Why is longer faster and why are skis faster

[Missionman]

Why is longer faster?

Hey all:

Just thought I would throw this question out there – I imagine it may have already been answered before on this site, and as snow sports go, you guys are probably more technical than anyone out there.  But I go out on the internet and seem to get different answers – nothing seem definitive.  Even this article ( https://www.economist.com/blogs/babbage/2010/12/babbage_hits_slopes  ) from a reputable publication speaks of the thin film of melted snow beneath the board or skis –which in my mind is false!  There simply isn’t enough pressure from the width of a board to cause a phase change like you get on skates.  Moreover, a snowboard will fly down an icy surface without any weight on it at all.

Am I wrong about the pressure/thin film theory?

I understand the benefits of length and stability (esp with respect to turning) but why is longer faster, if at all?  And why are skis faster?

I want to understand the physics behind this.     

As an aside - hate me if you want- I agree with almost everything in this article, esp the helicopter thing :)  (what this article neglects is the ‘feel’ of snowboarding, which will always be cooler to me than skiing)  https://mpora.com/skiing/12-reasons-why-skiing-is-cooler-than-snowboarding

[MNSurfer]

My non-professional/uneducated view. Sorry, no physics, here.

If we're talking about racing, or folk who haul a$$ down the hill...

Skis aren't necessarily faster. As a matter of self-preservation, you simply wouldn't want to go that fast on one plank, as it's much easier to make corrections when your feet are separate. Of course needing make said corrections at-speed is unavoidable.

Also, wouldn't there be more drag/friction from one, deep rail pushing into the ground, than two rails skimming the snow? I don't know about putting the board/skis flat, as I'd never do that, at high speed. In theory, if the surface area is essentially the same, my guess is they'd go relatively the same speed. But...

The skier himself also has the distinct advantage of being able to become more aerodynamic.

 

I follow my ski friends around, we probably all have the same length tools. They're aggressive, to be sure, and I can keep-up. Just not for that long. Fatigue and errors are far more dangerous (and costly) on a board. It's those repercussions that have me pulling back about mid-way through a bombie run, or not bombing at all if your tired. 

 

 

[workshop7]

Every ski tech I’ve talked to for the last 30 years all subscribe to some version of the ‘skiing on a thin layer of water’ theory.  I believe it.  Mostly because it’s all I’ve ever heard.

Edited February 21, 2018 by workshop7

[John E]

Maybe this has no physical reference but I think that when the curlers are sweeping ahead of the stone, they are momentarily melting the surface of the ice just ahead. This is what speeds up or steers the stone. If a sweeping broom can do this, a snowboard should be able to do so as well. 

[Neil Gendzwill]

A skier's tuck is more aerodynamic than a snowboarder's. The independent legs and more natural knee action mean the skier can keep that tuck through more terrain variation than a snowboarder.  That same independant leg action allows for quicker correction of line, not to mention much faster edge changes. 

The width of the board is likely to cause the snowboarder to take more unintentional air on jumps. I'd wager the narrower profile allows thicker layups so you can make skis stiffer and more damp without getting into ridiculous weight. 

Edited February 21, 2018 by Neil Gendzwill

[Corey]

I buy into the melting snow theory as you can see the shiny paths laid by straight-lining skis or boards even in very cold conditions.  

[lowrider]

I often over take skiers when on my Skwal.

[Neil Gendzwill]

13 minutes ago, lowrider said:

I often over take skiers when on my Skwal.

I often out-glide them on the flats.  But I just attribute that to better prep, i.e. I waxed and they didn't.

[michael.a]

I agree with Neil, not just wax but a better base grind. 

[JRAZZ]

7 hours ago, Missionman said:

There simply isn’t enough pressure from the width of a board to cause a phase change like you get on skates.

Pressure isn't the only thing acting on the snow.  There's friction, and plenty of it.  Certainly enough to create a microscopic layer of water to glide over.

