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# momentum created by rebound on carving

is it linear or angular?
What are you trying to find out? Could you please be more specific?

Quote:
 Originally Posted by carver_hk is it linear or angular?
Quote:
 Originally Posted by simplyfast What are you trying to find out? Could you please be more specific?
Just to explore what's the effect of rebound on carving turns. My simple minded imagination is like:

- linear momentum = sent the skier to the other side straight
- angular = turning the skier uphill with the center of rotation of the radius of the bended skis. the skier have to do something to make use of the angular momentum generated by rebound to make a good start on next turn. It is more true for shorter carving turns.

Quote:
 Originally Posted by carver_hk Just to explore what's the effect of rebound on carving turns. My simple minded imagination is like: - linear momentum = sent the skier to the other side straight - angular = turning the skier uphill with the center of rotation of the radius of the bended skis. the skier have to do something to make use of the angular momentum generated by rebound to make a good start on next turn. It is more true for shorter carving turns. and ultimately the question is asking what to do about rebound.
control it with absorption and let the skis travel underneath and away from you to the other side.
Quote:
 Originally Posted by BushwackerinPA control it with absorption.
May I ask what is the specific absorption to use in this context?
Both.

There will be a vector force acting on you; it will vary over the duration of the turn. Unless it is all precisely directed through your centre of mass then it will impact your linear movement and it will impact your rotational movement.

IMO the majority of the force will be 'linear'.
Quote:
 Originally Posted by carver_hk May I ask what is the specific absorption to use in this context?
you said short carved turns if you compress and get you COM down the hill very aggressively those will be the shortest arced/carved turns you can make.

extension is great for GS style turns but for SL turns and shorter retraction is the only way to go. IMO retraction is easy to stay in balance in more varied conditions as well so not only is it a great technique it also becomes a great tactic in crud, powder, steeps and bumps.
I learn (theoretically and still working on it) both recently from an expert in this forum. From what I understand both are not mean to be absorption but a way to release (without killing momentum) so that the hip can get through the transition without being pushed in a less than desirable manor. What reminds me of absorption is more in the sense of reducing the effect of unwanted force or impact.

Quote:
 Originally Posted by BushwackerinPA you said short carved turns if you compress and get you COM down the hill very aggressively those will be the shortest arced/carved turns you can make. extension is great for GS style turns but for SL turns and shorter retraction is the only way to go. IMO retraction is easy to stay in balance in more varied conditions as well so not only is it a great technique it also becomes a great tactic in crud, powder, steeps and bumps.
Quote:
 Originally Posted by lbt Both. There will be a vector force acting on you; it will vary over the duration of the turn. Unless it is all precisely directed through your centre of mass then it will impact your linear movement and it will impact your rotational movement. IMO the majority of the force will be 'linear'.
Thanks. It's very inspiring to think about force before momentum. So where is the center of force and where is the center of momentum so created?
Quote:
 Originally Posted by carver_hk is it linear or angular?
If we look at momentum referenced to the skier's center of mass then I'm of the opinion the quantity greater changed by "rebound" will be angular momentum.

"rebound" occurs at the transition, as I understand it. The energy stored in a flexed ski (plus that stored in elastically deformed snow) produces a relatively small restoring force compared to other forces we experience in skiing. Yet "rebound" is often perceived as a powerful, even violent event.

IMO, this experience occurs when we put our body into a position where relaxing certain muscles rapidly allows a loaded ski to unload while acting through our legs at a large angle to our center of mass.

If I'm right, purposely skiing without any angulation should eliminate the feeling of "rebound". I'll test this over the weekend and let you know how far off base I am.
No you do not want to absorb anything there by for example dropping the hips just before the transition. That would kill any momentum, so you have to resist that. Also you do want angulation, not Inclination with a stretched leg. Nothing will happen then either.

It is like a stick that is just stiff and does nothing. Does that answer a bit your question?

Quote:
 Originally Posted by carver_hk I learn (theoretically and still working on it) both recently from an expert in this forum. From what I understand both are not mean to be absorption but a way to release (without killing momentum) so that the hip can get through the transition without being pushed in a less than desirable manor. What reminds me of absorption is more in the sense of reducing the effect of unwanted force or impact.
Quote:
 Originally Posted by Garrett If I'm right, purposely skiing without any angulation should eliminate the feeling of "rebound". I'll test this over the weekend and let you know how far off base I am.
I suppose by 'without any angulation' you mean right CM above both skis vertically?

simplyfast - your posting is great for me. very appropriate to my skiing. Thanks. But it seems you have not contributed to the original question.
Quote:
 Originally Posted by carver_hk I suppose by 'without any angulation' you mean right CM above both skis vertically?
Yes. For completeness, I should have also said no counter.

