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Ski Design: Science or VooDoo

post #1 of 7
Thread Starter 
“The designing of snow skis is a subjective process, for the forces between the ski and snow depend on the orientation of the ski in relation to the snow, the ski geometry and the material properties of the snow, and the ski speed, all in an unknown manner. The design process relies heavily on designer intuition to identify the contributions of ski design and of skiing technique to the qualitative interpretations of skiing performance. The mystique and art of ski design add to the glamour of the skiing process, but they do not necessarily lead to a physical understanding of skiing or to better ski designs.

“Surprisingly few references can be found in the technical literature on the process of skiing.”

Excerpted from the paper “Mechanics of the Turning Snow Ski” – Dennis k. Lieu and C.D. Mote Jr. (C.D. Mote Jr. was formerly a Professor and vice-chairman of the Department of Mechanical Engineering at U.C. Berkeley. Mote and his associate at UC Davis, Maury Hull, wrote the current standards for ski bindings. Mote has also served on the ASTM-F27 Ski Safety Committee and the International Society of Ski Safety.)

DM: Well that certainly makes ski design sound like a combination of witchcraft and voodoo doesn’t it?

Originally posted by Kneale Brownson:
Won't your ski design have something to do with how things feel in your boot AND body?
I know I experience several different sensations when I ski my 181 K2 Axis XP from when I ski my 174 K2 Mach-S.

Arcmeister: Kneale, My body tells me equip differences do make an alignment difference. (excerpt of my post in SI's "Alignment and soft snow" thread)
Even as a well aligned (from slightly bowlegged) skier I was very aware of the leverage variations when on a 78mm waist K2 XP in conditions varying from packed to soft snow. In those same conditions on my 64mm waist K2 XR the variation was negligible. On the fat boys my edges are 7mm further inside my knees on hard snow, but knee position relative to center axis of ski base was unchanged so in soft snow they felt fine. After a day a Copper that included some very fast mileage on hard pack I felt additional soreness in knees and new soreness in my hip flexors from the magnified leverage of the wider waist. Even though my boots are ground, I'm considering canting my wide skis to move my knee in a little to a stronger relationship to the edge location.

DM: To paraphrase Shakespeare, “Arc the problem lies not in the position of your knees but in the width of the ski waist which at 78 mm is well beyond the limits of the ball of your foot whereas 64 mm puts the ball of the foot on the inside turn aspect of the edge.”

The pain you are experiencing is caused by the forces acting on your ski tending to rotate your feet away from the hill. This force is translated through the subtalar joint in the ankle complex into axial rotation of the tibia also towards the outside of the turn. Since you don’t want this to take place you resist the axial rotation with opposing rotation of the femur. Three guesses where the forces meet. The knee of course.

Wide skis on hard snow = sore knees (and hips and back). Sometimes things break, like bones and knees. This is why skiing on hard pack with wide bodies is bad news.
post #2 of 7
Blond questions [img]smile.gif[/img] what are the measurement parameters for what you would consider wide? Is there a median width that would be acceptable for someone who usually who usually skis hard pack, but makes one trip a year out west, or do they need 2 different pairs of skis?

I mentioned in my Ski, Length or Me thread that it was incredibly easy to get the Nordicas, with a 65 waist on edge, even though they were not my favorite ski. Is there any benefit for someone who is just learning the dynamics of edging to occaisonally practice on a narrow waisted ski? Would there be a carry over of the learned kinesthetic input when they went back to a slightly wider waisted ski?
Thanks!
post #3 of 7
Thread Starter 
Lisamarie: Blond questions [img]smile.gif[/img] what are the measurement parameters for what you would consider wide? Is there a median width that would be acceptable for someone who usually who usually skis hard pack, but makes one trip a year out west, or do they need 2 different pairs of skis?
DM: If these are blond questions then blonds are pretty smart. I think it depends on what the predominant snow conditions are that you ski on.
For example if one skis exclusively off piste in powder I would suggest a wide body ski with minimal binding elevation and not terribly long (maybe 170 for myself).
However, if one skis mostly hard hard pack tending towards ice then someone like you should use a ski with a waist under 62 mm (but they don't make skis with waists this narrow that I know of).
I have two skis, one for hard pack and one for soft pack and spring moosh. If I skied more powder I would have a third ski that was a wide body.

