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# Accelerating out of a turn - Page 5

Quote:
Originally Posted by majortato

Yes! you almost got it.  You just have to tie it together.  The skis want to rebound at 90 degrees to the direction of the bend...but this is NOT the direction of the original force.  Remember, the ski is turning and on edge so that direction is constantly changing.  When the ski is facing down the fall line, the tops of the skis face the inside of the turn.  If you unload the skis right now, it will try to shoot across the hill.  This is the thing we want to take advantage of!

Remember, in high speed dynamic turns, the skis are most loaded when facing the fall line...and that or slightly after that is when the energy is released.  At low levels of skiing, this happens much later (skiers rely on gravity to generate the force/bend rather than high speed turns), so it's hard to get the acceleration and pop.  The better the skier, the earlier that skier can get edge pressure early in the turn so it can be released across the hill rather than up the hill (if the release happens way after fall line).

Let's simplify as much as possible...skis are facing directly down the fall line and bent.  The instantaneous velocity of those skis at this point is also directly down the fall line.  They are on super high edge angle so the skis want to rebound 90 degrees across the hill.  The racer releases the energy at this point.  The rebound acts as a pushing force across the hill.  Because the ski still has momentum to go down the hill, it ends up going lets just say 45 degrees to the fall line.  The skier allows this to happen and makes sure the skis stay pointed in the direction of travel...the acceleration of the skis drag the body with it across and down the hill and the turn starts all over.  The final result is skier going faster both across the hill (to make the next gate) and down the hill to the finish line.

I alsow want to point out that muscle movements can also be used to help direct the rebound energy forward even on a flat hill.  If the ski unloads with a dead weight on it, then it would just pop up and not move forward.  If you put a person on it that knows what they're doing.  Then they can direct that energy to drive the body forward...the end result is actually the body going forward and pulling the skis along.

Finally, even without all that, as long as energy is introduced into the system, regardless of the direction of that energy, we can use movements of the body to redirect that energy somehow.  Think about a person sitting on a swing with no one to push them.  Yet, just by using muscles of the body, we can get the swing to go higher and higher without ever pushing off against anything.  This is the power of the human body.

Majortato

How?  How is this energy released? What move is made?

Hint: you have a bent ski which does have some energy in it.  When the ski returns to its orginal shape, that energy is fully "restored"...at what point in the turn does the ski return to its original shape?  How long does it take to get there..ie at what part of the turn does the energy start to be restored, and at what point is it fully restored?  Is this instantenous...or does it happen progressivley over a part of the turn?  Have a look at some race montages...

Second Hint (just a point of clarification): Do you understand that the skis accelerate and decelerate...yet their speed never changes?  And how this is possible?  Question: If a car goes around a track, (so the start and finish line, are the same line), and the track is 60miles around, it took him one hour to do?  What was his average velocity? What was his average speed?

Quote:
Originally Posted by zentune

Can't wait for more of this discussion! btw, Jamt, I believe, is a race coach and physicist;) hope you don't mind me saying Jamt...

I would recommend reading the aforementioned threads regarding VB, and also one titled "reasons for finishing aft". Good stuff there, to be sure! I also used to think of the skis as being a spring in which we can store energy, which can then be released...but there's more to it than that. The whole body (with various incarnations of upper/lower separation, angulation, rotational foot steering, etc...), the type of transition we use (whether retracted or extended, or somewhere in between), and the forces generated by such movements/positions when exiting a turn and moving towards transition, all play a major role in the way a racer, or good skier, experiences this "rebound phenomena"...

Good discussion!

zenny

Another thing to consider when thinking about this is the relationship, in regards to speed, that the body/CoM and the skis/BoS have with one another--turns out the skis are always traveling faster than the CoM, as the skis are on the "outside" of the arc created, and the CoM is on the inside. This means that at transition (x-move), the skis come speeding from "behind" and pass the body and, (for a short time, anyway) continue on their path. At the same moment(s), the CoM makes its own movement more or less towards the fall line, to "get ahead" at the top of the turn. This can be a seemingly instantaneous encounter between the two. Now, add a strong retraction movement while limiting vaulting, and BLAMO, you've experienced major "rebound". At least that's how I've come to understand it.

zenny

Quote:
Originally Posted by Skidude72

Majortato

How?  How is this energy released? What move is made?

Hint: you have a bent ski which does have some energy in it.  When the ski returns to its orginal shape, that energy is fully "restored"...at what point in the turn does the ski return to its original shape?  How long does it take to get there..ie at what part of the turn does the energy start to be restored, and at what point is it fully restored?  Is this instantenous...or does it happen progressivley over a part of the turn?  Have a look at some race montages...

