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The virtual bump, revisited. - Page 2

post #31 of 56

 

Quote:
Originally Posted by Ghost View Post

How I used to do turn into the air (it's been a while) is pick a spot, drive my ski tips into it to create sufficient bend in the tips, tip 'em and drive my skis through the apex, focusing on the spot and the portion of my skis that were going over it, ending by pushing away from that spot on my tails.

 

 

That's called an ollie if you ever tried a skateboard.

post #32 of 56
Quote:
Originally Posted by justanotherskipro View Post

 I also question your ability to make a short radius (say ten meter radius) turn at 100kph. Your traveling at 27.77 meters per second. I'm not going to do the math but off the top of my head I'm thinking a complete turn in a little more than a second? To the Apex in about half a second? Really? Yup you would probably get tossed but not by the VB.


Quite right JASP.  You won't turn on a dime at 100+kph.  I said I was going from memory.  I was remembering slower speed turns.  IIRC, the procedure changed by expanding the "spot" of focus as speed increased.  Focus was on the tips and moved back to the tails, but the snow in contact with that part of the skis was moving past the skis, (edit: at slow speed the spot was fixed on the snow, but at high speed the "spot" was also moving forward on the snow at a rate that increased with ski speed).  At 50 kph you could do it.  At 100 kph you would be airbourne with no edge grip before the turn was completed and have to ease up on the turn down the hill.

 

Damn!  No wonder I had such trouble keeping my edges biting into the snow!  It's a good thing to realize if you like to carve on narrow trails: turning into the traverse and pointing at the trees is easier than turning back down the hill.  (Edit: there's the real succubus!)


Edited by Ghost - 1/13/11 at 4:27am
post #33 of 56

Rick, is up-unweighting creating a virtual bump?

post #34 of 56

Very interesting posts - hope you don't mind if I post my take on things...

 

The VB is probably fine as a rough analogy/visualisation for the forces we feel. Low pressure at the begining of the turn, pressure builds, pressure is released, repeat. It even has a bit of the analogy for the rebound from our skis when we stop turning.

 

Where I think it causes problems is if it tries to explain the forces felt in terms of changing gradient. If we solely look at gradient changes then:

  • We start with a gradient of zero
  • We trend to a max (fall line, let's say 45)
  • We trend back to a gradient of zero again.

That ain't a bump, that's a dip. Maybe something like this:

bump.JPG

So, it seems that the gradient change on the real slope alone doesn't marry well with what we feel. Why? Because the forces we feel are more complex.

 

For example, in the top half of the turn there is a component of gravity that is helping provide the centripetal force. In the bottom half gravity is against us as we try to follow a curve. Our legs and skis must both fight gravity and provide centripetal force.

 

In addition, the centripetal force required to keep us in circular motion increases as speed increases.*

 

Once we hit the traverse we end the turn. No more centripetal force required. Force on skis drops to m x g. Ski's may rebound.

 

As discussed in above posts, the only thing that can get us (or the marble) airborne is a force acting in a plane non-parallel to the slope. E.G. rebound or pushing with our legs. In the marble analogy, this could be a guiding wall that is not perpendicular to the plane of travel - IE one that points out from the board.

 

The key thing is that whilst the VB may help us imagine the forces we feel, it doesn't explain why we feel them. I am not sure if it was ever intended to, but when people try to explain the real-life forces using the VB analogy then confusion ensues.

 

 

 

* Interesting point would be when do we start to lose speed. There's a lot going on here, changing component of gravity vs friction on skis (also changing as pressure on skis change).Remember that the centripetal force is tangential to our velocity. So this should not affect our speed. Probably this is for another day, another thread...

