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Torsional stiffness, strange observation...

post #1 of 21
Thread Starter 
I'm futzing around with my skis at home, admiring their construction and all.

I'm sure we've all flexed skis by setting the tail on the floor, holding tip up with one hand, and pushing the center down with the other hand.

Well, I got the bright idea to hold one end (at the widest points of the shovel /tail) in each hand and twist. The results were kind of mixed.

I have a pair of 170cm Atomic LT:11m (non-FIS GS race skis for lighter/women racers) that I just bought and have yet to mount bindings to and use. These were the most torsionally rigid and took a good amount of force to deflect.

I also have a pair of 165cm Salomon Scrambler 7 Pilot (highly-shaped all-mountain skis shaped like the Metron, but not nearly as stiff and targeted toward low intermediates) that I first bought to get me back into skiing. I retired them in favor of the subsequent Volkl 4-stars. Unsurprisingly, these were somewhat less torsionally rigid than the LT:11m's.

What really surprised me was my pair of 161cm Volkl Supersport 4-stars (on-piste carver targeted toward intermediates/advanced) that I've been riding on for awhile. These were the least torsionally rigid.

Honestly, I did not expect the 4-stars to be as torsionally flexy as they were, neither did I expect the Scrambler 7's to be as rigid as they were for their intention. That said, I've been doing significantly more skiing on the 4-stars than I ever did with the Scrambler 7's, and also pushing the 4-stars harder than I ever did with the 7's.

Have I totally missed the boat with this "test", or could it simply be that my 4-stars are worn?
post #2 of 21
Torsional rigidity is the first thing to degrade in my skis. Therefore I am not surprised by your results but I am surprised you can feel the difference because it is hard to grab both the tip and tail of a ski and still twist with authority. I wonder if it would be easier if you clamped a tail in a vice and twisted the tip.

dt
post #3 of 21
I got the impression you did put it in a vice. Nonetheless, you've got me looking for a vice... will test tonight.
post #4 of 21
What I do is jam the tail where the wall and floor meet and twist the tip. One of my big strong friends can twist the crap out of some skis, it's pretty funny.
post #5 of 21
The other thing to consider is that some skis, even race stock, are designed to be quite torsionally stiff underfoot but a bit torsionally softer at tips and tails to allow the skis to ease in and out of turns. So your test doesn't tell the whole story.
post #6 of 21
Quote:
Originally Posted by DtEW View Post
I've been doing significantly more skiing on the 4-stars than I ever did with the Scrambler 7's, and also pushing the 4-stars harder than I ever did with the 7's.
Maybe they let you push without pushing back?

Torsional flexibility can often be part of "forgiveness".
post #7 of 21
I'm curious how you actually measured torsional flex. Eyeballing it? Are you sure you can tell the difference between the ski twisting and your hands moving? Unless you're Brian Urlacher, I doubt that you're strong enough to a) grip a shovel and tail securely under serious torque, or b) exert enough torque to twist a ski instead of your wrists (which is the joint most often injured by elite power lifters). Sorry, I just can't see how you did this. :
post #8 of 21
Oh, and I'll add that given that the average male has a total armspan, from finger tip to finger tip of roughly 190 cm, that means that you had only 40 cm to play with (assuming you gripped each end of a 170 cm ski 10 cm in.) Since each of your hands is about 20 cm from middle finger tip to wrist joint, that works out to mean you had to have been holding the shovel and the tail by your fingertips. OK, well say you had the ski vertical, or had your body at an angle to the ski. Your functional armspan becomes far less and any grip at the ends is impossible.

Alternatively, say you're mis-remembering and you gripped each end much closer to the ski's midline. The ski is that much thicker and far more resistant to twisting torque. But you said "shovel/tail" and "widest point," so we'll take you at your word. Not.

post #9 of 21
Quote:
Originally Posted by beyond View Post
Oh, and I'll add that given that the average male has a total armspan, from finger tip to finger tip of roughly 190 cm, that means that you had only 40 cm to play with (assuming you gripped each end of a 170 cm ski 10 cm in.) Since each of your hands is about 20 cm from middle finger tip to wrist joint, that works out to mean you had to have been holding the shovel and the tail by your fingertips. OK, well say you had the ski vertical, or had your body at an angle to the ski. Your functional armspan becomes far less and any grip at the ends is impossible.

