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Releatonship of Ski Rebound and Technique

post #1 of 22
Thread Starter 
In the ski reviews, some skis are described as having more or less rebound. Some testers will state that a ski lacks the rebound they wuld prefer.

What are the advantages or disadvantages of ski rebound on technique? I'm not as smart as the average [barking] bear, but it would seem to me that any energy from a ski rebounding as the pressure is reduced in it will be directed either uphill or vertically. If this is wrong, what techniques are used to properly channel the ski energy?
post #2 of 22

There is no such thing as a free launch

Interesting question SnoWonder,

The secret behind "snappy" turns is using the skis rebound to help launch yourself into the next turn. One hallmark of efficient technique is the ability to bend the ski in the middle of a turn to store energy and control speed from one turn and then use the corresponding unbending of the ski to speed up the finish and entry into the next turn. A less skilled skier would bend the ski entering a turn finish, but lose the energy in a subsequent skid before the start of the next turn. If you are balanced over your skis properly, the "up motion" of unbending will translate into additional forward momentum. Racers will sometimes sit back on their tails at the end of a turn to add tail pressure acceleration to the unbending acceleration and then use their core muscles to pull their upper body back into balance. For normal skiers, being in the back seat will make you unable to generate or take advantage of ski rebound. For advanced skiers a subtle change of the fore/aft balance point underneath the feet throughout the turn is required to take advantage of ski rebound. Early in the turn forward pressure is used to engage the ski tip to start the bending process. Late in the turn, letting off the foreward pressure helps to guide the rebound into forward acceleration. Your camber may vary.
post #3 of 22
Thread Starter 
How is that energy directed downhill? The ski is bent in the beginning to middle of the turn. Before starting the new turn the skis are flat on the ground. At this point the body is roughly perpendicular to the hill. By the time the skis are flat, the energy or rebound is lost. te rebound energy must occur before neutral is reached. So the vector of the rebound energy would seem to be pointed more uphill than perpendicular. Is this the case? Is this desired?

One avenue I'd like to discuss is should it be a quick unleashing of the energy so you feel the rebound, or should the movements be slow and controlled so as to mostly absorb the energy.

In the old days on straight skis and rotary skills, I can see where rebound up would allow one to pivot the skis easier as the rebound lightened the skis and would allow the ski to be pivoted to start the turn to make it tighter. In modern edge-to-edge skiing, is it as desirableto have rebound?
post #4 of 22
It's not directed downhill so much as it's directed laterally across the hill into the new turn. You are correct that the rebound occurs perpendicular to the skis and that it occurs when the skis are on edge. With shaped skis, rebound is useful for facilitating cross under. For larger turns with mostly cross over movements, you generally don't want snappiness in your turns, but I suspect that you can use the rebound energy through slow absorption.
post #5 of 22
Take a run with a very skilled skier/ racer on a very mellow pitch. Ski in a straight line down the hill, have the racer turn. The racer will get to the bottom first by generating acceleration with rebound.
post #6 of 22
It all depends on where the turn is released. If it's just after the fall line, the rebound directs the skier across the hill. If it's all the way across the fall line, it's uphill. It's certainly possible to manipulate that energy into a different direction by moving the body in certain ways.
post #7 of 22
And when not wanted, when continuous ski to snow contact is desired, an Inside Leg Extension transition will totally eliminate it. Cause/allow it to dissipate into nothingness.
post #8 of 22
SnoWonder,

I think you're asking exactly the right questions about this long time popular perception.

In fact... I think I'll go on record with my own belief that any Rebound we perceive from a ski (even a stiff ski) provides minimal (if any) overall acceleration to a skier either down or across the slope.

I believe any thrust we feel (perceived to be from 'ski rebound') is actually a result of the sharp increase in centripetal acceleration (of the carving ski) felt at our feet late in a spiral-in turn, combined with muscular involvement of the skier. A stiff ski will add a tiny fraction of extra oomph to be sensed by the skier's legs - but only a tiny fraction - and aimed in the wrong direction to assist the skier in speeding up.


For proof, imagine holding yourself stationary (by leaning on your ski poles) in an inclined (45-degrees) position on some well-bent, stiff skis. Then, you release the holding-pressure on your poles (but continue to support yourself vertically).

