longitudinal & torsional stiffness of SL skis?

[AlexisLD]

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.

Lot of good thoughts in there that I tend to agree with. You are working with the holding limit of the snow/ice. You want some penetration to create a channel into which the ski will hold, and for that the ski needs to apply force at the right spots. However, too much force might break the snow/channel that you just created. The snow has an "edge grip" limit too!

That being said, I would add two things:
1. If you take a beginner vs expert ski, the torsional stiffness varies by 2-3x. Edge grip is clearly increasing over that span. However, there is too much edge grip in an expert ski for a beginner, but not too much for an expert. We have to differentiate "so much torsional stiffness that it would decrease the edge grip" (not 100% sure I would agree with that because of #2) and "too much torsional stiffness for most people" (100% agree with that one).
2. Bending stiffness and the shape of the ski are the main thing that determine the pressure distribution on the edge. Torsional stiffness doesn't change the pressure distribution by much.

The reason I am claiming #2 is that we calculated it. It is not too hard to visualize. When you think about it, a ski really doesn't twist much when you compare that to how much it flex. Maybe torsional stiffness just contributes to how the ski engages with the snow under a certain amount of pressure? There might also be other dynamical effects.

My running theory for now is that a good ski for you should give you about X deg of twist when you ski it, where X on the order of 2-3 degrees. If the ski is too stiff, it will twist less and feel overly tuned (probably like a 0 deg base edge angle). This probably comes from the fact that you want the tip/tail to release before the underfoot section during transitions so that it doesn't send you spinning (you want the same behavior in an airplane with regard to the stall characteristics of the wing tip). If the tip/tail twist too much, it will feel like it is missing some edge grip (engage later, release too early, ... like for a higher base angle). Reduced edge grip at the tip/tail will not make for a very stable ski either, but in a different way.

This all becomes very personal though, because the twist will depends a lot on the load that you apply to the ski. More load will require more torsional stiffness...

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.

 

[Vitamin I]

...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.

That is an interesting observation; especially since the skis are a clearly "marked" left and right and were probably skied that way predominantly (if not always). I trust your tuning ability, so I would discount any possibility of the tune being asymmetrical (especially on both skis). So I think all that leaves would be that the skis actually are physically worn out in a particular pattern and can be "revived" by utilizing parts of the skis' internals that were less "used up". That's what you're implying here, correct?

Maybe. Or maybe he didn't rule out asymmetric binding wear. For example, maybe the outside toe wings developed more loose play than the inside toe wings. If so, after that wear, then when you switch Right/Left, then you might get tighter lateral transmission for better edge grip. ...Maybe. Or, on old skis, the outside binding screws might even become looser than the inside binding screws. Or maybe other binding wear asymmetries. Etc.

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.

Is it valid to completely neglect lateral rigidity of the ski underfoot? Maybe yes, if this thread is only about narrow SL skis. But I wonder... Given that some of you guys can sense the tiny difference between 0.25 and 0.50 degrees base bevel, then maybe it takes only a tiny bit of undesired lateral bending underfoot before you can feel that tiny difference? (Disclaimer: I am clueless about SL skis. I'm just spewing ideas here.)

.

 

[Vitamin I]

...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.

As I wrote above, @AlexisLD, do you think it is valid to completely neglect lateral stiffness underfoot? After 50-100 days of wear? If yes, then maybe I would start accusing the worn bindings for any decrease of edge grip. Not sure.

This is so hard to explain... if someone has a better way, please tell me...

...We flex only at one point, but we measure the deformation at all points and can thus calculate the stiffness at all points.

Maybe if you say the words "angular deformation", then new people might think "This sounds more advanced than the methods I saw before." hahaha

.

 

[Swede]

Re lateral under foot, for race skis, the ski is only a part of the ”ski”. A race plate and the binding makes up a lot of the ”skis” flex performance under foot. Just taking the ski into account and leaving the other parts on the table will only give you half of the story.

@AlexisLD nails it in previous post. Surface tends to be s limiting facor (why race courses are injected). Can’t really have too much torsional stiffness, unless you want skis to loose grip and skid at some point (not really relevant for racing).

 

[oldschoolskier]

Something that is important is the difference between True Race skis (R) and Consumer Race skis (C) is the consistency of the flex and torsion pattern and its progression.

So R may take more effort to flex or twist X degrees at distance Y from the tip, C on the other hand takes less.

Start looking at changing these variables and you will find that the R skis are more predictable in response compared to the C skis.

So what does mean in ski ability, R may take more effort but C is more difficult to find the right effort involved to get a consistent performance. This changes the view from strength to ease at which the ski is learned to ski, as long as the ski is within the flex range of the skier (and other equipment, boots).

This explains the FIS SL ski compared to consumer SL ski comments about FIS being more pleasant and easier to ski (if they are not too stiff).

 

[AlexisLD]

As I wrote above, @AlexisLD, do you think it is valid to completely neglect lateral stiffness underfoot? After 50-100 days of wear? If yes, then maybe I would start accusing the worn bindings for any decrease of edge grip. Not sure.

The reason I am not thinking about much is because I would assume that this deformation is on the order of 0.1 mm. This is really small!

Ski flexing will get you on the order of 10 cm of deflection at the tip.

Ski twisting will get you a few degrees of twist, which along the full length of the ski will get you on the order of sub-cm deflection at the tip.

I would think you would get more play in your binding, boot, ankle than in the lateral stiffness. But I haven't measured any of that. I am trying to go from the simpler problems (low hanging fruit) to the more difficult. When doing sensory analysis (evaluation what people can feel for real), it is good to go from the obvious to the less obvious... it is surprising by how much people are not that sensitive in blind tests.

I like the theory that a "edge that has been hit repeatedly" will never be as truly as sharp as a new edge...

 

[KingGrump]

Maybe. Or maybe he didn't rule out asymmetric binding wear. For example, maybe the outside toe wings developed more loose play than the inside toe wings. If so, after that wear, then when you switch Right/Left, then you might get tighter lateral transmission for better edge grip. ...Maybe. Or, on old skis, the outside binding screws might even become looser than the inside binding screws. Or maybe other binding wear asymmetries. Etc.

Maybe it's asymmetrical wear of my various body parts. Then there is that right brain, left brain crap.
Too much thinking. Sucks the fun out of skiing.

I much rather ski than think.

 

[Bruno Schull]

@ Noodler--you had a nice list here of things that might break down/change over the life of a ski:

"So for the ski "life" critical aspects, we've got the flexural and torsional stiffness, amount of camber, and edge material."

Maybe we could add the structural integrity or other physical characteristics of the resins and glues and carbon and other layers? This would effect the stiffness, of course, but maybe it would also effect elusive quantities like "dampness" or "liveliness" and so on.

And boot and binding wear...

 

[Jaakko Hyvönen]

Revisiting this thread started 3 yrs ago. A variable that I was NOT thinking originally is the camber of the skis and I think that is the culprit in my original experience with Atomic vs. Nordica. The latter have now practically lost it totally while the former still have it intact, I guess due to their construction with carbon (?) longitudinal inserts to make skis stiffer and hold their form. Originally was very happy to be able to order a pair of Nordica 165 softer than normal but probably the reason for their softness was that the core had more resin and gradually disintegrated easier than stiffer ones. A total dilettante with these matters and thus would appreciate comments from those who really are (former) insiders, or otherwise know these matters.

 

[breck]

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

Just wow, these folks are doing very unexpected and hopefully useful work. I'll be donating $20.