Carbon Fiber?

[numpty]

I have now managed to successfully build skis and ride them, a fine experience and well worth the ton of time and money it took.
Now that I have skis for all kinds of conditions I am curious about what to do next and was interested in carbon fiber.
I am looking for a ski that has a soft flex but can hold an edge at speed. I am looking to make an 'easy' ride as my knees get older and more grumpy. I am not so concerned about weight since I don't hike for it.
I have read that carbon can shatter under impact, not so good in a ski.
I have seen designs with stringers and some like Goode that claim to be mostly fiber.
I was thinking of a sheet of carbon under the core and nothing above the core. This makes some sense to me since I am not sure on a very basic ski design point, what does the glass above the core do? Glass is not so good in compression which is what I assume the top sheet is under when the ski bends. Is it there just to sandwich the core and protect it?
Any advise on carbon and the type, plain, 90 degree or whatever weave?
What weight of cloth etc?
Thanks for the help.

[MontuckyMadman]

http://www.skibuilders.com/phpBB2/viewt ... 43113e338d

http://www.google.com/search?hl=en&lr=& ... =&gs_rfai=

[Alex13]

It's not as simple as "the top sheet is under compression when the ski bends".

If you think of a U shape bend, the inner curve of the U is under compression, and the outer curve is under tension... Such is true of the top sheet (the upper fibres are under compression, the lower ones under tension, and the middle ones are neutral). It is true that in a ski or board, the top sheet when flexing upwards will be under more compression and less tension than the bottom sheet, however, as the radius of the arc for the bottom sheet is larger than that of the top sheet.

[davide]

Glass is not so good in compression which is what I assume the top sheet is under when the ski bends. Is it there just to sandwich the core and protect it?
Any advise on carbon and the type, plain, 90 degree or whatever weave?
What weight of cloth etc?
Thanks for the help.

Actually in compression glass is not that bad and it is stronger than carbon.
You may consider to use UD carbon and biax glass under the core and triax glass above the core: it will be a cheap test.
A further step is to use triax carbon under the core, UD glass and biax carbon above the core: more expensive but lighter.
Weight should be about 1/3 or 1/2 of the glass you want to replace.

Using reinforcement fabric only under the core, and nothing above, is not efficient (I did once).

[twizzstyle]

Keep in mind if you do a different layup on the bottom than you do on top, if you are doing a heated cure its likely you will get camber changes after curing. Ideally you do the same layup on top as you do bottom, that will yield no camber changes.

[AlexF]

You really need reinforcement on both the top and bottom of the ski.

Skis are sandwich structures which rely on stiff face sheets to take the majority of the tensile and compressive loads. Mechanics requires that for the ski to be in equilibrium in bending the sum of the compressive forces has to be equal to the sum of the tensile forces in the ski.

If you have a stiff sheet on the bottom then that surface will be stiff. But If you don't have something similarly stiff on the other surface then that surface will not be able to take the same (but opposite sign) sort of loads. As a result the whole ski will be a lot less stiff.

Its a little bit more complicated than that (to do with deviation of whats called the elastic centroid and the weighted second moment of area) but I won't bore you

[AlexF]

Oh yeah, about carbon fibre reinforced composites (CFRPs). CFRPs can be very brittle when subject to out-of-plane loads (i.e. when you hit a sheet of it at a perpendicular angle), but glass fibre reinforced composites are pretty brittle too.

In the loadings you going to get on a ski it will never shatter! Delaminating, cracking etc...is still quite possible.

[Alex13]

Its a little bit more complicated than that (to do with deviation of whats called the elastic centroid and the weighted second moment of area) but I won't bore you

Feel free to elaborate, a lot of us in here are studying or qualified engineers and we can follow you.

[AlexF]

I've put together a short document on bending of sandwich beams. I've put in a couple of examples too. As this thread started on the subject of a ski with composite only on one side of the core I did examples for this, a core by itself and a core with composite on the top and bottom.

Hopefully this will help explain why the top and bottom skin of a ski should be the same:

http://files.me.com/alexanderfergusson/uzdvqg

[doughboyshredder]

I've put together a short document on bending of sandwich beams. I've put in a couple of examples too. As this thread started on the subject of a ski with composite only on one side of the core I did examples for this, a core by itself and a core with composite on the top and bottom.

Hopefully this will help explain why the top and bottom skin of a ski should be the same:

http://files.me.com/alexanderfergusson/uzdvqg

Uhhhh, o.k.

Impressive.

[skidesmond]

All that talk about sandwiches made me hungry! Just kidding.

Alex - I'm impressed by the people who understand how a ski should be made and can show it mathematically and can express changes in the ski composites mathematically. It's nice to know (generally speaking) a head of time what the changes will be in the ski if you change a composite or the thickness of the core.

There have been other technical/engineering documents in the forum that I struggle to understand. What I get mostly is that a ski will be X times stronger/stiffer or softer. But I think part of the key to understanding a lot of this is to hold a 10mm core and feel what the stiffness so the X will have more meaning in the end.

It also tells me that I made the right decision years ago to switch my major from engineering to computer science.

