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#51
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Zach,
If you shorties (I'd typically advise you a mountain bike size Small or Medium, I ride X-Large or XX-Large) are on 192's, by rules of proportion (194cm dicided by 170cm, multiplied with 192 ski's), for height I'd be on.....hold on now people...217cm ski's! And then, I may still have relatively longer legs so I could "handle" even longer still? This really sucks, I though at first that my body style might actually be super suited for skate-skiing, and now I hear I may have to do with ski's 10% shorter than ideal. Exactly as in bikes, until 29" wheels hit that market and made me from a very so-so technical rider to one that drops anyone anytimes when it comes to technique. From your explanation, I get the impression that wider ski's might actually be just the ticket after all, to make longer ones that are still stiff enough for heavier/stronger legged athletes. I'm no phisicist, but I would guess that if you add 10mm to the 40mm width of a ski, that immediately adds 25% of stiffness. Yes, also weight, but not a full 25% of what sits on the foot (binding, boot, etc). Skiers that can put more pressure on a ski can also pull on it harder, I would say. Assuming a whopping 25% wider ski, longer skiers like your friend and myself might be able to have 220cm ski's (10% longer than his rare Madshus') that close at appopriate pressure, and with a huge surface area, be it good or bad for glide. If width is such a small factor in a ski's performance that it's even disregarded lately, at least it wouldn't kill the advantages of the perfectly fitting-length and flex of the ski's? I didn't quite understand your "torque on edge" argument against wider ski's (language barrier and technically under educated, I'm afraid). And I appreciate that wider as well as narrower-than-standard have been tried over the years, and rejected. But wouldn't a couple decades of ski development have come up with some tricks to make it work in 2006? I hope you (and others) won't confuse my fresh look at the technical side of this sport being the total newbie that I am, with being cocky. I get really cocky, when I actually know what I'm talking about. Not there yet with ski's, just trying to learn. Looks like that if I would try biathlon, the extra 4kg of the rifle would make my future ski's even less desirable. My current 81kg is in pretty bad shape, I might actually weigh 85kg in true race shape, adding some upper body muscle this summer rollerskiing. And I actually thought being a strong guy made it an advantage to carry the same 4kg rifle as people weighing much less themselves. That would make me 89kg already... Thanks for your insight. J "Zach Caldwell" schreef in bericht oups.com... Hi Jan - you're definitely not heavy, given your height. But at 81kg (regardless of height) you're certainly in the category that will be best served by a full-sized ski. From any manufacturer you'll be on the stiffest flex category of the longest ski. You won't be looking for the stiffest pair in that category, by any means. But that's where you belong. In general when you get close to 90kg it gets quite difficult to find an appropriate skate ski for an agressive and technically competent racer in general inventory. The skis exist, but they're not nearly as common, and they're on the very outside of the design spec for the skis. Interestingly nobody makes anything longer than a 195 (Madshus) for skate skis. That's been the case for several years. Anyway, I had one 88kg customer this year who wanted four pairs of well matched skate skis. He's a strong and aggressive skater, and it took me a lot of tries to find skis for him. A lot of this depends on what brand and model of skis your're looking at. A Fischer 610 gets very stiff near the close and can fit a very broad range of skier weights. I'm 170cm and 65kg and my prefered Fischer ski is a 192 stiff - the same size you'd be looking for. My wife is the same height as me (actually, probably a CM or so taller) and closer to 58kg and she prefers my skis to her own 187s. The skis I use as test skis are a 96kg factory flex (145% of my body weight). That's generally a ski fit for somebody closer to 73-75kg, but it's what I like to ski on. For you I'd want to find something like a 105-110kg ski, depending on a bunch of specifics that aren't important here. Madshus, on the other hand, makes skis with a very soft and supple finish. A ski that is well fit for a 75kg guy would be pretty-much unskiable for me. And my skis would turn inside out under you. A Madshus (or similar design) has to be fit much more carefully. But it's also possible to fine-tune the feel and feedback that the skier will get to a much higher degree. And you'd be on a ski that closes at something more like 85kg than 105kg. I have a good friend who helps me grind skis from time to time when I get very busy, and who is almost exactly your size. Really tall and skinny with a background as a ski racer and cyclist. We actually found a couple of old pairs of 200cm Madshus skate skis from Peter Hale - a long-standing Madshus rep in this country - to set up for him this year. Then we ran out of snow and never got them onto the grinder. So next year. Anyway, this guy has been frustrated by the lack of longer skis on the market. He can find skis that are stiff enough, but it's difficult, and he feels that he can definitely handle longer ski. Best of luck getting well set-up. I've never felt it was a great advantage to be short - but it sure beats being too tall! Zach |
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#52
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Hey Jan - this is turning into a two-way conversation, which I'd be
happy to continue offline if you'd like. Interesting that you note 29 inch mt bike tires. I like those as well! As a short guy it's hard to find a bike that can fit the larger wheels gracefully, but they sure do roll over things nicely. my opinion would be that advantages of larger wheels on a mt bike might be analagous to the advantages of longer ski. Smoother transitions, lower "rolling" resistance, plain and simple. The ergonomic consideration is how to build a bike frame to fit the rider... Anyway, my e-mail address if you want to continue to conversation is Zach |
#53
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Cheers Zach, you obviously put serious time and efforts in these replies,
