or Connect
New Posts  All Forums:Forum Nav:

# Accelerating out of a turn - Page 6

if we release this pressure too slowly, then we pass (or at least are late for) our next gate. we retract with limited vault so that we can begin our next turn as early as possible...taking the shortest possible route with our com. the reason we experience the sensation of rebound from the skis is that they are quickly moving from a laterally extended position to the outside of our com, passing underneath us--which requires that we, the skier, retaract our legs (or risk too much vaulting, causing us to travel further across)--and then extending out laterally again. we the skiers are going from long, short, then back to long. building/bracing against forces, releasing, and then building again...all in the proverbial "blink of an eye". passive retraction which you mentioned in post # 138 just doesnt hold water.

i trained gs today where crossunders were definitely advantageous for a lot of the course. at one point, i "passively retracted" letting the forces do the work, without actively absorbing VB, and BLEW OUT! dont worry, i'm a trained proffessional

p.s. i did this on purpose...as a real world experiment
zenny

We are going around in circles. I could answer with the same reply again but it would not lead us anywhere.

There is nothing like an instantaneous force. Didn´t you read my argument about increasing from 3 to 4 g? Impossible, and now you say instantaneous. That would mean we would be talking about a force of a different order of magnitude. 40 g? How? The force you are talking about is not available until you recamber the skis, and it can help a bit, a very small bit, in the retraction move. At this time the turning forces are virtually zero.

The only way to increase speed in other ways than gravity is to push with your muscles. That is pumping. Pumping increases pressure early and removes pressure late. What you are arguing is the opposite.

If what you say was even close to true skis would have massive camber. The last ten years the camber has disappeared more and more. Most racing skis even have reverse camber (rocker) now.

Why dont you edge the skis 60 degrees and push them down into a carpet instead. release them during 0.3 seconds. That is more similar to what really happens.

For arguments sake, say that your reasoning is correct (it is not). You gain speed when you release energy stored in the ski. How would you avoid loosing speed when you load the skis? Wouldn´t that be the reverse process?

By the way, you are mixing vectors and component.

I feel like I am repeating myself with different words. I advice you to challenge your own thoughts, and if you insist that you are correct please come with concrete proof.

Quote:
Originally Posted by Jamt

For arguments sake, say that your reasoning is correct (it is not). You gain speed when you release energy stored in the ski. How would you avoid loosing speed when you load the skis? Wouldn´t that be the reverse process?

This is the obvious point.

I have refrained from making it, because I believe Major does beleive we do infact speed up and slow down in a turn...of course this is false.  His arguement will be, we dont slow down because we add energy in from our legs.  Of course, this cant be true, because we are pushing the skis one way, and our COM the opposite way.....

Obviously there's no such thing as instantaneous force.  I was simplifying when I said instantaneous...the point is it happens pretty quickly and mostly in a direction that is across the hill.  You keep saying this recambering action happens super late close to transition where there are no turning forces and therefore useless....yet this is not what the picture montages show.

Indeed we can avoid losing speed when we push on the skis because of muscle input.  Muscles can add energy into a system and convert it to a force that is useful.  How does a person on a trampoline keep getting higher and higher?  They push the trampoline down with their muscles while the trampoline pushes them up...yet with every jump they get higher and higher.  How does a person sitting on a swing make movements to swing higher and higher?  By your logic, this cannot happen.  It shouldn't be hard to conceive the idea that if we use our muscles in some way, we can accelerate our bodies.

We are going in circles for sure.  I have tried to open my mind and look at it from your viewpoints.  I'm past the point where I'm basing my viewpoints on the "feel" aspect of it.  I understand that a lot of the "feel" of this acceleration is possibly due to something else.  I've thought hard about what's been discussed and a lot of what you said makes sense...however, there's always aspects that do not draw the whole picture.  A lot of the points that I made have been ignored or not been well rebutted by you other than a general statement that says I'm wrong.  Sorry but this is not sufficient.

Here's a thought experiment.  Let's take 2 identical sized skiers.  Skier 1 has a 18m radius ski and really bends and drives those skis in turns and makes 14m radius turns.  Skier 2 has 14m radius skis and just rides the sidecut to make 14m radius turns.  The 2 skiers end up making the exact same turns down the hill.  Assuming you are correct that the ski rebound plays no role, then theoretically both these skiers will make it down to the bottom of the hill in the same amount of time because the only thing that is accelerating them is gravity.  If you can prove this is the case, then I concede my points.  However, if skier 1 is faster...then you have to ask, where is this extra acceleration coming from if not generated by the skier somehow?

