 Poster: A snowHead
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A post on another thread got me thinking about how to initiate the traditional summer long physics spat.
Anyone got any ideas on an equation for ski suface area, velocity ,gradient, snow density & skier mass to "prove" what the optimum ski width & length is in powder?
For the more advanced paper we will move onto rocker, limb length, tip profile and optimum penetration into snow pack.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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sure i saw something on gapic "proving" that a 60kg lady only needed x mm underfoot to get as much float as her 90kg hubby with y under foot
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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| fatbob wrote: |
| ... got me thinking about how to initiate the traditional summer long physics spat. |
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Guys, just a quick one here.....I am looking to buy some new skis on budget and was wondering if Rossignol Scratch Sprayer BC 170 would suit someone like me. I am 6ft 1 and weigh 95kg. Thanks
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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I am an advanced skier but this is my first purchase 
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You'll need to Register first of course.
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sweats, don't know what advanced really means...but if you can ski well, then 95kgs and 6ft or so means you should be looking at a 185, so head height ...and another 5cms on that if a TT.
So, without really knowing that ski...then I'd say 170 is waaaay too short, IMO.
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| fatbob wrote: |
Anyone got any ideas on an equation for ski suface area, velocity ,gradient, snow density & skier mass to "prove" what the optimum ski width & length is in powder?
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The width and length of the ski is directly proportional to the number of little bunny rabbits available to push the ski up from below.
eg XWidth + YLength = ZBunnies
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Kieran, I think you'll find it is too subject to the part equasion that the buunies are then inversely proportional to the power of the 4th wave length.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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and how tightly packed the bunnies are. They can get pretty close apparantly 
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JT much appreciated, thank you!
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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IIRC physicsman posted a 'width underfoot v weight' equation on epic some time back.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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| fatbob wrote: |
| Yeah but did his factor in the bunnies? |
Typical, you're splitting hares now 
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You know it makes sense.
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Mosha Marc, judgement of Salomon.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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Poster: A snowHead
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| Arno wrote: |
| sure i saw something on gapic "proving" that a 60kg lady only needed x mm underfoot to get as much float as her 90kg hubby with y under foot |
Actually, if you're just comparing a single parameter (in this case skier weight), it's very easy to calculate equivalent ski widths for equal floating ability. You just have to calculate the width required to achieve equal pressure on the snow surface (Pressure = Force / Area)
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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probably less so -if you are a fat person in the first place 
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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| spyderjon wrote: |
| IIRC physicsman posted a 'width underfoot v weight' equation on epic some time back. |
And here it is http://www.epicski.com/wiki/sidecut-radius-calculator
Its also come up in a discussion about sidecut in another thread.
IMO a lot of the lift will come from the shape of the skis (I'm not refering to side cut here but more so the upturn of the tip and amount of rocker) and the stiffness and of course how well you ski powder so as to facilitate lift ocurring. To ski on powder the snow needs to get under the noses of the skis. The more snow that gets under them the more there is to support or lift the ski. Big up turned noses can collect more snow which will be compressed as it passes under the ski. The pumping or bouncing used in skiing powder will be doing a similar thing collecting as you flex the legs and lighten the ski compressing it as you extend them. And of course a bit of speed works wonders in getting and keeping the skis floating realising this was my biggest breakthrough in powder skiing, the more cautiously I approached it the more my tips dived and the more I tripped .
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| fatbob wrote: |
| optimum penetration |
huh-huh, you said optimum penetration, huh-huh heh-heh
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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horizon, Well what else do the terms "balls deep" and "blower" mean?
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You'll need to Register first of course.
You'll need to Register first of course.
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Not to mention face shots.
Let alone virgin snow.
And I thought skiing powder is one of those few times when a man doesn't think about sex...
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Thanks comprex,
Physics man makes some good observations here.
The link I put up will calculate the foot print of the ski as well as sidecut and radius. This Footprint and the weight of skier as well as force exerted by skierr would influence how a given ski would float on snow of a given quality. Lighter skiers would require a smaller foot print.
Maybe a footprint (rather than waist width) to skier weight would be a more realistic tool skiers could use when trying to predict how a ski will float as a lot of skiers up the length to increase the surface on their powder skis.
To be honest IMO if we focus too much on one aspect we could miss others. I like all mountain skis as I have to ski a variety of conditions on any given trip and find many all mountain skis do this admirably, others less so and their manufactures descriptions are bizare. When trying to predict what a ski might be like I just think wider longer than some skis I'm used to should probably float better narrower shorter probably float a bit less. Is it stiffer softer than what I'm used to? Then I try em and see if my prdictions are correct. I found I can make reasonable predictions but usually get a few surprises.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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RPF, I don't disagree with you.
I do have another way for you to look at this whole footprint idea:
Every time someone asks "I'm 6'2", what ski length is right for me" what they're really asking is
"I'm 6'2", please assume my BMI is within expected norms around 14st, what footprint is right for me"
and they've gotten away with the elision for decades /because a wide range of waist sizes was not truly available until 5 years ago/.
Yours to prove or disprove as you will.
