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# Why be patient at turn transition? - Page 5

Quote:
Originally Posted by patrickjchase

This doesn't address my specific claim that the rebond energy can drive the skis/boots/feet through the turn transition. I never claimed that it can lift the entire skier like a trampoline, so I would appreciate it if you would address my actual argument instead of a strawman..

I've shown that the energy required to drive the skis/feet/boots under the body durng the turn transition is on the order of 250J.

I've also shown that the energy contained in the skis is on the order of 100J, i.e. a significant fraction of the requried energy

Which one do you believe to be wrong and why?

While NEC goes to find an answer for you....let me explain....the answer is in the video below.

Note the amount of energy there?  A fair bit wouldnt you say?  But there is two problems with it:

1) we dont get a ski that bent ever

2) we dont ski with a strap - this is significant difference because:

• With the strap 100% of the energy can be realised instantly, this creates the "snap" -
• in skiing the energy is released by rolling the skis flat - this takes time, meanning the release of energy is much slower, thus less significant

Look at this shot for example:

Look at how bent the ski is (ie how much potential energy is in it), then look at how many frames it takes to release it 3.5/4?

So repeat the experiment in the video below, but put the same amount of bend into it....and release it at the same rate as can be done skiing....what do you get?

A few more shots to compare:

Note the skis are not bent like the video...and the energy is released  over 3-4 frames...not instant.

Finally - note the direction the energy is released.....it acts in a direction 90 degrees to the ski base.....so it cant push the feet "throught" or forward etc.

Quote:
Originally Posted by L&AirC

The ski does store energy...

...but this is just like the energy that a bow has.  A bow will launch arrows but not a person.  Even when you consider a trampoline, to get maximum lift from it, you have to work with the trampoline and time everything right.  I think some people expect the ski to just haul them down the mountain or into the next turn.  In my head, it is more like a well timed judo throw where you take your opponents energy and keep it moving in the direction it wants to go, though your opponent might disagree.  If you don't do the move correctly, it doesn't work.

Same with skiing. I think more of the energy comes from the person than from the skis.  Even in the bent ski in the moguls mentioned previously, I can jump higher than the bent skis can lift me, but if I time it right, using the ski I can get higher than just my legs alone.

Ken

### Gear mentioned in this thread:

Quote:
Originally Posted by patrickjchase

This doesn't address my specific claim that the rebond energy can drive the skis/boots/feet through the turn transition. I never claimed that it can lift the entire skier like a trampoline, so I would appreciate it if you would address my actual argument instead of a strawman..

I've shown that the energy required to drive the skis/feet/boots under the body durng the turn transition is on the order of 250J.

I've also shown that the energy contained in the skis is on the order of 100J, i.e. a significant fraction of the requried energy

Which one do you believe to be wrong and why?

With respect to the trampoline question, the math to settle it once and for all is incredibly simple, so here goes.

To raise a person of mass m (in kg) by height h (in meters) you need an energy (in Joules, henceforth J) of m*g*h, where g is 9.81 m/sec^2 (the Earth's gravity).

Per my post above a VERY deeply reverse-cambered pair of skis contains on the order of 100J of energy

Assuming an 80 kg skier we get h = 100/(80*9.81) = 0.1 meters. In other words you get a whopping 4" of trampoline effect. Skiers who aren't named Ligety, Raich, or Kostelich will get much, much less. I agree with NECoach that this is insignificant under most real-world situations, and that's precisely why I've never claimed that the energy in the skis could lift the entire skier. I only said it could drive the skis across the body in a cross-under transition.

Before replying, please note that it takes a lot less energy to accelerate a given mass sideways than it does to raise it straight up. This plus the fact that feet+boots+skis weight much less than the entire skier means that a lateral transition takes an order of magnitude less energy than moving the whole skier up.

Quote:
Originally Posted by patrickjchase

With respect to the trampoline question, the math to settle it once and for all is incredibly simple, so here goes.

To raise a person of mass m (in kg) by height h (in meters) you need an energy (in Joules, henceforth J) of m*g*h, where g is 9.81 m/sec^2 (the Earth's gravity).

Per my post above a VERY deeply reverse-cambered pair of skis contains on the order of 100J of energy

Assuming an 80 kg skier we get h = 100/(80*9.81) = 0.1 meters. In other words you get a whopping 4" of trampoline effect. Skiers who aren't named Ligety, Raich, or Kostelich will get much, much less. I agree with NECoach that this is insignificant under most real-world situations, and that's precisely why I've never claimed that the energy in the skis could lift the entire skier. I only said it could drive the skis across the body in a cross-under transition.

