Stuff I've learned from research! (New material)

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brianmwaters
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Joined: Thu Dec 27, 2007 10:12 pm

Stuff I've learned from research! (New material)

Post by brianmwaters »

So I've been scouring this site for months -- reading, learning, expanding my understanding of ski construction and design. I want to thank Kelvin, Kam and Kam and everyone who has contributed to this site. The wealth of knowledge we have here is truly amazing!

That said, I have come to realize that this site is not the end-all be-all when it comes to ski building. I recently got my hands on a copy of Lind and Sanders' book "The Physics of Skiing: Skiing at the Triple Point." All of you should have a copy of it. The chapter on equipment is completely invaluable.

The book talks about a lot of things that are not discussed on this site, or that have only been mentioned in passing. I'm gonna introduce those ideas here -- I hope that this thread blossoms into others, so that we can have some good in-depth discussions about this stuff.

1. The role of laminates
I'm not sure how many people on this site understand just how the strength of the fiberglass layers comes around. A sheet of cured fiberglass on its own is pretty flimsy. Skis work sort of like steel I-beams, whereby the stiffness comes from the *separation* between two flat pieces of steel (or fiberglass). I think the stiffness goes up cubically with separation, so a small change in core thickness can make a big difference. One model I made showed that the fiberglass was responsible for about 50% of the stiffness of the ski, and the core the other 50%. Lind and Sanders go so far as to describe the core as to say "the core of a ski serves chiefly as a separator for the ski's structural elements."

2. Pressure distribution
When you stand on a ski that has camber, more of your bodyweight is supported at the tips and tails than at the center. Same when you're turning. This pressure distribution changes when we lean forward or back, and it has huge implications for ski performance. So camber has a lot more to do with the dynamic properties of a ski than we thought before. The book doesn't say too much about calculating this distribution though. I'm working on it.

3. Taper (you guys probably already knew this one, but what the heck)
Taper is defined as the shovel (width - tail width)/2. It's bigger when the tip is fat and the tail is skinny. A ski with low taper will want to hook up and carve through the whole turn. One with high taper will be easier to release at the end of the turn. This is why some skis (my K2s) have "progressive sidecut" -- because it raises the taper, making the tails easy to release.

4. Tail stiffness
You can measure the stiffness of a ski tip or tail by clamping it at the center, loading the tip/tail, and measuring the deflection. The stiffness depends on the deflection and the distance between the tip/tail and the clamping point. Most skis have about the same stiffness in the tip and tail, but racers will want a stiffer tail so that it will continue to carve as they lean back at the end of the turn.

5. Mount point (this one is interesting)
The above-mentioned fore/aft spring constants vary cubically with distance from the clamping point (which models the boot/binding pretty well). So the turn-finishing properties of the ski are extremely sensitive to small changes in mount point. The book says a 3 cm change in mount point can make the tail's spring constant 25% higher.

6. Damping
This is a big one. I'm starting another thread for it.

7. Flexure and Vibration Modes
I started another thread for this one too.
alloyguitar
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Joined: Mon Feb 25, 2008 5:29 pm

Post by alloyguitar »

you do pose some interesting points.

I like the concept of the laminate construction thing, mainly because it means that my design for a chambered-esque core will not have as much of a detrimental effect on strength as I had originally thought.
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telepariah
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Location: Sapporo, Japan

Post by telepariah »

My first post here. This site is an amazing resource for someone just getting interested in ski building.

I have the book you are describing and will need to take another look at it. I actually went on a hut trip years -- I mean decades -- ago with Dave Lind. Got to spend some quality time with the man. He was pretty old then -- I think he was over 70 at the time. I haven't heard anything about him in a long time but if he isn't 90 yet he must be close. He was a VERY strong skier and a delightful gentleman to share a hut and tour with.

I will be checking in around the site from time to time even though my cave is already too cluttered to use for building skis.

Kampai!!!

P.S. Thanks G-man for introducing me to this site!
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bigKam
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Post by bigKam »

W. Shorthill from the Univ. of Utah, Mech. Eng. Dept., published a few papers on ski dynamics, in particular, on mode shapes. I recall they were interesting to read. Shorthill is retired, but I do remember him when I was a student there. J. Howe's book also has some good info, but not as technical as Lind and Sanders' book. Also, I remember thumbing through a couple MS theses at the Univ. of Washington's engineering library on ski design. I don't recall the advisor or the titles of the work, but the info was valuable. I'll see about getting copies of the work to share.