Do you suddenly get forcefully catapulted to the side? Or do you just feel a teeny-tiny 'push' from the skis as they straighten out (just a bit) under your continuing weight?

Now imagine that you have your whole body weight plus centrifugal force compressing your skis into a deep bend into the snow laterally during a normal turn... just how much upward thrust can they actually deliver against your Mass here?

I believe the only 'catapult' we might feel from our bent skis will be if we suddenly relax a leg - and then we'll feel that ski 'kick back' - but perpendicular to the ski top plate and into our relaxing leg.

I do agree with the comment above about using 'acceleration turns' to increase ones speed. But that's not a result of ski rebound - it's a result of muscular activity used to actively extend ones Mass toward the inside of a turn at the right moment.

On the tail jetting idea, I suspect the skier might perceive a 'thrust forward' by the tail but I don't believe they will actually speed up. Instead, conservation of momentum suggests the skier's Mass will instead be rotated forward. In other words the skier's upper body will be briefly driven forward while the feet are equally ejected backward - creating a Fore/Aft whole-body rotation.

Any skier sensing their head and shoulders move suddenly forward from this is likely to perceive an overall acceleration - but it seems unlikely that such a thing really did occur. This move does reposition the skier into a better whole-body attitude for the gravity-induced acceleration they're about to incur though...

I'm not saying ski rebound doesn't exist - just that it can't be used in the manner most often described.

Such rebound can be used if our desire is to have our feet thrust in the direction the top plate is currently perpendicular to. For instance, the skier might currently be using a lot of knee-angulation and want their feet to be ejected away from the snow to 'whip' under them to the other set of edges (instant cross-under).

.ma
<now just where did I put those Nomex bunny slippers...>
post #9 of 22
Rebound works like a sailboat tacking in to the wind or a skateboarder "pumping" themselves forward. A skateboarder with both feet on the board can self propel forward on a flat surface until they get tired. You want proof of the power of rebound? Place your ski tips and tails on top of a couple (of sturdy) chairs and bounce up and down on them (use a chair in the middle to get started and use a spotter).

Tail jetting rockets the skis forward. The feet, being attached firmly to the skis, must go with them. The upper body, not being firmly attached, will get dragged BEHIND unless muscle power is used to keep it over the feet (with respect to travel down the slope). Momentum is not being conserved because muscle power is being applied and the effects are amplified because the skis are being used as a lever.

If a sailboat can use power directly applied against it's overall direction of travel and a skateboarder can generate forward push, is it so hard to believe that skis can also generate forward speed when they are operated with a similar technique?
post #10 of 22
Quote:
Originally Posted by SnoWonder View Post
In the ski reviews, some skis are described as having more or less rebound. Some testers will state that a ski lacks the rebound they wuld prefer.

What are the advantages or disadvantages of ski rebound on technique? I'm not as smart as the average [barking] bear, but it would seem to me that any energy from a ski rebounding as the pressure is reduced in it will be directed either uphill or vertically. If this is wrong, what techniques are used to properly channel the ski energy?
Interesting question. Let me share my perseption of the effects of a 'stiff' or high rebound ski.

I use a ski with a particularly stiff tail. I really feel it's rebound effects in short, high energy turns in crud conditions on steeps when the extra pop of rebound allows me to unload my skis from the broken snow and get them turning into the fall line before the edges reingage in the next turn. I have tried this with a softer set of skis with less rebound and I find that I work harder to make turns in the same snow conditions even though the more flexable skis are lighter in weight.
In short, higher rebound makes for quicker turns under high loading.
post #11 of 22
Rusty, That was a great explanation and example of rebound energy.
post #12 of 22
Quote:
Originally Posted by Rusty
Rebound works like a sailboat tacking in to the wind...
I think a sailboat tacking into the wind may be unrelated in mechanics. In such a case momentum alone is used to ‘get thru’ the moments of weak (or no) thrust from the wind. Then it is the continuous deflection of wind off an angled sail that continues to drive the boat forward. Where do you see any elements of a ‘springy rebound’ in the sailboat example?

