Ski Flex Prediction...

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

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Bambi
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Ski Flex Prediction...

Post by Bambi »

<I am no longer hosting this spreadsheet> PM me if you want a copy - if some kind soul could make it publicly available then that would be great - distribute it freely.

To use it you can enter values in any of the yellow highlighted boxes. Key features are:

Specify your ski dimms
Specify your core profile
Specify the laminate (only works for 3 layer system, but you can fudge it if you want)
See your ski plan and core dimms
See your stiffness profile
View a deflection profile for a point load with simply supported end conditions. (note this is the deflection profile, not the final shape which would be influenced by the initial camber).

You may need to scroll down to see all the graphs depending on the size of your monitor.

It is a bit rough at the minute, but feel free to feedback suggestions/requests to me. I will hapilly answer questions on how to use it as I didn't put a lot of time into making it easy to use!

There is only one value for the youngs modulus of the wood core - if you are using multiple woods then you can just average the youngs modulus (but weighted according to the relative area of each wood) - only valid if laminated vertically, not horizontally like plywood.

As far as correctness goes - I have tested it against the analitical solution for a simple beam in bending and the results matched, so I am reasonably confident that the numbers are basically correct, but the model makes some small angle approximations and ignores the effect of deflection due to shear (which should be a second order effect in a beam with these dimensions). If you do find any mistakes please feed them back to me and I will update it accordingly.

I am quite happy for people to download / modify / redistribute as they see fit.


B.
Last edited by Bambi on Mon Nov 23, 2009 11:47 am, edited 1 time in total.

MLReed05
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Post by MLReed05 »

Great Job! I played around with it for a few mins and it seems very accurate when compared to measurements I have taken. Thanks for sharing that! I am sure it will come in handy in the future.

kelvin
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Post by kelvin »

Wow. I was only able to take a quick look, but from what I saw, it looked great and like a lot of work. Thanks for sharing. I'll have to look at is more closely later.

-kelvin

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littleKam
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Post by littleKam »

awesome! thanks for sharing!
- Kam S Leang (aka Little Kam)

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bigKam
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Post by bigKam »

Bambi:

excellent work! i enjoyed looking at it, and it's a very, very useful tool. would you consider writing up a short article that includes the Excel file for us to post on the "articles" page of the SB site? we'll definitely give you full credit.

question: how difficult would it be to incorporate determining the resonant frequency?

Bambi
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Post by Bambi »

I would be happy to add an article and for you to post it - I'll put something together and then send it to you.

Regarding frequency - I was interested in looking at this, along with trying to understand damping characteristics a bit more. The frequency thing should be possible from pure theory, but the damping will involve some experimental work to evauate constants. I will think more on this.

B.

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bigKam
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Post by bigKam »

Bambi:

great, send me your work when you get a chance: leang k k 'ut' G mail dat c o m.

the damping coefficient would be a very good number to know, and to compare over various designs, provided we have some point of reference. it would be a nice project to survey skis on the market to get these numbers and then compare them with homebuilt skis.

can you make a few approximations to back out the damping, zeta?

there's one simple way to experimentally measure zeta: mount a strain gauage to the ski; clamp the ski to a bench at it's waist; perturb with a hammer; record the strain vs. time profile; assume a 2nd order response and fit the data to find zeta. likewise one can easily find the natural frequency, Wn. i'll see about doing it soon, but if you have the equipment, investigate.

again, what you've created is very nice, and it will come in handy for builders. thanks for your contribution!

Bambi
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Post by Bambi »

I think that your expermental method is the way to go. Knowing the natural damping in each material and trying to do something theoretical will not be very accurate as there will be a lot of damping produced by the various bonds in teh system. The interesting challenge would be to try to work out how much the damping is effect by the presence of VDS etc...

hafte
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Post by hafte »

Bambi, I have been playing with the SS and really like what can be done to affect the flex in the ski. It gives great guide lines for core construction and with some of the comments G-man has made in other threads will be a very useful tool to expieriment with core design.

