Ski Stiffness - Hero, Whiteout and #1SC Compared

[Dakine]

Best skis for high performance on hard surfaces given a skilled pilot are stiff and have camber. Less skilled pilots may want something less stiff. Highly skilled heavier pilots may want more stiffness. Lighter skilled pilots, having less force to move may want slightly less stiffness. There is also a balance to how far along the ski you want to move those forces, and how much you need to still have underfoot.

Longitudinal stiffness and torsional stiffness are different animals.
I'm getting a real lesson on this from my Blossom #1 SCs.
They are longitudinally limp and torsionally stiff.
I thought they would be a challenge on ice but I'm wrong.

 

[François Pugh]

There is the other side of the coin too; you can think of stiffness as how far the ski has to displace before it provides the required loading. Without torsional stiffness tips and tails will just twist without supplying the grip, even though the underfoot portion of the ski might be tipped appropriately and doing all it can, without the help from the rest of the ski you won't accomplish much.

 

[AlexisLD]

Longitudinal stiffness and torsional stiffness are different animals.
I'm getting a real lesson on this from my Blossom #1 SCs.
They are longitudinally limp and torsionally stiff.
I thought they would be a challenge on ice but I'm wrong.

People are saying that you want a somewhat uniform pressure distribution under your ski when you ski on ice. The rational for that is that if your weight is distributed uniformly, you have less change of reaching the structural limit of the ice at any point and having the ski slip. To have a uniform pressure distribution, you need a ski stiffness distribution (camber and sidecut are also important) that match your weight (as in F = m*a). This involves your mass, but also your acceleration. Acceleration depends on turn shape and speed. Too soft of a ski and it will feel like you are skiing snowblade as all the pressure will be underfoot. Too stiff of a ski and the pressure will be mostly in tip and tail, and you will have a hard time turning. This is why some skis only become "lively" at higher speeds.

I think most of us have experienced that torsionally stiff skis are great on ice (i.e., many "Ti" model are torsionally stiffer, although that is not always the case). However, it is a little hard for me to explain/understand why. The tip of a ski will twist of only 2-3 degrees with respect to underfoot. This doesn't change the pressure distribution much as this displacement is relatively small when compared to the flexion. It shouldn't also change significantly the holding force that the snow can support, as this holding force is supposed to be a linear function of edge angle. So twisting by 2-3 deg less than 30-45 deg shouldn't make a big difference on how much force a ski can hold. I would be surprised if most of us can feel a 5% difference in a ski holding force. Torsional stiffness also changes the vibration frequency of the torsional mode. However, that mode is typically >70Hz. I would be surprised if you can feel that very well through your feet, specially given that this is the tip vibration mode and the underfoot section creates plenty of vibration of its own! I am not saying torsion is not important, it is what people even seem to feel the most. I think however that something more complex is going on with the torsion of a ski. We are still trying to explain why torsional stiffness is important... I would love to get your inputs.

Francois, most/all skis are torsionally stiff enough to allow plenty of bending. What you say is true and helpful to understand how all that works, but I doubt that you can find a ski soft enough in torsion in real live to prevent its bending. You typically will get sub-mm deflexion of the tip (2-3 deg) due to the torsion but you easily get a few-cm of bending deflexion when you put a ski on edge...

 

[François Pugh]

Longitudinal stiffness controls how easily the ski is bent into a curve, and thus indirectly what angle of attack the leading edge of the ski is at (changing the force vectors acting on each minute part of the edge, but let's not get too complicated just yet).

Torsional stiffness controls how little the ski twists from the angle set at the bindings. 3 degrees might not be much, but if it's the difference between 30 degrees and 27 degrees where the critical angle is 29 degrees, it is the difference between holding the carve and slipping out. BTW, I've hand twisted some noodles far more than 3 degrees, and although my grip strength is still ok, it never was anywhere near the force readily available when skiing at speed.

On a related note, I did not like the balance of longitudinal softness to torsional rigidity of my P50s; the torsional rigidity led me to expect grip that wasn't there because the tip developed too high an angle of attack.

