super light freetourer

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SleepingAwake
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super light freetourer

Post by SleepingAwake »

Hello Everyone!

Wanted to show off my latest build project. I like to try things which resulted in this weird contraption.

The goal was to make a freetourer with an arbitrary weight limit of 1kg per ski. It all started with me questioning the traditional triax layup.

The hand-wavy explanation to my idea is as follows: The modulus of a UD laminate falls of drastically when loaded off-axis. This is why we can independently tune torsional and longitudinal flex of our skis. But there is another way to achieve the same thing by using a biax laminate and instead you change the fibre angle of your layup. Let's say a +/- 10° angle will result in the laminate working more lengthwise and you get a softer torsional flex, or you choose a +/-20° angle for a stiffer torsional flex. The longitudinal flex is influenced by this as well but this can easily be adjusted for in the core thickness.
I basically calculated that with a fibre angle of +/- 17.5° to the main axis, I get the longitudinal and torsional stiffness of my last build, while shaving off a lot of weight.

The lightest ski in similar dimensions that i found are the Movement ALP TRACKS 89 LTD 2020 which come in at 1170g at 1850x89mm. I managed to get these to just under 1 kg with full size edges and a standard base. Also my sidewalls are junky.

Had to pull out all the tricks for the core tho and used a Bcomp balsa core underfoot and Rohacell in tip and tail.

I will report back to let you know how they ski. I honestly don't expect too much of them as I usually prefer damp skis. We will see.

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chrismp
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Re: super light freetourer

Post by chrismp »

Great idea! Really looking forward to your ride report!

Is that a veneer topsheet? Kinda hard to tell from the photos.

pmg
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Location: Sonthofen

Re: super light freetourer

Post by pmg »

Wow, really nice! How long is your Ski?
And what happened to your edges? :) Will you cut your climbing skins in the exact shape or leave them straight in the "uneven" part of the edges underfoot?

I currently am on a similar project, though my ski will be a bit smaller. I also use Bcomp Core and hope it will survive the planer :)

Best regards
Philipp

pmg
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Re: super light freetourer

Post by pmg »

I'd also be interested what UD you used in +- 17.5°. Surely carbon, but what weight? If you want it damper (though a bit heavier), there is some nice carbon/flax mix UD from bcomp: https://www.bcomp.ch/products/amplitex/.
I really like the feel of carbon and flax mix.

SleepingAwake
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Re: super light freetourer

Post by SleepingAwake »

Topsheet is visible carbon

The dimensions are 1850x90 underfoot. And wanted to give Magne traction a go on skis for a long time. it worked well on my snowboard so went for it with this built. The skins are cut to the same wavy shape.

Carbon or Glass UD with biax flax is my go-to layup i have a lot of experience with.

The layup is 500gsm on top, 400gsm on the bottom.

And @pmg: I would advise not using the planar on end grain balsa. If you have a fancy helicoidal cutter you might get away with it, but i wouldn't recommend. Thickness sander would be best, otherwise router sled with template or CNC machining.

Thelongride
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Location: Washington

Re: super light freetourer

Post by Thelongride »

Let me know how that rohacell holds up, really curious!

SleepingAwake
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Re: super light freetourer

Post by SleepingAwake »

Yeah this is the one point I'm really not sure about. Let you know!

pmg
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Location: Sonthofen

Re: super light freetourer

Post by pmg »

Hi,

thanks for the hint. As I only have a planer and nothing else, I will just give it a try and be really careful...

regards

carlduke
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Joined: Mon Sep 30, 2019 6:22 am

Re: super light freetourer

Post by carlduke »

Thank you for posting this! It is a beautiful ski and I'm anxious to hear how it performs and if you were building it again if you would use something more substantial in place of rohacell and instead take some weight off the base and edges.

Did you reinforce the binding area at all?

mammuth
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Joined: Tue Oct 28, 2014 3:48 am
Location: somewhere in the alps

Re: super light freetourer

Post by mammuth »

When changing the direction angle you change the torsional stiffness as well. Looking forward to see how your experiment will ride. I like thinking out of the box!


Btw. the top looks excellent. Do you have a pic of the raw carbon you did use?
Tom

SleepingAwake
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Re: super light freetourer

Post by SleepingAwake »

Alright I tested the ski today in varied conditions.

Aaaand the verdict: meh. or meh-ish...

