Cap skis.... yo G-man!

[collin]

One area where the newer skis have shown some real improvement is in the quality of their flex arc. Skis turn most smoothly, whether on soft or firm snow, as the arc of their flex becomes more circular. Most factory built skis use the cap construction technique, which inherently produces a ski with a flex that is stiffer at the waist of the ski than it is through the fore and aft body portions. Because the top fiberglass layer wraps around the side-walls of the ski (torsion box design), the thicker waist portion ends up being much stiffer than the thinner tip and tail portins of the ski.

This resulting ski does not flex circularly, so, when skiing, not all portions of the ski are headed for the same destination during a turn. Ski makers don't talk about this openly, but if you check out some of the patent applications over the last 10 years, or so, you can see that there has been a lot of focus on mitigating this undesirable charactoristic. If you check out some of the newer skis, you will notice that skis have become much thinner at the waist, which reduces some of the unwanted torsion box stiffening effects (but doesn't leave much room for binding screws). Other mitigations include the addition of contoured top sheet stiffening ridges that taper into the tip and tail sections of the ski. These design elements may initially appear to be included just for aesthetic reasons, but their main purpose is to stiffen up the fore and aft bodies so that they flex more similarly to the waist portion of the ski. These stiffening elements make for a bit heavier ski, but, if applied well, they do improve the flex arc.

One popular ski maker uses polyethylene side-walls under the cap construction, then, after the ski is pressed, machines away the side-wall fiber glass at the waist to produce a ski with a more uniform flex. The newly exposed polyethylene side-walls prevent moisture intrusion into the wooden core. This techinique requires more time and materials to produce the ski, resulting in a more coslty end product.

The best flex arc still results from a ski that is built via 'sandwich' lay-up because, with this technique, there is no fiberglass wrap over the side-walls. The downside is that sandwich construction is much more labor intensive, and durabilty is often a trade-off because of bonding difficulties.

Ok, you didn't say that here. But I figured I'd cut-&-paste.

I follow what you're saying, but not the "why". Since this would be applicable to sandwich construction with a torsion box (which I've been thinking about) or cap construction with a vacuum press I think it'd be good to flesh this out. Since I've been thinking about torsion boxes, since well they're torsionally stiff.

I understand that with a "box" of glass, the vertical portion near the sidewalls is going to dramatically increase the stiffness. But what I don't see is how this isn't the same as just having a stiffer core. Wouldn't it just average together? How is it fundamentally different than say a mixed wood core, eg aspen/maple?

[G-man]

Hey collin,

Okay, I'll give this a shot without pictures, which would be quite helpful if I had some, but I don't have any.

I built this contraption that I call my 'flex analyzer'. Basically, it allows me to compare the flex of a weighted ski to a perfectly circular arc. When I place one of my own homemade skis on the analyzer, they 'arc-out' just perfectly, regardless of how much or how little weight is applied (as long as the weight is enough to get the ski decambered). If I put one of my factory cap skis on the anylyzer and add weight, the tip and tail will 'arc-out' long before the waist of the ski will. I have to apply a lot more weight to get the waist portion to the arc line, and by then, the tip and tail portions are actually starting to lift away from the arc line.

So, what does this all mean in real skiing terms? Well, as I've warned before, I'm just a garage tinkerer, but here goes. The flex arc of a ski is the most important parameter of ski design when it comes to the turn. Everybody talks about sidecut, but all that the sidecut really does is allow the ski to attain a flex arc on firm snow. As an example of this, take one of your skis, place it about 45 degrees on edge on your work bench, hold the angle and push on the waist until the waist edge touches the bench surface... the more sidecut the ski has, the more flex arc you'll get. If the ski didn't have any sidecut, you wouldn't get any flexing with the weighting. On firm snow, which is where the ski edge actually engages the snow in the turn, you'll get tighter 'carving' turns as the amount if sidecut increases... and the smoother the flex arc is, the smoother the turn will be.

On soft snow, the ski edge doesn't actually 'engage' the snow because the snow doesn't have enough integrity to push back against the pressure that is exerted by the edge. So, on soft snow, it's the bottom surface of the ski that floats or 'surfs' on the snow. Here, the more flex arc that you have, the shorter your turn will be, and, very importantly, the smoother the flex arc is, the smoother the turn will be. Along this line of thinking, consider the recent popularity of the reverse sidcut ski for soft snow conditions... it floats on it's bottom surface rather than carving on it's edge.

