The dynamics of skiing

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

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bcohen5055
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The dynamics of skiing

Post by bcohen5055 »

Hello, everyone. I’ve been lurking on the skibuilders forum for almost 2 years now and I’ve learned a lot of great information. I do plan on eventually building skis but currently I’m more interested in truly understanding ski design. I’m a recent grad of a mechanical engineering program and while in school I spent a great deal of time working with composites and race vehicles. I have a foundational understanding of vehicle dynamics and I’m hoping to develop a similar understanding of the dynamics applicable to skiing.

I’d like to use this thread as a place to share information that will enable ski builders to make quantitative decisions when deciding on ski geometry and materials. I realize that skis are very sensitive to the skier itself and that may even be a dominant factor in ski performance, but I still think by understanding the physics we can all develop better skis that are matched closely to the skiers style.


So now where to begin? I’d like to possibly begin by just discussing the different loads and reactions skis see in various scenarios. We can start breaking things down but maybe if someone has a good link to an introductory article on the physics of skiing that may be better.
Richuk
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Post by Richuk »

The forum consists of many hackers, a few pioneers and one or two engineers, so how do you intend to ensure that everyone can contribute and or confirm the results.
Last edited by Richuk on Mon Nov 11, 2013 4:41 pm, edited 1 time in total.
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Head Monkey
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Post by Head Monkey »

I used to have a copy of this book, but I can't find it anymore. IIRC it was interesting:

http://www.real-world-physics-problems. ... kiing.html

You should also check out many discussions on this topic over at bomberonilne.com where they regularly geek-out on the physics of a carved turn.

Honestly, though, my opinion on the value of much of this kind of stuff is summed up here: :)
Everything I know about snowboard building, almost: MonkeyWiki, a guide to snowboard construction
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bcohen5055
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Post by bcohen5055 »

Richuck, maybe I wasn't so clear in what a was thinking about. I'm not looking for high level equations and calculations to design skis around but I'd like to have a conversation over what each variable does. A good example of this is stiffness. There is always a thread going on abut someone asking about making a ski stiffer. I would agree that in general once you have skied a soft ski and a hard ski you can begin to tell what you like but what gets tricky is if you ask yourself why do you like the stiffer ski? Is it helping with turn initation, "pop", how much do you weigh? What would happen if you just changed the length? How does this effect the damping? Could it be too stiff? Etc...There are a lot of variables at play and I'd just like to discuss how they are related and how they effect ski performance.

If you read though build logs there are a lot of experienced builders who have iterated through designs and found what they like for various reasons. I'd like to think this discussion may lead to quicker iterations and more fine tuning for future builders. I know for me personally that's where I'd like to be.

Head Monkey thanks for the links they are a great place to start. Also, I'd agree that modeling a ski in FEA or performing rigorous calculations won't have a lot of real world correlation. But more general discussion will. At the very least it may just be mental masterbation, but even that can be helpful for understanding some of the basic physical relationships in skiing.
PTTR
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Post by PTTR »

Richuk wrote:Most of the forum consists of many hackers, a few pioneers and one or two engineers, so how do you intend to ensure that everyone can contribute and or confirm the results.
I Will follow this with great interest. I don't really care about measurable results but I am looking forward to get inspired. I think most of us in here are interested in skis and boards that are fun to use compared to optimal racing gear. I love it when my skis kick me out of turns and I want them to shatter and rattle. The feeling of speed is far more important then the actual speed. A very S shaped core, with a thick midsection and long tipp and tail, does it for me. The thin tips react instantly on pressure or bumps while the mid section gets a firm grip and pop.
But there are so many parameters to play with: boot center and waist placement, taper, radius, rocker/camber. I would love to see a discussion about them and what people think they get out of doing this and that.
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vinman
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Post by vinman »

Ok I'll play so how about taper and it's effect on float in a powder ski.
Meaning the difference in mm when you subtract the tail dimension from the tip dimension.

When I design a ski for freeride/powder I usually start with at least 13mm upto 17-18mm. This allows the tail to sink and the tail to rise.

For an all mtn ski I use less taper to improve edge hold, maybe in the range of 10-12mm
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Post by PTTR »

Yep. so do I
but as I see it there are 3 different ways of getting there. And what are the pros and cons?

Lets talk about allmountain skis as they are both pist and powder designed.
An average pist ski usually has this 55-45% boot/waist -center placement. and lets assume that that is just perfect.

#1 way of getting a desired taper is to move the waist backwards, thus having different boot and waist.

#2 is to keep the waist at boot center and then "cut" off the edge to get the desired dimensions.

#3 is to keep the waist at boot but change the radius behinde the foot

I have only played with #1 and #2. I don't like #1. I think the skis get less aggressive and I belive it has to do with having the pivot point(waist) further back. I want to "push" the skis into turns instead of "follow" them.
Is it religion og fact? I have no idea, but it feels this way.

