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longitudinal & torsional stiffness of SL skis?

AlexisLD

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So are torsional stiffness measurements not precise enough to show a difference? There’s little doubt amongst long term skiers that skis lose their grip with advanced age of use.
Where would you measure it? I’d imagine measuring torsional stiffness in front of the binding piece is not easy.
Bending and torsional stiffnesses measurements are both within 5%. We measure stiffness distributions, so all along the full length of the ski.

The original idea (one of them) was that we would measure a ski from the factory, and that stiffness changes would be >20%. We could then easily compare skis out there to the reference. But we never found that to be true...

On thing that can change with use is camber. I also agree that torsional stiffness is part of the equation that described edge grip, but it might not be the only thing. If torsional stiffness doesn't change with time, it must be something else.
 

Noodler

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Bending and torsional stiffnesses measurements are both within 5%. We measure stiffness distributions, so all along the full length of the ski.

The original idea (one of them) was that we would measure a ski from the factory, and that stiffness changes would be >20%. We could then easily compare skis out there to the reference. But we never found that to be true...

On thing that can change with use is camber. I also agree that torsional stiffness is part of the equation that described edge grip, but it might not be the only thing. If torsional stiffness doesn't change with time, it must be something else.

Good point on not only measuring stiffness changes, but camber changes as well. If any company was in a position to actually do this, it's SoothSki, but as you noted, there has to be a good reason to do so (aka "is there some kind of revenue potential").
 

Noodler

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All this assumes that stiffness is all that matters in judging ski life…

I'm all ears. What else would provide the "power" for the ski? The flexural and torsional stiffness are the engine of the ski's rebound and edge hold. When we judge how much "life" is left in the ski, what else should we look at?
 

Swede

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And when you can’t—the ski is finito. Inme that happens before a race ski ”die”. Unless you have been skidding around with dull edges.
 

mdf

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Hmmm... I've got three pairs of thee same ski (different years but I don't think they changed). Worn out, skied 2 weeks, and never used. Unfortunately I don't have a spring scale.
 

KingGrump

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That's what you're implying here, correct?

Yup. This is the first time during early season where I have enough time on firm snow to discern the difference.

On thing that can change with use is camber. I also agree that torsional stiffness is part of the equation that described edge grip, but it might not be the only thing. If torsional stiffness doesn't change with time, it must be something else.

The camber on a pair of race ski is definitely reduced after a season. Some even develop tail rockers. :ogbiggrin:
We do not race. We free ski a lot on race skis.
 

Noodler

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OK, I think we can all agree that skis need edges; at least enough material to be sharpened and sufficiently provide grip on the snow surface. So for the ski "life" critical aspects, we've got the flexural and torsional stiffness, amount of camber, and edge material. I think that pretty much covers it. It's the torsional stiffness that is the most difficult to see of test by hand. That's the one that needs specialized equipment for measurement.
 

AlexisLD

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It's the torsional stiffness that is the most difficult to see of test by hand. That's the one that needs specialized equipment for measurement.

You could build something quite easily to test that if you are just interested in the change of total torsional stiffness over time (vs the full torsional stiffness profile of the ski to compare with other skis).

You could just put a bar perpendicular to the ski and apply a weight to twist it. You could then use your phone to measure the twist. Phones have a resolution of about 1 deg. You would thus need to create at least 20 deg of twist to keep the accuracy below 5%. You would also need to keep everything else (weight, bar, clamp) exactly the same. It is even simpler with flex!

But again, if someone is interested in running this comparison I am happy to do the measurements with our machine.
 

slow-line-fast

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Strange thing happened today. Took out an older pair of FIS SL today. Ski had about 50 days on them. Skis have nose picker tips. The skis had a fresh base grind and my usual hand tune. Normally, the skis are skied with the nose pickers pointing in. So there is a definite preference which edges are the inside edges.

Snow was very firm today. Skied the first run with nose pickers pointing in. There was definitely a lack of grip feel. Second run, the same lack of grip feeling persisted. Switch skis left to right (nose picker pointing out) on the third run. Felt an improvement with grip.
Half way through the day, switched back. That lack of grip feeling returned. Switched again. All is right with the world again. Finished the day with the nose picker pointing out.

