The Magic of Metal

[Tanglefoot]

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?

 

 

Edited August 13, 2016 by Tanglefoot

[west carven]

howdy

how about a thicker softer wood core with bamboo-rubber-carbon.

thickness is the key word for less floppy frequencies where you get a nice smooth flex from tip to tail.

like a nice Cadillac instead of a vw bug.

[lowrider]

Tanglefoot Excellent analysis. The limiting factor in my opinion is $$$  Since it is possible to construct infinite variables the challenge is to come up with a specific

 combination to meet the desired outcome without having to build 1000 prototypes. I think west carven hit on basic principal that represents the delemma of snowboard construction.

 Not all manufacturers even start with similar core construction. The variables multiply from there. I know from speaking to Bruce Vasarva     (Coiler Snowboards)

that trying to use previous date to predict future success with different materials is proving to be difficult. The ability to influence board behavior by manipulating just the core has profound results.  The very fact that a snowboard is a composite of materials compounds the problem.. You are correct it isn't magic that makes the perfect board it's magic to find the correct composition of materials given the variables. I'm still holding out hope tenssioned carbon stringers and rubber strips under the edges can produce some positive results. Time will tell. 

[neanderthal]

I am happy to see this level of thought going into development. It sure is scary to think I will not be able to just order a new Nirvana from Bruce when my  quiver looses some pop :(

[Tanglefoot]

Altering the core thickness will certainly alter the natural frequencies - as will altering the stiffness of the skins. However, these measures will also change the flexural and torsional stiffness of the board, so not a viable option if your board already has the correct flex. And the damping would not be greatly affected by doing this either. 

Lowrider has a very good point in that there are far too many parameters for comfort, and that it would be too costly to build enough prototypes to map them all. However, I think you can get very close to the target with a systematic design process and a benchmark board. You need a materials database, suitable analysis tools and some test equipment - nothing your local engineering college or university couldn't handle.

If I were in the business of manufacturing edge hold oriented snowboards, I would turn up with my favourite metal board and see if any of the students would be interested in reverse engineering it with different materials. Measure as many of the static and dynamic properties as possible, and see if these properties can be replicated without aluminium. If this approach proves successful, then you can start playing with the different variables one by one. The starting point for a whole new family of non-metal boards?

Again, I think the secret to success will be in finding the best materials and thicknesses for the damping layers.

[west carven]

howdy Tanglefoot

8 hours ago, Tanglefoot said:

so not a viable option if your board already has the correct flex.

you are already wrong!

need to start from scatch. I believe there is the perfect thickness to length where the floppy frequency will be eliminated.

[west carven]

howdy

let me say that thickness equals all of the materials used to make a finished board without the metal

say ash or whatever wood, bamboo, rubber, carbon, p-tex  and yes there is a lot of variables, but there is a perfect

thickness to length ratio.

[Neil Gendzwill]

Tanglefoot, you should probably buy Sean or Bruce a few beers and pick their brains over this. I get the impression that Sean takes a more calculated approach whereas Bruce is all about emperical.  

[Corey]

Most definitely.  Sean has done lots of experimenting on various construction methods at very high levels with racers and recreational riders.  While I play an Engineer at times, sometimes a mathematical model is not the solution if the problem has enough confounding factors and you don't have the resources to devote thousands of people-hours and millions of dollars to the project.  See automotive tire design, wind tunnel testing, and probably alpine snowboards.  

Note that the real secret of a 'metal' board is actually in the rubber layer.  I had interesting discussions with both Sean and Bruce about this some time ago.  I'll let them write up their thoughts if they want.  

[daveo]

On 22/07/2016 at 10:27 AM, Donek said:

Never Summer uses pre tensioned s glass precure laminates.  Carbon has a short elastic curve to begin with.  Pre tensioning it will only shorten it further and increase the likelihood of a catastrophic failure.  A balanced laminate stack becomes a necessity to eliminate changes in shape with temperature variations and de molding.  There are many ways of generating a damper board, but Carbon fiber is unlikely to achieve that goal.  You could look at wavy fiber technology, but the patent is probably still in effect.  Metal construction boards would be better described as rubber construction boards.  The metal generates a certain perception of dampness, but it's the rubber laminates that do most of that work.  The rubber is there because the boards will fall apart without it.

 

I think what Corey is referring to is the post above in this thread.

[lowrider]

I will probably be lambasted for saying this but the greatest dampening device to date is a plate. It is the ultimate device that guarantees a more damp ride on any board. I have seen video of a board ridden with and without a plate and the contrasts are remarkable. I know most people seem to harbour negative feeling towards plates for many reasons but if dampness is a priority you shouldn't overlook a proven tool.

[daveo]

But I don't think a plate has any bearing on how the board handles ruts.

[lowrider]

9 hours ago, daveo said:

But I don't think a plate has any bearing on how the board handles ruts.

I will not respond to sticks and stones.

[Corey]

Plates definitely do have their plusses and minuses (and not many minuses!), but these guys are talking about optimizing boards.  