 

[ursle]

Yes, the friction effects the snow under the board or ski, yes the skier  is faster in a "course", the skier can bend (de-camber) the ski and step off it onto the other ski getting "rebound" on each turn, as well as pressure the tail (drop the hips, push down) for speed, snowboarders can only pressure the tail, yes, skiers also have poles and can skate the beginning of a course, in Nastar snowboarders are give a Handicap to make things equal. Hard to test using a glide course, 

Aerodynamically, a snowboarder can get tiny and be still.

Longer is faster because the riders weight is more spread out and the sidecut is also more spread out, for example, my madd 158 couldn't go straight, the tail stopped it, it had to be on edge, a drag on very thin flat trails, longer absorbes irregularities better, more damp, and longer doesn't necessarily mean only large turns, bending a long board, then "de-cambering" it makes for some tight turns.

[Missionman]

i buy it all!  I read this in Wikipedia - which i think I agree with -so its not quite the high pressure of the skate that causes the surface to be slippery, although the pressure can melt the ice (hence the deep cuts in the ice when cornering on single skates or at high speeds). 

"Physical mechanics of skating[edit]

A skate can glide over ice because there is a layer of ice molecules at the surface that are not as tightly bound as the molecules of the mass of ice beneath. These molecules are in a semiliquid state, providing lubrication. The molecules in this "quasi-fluid" or "water-like" layer are less mobile than liquid water, but are much more mobile than the molecules deeper in the ice. At about −250 °F (−157 °C) the slippery layer is one molecule thick; as the temperature increases the slippery layer becomes thicker.[14][15][16][17][18]

It had long been believed that ice is slippery because the pressure of an object in contact with it causes a thin layer to melt. The hypothesis was that the blade of an ice skate, exerting pressure on the ice, melts a thin layer, providing lubrication between the ice and the blade. This explanation, called "pressure melting", originated in the 19th century. This, however, did not account for skating on ice temperatures lower than −3.5 °C, whereas skaters often skate on lower-temperature ice. In the 20th century, an alternative explanation, called "friction heating", was proposed, whereby friction of the material was causing the ice layer melting. However, this theory also failed to explain skating at low temperature. In fact, neither explanation explained why ice is slippery when standing still even at below-zero temperatures.[19]"

The analogy to curling sold me too.  There isn't the level of pressure exerted by a skate on a broom to melt the ice, but the effect of sweeping is real, having curled myself.

9 minutes ago, ursle said:

Longer is faster because the riders weight is more spread out and the sidecut is also more spread out, for example, my madd 158 couldn't go straight, the tail stopped it, it had to be on edge, a drag on very thin flat trails, longer absorbes irregularities better, more damp, and longer doesn't necessarily mean only large turns, bending a long board, then "de-cambering" it makes for some tight turns.

^I actually this this is key now (having read a little bit more).  A corollary of this then is that stiffer boards probably absorb the irregularities even more - since they would smooth out all of the bumps.  This would double the impact of speed, since longer (non-custom) skis/boards are typically made stiffer.

 

2 hours ago, Neil Gendzwill said:

I often out-glide them on the flats.  But I just attribute that to better prep, i.e. I waxed and they didn't.

And yes - the impact of length probably doesn't have as much an impact on flats, where there are less 'irregularities'.  Hence my own experience of out-gliding skiers as well (and probably attributed to better prep which I have experienced against my skier friends since they don't wax like I do

 

5 hours ago, MNSurfer said:

Skis aren't necessarily faster. As a matter of self-preservation, you simply wouldn't want to go that fast on one plank, as it's much easier to make corrections when your feet are separate. Of course needing make said corrections at-speed is unavoidable.

I also agree with this - lose an edge on snowboard and you slide out.  Lose an edge on skis and your other ski can compensate long enough to get the other ski corrected. 

So I think I am satisfied!!  Thanks all

[philw]

17 hours ago, Missionman said:

Why is longer faster?

..... Am I wrong about the pressure/thin film theory?