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| Quote: |
Every time someone asks "I'm 6'2", what ski length is right for me" what they're really asking is
"I'm 6'2", please assume my BMI is within expected norms around 14st, what footprint is right for me"
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totally agree
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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You have, of course, to calculate the longitudinal bending moment of the paired skis and include that as influenced change to the frictional co-efficient over the length of the skis against the density, temperature and crystal type morphology of the snow and include this in your calculation parameters . . .
Alternatively. you could fling fresh bunny entrails at the wall and sell the results to Epic as sage divined wisdom . . . or just call it a Jackson Bollock
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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| RPF wrote: |
The link I put up will calculate the foot print of the ski as well as sidecut and radius. This Footprint and the weight of skier as well as force exerted by skierr would influence how a given ski would float on snow of a given quality. Lighter skiers would require a smaller foot print.
Maybe a footprint (rather than waist width) to skier weight would be a more realistic tool skiers could use when trying to predict how a ski will float as a lot of skiers up the length to increase the surface on their powder skis.
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Yes, all you have to do is calculate the pressure on the base of the skis from the skier's weight and ski surface area (footprint). Pressure (measure of float) = Force (weight) / surface area (ski footprint). The lower the pressure, the more you will float.
A heavier skier will need a larger footprint to maintain the same pressure and hence float.
There are obviously other factors determining how much you will float in a given situation, but this is a simple and fundamental way of measuring the effect of skier weight alone. So it makes perfect sense for bigger guys to ski wider skis in soft snow/powder.
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| Quote: |
There are obviously other factors determining how much you will float in a given situation, but this is a simple and fundamental way of measuring the effect of skier weight alone. So it makes perfect sense for bigger guys to ski wider skis in soft snow/powder.
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and/or longer
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You know it makes sense.
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How resistant is the snow? What speed are we travelling at? What is the steepness of the slope? We are going down a path that has way too many variable parameters to push this any further than the manufacturers have already. Skis are often in development between 2-3 years. The market that the product is destined for, help with concept, development and testing.
So...................................are we trying to find a formula to give an exact ski, dimensions or program? We will never be able to be precise in such, so let's all buy Black Crows.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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| SMALLZOOKEEPER wrote: |
So...................................are we trying to find a formula to give an exact ski, dimensions or program? |
No. We are trying to find a reliable starting point for interpreting other peoples' reviews.
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Poster: A snowHead
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Icelantic put the surface area of the ski in their specs. It does make for interesting comparisons between different models or lengths of the same model.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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| RPF wrote: |
| Icelantic put the surface area of the ski in their specs. It does make for interesting comparisons between different models or lengths of the same model. |
All manufacturers should. Then people of different thicknesses would have more of an idea of what they actually needed.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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dunno about that - it's where the surface area is (if you get my meaning that makes the difference). extra surface area in the tail of the ski wouldn't do you much good if you want to float
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You need to Login to know who's really who.
You need to Login to know who's really who.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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You'll need to Register first of course.
You'll need to Register first of course.
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| Quote: |
dunno about that - it's where the surface area is (if you get my meaning that makes the difference). extra surface area in the tail of the ski wouldn't do you much good if you want to float
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A good point this, as discussed many times on the forum, could be influenced by binding position too.
As has come up above there are a lot more factors that will determine how a ski feels and skis under different conditions. It is this that gives them their individual qualities. And no matter how many mathematical descriptors we have only getting out and skiing them lets us know wht they are like.
The individual using them will affect how any ski performs. This could be taken beyond height and weight and is probably best adressed with allignment and binding position analysis.
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RPF, data is available to provide objective information, such as it exists. To imagine that making decisions on such crude information alone will yield perfect results is obviously at worst crass and at best naive. However such information may be indicative and supportive which is one of the reasons why data is so often published alongside analysis, interpretation and conclusions.
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slikedges, I agree with you, and as comprex, mentioned above this (and other measures) give us a good starting point for comparing skis before trying, Often much more useful than the manufacturers descriptions of their skis capabilities.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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I have discussed this on the Ski Club GB forum (am I allowed to mention them??!)
RPF: good spreadsheet but not always available in a shop!
Generally the mid dimension - underfoot - gives a rough but effective guide to flotation as thats where the pressure is centred. So I just take (Ski Mid Width x Ski Length) and divide by Skiers weight in Kg. This gives what I call Flotation Factor (FF) . The higher the FF, the more float on the snow. From trial and error, FF under 150 will be a piste ski inadequate for powder for that skier. 200 and over will be floating on top! Around 160-180 will be good compromise for mix of piste and powder.
eg.Skis 176 x 82 = 14432, divide by Skier weight 80 Kg, = 180. So good for say 70/30 powder/piste.
165 x 72 / 85kg = 139. So inadequate flotation for powder for this skier.
176 x 100 / 80kg = 220. Fat boys, float on!
Rough and ready guide, and obviously a very long thin ski could give a high theoretical FF but sure wont be easy to ski in powder.
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dickyb Perfect, let's use that, simple and realisitic, the industry should standardise it as such.
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FAQ
which as just proved is definitely not a contstant. Yep! looks like this could take most of the summer after all 