 

 

 

 

post #35 of 56
Quote:
Originally Posted by Rick View Post

 

Using the principle of the Virtual Bump to control speed through the bottom half of the turn is different.  It's another and separate speed control tool.  I generally just refer to it as "using turn shape to control speed", but it's based on the Virtual Bump principle of the changing degree of pitch that happens during a turn.  As your ski through the bottom half of a turn, you are progressively turning further and further away from pointing downhill.  The angle at which you're traveling becomes less and less steep. In essence the hill you're skiing on is getting flatter, and you slow down.   The longer you continue to turn, the flatter the hill becomes, and the more you slow down.  I refer to how long you keep turning as "Degree of Turn".  See the following link:  http://www.yourskicoach.com/SkiGlossary/Turn_Shape.html

 

The other speed control tool in the turn shape family is "radius".  The smaller the radius of your turns, the less time you spend accelerating during the top half of the turn, so skier speed remains slower than that experienced in longer radius turns of a similar degree of turn.  (See turn shape link for an explanation of "degree of turn")

 

Thanks a lot Rick - really helpful.

 

I can see how the tapering of angle in the bottom half of the turn reduces the ability of gravity to speed us up (meanwhile pesky friction is slowing us down).

 

I think I have previously underestimated friction,  in a carve. When I think more about it, I know that I don't finish my turns on mild slopes, because I want to retain my speed. I just never put 2+2 together. (And on steep slopes I am too rubbish to carve anyway!)

 

Thanks again.

Malco.

 

 

 

 



 

post #36 of 56
Good post in #34 above Malcomtento.

I think your re-visualization as a 'Virtual Dip' probably works as well or better than a Virtual Bump. Somewhere in this thread Skidude72 suggested it was a 'Virtual Rollover' which also seems accurate. Lots of Rebound perspectives in that thread but kind of messy because the Post Quoting feature had died and people went into repetitious color-code-mode.

The subject of Rebound was examined a bit more in this recent thread while an older thread specifically explored rebound. My own view of it is in post #8 of that thread.


Looking at the geometry of a turn along with the geometry of a bent ski vs direction of travel I've come to the conclusion that the sensation of 'rebound' is 99.9% caused by the Vaulting process along with muscular tension release when stress is unloaded quickly near the end of a turn. Any physical tension stored in the bent ski is minimal, delivered gradually and delivered mostly in unhelpful directions anyway. It simply doesn't contribute a meaningful upward thrust to the skier ... and no thrust at all in an overall forward direction.

---
Hey Ghost - have you banished that alluring Demon by destroying the Sumerian Tablet yet? (I can't tell from your soft and fuzzy post above tongue.gif)


.ma

 

post #37 of 56
Quote:
Originally Posted by michaelA View Post


---
Hey Ghost - have you banished that alluring Demon by destroying the Sumerian Tablet yet? (I can't tell from your soft and fuzzy post above tongue.gif)


.ma

 

Yeah Tony,.  She was a real Deamon, and I had her in my sights.  Unfortunately she broke the camera, so I got no pics.  Will you run the story anyway?

 

I've determined, that while you can turn out away from the slope and catch some air at slow speeds, you cannot do it at high speed; getting airbourne causes you skis to loose their grip and their ability to turn you and get you airbourne. 

 

More importantly this presents a real-life-or-death application to the virtual bump (or dip if you prefer).  At high speeds, turning downhill from a horizontal direction will indeed cause you to loose traction as the apparent slope increases back from 0 to fall line slope and your momentum caries you on a gravity trajectory.  This does not happen when you turn out of the slope (although more traction might be needed to resist gravity and turn you too).  This is very important information to keep in mind if you like high speeds, and you like g forces.  You could find yourself pointing at the trees on the side of the run at 50 mph after making a great turn and expect to be able to make another great turn back down, but by the time you realize what's going on, it's too late to turn back up and you can't make the turn back down. 

 

I've wondered why I always seemed to allow a larger margin of safety than seemed necessary when turning back down a run after making a tight turn for the fun of it on a narrow trail, that is it seemed conservative given the turns I had just completed out of the fall line.  Now I know.
 

post #38 of 56

 

MichaelA,

 

On the rebound thing I think that sounds right. Ski rebound for these sorts of turns should be (as you say) mild and gradual compared to the other stuff at play.