But #3 should, in my mind, be the perfect solution because 1 and 2 changes the 55-45% division of the edge in relation to waist. That said, I think that the salomon BBR is a dull experience, but it might be something else at play.
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vinman
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Post by vinman »

I use #3, I use 2 different radii that converge at the waist. I like this design for powder/freeride. It allows you to initiate the turn easily and improves the looseness/smearability of the tail, allows enough taper to make the tail sink as desired in deep snow.

This combined with a rocker-reverse sidecut tip is a great combo.
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pmg
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Post by pmg »

I had a lot of thoughts about different radius over the ski. Dynafit does a lot of that, often using the narrower radius in the back of the ski. At first this sounded quite unlogical to me, but after some thinking about it I really like the Idea and will test it someday.

My thought why it could be very good:
When riding a normal ski in a turn, the amount of steering the ski does by itself (carving) is determined by the angle of the ski to the snow and where the main force is put on the ski: The more force that comes to the tip of the ski, the more the ski steers.

We usually want to stand quite centered on the ski, so we can balance the unevenness of the terrain. If our weight comes too much forward, we want to move back, if it comes too much backward, we want to move forward.

A normal ski with a single radius does not help us with this at all, it even does the contrary:
When we go too much forward with our weight, the ski steers even more, making us fall further forward if we don't release the pressure we put on the ski.
When we go too much backward on the ski, the ski steers less (going straighter), making us fall backwards even more - we turn the ski with our ankle or the whole body to avoid falling down.

In theory, a ski with a wider radius at the front and a narrower one at the back would help us with this much more:
If we go too far forward, the wider radius makes the ski steer less, and we automatically fall backwards because the ski runs straighter.
If we go too far backward, the narrower radius makes the ski steer more, making us coming further forward again.

Very nice in theory, isn't it?

I programmed some small tool (in German only at the moment), which calculates each mm of width over the ski length for skis with different radiuses at front and back: http://www.spame.de/skirechner_dyn.php

Cheers
Philipp
Richuk
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Post by Richuk »

PTTR,
Inspiration; I took another look at your website. I like the ethos you describe, but I think the numbers are important, as demonstrated by pmg.
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Post by PTTR »

I am sorry if I offended you Richuk in my last qoute, I always enjoy reading your views and thoughts and don't get me wrong. I am very concerned about numbers when designing. Everything I do is measured and written down and used as future reference. But I am more interested in turning theories like yours pmg then in the numbers in your calculator but I think that is also because of my way of designing. I never use the computer but draw everything in 1:1 and make 1:1 or 10:1 graphs. Analog-line-smoothness-eyeballing-trust-your-guts-and-experience designing.
I belive that measurable results will be hard to share in between all of us because of different setups, materials and technics but thoughts, ideas and stories about results are very helpful. For example the the idea from above. Designing a ski that will automaticly catch you if you are about to fall is almost the exact opposite of what I am aiming for. If this works for you pmg, then I can turn your theory on its head and design a ski for myself. Inspiration!

and to turn this very long post back on track:
Last year we designed a ski with 15 meter radius and the waist far behind the heel as a test. In retrospect in maybe isn't that strange but I was very surprised when skiing on it. It can only ski on edge. As long as you do soft round turns it performs super well but it can not do anything else. The grip is so firm that a stopturn is almost impossible and you have to jump it out of carve. It was a scary ride when there were lots of people in the slope. But if I was asked to make a supercarver then I would probably place the waist with a few cm setback to get a smoother and firmer carveturn.
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Post by Richuk »

No man. I thought it was just making the point that numbers are a common language and an easy way to share within a group. Please continue to drive this thread forward, as its always a pleasure to read your comments.
Cadman
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Numbers

Post by Cadman »

I mostly agree with both of you on the "gut feel" and the numbers. Right now we are working on an issue of trying to figure out if a wood sidewall is as strong as an ABS sidewall. I know there are a lot of different issues with wood versus ABS but this is a case where you do an impact test and the numbers tell you the answer. It just takes the guess out of it. In other situations, I trust my gut feel. Stimulating comments here. Keep at it!
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MontuckyMadman
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Re: Numbers

Post by MontuckyMadman »

Cadman wrote:I mostly agree with both of you on the "gut feel" and the numbers. Right now we are working on an issue of trying to figure out if a wood sidewall is as strong as an ABS sidewall. I know there are a lot of different issues with wood versus ABS but this is a case where you do an impact test and the numbers tell you the answer. It just takes the guess out of it. In other situations, I trust my gut feel. Stimulating comments here. Keep at it!
Pretty simple with uhmw the answer is almost always no if a good bond us acheived.
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