I have a couple of questions about the modulus's that are currently entered. Where did you find them? I have been doing some searches and have come up with very little. Here is one site that lists several wood species, which are much lower than what is in the SS.

http://www.pathnet.org/si.asp?id=490&adultfilter=heavy

Hard maple is listed at 1,600,000, which should be close to the ash I'm using. Was the entery in the original SS for Steel (10,000,000,000) or some other high strength material? Or am I just seeing/using the wrong thing?

Here is the tech sheet for the resin I use.

http://resinresearch.net/_wsn/page4.html

Would I enter the tensile modulus psi of 290,000 from them in the resin section? I'm thinking not since the units may not be the same.

I have noticed that making changes to young's modulus for the core makes a big differance in the flex pattern over the length of the ski.

And can the wood data in this site be used some how?

Thanks

Hafte

Bambi
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Post by Bambi »

Hafte,

The only time I spent checking was that the equations were correct - all the figures that I put in were just grabbed from the nearest place - I didn't take any time checking that the values that were in there were really accurate as I assumed that people would want to add their own and to be honest i haven't had much time to play with this yet myself!

Consequently there may well be some inaccuracies due to conversion of units - please do check that the values that are there make sense before proceding!

Having said that I got the various figures from the following sources:

Wood modulus - grabbed this from the figures off the Kams data sheet on this site. The value was an everage. had to convert units - I will take a look at the conversion this weekend. Perhaps I can put it in the sheet so that the sheet takes the values off the Kams data directly.

Glass fiber - Values for E glass from Mechanics of materials book
Resin - typical plasitc modulus - shouldn't have a big impact on things.

one thing that may also not be clear - for the glass fiber I have assumed a 3 fiber directions in one layer. The weight is the weight for the total for one side of the core and then the 3 layers refer to the 3 fiber directions within the overall cloth which I have assumed to be equal weight. So as dowloaded the figures should be right if you use one layer of the 22 oz Triax cloth above and below your core. I will try to make this a bit more flexible / clear and update the spreadsheet.


I'll take a look in more detail this weekend and get back to you on the specifica questions that you have.

B.

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Head Monkey
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Post by Head Monkey »

FYI, I believe most triax fiberglass does not have an equal distribution of fibers in all 3 directions. It’s typically 50% in 0, and 25% in each of 45/-45. I know this is the case for Vectorply E-TLX 2200 which is what I suspect everyone is using. (If you get your glass from snowboardmaterials.com, then I’m 99% sure this is what you’ve got. If he’s not selling Vectorply, then it’s something that looks, feels, weighs, and is woven exactly the same :)) Also, the “22oz” cloth is really about 23.55oz/sqyd. Here’s the data sheet, which includes a bunch of stiffness data: http://vectorply.com/pdf/e-tlx%202200.pdf.

Nice spreadsheet, by the way… for someone like me with little to no engineering background, it’s very helpful.
Everything I know about snowboard building, almost: MonkeyWiki, a guide to snowboard construction
Free open source ski and snowboard CADCAM: MonkeyCAM, snoCAD-X

Bambi
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Post by Bambi »

Thanks for the info regarding the vectorply. You can change the weighting of the relative strands by changing the equations - if you look at where the final youngs modulus is calculated it takes the average of the 3 layers. If you multiply the weighting of the 0 degree layer by 2 and then divide by four (rahter than just averging the 3) then you will get the correct number based on the info that you have supplied (I.e 50% of the weight is in the 0 degree direction).

I will try to make a more slick interface to this bit of the sheet and send out an updated version soon!

B.

sam
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Post by sam »

When it sais 1.00E+010 does this mean 1.00*10^10 or am really dumb and not getting it.

Bambi
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Post by Bambi »

yes that is correct

sam
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Post by sam »

so does 1.00E+010 correspond to 1000 kg/square millimeter on the page that lists the stiffneses of varios woods. I'm asuming that the 10,000,000 is for a square centimeter which explains why it is 10,000 times that of a square milimeter.
http://www.skibuilders.com/howto/skicon/woodcore.shtml

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