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longitudinal & torsional stiffness of SL skis?

Jaakko Hyvönen

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Hi fellow gate bashers, I recently compared two pairs of SL skis and was surprised by their differences on a hard/icy coarse (comparable to injected one). Their bending/longitudinal stiffness is almost equal (1 mm difference as loaded with 20kg) but there was surprisingly large difference in terms of the edge hold on coarse. The Atomic SL12 (beta construction of 2007) had MUCH better hold. So much so that I first thought that the edges of the other pair (Nordica RD WC 2014) were dull, but not so. I know that some brands have a reputation of being torsionally VERY stiff (Völkl) but do not know if this is only a rumour, or based on general characteristics of their whole line-up as compared to some others.
I haven´t, as yet, tried to measure the torsional stiffness of these skis, should be doable but not as easy as to measure bending. Your advice, advice, experiences would be greatly appreciated. All in all my own advice to anyone planning to buy a new pair of SL skis is that you test them in really hard conditions on a coarse before buying, all the best, jaakko
 

Ivan

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So much so that I first thought that the edges of the other pair (Nordica RD WC 2014) were dull, but not so.
How did you determine whether the edges were dull? Did you check both base and side angles? My 2006 Dynastar Speed Omeglass Comp 64 (race stock) felt like they completely lost ice grip when the base angle was set at 1 degree. I recently had them tuned at 0.5/3, and they became super-grippy again, just as they should be.
 

Ivan

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I guess I would add one more thing. I think Atomic race skis have the reputation for being great on ice, but presumably all race stock skis should be close to each other. I believe @ScotsSkier did several reviews of both GS and SL skis where he said that the difference between different brands is minimal and most (if not all) race stock skis are excellent these days.
 

James

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Despised the feel of the Beta tubes.

You're comparing used skis of unknown mileage? Slalom skis wear out. If raced, they receive more punishment than just about any ski save big air and cliff droppers.
 

Dakine

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Torsional Test Setup.jpg
I did a bunch of home shop testing and wrote a thread on the results for EPIC.
It is gone but I could recover some of it if anyone is interested.
My results were similar, most skis have a longitudinal flex in a small range but the torsional flex is different.
The balance of longitudinal stiffness, front stiffness v. back half stiffness also varies and the shape of the flexed ski also varies.
I didn't get enough data to really draw conclusions but I did conclude that hand flexing skis it totally worthless unless done by a world class tech.

Tip Flex Setup
Tip Flex Setup.jpg


Torsional Flex Setup Above
 

Karl B

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Looks like Dakine has a Starrett protractor there. I understand the design of experiment but not sure why the tapered shim. Why not adjust the protractor to a different angle?
 

ScotsSkier

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The Beta lobe cores were some of the torsionally stiffest Atomic ever made.

Despised the feel of the Beta tubes.

You're comparing used skis of unknown mileage? Slalom skis wear out. If raced, they receive more punishment than just about any ski save big air and cliff droppers.
. Winner winner, chicken dinner...

And longitudinal stiffness is not necessarily a good attribute in a slalom ski. If you can not bend the ski you will not get it to grip or to turn.

For the OP, too many unidentifeid variables here to pass any comment. For example boot, skier weight, ski size, tune, condition of ski etc.
 

Calbearski

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I believe having some objective data on flex would be valuable. For example I ski a 155cm slalom ski and weigh approx. 148 lbs. so my point being would a 110 lb. skier get the same performance bending the ski in a turn. I seriously doubt it, so flex can serve as a roadmap to skis that would work for a particular skier keeping in mind there are other factors to consider.
 

Dakine

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Looks like Dakine has a Starrett protractor there. I understand the design of experiment but not sure why the tapered shim. Why not adjust the protractor to a different angle?

I used the shim to set the protractor to zero angle at zero load.
Then I set the protractor to 10 degrees (maybe) and added load until the bubble zeroed.
So I measured the force necessary to twist the ski 10 degrees.
By zeroing the protractor I didn't have to remember the initial angle while adjusting my attitude.
 

Dakine

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One thing I could never measure was any degradation of the stiffness in well used but not abused skis.
Maybe it happens but I couldn't find it by comparing a relatively new ski to a well used one of the same model.
IMO, performance degradation in used but not abused skis is probably about vibration and damping rather than measurable structural failure.
 

hbear

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I believe having some objective data on flex would be valuable. For example I ski a 155cm slalom ski and weigh approx. 148 lbs. so my point being would a 110 lb. skier get the same performance bending the ski in a turn. I seriously doubt it, so flex can serve as a roadmap to skis that would work for a particular skier keeping in mind there are other factors to consider.
Well that is where we might disagree.

Weight is a factor, but I’d say speed and technique matter much more. Lighter skier can easily still ski the same ski provided their technical ability and speed they ski at overcomes this. The ski has no idea how heavy a person is...just the force vector that is required to bend the ski. (Never mind lever lengths and so forth)

Eg Shiffrin is not 148lbs and I suspect she can get more performance out of the 155 than you can.
 

