on this type surface. 

 

 

Important point, this is what Lind says:

 

Quote:
 "The difference between F-reac and F-n equals the inertial force from acceleration that is normal to the slope plane"
 

As Jamt already pointed out, you may be getting slightly confused about the difference between momentum and acceleration, its not clear to me you are, but just to be clear: Zero acceleration does not mean slower.  It means unchanged.  An acceleration can be positive or negative; positive speeds you up, negative slows you down.  Braking is a form of acceleration.  zero acceleration means no braking.  Gravity is always working on us as a constant force, so in a way, just to keep us from speeding up endlessly to some infinite speed, some other force must be working on us in an opposite direction by some amount.  To actually slow down you have to increase that opposing force otherwise gravity would speed us to supersonic speeds if it had its way.  Friction is usually providing enough opposing force to prevent that from happening.  If you want to slow down more immediately then you need more opposing forces.

 

More from Lind while talking about the first example of not flexing/extending through the bumps:

 

Quote:
 "The sum of all forces on a mass body, including the inertial forces, must always equal zero"
Quote:
"When the  curvature of the path of the skis is upward, as it is when the skier is in one of the troughs between two moguls, F-reac is greater than then F-n, so the skis press into the surface of the snow with greater force."

 

To put it in plain words, if the pressure increases, we have accelerating forces acting on us that match the increase in pressure.  The question is what direction is that accelerating force and how does it relate to gravity which is constantly acting on us?

 

I would say, that without flexing and extending through the bumps, these reactionary forces would slow you down on the face of bumps and speed you up on the backsides.  You'd be heavy on the face and weightless on the back.  Doesn't take a physics degree to know that.

 

 

More from Lind about example two, flexing and extending in the bumps to keep pressure (F-reac) constant:

 

Quote:
 "the skier extends her legs going into the dip and retracts her legs on coming out of the dip to keep her center of mass moving in a straight line.  When this is so., F-reac remains constant"

 

According to everything presented by Lind here, and everything we experience as skiers, flexing and extending to change the  position of the CoM does the OPPOSITE of what you have suggested.  It keeps the pressure more constant.  The reactionary force constant, and thus the corresponding accelerating forces are minimized , allowing only gravity to continue to work on you.  No pumping, no braking, just good old gravity.

 

 

Quote:

Originally Posted by jack97 View Post
 

 

If you have pressure when the mogul is steep this horizontal speed is decreased because you have a force pushing you backwards.

 

 

This seems fairly in line with what we have been trying to explain all along.  I don't see how you are correlating that to absorbing to slow down.  Pressure on the face slows you.

 

But according to Lind, if we absorb the crests and extend into the troughs, we will not have any new reactionary forces slowing us, so how then can someone slow down on the face of a bump while absorbing it as suggested by Mogul Logic and by you anti-pumpers?

 

According to Lind it simply doesn't work that way.  I do not think Lind has all the answers by the way.  But I'm just saying, Lind's chapter here has not justified in any way at all the notion of flexing to slow down, it has presented the idea that flexing reduces pressure, which removes braking if anything.

 

We have made the point numerous times that its possible to accept those additional pressures from the upslope of a mogul face while also flexing to absorb the impact of it.  In other words, imperfect absorption.  

post #724 of 736
Quote:
Originally Posted by Jamt View Post
 

 

It is not the movement of the COM, it is the acceleration. Important difference.

MOvement corresponds to distance, rate of movment corresponds to speed, rate of change of speed is acceleration.

 

 

Yes, I agree. I was speaking toward a set of figures in Lind's book where he plotted out the COM in cases where it form a straight line and in another case where it was offset to the crest and troughs of the terrain. It was in that context that Lind shows how places the com keeps the external force constant. 

post #725 of 736
Quote:
Originally Posted by jack97 View Post
 

 

Yes, I agree. I was speaking toward a set of figures in Lind's book where he plotted out the COM in cases where it form a straight line and in another case where it was offset to the crest and troughs of the terrain. It was in that context that Lind shows how places the com keeps the external force constant. 

