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Do rebound exist in short turn? - Page 4

post #91 of 104
 JASP,

Is it your conclusion that reaction force is responsible for rebound?
post #92 of 104
A deeper sidecut enables a shaped ski to arc more/dip deeper when tipped and pressed.  That deeper arc provides more bounce when throwing it side to side skiing really fast shortswing like you would in a zipperline  At least it feels that way to me.  My softer CaBrawlers seem to have more spring than my stiffer mid 90s F-17s do when I'm cranking out turns as fast as I can.  As for straight up riding a rail carving without the down-up-down I don't think it much matters, but you're not going to get a real shortswing going without the up motion, or at least not a really fast one.
post #93 of 104
Quote:
Originally Posted by justanotherskipro View Post

Sd, like I said in my response to the e-mail you sent me. Everything you highlighted is Vagners work not mine. Those are direct quotes so if anything you disagree with his conclusions that he wrote and I copied without adding anything to his work. I thought I made this clear in my last post but just in case some of you missed that let me repeat it slowly. The first two sentences are mine and I carefully crafted them as to not reveal who's opinion I was sharing. Although the quotation marks preceeding the next sentence and after the word powder (in the last sentence of the quote) should have given you a clue that I was quoting someone elses work. Not to mention his totally different writing style and word use. I told you I was going to re-read the book you suggested as further reading, I would ask that you do the same and read the books I mentioned. Maybe then you will realize that I'm telling you the truth here.

E the smug "nice catch comment" needs to be redirected right back at you. You guys can disagree with Vagners all you want I really don't care. But let me say this again, HIS CONCLUSIONS ARE HIS CONCLUSIONS. YOUR CONCLUSIONS ARE YOURS. THE FACT THAT THEY ARE INCONGRUENT IS GOOD IN THAT IT JUST MIGHT MOTIVATE YOU TO ACTUALL READ HIS WORK AND TRY TO UNDERSTAND WHY HE DRAWS THE CONCLUSIONS HE DRAWS. Something I doubt you will do since you seem content pontificating and suggesting you somehow have a patent on facts and that only your opinions are valid. 
    

Thanks, JASP

I think that was clear before....but I agree with Big E...can you find the quote in Vagners that direclty links reaction force to rebound?

I dont recall any work where he specifically discusses the reaction forces and how these work to generate the rebound we all perceive at the end of turns....

What chapters/pages where your quotes from?
post #94 of 104
Force and it's equal and opposite reaction force act on different objects.

For every force of the ski acting on the snow there is an equal in magnitude force acting on the ski.

If you consider the total force acting on a skier and ski taken as a free body, this total force will accelerate the cm according to F=ma.

Just because your pushing down on the snow and the snow is pushing you back, doesn't negate rebound.

Rebound at the end of short short turns refers to that portion of the total force that is coming from the ski's stored elastic energy due to the ski trying to regain it's normal shape after being bent.  You pushed the ski into this shape earlier.  It's still trying to unbend, and you are no longer pushing it into that shape.  It pushes down on the snow as it unbends and the snow pushes it back.  When the snow pushes the ski back, you may resist and get pushed up, or relax and let the skis jump up.  If you resist, then you must exert a force that is in fact a reaction force to the rebound force.  I can see where this can get confusing.


Here's a thought experiment for you.  Suppose you took a cable and restrained the ski in it's deformed shape like stringing a bow, then tipped the ski so that it's tail was on the ground, but not the tips, and then suddenly cut the cable.  The ski would jump.  That's rebound. 

How usefull is it considering how much much more likely you are to loose it if you ski with your tails on the snow and your tips in the air at the end of a tight hard turn is debatable, but it is another fun thing to investigate.
post #95 of 104
Post 75 has all of that in there. Chapter six is all about what he describes as the misconceptions about the physics in skiing. His opinion about reaction forces and their connection to rebound are there as well. It's a repeat of things he wrote in chapters 3-4 but with much stronger opinions about how he feels so many people get this stuff wrong.
for my part The whole inclusion of his work was meant to spur debate as much as offer where I based most of my opinion about rebound. Although I must say for a textbook he certainly expresses a strong opinion about rebound. I'm sort of through with all of this though, I obviously am having trouble expressing any opinion here without it immediately being discounted. Even if that opinion is from a very respected and accepted source like Vagners. Post 67 was a test to see just to what extent this is happening. IMO everything subsequent to that post proves just how much this is happening. I don't have Vagners misquoted or did I misrepresent his work in any way. I fully understand what he wrote. I doubt I can convince you guys of that though. I'm outta here. 
Edited by justanotherskipro - 10/31/09 at 3:28pm
post #96 of 104
 post 75 talks about a large scale snowpack.  Not about the compression under my skis.
post #97 of 104
 JASP,

Does the video I posted show rebound from the snow or from the skier?
post #98 of 104
Quote:
Originally Posted by justanotherskipro View Post