We are kind of going in circles.  You're right that I don't have concrete proof...but neither do you.  I say we call it a stalemate and go do some real skiing before the season is over.  :-)

Cheers!

inline

Quote:
Originally Posted by majortato

Obviously there's no such thing as instantaneous force.  I was simplifying when I said instantaneous...the point is it happens pretty quickly and mostly in a direction that is across the hill.  You keep saying this recambering action happens super late close to transition where there are no turning forces and therefore useless....yet this is not what the picture montages show.

Question for you. If you have one ski with camber and one with reverse camber. They bend the same amount in apex and have the same spring force. The skis with camber has more energy stored in it. Which ski has the largest force against the surface?

It should be clear that the forces are the same. This is the moment you claim that there is a big extra force from the spring? Just like the Skidudes ball point example, the spring is pressed to the "ground". When you start to lessen the edge the spring slowly releases the small amount of energy all the way to the moment when the ski has left the ground.

Indeed we can avoid losing speed when we push on the skis because of muscle input.  Muscles can add energy into a system and convert it to a force that is useful.  How does a person on a trampoline keep getting higher and higher?  They push the trampoline down with their muscles while the trampoline pushes them up...yet with every jump they get higher and higher.  How does a person sitting on a swing make movements to swing higher and higher?  By your logic, this cannot happen.  It shouldn't be hard to conceive the idea that if we use our muscles in some way, we can accelerate our bodies.

I have not said it is not possible to accelerate, not just in the way you describe it. What happens if the trampoline is mounted 2 inches above the ground?

We are going in circles for sure.  I have tried to open my mind and look at it from your viewpoints.  I'm past the point where I'm basing my viewpoints on the "feel" aspect of it.  I understand that a lot of the "feel" of this acceleration is possibly due to something else.  I've thought hard about what's been discussed and a lot of what you said makes sense...however, there's always aspects that do not draw the whole picture.  A lot of the points that I made have been ignored or not been well rebutted by you other than a general statement that says I'm wrong.  Sorry but this is not sufficient.

What point would that be? I think most of them were adressed.

Here's a thought experiment.  Let's take 2 identical sized skiers.  Skier 1 has a 18m radius ski and really bends and drives those skis in turns and makes 14m radius turns.  Skier 2 has 14m radius skis and just rides the sidecut to make 14m radius turns.  The 2 skiers end up making the exact same turns down the hill.  Assuming you are correct that the ski rebound plays no role, then theoretically both these skiers will make it down to the bottom of the hill in the same amount of time because the only thing that is accelerating them is gravity.  If you can prove this is the case, then I concede my points.  However, if skier 1 is faster...then you have to ask, where is this extra acceleration coming from if not generated by the skier somehow?

That is completely wrong. Read e.g. the recent MA of ChuckT. (It is also applicable to your skiing)

We are kind of going in circles.  You're right that I don't have concrete proof...but neither do you.  I say we call it a stalemate and go do some real skiing before the season is over.  :-)

Cheers!

I guess you are a  beer league racer? Do you try to maximize pressure early in the turn or late in the turn?

There is significant energy - here is a visual of how much energy there is: http://www.youtube.com/watch?v=I19etFrGoXQ&sns=em

Cheers

(Edit) my app is stupid - i was replying to an earlier post i thought was the last post... The video is the one where i was shooting boots from a ski...

Which of the following statements do you think are false?

A decambered ski stores energy

The amount of energy stored in a ski is not enough to send a skier cross the hill.

The amount of energy stored in a ski is enough so send a childs boot across a slippery floor.

The energy stored in the ski can help in a retraction move so that the necessary retraction force is somewhat smaller

The energy stored in the ski is released progressively, not instantaneously.

Progressively release energy means smaller forces.

A force perpendicular to the direction of travel is a turning force.

If there is an external force pressing the ski into the surface, the ski "spring" cannot generate an opposite force that is larger than this force. At no point in the turn.

The external force pressing the ski into the surface is larger than the spring force in most of the turning phase.

If more camber made racers faster skis would have a massive camber.

Modern race skis have almost no central camber and reverse tip camber.

Generating pressure early and releasing pressure early is fast in racing.

A spring system in a ski delays pressure input output function.

It is possible to accelerate out of a turn by applying muscular force, but not in a direction that increases speed or brings the skier faster to the bottom.