 

Right... so if adjusting the BoS to CoM relationship keeps a constant force....how are you slowing yourself?

post #726 of 736
Quote:
Originally Posted by borntoski683 View Post
 

 

Yes have read it.  Read it again just now.  It only substantiates everything we have been trying to say so far, and does not in any way support the theory of anti-pumping put forth by some of you, nor does it make any mention to angular physics as put forth earlier in the thread.  Its all about pressure management, nothing more.

 

Just to be clear, the section makes no mention at all about slowing down through absorption, rather the section is about normalizing pressure under the feet, by lifting the feet on the crests of bumps and extending into the troughs.  Pretty basic concept overall....

 

But I think you have tried to extrapolate more out of it then what Lind actually presented, so let's get into the details and see if that holds water.

 

This is where you're going wrong:

 

 

Let me explain Lind's presentation; for why you have it backwards.

 

 

First an example is given where a skier does not flex and extend through the bumps, rather skis through statically posed, showing that a skier feels heavier in the troughs and lighter on the crests.  More pressure in the troughs, less pressure on the crests.  More and less reactionary forces.  Note this it not how any of us have been taught to ski bumps, the example is to make a point.  That increase and decrease in pressure creates accelerating forces on the skier up and down.  

 

Lind then gives an example of a skier who is flexing to absorb the crests and extending into the troughs.  The point is made that the reactional force remains constant this way, ie, no additional accelerations besides gravity.

 

I believe that alone is opposite of the point you made above, that moving the CoM makes reactionary force non constant.  Quite the opposite, flexing and extending maintains constant reactionary force, while being statically posed would allow non-constant reactionary forces (ie, pressures) to change while going through 3D terrain.

 

More detail......

 

 

 

Important point, this is what Lind says:

 

 

This seems fairly in line with what we have been trying to explain all along.  I don't see how you are correlating that to absorbing to slow down.  Pressure on the face slows you.

 

But according to Lind, if we absorb the crests and extend into the troughs, we will not have any new reactionary forces slowing us, so how then can someone slow down on the face of a bump while absorbing it as suggested by Mogul Logic and by you anti-pumpers?

 

According to Lind it simply doesn't work that way.  I do not think Lind has all the answers by the way.  But I'm just saying, Lind's chapter here has not justified in any way at all the notion of flexing to slow down, it has presented the idea that flexing reduces pressure, which removes braking if anything.

 

We have made the point numerous times that its possible to accept those additional pressures from the upslope of a mogul face while also flexing to absorb the impact of it.  In other words, imperfect absorption.  

 

I don't have the time to go item by item. The concept he talks about may be thought of as pressure management but I think that is myoptic. Think about this example, the same F_react (the external force in the system) can jet your ski from under you or launch you across the slope if you go onto the frontside and up the crest with stiff legs. By retracting/absorbing, the com has moved you do not because you have slowed down. Try that on a set of rollers, just use pressure management with stiff legs only and see how far that gets you. Lind describe extending and retraction by the skier's interia so that it fits within the force approach, angular momentum is covered in other chaps/sections

 

In addition, please re-read post 696 , it aligns with Lind's concepts using examples within the context of this thread. 

post #727 of 736
Quote:
Originally Posted by jack97 View Post
 

 Think about this example, the same F_react (the external force in the system) can jet your ski from under you or launch you across the slope if you go onto the frontside and up the crest with stiff legs. By retracting/absorbing, the com has moved you do not because you have slowed down. Try that on a set of rollers, just use pressure management with stiff legs only and see how far that gets you. Lind describe extending and retraction by the skier's interia so that it fits within the force approach, angular momentum is covered in other chaps/sections

 

In addition, please re-read post 696 , it aligns with Lind's concepts using examples within the context of this thread. 

 

You're all over the map here Jack.  Try better to explain when you get home from work. You referenced Lind's section on 3D terrain and I have explained it perfectly.  You are extrapolating some ideas which Lind did not feel compelled to suggest directly.

