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Ski "rebound" or "pop"

Seldomski

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Some posts about the "ski scaling" article reminding me of something a ski instructor told me last season.

He told me that ski rebound or pop did not have much to do with the longitudinal flex of a ski. He said a common misconception is that ski stiffness leads to a feeling of pop out of the turn. That many skiers think the pop you feel as a skier at turn completion was energy released by the ski. He said this was incorrect.

Instead, rebound/pop have more to do with the shape you draw with the ski on the snow. By nature of arcing turns quickly at high angles, the ski comes quickly up below you after apex and pops up toward your body at transition. The energy released by a ski in a turn is insignificant compared to the ground reaction force you are feeling.

So, a ski with high rebound will be good at staying in a carved track through a short radius turn, and I suppose specifically at turn completion?

Thoughts? Is this the correct way to think of rebound? Why do some skis feel like they have more rebound than others? What aspects of the ski design lead to 'high rebound'? Things that maximize grip at the end of a turn such as stiff tail, sidecut, and high torsional stiffness?
 
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tomahawkins

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I think this is right and that talk of a skis' "energy" is somewhat conflated. As for energy storage, skis probably make poor mechanical batteries. As a test, put your most "powerful" ski on a couple of 5-gallon buckets at the tip and tail, place a 10 lbs weight on the bindings, bend the ski into a typical turn shape, release, and measure how high it launches the weight. From there you can calculate the potential energy of the raised mass and hence the total energy delivered by the ski in a turn. I bet this value pales in comparison to a skier's lateral kinetic energy as they cross the fall line.

I think "energy" is really a ski's ability to bend into a short radius turn (flex + sidecut) and how well it will hold an edge at the apex at speed.
 

oldschoolskier

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Comments like this are by non-engineers that don't understand simple engineering principles.

Pop, damp, lively, dead, stiff, soft and so on are functions of construction and materials used as to how they react to energy input and the resultant of energy release along with the speed at which it occurs.

Easy comparision, think of springs, shocks and suspension travel in a car. Depending on application you want lots of travel (desert racer), extremely stiff damp response (F1), or lively limited stiff travel (Rally). Skiing application is no different.

Additionally manufacturers have different ideas of what is best, therefore we see variations in them.

To make matters worse skier weight and ability come into play along with the variables of the boot and finally binding.

So yes its actually simple or complex depend on how detailed you want to see it.

Hopefully to enlightened your view without confusing it further.
 

François Pugh

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Camber is pre-stressing the beam. A cambered ski when flat on the snow is already stressed so that the tip and tail are already bent a bit and pushing back against the bending required just to make the ski straight. It moves the starting point; as you bend the ski more (i.e. into a turn) it takes more stress. In essence you have more stress at tip and tail to play with through out the entire process.
 

François Pugh

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Some posts about the "ski scaling" article reminding me of something a ski instructor told me last season.

He told me that ski rebound or pop did not have much to do with the longitudinal flex of a ski. He said a common misconception is that ski stiffness leads to a feeling of pop out of the turn. That many skiers think the pop you feel as a skier at turn completion was energy released by the ski. He said this was incorrect.

Instead, rebound/pop have more to do with the shape you draw with the ski on the snow. By nature of arcing turns quickly at high angles, the ski comes quickly up below you after apex and pops up toward your body at transition. The energy released by a ski in a turn is insignificant compared to the ground reaction force you are feeling.

So, a ski with high rebound will be good at staying in a carved track through a short radius turn, and I suppose specifically at turn completion?

Thoughts? Is this the correct way to think of rebound? Why do some skis feel like they have more rebound than others? What aspects of the ski design lead to 'high rebound'? Things that maximize grip at the end of a turn such as stiff tail, sidecut, and high torsional stiffness?
That's sort of true, but a little knowledge is a dangerous thing. How you get the load is by vaulting - true enough. Whether you can feel the difference in what the ski does as the load is removed is a different question all together. There is a difference between how easily a ski returns to it's initial shape compared to another ski (true). This is what makes one ski have more pop than another. You are unlikely to notice this difference if there isn't very much stress in the ski to begin with, but more likely to notice it on higher stressed skis in higher stress situations (stiffer skis in harder carved shorter turns), just because the total stress is higher, as is everything else.
 

Dakine

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Most of the energy stored during the loading phase of a turn is stored in your muscles and tendons.
Pop come from the way the ski releases with torsionally stiff skis giving a bigger pop because the energy can be released more quickly.
Energy is stored in this order.
1. Muscles and tendons,
2. The snow that is compressed under the skis,
3. The bent ski.
 

markojp

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I like to use my skeleton, too.
 

cantunamunch

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Well, folks, don't stop now. Those are all easy statements. All sorts of harder questions are still begging to be answered. For example:

1 "Do skis with more POP become noodles faster?" 1A: "Torsionally or longitudinally?"

2 "If torsional flex affects POP then the specific sidecut shape affects POP because you can't engage torsion otherwise. What is an optimal sidecut shape for POP? "

3 "Do reverse cambered skis by definition have no POP?"
 

