ski width & math

I'm trying to make some sense of ski width and length, compared to the size of the people skiing.  Take a Mantra for example, which comes in lengths from 170 - 191cm.  I'm guessing that the 170 is aimed at a lighter guy, say 5'5/140 lbs, and the 191 is aimed at the larger end of normal, say 6'2/220 lbs.  The difference in length between 171 and 191 is 10%.  But the difference in weight between 140lbs and 220lbs is 37%, so hmm... maybe height is more relevant than weight.  The difference between 5'5" and 6'2" is 9% - bingo!  (I did not make these numbers up so the math would work out, really).   But now comes my real question - width.  All these are 96 under foot.  But going by percentages if the 170 is 96 the 191 should be 106 underfoot in order for the big guy to have the same skiing experience as the small guy.  So when you read a review about ski X holding great on groomers, maybe it's important to look at the size and weight of the tester and the length he was testing.  A big guy may have no problem arcing turns on a 96, but he might have problems arcing on a 106, which is what that 96 will feel like to a small guy.  I might be full of it, probably am.  But how would you really know that the two sizes ski alike, since each tester is just whatever weight he is, you never really get a comparison of the 170 to the 191 by the same person.  This all came about because I was thinking about the width of the Mantra Jr for my kid, who weighs 75 lbs, and I did the math and decided it would be like me skiing on something thats 140 underfoot - not exactly an all mountain experience.  Please, make some sense of this for me. Thanks.

An interesting topic. Good luck making sense of it... Especially given the discussion likely to ensue.

Nonetheless, the short answer to the real question at the heart of this is that you should get your kid the new kid's Gotama. The only modern design kid's ski on the planet that I know of. You might as well start your kid out right. 

The only sense to  be made of this is that if you chase your tail persistently enough, you may catch it and give yourself a nip. If you don't catch it, you stand a fair chance of tying yourself in knots or at least getting very dizzy.

SJ

Where do you ski  ? Both of those sound great  at 113/80/105 rocker or not with the Mantra. If the kid has a hard surface ski let him rock the Gotama if he needs to navigate more on piste  or those between dump days then get the Mantra
The steeze value for a kid on a Gotama would be priceless and the learning experience would even be more worthy. Steeze and application aside it depends on where he will ski these . So there are  some more options to explore.

A ski is a beam with a load applied in a relatively small spot on the top and distributed load along the bottom.  The only thing that matters is the load.  Weight directly affects the magnitude of the load, height affects the load to a lesser degree as the skier can change the load on tip and tail and apply more torque through more leverage.

Normally, you take the skier's height/weight/skill into account when choosing the ski length.  Width, on the other hand, falls into the same pot of characteristics as shape, camber, flex, materials, etc... which speak more about the type of ski and its mission/focus.  You'd choose a narrow waist ski for hardpack carving, a fat ski for powder, and all sorts of choices in between.  

You sort of need to trust that the ski maker designed, constructed, and built the ski such that all the characteristics work out right together.  It's up to you to fine tune it by choosing the right length.

Most reviews here will tell you the skier's height/weight and the ski length chosen.  Some people try multiple lengths, but normally it's just one.  Having demoed a lot of skis for fun, I've gotten to the point where I go chin to noise height for hardpack carvers and nose to head height for everything else.  At 6'1 / 200lbs, that pretty much always means the longest ski in whatever model I'm looking at.  That's as  complicated as I care to make it.

FYI, we have an interesting article on the subject by Physicsman here.

steeze who the hell cares how you look?

just know this in 3d snow more float is better no need to get all nerdy about it like physicsman.

float can be size, shape and flex as well.

That new Gotama Jr. is the sh&t. I wish it had been around when my  kid was skiing powder on 9/22's. A kid's pow ski! What could be more fun! This is going to be a major factor in changing the way people ski.

About width and length: many skis now vary the width with the length. Makes sense. I always figured I was getting a wider feel riding a short ski that stil had the full width under foot.

In the end, we learn to use the tools we have.

Both the Gotama Jr and the Mantra Jr are 80 underfoot.  I agree that they both seem like great pow skis for kids, but I'm looking for a quiver-of-one kid's ski, not a pure pow ski.  It's tempting to think a Mantra would be that based on the name, since that's exactly what it is at adult sizes.  But by my calcs, an 80mm Mantra for my kid is comparable to a 130 mm ski for me, and I definitely would not call that a quiver-of-one width.  Perhaps my analysis is wrong.  For those of you who think that 80mm is a great all-around width for a kid, I'm curious to know what makes you think so.  Thanks for the interesting discussion. 

 I don't know the physics of it, but when my 10 year old is skiing with me on his 85mm K2 Bad Seed skis, he is at about the same approximate depth in the powder as I am on my 95mm wide skis. He loves them in powder, but also thinks they're fine when there isn't any fresh. That said, if it looks like it's going to be hard packed, then he usually skis his (wood core) Apaches though even on those days he sometimes skis the Bad Seeds. I think that you're thinking too hard about it. Skiing is fun and kids are very adaptable. I'll bet that your kid will enjoy and adapt to whatever boards you strap to his feet.

kids handle bumps and groomers with ease. It's the powder that used to be a problem, partly an equiptment gap for them. The kid's Gotama is slightly rockered, the Mantra not, and both are going to be so cool in pow and crud. Unless your kid is headed toward racing and needs a one-quiver that keeps him honest to that technique, I'd get those new-school boards for him. IMO, the 80 = 130 equation does not function in real snow scenarios.