and although I'd love to pick your brain, it seems unfair. Other people's thought would be great here too obviously, if only to answer original question with personal experiences. Most I know about bikes I learned lurking forums, I don't even buy mags. If you ever need a design for a small 29" bike that fits and handles like you want it do, do drop me a line, it's sorta my gig to figure that sort of thing out. what you do with ski's, on a simpler level I do with 29" bikes. Sometimes manufacturers actually listen to me and give me carte blanche and a proto to try, way cool. Happy trails, J "Zach Caldwell" schreef in bericht oups.com... Hey Jan - this is turning into a two-way conversation, which I'd be happy to continue offline if you'd like. Interesting that you note 29 inch mt bike tires. I like those as well! As a short guy it's hard to find a bike that can fit the larger wheels gracefully, but they sure do roll over things nicely. my opinion would be that advantages of larger wheels on a mt bike might be analagous to the advantages of longer ski. Smoother transitions, lower "rolling" resistance, plain and simple. The ergonomic consideration is how to build a bike frame to fit the rider... Anyway, my e-mail address if you want to continue to conversation is Zach |
#54
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Zach
Too pity you can't find those valuable measurements of kicking forces along the footbed. You said the forces were predominantly going through the ball of the foot. All of the time? I mean, they didn't register the resulting force move along the footbed with the time of the kick? By the way, you have the opportunity to talk to Kris and Justin. What do they say, do they initiate the kick with the heel pressed against the ski (and I as I have said this doesn't mean that the center of mass is above the heel), or do they kick with the toe only? I may seem too pushy asking these questions, but the fact that good skiers have such different views at this crucial point of ski technic surpises me. Vladimir |
#55
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Those guys don't spend much time thinking about their heels. My feeling
is that the differences between what top skiers talk about are primarily differences between what they talk about, not so much the way they ski. I know of no skiers who actually initiate a kick without the foot flat on the ski (or heel pressed against the ski, as you say). Whether they focus on loading the ski through the heel or through the ball of the foot is of relatively little importance because we're talking about mental imagery much more than actual biomechanics. I've seen coaches nearly come to blows over this issue, and other similar ones. And in fact I wouldn't call the difference crucial at all. There are very few crucial differences in technique among the best skiers. There are stylistic differences and personal strengths. The truly crucial differences occur and lower levels of the sport and have to do with body position. A skier who is trying to initiate a forceful kick with the kicking foot well in front of the center of mass will struggle compared to a skier who initiates a kick with the center of mass forward over the kicking foot. But good luck finding that first skier on the world cup! Once you refine the population of skiers being examined to those top level skiers who all initiate a kick with their center of mass forward, the range of actual loading positions narrows a great deal. There is still a range, and there are ski selection implications to be sure. But the range is fairly small and skewed well toward the agressive (forward) end of things compared to the general population of non world-cup ski racers. Zach |
#56
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Zach Caldwell wrote: .... we're talking about mental imagery much more than actual biomechanics. I've seen coaches nearly come to blows over this issue, and other similar ones. And in fact I wouldn't call the difference crucial at all. There are very few crucial differences in technique among the best skiers. There are stylistic differences and personal strengths. Excellent point that we can't be reminded of enough! Thanks. |
#57
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Zach Caldwell wrote
. . . A skier who is trying to initiate a forceful kick with the kicking foot well in front of the center of mass will struggle compared to a skier who initiates a kick with the center of mass forward over the kicking foot. This seems very helpful -- it's going to change how I ski -- but also rather complicated. Thanks a lot for sharing it, Zach. It's complicated because: (a) a key bottleneck which limits power in classic diagonal striding (compared with skating) is the short _length_ of the leg-push while the grip zone is stopped against the snow, so I've long thought that by lengthening the leg-push out in front should add power. (b) elite racers (including Kris Freeman) in side-view videos do move their ankle joint significantly out in _front_ of both the hip joint and the knee joint before starting their leg-push. (c) "center of mass" is a "virtual" concept. Though precisely defined in physics, it's not located in any specific location of the body -- its virtual position moves with different body configurations, might even be outside the body -- pretty difficult to tell where the skier's center-of-mass is located from video analysis. Here's my response to those complexities: (a) starting the propulsive leg-push with ankle-joint out in front is usually not effective for elite racers on moderate terrain -- because they've got another more important limiting bottleneck to deal with, namely reducing the "dead spot" in time (thus increasing stroke turnover frequency). (a) non-racers who don't want to try to "rush" their turnover frequency might benefit from pushing with the ankle joint out in front. (a) serious racers in special terrain or snow situations might sometimes push from out in front (b) elite racers have two physical reasons for first moving the ankle joint out front before they push, even though they do not start their actual propulsive push until the ankle joint is closer to underneath their hip joint. (c) "center of mass" is not very important to the key physical arguments here, so let's talk instead about locations relative to the hip joint of the currently-pushing leg, which _is_ readily observable in side-view video. Details: (a) power bottlenecks for serious racers. I'm thinking now that the reason elite racers