...And why are skis faster?

I want to understand the physics behind this.     ...

Multiple questions. My italics. You can look up the "why do things slide" stuff on the net so I'll ignore that. 

Why is longer faster? I'm not sure, ...assuming you're on hard pack (not powder), there's gravity and drag ("air resistance") plus friction. If you keep the surface area the same, don't change the drag coefficient, and don't change the weight, just increase the length, then "the physics" haven't changed. Which suggests that length is going to help with"stability" only, I'd say. Which is probably why metal short boards ride faster than their glass equivalents.

Speed skiing & snowboarding wasn't a full FIS sport, but they do publish rules, which include min/max lengths for skis, along with a max weight. Less than the minimum length would be unstable at those speeds. The mass limit makes sense from the physics (weighting skis without adding drag would give more speed). I'm not sure why the upper limit exists... 

Skis are definitely faster than boards for speed ski/ snowboard. That's a straight course (no turning allowed or possible), and all ice. That could be because the stance is more aerodynamic (less drag), or it could be independent leg action as described, but the difference is significant. 

 

[Missionman]

It would be interesting to get speed  data from a mono-ski and compare that to standard skis.  It could definitely give some idea about independent skis (perhaps more effective running length), but take out the aerodynamic aspect of the comparison with snowboards.  Also, could get information on how feet separated down the length of the snowboard vs together on a monoski and how the board flexing over terrain differs in speed vs skis/monoski.

The monoski is the key to finding out lol.

[Corey]

Monoski ManXavier Cousseau (France) - 212.26 km/h - Les Arcs, Pro Mondial 2006

from http://www.speedski-info.com/English/Records.htm

Closer to snowboard speeds than ski speeds on that page.   I wouldn't have guessed that based on aero alone.  I'm guessing the wider/stable base means it's easier to hold a proper tuck.  

[Allee]

4 hours ago, philw said:

That could be because the stance is more aerodynamic (less drag), or it could be independent leg action as described, but the difference is significant. 
 

You could see it on the boardercross v the skicross at the Olympics. That was a great course for boardercross. It was borderline breakneck for the skiers (as I'm sure Chris Del Bosco will tell you). The speed differential was very obvious.

[BlueB]

36 minutes ago, Corey said:

Monoski ManXavier Cousseau (France) - 212.26 km/h - Les Arcs, Pro Mondial 2006

from http://www.speedski-info.com/English/Records.htm

Closer to snowboard speeds than ski speeds on that page.   I wouldn't have guessed that based on aero alone.  I'm guessing the wider/stable base means it's easier to hold a proper tuck.  

On skis one can hold the high tuck (bearly a tuck, but it's still called so), medium tuck known as "egg", and low tuck "frog". The last one is the most aerodynamic, it's done by separating the legs enough for upper body to fit in between the thighs. Monoskier can't get into this position. 

Next thing is the frontal contact with snow and surface area vs. length. Maximum friction is right at the tip, where melting of boundry layer happens. Once melted, the rest of the device glides on that. With skis it's easier to make this ratio ideal. Monoski and board are always wider then a pair of skinny downhill skis... 

[BlueB]

Also, an interesting anecdote: 

Last year, Scooby, Luka and I went to Sun Peaks, for Luka's U12 ski racing. There's a "little" run called Headwall, where they often hold speed contests. Starts at about 45° fall line, but mellows out to almost flat - ideal. Now, Scoob and I were not even thinking of bombing that, but all the little racers did, without thinking twice, on their 130-150cm long skis... 

[Neil Gendzwill]

11 minutes ago, Allee said:

You could see it on the boardercross v the skicross at the Olympics. That was a great course for boardercross. It was borderline breakneck for the skiers (as I'm sure Chris Del Bosco will tell you). The speed differential was very obvious.

That was one of the most frightening crashes I've ever seen.  Latest news is that it is 4 rib fractures, a bruised lung and a pelvis fracture.  Holy Kittens! as Craig McMorris would say.