Rebound

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Simplified?

Postby BigE » Fri Mar 02, 2007 8:15 am

Here is my take on what is being said, from a slightly different perspective, although I believe consistent.

It has been stated that it is all about the centripetal forces. Yes, I agree.

We feel acceleration in the turn when we keep our CM over or sllightly infront of the feet -- the edged carving ski deflecting our mass to a new direction is sensed as acceleration.

When we stop resisting or balancing against this deflecting force, (at release through OLR say ) the deflecting forces of the turn now only push against the skis, instead of the ski & body system. The turn forces overwhelm this resistance, and the skis move under us, apparently "of their own accord".

As we know, it does not take a lot of force to decamber a ski. You also know that if you are in the back seat, there will not be a lot of rebound created, because your bent legs absorb what would otherwise be manifest as acceleration.

So, it is the edged and arcing ski creating that deflecting force that drives the rebound, not the straightening of the ski. You ain't gonna get a whole lot of rebound twisting the skis -- edge grip is the key component.
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Postby h.harb » Fri Mar 02, 2007 11:12 am

There is also the muscle contraction issue to keep in mind. A quad muscle that can be fully contracted with a strong platform under the foot, has it?s own spring built in, like plyometrics. Yes, when I do the hop change exercise a strong platform yields more efficient hops. I always try to make my hops by using a strong retraction move rather than a push off. This is hard to do at slow speed and much easier a higher speed, the acceleration equation is still at work.

When I feel, I have to use more extension to hop, it is usually because all the stars haven?t lined up properly, edge angle, edge grip, muscle contraction and arc radius tightening. Whether the ski rebounds more with all these components in place or whether the combing of all forces results in better rebound , is yet to be measured or proven. Our overall theory, I think is right, but how much does the ski actually contribute to the push or flexion is hard to determine. I know if I am not completely balanced or have a relatively weak carving outside ski edge, I can?t hop without effort. If my timing and arc are perfect I can hop without pushing or even retraction effort.

This gives the hop exercise tremendous validity on two fronts. Not only for developing clean balanced edge changes, but for developing timing with arc radius and release.
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Postby BigE » Fri Mar 02, 2007 12:28 pm

serious wrote:I still am struggling with an explanation for the feeling you get when you jump up. A good rebound certainly makes it feel easier. Perhaps the example given by Harald (jumping on the ski while parked in the bumps) makes most sense for this scenario.


I think the spring from the legs is key. If you are stacked/aligned perfectly to resist the centripetal force of the turn, your muscular effort will appear much easier, as if assisted by the ski itself.

I recall from somewhere, that the bulk of the rebound is derived from tension within your own body. Harnessing that energy is a matter of alignment over the engaged edge.
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timing

Postby François » Sat Mar 03, 2007 7:28 am

I have no historical background in ski instruction, so what the words mean to me may be quite different form what they mean to experienced teachers and students of skiing. I think of rebound as what a diving board does when you jump off the end of it.

I have been carving turns for a long time, though it is only in the last five or ten years that I have played with short turns. I have tried different things with absorption, flexing, standing on the skis, etc. at different parts of the turn. I like to play at keeping the skis at the limit of their maximum grip. Sometimes that play requires a little flexing for pressure control at the end of the turn. A carefully balanced slow flexing release at crossunder can absorb all the rebound. Doing the opposite, really loading up the ski and then suddenly removing the load, gets you rebound. You need to be carving tight fast turns to load up the skis.

The game gets better when you play with fore-aft balance. It's not just the bow straightening to propel the arrow; it's the bow being bent while touching the ground and jumping into the air when released. I find it helps to think of the movement of a cracking whip. Imagine the side of the whip near the end being in contact with something. The motion of the whip is like that of a snake. If you load up the front and move the load to the rear as you complete the turn really standing on the tail at the end and then when that tail is wound up tight, suddenly let go, your skis will jump ahead to the other side as you go straight through detached as it were from the skis. Careful! You might be left behind.

The force I can generate from "rebound" isn't much if it has to move me and my skis, but it's a lot when it only needs to move the skis across.
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Postby ramshackle » Sat Mar 03, 2007 11:52 am

Max_501 wrote:
serious wrote:The amount of rebound you get from the ski depends only on how decambered the ski is at the moment of release.


Is the rebound from the ski or from the turn forces? I suspect its mainly from turn forces.


Yeah but the turn forces are stored up in the ski and in your body and the ones in your body are there in the form of tense muscles and its your muscle management that determines how quickly you release the turn.

