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Ski Sizing - What "body" measurements should be used?

post #1 of 34
Thread Starter 
Rather than bury this discussion in the other thread, I'm really interested in thoughts about how ski sizing on modern skis should be determined.

What body measurements matter? I still believe that height plays a factor along with weight, but am I just being "old school"? No doubt most everyone is skiing shorter more shapely skis now, but how does that change coming up with a recommended length?

Elan doesn't even mention weight on their web site for length recommendations. They only use height. I've seen some posters argue that only weight matters. How about a discussion with some scientific physics basis on why (or why not) height should be included.

Obviously there is a skill component involved too. I'm surprised how many skiers still resist going shorter to avoid "insulting their testosterone" level.

I've pretty much stuck to listening to the manufacturers recommendations and demoing, but you can't demo every lenght (usually). So how do you narrow it down to what size is most likely the right size?
post #2 of 34
Interesting question. I'd say BOTH height and weight matter. Height because it helps determine your lever arm working against the pivot (ski edge). Weight because it helps determine how much you can deform the ski surface.

Muscularity is the gorilla in the closet. Someone who can leg press 2.5x body weight will bend a ski more with less effort regardless. Some male testers for the ski mags are like that; small, but you can bet they torque Volkls.

I'd guess sites go with height because they assume a narrow range of weights for each height. Wrong. IMO a better approach would be chart of weight for height, like in a doctor's office. That would also cover some variance due to strength, since muscular folks weigh more at the same height than fat folks.

I've played around at the Elan site by plugging in false data. Seems like if I take 2-3 cm off my height, the ski lengths are more realistic. Probably because I'm light for my stature.
post #3 of 34

Calling Physicsman

This is an interesting question, and perhaps we will hear from Physicsman before it is said and done. I think height has more to do with selecting length than weight. I will toy with some ideas here, but I hope more knowlegable bears will help refine this, or even blow it out of the water.

Beyond raises the critical concept of a lever working to impart force on a ski. So where is the fulcrum? The body working as a lever on the ski works in all directions, fore, aft, left and right to exert tip and tail pressure, and to engage edges with yaw. Going out on a limb, the effective lever extends from the ankle to the hip, not the full height of the skier. A skier is articulated and often counters or rotates above the waist, so the effective lever is the lower body. For a skier using a balanced style, mass and strength in that lever are less important in applying force than its length. Could the distance from ankle to hip be the most important determinant of ski length?

Mass (weight) can come from physical body weight, or by inertia created by changing directions (some might call centrifugal force). Higher speeds and greater changes of direction can both increase the G-force a skier can apply to skis, through the lower body lever. So an "aggressive" or fast lightweight skier can effect similar forces as a heavier skier not using kinetic energy/ inertia to their advantage.

So, my GUESS is that the effective lever is the lower body, counter-balanced by the upper body exerting force and torque through a fulcurm approximately at the center of the foot. Ski boots of course translate the force to the center of the ski and reinforce the effect of the leg to the ski. Height is important because the length of the leverage (hip to ankle) is similar for people of similar height. As we all know, using good leverage, relatively little force (weight) can be amplified through an effective lever to apply greater force at the short end of the fulcrum. So a light-weight skier with long legs may be able to use the same or longer length ski as a heavier weight skier with shorter legs because they can leverage more force. Force in this case being foot-pounds levered against the fulcrum. The ski is also a lever, and as it becomes longer, it can exert more force on its side of the fulcrum. So when choosng between lengths of ski, is it better to have an over-leveraged ski, or an over-leveraged skier?

Man, I really should have stayed awake in physics class
post #4 of 34
Weight, primarily.

Height if it is out of "normal" proportion to weight (you are either taller or shorter than is typical for your weight).
post #5 of 34
Thread Starter 
Cirque - so take your theory one step further and give us a process to determine proper ski length. Are we going to use the "lower body lever" measurement along with weight and physical strength/skill? I'm following your theory and it sounds good. How do we put it into practice?

ssh - I think you're right about body proportions. We could probably work out something that provides guidelines for the "norm" and then provide a +/- factor depending on if you are lighter/smaller or heavier/bigger than the norm.
post #6 of 34
2 x L (floor to hip). I get 184 cm. Oh coincidentally it works!

I'm actually clueless about this, but it might be interesting to compare answers. Anyone want to have an ability multiplyer? Type I= .85 Type II= .9, Type III= 1 ....Could I be more arbitrary?