Before replying, please note that it takes a lot less energy to accelerate a given mass sideways than it does to raise it straight up. This plus the fact that feet+boots+skis weight much less than the entire skier means that a lateral transition takes an order of magnitude less energy than moving the whole skier up.

That formula just provides potential energy from gravity.

For a ski you need to look here and consider:

As I wrote before, you need to consider time - total energy is meaningless without the concept of "work" and "power".

Also "work" explained here:

http://en.wikipedia.org/wiki/Work_%28physics%29

Time for JamT!

Let me preface my remarks by saying that I never claimed that the energy drives the skier or the skis forward. I said it drives the skies under the skier's body, i.e. side to side. In a highly angulated turn that is in fact very close to 90 degrees from the ski base, so all of your points about direction merely support my argument. In fact what matters is the projection of the normal vector of the ski along the snow surface, which is simply the sin() of the edge angle. For example, if the ski is at 60 degrees then sin(60) = ~70% of the energy is directed from side to side. If you work through th math for spring potential you'll see that the vast majority of the energy gets released in the first inch or two of de-decambering and therefore happens at the highest edge angles (this addresses your point about the ski being flat in the middle of the transition).

Quote:

Originally Posted by Skidude72

While NEC goes to find an answer for you....let me explain....the answer is in the video below.

Note the amount of energy there?  A fair bit wouldnt you say?  But there is two problems with it:

1) we dont get a ski that bent ever

2) we dont ski with a strap - this is significant difference because:

• With the strap 100% of the energy can be realised instantly, this creates the "snap" -
• in skiing the energy is released by rolling the skis flat - this takes time, meanning the release of energy is much slower, thus less significant

In this and the remainder of your post you are consistently confusing power (the first derivative of energy with respect to time, i.e. rate at which energy is released) with total energy. The fact that the energy is released over a long-ish period of time is actually irrelevant to this discussion. Imagine going 0-60 in 5 seconds vs 10 in your car - The energy required is about the same either way, it's just a question of how fast it's converted from chemical to kinetic energy. In other words, you need an engine that burns gas twice as fast to do it in 5.

In thie case of skis, the spring potential energy in the ski is precisely equivalent (by "precisely" I mean that they are both potential energies, denominated in Joules) to the chemical energy in gasoline, or the energy in a modern F1 car's KERS flywheel. That energy can be used to do exactly the same amount of work (i.e. accelerate the skis/boots/feet to the same lateral velocity), regardless of whether it takes 2 frames or 10. All that matters is whether the total energy in the skis is enough to accomplish what I claim. "snap", "straps", etc are completely irrelevant. I'm not replying to the rest of your post because they are all addressed by either or both of my foregoing remarks about angles or power/energy.

Addressing your points about power for a moment, those sequences were taken at ~10 fps, so 2-3 frames is 200-300 msec. If the skis contain 100J that means that the instantaneous power is on the order of 300-500 watts (probably more). That's actually a fair bit, as any cyclist can tell you.

I do agree with your statement that we (the sort of people who post on this forum) don't get a ski bent that far (any more), and I acknowledged that in at least 2 previous posts. NEcoach claimed that rebound doesn't matter without exception, though, so from a logical argument perspective all I need is one counterexample. It doesn't matter if that counterexample is named "Ligety" or "Miller".

Quote:
Originally Posted by patrickjchase

Let me preface my remarks by saying that I never claimed that the energy drives the skier or the skis forward. I said it drives the skies under the skier's body, i.e. side to side. In a highly angulated turn that is in fact very close to 90 degrees from the ski base, so all of your points about direction merely support my argument. In fact what matters is the projection of the normal vector of the ski along the snow surface, which is simply the sin() of the edge angle. For example, if the ski is at 60 degrees then sin(60) = ~70% of the energy is directed from side to side. If you work through th math for spring potential you'll see that the vast majority of the energy gets released in the first inch or two of de-decambering and therefore happens at the highest edge angles (this addresses your point about the ski being flat in the middle of the transition).

