Stiffness Calculation

For discussions related to ski/snowboard construction/design methods and techniques.

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kiwigrabber
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Stiffness Calculation

Post by kiwigrabber » Sun May 08, 2016 2:44 am

Hey there,

Me and my brother built our first ski last season and it worked out pretty well, even though we didn't had any experience in building skis. The only thing is, that the ski is a little bit too stiff. So we were asking, if you guys know how to calculate the ski, that it's getting better the next time. I am searching for something to make a simulation on the Computer, and where you can see where the ski is bending at wich force, ... I know that there are such simulations for frames in the autocad inventor, but this program is more for mechanical things.
I am searching for something to make a finite element analysis where I can simulate the ski with everything that is in there (Core, fiberglass, carbon,...). Do you guys know a program that can deal with that?

Or is there another way to calculate such things and how are you doing it :)

Maybe some people could think that calculating such things is overrated, and deal with that with trial and error, but I think that's just a fun thing to see which forces are on the skis, etc... It's just a extra gimmick wich could be fun ;)

Greetings,
Wolfi

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MontuckyMadman
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Post by MontuckyMadman » Sun May 08, 2016 11:48 am

Any of this software can do fea
http://www.3ds.com/products-services/
Let us know when you provide this as a service
Its always a goldilocks situation. This ones too stiff this ones too soft this ones too heavy this ones too light.
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richie
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Post by richie » Sun May 08, 2016 2:21 pm

http://www.junksupply.com/ski-calculator/

This looks well thought out and there is a lot of research and engineering knowledge in it, while this won't necessarily be the silver bullet still with so many variables it is good to understand the influence of them all rather than relying on gut feel and testing. Designing in the properties you want sounds sensible after all!
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kiwigrabber
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Post by kiwigrabber » Mon May 09, 2016 3:44 am

Thank you for your replys!

I am going to try this products and how I can manage to get a good FEA Analysis.

And thank you for the junksupply calculator, this one is really great, just to understand how different materials affect the ski! Awesome!

I hope this helps a little to get a very good ski.

Greetings,
Wolfi

chrislandy
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Post by chrislandy » Mon May 09, 2016 4:52 am

Standard linear elastic laminate analysis (or classical laminate theory) is perfectly good, I really wouldn't bother with FEA unless you have a barrel load of time to kill, it takes a lot of insight and time to get the correct answer out. You won't get that much more accurate, in fact you'd probably get a less accurate calculation, using FEA over any other method unless you have actual test data from your process and materials.

FEA is complicated enough as it is with isotropic materials let alone laminated anisotropic

Sure, if you want some fancy pictures for the website then crack on but it's really not necessary.

my 2p (as a structural engineer)

24Dave
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Post by 24Dave » Mon May 09, 2016 12:28 pm

I know this is the opposite to what you are asking, but this has helped me figure out flex for different weight riders immensely. When I have a flex that I really like, I set the board in a frame wide enough no not involve the nose and tail, stand on it and measure the deflection. Then for a lighter or heavier rider I have the same weight applied as the target rider's weight and note the degree that that board bends. I target the same radius as what I like in my favorite boards under a heavier or lighter rider.

After more and more boards (or skis) you start to develop your core profile, wood types, composite types for the different weights of your riders as real proven data. Developing this book is pretty crucial to make a favorite board for someone of significantly different weight than yourself. I'd stick to the same composites and resin and core material for several and just vary the thickness profile to affect flex first. Then after you have the profiles that work for different weights it makes more sense to begin to tweak your composites or core material or epoxy modulus.

It is pretty primitive but then your only real feedback that you get from riders is also pretty subjective so perhaps lots of data up front is of little value in the end.

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vinman
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Post by vinman » Mon May 09, 2016 3:37 pm

^^The art of ski/board construction! Love it. I do things very similar to you.

I find that varying stiffness is more difficult when varying ski length than it is for skier weight. With a given core thickness, the shorter the ski the more stiff it becomes.

Thinning out the core and or using lighter glass is very important when you begin to build shorter skis/boards.
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Post by skidesmond » Tue May 10, 2016 2:16 pm

It sure would be nice to have a formula spit out the perfect board. But the formula only takes you so far, because the perfect board is different for everybody. What I've done is to make sample layups with different configurations of materials. Then measure the deflection with a the same amount of force (twisting and bending) to each sample. I record and compare the data of each sample.