 

[Noodler]

People are saying that you want a somewhat uniform pressure distribution under your ski when you ski on ice. The rational for that is that if your weight is distributed uniformly, you have less change of reaching the structural limit of the ice at any point and having the ski slip. To have a uniform pressure distribution, you need a ski stiffness distribution (camber and sidecut are also important) that match your weight (as in F = m*a). This involves your mass, but also your acceleration. Acceleration depends on turn shape and speed. Too soft of a ski and it will feel like you are skiing snowblade as all the pressure will be underfoot. Too stiff of a ski and the pressure will be mostly in tip and tail, and you will have a hard time turning. This is why some skis only become "lively" at higher speeds.

I think most of us have experienced that torsionally stiff skis are great on ice (i.e., many "Ti" model are torsionally stiffer, although that is not always the case). However, it is a little hard for me to explain/understand why. The tip of a ski will twist of only 2-3 degrees with respect to underfoot. This doesn't change the pressure distribution much as this displacement is relatively small when compared to the flexion. It shouldn't also change significantly the holding force that the snow can support, as this holding force is supposed to be a linear function of edge angle. So twisting by 2-3 deg less than 30-45 deg shouldn't make a big difference on how much force a ski can hold. I would be surprised if most of us can feel a 5% difference in a ski holding force. Torsional stiffness also changes the vibration frequency of the torsional mode. However, that mode is typically >70Hz. I would be surprised if you can feel that very well through your feet, specially given that this is the tip vibration mode and the underfoot section creates plenty of vibration of its own! I am not saying torsion is not important, it is what people even seem to feel the most. I think however that something more complex is going on with the torsion of a ski. We are still trying to explain why torsional stiffness is important... I would love to get your inputs.

Francois, most/all skis are torsionally stiff enough to allow plenty of bending. What you say is true and helpful to understand how all that works, but I doubt that you can find a ski soft enough in torsion in real live to prevent its bending. You typically will get sub-mm deflexion of the tip (2-3 deg) due to the torsion but you easily get a few-cm of bending deflexion when you put a ski on edge...

If you guys would add measurements of ski damping effectiveness, that would really put this over the top for me. I often do what I call the "ping test" when I pick up a ski for the first time. I will hold the ski lightly in the middle (on the sidewalls) and then use successive taps down the ski while holding it near my ear. After doing this for over 20 years I've become fairly adept at understanding how much vibration damping I can expect from the ski when I get it on snow.

I am definitely picky when it comes to how "quiet" a ski feels, so having objective measurements of this characteristic would really make ski selection based on the data even more accurate for me.

 

[Noodler]

@SoothSkier / @AlexisLD - maybe I'm missing something (or maybe my browser isn't working correctly with your web site), but I cannot figure out how to generate my own graphs to compare skis. The ski finder brings up skis, but then how can I have it graph the measurements? There is mention of a guide for new users to use and interpret the graphs, but it's supposed to be on the members page and I don't see anything there. Please help...

 

[AlexisLD]

If you guys would add measurements of ski damping effectiveness, that would really put this over the top for me. I often do what I call the "ping test" when I pick up a ski for the first time. I will hold the ski lightly in the middle (on the sidewalls) and then use successive taps down the ski while holding it near my ear. After doing this for over 20 years I've become fairly adept at understanding how much vibration damping I can expect from the ski when I get it on snow.

I am definitely picky when it comes to how "quiet" a ski feels, so having objective measurements of this characteristic would really make ski selection based on the data even more accurate for me.

I am really curious to hear more about how you can correlate "ping test" measurement with on-snow damping? Can you describe more what you like and don't like with respect to the vibration behavior of a ski on the snow? How does a quiet ski feels on the snow?

 

[AlexisLD]

@SoothSkier / @AlexisLD - maybe I'm missing something (or maybe my browser isn't working correctly with your web site), but I cannot figure out how to generate my own graphs to compare skis. The ski finder brings up skis, but then how can I have it graph the measurements? There is mention of a guide for new users to use and interpret the graphs, but it's supposed to be on the members page and I don't see anything there. Please help...

Marius might correct me, but I think we just have the table on the website for now. If you tell us which skis you are interested in, just paste the list here and will will post the graph...