LM: I mentioned in my Ski, Length or Me thread that it was incredibly easy to get the Nordicas, with a 65 waist on edge, even though they were not my favorite ski.
DM: This issue brings me to what I call one of skiing's dirty little secrets. Skates have a reference point for positioning the blade based on the balance point of the foot when standing on two feet with the ball of the 2nd toe under the head of the femur. This is called the 'mechanical line' of the lower limbs. The balance point runs through the center of the heel and the ball of the 2nd toe.
Skis have this reference as well. The ball of your second toe lies on the proximate center of the ski. It needs to so the skis will run flat when running on two skis on the flats.
BUT, skis have a 2nd reference, the uphill edge. This will be in a different position in relation to the ball of your foot based on what? The size and width of your foot. The industry standard is a US size 9 MEN's (not women's) foot. As feet get smaller the skiers suffer a huge mechanical penalty. But what the heck, size 9 men are a big market.
The long answer is if you ski mostly hardpack then find a ski that works with a 62 mm waist and a shovel in the area of 100-105 mm plus or minus a few mm. Lifters act to multiply the force applied by the ball of your foot. But one should have good tecnique as they can also increase the risk of injury. There can be huge differences in skis with essentially the same geometry. So test before you buy.

LM: Is there any benefit for someone who is just learning the dynamics of edging to occaisonally practice on a narrow waisted ski? Would there be a carry over of the learned kinesthetic input when they went back to a slightly wider waisted ski?
Thanks
LM: The benefit will be there for a ski with the right geometry for hardpack if that is what you ski. In soft pack you will derive more benefit from a wider waist. There will be an initial kinesthetic carry over if you go from a narrow waisted ski to a wider waisted ski on hardpack. But it will quickly degrade because the mechanics will not work.

If you have more questions please post them. Good one LM! [img]graemlins/thumbsup.gif[/img]
post #4 of 7
DM: Not trying to be TOO critical here, but I'm not seeing an answer to the question of how to define wide. You said for hardpack you should use a 62mm waist, which you mentioned doesn't exist, and then said for soft snow you'd use a "wide body ski". So the definition of wide = wide body ski?

LM: in my non-technical opinion, I think once you really get the feel for getting a ski on edge, you can get any ski on edge, regardless of the waist. Obviously you should use one where the dimensions are suitable for the snow conditions you will be skiing in for the best results.
post #5 of 7
Thread Starter 
altagirlM: Not trying to be TOO critical here, but I'm not seeing an answer to the question of how to define wide. You said for hardpack you should use a 62mm waist, which you mentioned doesn't exist, and then said for soft snow you'd use a "wide body ski". So the definition of wide = wide body ski?
DM: You are not being too critical. I actually said under 60 mm for those with small feet (I am thinking around size 6 womens). I would define wide as waist width that puts the center of the ball of the foot on the outside turn aspect of the waist edge. In order to have the forces controlling the edge angle of the ski in the same plane (i.e. horizontal) they must act across the inside edge of the ski. This can be done when the center of the heel bone is on the outside turn aspect of the waist edge and the ball of the big toe is on the inside turn aspect.
When the ball of the foot and the heel are on the outside turn aspect of the waist edge (i.e the same side) then the force acting on the inside turn aspect of the waist must be generated by the application of force applied to the inside turn aspect of the boot cuff. In this scenario you have forces acting in 2 different planes with the external forces acting on the skier changing as the turn progresses. I see this as a complex situation for the balance system to manage compared to the former scenario.

Altagirl: LM: in my non-technical opinion, I think once you really get the feel for getting a ski on edge, you can get any ski on edge, regardless of the waist.
DM: There is a physical limit to how much waist width one can manage. But I agree that one can put ski on edge even a wide one. But keep in mind that a different mechanism kicks in once waist width precludes the possibility of applying force across the ski edge with the foot. As I pointed out waist widths outside the ball of the foot can apply large stress loads to the ankle and leg. Unlike forward bending the leg has little defense against such forces.
post #6 of 7
Lm & Altagirl,
Convential waists are generally approx 60-67 mm, Mid-fats approx 68-80mm, Fats anything wider. I repeat, generally. Each mfgr's marketing sets their own definitions.

60mm is minimum width for WC racing so no one makes them any narrower, except a couple really narrow Elans of years past.

Problem is, narrower scientifically appears better, but boot max shell width sets a practicle limit as to how far you can edge a narrow ski before it "boots out" and you crash big time. Hense the advent of lifters to mitigate this issue (with their own set of problems, as pointed out). There are some narrower skis out there. K2 Spire Hi-perf womens ski is 64mm for example.

Note of caution, if you are knockneed, make sure you are aligned before you go narrow. Knockneed skiers already have end of femur too far inside that 2nd toe-ball. This is why non-aligned knockneed skiers like wide skis (and non-aligned bowlegged skiers like narrow skis).

Hope this helps clarify.
[img]smile.gif[/img]
post #7 of 7
Yes it does help. My instructor, as I mentioned in the other thread, is knock kneed. The folks at Green Mountain Orthotics told her to go any narrower than 66.
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