Core profile
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Core profile
What core profile is everyone using? Are most of you using a similar design to what is posted from the K’s in there descriptions, plateau of 12cm in front and 20 behind CC seams to be there standard for many of there skis. Would be really interested in some comments on how people are thinking in terms of this.
Kronc, I have been using Bambi’s SS to help see this better. I have input most of the skis from the gallery as best I could to see how those skis have been working and to help me gage how I need to profile the core for me and the skiing I do. The results have been interesting. Most have a nice smooth curve and are nice to ski according to the TR provided. A couple had uneven flex tip to tail and were difficult to ski or made the skier work to hard to control them (the shit skis).
See this thread http://www.skibuilders.com/phpBB2/viewt ... highlight=
It uses the center of the running length so you need to adjust a bit find the CC. I have been making my mount area a bit bigger to get a bit stiffer center flex and softer tip flex like an alpine ski. 200 mm forward of the running length center and 250 behind on one pair, and 200 and 257 on the other pair.
The thickness and the taper of the core to the tips will make a huge difference in how the ski feels also. I have been working towards making two pairs of skis with differing thickness and taper but with the same flex profile and deflection according to the SS to see how much affect this will have on the performance of the ski. One ski will have no plateau (or very little) and be thicker. The other will have more plateau and be thinner. The actual ski testing will have to wait until next year though since the snow is going so fast here.
Hafte
See this thread http://www.skibuilders.com/phpBB2/viewt ... highlight=
It uses the center of the running length so you need to adjust a bit find the CC. I have been making my mount area a bit bigger to get a bit stiffer center flex and softer tip flex like an alpine ski. 200 mm forward of the running length center and 250 behind on one pair, and 200 and 257 on the other pair.
The thickness and the taper of the core to the tips will make a huge difference in how the ski feels also. I have been working towards making two pairs of skis with differing thickness and taper but with the same flex profile and deflection according to the SS to see how much affect this will have on the performance of the ski. One ski will have no plateau (or very little) and be thicker. The other will have more plateau and be thinner. The actual ski testing will have to wait until next year though since the snow is going so fast here.
Hafte
Apologies for posting this twice - I didn't notice this thread before posting a new one! Anyway:
I have done a bit of work looking at the deflection profiles of some of my commercial skis in comparison with my theoretical model before my first build.
Two things have come out of this:
1) The commercial skis that I have tested vary surprisingly little. Differences seem to be confined to having softer tips or a more rounded flex, but the gross deflections are remarkably similar (approx 10mm with a 3.5Kg load at the center).
2) The predicted central deflection for a ski of core dimmesions ~10mm seem to be about double that of typical commercial skis. This could be inaccuracy in my model (I ignore the plastic topsheet and base effects for example). To get closest to commercial ski deflections my model is suggesting 2mm tips/tails rising to a 13mm mid section (running 200mm in front and behind the CC).
So here is the question for people who have made skis - do you find that your ~10mm core is more flexible than commercial skis? If you have made ~13mm cores are these ridiculously stiff? I'd be interested to know.
Thanks for your thoughts...
B.
Edit: The model is extremely sensitive to the ratio of glass to epoxy ratio that is predicted. Changing this ratio impacts the overall glass thickness layer. A ratio of 0.3 gives a layer thickness of 1mm for a 22oz fabric. Does anyone have a feel for the final glass thickness layer?
I have done a bit of work looking at the deflection profiles of some of my commercial skis in comparison with my theoretical model before my first build.
Two things have come out of this:
1) The commercial skis that I have tested vary surprisingly little. Differences seem to be confined to having softer tips or a more rounded flex, but the gross deflections are remarkably similar (approx 10mm with a 3.5Kg load at the center).
2) The predicted central deflection for a ski of core dimmesions ~10mm seem to be about double that of typical commercial skis. This could be inaccuracy in my model (I ignore the plastic topsheet and base effects for example). To get closest to commercial ski deflections my model is suggesting 2mm tips/tails rising to a 13mm mid section (running 200mm in front and behind the CC).
So here is the question for people who have made skis - do you find that your ~10mm core is more flexible than commercial skis? If you have made ~13mm cores are these ridiculously stiff? I'd be interested to know.
Thanks for your thoughts...
B.
Edit: The model is extremely sensitive to the ratio of glass to epoxy ratio that is predicted. Changing this ratio impacts the overall glass thickness layer. A ratio of 0.3 gives a layer thickness of 1mm for a 22oz fabric. Does anyone have a feel for the final glass thickness layer?
P.S. - I have now updated the SS to address a couple of things (wrong weighting of fiberglass layers). and added the plots of a couple of outliers from commercial skis.
