 Poster: A snowHead
|
| comprex wrote: |
| ssh, my frame of reference deliberately starts at the ski, and assumes identical skier CoM, one skier being merely taller than the other. |
So how can you have identical CoM, but merely be taller?
Edit: Maybe someone the shape of ET with a huge long neck and short fat body?
|
|
|
|
|
|
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
|
| uktrailmonster wrote: |
| But as I mentioned earlier a shorter ski will have to produce a higher reaction force to create this moment, therefore it will be loaded more heavily. That's ok if the ski and snow surface can both support that load. |
Ok. We agree on this. Let's start there, as I see a lot of confusion with regard to frames of reference. Allow me to restate my question from the first post in this thread:
Consider the same ski under skier A then under skier B of identical mass but different height. Both skiers have edged the ski to, say, 30 degrees roll angle. The ski in either case is decambered by snow to provide 200 lbs force up through the boot. That's how much there is and there ain't no more.
Case 1:
Skier A is taller but has the same CoM as skier B (maybe skier B is a short weightlifter or is wearing a pack or skier A has thunderthighs or whatever).
Both have their CoM aligned with the resultant force up through the boot. Which is better balanced?
Case 2:
Skier A is taller and has a higher CoM than skier B
Both have their CoM aligned with the resultant force up through the boot. Which is better balanced?
|
|
|
|
|
|
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?
|
| comprex wrote: |
| ssh, my frame of reference deliberately starts at the ski, and assumes identical skier CoM, one skier being merely taller than the other. |
If one skier is taller than the other, it is highly likely that his CoM will be farther from the snow (keeping in mind that a close approximation is just behind the belly button). Other than that, I see no difference.
|
|
|
|
|
|
You need to Login to know who's really who.
You need to Login to know who's really who.
|
|
comprex, they are both equally balanced.
|
|
|
|
|
|
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
|
Case 1: They are mathematically identical if the CoM is the same and they have the same mass. Any force calculations you make will be the same.
Case 2: It's not really a question of which is "better" balanced. They can both be balanced, but the taller skier will exert a larger moment on the skis from any acceleration acting through his CoM. The reaction loads from the ski will be higher to balance this moment.
|
|
|
|
|
|
You'll need to Register first of course.
You'll need to Register first of course.
|
| comprex wrote: |
Consider the same ski under skier A then under skier B of identical mass but different height. Both skiers have edged the ski to, say, 30 degrees roll angle. The ski in either case is decambered by snow to provide 200 lbs force up through the boot. That's how much there is and there ain't no more.
|
I'm starting to see where you're coming from here. But to achieve equilibrium (or balance if you like) you not only have to balance loads in the vertical plane i.e 200 lbf, you also have to balance moments around any reference point eg boot/ski interface. This is what changes with CoM height. A higher CoM means a higher moment to balance out.
Probably the easiest way to represent these reaction moments from the ski is to resolve the distributed load along the ski into 2 point loads, one acting near the ski tip, the other near the tail. The shorter the ski, the larger these forces will become to balance the moment exerted from the skier. The higher the skiers CoM, the larger his exerted moment will become.
|
|
|
|
|
|
|
|
uktrailmonster, I could agree completely if we look at the meaning behind that last sentence,
| Quote: |
| The higher the skiers CoM, the larger his exerted moment will become. |
.
I would prefer to read that as "need to become to achieve equilibrium from a similarly perturbed state" which then leaves a very good idea of how skier skill enters into the issue.
For the record, I still hold "a taller skier has a higher CoM" as unproven. Backpack, leg/torso ratios, muscular development, brassiere cup size, calves (or cows), foot size and hence boot weight, sheesh.
Leaving that aside for now, I had hoped someone would notice a little-mentioned factor, particularly in regards to vision vs. CoM motion in threads such as
V8's wonderful arc analysis
Again, all these simplistic CoM models get us into trouble. Vision and balance organs are not located at the CoM. Thus, they follow a different path. For a taller skier, they might even be further away.
|
|
|
|
|
|
|
|
An empirical observation - shorter/stockier folks seem to find it easier to look graceful on skis than taller people. This is a highly subjective/anecdotal piece of evidence but is my experience from watching other skiers.
Also - in general or on average - tall skiers will have higher CMs than short skiers
I think uktrailmonster has it about right.
Just thought of an analogy - imagine a coat stand. Does anyone dispute that the taller it is the broader the base need to be? It's got nothing to do with floating in powder and everything to do with giving it stability in the event of it being knocked (a "perturbation").
