New deisgn, finished ski results

[SCHÜSS]

Hi there. It has come to that time of year again here in Australia, the time to build more skis. Last year with the help of you guys we finished one design, tested it, made 'mark2' and was very happy with it.

So i have come up with a new design. very different from our last one and i was after the much respected thoughts of you guys.

The construction goes as follows. Wood core will be either maple, oak or jarrah. thats what they are called here in aus, so yeah. We will laminate vertically with carbon fibre inbetween. then 3 layers of carbon above the core and finished with a clear glossy 1.2mm ptex topsheet. Ptex side walls will be used and the edges will be routed into the base to ensure a flat base is achieved (was a problem with last ski)

Here is a pic:



We want to start building soon. but any thoughts and comments are most welcome. how do you think the ski will behave from that construction? any problems you could invisage?

the ski will have the dimensions 115-64-100 @165cm. It will be used as a slalom carving ski with the hope of max grib on hard snow.

Here are some pics of our last ski (thought i would kill 2 birds with 1 stone)

Construction:
1mm alluminium topsheet
12mm vertically laminated tasmanian oak core.
3mm aluminium 2nd core
base n edges. fibreglass in tip as well as the allu.
12mm binding plate w/ salomon bindings to din 11

pics are from thredbo australia in june.

[SCHÜSS]

[alexisg1]

whaouh !
congrats ! Nice surves indeed

[plywood]

hehe and how heavy were your aluminium skis? at the moment i`m riding some volants (if you don`t know them: also some sort of metal topsheet) and they are heavy as hell.

to your plans about the new ski. looks interesting. the carbon between the wood in the core sure will work. if the carbon really increases the stiffness...i don`t know if it`s noticeable.
but: why do you want to place 3 layers of carbon JUST ON THE TOP of the core?

well, it`s a sandwich construction, with compression chord (on the top) and a traction chord (bottom). an all the composite materials have a better performance in traction than in compression. they can absorb more traction power.

so, i`m not an experienced skibuilder, just finished my first ride - due to lack of snow not yet tested...(but i got some experiences with skateboards). on my ride i kind of used 2 layers of unidirectional carbon (about 140g/m2) between base and core and just one layer of normal woven tissue on the top (looks great, but longitudinal a bit "weaker" than the unidirectional one). but it`s difficult to give you a good advise because we don`t know which kind of carbon fibre you`ll use, if unidirectional, biaxial or a "normal" tissue. and it`s also difficult to predict the stiffnes of your core type

[SCHÜSS]

Hey thanks for the help.
well we should be using just plain woven carbon. We can get the twill woven stuff but we have to order that in. so for now just the plain checker board type.

The reason for putting the carbon on top is as follows (well we think its a reason)

1. carbon is good in compression (top side) and bad and tension (bottom) so to give the ski more pop as well as stiffness wouldnt it be better ontop?
glass is oposite right?

2. if we put it on top we will have a cleaner more precise base to route the groove for the .7mm edge prongs (or around about that size)

I am thinking about going a harder wood like Jarrah as has good flex but is heavier too. ( personal taste, dont like skis that feel like feathers)
However this is easily adjustable.

My thoughts are the carbon inbetween the vertical laminates in the core will not only increase the bond strenth but also provide better torsion stiffness and ski stiffness. ??

And as for our alluminium skis, well with both the topsheet and a 2nd core out of allu, ur right the ski was damn heavy. the weight of 2 normal SL skis at least! The carved like rails which was great. but sliding turns they were shocking

[plywood]

well, the woven thing.... (as far as i know)
unidirectional would be better for the use in a ski. because of following reason: in the woven tissues the sinlge fibres go kind of "up" and "down" in the tissue to cross the fibres which lay orthoganal to them. such laminates aren`t that stiff as unidirectional ones: first of all you get a heavier laminate because you have a lot of fibres orthoganal to the main direction of the load which are "useless" (they actually don`t absorb any of the load because the lay orthogonal to it) and secondly you become a weaker laminate because of the "up and down" of the fibres described above.
the fibres lay in a wavy line, viewed from the site...and a waved line always is weaker than a straight one.

to your reasons:
i`m nearly sure, that also carbon behaves similar to glas: stronger in tension than in compression. but my pc broke down, so i can`t look up my tables with the mecanical characteristics of carbon...

the thing with the edge prongs.... for me this would be a reason to put at least one layer of carbon between base and woodcore. the soaked tissue helps to keep the epoxy in position and it easily bends "around" the corners of the routed channel in the core for the edges. so it would improve the strenght of the edges. i personally would never try to glue the edges on the naked wood - damit, well...i hope you understand what i mean. my english feels kind of bad at the moment last try: the soaked tissue builds kind of a reservoir with epoxy and if you press the whole thing, some of the epoxy gets squeezed out and fills some gaps around the edges which probabely can result. if you used wood, the epoxy just would kind of flew away. also wood is much less elastic, so you can use the carbon to level out the tiny little gaps of the edge prongs to fit to the straight surface of the wood. hopefully it`s clearer now.

if a carbon/wood core would provide torsional stiffness i don`t know...never tried a vertical laminated core i guess it would be a better use if you made some kind of "forks"...but you can try things out! i won`t avert a great invention

[kelvin]

Very nice skis! and interesting layup. How did you prepare the aluminum for bonding? How is it holding up?