### Rebound

Interesting reading. I probably shouldn't............but I rebound my skis when I want to.......usually in the fall line doing short turns. Although to be honest I have never dissected the geometry of what I am doing. Like Bushwackers earlier post, I move the skis sideways which would be angular to me.

Is it necessary to understand the geometry of what is happening or is it sufficient and worthwhile to just "feel" the dynamics of the rebound and have the ability to "do it" when I want to?

I am not trying to be cute or onery but really would appreciate some input. My skiing has always been feel oriented. This has always been the case even though I have taken many lessons, done a lot of clinics both racing and otherwise. If this question hijacks then disregard that was not my intent.

I just feel that there are a lot of Bears and lurkers on Epic that think, believe, know this is technical jargon over their heads.
Totally agree with you about all that Pete. Definitely sufficient, more useful even, to have a good feel for skiing than some technical explanation for it.
Quote:
 Originally Posted by Garrett Yes. For completeness, I should have also said no counter.
Then I don't see how you can get any rebound at all because your skis are flat on the snow.

Quote:
 Originally Posted by Pete Is it necessary to understand the geometry of what is happening or is it sufficient and worthwhile to just "feel" the dynamics of the rebound and have the ability to "do it" when I want to?
I think it is just another way to learn skiing. Of course the better way is go out and seek a great coach and have fun on the big & beautiful mountain while learning. For me , even that I ll ask the coach this kind of question. I m just too curious about how skiing works.

### Rebound

Quote:
 Originally Posted by carver_hk Then I don't see how you can get any rebound at all because your skis are flat on the snow. I think it is just another way to learn skiing. Of course the better way is go out and seek a great coach and have fun on the big & beautiful mountain while learning. For me , even that I ll ask the coach this kind of question. I m just too curious about how skiing works.
Don't get me wrong I am not saying learning is bad but sometimes I question (in my own mind and experience) the mechanical approach. However I am open to learning of all types it is just that sometimes I just don't understand the specific mechanics and their relevance. To simplify: Does it make any difference if we label rebound as linear or angular if we can do it and it works?
Quote:
 Originally Posted by Pete No. Idaho Does it make any difference if we label rebound as linear or angular if we can do it and it works?
To me, it matters. That's why I ask. Because angular momentum means turning the skier about his CM. The skier have to do something to make use of this angular momentum or to neutralize the momentum so that the skier wonld not got swing uphill.
Quote:
 Originally Posted by Garrett If we look at momentum referenced to the skier's center of mass then I'm of the opinion the quantity greater changed by "rebound" will be angular momentum. "rebound" occurs at the transition, as I understand it. The energy stored in a flexed ski (plus that stored in elastically deformed snow) produces a relatively small restoring force compared to other forces we experience in skiing. Yet "rebound" is often perceived as a powerful, even violent event. IMO, this experience occurs when we put our body into a position where relaxing certain muscles rapidly allows a loaded ski to unload while acting through our legs at a large angle to our center of mass. If I'm right, purposely skiing without any angulation should eliminate the feeling of "rebound". I'll test this over the weekend and let you know how far off base I am.
you are correct that a ski deflexing is a very small force. the force of skis coming across the hill is what you feel mostly has everything to do with its rate of travel of the skis compared to the rate of travel of your COM and almost nothing to do with a deflexing ski.

easy math....

say your moving at 13 meters a second down the hill(roughly 29 mph). you travel a 100 meters in 7.69 second. then lets say you have a 15 meter sidecut on you ski, or better yet lets say your arc is 15 meters.

(15 - 2) - Ï€ = 94.24.
94.24/2 = 47.12

47.12 is each half circle you make assuming complete at a 90 degree angle turns

each semi circle will be 30 meters long. 100/30 = 3.333 total semi circles.

3.333 - 47.12 = 157.07 meters the ski travel while you traveled 100 meters.

and finally

157.07 meters traveled in 7.69 is 20.43 meters per second. which is roughly 46 mph.

So while your body is traveling at 29mph you skis are traveling at an average speed of 46 mph. pretty significant if you ask me. I made some assumptions to make the math easy but its still a good estimate of the difference in speed between your skis and your body.

### Rebound

Quote:
 Originally Posted by carver_hk To me, it matters. That's why I ask. Because angular momentum means turning the skier about his CM. The skier have to do something to make use of this angular momentum or to neutralize the momentum so that the skier wonld not got swing uphill. I am not sure about this. Let's see what the experts say.
Skier going downhill, hips slightly ahead of COM, ski's rebounded up and over and down (because you're moving down the hill). So angulation to the side and down the hilll. Just Happens when I do rebound turns.