 

I actually don't like the idea of the virtual dip. I think the VB is great for helping people visual and anticipate what they should be feeling. But as evidenced by all the discussion so far, it does tend to muddy things a little when taken more literally - which isn't helpful for learners like me.

 

Meanwhile, it is 9 celsius (48F) in the ski resort I am going to next week. So i should probably be thinking about the physics of waterskiing instead.

post #39 of 56

I thought it might be helpful to post some of Ron's pictures to give the rebound, VB, and vaulting some clarity.

 

barnes-rebound-02-final.jpg

(Rebound)

 

Selected%20Images%20from%20Ultimate%20Skiingberg-straight-up-and-down-02-final.jpg

Absorbing actual bumps

 

alcott-st-anton-2007-pole-vault-final.jpg

Vaulting

post #40 of 56

I think it's clear that the VB doesn't launch Barnes or Alcott, they go airborne because of their tactical choice to use a strong outside leg followed by flexing once the CoM get redirected upward. Both could have easily extended to maintain contact with the snow but they chose to create separation from the snow. I can't say this strongly enough, In both cases the slope doesn't fall away from them, they chose to create a ramp and launch off that platform.

 

Contrast that with Berg who in real bumps makes the tactical choice to maintain as much contact as possible. The flattest part of the bump is a ramp that if he didn't choose to absorb, would launch him because the slope actually falls away from him as he crosses the crest of the moguls.

 

That's the biggest problem I see when we talk about the VB. Yes the ski tips dip below the feet as the skis turn towards the fall line but getting launched has more to do with redirecting the CoM upward than the VB and the slope falling away from the skier. Here is another example, it's of a less severe upward displacement of the CoM and no loss of contact with the snow occurs because the slope falls away from the skier.

zettel-aspen-2006-gs-2A.jpg

 

Again it comes down to a tactical choice of how parallel to the slope the skier wants their CoM to travel during the transition. The VB idea addresses the incidental slope angle in two planes and can be seen as the amount the tips drop below the feet as we turn into the fall line and how they return to level with the feet as we turn across the hill. Any redirection of the CoM upwards happens as a result of how we use our legs to manage the forces involved. If we create a strong enough platform we can redirect our CoM upward enough to lose contact with the snow but it's not the slope falling away from the skier, it's the skier making a movement to rise away from the snow.

post #41 of 56

JASP,  If Bob had chosen to keep ski snow contact in the above montage, what would he have done differently?

post #42 of 56

He had a lot of options Bud, Here's three off the top of my head;

  • Absorb so the rebound didn't occur in the first place,
  • Stay Extended more after the rebound, or not flex so his skis lose contact,
  • or do the squirt move he's showing so many people nowdays.
  •  

My point is that Bob lauching off the snow was a willfull tactical choice (to redirect himself off the snow). Berg chose the opposite tactic because it suited his objective better. It's worth noting that the slope fell away from Berg much faster than it could in Bob's montage but even then Jerry was able to maintain ski snow contact. This IMO should put to rest the notion that on the back side of the virtual (imaginary) bump the slope falls away so fast that the skier cannot maintain contact. I've made a million high speed turns and finished them across the hill but the only time I've ever been tossed in the air is when I redirected my body there.

 

Sorry to spend so much time on my soapbox but I say with respect this idea, even good skiers and inteligent men like Ghost have fallen for this myth about the VB being more than it is. It is simply a way to understand our relationship to the snow and how that changes through a turn. It's really no different than the effective edge angle changing as we change directions in a turn. Do we make adjustments for the slope falling away from us (laterally) as we turn across the hill? I'd say yes since we also would be transitioning to a new turn at the same time. I'm sure you would agree that doesn't mean we would automatically lose contact with the snow because that edge angle changed. We still would need to do something to make our CoM rise off the snow to the point we lose contact with the snow.

post #43 of 56
Quote:
Originally Posted by malcomtento View Post

That ain't a bump, that's a dip. Maybe something like this:

bump.JPG

 

 


You are right that it doesent look like a bump but if you link turns it looks like this:

DipBump.jpg
 

  



Quote:
Originally Posted by malcomtento View Post

 

For example, in the top half of the turn there is a component of gravity that is helping provide the centripetal force. In the bottom half gravity is against us as we try to follow a curve. Our legs and skis must both fight gravity and provide centripetal force.

