Touring ski design

[Eirik Hanes]

Hi

Just made a new touring ski design. This is going to be my allround touring ski. And I'd like your opinions on the side-cut design.



The core is going to be a combination of aluminium honycomb and aspen. And the laminates are going to be ~450g/m quad axial carbon.

[twizzstyle]

Shape looks good (although I'm not sidecut expert), and the core sounds very intriguing... I am curious why you are using quad-axial CF though? Seems like the lateral fibers will do nothing but add weight? The ski isn't really wide enough to worry about lateral bending.

[Eirik Hanes]

For the moment I only have a biaxial 320 g 45 -45 CF and the quad axial 450 g CF, and acording to my calculations for the same stiffness there is really no weight difference.

I guess i could do a strip of uniaxial fiber togeter with the ligth biax fabric to optimize for weight. But I really like the togh feel in the quadaxial fabric. But I'll see, still some time to layup and core profiling. Got to be sure I have the right sidecut before I move on.

I have a few side-cut questions:

- Should the narrovest point on the ski be at boot center?
- How does the difference in width between tip and tail affect the ski performance (I dont care about the switch riding performance), currently its 9 mm
- If a dual radi is used in the front ski should the smalest radi be the one closest to bootcentre?

[MontuckyMadman]

I noticed all the trab skis use a quad axial fabric as well as all g3 big mountain skis have a 0/90 weave with the 45/45. they must be able to shave weight in the 45/45 by adding to the 90 to increase torsional stiffness. I know it doesn't make sense but some big companies are doing it so...

[G-man]

Hey Eirik,

I have just a couple of comments regarding the basic shape of your proposed design.

Since you indicated that this ski is to be used for all round 'touring' (as opposed to all mountain 'back-county', where you might be landing switch at times), I'm assuming you're going to be doing as much climbing as you will be doing turns. A tail curve doesn't offer much benefit on a touring ski. It can allow you to do a 'fallen leaf' move in a tight, steep chute, but a relatively short curve will suffice for that. Here's some things that aren't so good about a tail curve on a touring ski:

- it reduces efficiency of the climbing skins because of the reduced surface contact and surface pressure at the tail curve.
- the tail curve gets in the way when you're doing kick-turns on steep slopes.
- it adds weight, without offering much benefit.
- it makes it very difficult to stick your skis into the snow (for using your skis as a back rest on your lunch break or when sitting around the campfire on over-nighters)

On a touring ski, I'd recommend a flat tail, with the side-cut ending very near the tail end of the ski. This will either provide you with a shorter ski than you have pictured (more maneuverable in the trees), or with a longer effective edge if you keep the ski at it's current length. I'd also recommend a fair amount of camber on a touring ski so that you can get good edge pressure transfer to the tip and tail edges, for long, icy, exposed side-hill traverses.

Regarding 'narrowest point on the ski at boot center", it depends much on how you ski. If you ski with your center of gravity at the balls of your feet (lots of varying opinions on this subject), your skis will perform better with the boot center rearward from the waist of the ski (ball of foot at narrowest point on ski) . However, there's a lot of other design parameters to factor in here, as well, such as the ski's flex profile.

My experience is that a tail that is about 10mm narrower than the tip is very good for an all round ski, so your 9mm is good.

I'd recommend that you stay away from dual radius side-cuts. The concept is mostly industry hype, and, at best, should be used only for very specialized ski designs.

I suggest that you take Twizz's advice on the quad-axial CF. The 90 degree fiber orientation will do very little for this design... it might be different if the shovel were 150mm or more, where the tip might tend to 'go convex' when the tip edge was placed under a lot of load. Also, on a touring ski, I'd personally probably go very light on the CF usage. Yes, carbon fiber is light and stiff, but it has a very small flex range... it will flex a certain amount, then the flex range suddenly 'hits the wall'. On a touring ski, you want to be able to 'feather' the flex response of the ski, depending on the various conditions that you'll be encountering. A straight FG ski will give you a broader range of options.

Ooops... I guess that was more than 'a couple of comments'.

G-man

[Eirik Hanes]

Hey, thanks for the feedback. I will make some changes to the sidecut based on your info.

Do you mean that I should shift the narrowest point back as much as 100 (or more) mm ? (I have size 28 cm boots). Sounds like a lot

Regarding CF i don't know if I agree with you. I have made skis with that fabric before, and I think they are amazing. I don't recognize the "stress stiffening" effect you are talking about. When I did my flexual tests, both the FG and the CF sandwich samples had a pretty linear curve all the way to failiure. But I will look at it again.