Quote:
Originally Posted by Rusty
A skateboarder with both feet on the board can self propel forward on a flat surface until they get tired.
A skateboard primarily uses the same ‘acceleration turn’ technique described so well by PhysicsMan Here. It is the muscular energy directed at moving Mass toward the inside of the turn that does the real work and not the springiness of the skateboard. (although I suspect that the ‘give and return’ of a flexible platform helps progressively distribute the muscular ‘impulse’ over a longer time duration - thereby assisting balance and flow of the activity... Probably the same thing is true on skis.)

Quote:
Originally Posted by Rusty
You want proof of the power of rebound? Place your ski tips and tails on top of a couple (of sturdy) chairs and bounce up and down on them
In the case of skis spanning two chairs - our skis once again spring back perpendicular to the top plate. How would this directly upward thrust propel you (en mass) forward? Even if you leaned forward just as you were ejected upward, the result would be a Fore/Aft rotational impulse - you’d somersault forward (assuming you didn’t cheat and add in some deliberate ‘forward thrust’ using your legs against the friction of the chairs - on snow such useable friction would be absent.)

In the interests of full disclosure - there is a bit of forward thrust that can result from the 'pinch' between gravity and the off-centered upward thrust of ski rebound. In the chair example the skis are pushing upward while gravity is pulling downward - and having leaned forward, our CM is no longer directly between them. Our CM is therefore 'squished' forward a bit - just as squeezing a marble between two fingers can cause it to squirt out of the finger pinch.

---
The example of exiting Crud Snow by WmCraven above is a good example of using Ski Rebound effectively - he makes use of the bent ski's ‘lifting’ ability perpendicular to the top plates (as with across the chairs above) to get away from the snow.

This leads directly to the Tail Jetting thing again. On snow, we have no real forward/backward friction to push against so any thrust forward by the tail of a sliding ski is canceled out by its own sliding backward. We instead rotate about our CM. We only feel a forward thrust if we also ‘jumped’ forward from against a platform. Any ‘Lever’ applied off-center to a given free-floating Mass causes a torque on that Mass - primarily causing rotation. The same idea applies to a forward sliding Mass when that lever is applied in the Fore/Aft plane (sans any other 'pinching' accomplice).

The same concept applies to 'Pulling the Feet Back' vs 'Leaning Forward' while sliding forward on relatively flat skis. Whichever technique we consciously employ, we actually re-orient our whole body about our CM. Either way, when sliding forward we end up pushing our tails downward and lifting our tips upward - causing the skis to slip back while our upper-body rotates forward around its CM. (The skis might not actually 'go backward' - they may simply 'stall' or slow down imperceptibly as compared to the CM.)

Of course, with stationary skis static friction holds us in place so we can actually lean forward without our feet moving back. Still, if we lean forward too quickly our skis will still release and slide backward (having overcome their static friction).

---
In the case of jumping or thrusting to the side from well-set ski edges at turn apex (which does incorporate the tiny amount of added spring from our skis) we are primarily using muscular power along with centripetal force to accomplish the burst of thrust to the side - at which point we then twist our skis into that direction after or during the thrust - and believe the thrusting sensation came primarily from our skis when in reality most of it comes from the centripetal force of the skis.

Also consider that any time we bend our skis it consumes energy to do so.

Due to friction (surface scraping, snow compression and snow displacement) the mere act of bending our skis consumes energy even when carving perfectly into a turn. When the skis unbend, the un-bending also consumes energy. We never get out what we put into it because the whole cycle leaks energy.

Again, I'm not saying ski rebound doesn't exist. Only that we attribute far too much to it. ‘Releasing’ the partially stored energy in our skis to assist in un-weighting certainly works. So does the pop-under transition using ski rebound to thrust our skis across under us (so long as the 'pop' is released while still on old inside-edges).

FWIW… I too believed in the Rebound Myth for a long time. About six years ago a clinician was describing how it worked to us when I detected the same fly in the ointment as SnoWonder did. Careful on-snow testing revealed to me that the spring-back (by itself) did not actually contribute what was claimed. I spent a lot of time pecking away at mechanical investigations to find what might really be happening and my thoughts eventually evolved into the ideas above.

So, Yes - I believe ski rebound does exist and that it can be useful. Just not in the ways so commonly believed.

.ma
<By all means, launch those slings and arrows of outrage >
post #13 of 22
Trust the force Luke, er, Michael.