Second Hint (just a point of clarification): Do you understand that the skis accelerate and decelerate...yet their speed never changes?  And how this is possible?  Question: If a car goes around a track, (so the start and finish line, are the same line), and the track is 60miles around, it took him one hour to do?  What was his average velocity? What was his average speed?

The energy is created by centrifugal force (observed in a non-inertial reference frame) acting on the ski.  As we exit the turn, that centrifugal force decreases and energy is released.  This happens with the edge release and a relief of pressure from the outside leg through either inside leg extension or outside leg retraction or combination of both.

The ski starts to unbend at fall line and returns to original state at transition.  Race montages show this clearly.  The release of energy is non instantaneous, however it is non-linear...with most of it happening earlier.

I've for the most part referred to acceleration in my posts as increase of speed.  I understand in a purely physics point of view, any change in velocity (which includes direction) constitutes as acceleration.

His average speed was 60mph.  Velocity is merely an instantaneous measurement of speed and direction.  Your question is technically a bit vague, but I believe the answer you're looking for is his average velocity was 0 since his delta x was 0 for that hour.

I'm an engineer btw...did I answer all your physics questions satisfactorily?

Quote:
Originally Posted by zentune

Another thing to consider when thinking about this is the relationship, in regards to speed, that the body/CoM and the skis/BoS have with one another--turns out the skis are always traveling faster than the CoM, as the skis are on the "outside" of the arc created, and the CoM is on the inside. This means that at transition (x-move), the skis come speeding from "behind" and pass the body and, (for a short time, anyway) continue on their path. At the same moment(s), the CoM makes its own movement more or less towards the fall line, to "get ahead" at the top of the turn. This can be a seemingly instantaneous encounter between the two. Now, add a strong retraction movement while limiting vaulting, and BLAMO, you've experienced major "rebound". At least that's how I've come to understand it.

zenny

This is exactly how I picture it.

The skis and COM share a delicate balance.  If you can make the skis go faster, then the COM will/has to move faster as well...otherwise the skier falls.  There are 2 forces acting on the skis: gravity, and your muscles.  With a lot of complicated movements, balance, and strength, you could take full advantage of gravity and your own strength to add speed to your turns.

Quote:
Originally Posted by majortato

The energy is created by centrifugal force (observed in a non-inertial reference frame) acting on the ski.  As we exit the turn, that centrifugal force decreases and energy is released.  This happens with the edge release and a relief of pressure from the outside leg through either inside leg extension or outside leg retraction or combination of both.

The ski starts to unbend at fall line and returns to original state at transition.  Race montages show this clearly.  The release of energy is non instantaneous, however it is non-linear...with most of it happening earlier.

I've for the most part referred to acceleration in my posts as increase of speed.  I understand in a purely physics point of view, any change in velocity (which includes direction) constitutes as acceleration.

His average speed was 60mph.  Velocity is merely an instantaneous measurement of speed and direction.  Your question is technically a bit vague, but I believe the answer you're looking for is his average velocity was 0 since his delta x was 0 for that hour.

I'm an engineer btw...did I answer all your physics questions satisfactorily?

Your answers are correct.  Most people dont understand what acceleration really means.

You are right about the release....so as an engineer you should be able to understand the ski can only restore the energy that it was able to absorb...which isnt much (it only stores what can be held in its is "deflection"), then when you consider that energy is released over a relativley long period of time, and for the most part in the wrong direction....the net effect is near zero.

Your ideas are not new to anyone here...I believed what you are preaching as did most people at one point...your concept was conventional wisdom back in the early 90s particularily around the introduction of shape skis.  However by the mid to late 90s ski coaches/physics types starting looking at this idea closer, and proved it is bunk.  The amount of energy stored is too small, and it is released too slowly to have any appreciable effect.

The "float" we feel is largley from "vaulting"...accentuated by virtual bump effects post "skis flat".

The "accleration" we feel is largely from converging paths of COM and BOS combined with virtual bump pre "skis flat".

Quote:
Originally Posted by Skidude72

Your answers are correct.  Most people dont understand what acceleration really means.

You are right about the release....so as an engineer you should be able to understand the ski can only restore the energy that it was able to absorb...which isnt much (it only stores what can be held in its is "deflection"), then when you consider that energy is released over a relativley long period of time, and for the most part in the wrong direction....the net effect is near zero.