Quote:
Originally Posted by razie

There is significant energy - here is a visual of how much energy there is: http://www.youtube.com/watch?v=I19etFrGoXQ&sns=em

Cheers

(Edit) my app is stupid - i was replying to an earlier post i thought was the last post... The video is the one where i was shooting boots from a ski...

That's the bow and arrow/trampoline effect....skis aren't supported on the snow at tip and tail only (usually), but rather along their entire length. Also, as we ski this pressure isn't released instantaneously (like letting the bow string go at full draw), rather the pressure is released relatively slowly...because the process of un-weighting the ski takes time--granted not much time in some cases, but much more relative to the shooting of an arrow from a fully drawn bow (or ski boot)  Of course I'm just repeating what's already been said before....

zenny

Skis have less camber as they get stiffer and use better materials

A "slippery" basement floor has more friction than the air

Boots slide on floor not pushed by god but by the horizontal component of same original impulse

Skis are released early, way before they are perpendicular to the direction of travel

Thus, the energy stored in them will help the retraction and floating the skis over to the other side

PS
The release in my video was about third of a second that is how long it took the skis to return to camber and unload sending the boot and themselves flying ( yes, my son couldn't hold the ski, which ski flew about a foot before landing on my helmet), that magnitude anyways - how does that compare to the speed of a regular race release, say a slalom turn?

My point is that the energy there is significant, i.e. not to be ignored. You can use it or waste it, but any advanced skier and especially racers can tell you the difference between an older, lifeless ski and a new springy one. That's part of why we keep buying new skis, right?
yes razie...but how much does a ski boot weigh? now, how much does a skier in motion weigh? i don't know (i bet Jamt could give us an educated guess!), but of course its a lot more than a pair of boots...as the skier in question would presumably be wearing them. if a boot in a basement is flung only a few feet, how much could a skier on the slope be affected?

as for new race skis...i just got a pair:) they are, shall we say, "wicked". there is a noticable difference between them and my older ones. i suspect that its the torsional degredation combined with the logitudinal detradation which we notice. longitudinal flex helps mitigate deflection. torsion is for edge grip.

i dont think any one here is saying the skis dont add something...its just that if we DO become airborne during transition, its either from a very active retraction on the part of the skier...or an error on the part of the skier.

zenny
also remember razie...an active retraction, especially in a race course, to absorb "VB", is used as an UNWEIGHTING movement (unload the skis, extend and reload). the skis simply cannot unload themselves--we have to do it, if we are to quickly change edges.

zenny
Quote:
Originally Posted by zentune

also remember razie...an active retraction, especially in a race course, to absorb "VB", is used as an UNWEIGHTING movement (unload the skis, extend and reload). the skis simply cannot unload themselves--we have to do it, if we are to quickly change edges.

zenny
Agreed - this is my point, and we all know it: the spring is there. I can use it and "push off it" or absorb it. It is all in how i use my muscles and the timing. Agreed also - i don't think it is enough to propel an entire body any significant distance upwards.

However, and honestly i didn't read the entire thread so forgive me if i am off, but i can certainly accelerate on the flats, by turning, in several ways (not just skating) and the skis do help there as well. I think my body does most of the work there though.
go back and read the recent VB thread, razie. what we absorb during retraction contains very little ski energy. what we are absorbing is a "bump" that isnt there. because our head stays lower than in a crossover style turn...therefore, for our skis to pass under our com we suck them up, much like encountering a terrain feature.

as far as pushing off? i always thought we were "pushing" against the snow.
zenny

Zenny, you prove that common sense goes a long.

Razie, my statements were not just random, like most or yours seem to be. Releasing the edges and releasing the energy stored in the ski are two different things.

I´ll ask you again which ones are false. because they cannot be true if the statements made by you and others are.

Here is something "perhaps intuitive"... to consider for Razie and Majortato,

Picture yourself riding an old school mountain bike (ie no suspension) down a ski run, making turns back and forth like a skier doing GS turns might.

Can you envision where the rider might get "highsided"...ie "chucked".

Yes or no?

If yes, where in the turn would it be?

Similair to skiing?

Since the bike has no "springs"...what could be causing this?

Ok here we go in red:

Quote:
Originally Posted by Jamt

Which of the following statements do you think are false?

A decambered ski stores energy

Yes

The amount of energy stored in a ski is not enough to send a skier cross the hill.

The amount of energy stored in a ski is significant...even if it is not enough to send the skier across the hill, it can send itself across the hill faster.  That's all we need!  If the ski can go faster, then the skier's COM can take a more aggressive or shorter line....we don't need enough energy to send a 180lb skier across the hill..only enough energy to send a 5lb ski across the hill.

