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# Pop at end of turn - Page 2

Ydnar says:
Quote:
 Originally Posted by ydnar Rebound-schmebound. Ever wonder just how much energy is stored in a flexed ski. Here's a way to see. Take two sawhorses and place a ski between them. Put a light object such as a five inch length of 2x4 on the ski. De-camber the ski by pulling it down with your hand. Release the ski and see how far the piece of 2x4 is thrown into the air. A foot? Maybe two feet? If the ski can only move a few ounces of wood a foot or two how is the poor weak thing supposed to supply enough force to move my fat old mans body around? Methinks that the sensation of pop/rebound/whatever must come from some other source, yd
1. I'm not sure the test proves what you say it does: When you pull down a ski with your hand, you're using, say, 30 pounds of pressure (less than one half of what you'd exert on a triceps pulldown machine, anyway.) What Ron LeMaster says in one of his on-line slide shows is that if you go from 60 degrees inclination to 70 degrees of inclination, you increase the force acting on the skis from 2 Gs to 3 Gs: For 180 pound skier, that's 540 pounds of force. Given unequal force on the inside and outside ski, that's over 270 pounds of force on the outside ski, bending it. I don't remember which of LeMaster's talks had the Gs reference, but it was either:

LeMaster, Alpine racing techniquehttp://www.ronlemaster.com/presentat...nique-2003.pdf

And (in my recollection) the books on plyometric training (Chu's Jumping Into Plyometrics and Radcliffe, High Powered Plyometrics) refer to studies showing that in jumping exercises like depth jumps and box jumps, in braking the body and absorbing the jump, we briefly (fractions of a second) experience loads of over a thousand pounds of force. It would make sense that similar (or greater, given the additional weight of the equipment) forces are exerted on the skier and the ski in jump turns on the hill. If you use 1000 pounds of force to bend the ski, I bet you've got a pretty good ballista to launch your wooden block.

2. You may be right, though, that it's a combination of the rebound from the ski and some force applied by the skier's legs to launch the pop. I once sat, horrified, with a group of other ski racers watching myself on video, having gotten back in a slalom course, completing the last four gates by jumping the skis from one side to the other.

3. Many of us have experienced the "pop". If you want to see it in action, go watch a masters or club GS race, after it's all rutted out, and when some guy going all-out gets his weight a little back, he will get absolutely launched both into the air and, because his weight's back, onto his tail.

4. Ron LeMaster describes it in Skier's Edge, and explains that intermediates exaggerate the pop to create an unweighting between turns, but that advanced skiers often absorb the invisible bump between turns by leg retraction during the crossunder. In one of his online slideshows, LeMaster mentions that World Cup atheletes train by jumping laterally between trampolines to practice the rebound/retraction/extension movement pattern where the legs cover much more distance laterally than the upper body. IMHO (and based on no direct experience) that seems to be the movement pattern reinforced by the Skier's Edge(tm) training machine.

5. That said, I'm not expert enough to answer the question about whether the pop is a good, or useful, thing. In general, ski racers these days are taught to keep the skis on the snow as much as possible, since air time is both slow and a place that you can't redirect your line. But the pop feels dynamic and fast, and when used with retraction and extension is a great way to get your legs out to the other side.

5. Regarding how to create the pop: If you really, really want to experience the pop (and maybe get a sled ride down the hill--I don't actually recommend this) put on your slalom skis and do high energy hop turns with some speed and redirection of the skis (i.e., dynamic fast but tight pivot entry turns) with your weight a little further back than it should be (e.g. go over the lip to a steeper section of a steeper intermediate slope without making a forward move). Pop. Splat. Tumble. I do have some relevant direct experience there.
skiingman: When you release the edgeset in some particular way on some skis in some turns, it is enough to launch skis above your head if you don't control it.

Only if your CM is already well on its way out of that turn. If the CM is still over the skis you won't get that kind of reaction. A bent ski simply does not generate that kind of force - especially when the body weight against it is increased by the centrifugal force created by the turn.

sfdean,

If you use 1000lbs to bend a ski, then the ski will break! The huge G-forces in a turn are not only against the ski, but also against the snow.

------------

In my opinion, rebound does happen, but a bent ski alone cannot do miracles. You need both a bent ski with some stored energy and you need to place your CM in a position which eliminates the centrifugal force that keeps the CM in the turn.