I don't like hitting sticks or stones with my boards, regardless of construction!  ;) 

[b0ardski]

Mark Miller of Thirst snowboards has managed to create non metal boards with amazing dampness. His boards are heavy, but so is a Cadillac and the ride is similar. I do think overall weight is an often overlooked factor in a boards dampened trackability, not a determining factor but  contributing one.

When I demoed the Thirst X7 the weight and apparent extreme stiffness (thick core, lots of carbon) scared me off a bit, but the totally intuitive, predictable, chop and rut proof Cadillac ride was a pleasure at fast or slow speeds unlike the Identity carbon 185gs that needs speed to come alive.

in short, I do believe tanglefoot is right, dampness is not necessarily metal dependent.

Edited August 16, 2016 by b0ardski

[daveo]

2 hours ago, lowrider said:

I will not respond to sticks and stones.

What does that even mean? Whatever it means, I'm glad you responded :)

But as Corey and I have both said now, this is about how damp a board it, the plate has nothing to do with it.

 

[BlueB]

On 8/14/2016 at 9:49 PM, west carven said:

howdy Tanglefoot

you are already wrong!

need to start from scatch. I believe there is the perfect thickness to length where the floppy frequency will be eliminated.

Howdy Westy! 

You can't oversimplify things like that... Thickness is the parameter that directly influences the stiffness of a beam. It can not just randomly be changed to address the dampness - it would kill the designed flex. It is not the thickness, but the mass that dampens the vibration, therefore you perceive the thickness as the dampening factor, while it is really just the extra mass that thicker board has. 

[BlueB]

Also, we need to figure out what is it, that we perceive as "dampness". I think that 2, or even 3, properties get often mixed up and looked at as "dampness". 

One is how much vibration and what frequency the board transfers to the rider. The other one is the general ability of a board to track in a less then perfect conditions (don't jump in with the plates here LR, please, we are looking at the boards only). The third is how much pop / controlled pop the board gives. 

[BlueB]

I've got a feeling that metal was an easy way to produce a torsionally stable board, without making it too stiff longitudinaly. The byproduct was added rubber shear layer for bonding, extra mass and more dissimilar materials used (wood, lots of rubber, carbon composite and metal, compared the traditional boards (wood and fiber composite, little bit of rubber). This resulted in extra mass and extra rubber dampening the frequencies; more materials of dissimilar natural frequencies also dampening the vibration; softer flex lowering the vibration frequency, while combined with torsional stability allowed for better tracking; plus the heavier and softer boards became less poppy. On top of that, the new geometries came along and improved the initiation/release and tracking further, resulting in the super boards we have nowadays. 

I'm pretty confident that a similar board can be produced without metal... 

[lowrider]

Nobody has come up with a little blob of plastic on the tip of a snowboard  like Dynastar  did with their skis and Dodge is doing with their  2016  trucks. Counter weights hanging off the frame to reduce vibration so your 43 coffee cup holders don't vibrate. BlueB i didn't know you rode in less than perfect conditions.

[BlueB]

We are coastal here at Cypress. You can get everything from nasty refrozen stuff, to heavy wet chowder. Grooming quality also went down over the years. Also, the proximity of huge city can make for huge crowds and chewed up slopes. 

Edited August 17, 2016 by BlueB

[Beckmann AG]

Thank you for taking the time to insert a navigable breeze into the summer doldrums. The ship’s pony lives to sail another day.

 

 

You may have overlooked/missed a few things, some historical, the other consumer based.

 

A snowboard is essentially a wide ski.

So a brief look at historic ski manufacture, from the first metal skis into the shaped ski era might be of use.

Head introduced the first metal skis in, I think, the mid to late 60’s. I have a pair of ’slaloms’, and as I recall, they skied reasonably well given materials science of the day.

 

Rossignol introduced a variety of dampening agents in their long running VAS series of skis in the early 80’s, complete with computer modeled ad copy. This began internally, (stressed carbon stringers? No.) and concluded with external aluminum plates bonded to the top sheet with a layer of rubber. Rossignol have often trended toward being overly damp, almost to the point of feeling ‘limp’.

 

Dynastar employed tip mounted weights floating between foam pucks introduced in time for the ’84 Olympics, and airflow devices/rubber tip guards to the same end on the recreational skis. At some point they used a capsule of small ball bearings. The first generation of tip hardware was dropped around 87?, with the introduction of the coupe du monde construction.

Despite being part of the same company, Dynastar tends to have a brighter signature than Rossignol.

 

I’d seen at least one set of aluminum blocks mounted to the ski tip and tail to reduce vibration/improve grip, and while they worked well, the skier claimed the added weight affected his knees, and not in a good way.

 

Meanwhile, K2 was introducing triaxial weave fibreglass in their KVC series, as a means of reducing torsional deflection. Dynastar had used a full length, omega-shaped (say ‘omeega’, like 007), zicral aluminum beam as  part of their core construction for years prior, for the same purpose. 

As the skis became torsionally stiffer, edge grip improved, with corresponding changes to the vibration profile.