 

Lind described extending and retracting the legs to keep the inertial force constant....  Conversely, keeping the legs static so that the CoM move with the BoS, results in pressure on the face.  But we don't ski bumps that way.

 

I hear what you are saying about absorbing, down-unweighting, etc to relieve momentarily some of F-reac.  F-reac is what provides acceleration to speed us down and across the surface of the snow, OR TO SLOW US DOWN IF THE REACTION IS AGAINST THE DIRECTION OF MOMENTUM

 

Let me try to explain it another way.  Think of F-reac like it can be either a gas pedal or a brake, depending on the direction of F-reac in relationship to your direction of momentum and the direction of accelerations from gravity and/or turn forces.  

 

If you are on the backside of a bump, then more F-react equates to stepping on the gas.  If you're on the frontside, its like stepping on the brake, depending on your angle of attack into the face.

 

And conversely, if you relieve F-react on the backside of a bump, its like taking your foot off the gas.  If you relieve F-react on the frontside, its like taking your foot off the brake and putting the car into neutral.

 

However what you need to realize is that, all that relieving F-reac through flexion does is very briefly and momentarily interrupt the gas pedal.  It does not slow your momentum, rather it only interrupts accelerations of gravity and/or turn forces, and only very briefly and partially, to the point of being inconsequential.  Its not a brake.  Momentum is not reduced or lost in any way at all.   Once you have reached the end of your range of motion all the momentum is still there and you will be going just as fast, and gravity never gives up.  And your momentum never gives up either until new forces slow it down.

 

Its like taking your foot off the gas pedal of your car while flying down the highway downhill, for only about 1 second and expecting your car to slow down.  It will a bit due to friction, but not noticeably or substantially enough to be relevant.

 

On the other hand, increasing F-react forces can be a lot more effective for slowing down, especially when you have the face of a bump in front of you or something you can use as a brake that way.  

 

Absorbing a bump completely is like taking your foot off the gas and not using any brake.  Your momentum continues un-slowed by any amount that matters.

 

On the backside of the bump, create more pressure and excluding skidding activities, you'll speed up, you're pressing the gas.  If you step off the gas there, you will not slow down appreciably in the amount of time you have.  Accelerate a bit less for a brief period of time, but that's it.   In terms of practical bump technique, what you actually want to do is create F-reac on the backside, but in the proper direction so that its against your direction of travel and/or gravity......

 

in other words.........develop a skid angle and try to get some more F-reac out of it to use like a brake.

 

 

 

 

 

 

 

 

  .


Edited by borntoski683 - 9/16/13 at 3:58pm
post #728 of 736
Quote:
Originally Posted by borntoski683 View Post
 

 

You're all over the map here Jack.  Try better to explain when you get home from work. You referenced Lind's section on 3D terrain and I have explained it perfectly.  You are extrapolating some ideas which Lind did not feel compelled to suggest directly.

 

Lind described extending and retracting the legs to keep the inertial force constant....  Conversely, keeping the legs static so that the CoM move with the BoS, results in pressure on the face.  But we don't ski bumps that way.

 

I hear what you are saying about absorbing, down-unweighting, etc to relieve momentarily some of F-reac.  F-reac is what provides acceleration to speed us down and across the surface of the snow, OR TO SLOW US DOWN IF THE REACTION IS AGAINST THE DIRECTION OF MOMENTUM

 

Let me try to explain it another way.  Think of F-reac like it can be either a gas pedal or a brake, depending on the direction of F-reac in relationship to your direction of momentum and the direction of accelerations from gravity and/or turn forces.  The gas pedal is more related to the direction of gravity and/or turn forces, the brake pedal is more related to momentum.

 

If you are on the backside of a bump, then more F-react equates to stepping on the gas.  If you're on the frontside, its like stepping on the brake.  And if the face of the bump is uphill its also a bit like putting the car into reverse at the same time as using the brake.