In this and the remainder of your post you are consistently confusing power (the first derivative of energy with respect to time, i.e. rate at which energy is released) with total energy. The fact that the energy is released over a long-ish period of time is actually irrelevant to this discussion. Imagine going 0-60 in 5 seconds vs 10 in your car - The energy required is about the same either way, it's just a question of how fast it's converted from chemical to kinetic energy. In other words, you need an engine that burns gas twice as fast to do it in 5.

In thie case of skis, the spring potential energy in the ski is precisely equivalent (by "precisely" I mean that they are both potential energies, denominated in Joules) to the chemical energy in gasoline, or the energy in a modern F1 car's KERS flywheel. That energy can be used to do exactly the same amount of work (i.e. accelerate the skis/boots/feet to the same lateral velocity), regardless of whether it takes 2 frames or 10. All that matters is whether the total energy in the skis is enough to accomplish what I claim. "snap", "straps", etc are completely irrelevant. I'm not replying to the rest of your post because they are all addressed by either or both of my foregoing remarks about angles or power/energy.

Addressing your points about power for a moment, those sequences were taken at ~10 fps, so 2-3 frames is 200-300 msec. If the skis contain 100J that means that the instantaneous power is on the order of 300-500 watts (probably more). That's actually a fair bit, as any cyclist can tell you.

I do agree with your statement that we (the sort of people who post on this forum) don't get a ski bent that far (any more), and I acknowledged that in at least 2 previous posts. NEcoach claimed that rebound doesn't matter without exception, though, so from a logical argument perspective all I need is one counterexample. It doesn't matter if that counterexample is named "Ligety" or "Miller".

I think you need to do some more study.  You are right, I am talking power...it matters a great deal, because we are talking about moving somthing over time.

Take the energy stored in a bullet...set it off all at once, and it can launch a projectile with leathal force.  Release that same amount of energy over a year....and at most it will keep the bullet nice and warm.  Big difference.

As for frames, 1/10 of a second per frame....ok...so 2 to 3 to 4 tenths of a second...vs. 1/100th of a second with the strap?  So even at 2 tenths...that is 20 times longer!

Further your "Sin(x)" argument...again, you are missing time...you math is right, but the skier only gets the energy that is released at "60" during the time they are at "60"...so 70% of almost nothing...then its repeated 59,58,57,56 and so on to 0.

Think about it.

Quote:
Originally Posted by Skidude72

That formula just provides potential energy from gravity.

For a ski you need to look here and consider:

As I wrote before, you need to consider time - total energy is meaningless without the concept of "work" and "power".

Also "work" explained here:

http://en.wikipedia.org/wiki/Work_%28physics%29

Time for JamT!

Yes, the formula I provided is for gravitational potential. I did that to address the comments about "trampoline effects", i.e. the ability or lack thereof of the ski to launch the skier *upward*. I actually did so to try to demonstrate that a ski couldn't do that to any significant extent. You are absolutely correct that I ignored a directional issue there for the sake of simplicity, i.e. not all of that energy would actually be directed upward and not all of it would add to the skier's gravitational potential. That's actually a sideshow, though. I will helpfully repeat what I have said several times: Skis do not "push" the skier vertically when they de-decamber. There is no significant vertical trampoline effect. Happy?

Wok *is* energy, though not all energy is work. How much energy does useful work is often a tricky discussion topic because it depends on your definition of useful work. I think that's where we differ - My intiial post said that the energy in the skis can drive them from side-to-side, i.e. normal to the skier's direction, parallel to the snow surface. At very high edge angles almost all energy released from the ski IS useful work along that axis. Clear?

Quote:
Originally Posted by Skidude72

I think you need to do some more study.  You are right, I am talking power...it matters a great deal, because we are talking about moving somthing over time.

Take the energy stored in a bullet...set it off all at once, and it can launch a projectile with leathal force.  Release that same amount of energy over a year....and at most it will keep the bullet nice and warm.  Big difference.

As for frames, 1/10 of a second per frame....ok...so 2 to 3 to 4 tenths of a second...vs. 1/100th of a second with the strap?  So even at 2 tenths...that is 20 times longer!

Further your "Sin(x)" argument...again, you are missing time...you math is right, but the skier only gets the energy that is released at "60" during the time they are at "60"...so 70% of almost nothing...then its repeated 59,58,57,56 and so on to 0.

Think about it.

I DID think about it. It's a fairly simple integral, and I've actually done it many times. EDIT: Removed needless complexity. You can use trig functions with simple integrals for camber depth.