Actually you are getting the idea with the angled force concept. We are also in agreement that muscles play a large role in these turns. I won't quibble with which component is larger, I'm only describing that springiness can be used and plays a role in these turns.

The sailboat example does not demonstrate springiness, but it does demonstrate angled force. The skateboard example may or may not demonstrate springiness, but I know that I could not self propel when the skateboard was a solid and stiff piece of wood.

With regards to squishing forward .... EXACTLY! This is a good description of the sensation that is felt. When you do this, you can feel the skis propelling you forward.

The bottom line here is that short radius turns in the fall line with the feet extending laterally away from the body from turn to turn and driving a deep bend and rebound in the skis is a thrill to perform and a joy to watch. Simply put, it is high performance skiing.
post #14 of 22
Thread Starter 
Now this discussion is getting interesting.

I have felt rebound when skating down hill with inline skates. Clearly the skates contributed little, it was the muscular energy - the coiling and uncoiling.

I can see the value of rebound as stated by WmCraven to redirect the skis. I only started to sk in 1989, but my perception is that before shaped carving skis, this was a more important move to make a small radius turn. It was necessary to use rebound to lighten the skis and get them redirected before the edge and pressure was applied.

So my question, is this really as desirable a move now with slalom (or slalom-like skis) that carve a narrow radius turn going from edge-to-edge? Is it a specialty move to be used in certain tactical situations? If so, what are these situations other than what WmCraven mentioned?

Thanks for the input.
post #15 of 22
Heh, heh - Nope, there's no longer a 'trusting' bone in my body with regard to mechanical skiing theory. If it cannot be clearly documented and supported by genuinely accurate mechanical descriptions... it just aint valid. Too often clinicians have said "Just believe me..." only for me to hear totally contradictory statements from some other clinician saying more forcefully, "No, just believe me...!" ... and neither has it right!

I'm not just getting the idea - I'm trying to dispell the myths so tightly coupled to ski rebound.

Ski rebound itself simply does not propel a skier forward faster - and I think it's important to define what really is going on. For instance, a Racer who truly understands what's really going on will not waste their time trying to use 'ski rebound' to assist their forward speed - they will instead focus on its use in other, more practical ways (like assisting cross-over or under).

The 'squishing forth' concept is valid - but applies equally well to short, stout ski-blades which are not much longer than our boots - and have minimal flex to contribute (very much like an inflexible skateboard). Such equipment produces an identically perceived rebound effect - despite having little stored energy in its overall bend. The arc of the Ski Blade combined with Gravity is sufficient to 'squish' us into a forward rotation without the forceful unbending of anything necessary.

When this 'squishing' effect is understood for what it really is (and how it's actually created) we can then knowingly apply it for a variety of biomechanical assists. For instance we can use it to help us 'get forward' during transition: If we move our CM just a bit forward of our feet late in the old turn (and keep our legs rather stiff) our upper-body will be 'squished forward' while our feet are thrust back thru transition. The Virtual Bump will press our feet up whie Gravity pushes our CM down - rotating us forward into a position well suited to stand against the coming acceleration.


Quote:
Originally Posted by TheRusty
The bottom line here is that short radius turns in the fall line with the feet extending laterally away from the body from turn to turn and driving a deep bend and rebound in the skis is a thrill to perform and a joy to watch. Simply put, it is high performance skiing.
This is all true but how is it relevant to the actual mechanics being discussed? Is it a segway into something else closely related to the topic : ?

.ma
post #16 of 22
Quote:
Originally Posted by Whiteroom View Post
Take a run with a very skilled skier/ racer on a very mellow pitch. Ski in a straight line down the hill, have the racer turn. The racer will get to the bottom first by generating acceleration with rebound.
First and second sentence: True.

Third sentence: At least partially fallacious. An inline skater can accelerate while appearing to make parallel turns. I can assure you inline skaters do not generate this acceleration via rebound.

michaelA does a great job dispelling much of the myth around rebound.
post #17 of 22
SnoWonder,

Glad to be Evil... er, helpful. Your own In-line Skating example is an ideal test: With no ability to flex, do your skates still deliver that same 'Rebound' feeling at times? I'd bet they do.