The amount of energy stored in a ski is enough so send a childs boot across a slippery floor.

Look above...yes

The energy stored in the ski can help in a retraction move so that the necessary retraction force is somewhat smaller

yes, not all over the energy goes across the hill...some of it is wasted, some of it can used for other purposes like retraction.

The energy stored in the ski is released progressively, not instantaneously.

as discussed...there is no such thing as instantaneous force except in a theoretical mathematical world.  The stored energy is released relatively quickly...about 20% of the entire turn.

Progressively release energy means smaller forces.

overall forces are the same....but forces that can be used towards direction of travel might be diminished through progressive release.

A force perpendicular to the direction of travel is a turning force.

A sustained force applied perpendicular to the direction of travel is a turning force.  A force applied to the initial direction of travel is NOT a turning force (think a swimmer going across a river).  As a physicist, you of all people should understand this fairly easily.

If there is an external force pressing the ski into the surface, the ski "spring" cannot generate an opposite force that is larger than this force. At no point in the turn.

Agreed.  It doesn't need to.

The external force pressing the ski into the surface is larger than the spring force in most of the turning phase.

The external pressing force happens before the fall line when we pressure the skis.  After the fall line, the pressing force decreases and the spring force takes over.

If more camber made racers faster skis would have a massive camber.

More camber can add more spring action....it does not necessarily make a skier faster.  Don't confuse the two.  There is a delicate balance that needs to be achieved.

Modern race skis have almost no central camber and reverse tip camber.

reverse tip camber is to avoid tips digging into snow...this is obvious.  The tip itself doesn't do much on the ski.  All modern race skis I own or have owned have some camber.

Generating pressure early and releasing pressure early is fast in racing.

yes....sort of.... It's a loaded question.  There's a lot of variables involved in racing aside from this.  You can say tucking is faster than not tucking, but when you tuck, you give up the possibility of a clean line.

A spring system in a ski delays pressure input output function.

pressure input happens when you create pressure....it's not related to the ski.  How the ski responds to that pressure is a different story.

It is possible to accelerate out of a turn by applying muscular force, but not in a direction that increases speed or brings the skier faster to the bottom.

I disagree.  This is basically our debate.

But here's the real question.  I simplified it as much as possible by removing all the complicated physics and everything out of the equation:

Quote:

Here's a thought experiment.  Let's take 2 identical sized skiers.  Skier 1 has a 18m radius ski and really bends and drives those skis in turns and makes 14m radius turns.  Skier 2 has 14m radius skis and just rides the sidecut to make 14m radius turns.  The 2 skiers end up making the exact same turns down the hill.  Assuming you are correct that the ski rebound plays no role, then theoretically both these skiers will make it down to the bottom of the hill in the same amount of time because the only thing that is accelerating them is gravity.  If you can prove this is the case, then I concede my points.  However, if skier 1 is faster...then you have to ask, where is this extra acceleration coming from if not generated by the skier somehow?

That is completely wrong. Read e.g. the recent MA of ChuckT. (It is also applicable to your skiing)

This is what I mean by telling me I'm wrong without really supporting evidence.  It's a simple thought experiment with a simple answer.  Don't direct me to a new thread with some semi-related discussion...just answer the simple questions

• Who is faster?  (if the same...and you can prove it...then I have lost this debate)
• If skier 1 is faster...why?  where is the acceleration coming from?  (if the force is not from gravity, then you have to give me something else...it's not "magic")
Quote:
Originally Posted by majortato

This is what I mean by telling me I'm wrong without really supporting evidence.  It's a simple thought experiment with a simple answer.  Don't direct me to a new thread with some semi-related discussion...just answer the simple questions

• Who is faster?  (if the same...and you can prove it...then I have lost this debate)
• If skier 1 is faster...why?  where is the acceleration coming from?  (if the force is not from gravity, then you have to give me something else...it's not "magic")

You are wrong.

Here is the evidence.

When shape skis started to come out, WC skiers had no restirction on gear, as such they ran shorter and shorter skis, with more and more sidecut.  This was true for GS and SL.  They didnt do this because it was slower...they did it becuase it was faster.  Hence in your question there is no reason to believe the longer ski with the longer side cut would be inherently quicker.

FIS imposed rules, requiring minimum lengths, and maximum sidecuts. As this happened, skiers typically stayed as close to the minimums as possible.