Your ideas are not new to anyone here...I believed what you are preaching as did most people at one point...your concept was conventional wisdom back in the early 90s particularily around the introduction of shape skis.  However by the mid to late 90s ski coaches/physics types starting looking at this idea closer, and proved it is bunk.  The amount of energy stored is too small, and it is released too slowly to have any appreciable effect.

The "float" we feel is largley from "vaulting"...accentuated by virtual bump effects post "skis flat".

The "accleration" we feel is largely from converging paths of COM and BOS combined with virtual bump pre "skis flat".

I have to disagree with some of this.

The energy is released fairly quickly....most of it is released right after fall line within a fraction of a second.  The direction is not in the wrong direction because when the ski is pointed down the hill, the direction of the release is across the hill.  This is actually the direction we want the ski to go after the fall line because we need to move horizontally relative to the fall line as well....so it's perfectly in line with the direction of travel.

You say the amount of energy stored is too small...how do you figure?  The skis act like a spring...you can compress it and release it.  The forces being acted on the skis through a high speed turn can be 2-3x the weight of the skier.  If we assume a perfect non-slip carve, then the ski has to bear the full weight of the skier and the centrifugal forces acting on that skier in the middle of that turn.  You said that the ski can only store the energy you put into it...that's correct...and while it might not be 100% elastic, a good amount of that energy has to come back....2-3x the weight of a skier is a pretty decent amount of energy.  Imagine this scenario...put the two ends of the ski on some cinder blocks.  Put a 500lb weight on the middle of that ski.  Now remove that weight and I guarantee you the effect is far from insignificant.  How high will the ski bounce when we take the weight off?

Another thing to note is that the ski does not necessarily need to "pull or push" the weight of the skier.  It only needs to accelerate itself.  So even a little bit of energy helps.  We can use gravity and shorter line to drive the COM ahead of the skis....as long as the skis can keep up, then we can keep going and projecting forward.  If the ski can't keep up, then we can't project our COM as aggressively down the hill or we'd end up falling flat on our face.  So the faster we can make the ski go...even if it's just a little...then the more aggressive we can project our bodies down the hill with a net result of getting through the gates faster.

You are correct that a good part of the "acceleration feel" is from the converging paths of the COM and ski and also from the retraction movements we have to make.  However, the actual acceleration effect due to the skis though isn't something that is felt as acceleration...because you've managed it and stayed in balance with it.  Just like the acceleration forces of a turn (talking about direction now, not speed) isn't felt as acceleration..only perfect balance of all the forces.

Quote:
Originally Posted by majortato

You say the amount of energy stored is too small...how do you figure?  The skis act like a spring...you can compress it and release it.  The forces being acted on the skis through a high speed turn can be 2-3x the weight of the skier.  If we assume a perfect non-slip carve, then the ski has to bear the full weight of the skier and the centrifugal forces acting on that skier in the middle of that turn.  You said that the ski can only store the energy you put into it...that's correct...and while it might not be 100% elastic, a good amount of that energy has to come back....2-3x the weight of a skier is a pretty decent amount of energy.  Imagine this scenario...put the two ends of the ski on some cinder blocks.  Put a 500lb weight on the middle of that ski.  Now remove that weight and I guarantee you the effect is far from insignificant.  How high will the ski bounce when we take the weight off?

Lets start here, and hope the engineer in you kicks in.  We can go back to the other stuff later.

2-3x the weight of the skier? YES!  Centrigual forces are high.  But force and energy are not the same thing.

But park that for a second.

Springs store energy by being deformed.....their elastic properties mean they can return to their orginal shape, thus returning the energy that it took to distort them in the first place.

If you take a spring, like from a ball point pen, and push down on it, so it is fully compressed....will adding another 10lbs of force to a fully compressed spring, put more energy into the spring...for it to return when the weight is removed?  No.

Same with the ski...once it is fully deformed (ie supported by the snow from tip to tail)....adding more weight to it does nothing...the snow is supporting that extra mass...the only energy that comes back, is the energy that took to bend the ski to touch the snow....not alot.

Put a ski on edge in your house...see how much energy it takes to deflect the ski so it is touching the floor tip to tail...that is all you got coming back.....adding 300 more lbs of force, will do nothing....as it just gets supported by the floor/snow.

Think about it...and then we can move onto your other stuff.

Edited by Skidude72 - 2/15/13 at 2:59am
Quote:
Originally Posted by zentune

Can't wait for more of this discussion! btw, Jamt, I believe, is a race coach and physicist;) hope you don't mind me saying Jamt...