Hmmm. :huh:
This sounds like a little experiment that I have never done in any sort of controlled way but that might support my superstitions. I typically mark skis L and R, ski them that way most of the time, then if I need some extra edge grip (very icy), switch them - but only if I'm fairly sure I won't hit any rocks, etc. My spurious logic had been to concentrate the edge dings / wear and tear on one set of inside edges, keeping the other set cleaner. But this spurious logic is a bit undercut by the fact that I often ride the inside ski too much. I hadn't considered whether there could be any long term effect on material stiffness that would be specific to one side of a ski vs the other. Who knows.

The effect (if it exists) could also be placebo - if I think the ski will hold better, I may ski less defensively and voila, the ski holds better.
 

Noodler

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☝Placebo is strong. If you have faith in your gear, it miracolously holds better. It is a fantastic thing

It's only a "placebo" until it can be quantified via scientific measurement. We're getting there thanks to the folks at SoothSki. They're already providing a ton of valuable data and I believe that given some more time, we're going to see these questions get answered regarding the life span of skis and how they change over that time.
 

Swede

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It's only a "placebo" until it can be quantified via scientific measurement. We're getting there thanks to the folks at SoothSki. They're already providing a ton of valuable data and I believe that given some more time, we're going to see these questions get answered regarding the life span of skis and how they change over that time.

Great. But we already have a scientific measurment/tool that tells us when a pair is loosing its oomph — it’s called a clock.
:)
 

ScottB

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As I said before, we measured the "wear" on a few skis. We didn't find any change in stiffnesses. Can I say that nothing change? No, nobody could say that with such experiment. One could always come up with one more thing that you could have measured, or different test conditions. I just know that continuing this experiment would be very difficult/long, and there is a good chance that we would not find anything again. I was asking you guys to provide reasons to tackle this so that I can change my mind about this! :)

I been skiing for decades, and skis breakdown over time. I doubt most skiers who have owned a pair of skis for a long time would disagree (maybe I am wrong). The two things I notice the most are longitudinal flex gets softer (especially the front/tip area) and they loose their pop or rebound energy. I watched your video, and if you are just measuring flex profiles (or displacement) by bending the ski to the same position every time, you won't be actually measuring that. Do you actually measure force versus displacement at various positions on the ski? I don't want to make any incorrect assumptions about what your machine does.

To your question: I had a pair of K2 710 FO skis. They were foam cores and that was new at the time. They broke down and became really soft before the end of one season. It took a few years, but the process of natural selection determined foam cores don't hold up as long a wood cores. Your machine would have caught that right away. These days, I agree with basically everything you said about skis. They are almost all wood cores, and seem to last a reasonably similar time period. I will say, since I am 6'4", 240 LBS, and a race coach, I can break down a ski faster than most people can because I can stress them more than most. I have an old SL ski (not FIS model) that has significantly softened after about 150 days. I have another ski, fully cambered twin tip, that is in the same boat after the same time frame. I still ski both, but notice the change. I have never measured anything, this is just by my feel.


Strange thing happened today. Took out an older pair of FIS SL today. Ski had about 50 days on them. Skis have nose picker tips. The skis had a fresh base grind and my usual hand tune. Normally, the skis are skied with the nose pickers pointing in. So there is a definite preference which edges are the inside edges.

Snow was very firm today. Skied the first run with nose pickers pointing in. There was definitely a lack of grip feel. Second run, the same lack of grip feeling persisted. Switch skis left to right (nose picker pointing out) on the third run. Felt an improvement with grip.
Half way through the day, switched back. That lack of grip feeling returned. Switched again. All is right with the world again. Finished the day with the nose picker pointing out.

Hmmm. :huh:

I feel this quite often with almost all of my skis. Which ever ski I tend to favor for the right one, has slightly poorer edge grip. Nothing really to do with left or right feet, just the preferred inside set of edges. My inside edges almost always have more ding's and imperfections than my outside edges. When I flip the skis, I get a better grip out of the skis. I try to change them up now, I figured this out a number of years ago. You did say you had a fresh grind, so in theory they should have been the same, but maybe not in reality. I use magnifying goggles to inspect my edges and can see the minor imperfections. After a grind they are mostly gone, but the deep dings are still visible. I also believe your skis could be degraded some on the heavy usage edge side, too.

I sharpen my edges razor sharp, always. Sometimes they are too sharp. A few hockey stops mellows them out pretty quick. About half way through the day, on a boilerplate day, I switch left/right skis and always feel improved grip till they dull up a bit. I can feel a reduction in sharpness on a boiler plate day in about 4 hours. Not saying my edges are dull, but not as sharp as the first run.
 