Alternatively, say you're mis-remembering and you gripped each end much closer to the ski's midline. The ski is that much thicker and far more resistant to twisting torque. But you said "shovel/tail" and "widest point," so we'll take you at your word. Not.

Whoa!!! Be polite!

I think there is an avereage wingspan like there's an average height. Maybe that's the middle, but there are many on either end. There is no way without knowing the guy and his measurments that you can say that something like holding a ski is impossible...

I haven't measured my wingspan, but I wear a 36 inch shirt which means my wingspan from wrist (where my shirt ends) to the middle of my back has to be at least 72 inches right? Now he's talking about twisting a 161cm ski and a 170 sm ski. Say, like you said, he holds them 10 cm in at each side, that means he's working with a 140 and a 150cm ski or 55-62 inches.

I don't know, If my wrists are 72 inches apart, I'm pretty sure I can hold something 55 inchs long...
post #10 of 21
Actually it is possible to notice differences in torsional rigidity by twisting tips, even while holding the middle of the skis, and I've noticed that a lot of skis are torsionally weak. I guess these skis are aimed at beginning skiers and designed to be more forgiving. The 4* is not a 6*; it is not supposed to be torsionally stiff. The Srambler, I can't say much about, but if you have a wider ski then the deflection at the edge will be greater without the ski needing to be torsionally weaker. The wider ski needs to be a little stiffer torsionally to get the same amount of give at the edge. All of my skis are pretty stiff torsionally; it hurts my hands when I grab the at the middle and twist the tips around, but I can tell the difference between them.
post #11 of 21
I still haven't done it, but it would be kind of cool if people would post their quiver results... i will mention which is stiffer, my gotama or my jet fuel. seems obvious so far, but we'll see.

and your quiver results???
post #12 of 21
Thread Starter 
Quote:
Originally Posted by beyond View Post
I'm curious how you actually measured torsional flex. Eyeballing it? Are you sure you can tell the difference between the ski twisting and your hands moving? Unless you're Brian Urlacher, I doubt that you're strong enough to a) grip a shovel and tail securely under serious torque, or b) exert enough torque to twist a ski instead of your wrists (which is the joint most often injured by elite power lifters). Sorry, I just can't see how you did this. :
Quote:
Originally Posted by beyond View Post
Oh, and I'll add that given that the average male has a total armspan, from finger tip to finger tip of roughly 190 cm, that means that you had only 40 cm to play with (assuming you gripped each end of a 170 cm ski 10 cm in.) Since each of your hands is about 20 cm from middle finger tip to wrist joint, that works out to mean you had to have been holding the shovel and the tail by your fingertips. OK, well say you had the ski vertical, or had your body at an angle to the ski. Your functional armspan becomes far less and any grip at the ends is impossible.

Alternatively, say you're mis-remembering and you gripped each end much closer to the ski's midline. The ski is that much thicker and far more resistant to twisting torque. But you said "shovel/tail" and "widest point," so we'll take you at your word. Not.



Wingspan = 170cm

Any questions? :

(The kitchen is the brightest room in my house, so I can avoid using a flash. The flash kept introducing a shadow which was hard to tell from the black base of the ski.)
post #13 of 21
Thread Starter 
Quote:
Originally Posted by DT View Post
Torsional rigidity is the first thing to degrade in my skis. Therefore I am not surprised by your results but I am surprised you can feel the difference because it is hard to grab both the tip and tail of a ski and still twist with authority. I wonder if it would be easier if you clamped a tail in a vice and twisted the tip.
Well, I was too lazy to go downstairs and use the vise , but at the same time I was also concerned about twisting the ski too much to the point where I wasn't just gauging torsional rigidity, but actually damaging the ski. I figured that I was limiting the amount of force I was applying doing it this way, as I'm pretty sure I can destroy the ski if I anchored one end and twisted the other end with both hands. Personally, I like my skis.