Have some skiers thou, opted to go beyond the minimums?...some have, yes.  Why?  They had the skill and strength to run straighter line..so a straighter ski suited them....

However, by and large the majority worked to the minimums...

Quote:

Here's a thought experiment.  Let's take 2 identical sized skiers.  Skier 1 has a 18m radius ski and really bends and drives those skis in turns and makes 14m radius turns.  Skier 2 has 14m radius skis and just rides the sidecut to make 14m radius turns.  The 2 skiers end up making the exact same turns down the hill.  Assuming you are correct that the ski rebound plays no role, then theoretically both these skiers will make it down to the bottom of the hill in the same amount of time because the only thing that is accelerating them is gravity.  If you can prove this is the case, then I concede my points.  However, if skier 1 is faster...then you have to ask, where is this extra acceleration coming from if not generated by the skier somehow?

Specific to this example thou...your bolded bit is false...they cant ski the same line, because the skier on the 18m skis is inclining more then the skier on the 14m skis...meaning if they are on the same line, one of them will be out of lateral balance, and either fall inside (18m skier)...or lose grip, or fall to the outside (14m skier)...so they will ski different lines.

Edited by Skidude72 - 2/18/13 at 5:30pm

Sidecut and length and two different things...don't confuse them.  Shorter sidecut makes it easier to make shorter cleaner turns and generally aids in a race course where turns are tight.  WC courses are setup to push the limits of the skier and skis.  It's made so that clean carves are not possible to make turns, which helps separate the skill levels of the racer.  This is why they usually do bare minimum.

Quote:
Quote:

Here's a thought experiment.  Let's take 2 identical sized skiers.  Skier 1 has a 18m radius ski and really bends and drives those skis in turns and makes 14m radius turns.  Skier 2 has 14m radius skis and just rides the sidecut to make 14m radius turns.  The 2 skiers end up making the exact same turns down the hill.  Assuming you are correct that the ski rebound plays no role, then theoretically both these skiers will make it down to the bottom of the hill in the same amount of time because the only thing that is accelerating them is gravity.  If you can prove this is the case, then I concede my points.  However, if skier 1 is faster...then you have to ask, where is this extra acceleration coming from if not generated by the skier somehow?

Specific to this example thou...your bolded bit is false...they cant ski the same line, because the skier on the 18m skis is inclining more then the skier on the 14m skis...meaning if they are on the same line, one of them will be out of lateral balance, and either fall inside (18m skier)...or lose grip, or fall to the outside (14m skier)...so they will ski different lines.

So now you're saying it's impossible for a skier with bigger sidecut skis to ski the exact same radius turns as a skier with shorter sidecut skis?  That would mean cleanly carved turns are purely dictated by the sidecut of your skis....and that's just not true.

Quote:
Originally Posted by majortato

Sidecut and length and two different things...don't confuse them.  Shorter sidecut makes it easier to make shorter cleaner turns and generally aids in a race course where turns are tight.  WC courses are setup to push the limits of the skier and skis.  It's made so that clean carves are not possible to make turns, which helps separate the skill levels of the racer.  This is why they usually do bare minimum.

No one has confused them.  How did I confuse them?  I referred to both.  And provided the proof you were seeking that rebound energy is not a factor for racers choosing skis.

Quote:
Originally Posted by majortato

So now you're saying it's impossible for a skier with bigger sidecut skis to ski the exact same radius turns as a skier with shorter sidecut skis?  That would mean cleanly carved turns are purely dictated by the sidecut of your skis....and that's just not true.

Not at all...just saying its impossible for them to ski the same line, at the same speed.  The skier with the longer radius will need to be going faster....or the skier with the shorter radius going slower....either would work, and neither mean rebound is present or relevant.  In my previous post, I was assuming they were both racing, thus skiing as quickly as possible....in this case, they will be on different lines...you example makes no sense any other way, since you are referring to "who is faster"....

Try skiing behind a friend who is on a different side cut then you...will be very hard to follow his exact track at this exact speed.  The longer the radius, the more you need to tip the ski, meaning the more you need to incline, meaning the faster you need to be going.  Its this need for lateral balance that necessatates the longer radius skier to go faster...he doesnt go faster because of rebound....serioulsy.

This is basic.

Quote:
Originally Posted by majortato

Sidecut and length and two different things...don't confuse them Generally, a longer race ski will have less sidecut.   Shorter sidecut Do you mean "smaller radius"? makes it easier to make shorter cleaner turns and generally aids in a race course where turns are tight.  WC courses are setup to push the limits of the skier and skis.  It's made so that clean carves are not possible to make turns, which helps separate the skill levels of the racer.  This is why they usually do bare minimum.