I would recommend reading the aforementioned threads regarding VB, and also one titled "reasons for finishing aft". Good stuff there, to be sure! I also used to think of the skis as being a spring in which we can store energy, which can then be released...but there's more to it than that. The whole body (with various incarnations of upper/lower separation, angulation, rotational foot steering, etc...), the type of transition we use (whether retracted or extended, or somewhere in between), and the forces generated by such movements/positions when exiting a turn and moving towards transition, all play a major role in the way a racer, or good skier, experiences this "rebound phenomena"...

Good discussion!

zenny

Quote:
Originally Posted by zentune

Another thing to consider when thinking about this is the relationship, in regards to speed, that the body/CoM and the skis/BoS have with one another--turns out the skis are always traveling faster than the CoM, as the skis are on the "outside" of the arc created, and the CoM is on the inside. This means that at transition (x-move), the skis come speeding from "behind" and pass the body and, (for a short time, anyway) continue on their path. At the same moment(s), the CoM makes its own movement more or less towards the fall line, to "get ahead" at the top of the turn. This can be a seemingly instantaneous encounter between the two. Now, add a strong retraction movement while limiting vaulting, and BLAMO, you've experienced major "rebound". At least that's how I've come to understand it.

zenny

Majortato....BTW, what skidude is trying to say is also what I was trying to get across in these two quotes above--though I may not have expressed myself as well as he can. Try the experiment he mentions. It's suprising (in a disappointing kind of way). Most people view the releasing of energy stored in a ski as that of releasing an arrow from a fully drawn bow...but this isn't what happens when we ski. It's more like releasing the tension on the bow by keeping our fingers on the string until it returns almost all the way to it's original shape. Jamt taught us (or at least me) this one.

zenny

Edited by zentune - 2/15/13 at 7:20am

Thanks for carrying on the discussion. Not much to add really.

By the way, have you watched a GS in slow motion where the ski chatters, engages and disengages repeatedly? If the skis stored the amount of energy that some believe, this chattering would be like riding a bump field in mach speed (well not really but...).

@skidude,

you are totally correct..there's a limit to how much that ski can deform and therefore store.  I still feel it's a significant amount though.  I can't just rely on putting ski on edge on the floor and push until the edges touch the floor....because throughout a turn, the skis will deform even more than that.  Putting more pressure on the ski allows me to decrease the turn radius and make tighter turns.  This test only works if I simulate park-n-ride skiing which we can all agree is not very energetic.

So I do a similar test where I put weight on the ski to bend it as much as possible with my own muscles, which should still be less than the forces acting on it through a turn.  When I release this force, I get a good amount of rebound....enough to have a significant effect on my body.  Fine if it can't store 2-3x my weight's worth in potential energy....but even a little bit would be enough.  As I said before, the ski doesn't necessarily need enough energy to pull the body along...it just needs to accelerate itself so that we can be more aggressive projecting our COM down the hill.

The other thing to consider is a race ski is able to store much more energy than a non-race ski.  It's basically like having a stronger spring...you need more force to push it down, but it also can store more energy.  Isn't this what makes it faster than a non-race ski?  If we have 2 skis with identical dimensions..the one with the metal layers would be faster through a race course would it not?  If the stored amount of energy is insignificant, then why would the stiffer ski be faster?  What makes the race ski better?

@Jamt

I know what you're talking about and this is a pretty strong argument.  But there is still an "average" bend of the ski through the turn.  The tips/tails flopping back and forth cancel each other out, but the energy stored in the "average bend" still needs to be released somehow.  Also, the effect you're describing is more noticeable on downhill/superG speeds.  There's less of it in GS and slow motion videos of slalom does not show this happening.

btw, in the end, all this doesn't really matter and would not change the way we approach skiing.  Nevertheless, it's a fun and engaging discussion.  I understand where you guys are coming from.  I could maybe buy into the idea that for long turns with bigger and slower transitions and a requirement for the ski to bring our COM across the hill, maybe the effect is minimal.  But, I'm not sold yet on the idea that the ski's released energy is not significant or cannot be used in a positive manner in shorter turns.

of course one thing that makes a race ski faster on firmer snow than a non race ski is its torsional rigidty. thats the reason for the 2 metal sheets with a wood core sandwiched in between...to prevent it from twisting. a "softer ski" in firm-ish snow is simply going to deform along its length in these conditions, producing a skidded turn rather than a pure carve. this is on reasin why they are slower in a course.
as i understand, longitudinal stffnes mostly provides a modicum of dampness in the course.

i suspect theres a little more to it, but....

zenny

Tomba could get a bit of rebound out of his old school slalom skis.

Some of the most schooled racers I know talk about "juiceing" the tails of their skis by rocking back at the end of a turn to get that "pop" into the next turn.