 

In the transitional 'shaped' era, Salomon used struts anchored in rubber as a means of controlling tip and tail 'bounce', a variation on the Tinkler snowsticks.

 

Bear in mind that this began when skis were straight, and most turns by most skiers were largely skidded. As such, vibration tended to work across the ski, rather than down it’s length. The natural camber/decamber action was employed to shape a turn, but not to smooth that turn. Skis were longer, in part because more length meant more edge contact and more mass, both of which provided better grip in a largely pre-carve world.

The tendency was to use metal laminates in both GS and DH skis, and avoid that use in slalom skis so as to preserve the ‘quick twitch’ response beneficial for short duration turning.

The added mass and the resonant characteristics were of greater use in the speed events, where one favored stability over ‘response’. The success of Kessler, et al on the snowboard race circuit stands as validation.

 

To get an idea on resonance, thwack a fingernail against a bike top tube made of steel, aluminium, and crabon fibre.  You’ll notice that each has a distinct sound, from bright to flat.

Combine tonal quality with mass, and you have a means of smoothing the ride.

 

There were always a few athletes making better use of the tool, carving more of the turn, and enjoying a smoother, faster glide. Which suggests that the tool need not be heavy/damp to be effective. 

The industry, however, is very dependent on the ‘lesser’ athletes, those willing to spend money every year on new gears as a hopeful substitute for more effective technique. (That’s accounting, not judgement).

So keep in mind that what really works well for the top athlete, probably won’t work well for the average. And what works well for the average, can still be used to great effect by the more skilled.

 

And regarding suitability/durability, metal skis have been around for a very long time, and they hold up just fine, within reasonable bounds. 

 

Getting back to snowboarding, consider that in order to make a carved turn, a board needs to bend, but it does not need to twist. (That ski performance improved when they became torsionally stiffer should make this point obvious.)  When a board twists while in a full engagement turn, one end of the board is essentially trying to make a turn of different characteristic than the other end. This generates a dissonant waveform, and if the frequency and amplitude are significant, the rider can get ’chucked’ out of the turn. If you lower the resonant frequency, that same rider twisting the board to the same extent can essentially ‘hide’ between the wave peaks long enough to reach the effective ‘end’ of that particular turn.  This is evident by the tracks left behind, and also from what one can observe in real time.

 

If a rider can figure out how not to twist their sled, they can be assured of a smoother ride, material composition notwithstanding.

 

 If one does not introduce 'harmful' vibration, one has no need to damp or attenuate that vibration.

 

More consideration might be given to the how’s and why’s of dominant technique, as well as to the boards that support that technique.

How we ‘choose’ to ride is influenced not only by what goes through our heads, but by what we have on and under our feet. 

As you continue your analyses, consider that just because a board exhibits a particular vibration, doesn’t mean that vibration is a necessary part of riding. 

 

Given that cores can be made of both wood and foam, and that both find ready acceptance on the open market, it could be said that the core is little more than a coat rack, a place to hang the goods. The core profile helps establish the geometry of the spring pack comprised of material laminates, and that, in turn, likely determines the spring constant, which then helps determine how much the board can bend for a given rider weight, velocity, and sidecut. 

Assuming there is metal and rubber involved, one can also assume that the board will be slower to return to it’s original shape than a similar construction without the silver and goo.

That comes in handy if, for instance, the rider is not making effective use of their legs as a suspension system. One of the best damping systems available is hanging off your bones.

(In the early days of mountain bike suspensions forks, which took more edge off the wallet than the trail, tire manufacturers messed about with compounding as a means of keeping the tread more fully engaged with the dirt.)

 

So the net effect of a metal laminate is to use both mass and ‘tone’ as a means of tuning the resonant vibrations and rebound to the greatest benefit for the greater number of users.

In the absence of metal, a rider can achieve similar results by altering their technique/inputs to  match their behaviour to that favored by the board, rather than the converse. Which introduces the sticky question of "Why do I do what I do, when I'm doing it?"

It’s not like everyone will stop riding in the absence of metal boards. If some do, others will replace them in the marketplace, further innovating to ensure their continued enjoyment of the sport.

 

Metal isn’t magic. It’s a beneficial sleight of hand.

[nigelc]

I think Bluebee nailed it. Metal is bendy but not stretchy, so soft bend with good torsional stiffness. Also wood, frp, aluminium and rubber all have very different vibrational characteristics and so transmission of vibration between the different materials is hindered in the same way as an impedance mismatch attenuates signal transmission in electrical communication.

It seems to me there are two paths: either develop a means of treating aluminium sheet to permit bonding, or to find a way to make composite layup mimic the torsional and flexural characteristics of metal  

[Tanglefoot]

Beckmann AG, thanks for your input. Love the history lesson!

[Tanglefoot]

Seems like Rossignol have several patents related to vibration damping, some involving constrained viscoelastic layers. Other manufacturers are also thinking similar thoughts. Try googling "ski damping patent" or "ski damping patent rossignol" if you need some bedtime reading.