Quote:
so take your theory one step further and give us a process to determine proper ski length.
post #7 of 34
Doesn't it really mostly come down to personal preference? I haven't skied much newer stuff, but I'm realizing my days on the 200+ cm sticks are long gone. At 6'2", 210 lbs., I was amazed at how well I liked my teenager's 178 cm Dynastar SC9's. I thought for sure they'd be too short and soft to hold an edge, but I could ski hard on that ski just fine.

I think alot of it also depends on what you want to do with the ski. If blasting down steep groomers is your thing, perhaps a longer edge might be appealing. If darting through the trees or bumps is your idea of a great day, perhaps something a bit shorter might be more enjoyable and controllable.

The guys I really hate are the ones who blow by me in the steep bumps prancing like a deer on 205 cm straight sticks. Then there's the young racer who's going Mach 1 on a groomer wearing pseudo mini skis. That's just not right!

Good skiers can do most anything on any ski. I want the ski and length that gives me confidence to ski the stuff I want to ski. If that means something shorter to be more forgiving in the bumps and trees, then THAT's what I want - regardless what some chart recommends.

AM.
post #8 of 34
Thread Starter 
Attacking - sure it's personal preference, but that's not what we're after here. The issue at hand is how to formulate a recommendation for yourself or someone else that has more than just an educated guess behind it.

It's just not possible to demo every size or even demo at all sometimes. How then do you decide on length. That's what I'm looking for.

I'm still in the initial stages of my process, but I'm working on something that takes into account:
1. Height and weight (height may be replaced with "lower body lever" length)
2. Ski type
3. Ski flex
4. Ski sidecut
5. Boot sole length (still not sure about this one, depends on system bindings or not)
6. Skier skill
7. Skier strength

I'm trying to account for every factor that affects why someone is more comfortable on a certain length ski. And I well understand that this may be an unattainable goal, but the engineer in me refuses to give up!
post #9 of 34
Weight primarily, skis don't know how tall you are but they can feel how heavy you are. When I was sizing people, If someoen was out of proportion either by height or weight, I used it in a 2 to 1 ratio.
post #10 of 34
I thought it was inversely proportional to the size of your um...
post #11 of 34
There is no one right size for any skier, for all skis. For there to be such a metric, there would have to be a right size across all models, yes?
post #12 of 34
Thread Starter 
Quote:
Originally Posted by jstraw
There is no one right size for any skier, for all skis. For there to be such a metric, there would have to be a right size across all models, yes?
I'm not really sure what you're trying to convey here. I think I know what you're trying to say if I "read between the lines". Let me re-phrase the idea by saying that due to the differences across ski models there's no one right size that can be picked. Maybe I still have it wrong.

Anyhow I contend that although there are many, many variables that affect the proper sizing of skis, you can still distill everything down to some key metrics to get "in the ballpark" on a sizing recommendation (at least a good starting point). This process should work across all manufacturers and model lines if we get it right.
post #13 of 34
You got it...My Metron B5 length is different than my Legend 8800 length.
post #14 of 34
Thread Starter 
Quote:
Originally Posted by jstraw
You got it...My Metron B5 length is different than my Legend 8800 length.
That's exactly why I'm trying to include ski type and sidecut in the process. A Metron B5 and a Legend 8800 are very different types of skis. How can we best account for their differences without making things overly complex when calculating a recommended length?
post #15 of 34
I've been thinking about this exact topic a bit recently (as a result of helping a variety of my students pick skis to either rent or buy).

My conclusion on ski lengths:

In high end skis there is no longer ANY relevance in height, weight or ability.

The ONLY relevant factors are radius of turn sought and stability at speed.

Go out and test the same ski in different lengths. What determines the correct length for you will generally be whether the sidecut created by that length fits the way you like to ski and whether there is enough length for you to feel comfortable at speed.
post #16 of 34
In another thread on this subject the poster stated that your body weight had to be increased by about 20 lbs due to boots and gear that you wear. Do the suggested manufacturer's charts already add this xtra weight or do you have to add it to your weight prior to using the recommended charts?
post #17 of 34
thumb. for the application of the rule thereof.
post #18 of 34
Quote:
Originally Posted by Noodler
That's exactly why I'm trying to include ski type and sidecut in the process. A Metron B5 and a Legend 8800 are very different types of skis. How can we best account for their differences without making things overly complex when calculating a recommended length?
It kinda gets back to Salomons PR (Power Rating) system.
post #19 of 34
Quote:
Originally Posted by NZskier
I've been thinking about this exact topic a bit recently (as a result of helping a variety of my students pick skis to either rent or buy).