^^ LOL I tried to click on play to watch the video and got confused for a second.

So this is a good example...if the ski rebound doesn't hold much energy, what's causing his skis to launch into the air like that?

a skilled racer can, at the moment of transition, allow the bos to continue whilst letting the com linger behind for an instant. so much aftness can produce this image. the point of such a maneuver, if intentional, would be to have the com MORE inside when it is "finally" directed towards the fall line. one way to " cut off the line". positions such as this produce photo images of a racer way inside the gate with the ski tips just barely clearing. used for recovery as well, if you suddenly find yourself a tad late.
remember the bos is faster than the com, let com lag just a bit, and you can wheelie.

zenny

Zenny,

I agree with everything you said..but the result is that the skis wheelie.  The skier's intention is not to lift the ski up into the air....get them ahead of the COM...sure, but not up in the air.  The skier makes no movements to make these skis go airborne.  And the skier has no desire to have those skis go airborne...we know this is slower and less controlled.  That means whatever movements he made loaded the ski with energy and the ski popped up.  This shows that the ski can rebound with significant amounts of energy.

Or it could be rutted...in just the "right" spot, as sl courses so often are
zenny
Quote:
Originally Posted by majortato

Zenny,

I agree with everything you said..but the result is that the skis wheelie.  The skier's intention is not to lift the ski up into the air....get them ahead of the COM...sure, but not up in the air.  The skier makes no movements to make these skis go airborne.  And the skier has no desire to have those skis go airborne...we know this is slower and less controlled.  That means whatever movements he made loaded the ski with energy and the ski popped up.  This shows that the ski can rebound with significant amounts of energy.

I dont see that at all.  Ruts are one cause...and I believe the issue in the pic in question.  Further to say no moves are made to get the skis in the air is simply not true....we "retract" in transition, this pulls our COM down and our BOS up.

The fact that we only ever get airborne in transition....and no where else....should be giving your some hints here.

You are right thou, that skis in the air is not desirable for racing.

The majority of the retraction move is not a muscle move where we pull the skis towards us though.  Rather, the forces of the turn and the direction of the skis is pushing those skis into our body.  If you did the upper part of the turn right, then this is going to happen whether you like it or not.  We release the pressure on the skis and let the skis pass under us.  Most of that "knee to the chest" feel is the result of skiing mechanics and physics...not really a primary muscle movement to pull the skis up.  If that were the case, then there's no way I could ever transition like that...have you ever tried jumping with your skis on and pulling your skis up to your body?  You won't go very high.

Quote:
Originally Posted by majortato

The majority of the retraction move is not a muscle move where we pull the skis towards us though.  Rather, the forces of the turn and the direction of the skis is pushing those skis into our body.  If you did the upper part of the turn right, then this is going to happen whether you like it or not.  We release the pressure on the skis and let the skis pass under us.  Most of that "knee to the chest" feel is the result of skiing mechanics and physics...not really a primary muscle movement to pull the skis up.  If that were the case, then there's no way I could ever transition like that...have you ever tried jumping with your skis on and pulling your skis up to your body?  You won't go very high.

Totally agree. But you suggested there is no move being made that can possibly lift the skis....seems we both agree that is not the case of what is desirable, but that move is hapening...if you are abosrbing ruts, huge VB effects (as WC skiers expereince), want to get off edges and onto new edges ultra quick....its not just a softening of the legs...its a pull...and sometimes it gets overdone, and the skis pulled off the snow....but if you watch good skiers, the skis stay on the snow most of the time...but with vaulting, ruts, VB, etc etc...we all get some air from time to time in this turn and that.  But this is not because skis are magic springs that propel us faster and faster.  I can "umph" off my tails in the lift line....but I dont spring forward throught the line...in fact I dont actually move forward/backward at all.

You have to understand, and I will repeat this....I used to beleive everythign you are saying, for all the reasons you presented....but I have since learned its not reality.  Apply your physics knowledge from your engineering degree.....you will see that its pretty simple: skis are springs...but relativley weak, with a realtivley slow release (the energy is relased as the skis are rolled flat).....and in the wrong direction for the most part...

I'm certainly not a Phd physicist like Jamt, nor am I an engineer like  Majortato or (don't want to spill the beans without prior authorization...preferably in written form), I deliver furniture for a living. Talk about vectors!! I have been skiing 37yrs., since I was 4...and raced (now mostly "retired") for a good number of those...

