Do skis get stiffer with length?

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SkiTheNortheast
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Do skis get stiffer with length?

Post by SkiTheNortheast »

I read that most major ski companies make their skis stiffer for each length of the same ski. For example, a certain K2 ski won´t be as a stiff at 160cm than at 180 cm.

Are skis inherently stiffer as they get longer or do ski companies add more/thicker layers of fiberglass and carbon to make them more stiff?

WhitePine
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Post by WhitePine »

I think that typically the thought is that longer skis are targeted toward larger riders. Therefore they may change the core profile or other tweaks to change to flex of the ski to make this accommodation. Usually the sidecut radius changes as well.

twizzstyle
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Post by twizzstyle »

Define "stiffness". True beam bending stiffness is not a function of length, only modulus of elasticity and moment of inertia.

So for the same thickness, a longer ski won't be stiffer, although it will deflect more for the same force applied. It's a hard thing to compare, I think WhitePine has got it.

SkiTheNortheast
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Post by SkiTheNortheast »

so how do you think the major ski companies do it? do they just change the core profile to increase the stiffness in a small way? or if they were using composite would they use, for example, a 3" wide carbon tape in the longer ski compared to a 2" wide one in the shorter ski?

If the skis had the same type of wood in the core what would affect the stiffness the most?

twizzstyle
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Post by twizzstyle »

That's too general a question to answer. Depends on the thickness, depends on the width, depends on the composites used, depends on their orientation, probably depends on the moon cycle when the skis were laid up.

All I can say is that small changes in core thickness can make HUGE changes in stiffness. All else the same, the stiffness is proportional to the thickness cubed. Double the thickness, the stiffness goes up by 8. That seems like a cheaper way to increase stiffness vs adding more composites, but I'm not sure what the big guys do.

WhitePine
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Post by WhitePine »

Twizz beat me to it but I was writing this up at the same time.

I think its probably a mixed bag. Some probably add thickness to the core, some add some width, others may find they’d rather add more glass or carbon. As Twizz said, one of the most important factors of ski stiffness (or any beam for that matter) is the moment of inertia. When looking at the cross-section of the ski, the equation for moment of inertia is I = (b*h^3)/12 (assuming a rectangular cross-section).
b = base width
h = height (Thickness in the case of a ski)

This means that the height or thickness is much more significant than the width because it is to the 3rd power. So adding thickness stiffens the ski dramatically. If you read enough on here you’ll notice people saying how much 0.5 mm in core thickness adds to the stiffness. Another way is to add carbon fiber or extra glass to stiffen the ski which affects the modulus of elasticity.

I’m guessing most manufacturers will thicken the core of the ski rather than add more carbon because its much cheaper. The exception might be manufacturers who focus on backcountry gear where weight is an issue.

A good way to check would be to go into your favorite ski shop with a pair of calipers and measure the thickness at the recommended boot center, and at the tip and tails for each size of 1 model ski or snowboard.

SkiTheNortheast
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Post by SkiTheNortheast »

thanks for the info, looks like I'll probably just increase the core profile a tad

OAC
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Post by OAC »

Test, test, test and test, then you will find your favorite thickness.
Math and physics in all, but they doesn't beat feeling.

twizzstyle
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Post by twizzstyle »

OAC wrote:Test, test, test and test, then you will find your favorite thickness.
Math and physics in all, but they doesn't beat feeling.
Not to mention inherent errors and inaccuracies in hand-made skis. You can do as much analysis as you want, but if you cut something just slightly wrong, all of that goes out the window! :)

Richuk
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Post by Richuk »

What errors ... commercial skis, 20 g difference. Handmade skis 10g difference. I'd be surprised if this is the limit - I'm sure CO and Brazen etc are around 5g difference.

feldybikes
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Post by feldybikes »

My guess is that most companies don't actually change "anything" (I use this term..err..loosely) when they make skis longer, they just get stretched out. Sidecut dimensions don't change (e.g. a Mantra is something like 130/95/115 (or whatever) regardless of length -- I could be using a wrong example here, but i think my point is made), but the turn radius goes up as length goes up.

Likewise, my guess is that the tapered part of the core profile gets stretched. So a 160cm ski has, say 50cm of taper in the front, but a 180cm ski has 60cm of taper (totally making these numbers up). As the length of the taper goes up, the amount of ski that's a certain thickness goes up, and it's all stiffer per the equations WhitePine stated.

Not sure if I'm being coherent here, but I'm also a bit lazy at the moment. So let me know if this needs clarification.

Of course, if there are thickness changes due to length, it wouldn't be hard to figure out by going to a shop with a pair of calipers. Anyone???

AllgäuSki
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Post by AllgäuSki »

old topic, but as i was doing some gear advice inside a huge sports-shop today (first time i was doing something similar - how do the clerks manage to stand indoors all day) and didn't have too much to do, i couldn't resist to take a closer look at the topic.

Result: Yes, Skis do get thicker with length...

Image

Image

I think it's visible in the pictures. If not, just trust me, i used a highly scientific system by holding the scale of a compass next to the skis ;)

(As somebody here in the forum once said: "I'm not a scientist. Skier first, redneck second." Great Quote by the way!!!)

Hagan Skyraider Length 178, Thickness 16mm, Length 154, Thickness 12mm

Stöckli Stormrider Light Length 175, Thickness 17mm, Length 155, Thickness 12mm
Last edited by AllgäuSki on Sat Apr 07, 2012 7:19 am, edited 2 times in total.

AllgäuSki
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Post by AllgäuSki »

feldybikes wrote:My guess is that most companies don't actually change "anything" (I use this term..err..loosely) when they make skis longer, they just get stretched out. Sidecut dimensions don't change (e.g. a Mantra is something like 130/95/115 (or whatever) regardless of length
Thats totally true, though... most manufacturers don't change sidecuts which makes the radii grow with length. Stupid, if you ask me. A ski with a twelve meter radius isn't the same ski with 18m radius, if you ask me. There is a manufacturer that keeps radii and changes sidecuts, but i can't recall who it is...

edit: found it. It's Zweydinger. They change sidecuts for different length to keep the radii similar www.skimanufaktur.de

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Brazen
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Post by Brazen »

Yeah, I'm usually surprised at how close I get weight-wise...2-3 grams max difference.
"86% of the time it works 100% of the time".

skidesmond
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Post by skidesmond »

Brazen that's amazing you got it dialed in that closely. I'm all happy if I'm within 3-4oz per ski.

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