 

In addition, the centripetal force required to keep us in circular motion increases as speed increases.*

 

Once we hit the traverse we end the turn. No more centripetal force required. Force on skis drops to m x g. Ski's may rebound.

 

 

* Interesting point would be when do we start to lose speed. There's a lot going on here, changing component of gravity vs friction on skis (also changing as pressure on skis change).Remember that the centripetal force is tangential to our velocity. So this should not affect our speed. Probably this is for another day, another thread...

 

 

 

 


I dont quite understand this. Isnt it the other way arround? In the high C of the turn gravity and the centrifugal forces are pulling in different directions. Thats why we have problems establishing enough pressure on the outside ski early in the turn if we dont have enough speed or our skis are too straight. In the lower part of the turn, turnforces align with gravity causing the pull to the outside/downhill to quickly increase. Also, as we come through apex our speed is increasing adding to the forces acting upon us towards the outside/downhill. This is the reason its hard to finish our turns across the slope. Less skilled skiers tend to finish them less and less across causing the speed to accelerate. It might be more correct out of a physics standpoint to talk about the centripetal force but in my opinion its a bit more than confusing.

 

Quote:
Originally Posted by malcomtento View Post

As discussed in above posts, the only thing that can get us (or the marble) airborne is a force acting in a plane non-parallel to the slope. E.G. rebound or pushing with our legs. In the marble analogy, this could be a guiding wall that is not perpendicular to the plane of travel - IE one that points out from the board.

 

The key thing is that whilst the VB may help us imagine the forces we feel, it doesn't explain why we feel them. I am not sure if it was ever intended to, but when people try to explain the real-life forces using the VB analogy then confusion ensues.

  


I agree with this. The main reason why I get airborn in the transition is because I eather use vaulting to cause an upward force as I play my CoM path against the skis path while I flex my legs as soon as I reached max pressure or a simple leg extention.
 

post #44 of 56
Quote:
Originally Posted by bud heishman View Post

JASP,  If Bob had chosen to keep ski snow contact in the above montage, what would he have done differently?



He should have flexed his outside leg more and sooner.

post #45 of 56
Tdk6, you wrote,
Quote:
Originally Posted by tdk6 
I dont quite understand this. Isnt it the other way arround? In the high C of the turn gravity and the centrifugal forces are pulling in different directions.
whereas malcomtento wrote,
Quote:
Originally Posted by malcomtento 
in the top half of the turn there is a component of gravity that is helping provide the centripetal force.
He's expressing essentially the same idea but from a different perspective.

When the skier is going across the slope a component of Gravity is pulling the skier downhill (into the new turn), thus balancing some of the centrifugal force at that point in the turn.

.ma
post #46 of 56
Quote:
Originally Posted by justanotherskipro View Post

He had a lot of options Bud, Here's three off the top of my head;

  • Absorb so the rebound didn't occur in the first place,
  • Stay Extended more after the rebound, or not flex so his skis lose contact,
  • or do the squirt move he's showing so many people nowdays.
  •  

My point is that Bob lauching off the snow was a willfull tactical choice (to redirect himself off the snow). Berg chose the opposite tactic because it suited his objective better. It's worth noting that the slope fell away from Berg much faster than it could in Bob's montage but even then Jerry was able to maintain ski snow contact. This IMO should put to rest the notion that on the back side of the virtual (imaginary) bump the slope falls away so fast that the skier cannot maintain contact. I've made a million high speed turns and finished them across the hill but the only time I've ever been tossed in the air is when I redirected my body there.