do not push with the ankle joint out in front is because they would first have to wait for their whole body to stop relative to the snow before they started their leg-push. That's because in order for the grip zone of a classic ski to grip with static friction to transmit pushing into the snow, the ski must be stopped relative to the snow. So that's what some (many?) non-racers do: after making the previous leg-push, then Wait and slow down until the ski roughly stops, so they can push again with the other leg, then Wait . . . The "wait" part doesn't work for winning elite racers, because speed depends on power, and physical Power can be thought of as having three key physical drivers: pushing Force multipled by pushing Distance divided by Time. So the racers need to manage the Time aspect, which implies looking for ways to reduce the Wait. The trick is to start the _ski_ moving backward relative to the hip while the rest of the skier's body mass is still moving forward -- so the net resulting speed of the ski relative to the _ground_ goes to zero -- and static friction happens and the skier can start the propulsive leg-push which transmits force into the ground. (b) why elite racers move the ankle joint out in front Getting the ski moving backward requires some distance, and that's the first reason that elite racers move their ankle joint (and ski) out in front of the knee joint. If they start accelerating the mass of the ski and lower leg backward from out there, then by around the time the ankle joint passes underneath the knee joint and/or hip joint, the ski will have achieved enough relative backward speed to be temporarily stopped against the snow, in a good configuration to get lots of propulsive work from the main leg-push. Of course elite racers get very good at unconsciously sensing and controlling the instant and leg-joint-configuration when the ski stops and the neural impulses for the main leg-push muscle-push starts. And there's trade-offs between the Force - Distance - Time drivers of physical Power: If a racer got in the habit of starting the main push with the ankle joint back farther behind the hip joint, the backward ankle speed would be faster, so the wait Time could be cut further -- but the pushing Distance driver in the numerator of Power would be reduced, so the net impact might be to reduce overall stroke-cycle Power (and thus overall forward skiing speed). Each racer learns to make their own trade-offs based on their own specific muscular capabilities and different terrain and snow conditions. The second physical reason racers and many other skiers can add power by moving the ankle joint out in front of the knee joint is more fun in the skiing, but takes too much time to explain here (unbalancing the forward-backward reactive force pair in the leg-recovery move). I think it's still valuable for speed to move the ankle-joint way out front, and valuable for fun to "kick" the foot explosively forward -- even though in most situations like Zach says that's not where the propulsive push should start. What I want to learn now is how to start my leg moving back _early_, and learning how to "sense" the transition to static friction -- without the obvious signal of waiting for my body to stop. (x) more . . . * I think elite ski racers _can_ train their leg muscles (notably the knee-flexor "hamstrings") to push strongly with the ankle-joint starting out in front of the knee-joint. (Especially if they apply some help from the pole-push.) * When doing hill-bounding up steep hills on snow, I've seen videos where elite racers actually _do_ make their main leg-push with their ankle-joint in front of the knee-joint. They've learned to employ a different trade-off of Time versus Distance than on moderate terrain, because the Time cannot be reduced much anyway in bounding, because once both skis are up off the ground, there's no choice but to wait for gravity to do its thing. And unlike runners, skiers (and "nordic walkers") can use their poles to _help_ their leg muscles push through the ankle-joint out front phase. * Has anyone put force-sensors in cross-country ski bindings, with radio digital output synchronized with side-view video frames, so we could actually _measure_ from what ankle - knee - hip configurations different skiers stop the ski against the ground and start their main leg-push? And _measure_ how the starting leg-joint configuration might change in different snow and hill situations -- including steep hill-bounding? How is it different if the skiers do not use poles? Thanks for the insight, Zach. It will definitely change how I ski classic. Ken ____________________________________ Zach Caldwell wrote [ in response to a question about toe-ball versus heel pushing + pressure under the topic of "Grip physics"] Those guys don't spend much time thinking about their heels. My feeling is that the differences between what top skiers talk about are primarily differences between what they talk about, not so much the way they ski. I know of no skiers who actually initiate a kick without the foot flat on the ski (or heel pressed against the ski, as you say). Whether they focus on loading the ski through the heel or through the ball of the foot is of relatively little importance because we're talking about mental imagery much more than actual biomechanics. I've seen coaches nearly come to blows over this issue, and other similar ones. And in fact I wouldn't call the difference crucial at all. There are very few crucial differences in technique among the best skiers. There are stylistic differences and personal strengths. The truly crucial differences occur and lower levels of the sport and have to do with body position. A skier who is trying to initiate a forceful kick with the kicking foot well in front of the center of mass will struggle compared to a skier who initiates a kick with the center of mass forward over the kicking foot. But good luck finding that first skier on the world cup! Once you refine the population of skiers being examined to those top level skiers who all initiate a kick with their center of mass forward, the range of actual loading positions narrows a great deal. There is still a range, and there are ski selection implications to be sure. But the range is fairly small and skewed well toward the agressive (forward) end of things compared to the general population of non world-cup ski racers. Zach ____________________________________ |
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