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#11
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"Jrw" wrote in message news:wse4g.28055$HC3.24018@trnddc07... How can one put 120% of body weight on a ski? One can jump up and come down on a ski but one cannot push down harder than their weight since they have nothing from which to push? If you accelerate your body mass upward you have to exert a downward force of more than 100% of your body weight. Also, the kick is started with a "pre load" in which the ankles and knees flex so the body mass moves downward. You exert more the 100% of your body mass to stop the downward motion. The force required to stop the downward motion and start the upward motion plants the ski. |
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#12
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Stand on a scale, have your partner note your static weight, then jump
(spring) upward and have your assistant note the peak force registered by your scale. It's physics (Newton's 2nd and 3rd laws). If you spring upward from your scale (i.e. the upward hitch as you completely shift weight to the kick or grip ski as the trailing ski passes the weighted ski), the upward acceleration, if 2 m/sec/sec will add 0.2 times your mass as on opposite reaction to your upward implusle move. If you look at classic skiing body position, as you transition from glide with your ski forward of neutral to the "kick", your glide (forward) leg begins to straighten. That motion is the little upward hitch that applies a downward force to the ski (the equal and opposite reaction of Newton's third law). Edgar |
#13
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Edgar wrote:
Stand on a scale, have your partner note your static weight, then jump (spring) upward and have your assistant note the peak force registered by your scale. It's physics (Newton's 2nd and 3rd laws). If you spring upward from your scale (i.e. the upward hitch as you completely shift weight to the kick or grip ski as the trailing ski passes the weighted ski), the upward acceleration, if 2 m/sec/sec will add 0.2 times your mass as on opposite reaction to your upward implusle move. If you look at classic skiing body position, as you transition from glide with your ski forward of neutral to the "kick", your glide (forward) leg begins to straighten. That motion is the little upward hitch that applies a downward force to the ski (the equal and opposite reaction of Newton's third law). Edgar Jumping up and then coming down will exert more force, but I don't see how flexing your muscles will help it will just cause you to lift some of your weight. If you bound that might help and that is a reason classic skiing in bad wax/kick condition expends more energy with additional lifting. |
#14
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do it on a scale, I bet you'll see it happen: Stand on a scale
balanced on one foot. Squat and flex your knee so you can balance on that foot and hold this positionat least long enough to confirm that this is 100% of your weight. Then quickly and forcefully straighten both the hip and the knee (keep your balance!). You're pushing down on the scale to *move* your body weight upwards - and I think you'll see at least a momentary weight reading on the scale than your body weight. I don't know if I could personally achieve 120%, I might try it this weekend out of curiousity. On a ski the amount more than 100 is dependent on your ability to begin with 100% of your weight truely balanced on that one ski, and also the quickness and strength you use to force that foot down, move your body upwards as you straighten your hip and knee before you shift to the other ski. |
#15
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Camilo wrote:
do it on a scale, I bet you'll see it happen: Stand on a scale balanced on one foot. Squat and flex your knee so you can balance on that foot and hold this positionat least long enough to confirm that this is 100% of your weight. Then quickly and forcefully straighten both the hip and the knee (keep your balance!). You're pushing down on the scale to *move* your body weight upwards - and I think you'll see at least a momentary weight reading on the scale than your body weight. I don't know if I could personally achieve 120%, I might try it this weekend out of curiousity. On a ski the amount more than 100 is dependent on your ability to begin with 100% of your weight truely balanced on that one ski, and also the quickness and strength you use to force that foot down, move your body upwards as you straighten your hip and knee before you shift to the other ski. You can surely apply force to the ski beyond your weight, but it isn't a very efficient process, it must be very impulsive and a good deal of the effort uncoiling your muscles goes into lifting your body up. If the ski is flattened only for the millisecond when the greatest impulse pushes this overweight, then it might not really help in grip since one really wants grip/friction in the horizontal direction not the up and down which is wasted motion and energy. Running is a style that lifts your weight more than one has to and presses down hard surely isn't efficient and I guess that is why double poling comes in so much in classic skiing. The questions of how quickly one can apply impulse are important I am sure that impacts the different feel of the ski I lot more than some simple measure of flatness with 120% body weight since in real skiing this is very much different than a static measurement. I guess that means Zach can try to find more useful measures of ski performance. I guess trying the ski under different conditions is the only real test. |
#16
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Impulsive loading of the ski happens. I'm not trying to prescribe a
certain level of impulsive overload as being "correct" or anything like that. But overload at some level is a fact of skiing. I'll leave to all you technique experts to figure out how much/little might be just right. The thing about measuring skis is that, at best, we can establish benchmarks. We don't make any attempt to actualy replicate what happens to a ski when you're actually skiing. There are NO static loads in skiing. The reason we check to see what happens to the ski at a number of different loads is that it gives as some idea of the rate of change, and a sort of indication or prediction of what we might expect from the more subtle loading that occurs during skiing. A good example is testing the "closing" flex of a classic ski. It doesn't actually matter whether a ski closes at 55% or 65% of your body weight - you don't spend any time skiing with that much weight on the ski. But it gives us a reasonable prediction of how the ski will respond to real-world loading. You're right to expect, Jeremy, that trying the ski on snow is the only "real" test. But it's a test that we usually don't have the luxury to perform as completely as we'd like when we're selecting skis. So, when we've got an opportunity to go back and measure known good skis, and known bad skis, and see what characteristics they share, then we stand a chance of narrowing the field a bit prior to deciding which skis to put on the snow. And when we get the chance to measure several thousand pairs of skis we get a pretty good sense of the range of available values in those measurements. It's taken me several years of testing skis to understand how useful that 120% measurement can be (simply in the context of indicating the action of the ski beyond full weight). So, while I continue to try to find more useful measurements, I'm guessing that your dismissal of the idea might be a bit premature. Zach |
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