My conclusion on ski lengths:

In high end skis there is no longer ANY relevance in height, weight or ability.

The ONLY relevant factors are radius of turn sought and stability at speed.

Go out and test the same ski in different lengths. What determines the correct length for you will generally be whether the sidecut created by that length fits the way you like to ski and whether there is enough length for you to feel comfortable at speed.
this is the ONLY answer that has made any semblence of sense.

height and weight should have absolutely no bearing on deciding what length ski to purchase. decide what you want to do with the tool and where you want to use the tool.

you wouldn't use a hammer to drive a railroad spike and you wouldn't drive a nail with a sledge hammer.

i pro-repped for Fischer and skiied their wc sc in a 155. Bob Barnes and I looked at the dimension of that ski in it's early years and realized it would make a great powder ski in a longer length with it's 123mm tip. i always felt it may have been the predecessor to the metron.

the downside would have been that in a longer length the wc sc would not have the turn radius most folks sought.

where you want to ski, the way you want to shape your turns, and the speed you want to ski are the answer.

please forget height and weight.

last year one of my skis was a speedmachine. i skiied it a little longer (178 vs 170) so that i could have a little more platform at a higher speed. i tell folks adding five cm makes a ski more comfortable going 5-10 mph faster.
post #20 of 34
Radius-schmadius. What radius arc were the slalom skis of yesteryear? Certainly not anywhere near those of today. Would you say that Stenmark, Tomba, and the like DIDN'T actually "carve" a turn? They most certainly did.

Height, weight, aggro-factor, hill size, snow conditions, muscularity and skill are all factors, which is why there is no right answer or formula.

BTW - I find it ironic that for some people ski length seems to define the size of another certain body part ...
post #21 of 34
Quote:
Originally Posted by Woodee

BTW - I find it ironic that for some people ski length seems to define the size of another certain body part ...
from a poster named woodee?
post #22 of 34
Quote:
Originally Posted by Noodler
I'm trying to account for every factor that affects why someone is more comfortable on a certain length ski. And I well understand that this may be an unattainable goal, but the engineer in me refuses to give up!
Then you cannot forget snow condition -- soft/hard. Where you are going to ski will change the effects of length quite a bit. On hardpack, length really does not make all that much difference, since you won't post-hole even on snowblades...
post #23 of 34
Quote:
Originally Posted by Rusty Guy
from a poster named woodee?
: Hmm, not sure where this is coming from since I'm typically not the one telling people that they're wusses that ought to go longer.

Actually, I believe my mantra has been that short is the new long and you should ski the shortest ski that you can get away with.
post #24 of 34

Noodler and Cirquerider: May have an answer

Getting back to the original question that Noodler posed, I hauled out a biomechanics text, am trying to work out a model. Roughly, I can say this so far:

1) Leg is a linked lever arm, eg, series of smaller levers that move around each joint. You can sum these. So total leg length is decent estimate of total lever length if you're skiing upright. BUT if you're concerned about most of the muscular effort in hard skiing, you're looking at forces generated by the thigh muscles. Therefore the lower leg (tibia) becomes the relevant segment length.

2) Bending a ski into a turn in physical terms is a change in angular momentum that the ski resists (angular inertia and friction with snow). We'll ignore ski rebound, fact that ski doesn't actually rotate around boot except maybe in moguls, fact that ski also is exerting force as it bounces etc. cuz it gets into CAD-land. Only the manufacturer knows...

3) Change in angular momentum is proportional to the external torque exerted on it.

4) External torque is supplied by the muscles, in this case, and muscles become more efficient as the lever arm gets longer. Formula is: T=F*r where T=torque, F=Force, and r=moment arm (perpendicular distance between axis of rotation and line of action of the force) By inspection, as r increases, a constant force willl produce more torque.

5) So in a leg, where boot sole is axis of rotation and knee is roughly location of force (actually just below knee), greater distance between the two will produce greater torque at the boot sole. Or put another way, greater constant r (lower leg length) will require less F (quads, hams) to produce same T at the boot.

