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Height in determining ski length

post #1 of 13
Thread Starter 
This is a n00b question, but to what extent (and why) is height a determining factor for the correct length of ski? So many people post their height as well as weight here when looking for advice on ski choice. Why shouldn't a 70kg 5'8" person be on the same length ski as an 70kg 6'1" person assuming everything else is equal? Is it fore/aft balance related due to a higher centre of gravity rather than being able to flex the ski?
Edited by narc - 11/21/09 at 4:42am
post #2 of 13
Weight and ability, i don't see why height would make a difference.
post #3 of 13
A longer lever has more ability to make use of it's length to manipulate that lever. A tall person can use a longer ski because of that ability.however a heavier skier can flex that ski differently than a lighter person  and may either desire a longer ski or a stiffer ski to bear his weight.
Height is a factor as is weight and  also   your ability  is  while including  your use choices  to determine proper personal length choices.
post #4 of 13
I still don't see how height would effect how you flex a ski.  We aren't levers, unless i have a fulcrum somewhere on my body.
post #5 of 13
Suppose you want to really bend the front of the ski.  Having your weight forward (to say nothing of throwing your mass forward and using some dynamic loading) puts a load on the tip.  The length from boot to tip x the load on the tip (actually load x length from boot averaged along the entire length, but it's the same principle) must equal your weight times the distance you are able to move it forward, which will depend on your length.

Now for dynamic loading you also get your momentum which is your mass times forward velocity.  The force stopping your momentum times your weight-averaged height is what must be resisted by the tip times its distance from the boot.

I would say weight is the main determiner with height playing a small roll, and more of a roll as skill increases.  I don't think the difference between 5'8" and 6'1" at the same mass would change optimum ski lenght by 5", but I would give it an extra  3 cm.  
post #6 of 13
Thread Starter 
As an engineering grad I appreciate your explanation ghost
post #7 of 13
Quote:
Originally Posted by huhh View Post

I still don't see how height would effect how you flex a ski.  We aren't levers, unless i have a fulcrum somewhere on my body.
 

wow this post is full of fail.

A taller person can leverage the ends of the skis more than a shorter person or the same weight. This matters more for guy who COG is higher than a girl.

Weigh and max load put on the skis still matter alot but to say height doesnt matter is incredibly uninformed.
post #8 of 13
 A ski does not know how tall you are, but can feel how heavy you are. 
post #9 of 13
Quote:
Originally Posted by Philpug View Post

 A ski does not know how tall you are, but can feel how heavy you are. 

so your saying a 200 lb 5'6 skier can exert as much force as quickly to the tip(or tail) of a ski as 200lb 6'6 skier?

I
post #10 of 13
Quote:
Originally Posted by BushwackerinPA View Post




so your saying a 200 lb 5'6 skier can exert as much force as quickly to the tip(or tail) of a ski as 200lb 6'6 skier?

I


 Downward force, yes, but leverage comes into play. that 6'6" person does extend the force. It is a guideline, not a rule. 
post #11 of 13
 Remember how bindings are set, taking into account height, weight, boot sole length, and age/ability.  The first three go directly into the physics of how your body applies a moment/torque to the ski through the binding interface, while the last two (age/ability) are more about risk.  I think that constitutes recognition by the ski industry that both height *and* weight matter.  But you can do some relatively simple physics to come to the same exact conclusion.  Height definitely determines both how much leverage you can apply to the ski, and the reverse -- how much directional and longitudinal stability the ski can give to you.

I normally use height as the main factor when sizing skis -- chin height for hardpack carvers, and nose to head height for soft snow and free-ride skiing.  From that preliminary sorting, weight becomes a secondary input, which may drive me to a longer or shorter length of ski in a manufacturer's discrete lineup.  Since skis come in discrete sizes that may be 4-10cm apart, you often would want to use factors of height *and* weight to make the best choice (demo would be great too, but we often do not have the luxury of demoing many sizes).
post #12 of 13
huhh,

"I still don't see how height would effect how you flex a ski.  We aren't levers, unless i have a fulcrum somewhere on my body."


I believe the boot becomes the varying fulcrum (locked by the heel/toe binding), the lever is you from the ankle up.  Leaning forward, from your heel forward you are applying pressure towards the tip and when leaning back, from the toe rearwards toward the tail.

I would also say that height and weight are tied at the hip on this.  Weight dictates how much force can be applied to the lever and it also determines how much force has to be applied to the lever to move the pressure from the center of the ski to the tip.  A 50kg man can't apply the same (downward) force as a 100kg man with everything else being equal.  How ever, a 100kg man needs more force applied towards the tip to get the downward pressure from the center of the ski to the tip

If your job was to be the heel binding, would you rather be it for:

a) 6' 50kg man
or
b) 6' 100kg man

The heel binding for b) has to work harder (i.e. gets a higher setting).

If you still don't believe me, take an oar, put the handle end between your legs and set the shaft of the oar on something that is about the same height as your inseam and is positioned about 12 inches infront of you.  The paddle end should be a few feet in front of you.  Have some one set their ski boots on the paddle end... carefully .  You probably feel some pressure between your legs but you're OK.

Now have the same assistant put the ski boots on and with everything else as the first test, stand on the paddle end .  When you gain consciousness, tell us what you felt.

Length does matter
post #13 of 13
It's been a quarter century since I've done this, so let me try this...

Physics tells us that in order to have no effect caused by difference in height, the two equally weighed persons have to be skiing perfectly centered over the center of earth. Otherwise, the taller person will have to bend down so much that the centers of mass are at the same position all the time.
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