Thanks for the article. I'm going to try to wrap my little brain around it.

[AlexF]

All that talk about sandwiches made me hungry! Just kidding.

Alex - I'm impressed by the people who understand how a ski should be made and can show it mathematically and can express changes in the ski composites mathematically. It's nice to know (generally speaking) a head of time what the changes will be in the ski if you change a composite or the thickness of the core.

There have been other technical/engineering documents in the forum that I struggle to understand. What I get mostly is that a ski will be X times stronger/stiffer or softer. But I think part of the key to understanding a lot of this is to hold a 10mm core and feel what the stiffness so the X will have more meaning in the end.

It also tells me that I made the right decision years ago to switch my major from engineering to computer science.

Thanks for the article. I'm going to try to wrap my little brain around it.

^I'll help you on composites if you help me on computing! I've thought about putting a program together for modelling skis and my computing skills are crap so I might need some help down the line.

Anyway, I hope it the article provides some insight into the effect of laminate layup.

I'm just really stoked that I found this forum. And judging by the fact that I keep getting told that I can't post any more every day because I'm a new member and haven't been verified as a non-spammer yet, so does this damn intra-ma-web thing! Skiing is awesome...FACT. When I'm not skiing I think about skiing, which can get pretty distracting at work. But now I can think about skiing and work at the same time as I work in composites engineering!

The idea of this forum is awesome too, its like an open source project for skis. Some huge advances in computing have come from open-source projects and it'd be amazing if a project like this can have even a fraction of the impact on ski design and manufacture as open-sourcing did for computing. The are so many BS ideas and marketing gimmicks put out by ski manufacturer's and they've been able to get away with it because there's been no other source of info or experience. They haven't needed to take risks or innovate because no one's pushing them or calling them on their crap. But a site like this could change that. I've only just joined and a have only read a few of posts (still on the first page of threads!) but its really cool to see people taking risks and trying ideas.

I haven't ever made a ski or done a bunch of other ski related stuff because I've struggled to even know where to start. Now I know I'm definitely going to give it a go (once I'm done with my degree).

I haven't got any experience making skis but I think I can help out on laminate/design and mechanics related aspects. If I can help out I'd be stoked to do it. I'm currently writing up my PhD in Mechanical Engineering and my thesis is about the thermal/mechanical behaviour of 3D Woven Carbon Fibre Composites and a new model I've developed for predicting their behaviour. I've also got and MSc in Composites and a BEng in materials science. As a result I'm pretty good on the laminate design and mechanics of composites thing. If there any particular areas that people would like some more info on I'd be happy to help.

In a couple of months I'll a have a bit more time (finsihed my PhD) and will be able to give more detail on engineering methods for skis. I've thought about writing a program to predict the behaviour of a ski but never did it because I thought I'd be the only person to ever see or use it. Now that I know others might be interested I think I'll have a go at it down the line.

I was thinking about a program where you put in the dimensions of the core, the lay-up of the composites and their properties. From that the program would give you the in-plane and flexural behaviour (e.g longitudinal, transverse and torsional stiffness) for any point or section of the ski. I now know about sno-cad (haven't used it yet) so if I did this I'd have a look at a way of importing the core profile directly into the program. Anyone got other ideas for what they'd like to see included? As long as its not too complicated I'll have a go (my programming skills were crap when I learnt them and I've probably forgotten most of that anyway so don't expect a program with the polish of an iPOD!). Might have to call on skidesmond for help!

[Alex13]

That explains your knowledge.

I'd be interested more in the theory behind CLA, though I may be the only one :P Composites engineering is an area that holds a lot of interest for me (not least because of my snowboarding interest).

[MontuckyMadman]

hey alex f i doubt you have seen this yet its an excel program that bambi developed it does kinda what you speak of.
http://www.skibuilders.com/phpBB2/viewt ... 5125#15125
download from the link its clean. need an xls viewer ie excel.

[skidesmond]

MontuckyMadman -

That was the other document I was thinking of. I downloaded that awhile ago and it's pretty cool. Problem is that I didn't understand it because I didn't know the terminology or the meaning of a lot of the variables and abbreviations such as Pa, Pb, etc. and my math skills are weak and my algebra and calculus all but forgotten. But I've done more reading and starting to get a first grade understanding.... "Tweak this part and that happens....." And now that I've made some skis I have an idea of what a core flex feels like before and after lamination so the X factor is starting to have more meaning.

AlexF - This really is a great forum and contains a ton of info. I never could have made the progress I did w/o this place. If you haven't checked out the home page yet (www.skibuilders.com) under "HowTo" do it. That's gives the basic steps of ski building. The forum has a staggering amount of info of stuff that works, and doesn't work which is just as important. As well as variations of skis, boards, presses, etc. Extremely innovated people here.

This forum is a place for ski/snowboard addicts who also are either skiing/riding, thinking about it, building it or thinking about building it .

My computer experience is primarily mainframe databases and open systems (Unix, windows (ugh) ) but I'm always willing to try something new.

And yes the ski industry continues to dump more hype, gimmicks and marketing crap on the un-informed. But now you know the real story!

Ski On Bro!