 

Sorry to spend so much time on my soapbox but I say with respect this idea, even good skiers and inteligent men like Ghost have fallen for this myth about the VB being more than it is. It is simply a way to understand our relationship to the snow and how that changes through a turn. It's really no different than the effective edge angle changing as we change directions in a turn. Do we make adjustments for the slope falling away from us (laterally) as we turn across the hill? I'd say yes since we also would be transitioning to a new turn at the same time. I'm sure you would agree that doesn't mean we would automatically lose contact with the snow because that edge angle changed. We still would need to do something to make our CoM rise off the snow to the point we lose contact with the snow.


Wouldn't this be like a virtual bump absorption?...

post #47 of 56

Bud, Are we trying to absorb some of the sum of forces that are combining during the second half of the turn, or the VB. It's really no different than a water skier finishing a turn. The first half of their turn the rope helps turn them towards the boat but in the second half they are resisting that pull and their own linear momentum towards the boat. The boat creates a wake but otherwise the surface of the water is level. So there is no Virtual Bump where the tip of the water ski drops below the level of the feet then rises back up to level with the feet. So even though the skis create a narrow platform, any rebound, or vaulting is a function of how we use our legs to redirect our CoM upward, or to be more accurate, diagonally upward relative to the surface of the snow. Which is exactly what Bob did.

post #48 of 56


 

Quote:
Originally Posted by justanotherskipro View Post

Sorry to spend so much time on my soapbox but I say with respect this idea, even good skiers and inteligent men like Ghost have fallen for this myth about the VB being more than it is.

You are making an erroneous assumption Jasp.  I do not think the VB is more than it is.  It is pretty simple 3-D physics and geometry, but perhaps I did not explain my self well enough.  Vaulting is great at accelerating your body up.  The virtual bump cannot accelerate your body up.  Rebound is nothing compared to vaulting.  All fine and good so far?

Now let's get to the important part.

 

Newton's first law, a body will continue to travel in uniform motion unless it is acted upon by an external force.

Newton's second law, a body will accelerate in the direction of the net forces applied on that body, and it's acceleration will be equal to the forces divided by the mass (using the right units).

 

A skier travelling horizontally will continue to travel horizontally unless he is forced from his horizontal path.  Gravity is the only force that will accelerate our skier down, and he can only loose elevation with an acceleration of 9.81 m/s/s, or less if he pushes up against the ground/snow.

 

A skier travelling at 100 kph 90 degrees to the fall line is travelling horizontally, with zero vertical velocity, nada, zip.  The fastest he can loose altitude is given by his current vertical velocity (0) and acceleration due to gravity -9.81 m/s/s.  His position (relative to his current position at 0 m elevation ) even if he did not push up at all against the slope would be y = -0.5(9.81) t^2 m, where t is time in seconds.  He can not lose altitude any quicker than that.

 

Now let's say our skier is skiing on a 35 degree slope, and he got perpendicular to the fall line by pulling off an amazing 2g turn (at 100 kph); he is headed for the trees, but he is not worried because he thinks he can pull off another 2 g turn back down the slope (he can't).   That 2 g turn would, if he were able to pull it off, give him a turn radius of slightly more than 26 m.  He would travel along an arc of that radius, increasing his distance along that arc at 27.7 m/s and increasing his horizontal distance in the direction of the fall line's projection on the horizontal plane at the rate of x= 26.2 * (1-cosine(theta)) where theta is the angular distance around the arc in radians.  His trajectory can be plotted as elevation versus horizontal distance corresponding to the fall line direction.  A 35 degree slope can also be plotted.  Here they both are:

2gskier.jpg

 

You can see the problem,  If he were to turn in the required arc to miss those trees, falling as fast as possible, his skis would leave the ground.  What happens when you skis start to leave the ground\snow?  You stop turning.  You know this free skiing as balancing on the edge of traction turning down the hill as fast as you CAN.  You do not experience being airborne from the virtual jump, because you can't turn hard enough without your skis on the snow.  Our poor skier looses traction trying to make that arc and ends up turning too wide and into the trees, just like a motorcycle rider who doesn't know how to lean his bike in a corner.