Now, back to your main question. With cap skis, the 'torsion box' actually works in reverse of what the manufactures would like you to think. We'd like the increased torsional rigidity to be in the tip and tail sections of the ski so that we get increased tip and tail edge hold. But, in these areas, the profile of the ski is too thin and the ski is too wide to get much, if any, benefit from the cap construction. At the waist, however, the profile is thick enough and the width is narrow enough that the cap construction increases the ski resistance to flexing, which is what we don't want. This unwanted stiffness is caused by the fiberglass layer that 'drapes' over the side wall of the ski. With sandwich construction, there is no such layer, so it is the profile of the core that primarily determines the quality of the flex arc. You ask how cap construction differs in it's effect as compared to a stiffer core material. I believe that the answer is that, with a stiffer core material, the modulus of elasticity is the same per unit of material throughout the mass of the core. With cap construction, however, the effects of the cap construction is disproportional along the length of the ski due to the varying thickness of the core at the waist versus the tip and tail... it does not create a uniform increase in stiffness along the length of the ski.

All of this being said, it is certainly possible to profile a sandwich ski in such a manner as to end up with a ski that does not have a uniform circular flex arc. If the ski is profiled with a uniform thickness (flat) area at the waist of the ski that is given too thickness or too much length, then it will behave much the same as a cap ski, in that it will have a waist that will not flex uniformly with the tip and tail sections. Therein may be one of our big challenges as individual ski builders... to find that right melding of materials and profiles that result in a flex arc that provides us with the sweetest turn.

I sure hope that at least some of this makes sense and that some of it is helpful. If anybody thinks I'm way off base, I welcome any other perspectives... cause I just may be way off base. It's all about sharing and learning from one another.

G-man

[endre]

interesting theory g.

I have done a lot of flextesting of both cap- and sandwich skis, and I can tell you one thing for shure: Cap skis are definately not stiffer in the middle, though there is a general tendency that cap skis have less evenly didtributed flex than most sandwich skis (especially skis with profiled tops have unsmoth flexcurves). My theory is that cap skis in general are less expensive and made with less care, therefore these flexcurves are in general not that smooth.

[G-man]

endre,

Your comments are much appreciated. I know that you have done a lot of research in the area of flex charactoristics. I completely agree with you that a cap ski is not necessarily stiffer than a sandwich ski, but that it's just more difficult to get a smooth uniform flex arc with cap construction. Sandwich skis can be made with a very stiff flex, but, if profiled correctly, can still have a very smooth circular flex arc. I also think that a cap ski can come pretty close to having a good flex arc, it just has to be fiddled with a lot more to get it there. That's basically what my original post on the telemarktips site was about... that some companies are putting forth more effort lately to try to get their skis to flex more uniformally. It's just interesting to watch all of the different things that they're trying. Mostly, they just end up adding more materials and more weight. But, without cap construction, we probably wouldn't have a ski industry. Nothing's perfect.

I couldn't see your photos on the other thread. I'll keep checking in to see if I can get them. Thanks.

G-man

[Bambi]

The flexibility of the ski should be dominated by the thickess of the ski as the stiffness will be proportional to the thickness to the power 3. This means that a core section that is 12.5mm thick will be twice as stiff as a 10mm section.

Given the info above any designer has the ability to tailor the stiffness along the length of the ski by profiling the core to a suitable shape and consequently I don't believe that the bend profile that you see in a commercial ski is 'side effect' of the cap construction - it is really a function of the core profile, which has been designed that way.

I agree with your assessment that the benefit of a torsion box construction for torsional stability would be neglidgeable due to the aspect ratio of the ski profile, although torsion will be significant along the entire length of the ski - not just the tips. I haven't done any calcs on this.

- I am a day away from a spreadsheet which calculates all this for you and translates your core dimensions and fibrelass weight into a flex profile which you can compare with existing skis. Will post ASAP....

B.

[justin56]

One thing that is often missed in cap construction is the effect of the top sheet.

One should be able to build a good flexing ski without one--it just won't look pretty. But, it illustrates the point that the top sheet is usually only in builders minds as a means to cover or "pretty up" the ski with graphics.

but, it has functional considerations.

Changing topsheet material can free-up any deadlocks in figuring flex.\
Actually, it should be considered when choosing glass/fibers and other materials as a whole unit when designing.

[Bambi]

Flex Spreadsheet can be found here:

http://www.skibuilders.com/phpBB2/viewtopic.php?t=486

B.

P.S. sorry for the proliferation of links, but his subject seemed to come up again and again!