ScottB

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Hyvää!

All FIS SL skis are torsionally stiff; might of course differ a little (also from pair to pair) but they're all about as stiff as alpine skis come in that respect. Longitudal flex will vary in pattern and overall but they are all somewhat stiff. They all grip if edges are sharp (and you ski them the right way) but have a different feel to them (I haven't enough experience in all brands and year models to give a fair overview, someone like @Scotskier perhaps?). Some have a little more 'pop'/energy, some are damper/deader, but they are all designed to do the same job--so rather nuances than big differences inme. Tune makes a helluva difference. But personally I think people obsess about edge angles too much, a sharp 88 tune will bite very good, not a whole lot of difference too an 87 inmo--if it's good. Base bevel impacts how "fast" the ski will bite and ... the temper of the ski a lot more than side bevel--0.3, 0.5 or 0.7 has less impact on bite. Some athletes ski with (barely) no base bevel at all--which most find completely unskiable as the ski will hook up just by thinking about making a turn. You will have to be very skilled managing your pressure and be on edge all the time (which is basically the idea of a SL ski anyways). @Primoz will know most things about tuning, since he has real pro tech experience ... and he is from Slovenia (like most good techs)! You can order FIS SL:s in different (longitudal) flex from the factory. I have a pair of Salomon (Atomic) FIS LAB 16/17 in the soft version. But I am 48 y/o. It is a damp ski, with a very gradual flex, if that makes sense. Easy going. As stated earlier, your ability and intended usage is more important than your weight or height. Quite normal that strong U16 (14-15 y/o) girls ski the 155 cm FIS SL:s. And what is most fun for you.

EDIT: Most "normal" people never tune the base. Just polish it up with a fine stone to keep it smooth and get rid of burrs.

Agree with what is said above. I would like to add two things to this discussion.

1. There were some posts on Blister from a ski design engineer about what determines a skis' edge grip on firm snow. Besides sharpness, he said really high torsional stiffness was not always good. It was the pressure distribution profile over the entire edge length that made one ski grip better than another. My interpretation of this was: at each point along the length of the ski, it had a limit on how much force it could hold without letting go, or slipping. The ski design had to distribute the total force so that no one spot got over loaded. IF any one spot was overloaded, it would let go and the ski would slip. The limit varied from tip to tail, as the ski did not want a constant force along its length, but a certain force profile or curve shape. The ski with good torsional stiffness and turning force properly distributed to keep all points below their grip limit would have the best grip. He didn't get into the details of edge angle and all the variables, but he said the ski with the best torsional stiffness was not always the ski with the best grip.

I think I experienced this with my SL ski. It is not a FIS ski, consumer SL. It was stiff as a 2X4, both longitudinal and torsional, and it did not have great grip. I skied a FIS SL ski that was much shorter and softer and had much better grip.

2. Tune makes a huge difference on grip, and base bevel is a major factor in edge grip. Not quite as direct as side edge angle, but close. I have experimented personally with base edge angles from 1.0, .75, .5, .25 on the same ski. I found I liked the .25 angle best. It was not hard to modulate. The lower the angle the quicker the hook up, but not a huge difference between each set. It was noticeable, but a .25 deg base was not outrageously quick. I never did 0 deg base, but I predict it is just a little quicker than the .25. I skied 0 base on my old straight slalom skis all my life (0 base and 0 side). With each lower base angle, the edge hold or grip improved. All my trials were with a 4.0 degree side angle, that was never changed. The .25/4.0 was definitely better edge hold than the 1.0/4.0 tune.

I tried another racers ski (FIS GS ski) that was tuned to 2.0/7.0. That ski was hard to handle. When it hooked up (which took some rolling on edge to get past the 2.0 base) it was not going to let go at any speed on any surface. It had the most "unlimited" grip I have ever felt. You had to roll it flat to get it to release. That ski was hard to modulate and felt like an ACL tear just waiting to happen. Not a tune I would race with or recreational ski with. I will say if you want better grip on ice, going past 4.0 deg. side will definitely get you better edge grip. I think the 2.0 base was to compensate for, and make the 7.0 side angle releasable.
 

