Wednesday, March 30, 2011

Board Outline Drawing

Excel Workbook to plot board outline, rocker and concave I started to put together an excel workbook to help plot certain parts of the board. I've got no plan to build in any fluid dynamics (largely cause I don't know how to), its purely a drawing project. However, the value I can see in it is that it will provide a way to accurately (mathematically) describe the outline and so the impact of changing them can be studied more systematically (less anecdotal). The first cut at it was just for board outline and used only second order polynomials to describe the curves separately on each side. This was pretty limited and made drawing the curved tips really difficult to do other than through Excel's solver function to nudge the circle into where the short and long side curves intersect.

A better alternative is to use cubic splines which are piecewise third order polynomials that gives a hell of a lot more control of the curve.

To use the splines you need to pick the points you wish the curve to pass through and then generate a spline for each set of 3 points. Ideally you would put some smoothness boundary conditions on the splines so that the spline 'pieces' fitted smoothly together. However, this added another level of complexity so I left it to joining them by eye. Here's a first cut at piecing 2 cubic splines together to create one quarter of the board outline. One for each of the short edge, rounded tip , long side (mezzanine) and mid section.

There are a number VBA macro out there that you can just cut and paste into an excel workbook. The one I pulled down could easily be tweaked to spit out the co-efficients so that the whole outline of the board could be accurately described with the collection of 6 co-efficients used to generate the outline. Some of the other things I'll add are calculating the radius of curvature for continuous rockers, determining center rocker lines needed to get the desired concave.

Wednesday, March 23, 2011

Plastics in board making

Something that I've found particularly confusing in the research on board construction is what plastic materials can be made use of in construction. There is a stagering array of different plastics with varying chemical inertness, strength and abrasion resistance. It seems that at one end is PVC that epoxy will stick easily and at the other end of the spectrum is Polyethylene which according to most punters is very chemically inert and so epoxy (and most things) will not chemically bond to. ABS, very commonly used as rail material and, at least for Brokites, as stringer material, is towards the PVC end of the spectrum. However, there are two additional factors that appear to be important in working out whether epoxy is going to stick or not and depending on the application it may or may not be desirable - for example, High Density PE ( which apparently is the most widely used plastic in packing material) is perfect for using as vacuum film as epoxy wont stick to it. The additional factors are how rough the surface is and whether you have flame treated the surface (literally pass a flame over the surface). Roughing the surface allows a mechanical bond to occur between the epoxy and the plastic. One source states it as 'scratches are like grab handles for resin'. If you want the plastic to stick to it then rough it up with 40 grit sand paper first. Flame treating oxidizes the surface of the plastic which in turn improves the epoxies ability to chemically bond with the plastic. So despite HDPE being know for peeling easily off resin coated surfaces, according to one snowboard maker, roughing the surface and flame treating creates a great bond with the epoxy. This punter used 1/32" HDPE sheet as the protective bottom layer on the bottom of his snowboard. The other PE derived material which apparently is the dogs balls of plastic to use on the bottom surface has the catchy name UHMP - Ultra High Molecular Weight Polyethylene which is tough, abrasion resistance and slippery. Ideal for the bottom of a kiteboard ....... but apparently v. expensive. Have not tried to price it myself. HDPE is apparently much cheaper and easier to come by as PE ( the PE in PET plastic) is very common. Am going to try to track down some and try on the next board. If anyone knows of a supplier in Sydney that sells 1/32" sheet and thinner film then left me know because, despite it apparently being ubiquitous, I can't find a supplier. Maybe I'm just not looking for the right keywords but I thought that HDPE would have done it......... UPDATE - thanks to JMF on kiteforum I've got a great update to the search on bottom sheets. 'PBT' plastic film is the stuff used as bottom sheets on snowboards, wake boards, and most importantly Brokite boards!!!! Still looking for an AU supplier if anyone knows of one. Cheers Matt