Bushwacker, don't argue with the math but so the skiers are going xx MPH and the body is going a lot faster. What does this mean? How does it affect the art or science of doing rebound turns?

### Rebound

Quote:
 Originally Posted by carver_hk To me, it matters. That's why I ask. Because angular momentum means turning the skier about his CM. The skier have to do something to make use of this angular momentum or to neutralize the momentum so that the skier wonld not got swing uphill. I am not sure about this. Let's see what the experts say.
Skier going downhill, hips slightly ahead of COM, ski's rebounded up and over and down (because you're moving down the hill). So angulation to the side and down the hilll. Just Happens when I do rebound turns. Carver, never had any problem with COM twisting/going uphill, at least haven't noticed this problem.

Bushwacker, don't argue with the math but so the skiers are going xx MPH and the body is going a lot faster. What does this mean? How does it affect the art or science of doing rebound turns?
Quote:
 Originally Posted by carver_hk is it linear or angular?
What exactly do you man by linear and angular?
carver-hk rebound energy can only act in the direction of uncambering the ski. I matters little whether its linear or angular. What matters is what you do with it.

In a crossunder turn (shortest at transition and tallest at the apex) you use rebound to help you retract and guide the skis under the body. This might be what you and in mind when you were thinking angular.

In a crossover turn (transition is not when your are the shortest) you use rebound like the the movement used to start the downswing on a swingset. You use that rebound, combined it with an extension and follow through to effectively slingshot from one turn to the next like pumping a swingset. Loads of fun on a green groomer. This might be what you had in mind when you were thinking linear.
Carver,
Rebound is simply the elastic qualities in a ski. Bend a ski into reverse camber and it will seek to return to it's normal shape. A quick increase in load followed by a quick decrease in load will cause the skis to pop up off the snow. Years ago it was a way to release the skis during a transition. Especially in slalom racing. Watch Phil Mahre's gold medal slalom run for a good example of slalom turns involving rebound. It should give you a good visual image of the skis popping up off the snow after a strong edge set.
Here's the technical jargon involved: a strong edge set usually caused by a strong muscular push, bends the skis into more reverse camber. The reaction force in the snow and skis combine to push back through the skis causing them to leap off the snow.

Controlling this phenomenon involves both a well timed extension to load the skis in the first place, followed by relaxing the legs enough to allow the skis to quickly seek their normal shape. Although I would be quick to point out that if the skis are stiff enough they will act like a trampoline regardless of the relaxation of the legs and bounce you up off the snow. To redirect this forward, lever backwards on the tails and the skis will squirt forward since the tip is not bearing any load and offers no resistance while the tail which is still bearing load does offer resistance. Mind you today's skis have a very short tail and getting them hung up on a tail carve is one of the quickest ways to blow out your ACL. So there is a huge risk in trying this maneuver on short skis. Which is why you don't see it done to the same degree we used to see it on the race curcuits.
Is the pop (rebound) linear or angular momentum? If it bounces you up it would be linear because it is acting upon the CoM, ,or the legs to deflect it/them up and away from the skis (90 degrees to the top of the skis). If you allow the skis to squirt forward then the action is on a tangent and creates an acceleration of the feet. Imagine the CoM being the center of a circle and the feet are on the circle. Since the feet are connected to the CoM by the legs this causes the feet to be pulled into an arc, thus turning this linear acceleration into angular acceleration. More speed produces more momentum. Which is where momentum comes into this scene. Once we create more momentum we need to control or dissapate it by absorbing it or redirecting it in another direction. My favorite is to release the energy forward while also thrusting the body forward to keep up with the feet as they accelerate. In this example the legs don't really need to bend because the body is moving away from the skis and forward and in some cases the legs are actually extending as if we are hopping into the new turn.
Carver,
Buy Bob's encyclopedia and Vagners book on the physics of skiing. Brancasio's book on Sports science also has a section on levers, momentum, and acceleration as it pertains to a variety of sports.
The ski has certainly a lot to do with it, but the rebound effect is also depending how clean and how tight you can carve a turn. While you enter the turn from lets say coming from the left, it matters how you can influence to set your ski on edge, not allowing it to slide and get back into the same direction where you came from. (Let's assume the angel in that turn would be roughly 130 degrees?) In other threats I have tried to explain how much you can actually influence that with your technique and physique. (Like in a trampoline of course the angle would be 360 degrees)
Also the ski certainly will support that and the Industry is doing a great job to build in that rebound, like the new Doubledecker from Atomic for example. But you still have to work on it.