 

And conversely, if you relieve F-react on the backside of a bump, its like taking your foot off the gas.  If you relieve F-react on the frontside, its like taking your foot off the brake and putting the car into neutral.

 

However what you need to realize is that, all that relieving F-reac through flexion does is very briefly and momentarily interrupt the gas pedal.  It does not slow your momentum, rather it only interrupts accelerations of gravity and/or turn forces, and only very briefly and partially, to the point of being inconsequential.  Its not a brake.  Momentum is not reduced or lost in any way at all.   Once you have reached the end of your range of motion all the momentum is still there and you will be going just as fast, and gravity never gives up.  And your momentum never gives up either until new forces slow it down.

 

Its like taking your foot off the gas pedal of your car while flying down the highway downhill, for only about 1 second and expecting your car to slow down.  It will a bit due to friction, but not noticeably or substantially enough to be relevant.

 

On the other hand, increasing F-react forces can be a lot more effective for slowing down, especially when you have the face of a bump in front of you or something you can use as a brake that way.  

 

Absorbing a bump completely is like taking your foot off the gas and not using any brake.  Your momentum continues un-slowed by any amount that matters.

 

On the backside of the bump, create more pressure and excluding skidding activities, you'll speed up, you're pressing the gas.  If you step off the gas there, you will not slow down appreciably in the amount of time you have.  Accelerate a bit less for a brief period of time, but that's it.   In terms of practical bump technique, what you actually want to do is create F-reac on the backside, but in the proper direction so that its against your direction of travel and/or gravity......

 

in other words.........develop a skid angle and try to get some more F-reac out of it to use like a brake.

 

 

 

 

 

 

 

 

  .

 

lol.... you're over the map. Try the rollers with pressure management with alone, I have been consistent on that one. 

 
BTW, scientist and engineers are trained to extrapolate from concepts, the fact that Lind did not list them as examples does not mean they do not apply. 
post #729 of 736

well try harder to explain your hypothesis.  I'm only responding to your claims point by point as I can.  Lind does not present the ideas you are presenting, so its intellectual dishonesty to use him to justify your theory.  

post #730 of 736
Quote:
Originally Posted by borntoski683 View Post
 

well try harder to explain your hypothesis.  I'm only responding to your claims point by point as I can.  Lind does not present the ideas you are presenting, so its intellectual dishonesty to use him to justify your theory.  

 

The best hypothesis was written in 696, it is within the context of the examples used in this thread and does align with Lind's concepts. Especially the external forces of the bumps. In addition, using skier's interia to quantify extension and retraction is really clever imo. 

post #731 of 736

Jack if you are in agreement with Jamt, then I don't understand all the disagreement.  I agree with Jamt also.  So if you agree with him then we're in violent agreement!  :beercheer:

 

 

But I do not believe Jamt was suggesting its possible to slow down by absorbing the obstacle.  I do not think he was suggesting you can pump on the ramp to speed up horizontally.  

 

If you understood him that way when I think you mis understood him.   He was just trying to find some common ground for how flexing movements could at all cause you to slow down, or extending movements cause you to speed up, for reasons other than have been presented by you and others.

 

This entire lengthy debate has been about claims of anti-pumping to slow down through absorption.  He did try to present several situations where flexion movements could result in speed loss or increasing, by flexing or extending PRIOR to hitting the face and basically presenting your CoM with less or more of a hill to climb.  I believe he was just trying to lend some insights for how flexing/extending movements could result in speed increases or decreases, but not in the way that you and others have been presenting...(ie slow down through absorption itself), and not pumping on the face!  Myself and others have also attempted to present other similar cases for how flexing movements could be present while slowing, while not by absorption.

 

Pumping on an uphill slippery surface would simply result in your feet sliding backwards unless you  have lateral angle to gain some F-reac from that direction to push against, you would not gain anything.  As he said, if you're near the bottom, and have an angle close enough to 90 degrees to pump against, you can probably gain some vertical acceleration but not horizantal to speed up.  The face of the bump is facing the wrong direction with those F-reac forces to do so.