So now you're saying it's impossible for a skier with bigger sidecut Do you mean " larger radius"? skis to ski the exact same radius turns as a skier with shorter sidecut skis ? (Audible sigh...) That would mean cleanly carved turns are purely dictated by the sidecut of your skis....and that's just not true.?? No. He's saying that a skier on larger radius skis, say 27 meters in a 22 meter set will have to angulate more and/or cut off the line...the latter at the top of the turn with a stivot of one magnitude or another...a redirect so to speak. The Com of the larger radius skis will take a different path down the course...as will the skis.

zenny

Sorry Skidude...didn't know you already responded....

zenny

Blue

Quote:
Originally Posted by majortato

The amount of energy stored in a ski is not enough to send a skier cross the hill.

The amount of energy stored in a ski is significant...even if it is not enough to send the skier across the hill, it can send itself across the hill faster.  That's all we need!  If the ski can go faster, then the skier's COM can take a more aggressive or shorter line....we don't need enough energy to send a 180lb skier across the hill..only enough energy to send a 5lb ski across the hill.

Ok, so how much energy is stored? Make a model and estimate. Did you do the carpet test? Also do you understand that to send something across the hill it needs to fly?

A force perpendicular to the direction of travel is a turning force.

A sustained force applied perpendicular to the direction of travel is a turning force.  A force applied to the initial direction of travel is NOT a turning force (think a swimmer going across a river).  As a physicist, you of all people should understand this fairly easily.

What I wrote is the definition of a turning force. Don´t know what you are trying to argue.

What is the initial direction of travel? The CoM is basically going down the fall line crossing the center line as it turns. While you are going towards the center line, i.e. after the fall line, you claim that you can increase the speed by pushing. At this time you are basically pushing yourself up the hill. Do you see why this does not makes sense? We are not even using physics here. Common sense.

The external force pressing the ski into the surface is larger than the spring force in most of the turning phase.

The external pressing force happens before the fall line when we pressure the skis.  After the fall line, the pressing force decreases and the spring force takes over.

Well you agreed on: "If there is an external force pressing the ski into the surface, the ski "spring" cannot generate an opposite force that is larger than this force. At no point in the turn.

Agreed.  It doesn't need to."

This means that the spring force "takes over" a lot later than you think.

If more camber made racers faster skis would have a massive camber.

More camber can add more spring action....it does not necessarily make a skier faster.  Don't confuse the two.  There is a delicate balance that needs to be achieved.

You are the one claiming that the spring action accelerates the skier out of the turn. If that was true...

Modern race skis have almost no central camber and reverse tip camber.

reverse tip camber is to avoid tips digging into snow...this is obvious.  The tip itself doesn't do much on the ski.  All modern race skis I own or have owned have some camber.

Lol, you realize we are talking obout racing on ice/firm snow here? Reverse camber in race skis is used because you want the ski tip to be soft but torsionally stiff. This is very difficult to make but with some reverse camber you don´t have to make the tip as thin. The tip of a racing ski is very important.

A spring system in a ski delays pressure input output function.

pressure input happens when you create pressure....it's not related to the ski.  How the ski responds to that pressure is a different story.

What? You have been claiming all along that the spring force "takes over", where does the resulting pressure come from if "it´s not related to the ski"

It is possible to accelerate out of a turn by applying muscular force, but not in a direction that increases speed or brings the skier faster to the bottom.

I disagree.  This is basically our debate.

So, you are basically claiming that you accelerate out of a turn by pushing muscularly? Well, I claim that in order to accelerate in a desired direction you should apply force above the fall line and minimize it after the fall line.

The ski-snow interface does not care if the force comes from energy stored in the ski or your legs. This means that released energy from the skis after the fall line would have the same effect as pushing after the fall line. Following this argument along with your reasoning then would come to the conclusion that pushing after the fall line is a good thing, and please don´t come with the argument that this happens at the fall-line because we agreed that the release is not instantaneous. In the best of worlds it starts at the fall line, but most often a lot later.

Regarding the 14m/18m I think you lack some basic understanding of what goes on in a carved turn and the reason why different sidecuts are faster in different circumstances. That is why I directed you to the MA of ChuckT where it was discussed. I think you have some important realization to do before you can take your skiing to the next level. Pushing will not take you there.

