Ski "rebound" or "pop"

[tube77]

Physics 101:
Hook’s law tells spring force F=-kx where k is the spring constant and x is the displacement.
So either higher spring constant (or stiffer skis) or larger displacement (or more bending of the skis) would generate higher force (or higher rebound).
This is my understanding.

 

[tromano]

In transition of a turn the skis are flat and have already "popped". Rebound or pop doesn't really make turns happen. It makes the ski flatten out once pressure is released. The release is an active move from the skier that results from the skis and skiers com following different paths. The infinity move.

Skis that I describe as poppy generally are more pliable skis that accomodates turns in the fall line better and allow for variations in for aft balance to play with the turns. Also for popping off the lip of a jump. A ski that works well for dolphin turns. $.02

 

[geepers]

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?

In Ultimate Skiing Ron le Master writes on rebound turns:

The sudden force from the snow and accompanying deceleration of the skier’s feet produce two effects that cause the unweighting. The first is an effect something like a pole vault (figure 6.20). The box in which pole vaulters plant their poles is below their centers of gravity. The reaction of the box to the pole and the vaulter redirects the center of gravity into a circular arc with the box at its center.
The second source of rebound unweighting is a rubber-band effect of the leg and hip extensors: the thigh, buttock, and lower-back muscles. ..... When the contracting muscles catch the falling mass of the upper body and upper legs, they stretch a bit, then recoil, tossing the center of gravity back upward
He adds:
Contrary to common belief, the skis themselves don’t contribute appreciably to rebound through a trampoline effect. Compared to a skier’s weight, the skis are simply not stiff enough to store and return much energy. In addition, the skis are supported by the snow directly underneath the skier’s feet, unlike a trampoline, which is supported only along its edge.

My own experience is that the pole-vaulting effect is probably the main contributor. I've got rebound from the deadest of over-used skis and to be frank at 60+ there's not a lot of muscle back there to send anything skywards. So that leaves pole-vaulting.

Still, puzzled over why some skis feel more reboundy than others. Tend to the view that it the ability of the ski to establish a platform - the box part of the pole-vault.

 

[razie]

Its a somewhat complex equation.

The ski stiffness contributes some directly but mostly by holding a tight arc: noodles, fat skis and non-cambers cant hold an arc tight enough. They will over-bend, wash-out etc

Tightness of the arc needs judged in relation to the speed, of course as well as the ski. A SL ski in a GS turn will result in less of a contribution to rebound. Sitting on the tails won't bend the tips etc.

Snow condition matters a lot: Soft snow won't hold a tight arc.

The body does help, by storing energy and even pushing or just resisting, at the right time (vaulting).

The timing matters a lot : You can spend too much time on the outside ski and kill the rebound or release too early and not let it create. You bave to release at the right time to use the bottom of the turn and it depends on many factors even for the same turn - the bottom of the turn is nit absolute, but in relation to the impulse of the upper body entering that turn... otherwise the resulting forces are in the wrong direction.

The usage of the body matters a lot: You can relax the wrong parts at the wrong time and loose vaulting etc. Flexing the release with a good timing, will allow the lower body to receive the energy and feels stronger, as there is less mass to be impulsed. Counterbalancing, angulating hard for instance allows the body to contribute to the rebound more etc.

You also have to put it in context - if it felt like one foot, it was one inch!