Cutting a Swallow tail to soften up the tail

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petemorgan(pmoskico)
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Cutting a Swallow tail to soften up the tail

Post by petemorgan(pmoskico) »

I was playing around with a pair of skis that i had built that turned out to be way to stiff. I wanted to soften up the flex, and i thought maybe cutting a big swallow tail might help make the tail softer.

I cut the swallow tail about 6 inches deep from the tail, in a triangular shape starting with about the entire width of the tail. weirdly it didnt seem to make any noticeable difference in the tail flex.

So my conclusion is that the fiberglass/wood-core I-beam structure's stiffness is not effected by width. So a skinny ski of say 80mm underfoot compared to a fatty with say 120mm underfoot, if they have the same core thickness say 10mm then they will have the same flex.

am i right, or am i right?
SleepingAwake
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Post by SleepingAwake »

Nope, linear dependent. Your fatty should be 50% stiffer than the 80mm one...

But if a ski is flexed, tail on the ground, tip in one hand, the other hand bending ( 3 point bending), the bending moment on the ground is zero, max below the flexing hand and zero again on the tip with a linear distribution in between. So if the very end of the tip is softer, the difference is less noticeable by hand.

On snow it might be different thought, because the force is introduced differently.

Cheers, Reto
twizzstyle
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Post by twizzstyle »

You're wrong :)

Stiffness is proportional to the width, and proportional to the thickness cubed!
SleepingAwake
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Post by SleepingAwake »

Hmm, I think you might be wrong as well. :D

Stiffness is proportional to the thickness cubed for plain material, but not for a sandwich.

The sandwich stiffness is the sum of the stiffness of the core and the stiffness of the two face sheets. The core gets stiffer by the thickness cubed but the face sheets by the thickness (core thickness) squared.

So it's something between squared and cubed and not always the same depending of core thickness and thickness of face sheets...

Cheers, Reto
petemorgan(pmoskico)
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Post by petemorgan(pmoskico) »

"am i right or am i right" surprised montucky didnt jump all over that.

Thanks for the replies. That is some good analysis.

It is getting more complicated as i build a whole range of skis from 190 cm to 155cm and 90mm underfoot to 120mm underfoot. dialing in that stiffness is tricky.

One thing that i have noticed is that the skis do soften up a little bit after some use and then typically stay in a more or less same flex/stiffness without for a while after the initial break-in period.

One other piece of the puzzle is length. its seems like a 10.5mm thick core for a 190cm ski has a similar stiffness to a 9.5mm thick core to a 155 cm ski. what the formula for that guy?

I wish i had a reliable way to measure "stiffness" and measure the loss of stiffness over time. any good ideas? its mostly just a feel thing at this point.

thank again for the discussion
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MontuckyMadman
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Post by MontuckyMadman »

I have given up on you. You cannot be helped.
sammer wrote: I'm still a tang on top guy.
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LifeisRiding
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Post by LifeisRiding »

Thanks for the laughs~
Richuk
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Post by Richuk »

I wish i had a reliable way to measure "stiffness" and measure the loss of stiffness over time. any good ideas? its mostly just a feel thing at this point
You can build at test rig - per 56 sec: Drawing directly onto paper using a pencil held in a bicycle pump is a practical alternative to a laser. A simple rectangular frame will be sufficient. How the ski reacts under a continuous load is a problem, so unlike the video, the weight should not be left hanging. Use the weight to bend the ski and then set up a method to capture and maintain the original displacement is a better way to go. If you apply this method, you will gain a sense of how your skis stack up when compared to a commercially produced ski - its a bit rough and ready.

Loss of stiffness over time - fatigue, will need some further research i.e. failure modes of glass fibre. Assuming the elastic limits of the matrix have not been exceeded during use, you should be taking in terms of many 100,000's of cycles before the matrix begins to fail. So it sounds like you have something else going on, related to the design, choice of materials and or cure.

Hope this helps.
hamerhead
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Post by hamerhead »

We use a 3kg block placed at the boot position to measure the stiffness of the cores. Each end is placed on blocks and the resulting deflection is measured and recorded. Both cores can be placed side by side to ensure that they have the same flex pattern.
You can use this method to define how stiff the tails and tips are.
For the final product we use a 20kg block and measure the deflection.
This gets recorded and you can manipulate the next pair accordingly.
petemorgan(pmoskico)
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Post by petemorgan(pmoskico) »

http://precisionmotions.wordpress.com/s ... rousel-588



http://precisionmotions.wordpress.com/s ... rousel-589



What do you guys think?

Image

Image

I have riden two days on them. Both days during a temperature inversion on mount raineer, first day, we missed the corn window and ended up skiing down too late (we thought sunset might be perfect, we were wrong) and had really hard pack, pretty icy at times. tough to give a good review after that day.

second day, we hit the corn cycle just right and had great turns through wonderful corn. I did not notice the tails being any different then normal

because the tails are relatively stiff, i think that this may end up being okay. i could see if they were too thin that this may compromise the integrity of the tails and possible break. but they seem beefy enough.

i kept looking down at my tails to see if i noticed any flex difference while turning, but really could not notice anything.

It definitely lightened up the ski especially when you consider "swing weight"

Its my favorite skin clip so far! no skins clip will fall off these bad boys

do you think this may effect the torsional stiffness to a noticeable amount?

i think it will take a couple more days in different conditions and maybe some time cruising groomers to get a good report on it.

It was a crude cut of the slot. Can do better next time
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