Post 75 has all of that in there. Chapter six is all about what he describes as the misconceptions about the physics in skiing. His opinion about reaction forces and their connection to rebound are there as well. It's a repeat of things he wrote in chapters 3-4 but with much stronger opinions about how he feels so many people get this stuff wrong.
for my part The whole inclusion of his work was meant to spur debate as much as offer where I based most of my opinion about rebound. Although I must say for a textbook he certainly expresses a strong opinion about rebound. I'm sort of through with all of this though, I obviously am having trouble expressing any opinion here without it immediately being discounted. Even if that opinion is from a very respected and accepted source like Vagners. Post 67 was a test to see just to what extent this is happening. IMO everything subsequent to that post proves just how much this is happening. I don't have Vagners misquoted or did I misrepresent his work in any way. I fully understand what he wrote. I doubt I can convince you guys of that though. I'm outta here. 

Thanks for answering my page/chapter questions.

Just one more if I may.  Since Vagners did infact conclude that there is no appreciable rebound from the skis.....what does Vagners say is the cause then?  Am I reading you right that you are saying, he says it is the elasticity of the snow?
post #99 of 104
So can I cautiously ask the gentlemen here if there is now a rebound or not?
post #100 of 104
Quote:
Originally Posted by adrianflo View Post

 Rebound is from reversing the  camber plus reversing the side cut. The more of each the more the rebound.

see www.FloSkis.com
Do you really think the decambering of a ski can move a 200 # human in the finish of a turn with the additional force from gravity resisting it ?  The experience of rebound  a much stronger force than you suggest.


Quote:
Originally Posted by simplyfast View Post

So can I cautiously ask the gentlemen here if there is now a rebound or not?
Do you think it doesn't ? The fact it exists is a given  and I didn't notice anyone denying it's existence. How it can be manipulated is another question.

Skidude suggested line has an effect and that seems obvious given there is no rebound in a straightline. Also early pressure manipulation can change the effects of rebound . This helps us edge on hard surfaces .

E brought up the virtual bump . That is the mental expression of  the source of rebound also. It's what you feel . It is real and it affects your intentions unless you consider it's presence.
post #101 of 104
 Consider the bent ski at the bottom of the arc -- the tip of the ski can point upwards, higher than the foot, as if you are skiing over a bump,.  The effect of this virtual bump is strongly correlated to the quality of edge grip.  More bite = more virtual bump = more spring through the legs.  This is what I've been talking about, and the CSIA  video example clearly demonstrates -- the skier is creating impulse by pushing downwards on the flexed ski as it goes up the virtual bump.  Exactly like skiing through a field of very small bumps.

The floski videos clearly show the tails of the skis being severely loaded and rebounding as the load is removed/  That behaves a lot like a diving board.  That is conclusive evidence that the ski itself can provide significant rebound energy.  I mean think of a diving board,  I can flex that by hand too, yet if I put a LOT of force into it, it will rebound and lift me up.  Edit: To be honest, it does not look well controlled.  Regardless, it does demand that the snow is strong enough to support the energy.  In some spots on the videos, it looks like the snow gives way.
post #102 of 104
Garry:

You need to do some more research on the virtual bump.  You clearly do not understand the concept.

Big E:

You are closer but the effect of the virtual "bump" is not like a bump "up" .... the effect is more like coming over a roll.  Think stairs then landing, then stairs...the length of the landing/stairs is determined by your turn shape, the size of the bump you speed...as you correclty wrote, the faster you ski a given line the greater the effect, of course you need more skills to ski a given line faster and faster, hence the bump gets bigger and bigger...that is why it is "virtual" becuase the bump size will always be proportional to the dynamics of the turn.

Second taking your diving board analogy....sure with the diving board in the air, held fixed on one end it shoots you up...but what if you took the board, tipped it on edge, bent it, then released that bend by rolling the board flat again....like you do with skis....how much "rebound" do you think you would get?  Not much.

PS: those "fl@ skis" have no tail...that is the point of them...6inches behind the binding at most. 

post #103 of 104
The diving board analogy works only if you consider that you are going to come down hard on the tail of the ski and bounce on it.  The floskis video shows an exaggeration.  I still think there is some power stored in the ski that can be returned, but you won't get it just by rolling the ski off the edge.

Regardless, we do agree that boiler plate won't behave like a trampoline no matter how hard you step on it.
post #104 of 104
Well of course it exists, that is what it is all about in ski racing. I remember last year around that time it was still a big black hole here, especially when I brought up the "trampoline effect".

I was just curious by throwing in that question. And yes timing is very essential, if anyone gets a chance this year and watches a WC SL somewhere, look out for Reinfried (Herbst), nobody in our team or anyone else has that explosion on the gate, that guy is amazing.

Anyhow have a great season.
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