When you wait for the maximum forces felt underfoot and quickly release you get a strong rebound in a ski like the Supershape and its the kind that can make the ski shoot out from under you and sometimes this is that over-the-bars kind of fall when the ski crossed under you faster than you thought it would. That kind of thing reflects a skier who hasnt fully learned his ski yet.
If your skiing feels good to you then it is good for you but that doesnt mean you cant improve your skiing okay?
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Postby ramshackle » Sat Mar 03, 2007 11:54 am

BigE wrote:
serious wrote:I still am struggling with an explanation for the feeling you get when you jump up. A good rebound certainly makes it feel easier. Perhaps the example given by Harald (jumping on the ski while parked in the bumps) makes most sense for this scenario.


I think the spring from the legs is key. If you are stacked/aligned perfectly to resist the centripetal force of the turn, your muscular effort will appear much easier, as if assisted by the ski itself.

I recall from somewhere, that the bulk of the rebound is derived from tension within your own body. Harnessing that energy is a matter of alignment over the engaged edge.


It still takes a bit of muscular energy even when stacked correctly if youre going fast enough right BigE I mean thats why top skiers who ski fast tend to have good strength routines right?
If your skiing feels good to you then it is good for you but that doesnt mean you cant improve your skiing okay?
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Postby BigE » Sun Mar 04, 2007 5:40 pm

for sure. It's just easier, regardless of your strength level, when you are "doing it right". I mean try jumping straight up from a standstill. You better be stacked! Getting into the back seat will seriously limit your height, and make it really hard to do. If you are stacked, it can even feel "springy".
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Postby NoCleverName » Mon Mar 05, 2007 5:35 am

The only thing I have to add is that the case of "springing up and down between to moguls" and "returning to cambered shape during a turn" is different because in the first case the net force is automatically reduced by one "g" while in the other case the reduction is going to be dependent on angle from the vertical. So at high edge angles you would definitely feel more rebound from the same decamber than at lower ones.

Just a nit. Probably doesn't affect the argument. On the other hand, I have seen people hocky-stop so effectively that they literally pop back into the air. This is undoubtedly aided by stored quad energy.
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Postby NoCleverName » Mon Mar 05, 2007 6:00 am

On the otherhand, we can't just dismiss "stiff" vs. "soft" altogether.

A ski supported by two blocks a foot apart and subjected to 100kg isn't going to decamber very much compared to the same ski supported at the ends. The practical result is that in one case relatively more energy is stored versus the other. Why? Because kinetic energy = work and work is Fds (force over distance) so that the decambered ski has the ability to do more work on the skier as it "covers a greater distance" during its return to form. Thus it must store greater energy.

"Stiff" vs. "soft" would affect both the decamber distance and the rebound rate. Rebound rate would determine power (work divided by time) or in other terms F v (force times velocity).

So there would be a delicate balancing act in design (like in any spring) to get the right energy storage/power output combination for the application in question. That is, since energy is conserved, you have to decide if you want to have the ski rebound a short distance fast or a long distance slow.
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Re: Rebound

Postby h.harb » Tue Apr 16, 2019 5:47 pm

It doesn't matter how much you get out of the ski. That isn't the concern, the concern should be are you achieving the right movements for the best results. The right movements are all spelled out and documented. This question is purely academic. We know the forces act on us and on the ski. The more edge angle and tighter we can make the radius the more we produce forces.


My best rebound comes in the arc later than where the greatest gravitation and centripetal forces are measured. Why, because I can shorten the radius, right after the maximal loaded point. As we know from the acceleration equation, shortening the radius increases centripetal acceleration. Therefore, I get more rebound feel as I shorten the radius.


If you realize that rebound isn't like a springboard, you'll stop looking for it to be that way. Releasing, properly at the right time, lets any forces that are acting on your body take over. It is how you use your technique and establish balance for the transition that will determine who well the release worked for the next turn.
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Re: Rebound

Postby h.harb » Tue Apr 16, 2019 5:52 pm

Just a nit. Probably doesn't affect the argument. On the other hand, I have seen people hocky-stop so effectively that they literally pop back into the air. This is undoubtedly aided by stored quad energy.


This is just a case of linear momentum being changed to angular by the sudden edge engagement, not applicable in this case. It has nothing to do with rebound or ski bend.
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Re: Rebound

Postby NoCleverName » Wed Apr 17, 2019 6:54 am

... I'm not sure why this thread popped back to life :? But way back then in 2007 when I made my post I knew nothing. Now, 11 years later I know a bit more (i.e., next to nothing). But I have certainly moved away from any need to understand skiing "academically". It's like HH says, it's all just the movements. The fascinating thing about this system is that as you chip away at the movements, the results reveal themselves as surprising, unexpected outcomes. You then begin to have a lot more faith in the wisdom of the movements which in turn motivates you to get better at them.

I guess it demonstrates how well-chosen the essentials are.
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Re: Rebound

Postby Max_501 » Wed Apr 17, 2019 7:08 am

^^^Great post and valuable insight you've picked up over the years. This thread was bumped because it is related to another discussion.
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