http://epubl.ltu.se/1402-1757/2006/03/LTU-LIC-0603-SE.pdf
It pretty much changes everything according to old waxing theory.
Somehow I have suspected this, after waxing my skis over and over ten times, just to see them completely dried out the day after skiing. Why do they make sintered uhmwpe at all? Doesn't work!
Thanks for this excellent and informative paper. Are there any reasons the conclusions should not apply to downhill skis? How are "kick" and "glide" waxes different than those used on downhill skis?
If we still make our ski bases with UHMWPE (obviously better than many materials), should we buy a non-sintered version? Are ski shops set up to scrape the bases instead of stone grind them?
Did anybody in the US hear about this in the "news" [radio/paper/mag] recently? I think this is what my girlfriend [who doesn't ski] was telling me about yesterday. But she couldn't remember where she saw/heard it. Maybe she's just psychic.
Not sure where else it was covered, but GetOutdoors blogged about it.
More coverage of the issue, but no definitive answers to the alpine ski question - is a raw, scraped UHMWPE base better for downhill skiing?
http://www.economist.com/science/displaystory.cfm?story_id=5300005&no_na_tran=1
If have well understood, they suggest to remove wax from the base and just scrape it: the ski will be faster.
A friend of mine got a pair of secondhand Pocket Rocket 3 years ago. He uses them around 100 days per season, and he never waxed the skis in 3 years: the base is so dry it is almost white. He told me they are faster than brand new skis...
Hello waxers!
I read about this a year ago and started to think about my old Blizzard Firebird Comp wich I scraped with a sharpened steel scraper! Since then I have not had a pair skis with such glide. I waxed them once a year, did not matter I think, and I always had the best glide in all conditions of all I knew.
I do this on my skis now but it is hard work to get that finish. Conclusion: Why have a nice high tech base when trashing it with a stone grinder.
Well, I think wax and grinder companies did a good jobb because they survived with something that belong to wood bases!
Hendryx
@Henrik,
Can you explain the scraping procedure you are talking about. What are you scraping away?
I've been reading just a couple of minutes after I posted this message about the scraping:
http://www.jenex.com/tuning/star_tuning.html
Is the scraping taking a part off from the base? How much and why does it make the ski faster? Are those skis not worn faster than those we don't scrape?
Cheers,
Geoff
Hi, just wanted to share a few thoughts after reading the article.
One comment made why sintered UHMWPE. It doesnt work. (endre). My understanding is the sintering is a production process as apposed to extruding, and makes the material stronger/more durable. So buying and using sintered base shouldnt be a problem. Its how the base is prepared that makes the difference.
Sinter - to use pressure and heat below the melting point to bond and partly fuse masses of metal particles, or to be bonded in this way.
They also mention that additives like graphite make the UHMWPE less able to resist abrasion. The best stuff to get is the clear pure UHMWPE, which is what was used to conduct the tests.
The test were conducted by an XC pro, so to me that just means he has more interest in that type of skiing. Why would it make a difference for alpine skiing? Glide is glide isnt it? I mean their tests were done on a down hill slope on a XC track for 100 m. I dont see a difference.
The real conclusions to the thesis were in the part about dirt absorption with respect to stone ground (SG) waxed skis and scraped unwaxed skis. The dirt pick up on wax skis and SG skis made them lose their glide advantage, which was slight, within just a few kilometers.
For us I would think that getting the skis ground flat and the edges set to the angles you want is the place to start than take the skis home and scrape them yourself. Look here for a Swedish steel scraper only $6.95 US
http://www.tognar.com/base_flattening_tools_scrapers_ski_snowboard.html
They also talk about using a rotary steel wire brush after scraping. Im assuming that it is a structuring tool?
Yes the scraper will remove material. You also need to take care that you dont flex the scraper as you use it or you can put a concave in the ski base. If the scraper is sharp you should not have to apply too much presser and one or two passes should do the trick. All you would have to do after that is clean the ski once in a while to bring it back to the glide you got when you first scraped it.
This article pretty much throws out all of the stuff Ive been doing to skis for the past 35+ years. It sure is going to save me a bunch of money on ski prep costs.
Thanks really great stuff!
sintering uhmwpe is done to create pores in the material, the problem is that the pores are to tiny. I don't think sintered is stronger than not sintered because of the sintering, rather opposite i would think, but i might be wrong there.