EDIT AGAIN: w.r.t. power, I also addressed that. 2 frames at 10 fps s fast enough.

Your point about a bullet is irrelevant, because we're not dealing with time intervals where friction or drag come into play. You can accelerate a bullet as you describe in a vacuum...

Quote:
Originally Posted by patrickjchase

If you work through th math for spring potential you'll see that the vast majority of the energy gets released in the first inch or two of de-decambering and therefore happens at the highest edge angles (this addresses your point about the ski being flat in the middle of the transition).

Go do the integral, and then come back to this thread so that we can have a reasoned discussion. I'm done until then.

Quote:
Originally Posted by patrickjchase

Yes, the formula I provided is for gravitational potential. I did that to address the comments about "trampoline effects", i.e. the ability or lack thereof of the ski to launch the skier *upward*. I actually did so to try to demonstrate that a ski couldn't do that to any significant extent. You are absolutely correct that I ignored a directional issue there for the sake of simplicity, i.e. not all of that energy would actually be directed upward and not all of it would add to the skier's gravitational potential. That's actually a sideshow, though. I will helpfully repeat what I have said several times: Skis do not "push" the skier vertically when they de-decamber. There is no significant vertical trampoline effect. Happy?

Yes, I agree.

Quote:
Originally Posted by patrickjchase

Wok *is* energy, though not all energy is work. How much energy does useful work is often a tricky discussion topic because it depends on your definition of useful work. I think that's where we differ - My intiial post said that the energy in the skis can drive them from side-to-side, i.e. normal to the skier's direction, parallel to the snow surface. At very high edge angles almost all energy released from the ski IS useful work along that axis. Clear?

Oh I am clear what you are saying....I just dont agree with it.  Useful work, would be "pushing the feet parallel to the snow surface", as you put it...skis dont do that.....at very high edge angles....you only get the energy that is released at high edge angles...and then only the component of it which is in the right direction (which seems we both agree on)...as the energy is continued to be released, the edge angle is decreasing...meaning we get less and less force being generated "across the snow surface"....so we start with not much and end with even less......

Quote:
Originally Posted by patrickjchase

I DID think about it. It's a fairly simple integral, and I've actually done it many times. The only tricky part is the equation for camber "depth" as f(edge angle), but you can approximate that as a polynomial. That's why I wrote:

Quote:
Originally Posted by patrickjchase

If you work through th math for spring potential you'll see that the vast majority of the energy gets released in the first inch or two of de-decambering and therefore happens at the highest edge angles (this addresses your point about the ski being flat in the middle of the transition).

Go do the integral, and then come back to this thread so that we can have a reasoned discussion. I'm done until then.

I agree with the bold bit....its correct.  But it doesnt change anything...you still have time, to deal with.  So what, you want to say 1/10 of a second? Its still 10Xs longer then the strap...and you know that your tangent to the arc will be in the right direction for what 1/1000 of a second?

Anyway.....I think the light bulb went on for you with "power"......

Skis dont project our feet across the hill because they do not have a lot of energy to begin with, its released over a relativley long time frame (few tenths of a second), and almost all released in the wrong direction.

Quote:
Originally Posted by Skidude72

Oh I am clear what you are saying....I just dont agree with it.  Useful work, would be "pushing the feet parallel to the snow surface", as you put it...skis dont do that.....at very high edge angles....you only get the energy that is released at high edge angles...and then only the component of it which is in the right direction (which seems we both agree on)...as the energy is continued to be released, the edge angle is decreasing...meaning we get less and less force being generated "across the snow surface"....so we start with not much and end with even less......

OK, here are the equations: EDIT: Wrong trig function. Too late! Thanks for your patience SD72!

The energy contained in a spring (the ski) is 0.5*k*x*x, where x is the deflection, i.e. the depth to which the ski is decambered

The decamber depth x is approximately sidecut_depth/cos(angle), assuming perfect torsional stiffness (big assumption, but please go along with me for the moment)

The energy at any given angle is therefore:

0.5*k*sidecut_depth*sidecut_depth/(cos(angle)*cos(angle))

What do you think that inverse-cos-squared term does at high angles? (hint: infinite asymptote) Go ahead and plug some edge angles into that equation, and see how much of the energy is released going from, say, 75 to 60 degrees. Torsional flex will moderate that asymtotic behavior a bit, but the general trend is that the last few degrees "contain" much of the energy.