Speaking of information sharing, I think it's a good idea to organize all the available literature on ski design and related topics in one place, like a library so it's easy access.

It would be easy for me to build a page, but would need some help organizing the literature. Any thoughts and interests?
That said, I have come to realize that this site is not the end-all be-all when it comes to ski building.
Agreed. That's why we encourage people to share what they know so everyone can benefit from the information...
Buuk
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Location: The Netherlands

Re: Stuff I've learned from research! (New material)

Post by Buuk »

brianmwaters wrote: 1. The role of laminates
I'm not sure how many people on this site understand just how the strength of the fiberglass layers comes around. A sheet of cured fiberglass on its own is pretty flimsy. Skis work sort of like steel I-beams, whereby the stiffness comes from the *separation* between two flat pieces of steel (or fiberglass). I think the stiffness goes up cubically with separation, so a small change in core thickness can make a big difference. One model I made showed that the fiberglass was responsible for about 50% of the stiffness of the ski, and the core the other 50%. Lind and Sanders go so far as to describe the core as to say "the core of a ski serves chiefly as a separator for the ski's structural elements."
This really depends on which part of the ski you are looking at (tip, tail, waist) and so the ratio between core and fibre thickness. A while ago I calculated that using a 1mm thick fibre layer, the core is reponsible for the stiffness as follows:
13 mm wooden core: 41%
8 mm wooden core: 28%
2 mm wooden core: 14%

With E_wood is 15 GPa and E_fibre = 40 GPa

And maybe this pic is interesting for you as well:

Image

It clearly shows that at the smaller part of the ski, the waist in this case, the steel edges have more influence at the ski stiffness than in the tip and tail.

Buuk
Make things as simple as possible, but not too simple
Craig
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Joined: Wed Feb 20, 2008 9:32 pm
Location: Seattle, WA and Saratoga Springs, NY

Post by Craig »

Though this is indirectly related to structural properties, it could be important to consider in your design. I've never seen anything posted on this site about binding ramp angle (am I wrong). Ramp angle is the angle of inclination underfoot, and there is debate over which is better, a high or a low angle.

Ramp angle is mostly determined by the binding (older bindings tend to have higher ramp angles than newer ones) but I think it is worth considering for some builders, considering a mm or two of ramp angle change can make a difference in fore/aft pressure and turn initiation (I've had this explained to me by reps, maybe they are screwing with me; I've never tried comparisons of different angles, holding everything else constant.

I've been sure to keep the ramp angle neutral in my ski design by keeping the binding platform level, or at least the same core thickness at toe and heel - for me, common sense dictated I do this. Maybe considering this could have implications for attaining fine tuned adjustments for certain skis, without having to rely entirely on bindings for determining ramp angle.
yan0
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Joined: Wed Sep 17, 2008 3:36 pm

Stiffness/ramp angle

Post by yan0 »

Craig r,
The reps weren't screwing with you when they were talking about ramp angle. I come from a racing background and not only does ramp angle make a huge difference, but companies are beginning to design for it. The first time I remember people playing with ramp angles was six years ago. Like most things, the guys that first started to do it went to ridiculous lengths. The displacement of boot to snow is regulated by FIS, so the guys would lower the heel of their binding in order to raise the toe more. The idea is fairly simple - by raising your toe you are more able to apply pressure to the front of the boot from a "natural" position. On the other hand, because your stance on the ski is less inclined (forward) it's easier to get thrown back. The guys who first started playing with ramp angle found that extreme ramp angles worked great on moderate steeps to flats - but the ramp made it really hard for them to recover and stay over the ski on pitches. Some guys like more ramp than others - most reps tell their athletes to put a given number of shims under the toe. This usually worked for me (I ski Fischer), but I know guys who use none, and some who use as much as possible. One thing to keep in mind is that bindings come naturally ramped forward - the heel piece is notacably thicker than the toepeice.

brianmwaters - quick question regarding the plot you put up (.. it looks matlab-esque, but I could be wrong). Did the plot come from data or an equation, if so, what was it? Correct me if I'm wrong, but the relationship between core thickness and width would affect the shape of the curve. If so, how?

Thanks
Jed
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