Actually, sintered umhwpe in general is stronger than the standard method of making uhmwpe (extrusion). I dealt with this issue at work a few months back, and as I recall, the sintered uhmwpe had a tensile strength around 3000 psi, while the best that any supplier could give me for extruded material was about 1800 psi. Additionally, the extrusion process creates a lot of internal stresses in the material, which can really cause troubles when it is cut out. Sintered material on the other hand is generally less prone to this issue as the material starts as fine grains that are then cooked together.
There has been simular discussions about sintered and non sintered inserts, and as many of you might remember Dnek sold their stock of sintered inserts on ebay about a year ago. Sintering allows combinations of materials in orher configurations than what is possible in alloys at lower than melting temperature (f.ex titanium, which has an extremely high melting temperature) Sintering also makes a more or less "spungy" structure, with pores, that can be positive or negative depending on use. In some ball bearings etc. they use sintered parts containing lubricates, when the metal heats up and expands the oil gets squeezed out. I automatically assumed that by sintering UHMWPE, these were the characteristics they were after.
Hi
I've been mailing with little Kam about his use of Titanal in his skis and I was also wondering how I could make a stiff alpine ski.
What do you think about the use of titanal and would you use small titanal tubes to stiffen up some alpine skis. I'm planning to start building my own skis and would like to know which materials I would use to have a very stiff cross ski. I'm 203cm and weighs about 110kg. Now you know why I would need a stiff ski.
I like the Voelkl Supersport Superspeed or Allstar Skis. They are incredibly stiff and have a nice cut. They have hooked shaped raised parts at the left and right side. I think the bindings are set on a plate which is adapted to the raised sides.
Which materials do you think they are using?
Cheers,
Geoff
you don't need stiff materials to make a stiff ski. but a strong ski needs stronger materials. Metal in skis makes the skis bend before they brake, personally I don't like that. (some claim it's safer: when you'r out in nowhere and you break/bend your ski, it's better with a bent one) On the other hand metal in skis makes them heavier, more damp and strange-but-seems-to-be-true: more torsional stiff. About the tubes I don't really know, they are definately made to dampen the ski, and would have a negative effect on the torsional stiffness.. difficult to make in the garage too.
but if your goal is just to make a stiff ski (and I uderstad why you would need one!) I would just make the core 2mm. thicker.(that would make a huge difference in stiffness)
The little bit thicker core will be probably the best option.
I have however another issue. What about the binding plate? Binding plate is a sort of plate which is part of the ski (under the toplayers) and in which the bindings are screwed. Where can we find some of them or how can we make them?
Geoff
what do you mean? The alu sheet to prevent binding pullout or just steel inserts? I have put a piece of aluminum in the mid section of the skis to prevent pullout, worked fine. If not you can just use inserts, probably the strongest:
I've been thinking about this lately as well, mostly because I've been lusting after the Volkl Mantras. [If anybody knows of a shop that demos them in colorado or newhampshire/vermont let me know...]
There has to be an advantage to using a lot of metal in a ski, otherwise nobody would do it right? Some things that might be true:
1- Is it lighter for the stiffness? Holding the Mantras, they seem fairly light for their size and stiffness, comparable to other skis with similar dimensions B4's/Seth Pistols/Pocket Rockets/etc which are all softer. I don't know of another fat ski that's as stiff. Then again I haven't weighed them.
2- Are they more torsionally rigid? Not having actually skied on them I can't say for sure, but from holding them and from all the mag reviews/forum posts seem to say they're rediculous. That's not to say you couldn't make a glass ski that's as torsionally rigid.
3- Is it cheaper to manufacure them? For Volkl, since they make skis in Germany [right?], labor costs are a lot higher than for a ski made in China. I'd guess it's a lot quicker to throw a stamped piece of metal on each side of the core. But a lot more work if you're building them at home.
4- More durable? With a full layer of metal on each side of the core it seems like you could sit there and hit you skis with a hammer and not really hurt them.
5- Unique flex properties? Well this has to be true, doesn't it? A ski with a lot of metal is going to be "less damp" or have "pop" or be "more active", which are kinda the same thing. I don't know if you could do this with glass, carbon yes which means more $$$, like the DP/DB skis which I've also been lusting after.
6- History? With the Mantras being the replacement for the Explosives do they use metal just to keep it a similar ski?
As for the Volkls you mention I think it's just a shaped wood core. Well, additional wood on top of a regular core is what it looks like here. It doesn't say they have any metal, but I think the Supersport Superspeeds have a layer of carbon between the two wood parts.
I like your idea about using tubing though. What size were you thinking? Going from the thin wall/large diameter use of aluminum in bike frames maybe 2cm with a 2mm wall?