Do it wrong and the ski will literally pop from under you doing a cross under that is completely out of control. Do it right and the pop and cross under will be just right.
Quote:
 Originally Posted by TomB skiingman: When you release the edgeset in some particular way on some skis in some turns, it is enough to launch skis above your head if you don't control it. Only if your CM is already well on its way out of that turn. If the CM is still over the skis you won't get that kind of reaction. A bent ski simply does not generate that kind of force - especially when the body weight against it is increased by the centrifugal force created by the turn.
....

Quote:
 Originally Posted by me The force vector isn't vertical. Ott's description of jumping on a bed with one leg can demonstrate the force. The force rapidly swings your body around its center of mass in the axis perpendicular through your chest and back. -Garrett
The force needed to rotate my body about its center of mass quickly is an awful lot less than the force needed to accelerate my center of mass vertically.

The rebound force from the ski can, and will if you aren't careful, rotate your body so quickly about its CoM that you will land on your head. While this is happening, your CoM is proceeding in roughly the same direction through space without much acceleration acting upon it at all.

While picking up a bowling ball and throwing it may be difficult, you can spin it quickly with a single finger.
-Garrett
Well, to summarize, I think everyone's right:

1. Yes, the 540-1000 pounds is pushed against the snow, not just the ski.

2. As the skis go from turning, to straight, the force bending them is released, and they can rebound, releasing some potential energy like a spring.

3. Even though that potential energy is nowhere near enough to throw a skier high into the air, it is certainly enough (especially when augmented by a modest push with the legs) to rotate the skier from a 60+ degree inclination in one direction to a 60+ degree inclination in the other direction, when coupled with retraction and extension. The pop.

4. As described in 3, the pop is potentially useful, since it facilitates unweighting the skis, rapid crossunder, and relatively unweighted redirection for the next alternating opposite turn.

5. Observationally, if my weight is back, instead of forward, (A) there seems to be a bigger, more powerful launch from the pop (don't know why), and (B) the results, predictably, are more crash producing.
I just figured out the "tail pop" thing. Very few turns are pure carved turns; generally, the skis are going to skid to one degree or another. When I skid, it's the tails that are skidding around. If, at some time in the turn while the tails are skidding, the edge of the tail is set, it will stop the skid, and the skis will take a jump.

I still don't know how to intentionally make that happen, or why I would want to, but now I think I know what's going on.
Quote:
 Originally Posted by Colossus178 I've read about getting a pop at the end of a turn, a little kick that rockets you out of the turn. I even experienced it a couple of times last year, but I have no idea how I made it happen. I would appreciate it if someone would speak a little about it, or point me to a thread on the subject. How do you make it happen? When do you want to make it happen. I'm guessing that skis with stiff tails are better for it, but tht's just a guess.
My two cents:

It's not as complicated as it sounds. You're really just bending those sticks and bouncing off them like a diving board.

When you make a nice tight carved turn, your skis bend into arcs, somewhat like a bow being drawn. Usually the approach is to set the skis on edge and get the front part of the ski's edge to dig into the snow. The entire edge of the ski will then follow along cutting a curved groove in the snow. If you concentrate you can feel the reaction force being applied by the snow on the ski as you dig that edge in and it begins to turn you.

There are all kinds of variations and games you can play with applying force to the snow through the edge. With a little practice you can concentrate on how much force you are applying during the turn to which part of the edge. You can play games in a long turn using your balance to keep an even force throughout the turn trying to keep your whole edge from front to back firmly pressed into the turn, or maybe concentrate on one point and forcing mostly with the tip then the middle then the back. Force more pressure into the front to drive the ski forward and it will generally tighten up the corner's arc, provided there is enough grip. Allowing the front to relax a bit and spreading your weight to the midle lets the ski find another arc. There is no end of things you can do with forces and a pair of skis.

If you put a lot of force on the back of your skis near the end of the turn and non on the front the front will straigten up as there is now nothing pressing the front edge down into its groove, but the back is well bent. The ski will respond much like a diving board and give you a little kick when it rebounds and unbends. You can make this affect greater by first forcing the front into a very tight arc and following through with the middle and finally forcing the back into a similarly tight arc and really "pop" out of the turn, if your ski's have a stiff enough tail.

