You will hear about a lot of fancy mechanical terms but dont worry, it is up to you how much you wish to learn and work with it. I want describe many of them (Matrix or Flexural rigidity), but in general the best idea is to read on forums, and decide what to build by borrowing some ideals from others, and copying the build up and layup to start with. Dont let this scare you, its easy to pick up.
- Flexural rigidity of a ski or a snowboard is what we call stiffness. It is defined by your core material, core thickness and composite materials!
- Matrix of a diy ski or a snowboard, is the mixture of composite fabric and epoxy. To get best properties, you need right ratio. Typically you weigh your fabric and then you mix the same weight of epoxy!
- Pot life of a epoxy resin for skis or a snowboard is the time you have to work with your epoxy before it becomes gel and unusable. Make sure you dont exceed this! Be well organized!
- Cure time of the epoxy resin, is the time it takes epoxy to cure, where all molecules are bound, and for your ride to be usable.
Rules of thumb when building a DIY skis or a snowboard
- Use epoxy above 20 degrees C, minimum at room temperature. It is much easier to work with!
- For every 10 degrees you raise the temperature of your layup, the epoxy cure time will half. In general a full cure takes 7days or more. But in 24h typically you will have 90% or more of a cure. If you imagine that you cured at room temperature + 10 degrees C, the the same cure will be achieved in 12h. If you imagine you cured you layup at room temperature + 20 degrees C, the same cure would have been done in 6h. Many resins cure at 80 degrees C in just 30minutes or so. Further more when you cure at higher temperatures, your resin will be more viscous, and penetrate the composite fabric better, giving better result.
- The stiffness of your final product, total thickness of your ski/snowboard, is proportional (^1) to the composite material weight/stifness, but ^3 to the core thickness within the composite material! This means you can increase the stiffness/weight of you composite material to improve stiffness or you can increase the core thickness, however the core thickness is a much more effective way to do it. If you increase the core thickness by 10% your stiffness will go up ~30%. Read here. This also means that any thickness change within the composite sandwich is much more effective then thickness changes outside the composite sandwich (like adding some veneer on top or a topseet)!
- Weigh your fabric and mix the same weight of Epoxy. This is a good start to get optimum fabric/Epoxcy ratio.
Stiffness or flexural rigidity of a diy ski or a snowboard
Link to the simulator: https://www.junksupply.com/guide-to-the-simulator/
…basically how stiff will your design feel? It is probably the most important property of your ride. In order for you to be happy with it, the design has to be just right.
The final stiffness depends on many factors and it is very hard to guesstimate. You have so many variables that do influence the stiffness, mainly the core thickness, the composite material properties and type and also your design width and length etc. So before you start crafting your ride, it is best to do a few calculations in order to get an idea how stiff your design should be and how to build it. For this you will need to look at some rides that you already know, and use it as a reference/benchmark for the calculations/simulations. Then you can do a study on how the design changes will influence the final design properties, hence the performance, and you can tweak your design till you are happy with it.
Luckily we have a tool here that will help you estimate the stiffness/flexural rigidity and many other properties of your ride, go to our simulation page.
Please remember:
‘The stiffness of your final product if proportional to the composite material, but ^3 to the total core thickness! This means you can increase the stiffness/weight of you composite material to improve stiffness or you can increase the core thickness, however the core thickness is a much more effective way to do it. If you increase the core thickness by 10% your stiffness will go up ~30%. Read here.’
also note, that on wide rides, basically anything than skis, you will have to deal with the stiffness in width as well, which is pretty much unwanted and will only worsen your ride in most cases. In reality it is reflected by the flexing of the ride diagonally, that is around the length axis. So in general you can look at it as
– longitudinal or length stiffness: which is the stiffness of your ride that you wish to determine as much as possible. Do you wish a stiff and responsive ride, or a soft and sluggish ride or something in between these two extremes. This is why we do simulations/calculations.
– width stiffness: which is the stiffness across the width of your ride. In general we wish this to be as stiff as possible so that you dont have unwanted flexing in the width of your ride that will make it worse performing. In ideal situations it should be infinite, in reality we use multi-axial composite material to make it as good as it can be.
– diagonal or torsional stiffness: this is basically a combination of the first two (length and width), and since the product is always physically the longest diagonally, this flexing of the product in the diagonal direction is the result of a lack of stiffness in width. You can imagine the stiffness as vectors, and any diagonal is a combination of the two (widht and lenght, x and y). In practice you can test the ‘quality’ of your product, by testing for the torsional stiffness. Start by placing the one side of your product between your feet , tightly, and grabbing the opposite end by your hands in each corner. Then try to flex the board by your hands by pulling opposite by each hand, so that it is locked tight between your feet and is turned clockwise or counter clock wise with your hands. The better the torsional stiffness, the harder it will be to flex the ride without having to loosen your feet grip.
The most widely used multiaxial fabric, and the one proven to give best stiffness properties in width and length is 0 (zero) degree combined with +/-45 degree. It is the one with the longitudinal fibers going in the length direction, 0 (zero) degree direction, and the rest of the fibers going in the plus and minus +/-45 degree direction, which is the diagonal direction of the ride.
It turns out that just as with vectors, you can split the diagonal stiffness vector into the length and widht stiffness vectors. The contribution you can get by multiplying the magnitude (in this case the weight of the ply in +/-45 degrees) with the cosine to the angle. So if the angle is 45 degree, the cosine to 45 degree is 1/square-root(2) or 0.707, then you multiply the weight of the ply in the diagonal direction with 0.707 to get the weight contribution to the length (or 0 degree) stiffness and width stiffness.