Ski Builders Forum

First post --- great forum!

I am wondering if anyone has ever pre-bent their cores using steam and then did the lay up of materials before pressing. To me, using straight cores puts the epoxy and fiberglass in a contant state of tension even at rest, maybe even contributing to degradation of the camber. The wood is trying to return to the flat condition while the epoxy is trying to hold it in the shape made by the press. If the wood is prebent, the epoxy and fiberglass are in a neutral state after the skiis are pressed. When the skiis are flexed during use, the wood, epoxy and fiberglass all work together to return the ski to the designed profile. I would also suspect that the skiis would be less prone to delamination, particularly at the tips. Any thoughts?

Frenchtom

See the attached link to a thread discussing this topic

viewtopic.php?t=498&highlight=prebend

^ I've missed that one! Very interesting, thanks!

wouldn't adding water to your wood increase the water content of it and thereby the weight? Also im wondering if it would actually decrease the poppines.

Because the tip and tail cross sections are so thin (2mm or so), I've noticed that in just wetting them out with epoxy during layup, they become quite pliable. So, I really don't think that there is much actual inherent tension in those areas after pressing, especially if a heated press is used.

The waist of the ski may be a different story, particularly if a dense hardwood is used for the core and if the waist thickness approaches 11 to 13 mm. It seems that pressing a flat core into a cambered core certainly must result in at least some degree of built-in pre-tension in the finished ski. The opposing tension energies of the wood and the glass might indeed have some damping effect on the flex responsiveness of the ski, or in the 'pop' as some might say.

Frenchtom, your questions and suppositions are intriguing. Welcome to the forum.

G-man

G-man -

I know that wooden stair railings and other curved wood on furniture are typically bent using steam. I have seen a DIY show where what looked like a 3" x 3" cross section of oak was placed in a steam box for about 30 minutes and when it came out it was as flimsy as paper. The craftman bent the end of the wood into a tight spiral which became the beginning of the stair rail. My root question is whether the wood strength, rebound, torsional stiffness, etc is degraded. For furniture and railings, this is no big deal, but on a ski it is prety important.

Frenchtom

I read something about this somewhere once, I thought it was here. I don't remember the details but one of the mass produced ski companies puts the camber into the core before they lay it up. The result is more pop and a ski that hold camber better.

Back in the middle 90's we built snowboard molds machined out of aluminum which were pre bent before they went into the molds. At the time they were cap molds and were what they call hard tooling. ( No air bags).This made laying up a difficult situation. The company came up with a small heated tip and tail press that used a heat blanket on each side. They would spray the tips of the full length wood cores with water and then put each end separately into the appropriated shaped form and apply the heat for a given time. To make sure that the cores didn't have excess moisture they had a heated enclosure that they place the core into before placing it in the layup. The biggest problem with this type of molding was the components had to be very accurately made so everything fit when compressed.The other major problems were determining the stretch when forming the top mold so the close out ledge was correct when the molds came together. This took alot of heat scratching since everyone had their ideas about how thick the tips had to be and everytime thicknesses changed, the lengths of the flat machined molds had to be tweaked so they would bend correctly. Air bag presses are much more forgiving when it comes to parts being a little thin or a little thick. There is only a flat top sheet so getting your core length correct is the key. This could be one of the reasons that cap tooling became out of favor due to the problems stated above. The other factor could have been edge strength. The cap construction is more prone to edge damage due to edge support and the park riders were ruining a lot of stuff on the rails.

The company that put the camber into their cores was Volkl. They used to machine the camber into the cores which was more expensive since they had to use a thicker blank. It made good sense due to the fact that the fibers were all in a relaxed state before you flexed the ski. I am no expert here but I believe that the types of wood makes more difference that the "pre stress" in the core but again, I am not an expert in this area.

Cadman wrote: They used to machine the camber into the cores which was more expensive since they had to use a thicker blank. It made good sense due to the fact that the fibers were all in a relaxed state before you flexed the ski.

I like this idea..

Has anybody tried shaping the core with a router for camber, similar to the top side?

RoboGeek wrote:Has anybody tried shaping the core with a router for camber, similar to the top side?

i tried this
of course the shape is good
but we cut wood fibers in this case
so the core lost its "sping" properties