There was a lot going on and I have to ski them a lot more before I can give a better feedback. If skied aggressively and fast they performed well. super snappy, agile. But when half-assing things they can be a handful. really nice on the way up obviously tho.

There are a lot of possibilities why this could be:
  • weight (definitely a factor, i plan to play around with added weights to increase swing weight. Was the idea from the start actually)
    I'm just a hack that doesn't know how to ski "skinny" skis (that's a given)
    Magne-traction?
    when working with a biaxial layup, there are coupling effects that will result in a concave base when flexing. I don't think this was a big factor in today's condition, but will see if I ski them on a slope.
    the pure carbon layup vs my usual mixed layups with ampliTex
For now i will detune the edges in tip and tail a little and hope i can ski them in steeper terrain soon. With the avalanche conditions here we had to stay below 30° for the majority of the day and i have a feeling they will come to life in the steeper stuff. Sounds all rather negative but I am actually super excited as I am learning a shitload in the process.
mammuth wrote:
Sat Jan 23, 2021 3:51 am
When changing the direction angle you change the torsional stiffness as well.
(...)
Btw. the top looks excellent. Do you have a pic of the raw carbon you did use?
Yes maybe I have to elaborate a little more on this. The stiffness of the carbon drops dramatically when loaded off-axis, as can be seen in this graph:

Image

Torsion in 0° is the same as traction/compression at 45°. Therefore we can consider the 0° UD layer being 45° off-axis if we talk about a torsional loading. As the stiffness at 45° drops so much the UD layer in a normal ski contributes nearly nothing to the torsional stiffness and only the diagonal layers do any work. same but reverse for the lengthwise flex. So you have two different fibre orientations to do very differents tasks. In my design all I did is finding an angle where my only layer matches the stiffness of what i usually would use at 45°, and then compensate for the lost stiffness lengthwise with a modified core profile (still somewhat hand-wavey as the core thickness also influences the torsional stiffness. Therefore I now have a mixed loadcase in my skis with traction, compression and shear.

So my 17.5° off-axis carbon can be considered being 17.5° off axis to the main direction, and 45°-17.5°=27.5° off axis for the loading in torsion at 45°. This was all verified with a couple of bending tests on a universal test machine.

The carbon I used is a non-woven UD https://shop.swiss-composite.ch/pi/Verstaerkungsfasern/Kohlefasern/Gelege-Unidirektional/Carbongelege-UD-125g-m-500mm.html
I used 4 thin plies on top and bottom, which will improve mechanical properties over fewer thicker layers. I also added a 8gsm carbon fleece between the UD layers and the core and used a toughened bonding resin on the interface rather than a brittle laminating resin.
carlduke wrote:
Fri Jan 22, 2021 12:29 pm
Thank you for posting this! It is a beautiful ski and I'm anxious to hear how it performs and if you were building it again if you would use something more substantial in place of rohacell and instead take some weight off the base and edges.

Did you reinforce the binding area at all?
Yeah the Rohacell is just silly for such a ski. Even if it does hold up i find it hard to justify spending the money to safe maybe 60g per ski over a full balsa core. I just needed it to get under my self imposed 1000g mark...

Binding area is reinforced with a aviation quality 3mm birch plywood and two layers of 163gsm glass in a pocket. So glass at 0/90, +/-45 , birch plywood, then the top laminate.


That's it for now i think. I will keep you posted when i have some more news.

mammuth
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Location: somewhere in the alps

Re: super light freetourer

Post by mammuth »

How was that carbon to work with?
Tom

mammuth
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Joined: Tue Oct 28, 2014 3:48 am
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Re: super light freetourer

Post by mammuth »

Ah, i understand now. I was in the false direction thinking you modified the angle from center to tip ... was wondering how you do that ;)
Tom

SleepingAwake
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Re: super light freetourer

Post by SleepingAwake »

mammuth wrote:
Mon Jan 25, 2021 4:20 am
How was that carbon to work with?
I'm used to it and was actually expecting it to be harder than it was in the end. I am rather used working with those materials from other projects tho.

Compared to a single layer of triax fiber glass it is a lot more faffing

SleepingAwake
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Re: super light freetourer

Post by SleepingAwake »

Alright got to ride them again on a short after work night ride in amazing conditions and they just worked really well. Sorry I have a hard time describing all of this as I never really read any ski reviews and I am lacking the lingo :)

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