As a related topic, I've got an old pair of GS skis I got out of the trash that I want to disect. As allways it's easier to destroy than it is to create. I blame entropy. They obviously have a full layer of metal between the core and topsheet, maybe there's one below the core. Any bets on where the second sheet is if it's there? Above or below the edge tabs? I'm also curious if there's any composites in them and if they did anything besides roughing up the metal to get a good bond. I'll post pics when I get around to it.
Endre,
I would like to ask you what you are using a core for your skis or what would be the best core material and structure for my skis. You know what I need ;-) .
Geoff
I just built a pair of skis with thick maple cores, and they are really stiff, but they are quite heavy. I also just made a ski using a 12mm thick ash core, and it is pretty much the stiffest ski I have ever seen (it is also 130mm wide in the center).
Also, when I broke a salomon AK Rocket last year, the first part to break was the metal topsheet, followed by the foam core, then the wood core broke last. The ski was held together by the base. I would bet that just by using p-tex as a topsheet (which is a pretty strong material in tension, or at least flexible) and a hardwood core (like ash) with fiberglass tips and tails, you could get a really solid ski.
If I have well understood, the stiffness is proportional to the 3rd power of the thickness of the wood core: for example, increasing the wood-core thickness from 12mm to 14mm, the stiffness will be 50% more.
It is also proportional to the width of the ski, so the stiffness of 130mm wide ski is twice than a 65mm wide.
I made a reverse camber, 170mm wide ski (14mm thick core), and even if I put very few glass, it came out crazy stiff. I will machine the core to make it thinner.
With a wood-core 4/6mm thick at the tip and tail and 20/24mm thick in the centre, no glass reinforcement is needed anymore. The drawback is that such a thick core is more fragile, i.e. it breaks when subjected to a small bending strain.
I use Ash. not only because it has the stongest strength/wheight ratio of the wood I can get aroung here, but also because it is quite easy to get. Skis have been made from ash for thousands of years. A very cheep and good alternative is to use pine. If the wheight issue is a big deal it is an interesting fact that a lighter ski has a thicker core (!) this is because as you increase the core thickness a certain amount, you can reduce the wheight of reinforcement (glass, titanal, etc.) more than the added core wheight. When you do this, the ski gets more "brittle" and will not bend as far as the thinner, more reinforced and heavier ski before it breaks.
Endre,
You're in Europe, so I'm asking you if you are talking about American Ash as we call it here in Belgium? I think it's the most available ash over here but I can be wrong about that. Is it multiplexed ash board or 1 piece?
What about maple? Some of the persons here are speaking about it. Is there a website where we can find some reference material about weight and strength of the different sorts of wood?
Ok, the second part makes sense to me, but the first doesn't at all. Why wouldn't the stiffness of a material be linear with thickness?
Let's say you increase core thickness to get core X% stiffer, is it X% stiffer torsionally? I would guess not since the wood in the core has fibers running only in one direction and twisting the ski would create a lot of shear forces parallel to the grain. Right?
Is this why metal makes sense in a fat stiff ski? Since metal is non-directional you get stiffness in the two directions for less weight compared to a thick core and more glass. Looking at how thin the Mantras are I'd guess that the core is maybe 30% thinner than a comparable ski. That combined with not having to use any glass [maybe] might make a lighter ski for the stiffness.
Does this make sense to anyone else?
We use triaxial fibers for the outer layer of the ski +-45 an 0 degrees. The outer fiber of the structure takes almost all the load for tension and pressure. 0 degree while bending and 45 degree for torsion. There is not much left for the wood then shear forces.
Making the core thicker increases the section modulus. When i've got time i will write a little explanation (it's a little difficult for me because i only no the german terms for mechanics). Maybe you can take a look into a mechanics book to find out....
I have used both the american (white ash) and green ash, they have quite simular qualities. with maple though, you have to look out, because sugar maple, which is used for cores, skateboards, floors, etc. is not at all the same maple that grows here in europe, which is much lighter and quite soft. So be shure it's the canadian/hard/sugar maple, and not soft/european maple. actually ash is lighter compared to the hard maple. (and about the same strength) so I use that. (and maple is harder to get here.)
I've been looking for wood some time now but without success. I don't know where I have to look and what I have to look for?
Some are selling pressed ash plates but that's not the thing I need I presume.
I'm living in Belgium and the Netherlands. Is there someone who knows how I would be able to find what I need?
Geoff
Anyone have m5 magnetized cap inserts?