 

HOWEVER, if you pump not down into the ground, but exactly outwards away from the center of the radius of a curved ramp (not the flat one Jamt presented), then you can add work to the system and gain something, this is the case made by Lind for swings and halfpipes except he reckons the best place to do it would be at the bottom of the half pipe where the greatest centrifugal forces exist.  This is what I pointed out the other day, but you were too busy disagreeing with me to notice I think.  In a half pipe that has a flat bottom, you lose the centrifugal forces there and so you'd have to wait until it curves up a bit, but not too long up the wall because if you wait too long then you will slow down and lose centrifugal forces, it would have to be right at the spot where it starts to curve up, push  with a strong impulse and you'd get an angular pump.....work added to the system, by pushing against centrifugal force...and that could increase momentum, but since the half pipe is sending you skyward in order to so do, you've lost horizantal speed, not gained.   however you can combine with rocking to gain a bit of horizontal...I described this a few days ago also....also from Lind..but again you ignored it.

 

That is the same process that applies to pumping a ski turn.  That is what Lind described.

 

However, there is no inverse for slowing down as some have imagined.

post #732 of 736
Quote:
Originally Posted by borntoski683 View Post
 

 

 

But I do not believe Jamt was suggesting its possible to slow down by absorbing the obstacle.  I do not think he was suggesting you can pump on the ramp to speed up horizontally.  

 

 

 

 

 

Quote:
Originally Posted by Jamt View Post
 

So, you can both increase or decrease speed by extending/maximizing pressure into a bump. If you do it early you increase speed (compare jumping over mogul). If you do it later you slow down.

Similarly you can decrease or increase speed by flexing/sucking/absorbing up. If you suck up your legs early you will hit the bump lower and with later pressure and the speed will decrease. If you suck up your legs later when the natural pressure grows, the speed will not decrease. (Decrease/increase is in relative terms here)

 

 

 

 

 

I specifically agree with jamt's statement above. This aligns with Lind's concepts.

post #733 of 736
Quote:
Originally Posted by Josh Matta View Post
 

... I am currently running a little experiment with someone who does not even know its being run on them. I have level 6 skier under my wing who recently picked up MTBing. she's been riding at least 4-5 days a week with a pump track day every week. Freaking loves the pump track. I have video of her skiing last year and I ll have video of her skiing this year.  If my theory that MTB and learning to pump is important if not the most important thing to skiing 3d terrain I should have pretty conclusive results....

Just a bump to ask for an update on this once the season starts.

post #734 of 736

I predict that if she mountain biked downhill on a bike path through the woods at least a few dozen times, she will slay it on skis.

post #735 of 736
Quote:
Originally Posted by Jamt View Post
 

 

Depends on the timing CT, pumping can speed you up or slow you down. Read my post 696.

Tell me about it.  More than 50 years of being very good at skiing bumps in all conditions  tends to get me all balled up when pumping on the mountain bike.  I often start out just fine but on multiple pumps I often switch to smooth out mode from pump mode. At least I quit trying to clear trees by moving the Inside hand in  The damned handle bars don't tuck in with the hand like a ski pole does.

post #736 of 736
Quote:
Originally Posted by Josh Matta View Post
 

 

thanks ct I honestly think my post went over 99 percent of this board....... after reading the replies that were about me and not about what i was saying. I am currently running a little experiment with someone who does not even know its being run on them. I have level 6 skier under my wing who recently picked up MTBing. she's been riding at least 4-5 days a week with a pump track day every week. Freaking loves the pump track. I have video of her skiing last year and I ll have video of her skiing this year.  If my theory that MTB and learning to pump is important if not the most important thing to skiing 3d terrain I should have pretty conclusive results.

 

what is boils is down is this. Great skier do not just control pressure they use it to their advantage, and can accelerate and decelerate at will and also vastly smooth out terrain.

 

 

 

 

...

Now that the season is here, how is that Level 6 you mention doing?

 

Before and after video on-snow would be great to see.

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