Up until recently, I also firmly believed that rebound energy came mainly from the skis--that the skis were a spring which launched us. Jamt and Skidude helped show me the "light"...don't worry, we aren't members of some cult with nefarious intent! Well, at least I'm NOT--no too sure about those other two!! One thing I've realized on my own, however, is that the skis are in fact part of a much LARGER spring. They are the base (bos). The body, with proper upper/lower separation, is the rest of this spring. There is a coiling (countering, angulation, retraction) to resist centrifugal pull while exiting), and an uncoiling (brief "squaring" and extention, laterally) during  finishiation.  This whole system with all it's parts and positions is the true spring....it's not just the skis! They play a small part though...

zenny

Quote:
Originally Posted by Jamt

You are not accelerating out of a turn in racing. It could be argued that preservation of angular moment can increase the speed at the end of the turn, but this is not used in racing because it does not bring you faster down the slope. By pumping early in the turn you can accelerate, but only if you are going slow/flat. This is similar to skating.
interesting - see, i could swear my Heas have a chip there that stiffens the tails, accelerating me out of a turn. In fsct, here's a quote on tneir technology:

KERS Technology works like a turbo charger that provides additional power and acceleration by stiffening the tail of the ski in outturns.

The effect: a boost, catapulting the rider into the next turn. Just like when Formula 1 pilots push a button for that extra notch of speed.

@skidude

I think we agree pretty much on everything except: 1. skis act like springs and are weak (you) / strong (me), 2. energy released in wrong direction (you) / correct direction(me).  I think we've both said everything there is to say to support our sides.  I'm basing my views on my personal experiments and (not too shabby) understanding of physics.  At this point, I think we just have to agree to disagree.  Like I said, I don't think perceiving it either way will negatively affect the way we ski....so if I'm wrong, I'm wrong.

@zenny

I feel like all the things you said just supported my argument.  If the spring is not limited to the skis, but rather the skier and all the movements we make, wouldn't it make sense then that more energy could be generated by this "spring" and therefore produce "acceleration"?

@razie

you're supporting my argument...but I actually have to go against you regarding the details of the Head technology

I'm pretty sure it's just marketing mumbo jumbo that helps separate them from the rest.  Kind of like Fischer's hole skis.  I don't think it really does anything significant...if it did, I'm sure other manufacturers would be quickly following suit in some way even if there's a patent on it.

Quote:
Originally Posted by majortato

@skidude

I think we agree pretty much on everything except: 1. skis act like springs and are weak (you) / strong (me), 2. energy released in wrong direction (you) / correct direction(me).  I think we've both said everything there is to say to support our sides.  I'm basing my views on my personal experiments and (not too shabby) understanding of physics.  At this point, I think we just have to agree to disagree.  Like I said, I don't think perceiving it either way will negatively affect the way we ski....so if I'm wrong, I'm wrong.

Well for clarity -

My points are

1. skis act like springs, but only store the energy it took to deform them, and in real skiing, skis dont get deformed nearly as much as people think (3-4 inches of deflection at most)
2. Releasing that energy is progressive, just like it was put in, so its release is too slow to "pop" us
3. The energy is released in the wrong direction.

Say that we have energy stored in the ski. When that energy is release it will produce a force that is perpendicular to the ski surface. One component going straight up from the snow and one componenent going parellel to the snow. The first component will push your CoM upwards, and the second force component will push the CoM towards the inside of the turn, i.e. turn, or in other words decrease the turning radius. There is no component in the direction of travel. When the skis were loaded we must have had the oppsosite effect.

So then we have a system with the following characteristics.

Early in the turn when the ski is loaded the CoM is supported less and the turn radius is larger than without the loading. You are delaying the pressure.

Late in the turn when the energy is release the CoM is pushed up and the turn radius is smaller than without the release. You are delaying the release of pressue.

Sounds kind of the opposite to what you would want in a racing turn.

Kers marketing is BS IMO. Piezo can change material characteristics but there is no catapulting going on. Its kind of rediculous. They say that they store the energy in a chip. They store enough energy in a small microchip to catapult a grown up man? Hmm, well...

@zenny

I feel like all the things you said just supported my argument.  If the spring is not limited to the skis, but rather the skier and all the movements we make, wouldn't it make sense then that more energy could be generated by this "spring" and therefore produce "acceleration"?

Yes, majorito...it would. Minus the acceleration part. However, this whole time you have been arguing that it is the SKIS which store and then release the preponderance of this energy--and thats why we the skier experience a jet/rebound. I have been arguing that it is the entire system of skiing. Speed, turn shape, and ski/skier. The former two help compress this spring. It is up to us to allow this to happen. Park and ride intermediates all share some deficiencies if they are to become dynamic carvers...they don't flex or extend that much at all, they don't tip the skis on edge very far, generally, and they use much less counter and angulation in the belly/finish of their turns. Now, put one of them on a pair of slalom skis with park-n-ride, are they going to get tossed? Maybe, but not because the skis shot them into the air, but rather, because they are not employing the right movements at the right time to get the CoM ahead (relative to the fall line) at/during initiation.