If we employ the correct technique then *Yes* I'd say we can make use of that Boing! In Slalom the skier needs to quickly effect a cross-under move and a brief spiral-in turn finish can produce the desired effect even on in-line skates.

When the skier depends entirely on the ski's own rebound they're also depending on inconsistent and unpredictable terrain and snow conditions - after all, the ski's tip and tail must remain perfectly engaged to deliver any ski-based boost at all. If the skier instead depends on centripetal force alone - they only care about whether the ski section directly underfoot is properly engaged.

I often use this pop-under transition technique by suddenly increasing my edge-angle late in the old turn. On my very soft skis it works just fine. On my overly-stiff skis the kick is about the same - but less controllable because the kick-back is so directionally dependant on the exact angle my skis are tipped at the moment of release. In other words, the ski often 'pops' in a direction not quite suitable for my intent.

.ma
post #18 of 22

Rebound is not a myth

Michael,

When I look at my skis, the high point of the camber is underneath the midpoint of the bindings. If the skis are decambering and I'm standing on the forward side of the high point, I will get added forward momentum because the angle of the ski surface that I'm standing on has a forward facing component. Simple physics says it's not a lot, but it is measureable. I'd draw a force vector for you, but you wouldn't believe it.

Racers bend their skis. Racers go fast. Could assisting cross under make one ski faster? Nah, that's pretty tough to believe too.

It's the same principle as surfing. As the wave grows bigger you get a little extra kick forward instead of just straight up because you are not aligned perpendicular to the top of the wave. But because the wave is moving forward, you're going to find that hard to believe.

How does an inch worm propel itself forward? Even I find that one hard to believe.

I believe in my skis, my snowboards, my snowblades, my skateboards and my rollerblades. I believe what they tell me. They all tell me that you are wrong. We're all going to have to agree to disagree on this because they can't type and I'm going skiing tomorrow. With any luck, I will get to bend my skis a little and smile a lot.
post #19 of 22
TheRusty,

Well then, I'll cheerfully disagree with you as well!

I put up some very specific and carefully-worded mechanical descriptions to spell out exactly how I believe things really do happen WRT ski-rebound. I fully expected others to contest these ideas and to throw a few tomatoes my way.

Your own replies, though disenchanted with my ideas were courteous and well worded (as always). I certainly can't complain about that!

Still, I'll continue to investigate the actual mechanics of 'rebound' on my own time and hope to build an even more precise understanding of them over time. I also hope you'll find yourself doing the same! Perhaps we can compare notes in the future.

.ma
post #20 of 22
Quote:
Originally Posted by michaelA View Post

Still, I'll continue to investigate the actual mechanics of 'rebound' on my own time and hope to build an even more precise understanding of them over time.
FWIW, I would look into the book Physic of Skiing by Lind. There are sections which talks about how one can alter (rotational) speed by moving the center of mass with respect to axis of rotation. The author provides equations on how much kinetic energy and velocity you can be gain by pumping up while skiing in a trough. Conversely, you will decrease kinetic energy and velocity by pumping down (absorbing). Using this idea, there are other techniques that does leverage this principle (changing speed by moving the CM). You can think of each turn as linked rotational movements like a sinusoidal curve, each turn has a center of rotation. In this case, the rebound from the ski can be use to move the CM not up but more lateral, closer to the axis of rotation thus increasing rotational speed. The CM would be taking a crossover path using Foster (Art of Carving) definition.
post #21 of 22
Jack, you describe the physics so well in plain English I think I understand what you're saying! What a way to close out the week.

This is very similar to the bump backpedaling thread. Talk to a good bumper about absorption, they may or may not have made it to physics class but they intuitively know what works. In this thread, listening to the skater might be the way to go. There are specific contexts where you may want or not want to have that energy stored and then available to harness at the right time through rebound, which is why damp to the point of dead and poppy can both be good things, look at slalom skateboards versus pool boards, but when you want it with the right material science you can get it there.
post #22 of 22
snowwonder,

There we have it. The stored energy cna either be released up ward, or directed along the length of the side of the ski using diagional directional movements. This causes the ski to continue along it's path as the energy is directed, or directed along a new path if the directional movement is changed (like during a transition), or upward at the skier if not directed.

RW
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