Clearly Comprex is right, many other factors apart from height effect ideal ski length but, holding them constant, taller people will benefit from longer skis. A lot of the other factors are difficult to quickly assess in a ski shop but height is straitforward so worth factoring in explicitly?
|
|
|
|
|
|
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.
|
| jedster wrote: |
| An empirical observation - shorter/stockier folks seem to find it easier to look graceful than taller people. |
Another emprical observation.
|
|
|
|
|
|
|
|
| flicksta wrote: |
| jedster wrote: |
| An empirical observation - shorter/stockier folks seem to find it easier to look graceful than taller people. |
Another emprical observation. |
Time for another empirical observation:
Take two brooms, one with a stick at least 2 feet longer than the other.
Which is easier to balance bristles up, just the end in the palm of your hand?
Try it and see.
|
|
|
|
|
|
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
|
|
|
|
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.
|
|
|
|
|
|
comprex, Neither - just give me the answer! (I always believe everything men tell me  .)
But at a guess, the small one. I'm not sure it's got anything to do with skiing though as people tend to have different shapes (unlike broomsticks).
|
|
|
|
|
|
You know it makes sense.
|
maggi the longer broom is easier to balance on the palm.
I am trying to dispute the simplistic 'taller coatrack- wider base' explanation because I think it is irrelevant (in part because skiers have different shapes)..
Last edited by You know it makes sense. on Fri 5-01-07 0:00; edited 1 time in total
|
|
|
|
|
|
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
|
comprex, Well, there you go - I'm wrong again  .
|
|
|
|
|
|
Poster: A snowHead
|
| maggi wrote: |
| comprex, Well, there you go - I'm wrong again . |
That's OK. I find that most people, when asked about "balance" or "stability" associate it with 2 or 3 fairly simple contexts, and I'm trying to get beyond that.
|
|
|
|
|
|
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
|
comprex,
the broom example is flawed - unless you have a way of applying stabilising forces to your forehead
J
|
|
|
|
|
|
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?
|
|
jedster, still a bit early in the morning here. Care to explain that rebuttal?
|
|
|
|
|
|
You need to Login to know who's really who.
You need to Login to know who's really who.
|
| comprex wrote: |
maggi the longer broom is easier to balance on the palm.
I am trying to dispute the simplistic 'taller coatrack- wider base' explanation because I think it is irrelevant (in part because skiers have different shapes).. |
my unscientific brain thinks that the broom example lends weight to the "taller coatrack - wider base" explanation
|
|
|
|
|
|
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
|
|
Arno, and taller cyclists need wider bike tyres?
|
|
|
|
|
|
You'll need to Register first of course.
You'll need to Register first of course.
|
|
maybe wider tyres would be helpful, but i'm thinking about fore/aft balance so a longer wheelbase on the bike would be a more apt comparison
|
|
|
|
|
|
|
|
So, 23mm suffices laterally even for the likes of Indurain but 1050+mm for fore/aft? Sorry, Arno, that really doesn't make sense to me, and, unlike the ski arguments above where I didn't dispute the '2-footed' explanation as I should have, there is none to be made here.
In fact the coatrack/base argument can be seen to apply most to lateral balance in one footed skiing, since the boot can be regarded as locked in that direction.
The hinge in the boot allowing forward flex seems to make the longer broom-> easier to stabilise argument much more relevant, since the contact patch to the palm of the hand does not change.
|
|
|
|
|
|
|
|
Bikes with longer wheelbases (and cars too) are definitely more stable. It's not unreasonable to expect the same to be true for longer skis.
On road bikes, drag is very important, much more so than cornering grip They only have skinny 20 mm wide tyres to minimise drag and weight. That's why they often end up falling on their back bottoms! Mountain bikes have wider tyres because the compromise between cornering grip, traction and drag is different. I use 20 mm wide tyres on my road bike and 60 mm on my mountain bike.
|
|
|
|
|
|
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.
|
Isn't the point that the long dimension of skis or bikes is basically the direction is which they travel? And therefore the axis along which there will be the largest forces acting? And therefore the axis along which you need the most help with maintaining balance?
Seems pretty simple to me, but maybe that's because I ain't a scientist 
|
|
|
|
|
|
|
|
Shorter skier, shorter radius ski - for similar body angles and dynamics.
Scale up the skier, scale up the radius.