 

I'm sure you have pushed yourself out from a wall to skim over some rocks when skiing really steep terrain.  If you can get enough of a turn accomplished before your skis leave the ground you will be airborne.

 

A lot of variables: lower slope = don't  have to worry about edges leaving the ground the ground.  Ski slow = don't have to worry about edges leaving the ground.  Turn more gradually = don't leave the ground.

post #49 of 56
Quote:
Originally Posted by justanotherskipro View Post

Bud, Are we trying to absorb some of the sum of forces that are combining during the second half of the turn, or the VB. It's really no different than a water skier finishing a turn. The first half of their turn the rope helps turn them towards the boat but in the second half they are resisting that pull and their own linear momentum towards the boat. The boat creates a wake but otherwise the surface of the water is level. So there is no Virtual Bump where the tip of the water ski drops below the level of the feet then rises back up to level with the feet. So even though the skis create a narrow platform, any rebound, or vaulting is a function of how we use our legs to redirect our CoM upward, or to be more accurate, diagonally upward relative to the surface of the snow. Which is exactly what Bob did.



EXACTLY!  To me they are one and the same. The virtual bump is not real, it is virtual.  The movements I make and the sensations I feel under my skis are similar to skiing over a bump.  Nothing more nothing less.  I choose not to over think it I choose to use it to communicate with students as a metaphor.

post #50 of 56

Ghost, Let's slow down a second and consider the idea that we are not a single particle and we are free to project our body up, down, or even down the hill independent of the path of the feet. Additionally think about the statement that only Gravity will turn us down the hill. Nothing could be further from the truth. Ever do an upside down traverse? Ever see one done at speed? Here's one from a guy famous for big edge angles well before the fall line.

 

MaierBCGS.jpg

 

Notice his CoM rises and falls about two feet relative to the snow. At World Cup race speed no less. Is he doing sixty mph (100k)? Nah, but he's doing forty (65k). In other words, he's turned across the hill and into a traverse, chose to vault, then drops his CoM into the new turn, all within five ski lengths. Seems to me by your theory he shouldn't be able to drop his CoM so quickly, or establish the new edge so strongly coming out of a traverse. The fact that he does so is why I don't agree with the fly into the trees conclusion you offered.

post #51 of 56

Bud, I'm hoping the water skier example demonstrates the pressure building up is seperate from the tips dropping and coming back up to level but I certainly agree that the combination feels like a compression from a bump or a bench. Maybe not as strong but similar. That's why I don't even use the VB idea, like we see in this thread, it's an idea shrouded in half truths and misconceptions. It reminds me of the idea of creating knee angulation by bending your knees sideways.

post #52 of 56
Quote:
Originally Posted by michaelA View Post

Tdk6, you wrote,

Quote:
Originally Posted by tdk6 

I dont quite understand this. Isnt it the other way arround? In the high C of the turn gravity and the centrifugal forces are pulling in different directions.


whereas malcomtento wrote,

Quote:
Originally Posted by malcomtento 

in the top half of the turn there is a component of gravity that is helping provide the centripetal force.


He's expressing essentially the same idea but from a different perspective.

When the skier is going across the slope a component of Gravity is pulling the skier downhill (into the new turn), thus balancing some of the centrifugal force at that point in the turn.

.ma
 


What is wrong IMO here is the word "helping". Because its not helping. After apex it starts helping. Here is a pickture I made a long time ago to explain how the forces act on us during different turn phases. What makes our skis turn is how much they are tipped and bent. How much we tip is a matter of turn forces helping to balance our body inclining into the turn. So there is a great relationship. In the high C part of the turn the gravity component is working againt us. Not helping us.