ScotsSkier

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Well that is where we might disagree.

Weight is a factor, but I’d say speed and technique matter much more. Lighter skier can easily still ski the same ski provided their technical ability and speed they ski at overcomes this. The ski has no idea how heavy a person is...just the force vector that is required to bend the ski. (Never mind lever lengths and so forth)

Eg Shiffrin is not 148lbs and I suspect she can get more performance out of the 155 than you can.

Exactly! the 155 slalom is basically built for 110# girls these days

plus if someone is in the wrong boot /has poor technique no matter their weight they will struggle to bend it in the correct phase of the turn
 
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Jaakko Hyvönen

Jaakko Hyvönen

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Wonderful, many thanks to everyone for the input. This is absolutely one of the best things about the net: one gets in touch with total strangers with same interests. Well, naturally, there is a dark side, all the wackos are able to do the same. But I would guess some people would consider this kind of interest on technical details of sport equipment highly pervert ;-)! Anyway, I´m grateful, especially to Dakine for sharing photos of his setup for measuring torsional stiffness. I think I´ll be able to put up something similar in my man cave. Keep you posted about the results (4 pairs of SL & 2 GS to measure).

And as a reply to Ivan: both pairs filed to 87 side with Razor-Tune, haven´t checked base edge as yet (and never even tried to tune them myself). Don´t have any precision tool but I think bright light and true bar will give at least some info. And my default thinking agrees with Ivan´s: ALL race stock skis should have a good edge hold but that was exactly reason why I was so surprised and puzzled by the difference between the two, especially given the minimal difference in their longitudinal flex. But the pair of Atomic really felt like giving much better hold, when you engaged it felt like 100% sure that it will hold. Naturally a skier at my skill level will not be able to tell all the nuances but this is partly why I was so surprised that even I was able to feel the difference so clearly.

Good turns to everyone for the rest of the season.
 

Swede

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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.
 
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Jaakko Hyvönen

Jaakko Hyvönen

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Crude measurement of torsional stiffness now made on tails of four pairs of SL skis. With a static weight of 20 kg differences not large, and to my surprise NOT into direction that I expected: Atomic SL12 slightly softer than Nordica (for the latter more or less same value for both 165 & 156) but Head i.SL RD (2012) still clearly softer than Atomic although it was the stiffest pair longitudinally. What I learned? Well, at least that skis are a dynamic whole, more than sum of its separate parts. And this is logical, otherwise these kind of simple measurements would have replaced the long hrs that professional athletes spend testing their equipment prior season start. But there are also real scientific studies on the topic, if you google "fredrik wikerman characterisation" you´ll find a thesis from the Swedish digital archive (DiVA). Interesting read.
 

Noodler

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In the golf world, we often cite and reference Structural Rigidity information (EI profiles) for golf club shafts. EI profiling measures the flexibility of the "beam" at multiple points along its axis. These measurement points are taken together to form a profile curve that represents how the flex stiffness varies from end to end.

For many years, golf club builders only used the butt frequency or butt deflection of the club, but it was soon realized that only looking at what the butt of the club is doing isn't even half of the picture. I see similar parallels between golf and ski equipment and in the advances of measurements in the golf world when considering how to measure and assess skis.

If you want to learn more about what EI profiling is, Dave Tutelman has a nice page on the subject. Since he wrote that information, there are now electronic EI profiling machines that were created and are available.

Skis can be considered a beam (similar to a golf shaft) for the purpose of studying their structural rigidity profile and, as such, exhibit similar properties which all could be measured with the correct scientific tooling. The EI profiling machines currently available could be modified and used on skis.

Endre Hals has worked on a standardized "Ski Flex Index" for a number of years (up until about 2014). Here are some links to info on his work:

There also used to be a whole web site dedicated to ski flex.

In addition, here is some interesting reading on the concepts and their application:
So imagine a world where we actually had the EI profiles for all skis (in all lengths). We could then start approaching ski gear acquisition more like the golf world, where the actual performance of the equipment is objectively evaluated and not just subjectively reviewed. Your own understanding of skis that you like and have worked for your morphology, skiing skill, and preferred terrain/conditions would be more easily applicable to your next acquisition by comparing the data. I guess I can dream...
 

Noodler

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I had completely forgotten about this thread until my previous post just received a new "like". Obviously when I wrote that I was completely unaware of SoothSki.com and the new ski comparator tool. So for the sake of completeness (and finishing my thoughts) here is this update and a thread bump.

Now we have a company measuring and providing EI profiles for skis. It truly is a huge advancement in the world of ski equipment assessment.

 

geepers

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I had completely forgotten about this thread until my previous post just received a new "like". Obviously when I wrote that I was completely unaware of SoothSki.com and the new ski comparator tool. So for the sake of completeness (and finishing my thoughts) here is this update and a thread bump.

Now we have a company measuring and providing EI profiles for skis. It truly is a huge advancement in the world of ski equipment assessment.


Be interested if SoothSki.com measure any degradation in torsional stiffness of well loved skis that have reportedly lost their edge hold.
 

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