Jamt's bow and arrow analogy which approximates what happens when energy is released from a ski (longitudinally, releasing the fully drawn bow whilst keeping our fingers on the string), is how I view it as well. A few posts above, you were talking about using weights in the center with tip and tails supported, and then releasing suddenly. Others have argued the "jumping on a trampoline theory". This is not what happens when we ski.

Racing isn't just about speed, as so many think. It's also about traveling the shortest distance through the course--more so than anyone else.To do that, the racer employs various tactics to cut off the line. You can haul ass across the fall line all you want...but if you don't flex/extend your body at the right time(s), you'll be slow.

zenny

Edited by zentune - 2/16/13 at 6:53am

skidude and jamt,

you both say the direction of the release is perpendicular to the ski and in the wrong direction, but as I have said before, I agree it is perpendicular to the ski, but it's NOT the wrong direction.  Let me try to explain again.  The skis are on edge during fall line facing down the hill.  At this point, the top of the ski is pointed towards the inside of the turn, or across the hill, not up the hill.  If we release the energy at this moment, the ski would rebound across the hill.  But this is actually the direction we want to go at the end of the turn correct?  So the rebound force is in the correct direction.  The result is sending the ski/skier faster across the hill.  His vertical velocity may not have increased, but his horizontal velocity has.  Since the skier needs to move horizontally back and forth, the end result is increased velocity overall.

zenny,

you're right we sort of rat-holed into talking strictly about the performance of the ski...but my original point was about the skier as a whole accelerating, not just the ski.  Everything plays a role.

Quote:
Originally Posted by majortato

The skis are on edge during fall line facing down the hill.  At this point, the top of the ski is pointed towards the inside of the turn, or across the hill, not up the hill.  If we release the energy at this moment, the ski would rebound across the hill.

.......Ok so you rebound across the hill but what if you are doing 60mph? The rebound would need to be quicker than that to have a positive effect on the overall speed no?

Quote:
Originally Posted by majortato

skidude and jamt,

you both say the direction of the release is perpendicular to the ski and in the wrong direction, but as I have said before, I agree it is perpendicular to the ski, but it's NOT the wrong direction.  Let me try to explain again.  The skis are on edge during fall line facing down the hill.  At this point, the top of the ski is pointed towards the inside of the turn, or across the hill, not up the hill.  If we release the energy at this moment, the ski would rebound across the hill.  But this is actually the direction we want to go at the end of the turn correct?  So the rebound force is in the correct direction.  The result is sending the ski/skier faster across the hill.  His vertical velocity may not have increased, but his horizontal velocity has.  Since the skier needs to move horizontally back and forth, the end result is increased velocity overall.

zenny,

you're right we sort of rat-holed into talking strictly about the performance of the ski...but my original point was about the skier as a whole accelerating, not just the ski.  Everything plays a role.

Correct.

BUT...and this is a big BUT....the problem is you are right in saying "if"...but the reality is, the energy is not released there...at best it only starts to be released there.

Good skiers ski on a constant arc (ie no flat bits- except breifly in transitions)...so that moment you are referring to, while it is present, it is only a fractions of fractions of a second.  So the only energy that would work for us, is the energy that was released at that moment....virtually nil.

Sure you could argue, then that more energy is released after that, and while not perfect, is better then nothing....well again, true....its more then nothing....but not a lot.

If it was, we would feel that "pop" in this part of the turn....we dont....we feel it much later, at the actual transition.

Quote:
Originally Posted by majortato

skidude and jamt,

you both say the direction of the release is perpendicular to the ski and in the wrong direction, but as I have said before, I agree it is perpendicular to the ski, but it's NOT the wrong direction.  Let me try to explain again.  The skis are on edge during fall line facing down the hill.  At this point, the top of the ski is pointed towards the inside of the turn, or across the hill, not up the hill.  If we release the energy at this moment, the ski would rebound across the hill.  But this is actually the direction we want to go at the end of the turn correct?  So the rebound force is in the correct direction.  The result is sending the ski/skier faster across the hill.  His vertical velocity may not have increased, but his horizontal velocity has.  Since the skier needs to move horizontally back and forth, the end result is increased velocity overall.

zenny,

you're right we sort of rat-holed into talking strictly about the performance of the ski...but my original point was about the skier as a whole accelerating, not just the ski.  Everything plays a role.