 

Forcesatwork2.jpg

post #53 of 56

TDK6,

Gravity is helping in the high c part of the turn in the sense that we do not need to supply as much force from our skis to give us the required acceleration towards the centre of the arc (skis pushing downhill, gravity pulling down hill).  Gravity is not helping in the bottom part of the turn in the sense that we have to supply even more force with our skis so that the total force pointing at the centre of the arc will still be enough (gravity pulling down trying to pull us out of the turn, skis pushing up).

 

That's a lovely picture.  I make it about 25 degree slope?  Let's be generous and call it 30. Let's say 2g (being generous again.), say 70 kph.

70kph30degrees2g.JPG

No problem.  You will not find a turn on a race course that nobody will be able to make, so you can stop searching for it.  There are plenty of them out on the mountains, and too many bodies in the trees.  

 

I will concede though, that you can push yourself with a force that has a higher downward component than upward component, (slope angle + tipping ends up pointing more down than up), enough to make up for the virtual dip in many cases, but not all.    

post #54 of 56



 

Quote:
Originally Posted by bud heishman View Post

EXACTLY!  To me they are one and the same. The virtual bump is not real, it is virtual.  The movements I make and the sensations I feel under my skis are similar to skiing over a bump.  Nothing more nothing less.  I choose not to over think it I choose to use it to communicate with students as a metaphor.



This is my take on it too. Anything more than a metaphor and it loses it's value IMO.

post #55 of 56


Hi TDK
 

Quote:
Originally Posted by tdk6 View Post

Quote:What is wrong IMO here is the word "helping". Because its not helping. After apex it starts helping. Here is a pickture I made a long time ago to explain how the forces act on us during different turn phases. What makes our skis turn is how much they are tipped and bent. How much we tip is a matter of turn forces helping to balance our body inclining into the turn. So there is a great relationship. In the high C part of the turn the gravity component is working againt us. Not helping us.

 


I should have been more clear. When I say "helping" I am talking only about the force exerted by our legs - the changing force that the VB is a metaphor for. Pretty sure you and I agree that in the bottom half of the turn, our legs must fight gravity and product circular motion. A higher effort than at the half way point of the turn.  I totally agree that as far as coordination/tipping/bending skis goes the word 'helping' is probably... unhelpful ;) 

 

All I was trying to do is describe why the force exerted by our legs to produce circular motion varies. The centripetal force (this is the force required to produce circular motion) points towards the centre of the circle. So obviously as the direction changes, the amount by which gravity can help produce this force changes.
 

 Hope the above makes sense.

post #56 of 56
Quote:
Originally Posted by malcomtento View Post


Hi TDK
 

Quote:
Originally Posted by tdk6 View Post

Quote:What is wrong IMO here is the word "helping". Because its not helping. After apex it starts helping. Here is a pickture I made a long time ago to explain how the forces act on us during different turn phases. What makes our skis turn is how much they are tipped and bent. How much we tip is a matter of turn forces helping to balance our body inclining into the turn. So there is a great relationship. In the high C part of the turn the gravity component is working againt us. Not helping us.

 

 


I should have been more clear. When I say "helping" I am talking only about the force exerted by our legs - the changing force that the VB is a metaphor for. Pretty sure you and I agree that in the bottom half of the turn, our legs must fight gravity and product circular motion. A higher effort than at the half way point of the turn.  I totally agree that as far as coordination/tipping/bending skis goes the word 'helping' is probably... unhelpful ;) 

 

All I was trying to do is describe why the force exerted by our legs to produce circular motion varies. The centripetal force (this is the force required to produce circular motion) points towards the centre of the circle. So obviously as the direction changes, the amount by which gravity can help produce this force changes.
 

 Hope the above makes sense.


Thanks for the clarification icon14.gif

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EpicSki › The Barking Bear Forums › Ski Training and Pro Forums › Ski Instruction & Coaching › The virtual bump, revisited.