Edited by Jamt - 2/19/13 at 1:20am

Tried to edit my post but it did not work for some reason.

Regarding the acceleration out of the turn vs not there is some there is some truth to both points here and since this is the major disagreement I'll try elaborate and see if we can agree.

First assume that we have two systems, one where the ski stores a lot of energy, and one where it stores a little energy. Assume that the skis turn the same in all other regards. Lets disregard friction also because it will not make a major difference.
We start in a transition with the same kinetic energy. The skier makes the same movements in both cases.
Lets assume that in both cases the skis have their maximum amount of energy stored at the apex.
At the apex one system has higher potential energy due to the more stored energy in the spring. Since the total energy is the same this means that the system with the higher energy ski has lower kinetic energy, i.e. it is moving slower. After the apex both systems will start to return to the unloaded state and when the skis are completely flat both systems have the same kinetic energy. So in this regard you can say that the system with stronger spring has accelerated out of the turn more than the system with a weaker spring. However, the system with the weaker spring will be faster because it reached the end of the turn sooner. It's average speed was higher. This acceleration out of the turn is the preservation of angular moment I mentioned before.

Off course this is very simplified but it should give the basic idea..

So how about the following summary:
You can increase the skiing speed muscularly by extending when the skis and CoM paths are diverging
In order to be fast this diverging need to happen early in the turn
Energy stored in a ski makes the average speed slower, but allows a "certain" amount of increased speed out of the turn. (avoiding the turn acceleration here since it also includes direction)
This certain amount of increased speed is only possible because the speed decreased earlier.
We will never agree on what the level of "certain" is. I think it is negligible and you think it is not.

JAMT

You assume a lossless system.

In my view, skiing is as much a fluid mechanical process as it is a rigid body problem.

Perhaps the process of accelerating out of a turn is a case where the skier conserves more energy than the case of the skier who slows into the turn and doesn't recover any energy coming out.

I'm surprised you haven't commented on my post about the elastic properties of snow and my gedanken experiment where I reason that a skier can get the most pop out of a ski on soft but not too soft snow.

I saw your post about elastic properties of snow. To be honest I don't know much about that but my intuitive feeling is that snow is not very elastic. In soft snow the ski bends more than the Cos(alpha)*r formula indicates. This makes the energy stored in the ski a bit more, but it also increases the VB effect due to tighter turning radius. Don't know which property becomes more dominant. In really soft snow I suppose the non-elasticity of snow would cancel the elasticity of the ski. Actually its not just a virtual bump, when you press down on softer snow you create a small real bump. Perhaps this is what feels like springy snow? I think this effect would be larger than any elasticity of the snow.

Quote:
Originally Posted by dakine

JAMT

You assume a lossless system.

In my view, skiing is as much a fluid mechanical process as it is a rigid body problem.

Perhaps the process of accelerating out of a turn is a case where the skier conserves more energy than the case of the skier who slows into the turn and doesn't recover any energy coming out.

I'm surprised you haven't commented on my post about the elastic properties of snow and my gedanken experiment where I reason that a skier can get the most pop out of a ski on soft but not too soft snow.

I know that any fis. reg race ski doesn't feel particularly "poppy" in soft snow. Poopy, maybe .

zenny

Quote:
Not at all...just saying its impossible for them to ski the same line, at the same speed.  The skier with the longer radius will need to be going faster....or the skier with the shorter radius going slower....either would work, and neither mean rebound is present or relevant.  In my previous post, I was assuming they were both racing, thus skiing as quickly as possible....in this case, they will be on different lines...you example makes no sense any other way, since you are referring to "who is faster"....

I didn't say same speed did I?  I just said same line.  Gravity works the same on both skiers...it doesn't realize one has a longer ski and will therefore pull that person harder.  I think we both agree that the skier with the bigger sidecut needs to go faster somehow to make those turns.  Gravity provides the same forces on both skiers...this means the skier with bigger sidecut skis is adding energy into the system with the use of muscle movements.

Quote:
The Com of the larger radius skis will take a different path down the course...as will the skis.

Zenny provides a good argument to my example finally.  You are correct..the COM needs to take a different path to achieve this....but from a theoretical point of view, the skis don't.  Why does the ski need to take a different path?  The snow and gravity don't understand sidecut or ski bend...they only care about a couple things..the curvature of the edge of the ski to the snow.  If the 18m ski is on higher edge angle than the 14m ski, then they can theoretically both create the same bend along the contact point.  They should both be able to carve the same line.  But lets get back to the COM.....the 18m skier takes a slightly shorter line with the COM.  This would account for some of the increased speed for sure.  How much though?  Does it account for all of it?  He's still working a lot harder to bend those skis than the guy on the 14m skis.  Where is all this energy going?  Is he wasting it just to bend the skis and lay them on edge?