I know you said that before, but it is still wrong.

I think your feelings may be playing tricks with you Major. Forget you feelings and think like an engineer for a while. In a turn the turning force, or centripetal force, is

F=mv^/r.

This means that there is a instantaneous relationship between turning force and turn radius.

The tighter you turn the higher the force and vice versa. In the apex/fall line of a great turn this force may be about 3g. Now wouldn´t it be great if we could release a force/energy at the apex to increase this force to say 4g. That would certainly make the turn tighter according to the above relation. However, as we have tried to exlain this is simply not possible. how could you release this energy when your are pressing the ski? When you release the turn you remove the force, and thus the CoM continues in the direction it had when you released, it doesn´t launch across the hill, even if you feelings tell you that it does.

Also, look at your argument about horizontal and vertical "velocities". You are mixing velocity and speed again. Skidude mentioned this before.

If you have a force that is perpendicular to you direction of travel, or in other words perpendicular to you velocity, like in carving. What happens to the speed?

The simple answer is absolutely nothing. You are just turning in a circle. The velocity changes though.

If the force was perpendicular to the fall line the speed would increase, but that is only true for an infinitesimal amount of time.

So then, if you have some energy to release in the late part of the turn nothing happens to the speed, but the velocity changes. You might assume that since you released the turn you had an intention to turn less and thus this change in velocity is in the WRONG direction. Luckily, as has been argued, the contribution of this force is quite small so we have no major problem.

Quote:

Also, look at your argument about horizontal and vertical "velocities". You are mixing velocity and speed again. Skidude mentioned this before.

If you have a force that is perpendicular to you direction of travel, or in other words perpendicular to you velocity, like in carving. What happens to the speed?

The simple answer is absolutely nothing. You are just turning in a circle. The velocity changes though.

When I say horizontal and vertical, I mean the vectors of the overall velocity, not speed.  You're right that if we apply a sustained force that is constantly perpendicular to the direction of travel, then the speed would not change.  This is why the earth orbits the sun without falling in or flying away.  This is only the case though with a sustained force that is constantly changing in direction to be perpendicular to the direction of travel (basically a centripetal force).  But I'm not talking about a centripetal force.  I'm talking about an instantaneous force applied to the initial direction of travel.  Imagine a ball travelling straight on the vertical axis and I give it a quick push in the horizontal direction as it goes by.  If that happens, then the vertical vector of the velocity stays the same, but we have added a horizontal vector to the velocity.  The end result is an increase in speed of the ball.

So the real argument here is about whether that rebound force is strong enough and happens quick enough at fall line to cause what I described...or if its too weak and happens too slowly so that it has no effect or is always perpendicular to direction of travel so it just acts as a centripetal force.

If we take a look at some of Ron LeMaster's montages...then its obvious that in a lot of them (especially slalom), the skis are going from fully loaded at fall line (or slightly after fall line) to fully unloaded before transition in 1 or 2 frames.  This means this is happening in fractions of a second.  Then it takes 7-8 more frames for the skis to load up again from that point until the next fall line.  So relative to the whole turn, that "rebound" is happening pretty dang quick....I would say the pressure build up stage is about 70% of the turn and the pressure release is about 20% of the turn, and 10% is transition phase where neither is happening.  Why build it up slowly and release quickly?  Why not smooth it out?  The reason is to take advantage of that built up pressure...if we release too slow, then it's useless.  This also creates that comma shaped turn.  I feel like the montages provide enough evidence that

• this rebound happens quick enough and mostly when the ski is pointed towards the fall line - taking up a small percentage of the overall turn
• it is not sustained in a direction perpendicular to the direction of travel - by the time skis are pointed across the hill (which happens incredibly quick), there is 0 energy left in the ski

So the final question is...."is it strong enough?"  This is probably the most debatable aspect.  Do a test...lean the ski against the wall at an angle...now push as hard as you can on the ski.  Now release....how hard does it push back?  Well...it's not super strong.....but it's not super weak either.  Is it enough?  Is my test sufficient?  Have I sufficiently bent the ski with just the partial weight of my body?  I don't have the sufficient tools to perform this test and give a concrete answer.  My feel is this is significant....and I have not been able to fully compress the ski to its maximum.  Perhaps it only contributes to 1% of the total energy of a turn...but 1% is also about a full second on a 100 second race course.  Tell the WC guys that 1 second isn't significant.  And what if it's 2-3%?  That's pretty significant even for us mortal beer league racers.

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