By coincidence (?), shorter skis have smaller radii.
|
|
|
|
|
|
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
|
|
Not sure where this discussion is leading now, but I'm pretty convinced that optimum ski length can be derived fairly accurately from a simple combination of skier height and weight. There are other secondary factors, but height and weight are the two primaries. More accurately, it would be weight and CofG height, but most people don't know their CofG height! But it can be implied fairly well from their overall height, which they do tend to know. I don't think humans of equal height and weight have wildly differing CofG heights and rarely enough to change their choice of ski length, since it's an incremental choice anyway. The ski manufacturers would tend to agree from their recommendations. Many of them just go off height alone and imply weight or vice-versa. Personally I think weight is the main parameter, followed by height which is often linked anyway. You wouldn't recommend 150 cm skis for a 90 kg 6' bloke would you now?
|
|
|
|
|
|
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.
|
uktrailmonster, yes, but its a very small contribution compared to geometric trail and pneumatic trail.
| Quote: |
| I don't think humans of equal height and weight have wildly differing CofG heights |
Wasn't that part of the point of womens' skis?
| Quote: |
| You wouldn't recommend 150 cm skis for a 90 kg 6' bloke would you now? |
I have several pairs and so do quite a few of my skating friends. They've had no problems at full speed at Snowbird (except in pow- there's a video someplace amongst my snowmediazone files). Look at historical slalom ski lengths for men. 155cm with snow contact lengths shorter yet.
| Quote: |
| Not sure where this discussion is leading now |
To dismissal of the CofG model, but rather one that looks at moment of inertia and the coupling between joints. I see the failure of the coat rack model at the ankle joint (we are pivoted).
The failure with the broomstick model is that we are not rigid along the full length of our bodies but rather have additional pivots of knees, hips, spine, neck, shoulders.
I contend that a fitter skier (better able to muscularly stabilize those pivots) with more rigid boots can use as short a ski as the surface underneath will support, independent of height.
|
|
|
|
|
|
|
|
| Quote: |
| How does a ski know how tall you are? |
It trips you up and measures your length. At least that's how they've always done it with me.
|
|
|
|
|
|
You know it makes sense.
|
I'm 193 cm and went down from 181 apache crossfires to 174 K2 recons this year and after about a week of chattering trying to hold an edge on the ice they are working quite well. would prefer a longer ski for soft snow but for on piste go short is my answer.
I demoed loads of skis and struggled to make a decision but my ski guru buddy made it a simple choice and said "what ski was the most fun". ....
|
|
|
|
|
|
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
|
comprex, Trail is very important on bikes (less so on cars, but still relevant) but totally irrelevant in the analogy to skis. Incidentally, pneumatic trail on a bicycle tyre is insignificant due to the large diameter and very low sidewall.
There are two paragraphs in your link that are vaguely analogous to skis:-
"Another element to consider is the wheelbase of the bicycle. As the bicycle turns, the steering axis is pulled to the side, and the rest of the frame will twist on the rear wheel's contact patch. The longer the wheelbase, the less the frame will turn with a given sideways movement of the steering axis. Thus, a short wheelbase tends to turn quicker, while a long wheelbase takes a more severe turn to produce the same effect on the bike's frame.
The rider's position on the bicycle is also important. A higher bottom bracket will cause it to handle differently when leaning from one side to the other, since the rider's weight will have to be moved farther on a longer lever. Putting the rider farther back will unweight the front wheel, which greatly affects its ability to grab the pavement and pivot the bike."
Womens skis tend to be lighter, softer, are available in shorter lengths and have girly inspired graphics. The average woman is shorter, lighter and less powerful than the average man. Mostly a marketing exercise I suspect, since skis cannot sense gender as far as I'm aware - only forces and moments. There's a similar marketing trend going on with women's specific mountain bikes.
It would be a mistake to dismiss a CofG model, because that's where all the forces are acting. You could refine it to have greater degrees of freedom if you like i.e more accurate and dynamic CofG positioning and better force representation, but I very much doubt that you'll end up with an answer that says tall heavy skiers need a shorter ski.
Slalom skiers will always use shorter skis specifically to reduce stability for the quickest reactions. GS and downhillers go longer to gain stability. It seems pretty obvious to me.
|
|
|
|
|
|
Poster: A snowHead
|
How does a ski know how tall you are?
Answer: it does'nt, it's a ski.
Taller people are normally heavier, have a higher centre of gravity, and most importantly longer levers, all of which would overpower a shorter ski so a longer one is required to spread the load. I THINK.
|
|
|
|
|
|
|
|
FAQ
. (In America, of course, everything is bigger 
)