6) The downside is that torque works both ways. So it looks like a longer lower leg will also place more torque on the knee if the ski becomes the dominant force (as in a tip slamming into a bump) and the knee becomes the axis of rotation. Goodbye, ACL's. This may also suggest that taller skiers can place safer loads on their knees by skiing shorter skis, but I haven't worked that one out yet. It's a different issue than simply whether or not height matters.

7) I need an engineer to check above, but I think it all shows that a) height matters, because it serves as a decent proxy for lower leg length. (Tall people tend overall to have longer lower legs.)

8) I may be doing serious violence to the model by ignoring upper body F. And I still assume weight matters as well, because some of the F exerted by the thigh muscles is also fighting gravity, mass and acceleration of rest of body...
post #25 of 34
Thread Starter 
Beyond - you're my hero. You seem to be getting on the track that will lead us to something useful. There will always be those detractors that turn their backs to science, but in the end we're dealing with tools to ride snow with gravity's help - how those tools interact with the rider and the snow is science.

Rusty Guy & NZskier - please review the purpose of my thread. I totally understand where you're coming from, but that's not my current goal.

Hopefully I'll have some time this weekend to digest everything here and put something together regarding a specific process (or formula) to arrive at some sizing recommendations.
post #26 of 34
weight and ability (or style).

The majority of the technique for current technology is getting the skis on edge. Beyond that, you have to work to get more performance out of the ski.
post #27 of 34

What About Gravity

Hi All,

I've been lurking for a couple of months, though I'd get off of my butt and post something.

All of the posts thus far have been really good.

My only problem with these force models is that they are only looking at the rotational/leverage forces involved. What about gravity?

The force of gravity is also acting on the ski. A heavier person will bend a ski far easier than a lighter person. This is especially true as the carve passes the fall-line towards the end of the turn where you are not only fighting the centripital force, but also the force of gravity.

Thus, a heavier person will create more force on a ski than a lighter person (all other factors being equal of course!) So if the goal is to achieve a specific arc radius (which is a pretty narrow view of skiing i admit, but still within the scope of conversation) the heavier person may need a longer ski than the lighter person.
post #28 of 34
Quote:
Originally Posted by Noodler
Rather than bury this discussion in the other thread, I'm really interested in thoughts about how ski sizing on modern skis should be determined.

What body measurements matter? I still believe that height plays a factor along with weight, but am I just being "old school"? No doubt most everyone is skiing shorter more shapely skis now, but how does that change coming up with a recommended length?

Elan doesn't even mention weight on their web site for length recommendations. They only use height. I've seen some posters argue that only weight matters. How about a discussion with some scientific physics basis on why (or why not) height should be included.

Obviously there is a skill component involved too. I'm surprised how many skiers still resist going shorter to avoid "insulting their testosterone" level.

I've pretty much stuck to listening to the manufacturers recommendations and demoing, but you can't demo every lenght (usually). So how do you narrow it down to what size is most likely the right size?
I think some ski manufacturers are still using (Hight) because they make an assumption about weight, based on a persons hight.
Here in the good old US of A we are having a (weight problem)? that blows the curve for the rest of the worlds sking population.
post #29 of 34
Weight (or more properly mass) is the prime body measurement. Skiing style / technique / what you're planning to do with the ski is a bigger factor,* but it's not what the original post asked about.

It's not solely a matter of gravity, but also of inertia.

Lever length of the leg varies with height and might be a factor if it mattered for anything. I guess it could conceivably relate to width of the ski, but I don't really see what it has to do with length.

--------
*Consider:
- In the old days, expert adults pretty much skiied on the same length skis (c. 201-210) without much regard to their size. On the other hand, most women typically used shorter skis, as did kids or - sometimes - particularly small people (and the reverse, in the case of particularly large people).
- Adult (or near-adult) top racers pretty much use the same size skis for a particular event without regard to body size. The same is approximately true of Masters Racers: though skis size varies more, it usually doesn't seem to relate to body size. On the other hand - see women again.
- A particular racer (at all levels, except little kids) uses hugely different-sized skis for different events. That is, someone whose body doesn't change size might change from something around 160 cm, to something around 185 cm, to something around 200 cm, to something around 215 cm, depending on what event he's skiing in.
post #30 of 34

It's not a cut and dry thing

Quote:
Originally Posted by sjjohnston
Weight (or more properly mass) is the prime body measurement. Skiing style / technique / what you're planning to do with the ski is a bigger factor, but it's not what the original post asked about.

It's not solely a matter of gravity, but also of inertia.
+1

Well said.
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