Edited by patrickjchase - 5/11/13 at 12:36am
Quote:
Originally Posted by patrickjchase

OK, here are the equations: EDIT: Wrong trig function. Too late! Thanks for your patience SD72!

The energy contained in a spring (the ski) is 0.5*k*x*x, where x is the deflection, i.e. the depth to which the ski is decambered

The decamber depth x is approximately sidecut_depth/cos(angle), assuming perfect torsional stiffness (big assumption, but please go along with me for the moment)

The energy at any given angle is therefore:

0.5*k*sidecut_depth*sidecut_depth/(cos(angle)*cos(angle))

What do you think that inverse-cos-squared term does at high angles? (hint: infinite asymptote) Go ahead and plug some edge angles into that equation, and see how much of the energy is released going from, say, 75 to 60 degrees. Torsional flex will moderate that asymtotic behavior a bit, but the general trend is that the last few degrees "contain" much of the energy.

Note that the asymptotic behavior I described above also means that the instantaneous power can be quite high, on the order of kilowatts if you start from, say, Ligety's angles.

Nice math.

In reality where the energetics and dynamics set limits (no 90 degree edge angles) things are even more complicated.

Most racers I talk to (including myself) know that to really get pop out of  good race skis all you have to do is rock back at the end of the turn and release.

Pop happens.

Sometimes way more than I have a plan to deal with.

The primary cause of blown out ACL is a flexed tail unloading into the knee joint.

In some situations the energy stored in a flexed ski can be significant and useful.

In some situations the energy stored in a flexed plug boot could make it very easy to move that foot back.

Most of the time this stuff is secondary to what the skier is doing.

I'd still like to know more about the original topic of this thread.

The best part about a carved turn is that magical time when you are between turns and are starting to engage the new outside edge while still getting your mass where it is going to need to be in a fraction of a second.  (Like Gretzky skating to where the puck is going to be)

I learned to ski shaped skis by myself when they first came out and I called the patient time while you are waiting for the edge "finding the edge."

Once you find the edge then you can stand on it.

Fundamentally, I guess the ski takes a while to flex and dig enough of a trench that it will take a load.

How do you guys describe this phase of the turn (high, high C?) when you are teaching?

Bend that tail.

Quote:
Originally Posted by patrickjchase

OK, here are the equations: EDIT: Wrong trig function. Too late! Thanks for your patience SD72!

The energy contained in a spring (the ski) is 0.5*k*x*x, where x is the deflection, i.e. the depth to which the ski is decambered

The decamber depth x is approximately sidecut_depth/cos(angle), assuming perfect torsional stiffness (big assumption, but please go along with me for the moment)

The energy at any given angle is therefore:

0.5*k*sidecut_depth*sidecut_depth/(cos(angle)*cos(angle))

What do you think that inverse-cos-squared term does at high angles? (hint: infinite asymptote) Go ahead and plug some edge angles into that equation, and see how much of the energy is released going from, say, 75 to 60 degrees. Torsional flex will moderate that asymtotic behavior a bit, but the general trend is that the last few degrees "contain" much of the energy.

Patrick, there has been a number of thread where the energy stored in a ski has come up. In bumps and other terrain features the ski can be loaded with a lot of energy, but the question is what happens on flat snow. My conclusion has been something like:

The ski is loaded with some energy that can assist in the release, but this energy is significantly less than most people think.

Like skidude said, the release of this energy is gradual, there is no or very little launch effect.

What people normally mistake for energy stored in the ski is the vaulting or VB effect.

It seems that you are not like most people and that you have understood the mechanics so the question then is really how big the effect is and how noticeable is it?

I took a pair of FIS GS skis and did some measurements. They have a 15 mm sidecut and it takes 160 N to press them down to a hard and flat surface (my workbench) when they are edged 60 degrees (30 mm travel).

Assuming an ideal spring according to Hooke´s we get the potential energy

PE=0.5 * Force * decamber distance = 0.5 * 160 *0.03 =2.4 J

2.4 J is not so much.

So, what if we edge them more? When I put my knee to the inside boot I can edge 72 degrees. This is my physical limit in my normal stance.

Cos(72)=0.31 and cos(60)=0.5. We are not really approaching any asymptote. Besides the assumption regarding the ski is probably not very accurate above 70 degrees anyway.