AlexisLD

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It's only a "placebo" until it can be quantified via scientific measurement. We're getting there thanks to the folks at SoothSki. They're already providing a ton of valuable data and I believe that given some more time, we're going to see these questions get answered regarding the life span of skis and how they change over that time.

Just to be sure:

Placebo is to feel a difference when there is none but you think there is one.

Feeling a difference when there is something different, even if you haven't measure it, is not the same.

Both are real. But I can make you think whatever I want with placebo. It is a mindset game. This is not the case when there are real differences between skis (measured or not).
 

AlexisLD

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This sounds like a little experiment that I have never done in any sort of controlled way but that might support my superstitions. I typically mark skis L and R, ski them that way most of the time, then if I need some extra edge grip (very icy), switch them - but only if I'm fairly sure I won't hit any rocks, etc. My spurious logic had been to concentrate the edge dings / wear and tear on one set of inside edges, keeping the other set cleaner. But this spurious logic is a bit undercut by the fact that I often ride the inside ski too much. I hadn't considered whether there could be any long term effect on material stiffness that would be specific to one side of a ski vs the other. Who knows.

The effect (if it exists) could also be placebo - if I think the ski will hold better, I may ski less defensively and voila, the ski holds better.

That is really interesting. Can you tune them and try to ride them without knowing which one is L/R. You could have a friend switch them for you each run, and you could tell him which one is L/R. If you score higher than 50% (random) for 10+ runs, then we are onto something! ;-)

My experience doing this has been more on the side of "if I think the ski will hold better, then the ski holds better"...
 

AlexisLD

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I been skiing for decades, and skis breakdown over time. I doubt most skiers who have owned a pair of skis for a long time would disagree (maybe I am wrong). The two things I notice the most are longitudinal flex gets softer (especially the front/tip area) and they loose their pop or rebound energy. I watched your video, and if you are just measuring flex profiles (or displacement) by bending the ski to the same position every time, you won't be actually measuring that. Do you actually measure force versus displacement at various positions on the ski? I don't want to make any incorrect assumptions about what your machine does.
This is so hard to explain... if someone has a better way, please tell me. People seem to be too used to think only about the deformation, and not about the stiffness. The curve that we provide are not the deformation profile. They are the stiffness profile.

We flex the ski at one point, but what we are actually measuring when we talk about bending/torsional stiffness profiles is the measurement of the stiffnesses at each point along the full length of the ski. We flex only at one point, but we measure the deformation at all points and can thus calculate the stiffness at all points.

To your question: I had a pair of K2 710 FO skis. They were foam cores and that was new at the time. They broke down and became really soft before the end of one season. It took a few years, but the process of natural selection determined foam cores don't hold up as long a wood cores. Your machine would have caught that right away. These days, I agree with basically everything you said about skis. They are almost all wood cores, and seem to last a reasonably similar time period.

Yes, foam core from the 80' were not good. Manufacturers were trying to be cheap and people didn't like it. I think there is more foam than we think in current skis, but I also think that it is much better than we think.

I will say, since I am 6'4", 240 LBS, and a race coach, I can break down a ski faster than most people can because I can stress them more than most. I have an old SL ski (not FIS model) that has significantly softened after about 150 days. I have another ski, fully cambered twin tip, that is in the same boat after the same time frame. I still ski both, but notice the change. I have never measured anything, this is just by my feel.

If you want, we could measure your skis before and after you used them. If you think that it soften by more than 5%, we should be able to measure it.

I feel this quite often with almost all of my skis. Which ever ski I tend to favor for the right one, has slightly poorer edge grip. Nothing really to do with left or right feet, just the preferred inside set of edges. My inside edges almost always have more ding's and imperfections than my outside edges. When I flip the skis, I get a better grip out of the skis. I try to change them up now, I figured this out a number of years ago. You did say you had a fresh grind, so in theory they should have been the same, but maybe not in reality. I use magnifying goggles to inspect my edges and can see the minor imperfections. After a grind they are mostly gone, but the deep dings are still visible. I also believe your skis could be degraded some on the heavy usage edge side, too.

I sharpen my edges razor sharp, always. Sometimes they are too sharp. A few hockey stops mellows them out pretty quick. About half way through the day, on a boilerplate day, I switch left/right skis and always feel improved grip till they dull up a bit. I can feel a reduction in sharpness on a boiler plate day in about 4 hours. Not saying my edges are dull, but not as sharp as the first run.

Cool observations! I would love to be able to measure the edges!
 

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