I do recognize that the 90 deg layer is probably some dead weight, but overall the skis end up pretty light with this fabric.

Thanks again, love challenging feedback.

[twizzstyle]

I noticed all the trab skis use a quad axial fabric as well as all g3 big mountain skis have a 0/90 weave with the 45/45. they must be able to shave weight in the 45/45 by adding to the 90 to increase torsional stiffness. I know it doesn't make sense but some big companies are doing it so...

. 90 degree fibers will have no effect on torsional stiffness .

[twizzstyle]

And on the quad ax cf... Even though i voted against it, if you already have it, use what you have. No sense buying something else just to save a few grams of weight when this stuff will just lie around unused!

[MontuckyMadman]

They would not use it unless it provided some benefit. Cost or otherwise.

[OAC]

I used Quad CF(just because I had it.. ) in my last build.
Observation: Both longitudinal and torsional stiffness. Even thou I used ca. 30% Poulownia. Not bad.

Haven't skied them yet...but so far...

Backside w. CF: It consumes more epoxy than FG.

[G-man]

Do you mean that I should shift the narrowest point back as much as 100 (or more) mm ? (I have size 28 cm boots). Sounds like a lot

Hmmm... this might be a little tough to explain... and I'm sure that you already know some of this stuff, so bear with me here. For starters, for ski turns on firm snow, we ride the edge... and for turns on soft snow, we ride the base (for the most part on both counts). For the smoothest turns on firm snow, we want the arc that the ski edge makes (when the ski is weighted and placed on edge) to be the same radius in front and back of the pressure point on the ski... in other words, a consistent radius round arc. That way, the entire length of the ski edge passes through the same set of points on the snow surface (think perfectly carved turn). Likewise, for the smoothest turn on soft snow, we want the arc that the base makes to be the same in front and in back of the pressure point. To design a ski that does all this nearly perfectly requires a boatload of R&D time, and some fairly sophisticated testing equipment. The profile of the ski core and the radius (or radii) of the side-cut arc must be mated and fine tuned together. Dual radius side-cut arcs can be of benefit in this setting, but without proper analyzing equipment, multiple radius side-cuts will most likely create problems rather than provide benefits. However, for the hobby builder, simply using a single radius side-cut, placing the pressure point at the narrowest point of the side-cut, having a tail that is 10 mm narrower that the tip, and matching the ski's flex arc with the side-cut arc... will yield a nicely performing ski.

So, no, I'm not suggesting that you move the waist of your ski design back. I'm really just suggesting that you build a very basic ski this time, and then figure out a way to analyze the ski's final flex characteristics so that you can locate the center of the side-cut arc and the center of the flex arc (every 'back room' ski builder's big secret), mount the binding depending on the center of pressure of your particular skiing style, then test the skis to see if the skis feel balanced on the snow. If not, design another pair and move the arc centers fore or aft, then build and test the skis as before (if you simply moved the binding forward or back on the first pair, you'd alter the fore and aft arcs so that they would no longer be equal). Okay, yes, this is taking ski design to a pretty nerdy level. My point is that you really can't 'shot-gun' all of these design parameters on paper, and get it right the first time... and if you really do want to design and build a super nice turning ski, you really need to get all the various arcs working together (flex arc and side-cut arc at the same point on the ski, and same radius fore and aft), get the pressure point located correctly, and have the right amount of ski surface fore and aft of the pressure point. In reality, very few skis fit this strict criteria, but we still enjoy riding on them.

As far as pressure point goes, it helps if you can determine what part of your ski boot applies the most pressure to your skis. This is really more difficult than it sounds, as not all people have the same skiing 'style'. Tests have been done with high level skiers by placing pressure transducers in their ski boot footbeds, and then recording the point of highest pressure while skiing. Most athletes are quite surprised that they actually pressure in areas different than they predicted before the transducer tests. Some skiers pressure from the heels, some from the mid-foot, and some from the fore-foot. Tele skiers often do a bit of everything. The optimal bio-mechanical position of a skier's foot in a ski boot is way too big of a discussion (and way too opinionated) to get into, so I won't even start. Some skiers have a good intuitive sense of where their pressure point is, but I think lots of people have a difficult time being that in tune with their what their bodies are doing. But, in terms of getting your binding mount optimal, it may be useful to stand on your skis in your living room and make a mark on your boots where you think you're applying the most downward pressure. Then mount that part of the boot over the 'sweet spot' on the ski.