Don´t know if it was mentioned, but have you tried to decamber an edged ski on a carpet and then release it? You will feel that there is much less energy than what you would think. Most people are a bit surprised when they try this.

Here's the related concept pre-loading a motocross bike's suspension before a jump - everyone agrees that it makes you jump significantly higher and we're talking about +200 kg rider+bike coming from two/three little pogo springs.

skip to 3 min in to see it or listen to the talk before...

or

Since this is a much more clear-cut application of the same principle, guys, it would be interesting to see your math explaining that, just to double check. The compression we're talking about is about 3-4 inches and it's all done by the rider compressing the suspension by unloading and then loading it at the right time... perhaps less force than the one bending a GS ski at speed when you're pulling... not sure... 2-3g ? The guy in the first video is using the previous landing to load it as well, so maybe 2g there?

This is a classic technique in either motocross jumping or enduro, used to start wheelies, lift the front wheel over a log, whip the rear around and many other uses - bred and butter in offroad racing. The point is that itself will not really make the bike jump a lot, but it ads significant help to make everyone strive for it... again, in enduro is not as much about jumping (first video) while in motocross is only about jumping higher...

cheers,

Razie who just started his enduro racing season :)

 here's another cool visual for it, skip maybe 1:30 in: http://youtu.be/oKkhFgAwmZY

Edited by razie - 5/13/13 at 11:44am
Sorry - just to finish my thought here - aside from double checking the math, the point is that the energy is there. You can try it on your mountain bike and FEEL it. Try jumping up and down on your street bike or sidwalk and then do the same on your mountain bike. I assure you it will be there and you will feel the difference, as it ADDS to your jumping. Or lets you push against it. Or whatever you do with it.

Maybe it's so marginal in numbers that it's irrelevant - I seriously doubt it and your mountain bike will doubt it too, although it only has like tiny pogo springs... but the brain really registers the difference, thus everyone talks about "rebound".

If we assume for a second that it's not there and it's all virtual quantum physics (not saying there isn't any VB) pushing my legs up rather than a bent skis - what difference does that make in technique or result? how extremely useful is this debate?

cheers,
razie
Quote:
Originally Posted by NECoach

Bend that tail.

Nice tail bending.  Who is that?

Quote:
Originally Posted by razie

Since this is a much more clear-cut application of the same principle, guys, it would be interesting to see your math explaining that, just to double check. The compression we're talking about is about 3-4 inches and it's all done by the rider compressing the suspension by unloading and then loading it at the right time... perhaps less force than the one bending a GS ski at speed when you're pulling... not sure... 2-3g ? The guy in the first video is using the previous landing to load it as well, so maybe 2g there?

Razie, the dynamics in skiing is completely different. What if you have a bike with a spring that has a travel of 30mm and bottoms out when you push it with a 160 N force? Any additional force you put on it doesn´t load anything. You cannot preload that bike more than the 2.4 J. It is the same as the analogous ski if you are on a hard surface. It does not matter if you load it with 160 N or 1600 N, it still has the same amount of energy.

Interesting dynamics involved in Enduro though. To me it seems that a lot of the jump power is achieved by vaulting, just like rebound. The vaulting is around the back wheel and is powered by the engine. Also a lot of jump power from running into objects with the wheels/suspension, kind of like bump skiing. Also some interesting fore-aft dynamics.

Quote:
Originally Posted by razie

Here's the related concept pre-loading a motocross bike's suspension before a jump - everyone agrees that it makes you jump significantly higher and we're talking about +200 kg rider+bike coming from two/three little pogo springs.

skip to 3 min in to see it or listen to the talk before...

or

Since this is a much more clear-cut application of the same principle, guys, it would be interesting to see your math explaining that, just to double check. The compression we're talking about is about 3-4 inches and it's all done by the rider compressing the suspension by unloading and then loading it at the right time... perhaps less force than the one bending a GS ski at speed when you're pulling... not sure... 2-3g ? The guy in the first video is using the previous landing to load it as well, so maybe 2g there?

This is a classic technique in either motocross jumping or enduro, used to start wheelies, lift the front wheel over a log, whip the rear around and many other uses - bred and butter in offroad racing. The point is that itself will not really make the bike jump a lot, but it ads significant help to make everyone strive for it... again, in enduro is not as much about jumping (first video) while in motocross is only about jumping higher...

cheers,

Razie who just started his enduro racing season :)

 here's another cool visual for it, skip maybe 1:30 in: http://youtu.be/oKkhFgAwmZY

Totally agree with JamT....motox is totally different.