Does that make any sense to you?

Quote:
 Originally Posted by justanotherskipro Carver, Rebound is simply the elastic qualities in a ski. Bend a ski into reverse camber and it will seek to return to it's normal shape. A quick increase in load followed by a quick decrease in load will cause the skis to pop up off the snow. Years ago it was a way to release the skis during a transition. Especially in slalom racing. Watch Phil Mahre's gold medal slalom run for a good example of slalom turns involving rebound. It should give you a good visual image of the skis popping up off the snow after a strong edge set. Here's the technical jargon involved: a strong edge set usually caused by a strong muscular push, bends the skis into more reverse camber. The reaction force in the snow and skis combine to push back through the skis causing them to leap off the snow. Controlling this phenomenon involves both a well timed extension to load the skis in the first place, followed by relaxing the legs enough to allow the skis to quickly seek their normal shape. Although I would be quick to point out that if the skis are stiff enough they will act like a trampoline regardless of the relaxation of the legs and bounce you up off the snow. To redirect this forward, lever backwards on the tails and the skis will squirt forward since the tip is not bearing any load and offers no resistance while the tail which is still bearing load does offer resistance. Mind you today's skis have a very short tail and getting them hung up on a tail carve is one of the quickest ways to blow out your ACL. So there is a huge risk in trying this maneuver on short skis. Which is why you don't see it done to the same degree we used to see it on the race curcuits. Is the pop (rebound) linear or angular momentum? If it bounces you up it would be linear because it is acting upon the CoM, ,or the legs to deflect it/them up and away from the skis (90 degrees to the top of the skis). If you allow the skis to squirt forward then the action is on a tangent and creates an acceleration of the feet. Imagine the CoM being the center of a circle and the feet are on the circle. Since the feet are connected to the CoM by the legs this causes the feet to be pulled into an arc, thus turning this linear acceleration into angular acceleration. More speed produces more momentum. Which is where momentum comes into this scene. Once we create more momentum we need to control or dissapate it by absorbing it or redirecting it in another direction. My favorite is to release the energy forward while also thrusting the body forward to keep up with the feet as they accelerate. In this example the legs don't really need to bend because the body is moving away from the skis and forward and in some cases the legs are actually extending as if we are hopping into the new turn.
Not sure we can limit the rebound effect to carving SF. An example of skidding to a hard edge set in the moguls comes to mind. Especially when we load the ski in the trough and allow it to snap back to shape launching us over the next bump, and landing us on the back of that second mogul. I call that a porpoise turn and it is a easy way to adjust for a traverse line in a middle of a bump field. More than a gelande and almost the opposite of pre-jumping a knoll.
No great thinking, at moguls you have a natural help. Sort of a slanted slope like a bob sleigh track? that you can just push off of. In powder it is sort of similar although it comes with a lot of cushion. It is on the normal groomed slope where you have to work at it.

Thanks.

Quote:
 Originally Posted by justanotherskipro Not sure we can limit the rebound effect to carving SF. An example of skidding to a hard edge set in the moguls comes to mind. Especially when we load the ski in the trough and allow it to snap back to shape launching us over the next bump, and landing us on the back of that second mogul. I call that a porpoise turn and it is a easy way to adjust for a traverse line in a middle of a bump field. More than a gelande and almost the opposite of pre-jumping a knoll.
Thank you all for the contributions. There are more to learn then expected. I now got a much better picture of what is it all about.
Quote:
 Originally Posted by justanotherskipro Rebound is simply the elastic qualities in a ski. Bend a ski into reverse camber and it will seek to return to it's normal shape. A quick increase in load followed by a quick decrease in load will cause the skis to pop up off the snow.
This is pretty much it, except the skis do not always have to pop off the snow. If the decrease in load is managed the skis can still be on the snow, but very light.

Think of it all this way. The amount you can go UP on a trampoline is a result of how hard you come DOWN on the trampoline. Just like loading and decambering a ski.

Pierre said in his post it's all about what you do with this rebound, he's right and that's my next point. If you do NOT absorb when you make contact with the trampoline you will get more rebound, same with skiing. The more you absorb the more rebound you allow to dissipate.

As far as the direction of momentum goes, we have to decide whether we are talking about the ski or the CM. IMO,as far as the ski goes, it would be linear momentum if we were straight running and angular momentum in any turn.

It's the momentum that the CM travels here that is important when we consider rebound. The direction of momentum of the CM can't be anything but linear as I see it. I can't imagine (though I think I did it once in a spectacular fall) the CM moving in an angular fashion.
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