We are still trying to figure out what are the most interesting things that you would like to be able to do with these data so that we can deploy all that throughout the summer.

 

[Noodler]

Marius might correct me, but I think we just have the table on the website for now. If you tell us which skis you are interested in, just paste the list here and will will post the graph...

We are still trying to figure out what are the most interesting things that you would like to be able to do with these data so that we can deploy all that throughout the summer.

I sent an email to Marius this morning with the skis I would like to see graphed. It wasn't clear on the site that members cannot generate all the graphs shown in the blog posts. If you add that capability then the usefulness of the site would take a quantum leap. Even just permitting downloading of the data as a .CSV file would at least allow members to create the graphs themselves.

 

[Noodler]

I am really curious to hear more about how you can correlate "ping test" measurement with on-snow damping? Can you describe more what you like and don't like with respect to the vibration behavior of a ski on the snow? How does a quiet ski feels on the snow?

A ski that gives back a very loud ping sound and that you can feel the vibration in your hand is "highly reactive" and will tend to transmit on-slope vibrations into your boots/feet quite readily. Skis with superb damping capability feel smoother and more solid on the slope. The vibrations due to bad snow conditions and other snow factors aren't transmitted into your body.

I have quite a few Stockli skis in my quiver. Stockli is renowned for their incredibly silky smooth on-slope feel. If you do the ping test with most Stocklis all you hear is a bit of a "thud", not a ping and there aren't really any vibrations transmitted into your hand. Pick up most any other ski (there are a few exceptions) and this is not what you will sense when doing the ping test. Skis with really poor damping when ridden on poor snow can practically rattle your fillings out.

 

[AlexisLD]

A ski that gives back a very loud ping sound and that you can feel the vibration in your hand is "highly reactive" and will tend to transmit on-slope vibrations into your boots/feet quite readily. Skis with superb damping capability feel smoother and more solid on the slope. The vibrations due to bad snow conditions and other snow factors aren't transmitted into your body.

I have quite a few Stöckli skis in my quiver. Stöckli is renowned for their incredibly silky smooth on-slope feel. If you do the ping test with most Stöcklis all you hear is a bit of a "thud", not a ping and there aren't really any vibrations transmitted into your hand. Pick up most any other ski (there are a few exceptions) and this is not what you will sense when doing the ping test. Skis with really poor damping when ridden on poor snow can practically rattle your fillings out.

I get that. These "thud" are very impressive in the store! It definitely feels super solid!!

But here is what I think about vibrations. The challenge with skis is that if you, for exemple, clamp them at the boot and hit the tip, you will get 30+ cycles. This is very very little damping (1-2%). Even Stockli skis are doing that. Adding some rubber to a ski won't affect this response significantly. It adds maybe 1-2% of damping. In comparison, even the hardest World cup ice/snow won't let a ski oscillate for more than 2-3 cycles (probably on the order of 25%). A few percent more damping in a ski won't change its response to an impact. And the added damping in the ski won't have time to make a difference before the snow damps all the motion anyways...

There are a lot of study on aluminum, steel and carbon bike frames (and also golf shafts) that show that the comfort of all of these is exactly the same if you prevent the user from hearing the "sound" of the frame (kind of like a placebo effect). It could be different for skis, but there is no indication that the damping of the modes that you can hear might not be related to the modes that are important on snow. Damping is also highly non-linear with frequency, temperature, etc.

That is why we have chosen not to measure damping at this point for "all the skis in the world". We might add it eventually...

When you think about it, it is kind of strange that Titanal is often included in a damped construction. Titanal is an aluminum alloy. Aluminum has almost no damping. Titanal however adds mass to a ski mass for sure, and often it also add torsional stiffness.

From our stiffness distribution measurements and a rough estimate of the mass distribution of a ski, we can predict the full vibration response of a ski under different boundary conditions (bending and torsion modes, natural frequencies). We are using this to develop some new ways of thinking about a "damp ski"... more to come soon!