Go to your driveeway, and jump up and down in ski boots with skis on....feel free to use the ones you think are "springiest".

Do the same with a pogo stick

Do the same on a motox bike

You will note, the skis do nothing for you.

I'm sure all the smarter ones here have already realized this and is something I guess I knew, but didn't think about.  Everyone talks about vaulting and rebound while carving like you can come flying out of a turn.  Some, myself included, bring up the whole thing with the way a ski bends in between bumps and how that will make you "springier".  Reading Jamt and Skidude's post above, it hit me.  We don't land in the apex of a turn.  We ski through the apex.  I know for about 5 pages the village elders have been stating the whole time thing, but some times I do need a boot to the head like Ed Gruberman.

In Skidude's example above, the driveway is flat so the ski can't bend to get springy. You could always rig something like two cinder blocks; one under the tail and one under the tip (hope you don't like your skis).  Now the skis will bend, BUT, that is still "landing" and not traveling through.  In Bob Barne's mogul animation that shows the backwards pedalling motion (I think), do you think while skiing like that, you would be able to be launched over the next mogul from the bottom of the trough?  I'm not talking about catching air in the bumps because you use the bump as a ramp, but actually spring over it.

Just because something has energy doesn't mean it all gets released at once.  There has to be a trigger like for a bow and arrow.  Standing still and jumping between bumps is like pulling the bow's string back and letting the arrow fly.  Skiing through the apex or even through bumps, is like pulling the bow's string back and easing the string back to it's resting place.  The energy was there at the apex but you didn't release it all at once.

Ken

Edited by L&AirC - 7/7/13 at 8:58am
I can't tell 2 Joules from 3 Vernes, but I can now assure you that there is significant energy in the bent skis. You got me to the point of being on the fence, but now I know. And you can feel the power too, baby - cost you 6\$ in 2x4s ... And a lot of storage wax

You can certainly feel the difference. The spring is quite strong, it surprised me on the first two jumps.

QED as far as I am concerned: it won't throw a skier over the house, but there is significant energy and it greatly enhances the release, which takes half a second, whatever it takes. Perhaps PMTS style cant use as much of it, since they don't seem to want to push as hard against it and thus let some dissipate, but it's there!

Cheers
Quote:
Originally Posted by razie

I can't tell 2 Joules from 3 Vernes, but I can now assure you that there is significant energy in the bent skis. You got me to the point of being on the fence, but now I know. And you can feel the power too, baby - cost you 6\$ in 2x4s ... And a lot of storage wax

You can certainly feel the difference. The spring is quite strong, it surprised me on the first two jumps.

QED as far as I am concerned: it won't throw a skier over the house, but there is significant energy and it greatly enhances the release, which takes half a second, whatever it takes. Perhaps PMTS style cant use as much of it, since they don't seem to want to push as hard against it and thus let some dissipate, but it's there!

Cheers

I love your videos

But again...you dont ski with 2x4s...in your video all of the energy was released at once.  In skiing its released slower.  So even if it is released in 1/10 of a second (and it is likley longer then that likley 2/10) that means the "power" will be 1/10 or more likley 1/20th of what you experienced there.

So you did it....if you reduce that spring by a factor 10 or 20 what you got?

Quote:
Originally Posted by Skidude72

I love your videos

But again...you dont ski with 2x4s...in your video all of the energy was released at once.  In skiing its released slower.  So even if it is released in 1/10 of a second (and it is likley longer then that likley 2/10) that means the "power" will be 1/10 or more likley 1/20th of what you experienced there.

So you did it....if you reduce that spring by a factor 10 or 20 what you got?
I hear you, but the duration of the release in the video I think is consistent with an average SL... I am not trying particularly hard or fast to release it. And if instead of pushing my entire body with skis up, you reduce it to just floating the legs and skis, there is lots to be had. Perhaps less on GS and none in speed, given the duration, you are right there.

Ummmm, Razie...as a semi-retired race tech--watching you do that to your skis makes me want to adopt them!!

zenny

Quote:
Originally Posted by zentune

Ummmm, Razie...as a semi-retired race tech--watching you do that to your skis makes me want to adopt them!!

zenny
yeah, I know - figured that using the softwood and hard storage wax should be ok. You can tell that I like the Heads better though

I am not going to scrape them and check the base, will let you know in the fall - the wax took a beating, that much is visible
Quote:
Originally Posted by razie

I hear you, but the duration of the release in the video I think is consistent with an average SL... I am not trying particularly hard or fast to release it. And if instead of pushing my entire body with skis up, you reduce it to just floating the legs and skis, there is lots to be had. Perhaps less on GS and none in speed, given the duration, you are right there.

