torsional stiffness vs longitudinal stiffness

[trashcat]

Hey all,

As I think about ski design some more, I've taken to the over simple notion that we want to maximize torsional stiffness while controlling longitudinal stiffness for pop and grip in our stride and minimizing weight. I'm guessing that at a point, we end up with skis that are so torsionally stiff that they're crazy chattery on hardpack.

Is maximizing torsional stiffness while controlling longitudinal stiffness and weight still a main concern of ski design?

...basically I ran across this crazy patent from the 80's designed to maximize torsional stiffness:



and wondered in the modern age of composites and 3d printing if I couldn't actually make something similar in principal that actually made sense on a back country ski...think an upside down leafspring or somesuch.


...or have we already reached a point with our ski construction where torsional stiffness is not a limiting factor in design

[riel]

...or have we already reached a point with our ski construction where torsional stiffness is not a limiting factor in design

I don't know whether ski manufacturers are there already.

I do know that for the 2018 Alpina Discovery 80 skis (equivalent to Fischer Traverse 78), I have encountered a situation where the torsional stiffness was nowhere near enough.

Specifically, the snow was pretty hard, and I had to side step up a few steeper places. The flex and sidecut on that ski are such that side stepping in hardpack may result in just the tip and tail touching the snow, while the middle of the ski is floating in the air.

Several times the tip and tail just twisted away from me, resulting in me going from standing perfectly still to laying on the ground perfectly still, without having moved a muscle.

Extra torsional rigidity may have helped in that situation.

I have a suspicion that my Asnes Ingstad won't suffer from that same effect, but I have not tried yet since the short skins allow me to just go up in places where I had to sidestep with the Alpina Discovery skis.

[lowangle al]

I'm not sure if a ski can have too much torsional stiffness, Id say the more the better. The longitudinal stiffness is the big variable to dial in.

Riel, it sounds to me like you had too much longitudinal stiffness if you couldn't contact the snow with the belly of the ski. I wouldn't expect much of anything good to happen while the underfoot part of your ski is not in contact with the snow, regardless of torsional stiffness.

[riel]

Riel, it sounds to me like you had too much longitudinal stiffness if you couldn't contact the snow with the belly of the ski. I wouldn't expect much of anything good to happen while the underfoot part of your ski is not in contact with the snow, regardless of torsional stiffness.

I was side stepping up some steep slopes with hardpacked snow.

That puts the ski at a much greater angle than it would be flat on the ground. The skis in question flex down to the ground just fine on flat ground, they just don't flex very far "past flat" when trying to get the middle of the skis (with 20mm sidecut) to touch the ground when side stepping up a steeper slope.

[Stephen]

While skiing the Ingstad (one size above recommended) last winter, and traversing a fairly steep slope on hard, wind packed snow, I had the experience of having the skis wanting to wash out from under me. I had to really concentrate on hips into the hill to get the most edge I could, and it still felt tenuous. That day, I was really wishing for more torsional stiffness (not wanting to go for a long slide down a steep, rock-strewn slope).
In softer snow, I had some amazing days on those skis and don’t know if I would have wanted the same stiffness in the softer conditions.
At least for me, sometimes I want the skis to really carve on the edge, and other times a more skidded turn seems more desirable / fun / workable / within my skill level.
Wish I had the 10,000 hours on skis to see into that equation better…
The Gammes felt MUCH more secure in similar conditions (edge hold traversing steep slopes).

[trashcat]

The Gammes felt MUCH more secure in similar conditions (edge hold traversing steep slopes).

...Which makes some sense to me. Narrower and more longitudinally stiffer skis are going to be more torsionally stiff.

Anyway, I appreciate y'alls feedback. I was going to try to do some finite element analysis and maybe some funky parametric design on this as a way to learn about that stuff a bit more. I'm an architecture student and I've learned the very very basics, but nothing really useful.

I'm guessing all the real ski manufacturers have already poured tons of money and brain power into this, so I doubt anything good will come of it, but as an experiment it could be fun.

[Woodserson]

More torsional rigidity is possible, usually with more weight. Think of downhill skis right? but Christ they are heavy.

The question is getting the TR without the extra weight, especially in XCD pursuits where we are not powered solely by gravity.

I'm guessing the ski manufacturers have poured very little money and brain power into this, especially for this niche as they are all of 38 people who really think about this on the planet and half of them are on this site.

I wonder about a honeycomb matrix as an inner layer.

Keep brainstorming trashcat!!

[trashcat]

More torsional rigidity is possible, usually with more weight. Think of downhill skis right? but Christ they are heavy.

The question is getting the TR without the extra weight, especially in XCD pursuits where we are not powered solely by gravity.

That's exactly where I'm going. thanks for the encouragement!

[lowangle al]

Trashcat, what is the intended use for these skis? It sounds like you want them resort capable.

[phoenix]

Both torsional and longitudinal stiffness are key factors in a ski's performance. The balance of the two, along with camber, result in most of the ski's personality. Different degrees of the three will vary, depending on the intended result. Camber is related to longitudinal stiffness, but is also an element of design in itself.