Pajarito-bred

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I disagree with Dakine, I don't believe that when you flex your body that any energy is stored.
There is also no significant amount of energy stored by compressing snow, but skiing soft snow may contribute to feeling that the ski dampness is greater.
A ski can be thought of as a leaf spring, when it's bent a portion of the energy stored is released when un-bent. Skis are bent in two ways:
  • in a mogul trough, while turning or not;
  • on a smooth groomer, by carving a turn where the side-cut radius causes the ski to bend when on edge.
What is it about the stiffness, amount of sidecut, amount of damping (how much of the energy put into the ski by bending it is returned vs absorbed by the wood and rubber components) that makes a ski fun to ski.
Not too many rocket engineers get to have their designs critiqued by getting asked: But is it fun?
The ski flex and sidecut work together with skier weight, speed, edge angle and turn radius (and snow conditions) to generate the amount of "pop" or rebound that propels the skier out of one turn and into the next.
My explanation attempt.
 

cantunamunch

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Can a ski can have more "pop/rebound" in some snow conditions and not others? i.e. powder vs ice vs packed powder vs slush vs ...

Did I ever show you the video where the skier -unstoppably- rebounds back onto his heels after a sudden forward flex of the boot?


^THAT is one mechanism that will be completely affected by surface conditions.
 
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Dakine

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I disagree with Dakine, I don't believe that when you flex your body that any energy is stored.
There is also no significant amount of energy stored by compressing snow, but skiing soft snow may contribute to feeling that the ski dampness is greater.
A ski can be thought of as a leaf spring, when it's bent a portion of the energy stored is released when un-bent. Skis are bent in two ways:
  • in a mogul trough, while turning or not;
  • on a smooth groomer, by carving a turn where the side-cut radius causes the ski to bend when on edge.
What is it about the stiffness, amount of sidecut, amount of damping (how much of the energy put into the ski by bending it is returned vs absorbed by the wood and rubber components) that makes a ski fun to ski.
Not too many rocket engineers get to have their designs critiqued by getting asked: But is it fun?
The ski flex and sidecut work together with skier weight, speed, edge angle and turn radius (and snow conditions) to generate the amount of "pop" or rebound that propels the skier out of one turn and into the next.
My explanation attempt.
By calculation, you are quite wrong.
I worked this over with PhysicsMan IIRC and the numbers tell the story.
I can bend a ski into full reverse camber with one finger, the longitudinal modulus is so low they don't store significant energy until you bend them like a recurve bow.
The snow storage is counterintuitive but I can send you a long dissertation on the properties of snow.
We concluded that the right kind of snow can store about 30% of the energy accumulated during loading.
An interesting side point is that ice is so incompressible it doesn't store any significant energy.
That kind of snow that we call ego snow that is compressible but not packable is best for energy storage.
Another interesting point is that you can train for resilience so you can maximize the energy stored in the muscloskelatal system.
Big beefy guys are pretty dead in this respect but a gymnast would be really lively from an energy storage viewpoint.
 
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tomahawkins

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Easy comparison, think of springs, shocks and suspension travel in a car. Depending on application you want lots of travel (desert racer), extremely stiff damp response (F1), or lively limited stiff travel (Rally). Skiing application is no different.
This is a great analogy. How often in motorsports do we hear drivers comment on how they store up energy in their suspension in turns and then release that energy to launch them into the next corner? They don't because that's not the function of suspension. Yet this is how we talk about a ski's energy.
 

Dakine

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This is a great analogy. How often in motorsports do we hear drivers comment on how they store up energy in their suspension in turns and then release that energy to launch them into the next corner? They don't because that's not the function of suspension. Yet this is how we talk about a ski's energy.
The purpose of race car suspension is to maintain tire road contact and minimize shocks that can break traction.
Sound familiar?
 
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Seldomski

Seldomski

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The car analogy is useful but I feel like it downplays the role of the skiers legs' role in the "suspension".
 

tomahawkins

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The purpose of race car suspension is to maintain tire road contact and minimize shocks that can break traction.
Sound familiar?
Exactly. Ski design is predominantly a suspension problem. It's not an energy storage and delivery problem, at least not at human-in-a-turn scales.
 

François Pugh

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Automobile suspensions do store energy in the springs and do rebound a vehicle into a new turn after stopping an abrupt turn abruptly (brought on by sudden steering wheel input). A lot of drivers not expecting this are surprised enough to loose control after making a successful swerve to avoid a deer, but the correction to back straight ends up propelling them all the way back into the ditch or making them spin out instead of going straight down the road in the other lane (or rolling their SUV).

Some automobile suspensions and quite a few motorcycle suspensions have high speed and low speed damping that is separately adjustable. With skis I guess you have to tune them for the right lack of damping at just the correct "speed" while maintaining dampening at other frequencies. Yeah, it's complicated.
 

oldschoolskier

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This is a great analogy. How often in motorsports do we hear drivers comment on how they store up energy in their suspension in turns and then release that energy to launch them into the next corner? They don't because that's not the function of suspension. Yet this is how we talk about a ski's energy.
Yet a well designed suspension does just that in helping the car go faster, by allowing the motor the most efficient contact with the road surface to transfer energy.

Why we use in in skiing is we have no motor to go faster, gravity and retention or amplification energy when we change direction. Think of a surfer pumping the board.

Again the efficiency of maintaining contact and how the energy is transferred is whats important.
 

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