You might find this article interesting:
Comparative Study of Ski Damping Technologies by Accelerance Maps - https://www.mdpi.com/2504-3900/49/1/49

 

[Lvovsky /Pasha/Pavel]

@Noodler beat me to the question. But I’ll pile on. @AlexisLD and @SoothSkier looks like some of your models are dynamic. What are you assuming for damping constant/matrix (C)? If you ha EI and m, you should be able get a pretty descent estimate at C.

it would be great if I could search your data for the skis I like now and get recommendations for similar skis. Or get comparison to skis I am considering.

you may have found a good source of knowledgeable skiers here to refine your product. There will be no shortage of options and suggestions.

 

[Noodler]

I get that. These "thud" are very impressive in the store! It definitely feels super solid!!

But here is what I think about vibrations. The challenge with skis is that if you, for exemple, clamp them at the boot and hit the tip, you will get 30+ cycles. This is very very little damping (1-2%). Even Stöckli skis are doing that. Adding some rubber to a ski won't affect this response significantly. It adds maybe 1-2% of damping. In comparison, even the hardest World cup ice/snow won't let a ski oscillate for more than 2-3 cycles (probably on the order of 25%). A few percent more damping in a ski won't change its response to an impact. And the added damping in the ski won't have time to make a difference before the snow damps all the motion anyways...

There are a lot of study on aluminum, steel and carbon bike frames (and also golf shafts) that show that the comfort of all of these is exactly the same if you prevent the user from hearing the "sound" of the frame (kind of like a placebo effect). It could be different for skis, but there is no indication that the damping of the modes that you can hear might not be related to the modes that are important on snow. Damping is also highly non-linear with frequency, temperature, etc.

That is why we have chosen not to measure damping at this point for "all the skis in the world". We might add it eventually...

When you think about it, it is kind of strange that Titanal is often included in a damped construction. Titanal is an aluminum alloy. Aluminum has almost no damping. Titanal however adds mass to a ski mass for sure, and often it also add torsional stiffness.

From our stiffness distribution measurements and a rough estimate of the mass distribution of a ski, we can predict the full vibration response of a ski under different boundary conditions (bending and torsion modes, natural frequencies). We are using this to develop some new ways of thinking about a "damp ski"... more to come soon!

You might find this article interesting:
Comparative Study of Ski Damping Technologies by Accelerance Maps - https://www.mdpi.com/2504-3900/49/1/49

I think you're conflating two separate characteristics. I was specifically discussing the higher frequency vibrations that you can feel in your feet coming up from the skis interaction with the snow surface. I was not referring to the oscillations of ski tips or tails caused by terrain deflection (or the release of the ski bend). Ski damping that I want to see included in your data has nothing to do with how a ski flexes or how it oscillates upon release of bend. It needs to be how well a ski stops the transmission of vibration from one side of the ski to the other side (whether that be bottom to top or tip to tail).

BTW - The vibration you feel in your feet has nothing to do with what your ears are hearing. I'm well aware of that phenomena from the world of golf, where golfers quite often equate the feel of a club from what their ears are hearing at impact.

 

[Noodler]

I thought I'd throw two very good examples of how a ski's damping of vibrations is not directly tied to the longitudinal flex stiffness. The Stockli Spirit O Two is a fairly soft ski with 2 sheets of metal and wood sandwich construction. It also uses bamboo sidewalls. It's one of the silkiest and smoothest skis I've ever ridden. Many DPS skis that have their carbon layup over the years have been quite stiff, both torsionally and longitudinally, and readily transmit every vibration straight into your feet (although their latest Alchemist construction has certainly improved that feel). So I don't see how the stiffness of a ski directly relates to the vibrations that are felt up into your feet.

 

[AlexisLD]

@Noodler beat me to the question. But I’ll pile on. @AlexisLD and @SoothSkier looks like some of your models are dynamic. What are you assuming for damping constant/matrix (C)? If you ha EI and m, you should be able get a pretty descent estimate at C.

Pasha, as detailed above, I believe that ski motion when in contact with the snow (and thus feelings) are not heavily influenced by damping. That is still a working hypothesis. I am not saying that ski motion is not at all influenced by damping, just that so far we made the choice of focussing on collecting informations like EI and GJ distribution. We feel that by having these values we would have enough information to help a vast majority of skiers in choosing the right ski for them. It seems that you guys might be more sensitive than most people about what you ski. That is great and I wish we could build machine to measure everything. I really do.