Find the new one here:
http://ben.arlett.googlepages.com/Skidimmsexe.xls
B.
Find the new one here:
http://ben.arlett.googlepages.com/Skidimmsexe.xls
B.
Bambi, as you can read in the post about the deconstruction of the Dynastar troublemaker, the glassfibre layer is about 1 mm thick for biaxial fibre of probably 600gr/m^2 (22 oz right?).
It might be that this thickness differs from the composite thickness when building your own skis because in ski industry often prepreg composites are used which are thicker for some reason (fiber to total ratio?).
In my own ski, as far as I can measure it, the composite thickness is around 0,5 mm, which corresponds quite good to using a fibre to total ratio of 0,6.
About the inaccuracy of your model, the base and edges do influence the model pretty much ( http://www.skibuilders.com/phpBB2/viewtopic.php?t=669 ).
I measured my homebuild skis and compared them with my Matlab model (for which I was inspirated by your nice SS) and the results were within 1 mm accuracy for a deflection of about 20 mm. I didn't measure the deflection for different loads yet, but will do this later. I think it's also a good idea to collect date of our own skis to be able to compare it to our models. I will post a simple guide on how to measure the ski deflection and what data are needed soon.
Buuk
It might be that this thickness differs from the composite thickness when building your own skis because in ski industry often prepreg composites are used which are thicker for some reason (fiber to total ratio?).
In my own ski, as far as I can measure it, the composite thickness is around 0,5 mm, which corresponds quite good to using a fibre to total ratio of 0,6.
About the inaccuracy of your model, the base and edges do influence the model pretty much ( http://www.skibuilders.com/phpBB2/viewtopic.php?t=669 ).
I measured my homebuild skis and compared them with my Matlab model (for which I was inspirated by your nice SS) and the results were within 1 mm accuracy for a deflection of about 20 mm. I didn't measure the deflection for different loads yet, but will do this later. I think it's also a good idea to collect date of our own skis to be able to compare it to our models. I will post a simple guide on how to measure the ski deflection and what data are needed soon.
Buuk
Make things as simple as possible, but not too simple
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thetradwoodboat
- Posts: 22
- Joined: Mon Mar 12, 2007 3:42 am
- Location: ports., nh
- Contact:
bambi, in response to your core thickness querry. my first pair were built on cores with a plateau under the bindings at 10mm and had a straight taper to 2mm at tip and tail. flex tested with my own method (ski supported at the ends of running surface and 24lb of force applied at cord center, then measuring the vertical distance traveled at cc. and also tracing the curve of the base, giving a good look at the flex shape)these skis bent hard under the binding and were stiff on the ends. not what you see on pro built skis. they worked in soft snow but were horrible in anything else.
so my next cores were made 13mm thick just behind cc with a slightly convex taper curve(looking from above) down to 2mm tip and tail. layed up with 22 oz triax and nothing else fancy. these boards had a beautiful round flex curve and i loved them from the first run! the most comfortable ski i'd ever ridden.
i plan to soften up the tip with less core thickness next pair, but i'm a believer in nice round flex.
so far my cores are spruce with hardwood sidewalls. its light and stiff (and cheap) for its weight.
i'd love to hear more about cores also. keep the idears coming.
so my next cores were made 13mm thick just behind cc with a slightly convex taper curve(looking from above) down to 2mm tip and tail. layed up with 22 oz triax and nothing else fancy. these boards had a beautiful round flex curve and i loved them from the first run! the most comfortable ski i'd ever ridden.
i plan to soften up the tip with less core thickness next pair, but i'm a believer in nice round flex.
so far my cores are spruce with hardwood sidewalls. its light and stiff (and cheap) for its weight.
i'd love to hear more about cores also. keep the idears coming.
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thetradwoodboat
- Posts: 22
- Joined: Mon Mar 12, 2007 3:42 am
- Location: ports., nh
- Contact:
bambi, sorry i haven't checked in sooner. pretty nice corny weekend, now that my legs are toast i'm back at the house.
my core profile has no flat spot, that just doesn't make sense to me so i put the thickest spot of the core maybe 4-5 cm behind cord center. so far my skis have been simple, just triax and woodcore (plus base and edges) so it seemed a nice smooth profile would create the best flex. i think if you were using any metal or glass reinforcements you could use a flat spot under the binding and still create a nice round flex, might take more trial and error though.
nate
my core profile has no flat spot, that just doesn't make sense to me so i put the thickest spot of the core maybe 4-5 cm behind cord center. so far my skis have been simple, just triax and woodcore (plus base and edges) so it seemed a nice smooth profile would create the best flex. i think if you were using any metal or glass reinforcements you could use a flat spot under the binding and still create a nice round flex, might take more trial and error though.
nate