Quote:
Originally Posted by D(C) View Post
The other thing to consider is that some skis, even race stock, are designed to be quite torsionally stiff underfoot but a bit torsionally softer at tips and tails to allow the skis to ease in and out of turns. So your test doesn't tell the whole story.
You're probably right. In fact, I'm sure the shovel and tail ends of the ski are designed to be torsionally different. I could probably discern that if I mounted the middle of the ski in a vise.

Quote:
Originally Posted by BigE View Post
Maybe they let you push without pushing back? Torsional flexibility can often be part of "forgiveness".
True. But recently I have been yearning for more grip in icy conditions, which is why I bought those LT:11m's. Maybe I'm growing out of the 4-stars, hmm...

I also have a pair of 155cm Salomon Equipe 10 SCs coming in the mail. Those have superwide shovels so I'm now totally curious to see where their torsional rigidity lies within my quiver.
post #14 of 21
I believe the torsional stiffness that matters would be between the center of the ski and the tip, and the center and tail. For that test, you'd want to fix the tip and tail and twist the center (as if edging). Notice that the applied torque puts the tips and tails twisting in the same direction. The actual twist ends up being over each half the ski. Since the angle of twist is directly proportional to the length over which a torque is applied, the shorter sections will twist a lot less than the whole ski.

It might seem like a subtle difference, but fixing the ends of a ski and twisting the middle (the typical load when skiing), or even the opposite of fixing the middle and twisting the ends in the same direction, is fundamentally different than twisting the ends in opposite directions. Different parts of the ski would be in tension together.

By twisting the tip and tail in opposite directions, I think any ski will seem less torsionally stiff because there is a lot more slop along the whole length that way -- the twist is running the whole length of the ski. I can't think of a normal skiing situation where the ski gets twisted that way (at least on purpose). And I would imagine ski makers don't focus on that scenario either. When they talk about designing skis to be torsionally stiff, they probably consider the typical twisting mode.
post #15 of 21
I'm with skier219 on this. Skis aren't designed to handle being twisted like a piece of licorice. So they will most likely react differently if the testing force applied is closer to what actually happens on slope.

With that said though I think that this simple twisting test (as shown by DtEW) still can show you relative torsional stiffness between different skis. I'd be really surprised to see a ski that could be easily twisted by hand from both ends, but suddenly is super stiff torsionally once the ends were in a fixed position and the middle is twisted. I just don't think that's going to happen.
post #16 of 21
Interesting observation. I am guessing that it would be tough to relate directly to ski preformance as torsitional stiffness is just one of the variables that goes into affecting how the ski will preform. You would have to say something like: All other factors being equal, a torsitionaly stiff ski will __ (whatever)______.

I remember years ago (maybe the 80's) either Ski ir Skiing measured torsitional stiffness in their ski tests. They clamped the waist and used a bracked to twist teither end of the ski. That way they got both a tail and tip torsitional stiffness result.
post #17 of 21
Yeah, we don't get any "real" data on skis from the major mags these days. We've discussed this many times here and there's been multiple promises from other posters that we would someday soon see a site that actually tested the major characteristics of skis in a lab setting - but of course it never happened. About the most interesting thing I've seen this season is the ski flex comparisons from a European ski mag (it's in a TGR thread). They compared a ton of skis and then someone even put together a very nice PDF that allows you to compare multiple flex graphs together.

Anyhow, nothing substitutes for really getting out on skis to figure them out, but it would be great to get more data available so that once you find a pair of skis you like you could compare those characteristics to others that are available and immediately narrow down your choices.
post #18 of 21
I see it as a bit more applicable to sizing a particular model, so one would post center-clamped deflection tests for various lengths and then relate that to the indicated sidecut radius.
post #19 of 21

I was wrong, DtEW; sorry

I still can't reconcile the U.S. armspan data with the ski measurements, but I guess it's one of those times when the experiment blasts the theory out of the water. (I also tried it on a 172 cm ski, and while it's tough to pull off, it can be done.) Mea culpa.
post #20 of 21
Thread Starter 
Quote:
Originally Posted by beyond View Post
Mea culpa.
No harm, no foul.

So does anybody have any interesting observations about their own quiver?
post #21 of 21
void of vice
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