Regarding CF vs. FG, it sounds like you've done your homework and testing, so yes, go with what you've learned from your own good R&D work. My comments are based on simple and subjective hand flexing comparisons of skis I made with the different materials.

I look forward to seeing what you come up with. And ya, the jibber-jabber back and forth about ideas is always fun.

G-man

[G-man]

It consumes more epoxy than FG.

Good point QAC. I've felt that was the case intuitively, but I haven't had a chance to prove it out.

[Richuk]

Videos like these illustrate the point http://novel.de/multimedia/covideos.htm

Using comparable materials, in terms of weight and weave etc, then CF is going to want a disproportionate amount of force to be applied to produce the same displacement when measured against GF, but this doesn't mean you won't see a non-linear curve.

If you have adjusted the weight of the CF, to accommodate the force being applied to the ski in normal conditions, then you will gain more comparable results.

As soon as you mix the fibres in one ski - using 900 mm of tape or 200-300 if you are creating a bridge, you will detect the transition, which may lead you again to feel the stress stiffening. How obvious the result is depends on whether standard or high modulus CF is being used and or any differences caused by the matrix, inter-facial bond etc

[hallvardaase]

So, no, I'm not suggesting that you move the waist of your ski design back...........

As far as pressure point goes, it helps if you can determine what part of your ski boot applies the most pressure to your skis....

G-man: I guess you are right that there is no simple answer to where the location of the narrowest point should be.

I guess you are also right that it is pretty hard to calculate all parameters of a ski for a perfect design first try. But Eirik, myself and the rest of our skibuilder-crew have tried and we are still trying to do it

By measuring the mass-produced skis I have available it seems like recommended boot-center is located at approximatly 42% of the skis length (measuring from tail) The narrowest point seems to be at the same location as boot-center or a couple of centimeters rearward.

Has anyone tried skis with the narrowest point located more than a couple of centimeters rearward? (like 80mm or 100mm) And how did the skis feel?

And what about narrowest point in front of boot center? Experiences?

[G-man]

By measuring the mass-produced skis I have available it seems like recommended boot-center is located at approximatly 42% of the skis length (measuring from tail) The narrowest point seems to be at the same location as boot-center or a couple of centimeters rearward.

This so much depends on what else is going on with the ski design, and what the skis are going to be used for. Tip and tail curve lengths play a huge role in 'boot center' location. Example... if you have a 180cm ski with a 300mm tip curve that is being used on firm groomed snow, you essentially are skiing on 150cm of ski surface, so you can calculate your percentages based on the 'effective length' of the ski. But, if you soften up the flex on the same shape of ski, and take it in soft, deep snow, you will be riding on much more than 150cm of of the base of the ski, like nearly the entire 180cm of the ski length. So, the boot center calcs should reflect this difference in 'effective length'. Yes, I realize that you might not design a groomer ski with a 300mm tip curve, but I'm just using this possible scenario to present an example of how so many factors effect a good binding mount position.

Has anyone tried skis with the narrowest point located more than a couple of centimeters rearward? (like 80mm or 100mm) And how did the skis feel?

I ski exclusively tele. A huge focus of my life has been involved around activities that utilize a thing called 'chi' (you can google it if this is a new term for you). As a result, I just simply move differently than most people do. I do a number of chi related activities regularly, but my basic activity is my daily 'chi walk'. I've been doing if for years, and still, pretty much everyday, I learn something new from it. Of course, I also 'chi ski'. The result of this is that I mount my bindings differently than most people do, in that, I mount my 'ball of foot' over the waist of the ski (which places my boot about 80mm reward from a 'boot center' mount). This is because my center of pressure when I ski (or walk, run, ride) is over the BOF, or forefoot. Gradually, I've been mounting this same way for more and more friends who I build skis for (mostly tele skiers), and it seems to work well for them also (of course, they are gradually becoming 'chi skiers' also). I'm not trying to promote my style of skiing here... I'm just trying to present additional reason to consider that there are few hard and fast rules in ski building. If you've riden a ski that you like, I'd suggest that you do your best to copy it, because as soon as you start changing stuff, even little things, you can easily end up with a whole different animal... which is fine, and even fun, if you're willing to suffer the consequences of having a corner full of rather expensive mistakes that don't ski as well as you hoped they would.

G-man