compensating core thickness with top veneer

[PTTR]

Hepp

Does anyone of you compensate the thickness of the cores when using veneer tops instead of varnish or other tops? and if so, how much?
0.6 mm seems to be the standard thickness of veneers around here and a core with that added on would make a quite stiffer ski but would the same stiffness be achieved when the thickness (veneer) is added on top on the fiberglas?

[skidesmond]

The veneeer does not add much stiffness to the ski .

[twizzstyle]

Your last sentence is the key here... The fiberglass, where the majority of your stiffness comes from, is under the veneer. The distance from the center plane of the core to the fiberglass is really what matters. You should not be changing the thickness of your core to account for the veneer unless it was under the fiberglass (which would defeat the purpose of the aesthetics of the veneer!)

[Richuk]

There is an interesting question that flows from this, which is whether there is any noticeable none-structural effect due to the choice of veneer and what factors influence any conclusions.

[vinman]

I have a friend that works for a global snowboard company here in VT....he tells me to decrease my core thickness by .2 mm when using veneers for top sheets. I have no data on this advise, just his expertise in the snowboard biz. In our world 1/42 of an inch is a fairly large amount when we are talking about tenths of millimeters making a difference in core stiffness. 1/42 = 0.238 in = .6mm.

Noticeable difference is the key here. In deep snow will I notice a miniscule difference in ski flex caused by using one veneer or another? Doubt it.

Could it be in some way measurable? Probably if you have the right equipment.

[Richuk]

Spend $50.00, join a few wires, download a free program and you are there.

[teleman36]

We use the Alpi composite veneers for most of our skis so I guess we have already compensated for core thickness buy having it in the system and much trial and error.. We feel that it adds a more beefy,damp feel to the ski and not a noticeable amount of stiffness. However, on one set of skis that was designed to have a carbon top sheet the core seemed a little thin so we put in a layer of veneer over the top layer of glass and under the carbon layer on top.This really stiffened the ski up a LOT. It was like have an other mini core in the ski, as it was between the composite layer. This is just my two cents on this topic, but we plan on using veneer in the layup for special applications.. Good Luck, Mark

[emegram]

Yes, basically, adding a veneer will stiffen your ski significantly, if mostly because it is the farthest layer from the center of bending of the ski. This means in contributes to the bending stiffness more than a veneer closer to the center (the relationship is actually distance to the center squared).

The bending stiffness (EI) of a beam with horizontal layering (a good model for a ski) can be modeled by this equation:

EI = sum( E*( (w*h^3)/12 + w*h*r^2 ) )

where:

EI = bending stiffness; for reference, a maple core at 12mm thick and 100mm wide would have a stiffness around 200Nm^2
E = modulus of elasticity, or youngs modulus; about 10GPa for most woods
w = width of the layer, which should be about the width of your ski (100mm?)
h = thickness of the layer, probably somewhere less than 1mm for veneer
r = distance from center of bending of the ski (approximately the center of the core) to the center of area of the layer. This would be about half the thickness of the ski, so probably about 8mm for the veneer. Noticed how this is squared in the equation above

Then, to find the total bending stiffness of the ski, you would sum the bending stiffness for each layer.

(if you try running your own numbers, remember your units. 1Gpa = 1 BILLION Pascals)

I would guess your veneer would contribute about 50Nm^2 of stiffness, so about 1/4 the stiffness of your core. Therefore, using the equation above and my guesswork numbers again, decreasing a 12mm maple core to 11mm would successfully compensate for the veneer.

Remember, however, that as you get to the tips of your skis, the "r" from the equation above will get much smaller, and therefore your bending stiffness provided by the veneer will decrease quadratically, while the stiffness provided by the core will decrease linearly (because your core should have an "r" of 0)

There is a lot of approximation and hand-waving that goes into making this work (how do we get an accurate elastic modulus for materials? is this elastic modulus actually linear? Is the center of bending on the ski really the center of the core? Can I ignore flaws in the materials? What about torsion?) so make sure to take this for no more than it is, a model. The best way to find out is to just build them, but this might point you in the right direction.

Big takeaway: stiffness is relates to distance from layer to center of ski and thickness quadratically, and related to width, and modulus of stiffness linearly.

Hope this helps

[gozaimaas]

I am using veneer for my top sheet material and will be building multiple boards with differing core thickness stepping up 0.2mm per board. I will post my findings but this really only relates to my board shape which is far from the norm.

The answer you are looking for can only be found by building 2 boards the same with the exception of one having plastic top sheet, the other having veneer top sheet and being ridden by the same rider.

[Brazen]

Veneer topsheets add stiffness, actually a surprising amount compared to an Isosport topsheet. That's my experience anyway.

[skidesmond]

Veneer topsheets add stiffness, actually a surprising amount compared to an Isosport topsheet. That's my experience anyway.

Huh. I retract me previous post. Thanks for keeping on the straight an narrow guys.