-kelvin

[SCHÜSS]

Thanks kelvin. Well for the aluuminium both the bottom 3mm layer and the topsheet, we roughed it up with course sand paper, and as consistantly as possible we ran the drill along it (side ways) just to rough it up even more. We also did this to the core. however not as much with the drill.

When we tested mark1, we instantly had issues. the bond between the raw alluminium and the wood core was poor. and in places we delaminated instantly.In the tip where we used fibreglass it was great and we had no issues. we only had issues with the raw bond between alluminium and wood.

With mark 2 we improvised. We didnt want to use glass cause it would make it tooooo stiff. ( would require a layer between the topsheet and wood, as well as, the metal core and the wood core)
So we used flyscreen. dont laugh, u are now looking at the most durable home made skis ever. It did not add much stiffness at all, and it was like adding steel bars into concrete. it made the bond great. no problems as of yet.

We wont do this with our new design as we are using just composite and wood. and if we do use metal we have the composite aswell to help out so the emergency need for flyscreen wont be utilzed this time.

re. plywood.

your most likely correct. uni would be better. it turns out tho that we can only get normal plain weave and 2x2 twill weave. so that limits our options.

The only reason i thoght that carbon was better in compression (top) rather than tension (bottom) is that i was reading in another topic on this forum where i asked the question and they said that:

There are a couple of things to remember about some of the material. Some work better in tension others in compression. Where you put them in the composite can be important.
The info I have is from "Boatbuilders Manual" by Charles Walbridge. They are expressed in a scale from 1-5 5 being the highest rating in this order tension/compression

Kevlar does very well in tension and very poorly in compression, so keep it under the core. 5/3

Glass seems to be equal in both directions, so it can go on either side of the core. e-glass 4/4, s-glass 5/5

Carbon is good in compression and poor in tension. Keep on top of the core. 2/5+

I'm not sure how the placement will affect the performance of the ski without doing some testing. The top side (compression side) of the ski seems to be the weak link since most of the stresses in the ski are in that direction. Reinforcing the top would seem to be the thing to do to add strength and possible "pop" to the ski. I know in the surf world compression failures in the boards are one of the most common failures. The board gets loaded from the surfers weight in a turn or water. The top composite compresses, buckles, and delambinates from the foam core.

Hafte

that was what i was going with

[plywood]

well, today i had some time to look up the whole thing with carbon, tension and compression we were discussing about. so i`ll copy what i found:

tensile strenght (MPa):

glas:
bidirectional tissue: 330-400
unidirectional tissue: 590-680
carbon:
bidirectional tissue: 560-650
unidirectional tissue: 950-1100
aramid:
bidirectional tissue: 460-540
unidirectional tissue: 790-900


compressive strenght (MPa):
glas:
bidirectional tissue: 310-440
unidirectional tissue: 480-600
carbon:
bidirectional tissue: 450-520
unidirectional tissue: 600-800
aramid:
bidirectional tissue: 130-165
unidirectional tissue: 180-190

well, sadly they don`t write there how heavy the tissues they used were...but there stood also a DIN-norm...so they say there, that these are standard values which may depend on your epoxy resin, the tissue and the processing technology.
these table is copied from my composite supplier, so i think you can rely on these values. and the basic information here is: all materials are stronger in tension

[endre]

hmm.. allways believed that aramidfibre is stronger in tension than carbonfibre?

[plywood]

hmm.. allways believed that aramidfibre is stronger in tension than carbonfibre?

as far as i know: no. the thing with aramid is something different. it`s not that strong, but it`s high-impact resistant (in comparison to carbon which breaks). that`s why they use it in bulletproof vests etc.

[SCHÜSS]

Nice research. Interesting too.

I purchased some composites and more core material today to use simply as tests.

I got:

50x100cm of Kevlar
1m2 of 2x2 twill wave carbon fibre
1m2 of bidirectional 6oz fibreglass

While i was at the store, the people agreed with my initial thoughts and research. Which is strange as it conflicts with yours in places.

So this lead me to do the tests.
Controlled variables:

• all test pieces are vertically laminated strips of 19x19mm Tasmanian Oak wood. Similar to maple.
  
  All are 1m in length
  
  All feature profiling. ( thickness of 4mm at one end, then increasing in a linear fashion to the full 19mm thickness on the other end. This is done with a router and is on a template to ensure consistency)
  
  All are 12cm wide

At the current moment i only plan to do four tests. but it may increase.

1 with carbon on top (as to my research it is better in compression, and there fore better ontop of the core)

1 with kevlar under the core (as to my research it is better in tension, therefore below the core)

1 with glass under the core.

1. with glass above the core.

what do you guys think? i will then see whcih are better in flex, stiffness, rebound and torsional rigidy.

maybe make a few more with opposite placings or placings on both sides of core. i havnt really thought that far yet.

What do you guys think? any suggestions or ideas?

this should detirmin my construction for my ski which i would like to start building asap

[Orion]

Interesting discussion here.

One thing to point out is it is probably more usefull to compare material modulus/stiffness rather than strength for ski performance.

It is my understanding that Carbon Fiber is stiffer than Aramid Fiber which is stiffer than glass fiber (long - tension). And like was stated earlier, laminates of any fiber tend to perform better in tension than compression

matweb dot com is a website that has some data that may be useful. Look under 'other engineering materials' in the database search