We measure these properties on a static testbench, so we can't measure damping. We need another testbed to do that. However, we can use our stiffness and mass values to simulate a bunch of different test cases, e.g., a ski hitting the ground on edge, dragging a ski on ice, etc. That damping of the ski is mostly irrelevant to these simulations. You dissipate most of the energy through ground impact (non-elastic collision, chopping ice, etc). I can't say for sure if damping is important to what your feet feel on the snow, but I am close to 100% sure that even doubling the damping in a ski won't affect it motion.

You might be interested in this paper: http://www.sandv.com/downloads/0712foss.pdf
The authors have evaluated a number of methods used by different vendors to add damping to a ski. Some vendor claims are based on modal analysis testing with idealized (free-free) boundary conditions. The damping measured during such tests is usually around 0.5%. However, in the real world, boots, bindings and suspensions usually bring a ski’s baseline damping to 3% to 5%. Therefore, a 30% increase in free-free ski damping (say from 0.50% to 0.65% of critical damping) may not be very noticeable on the snow.

This paper basically claims that what you feel on the snow is not the damping but the shift in frequency in the torsion vibration mode.

it would be great if I could search your data for the skis I like now and get recommendations for similar skis. Or get comparison to skis I am considering.
you may have found a good source of knowledgeable skiers here to refine your product. There will be no shortage of options and suggestions.

We are working to deploy this. Thanks everyone for the enthusiasm, we just need time to build it! Bring on the suggestions and desires!

Our tools work really well to search for a similar ski or to compare a few models. Until we have them all available on our website so that everyone can give it a try, I am happy to do a telecon and show you how we can use it. If you are interested, PM me and we will find a time.

 

[Noodler]

@AlexisLD - you could quickly add the ability to have filtering by brand included in the ski finder. Not sure why that isn't an option.

 

[AlexisLD]

I think you're conflating two separate characteristics. I was specifically discussing the higher frequency vibrations that you can feel in your feet coming up from the skis interaction with the snow surface. I was not referring to the oscillations of ski tips or tails caused by terrain deflection (or the release of the ski bend). Ski damping that I want to see included in your data has nothing to do with how a ski flexes or how it oscillates upon release of bend. It needs to be how well a ski stops the transmission of vibration from one side of the ski to the other side (whether that be bottom to top or tip to tail).

What would be your guess of the frequencies that you can feel in your feet?

If you had very short skis with not tips/tails (i.e., boot/binding length), do you think that varying damping, stiffness and mass would change the vibration response that you feel?

 

[Noodler]

What would be your guess of the frequencies that you can feel in your feet?

If you had very short skis with not tips/tails (i.e., boot/binding length), do you think that varying damping, stiffness and mass would change the vibration response that you feel?

My suspicion is that these are "higher" frequency vibrations, but not necessarily actually considered "high" in the grand scheme of the frequency spectrum.

Ski mass clearly changes the frequency of the vibrations. Not so sure if stiffness has the effect most believe (at least I have had enough real world observations where stiffness does not have a clear correlation to how "quiet" the ski feels).

Renoun has developed some interesting designs using their non-Newtonian material they're now calling Vibe-Stop. So there are some manufacturers who understand how much this characteristic translates to a skier's impression of the ski performance. Are you also familiar with Sandwich Tech (another builder in the Northeast) who also focus on improving the ski feel through the elimination of these vibrations.

 

[AlexisLD]

to how "quiet" the ski feels

What do you mean by "quiet"? When do you feel that (when in the turn, on what kind of terrain, etc)? Can you describe what you feel?

 

[Noodler]

What do you mean by "quiet"? When do you feel that (when in the turn, on what kind of terrain, etc)? Can you describe what you feel?

I use the term quiet to relate the lack of the "higher" frequency vibrations that are felt through your feet. Not for the oscillations of the tip of the ski.

Great snow conditions will hide this concern for many skis, but get those skis on ice or other crappy snow and the difference between a ski that will absorb those types of vibrations and those that cannot is readily apparent.