Since AMAG has tightened up the supply of Titanal, there has been some discussion on this forum regarding alternative designs and materials. This has got me thinking about the fundamentals of board construction, and I have tried to find out exactly why sheets of aluminium are used in the construction of carving snowboards. I do not work in the snowboard industry, so my thoughts are based on general engineering principles. I have not read everything on the internet, so apologies if I am asking stupid questions or stating the obvious.
From various forum posts and from personal experience, the reason for using aluminium seems to be “better edge hold on ice and hardpack.” So far, so good. But why exactly should aluminium sheets improve this particular behaviour? This is where I struggle to find the answers, although the hardbooting community clearly agrees that metal boards possess magical properties. One aspect that is often mentioned is “better damping”, and another one is torsional stiffness. More on these later.
There are several drawbacks with aluminium: The coefficient of thermal expansion is different to those of glass and carbon, so the combination of these materials will cause interlaminar stresses in the structure. Aluminium is also notoriously difficult to bond to, and requires expensive surface treatment so that other materials will stick to it in the long term. In order to avoid disbonding, it is necessary to include “rubber” or elastomeric layers between the aluminium and the composite. Also, if you combine aluminium with composite, you end up with materials of different stiffness carrying loads in the same direction - which is structurally inefficient. Boards containing aluminium appear to be more fragile and less durable. In my view, there is a case for getting rid of aluminium altogether, not just replacing Titanal with something similar.
Dynamic behaviour is a relatively complex topic. A snowboard will exhibit several bending and torsional modes when modelled in isolation, and the dynamics will change significantly when you add bindings, boots and a rider to the equation. Different designs of race plates will also have an effect, both on the static behaviour and on the dynamic response. Tricky stuff, so I will only consider the board in isolation here - which is admittedly quite tricky in itself. It seems to me that a race board should be critically damped, but they all appear to be underdamped, i.e.. they oscillate for a little while after an impact. This is a bit like driving a racing car with sprung suspension but poor dampers - which would also cause poor grip. I have found a few papers on the internet regarding the dynamics of skis and snowboards, but none of them provide complete answers. The damping of torsional modes seems to be proven to improve edge hold, but this is not directly linked to the properties of aluminium. It is also unclear to me how much damping is needed for optimal ice grip. Is it the case that more damping is better, or can you have a board that is “too damp” to have good edge hold?
There are at least six different measures of damping that are in use in industry and science. I will just use the terms “more damping” or “less damping” in this post. Aluminium has less damping than composite, so this is not a reason to use it. In terms of structural properties, it is somewhere between glass and carbon, so let’s pursue this a little: Consider a metal board that we want to replicate with composite materials. We will keep the core the same, and the overall geometry the same. Now we can simplify the bending analysis, so that instead of calculating the bending stiffness in terms of EI for all sections of our sandwich beam, we can compare Et for the skins only, where E is the modulus of elasticity and t is the skin thickness. It is now easy to show that we can achieve the same bending stiffness as aluminium with unidirectional glass or carbon plies, but the glass board will be heavier and the carbon board will be lighter. In a similar manner, we can simplify the torsional stiffness of our torsion box from GJ down to Gt, where G is the shear modulus and t is the skin thickness. Now it is more of a struggle to match the aluminium with glass plies, but easy with carbon plies (at +45/-45 degrees). Again, the glass board will be heavier and the carbon board lighter than the aluminium version.
So by keeping everything else the same, including the rubber layers, we can match both the torsional stiffness and the bending stiffness of our aluminium board with triaxial carbon skins. However, our carbon board will be lighter, and this will lead to higher natural frequencies, which is probably bad for edge hold. Maybe it is just “lucky” that aluminium has the right density to create favourable natural frequencies? Mass is easy to add though. Some manufacturers use a P-Tex topsheet, which will add mass and hence lower the natural frequencies of the system.
There is clearly a difference between different makes and models of metal boards. I believe that the main difference is in the “rubber” layer. By selecting a suitable elastomer, preferably a viscoelastic material, it is possible to alter the damping of the board significantly. I believe this to be the most important trade secret of the successful manufacturers. The technique is known as “constrained layer damping” and is used in industry and aerospace.
Based on the ramblings above, I have the following thoughts:
There seems to be no reason why viscoelastic damping layers cannot be used to improve edge hold in an all composite construction. I am aware that some composite boards do have elastomeric layers, but I am not sure whether these have been tuned to provide the same dynamic behaviour as a metal board. In any case, here’s Tanglefoot’s Theorem for you: “By carefully engineering a composite board from scratch, and by careful selection of the viscoelastic layers, it is possible to replicate both the static and the dynamic behaviour of a metal board.”
The natural frequencies of the board can be tuned by means of small weights attached near the nose and the tail of the board. Mounted on viscoelastic pads, obviously. These should be adjustable in order to optimise the board for various courses, conditions, race plates and riders. This is analogous to tuning the suspension of a racing car to achieve the desired handling and balance. Nobody would design a racing car with no possibilities of adjusting the suspension.
So on the matter of metal in snowboards, I am with Vernon Dursley: “There’s no such thing as magic!” However, I have a nagging feeling that I am missing something. There are lots of clever people working on the development of snowboards. If the solution is this simple, why hasn’t it been done already? Or has it?