Well if you have a fancy camera that lets you take stills frame by frame you can detrmine for sure.  I tried on youtube, but not really possible...but if you can get how many frames from bent to "neutral"..and if you know the frame rate....you can work it out.  I dont believe that a SL skier can go from the gate (max deflection (more or less)) to skis flat as fast as your feet move those 4-5inches from skis flexed to skis neutral....WCers are not that fast!

Razzie starts his jump -

1:36:296 - maximum downward ski flex

1:36:430 - skis pass through flat

1:36:563 - skis launch and reach maximum elevation

What happens when Razzie lands back on the blocks? The skis bend deeply and then oscillate between flat and slightly bent but do not have the energy to move the skier much. There is not enough energy to launch back up without a jump.

The experiment does not model on snow dynamics because the snow's support inhibits the trampoline effect of jump induced loading and unloading.

Quote:
Originally Posted by razie

I can't tell 2 Joules from 3 Vernes, but I can now assure you that there is significant energy in the bent skis. You got me to the point of being on the fence, but now I know. And you can feel the power too, baby - cost you 6\$ in 2x4s ... And a lot of storage wax

You can certainly feel the difference. The spring is quite strong, it surprised me on the first two jumps.

QED as far as I am concerned: it won't throw a skier over the house, but there is significant energy and it greatly enhances the release, which takes half a second, whatever it takes. Perhaps PMTS style cant use as much of it, since they don't seem to want to push as hard against it and thus let some dissipate, but it's there!

Cheers

Cool! However what you have showed is that the skis can be loaded with significant energy, like e.g. in bumps. No one claimed it was not so. What was claimed was that they are not loaded with a lot of energy on a hard flat surface AND that the energy is release progressively.

If you want to make a more realistic case you can build an inclined plane that is inclining about 60 degrees. Stand on that plane with your egdes biting into the surface and untip the skis until they slide. Did you feel any energy released?

Look at this HQ slow motion clip,

Energy released from a ski that launches the ski off the snow does not give back any "launch" energy until the ski leaves the snow under foot. Before that the "spring" is pressed to the bottom and the energy is returned as "untipping energy" and friction. The ski follows its track until this point, it is not flying from side to side. Granted there is some energy in this untipping, although very little. However, this untipping moment that the released energy results in acts in the opposite direction compared to what you claim, it rotates the boot towards the outside of the turn, not launching across the slope towards the inside.

In the clip you see that when the ski under foot leaves the snow the angle is quite low and the bend of the ski is almost nonexistent. I claim that the energy released from this point forward to get the skis across is very small unless there is a terrain feature.

By the way, I can create rebound on hockey skates. I'm pretty sure the energy does not come from energy stored in the skate.

Quote:
Originally Posted by Jamt

Cool! However what you have showed is that the skis can be loaded with significant energy, like e.g. in bumps. No one claimed it was not so. What was claimed was that they are not loaded with a lot of energy on a hard flat surface AND that the energy is release progressively.

If you want to make a more realistic case you can build an inclined plane that is inclining about 60 degrees. Stand on that plane with your egdes biting into the surface and untip the skis until they slide. Did you feel any energy released?
In my mind, what I showed was that a ski decambered 3 inches has significant energy. We are talking about skis decambered avout 3 inches, shown in the photomontages... So skis decambered 3 inches have significant energy - no doubt anymore.

Mine are supported at the tips and middle, just like yours in the snow. The discussion is now around how to release and use that energy. If you kep yours at 70 deg and release, they will be supported at the tips and tail like mine and you should be able to do exactly what I did, except at 70 degrees. If at the same time you roll them to flat, you have options to use the energy or waste it, depending on how you time the movement.

Releasing at 70 degrees should allow you to use more of that energy, because you're not fighting gravity anymore... You may be fighting a centrifuge though... Need to think through that...

The release time of .15 that NEC calculated is consistent with SL, another .15 to load them and that's .3 of a turn of .8, sounds about right, with gliding to rise
Ine and etc.
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