Quote:
What is the initial direction of travel? The CoM is basically going down the fall line crossing the center line as it turns. While you are going towards the center line, i.e. after the fall line, you claim that you can increase the speed by pushing. At this time you are basically pushing yourself up the hill. Do you see why this does not makes sense? We are not even using physics here. Common sense.

No I'm not claiming that we push to accelerate out of a turn.  The push happens earlier (turn initiation).  We push to store energy, then we release.  There is no pushing after the fall line..I never claimed that.

Quote:
You are the one claiming that the spring action accelerates the skier out of the turn. If that was true...

If you've read my previous posts..I've said multiple times that this force might not accelerate the skier out of the turn.  It might just be accelerating the skis out of the turn and across the hill while the skier is unweighted.

Quote:

The tip of a racing ski is very important.

Which is why fischer put a big hole in their racing skis at the tip?  And no other company has followed suit after 3 years.  I don't buy that the ski tip does anything substantial other than help prevent the skis from digging into the snow.  I think the ski tip is just a remnant of classic ski design.  It doesn't hurt to have it there..so why remove it?  Racing course might be complete ice, but the stuff around it is probably fluff.  Imagine if you skied out of the course and hit the soft stuff, then flip over your skis because you have no tip and your skis dig in....pretty dangerous situation.

Quote:
So, you are basically claiming that you accelerate out of a turn by pushing muscularly? Well, I claim that in order to accelerate in a desired direction you should apply force above the fall line and minimize it after the fall line.

I never claimed you push muscularly after the fall line...please re-read what I wrote.  I agree that we apply force above the fall line...it is that application of the force above the fall line that allows us to accelerate after the fall line.

Quote:
Lets assume that in both cases the skis have their maximum amount of energy stored at the apex.
At the apex one system has higher potential energy due to the more stored energy in the spring. Since the total energy is the same this means that the system with the higher energy ski has lower kinetic energy, i.e. it is moving slower.

Why does the total energy have to be the same?  There's no reason for this restriction.  IF the total energy is the same..then your argument makes sense.  But in the case of the ski with higher potential energy, the skier needs to work harder to bend that ski.  This means the total energy in the system is higher for the ski with higher potential energy.

Quote:
So how about the following summary:
You can increase the skiing speed muscularly by extending when the skis and CoM paths are diverging
In order to be fast this diverging need to happen early in the turn
Energy stored in a ski makes the average speed slower, but allows a "certain" amount of increased speed out of the turn. (avoiding the turn acceleration here since it also includes direction)
This certain amount of increased speed is only possible because the speed decreased earlier.
We will never agree on what the level of "certain" is. I think it is negligible and you think it is not.

I pretty much agree with this completely except "energy stored makes the average speed slower."  The storing of energy is the result of a lot of things at play.  Reducing average speed to build this energy is not one of them.  So the last statement...a "certain" amount of increased speed is possible at the end of the turn.  I think this is NOT negligible.

a skier on a larger radius ski will enivitably have to redirect at some point, to stay on line...pitching the skis at the top of the turn...hence the skis taking a different line.

zenny
BTW, majortato, you do know that a smaller radius ski (same manufacturer/model) is going to have a bigger sidecut than the longer radius ski...relative to length?

the reason a racer is "adding more energy" on the larger radius skis is that he has to have higher edge angles...hes going faster, so he is resistin or bracing against the forces more.

zenny
Quote:
Originally Posted by dakine

JAMT

You assume a lossless system.

In my view, skiing is as much a fluid mechanical process as it is a rigid body problem.

Perhaps the process of accelerating out of a turn is a case where the skier conserves more energy than the case of the skier who slows into the turn and doesn't recover any energy coming out.

I'm surprised you haven't commented on my post about the elastic properties of snow and my gedanken experiment where I reason that a skier can get the most pop out of a ski on soft but not too soft snow.

Snow is not elastic.  Water is one of the most well understood and studied substnaces on the planet.  Snow is formed of crystals, which are brittle.

You feel more "pop" in those hero snow conditions, because they allow you to ski with more peformance...meaning you get bigger VB effects, and greater vaulting effects from quicker and more dynamically converging paths of COM and BOS.

New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: Ski Instruction & Coaching