So for example if you made a sample using biax FG and a sample using triax FG you'd see the difference and can correlate that difference to a board. Not a perfect system but it puts you in the ball park.

24Dave
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Post by 24Dave » Tue May 10, 2016 3:14 pm

Right Vinman, you have to account for the shorter stick being a shorter lever arm so the same stiffness piece does not bend as far into the same radius for a given weight.

I reckon for every significant change in length when you get a winner board for a given weight you have a new starting or reference point for tweaking to account for weight differences.

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Post by skidesmond » Wed May 11, 2016 3:05 am

You can always take the other approach... Once you get a ski that you like, enter the values into a http://www.junksupply.com/ski-calculator/ and get the profile characteristics of the ski. Then use that info and numbers to produce a a skis that's shorter/longer.. keep tweaking the numbers until you get a similar profile.

There used to be a excel spreadsheet calculator in the forum that did this as well.

twizzstyle
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Post by twizzstyle » Fri May 13, 2016 12:02 pm

Here's my take (full disclosure, I am an engineer, although I test airplanes, not structure)

The math is fairly straight forward for simple composite beam bending. You don't need FEA for something like this. Throw the maths into a spreadsheet and build some tables, make some easy plots, etc based on some reasonable distributed load. Not rocket science.

BUT, use any numbers you get from such an analysis as notional and relative. It will give you a good idea of a relative flex profile between shapes/thicknesses, but I wouldn't rely on the final numbers to mean much. There is a LOT of variability in handbuilt skis (depends on your tools, your methods, your skill level, your attention to detail, the quality of your composites, the quality of your epoxy, the temperature and humidity, the cycle of the moon, etc).

As an example, I made one pair of skis with a bamboo core. I wanted to make an identical pair with a foam core, and did some similar analysis to come up with a core profile for the foam that would give me the exact same stiffness as the bamboo skis. The math was easy, fool proof, I thought I was a genius. I made the skis, and they ended up being about 30% stiffer than the bamboo skis. Oh well. In the end they're the best skis I've ever made and are going strong after 3-4 years or something.

motoman
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Post by motoman » Thu Nov 17, 2016 6:36 am

Just try to go deep in calculation of stiffness.
I am stuck in calculation of torsional stiffness.
The calculation of bending stiffness if simple because of relative material uniformity along main axis. But in case of fiberglass it has to be take into account orthotropy. Has anybody the experience for calculation of fiberglass torsional stiffness?

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Post by chrislandy » Fri Nov 18, 2016 3:39 am

I did some work on this during my masters thesis, but can't remember all the bits now (it was a few years ago!)

You can model it but the most accurate was from empirical testing because it is so reliant on the quality of the layup and materials. i.e. you and I could layup the same part but it would exhibit a different response to the loads.

I know its for an isotropic material but a good publication to look at is "SCI Publication 057 1989/1997 - Design of members subject to combined bending and torsion" it goes through basic theory so a good starting point to apply first principles design to

motoman
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Post by motoman » Fri Nov 18, 2016 12:12 pm

Thank you for good advice. Now I am working of FEM modeling and analysis hope soon I will get some interesting results which may be will post here.

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Post by motoman » Tue Nov 22, 2016 1:04 am

I have a question for forum users.
Lets observe two snowboards or skis of one model.
What do you think, has the smaller ski or snowboard be softer then bigger?
Or they have to be the same stiff?
By the stiffness I do not mean the stiffness of cross section ( The bigger the product the bigger the stiffness).
I mean the deflection under the load.
I'm confused. I think that smaller product has to correspond to smaller weight range then the bigger one since smaller people usually weight less than bigger. For instance 151 height snowboards suits to weight ratio 120 -170 lbs (54-77 kg) according to height to weight tables. So the average weight is 145 lbs (65.5 kg). And 158 suits 150-240 lbs that is 195 on the average.
On the other hand if we would like to make the same product for wide range of users there has to be standard.
Yesterday I took two snowboards from the shop in order to make practical measurements. I have applied the same load in the center of each snowboard. Both were deflected on the same height.
This put me stumped.

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