Sag in the tip and tail-(not camber) to pre bend or not

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jonno
Posts: 11
Joined: Mon Jan 18, 2010 12:58 pm
Location: Chamonix

Sag in the tip and tail-(not camber) to pre bend or not

Post by jonno »

OK boys and girls, this is my first post so forgive me if i screw something up.

A lot has been written about camber and the loss thereof. But what of sag in the tip and tail of your skis. have they retained thier shape and hight and if so what process are you using.ie pre-bending or not? heated pneumatic press or unheated vacuum press? are you simple exaggerating the curve and height of your tip and if so by how much?

These are the questions that keep me awake at night. your input would be greatly appreciated.
twizzstyle
Posts: 2204
Joined: Tue Mar 07, 2006 8:25 pm
Location: Kenmore, Wa USA

Post by twizzstyle »

If you have good even heat, and your layup is symmetric about the core (fiberglass layers oriented the same way... i.e. if using triax, diagonal fibers away from the core top and bottom, or towards core top and bottom. Not both facing up or both facing down) you will have no changes in camber, or tip/tail shapes.

We use tip spacer material (p-tex) for much of our tips (2/3 of the tip about) and pre-bend it just slightly by hand, but that isn't so much to help keep the shape, as it is just to make the layup easier. Trying to mush everything down in the mold is tricky if its all flat.
doughboyshredder
Posts: 1354
Joined: Mon Sep 17, 2007 7:37 pm

Post by doughboyshredder »

I think it depends mainly on whether you are doing full length wood cores or not. That being said, I have had no relaxation of any of the tips on the boards I have built, and the wood core on last years boards went 1/2 way in to the tip, this year I used tip fill all the way to the contact points.

Twizz, I never thought of that with the glass. I always do that, but I thought it was more ocd then anything.
jonno
Posts: 11
Joined: Mon Jan 18, 2010 12:58 pm
Location: Chamonix

Post by jonno »

Thanks boys thats great. Im using a simple vacuum set up with no heat and full wood, so not sure if that will make a huge difference but im just trying to minimise the time that is spent with prep, and prebending with hot water, which is what i have done before means more time to wait for things before i can layup. I think too as it is really just the tip and tail and core thickness at those points is minimal it should be all good. great idea with the glass layup twizz. thanks again.
twizzstyle
Posts: 2204
Joined: Tue Mar 07, 2006 8:25 pm
Location: Kenmore, Wa USA

Post by twizzstyle »

doughboyshredder wrote: Twizz, I never thought of that with the glass. I always do that, but I thought it was more ocd then anything.
Yeah I think a lot of people forget this and might be the cause of camber changes some people get (the typical blame is uneven heating). The composite cures under heat, so when it cools to ambient temp it contracts. The fiberglass contracts along its length, with quite a bit of force. If you have fibers going lengthwise that are further away from the core on one side, they'll pull "harder" than the fibers on the opposite side of the core, warping the whole thing towards that side.

Way back in school I had a class that was something like "advanced composites in aerospace structures" or something like that (aero engineer). One of our projects was to layup a plate of pre-preg carbon fiber that after curing was as deformed as possible - it was the potato chip project :)
skidesmond
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Location: Western Mass, USA
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Post by skidesmond »

twizzstyle wrote:If you have good even heat, and your layup is symmetric about the core (fiberglass layers oriented the same way... i.e. if using triax, diagonal fibers away from the core top and bottom, or towards core top and bottom. Not both facing up or both facing down) you will have no changes in camber, or tip/tail shapes.
Ditto on the fiber glass lay up. I never gave that a thought. I will pay attention to that on my next pair... due out in a couple weeks. It will be interesting if the skis retain more camber or not. I don't use heat so I get some relaxation in camber but never measured the difference.

It's this kind of information/details that make this site a great forum.
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