How you use this impulse is up to you.
Hi:
Anyone reading this "pop" stuff tried a "Ski Fitter" ? -A great canadian invention...
Not the wimpy- hold the handrail- version from the US.
This thing ROCKS - literaly. Because of the rocker- it drops out from underneath you and the bungies give you the zip of the ski coming out of the turn.
You can up unweight or down unweight...
Also- try standing sideways so you are pushing and pulling the tray for fore/aft balance...
Which brings me to the real matter at hand...
Controlling the launch from one turn into the next...up-unweighting is too slow to deal with 30 miles and hour - because you can't pressure the ski early enough in the top of the turn...
The real problem I have is how to steer the ski the 34-45 degrees while its off the snow and land on a clean carving edge and then absorb those horrible G's again while staying centered.
Paul Whistler
Quote:
 Originally Posted by g-force Anyone reading this "pop" stuff tried a "Ski Fitter" ? -A great canadian invention... This thing ROCKS - literaly. Because of the rocker- it drops out from underneath you and the bungies give you the zip of the ski coming out of the turn. Which brings me to the real matter at hand... The real problem I have is how to steer the ski the 34-45 degrees while its off the snow and land on a clean carving edge and then absorb those horrible G's again while staying centered. Paul Whistler
On the latter, if I knew the answer for real, I'd be out there winning Masters races, not club racing and hanging out here, BUT if you look at Ron LeMaster's on-line slide shows (and his book Skier's Edge) he's got numerous montages of World Cup skiers redirecting their skis about 30-40 degrees while off the snow, and landing on a close-to-clean carving edge, and staying in the course. How do they do it? It seems to me, by having their CM travel a much, much shorter line between gates, so when those skis waaay out there engage (1) they're tipped on a very high edge angle, necessitating a less pronounced pivot-entry redirection to keep the same line, and (2) there's a more gradual redirection of the CM, resulting in less force than 40 degrees-pivot-40 degrees-pivot. The other thing you notice about those World Cup guys (and gals) is that they engage the snow with a pretty straight outside leg, so they're standing against those G forces with skeletal strength, not just their large quads.

And how do they stay balanced with all that going on? Hmmn: My new theory is that a little knee angulation and forward pressure on the outside ski to bend the shovel and feel the pressure on the big toe edge, coupled with pulling the inside foot back and tucking the inside knee up close to the chest keeps you balanced on two skis and forward, exactly where you want to be. At least that's how it looks when the World Cup guys do it.

Of course, as an alternative to the 40 degree pivot, now you could instead just claim lower-intermediate technique as cutting edge and simply hockey stop slide every other turn, as Bode Miller has advocated in blowing away all the other world class GS racers by over a second in Soelden, Austria, keeping a straighter line by not going as fast or as far across the hill:

http://www.skiracing.com/news/news_display.php/1905/

But getting back to what you first said, do you use actually use the Fitter equipment? Is it useful? Helpful? I always kind of figured it was a cut-price, portable down market cousin of the Skier's Edge(tm) machine (not to be confused with the very fine LeMaster skiing forces book of the same name.) Do you have one, do you use it often, and what kind of training is it--aerobics (e.g., 20 minutes at a time), or anaerobic threshhold (e.g., two minute intervals, with a break before doing it again?)

### pop vs acceleration

Quote:
 Originally Posted by Colossus178 Good discussion, folks. Thanks for the insights; hopefully, I'll be able to put them to use on the hill. TheRusty asks the same question I have.

I see two different "threads" in this thread. In my opinion the original post was related to the (seemingly sudden) acceleration feeling that can happen at the "bottom" of a turn. For me, it feels like someone hit the gas and my CM (ie rear end and torso) are now not keeping up with the skis. In todays SLALOM events, you can racers who land on their backsides by not keeping the up with the skis, too much in the backseat and they dq in an entertaining manner.

the other thread, also in my opinion, is about rebound energy and how it can be used to help effect an edge change. in mogul skiing there may be that infinitesimal moment where the skis stops, become unweighted and the skier "hops" onto the opposite edges. in this case both the edges AND the skiers direction change. in more wide open skiing, this rebound might be used to change only the edges, with the CM going from inside the skis (bottom of the turn) to ON the skis (transition/edge change) to inside the skis again, which are now on the "other" edges. as multiple people have pointed, this rebound gives a lighter feeling at the time of edge change/leg retraction.

now, if it would only snow in michigan
Quote:
 Originally Posted by MilesB The pop is really only useful if you are going to redirect the skis into the next turn. But it is fun.
Or scary, dpeending on wether you were expeting it or not.
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