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What Gives Skis Directional Stability?

post #1 of 25
Thread Starter 
I've always wondered about this question, but really haven't the slightest idea about an answer. Any physics people out there who know what makes a ski tend to "track" forward rather than turn? Better yet, if you want more directional stability in a ski, what should you look for? If you want less, what do you look for?

Thanks in advance for any answers, theories, or just plain guesses!
post #2 of 25
To make a ski stable make it,

Not have much sidecut, a lot of shape will make it "hooky" at speeds.

Stiff.

Long.

being 100mm+ wouldn't hurt either.
post #3 of 25
Torrisonal stiffness of the tips keeps a ski going where you point it and resists deflection.
post #4 of 25
Take a look at ski jumping skis for your answer.

long, straight (no sidecut), multiple grooves in base, stiff longitudinally & laterally, and edges railed. There, that should keep you stable and straight!

Now if you do want to turn....?
post #5 of 25
Thread Starter 
So does a ski with a lot of sidecut need to compensate somehow to keep from wobbling all over the place?????
post #6 of 25
The PILOT should be what gives them directional stability. Unfortunately, it's not always the case.

A coarse linear grind pattern (As Bud H mentioned) can help*.


*If you consider abandoning the ABILITY to turn 'Help'.
post #7 of 25
Quote:
Originally Posted by Trotski View Post
So does a ski with a lot of sidecut need to compensate somehow to keep from wobbling all over the place?????
Yes.

They need to be placed on edge, this gives them excellent stability.
post #8 of 25
Quote:
Originally Posted by Trotski View Post
Better yet, if you want more directional stability in a ski, what should you look for? If you want less, what do you look for?
Ski stability is actually a pretty complex topic. People tend to dramatically oversimplify these things. I think it would be difficult to describe the mechanics and aerodynamics that define a given ski/skier system's stability without a book. There are things all of us know from experience matter, and some of them haven't been mentioned yet. For instance, mounting point and camber.
post #9 of 25
Quote:
Originally Posted by Whiteroom View Post
The PILOT should be what gives them directional stability. Unfortunately, it's not always the case.
Great minds think alike! You beat me to it!
post #10 of 25
Quote:
Originally Posted by Whiteroom View Post
Yes.

They need to be placed on edge, this gives them excellent stability.
You got it!!!!!
post #11 of 25
Quote:
Originally Posted by MAGGOT View Post
To make a ski stable make it,

Not have much sidecut, a lot of shape will make it "hooky" at speeds.

Stiff.

Long.

being 100mm+ wouldn't hurt either.
I agree that the "stiff", "long" and "straight" aspects would certainly help make a ski stable. But why the width? I thought DH and SG skis were still pretty narrow, although they still have to turn.

What about speed skis? (i.e., the kind that Jeff Hamilton hit 150+mph on). Are those just super-long DH skis? They certainly aren't turning (except maybe to stop).
post #12 of 25
Yeah, I'd question the width assumption. Or at least qualify it: In pow or deep crud, only surfaces Maggot considers worth skiing, more width = more SA, probably = more stability. Ditto for landing air, whether off a jump or a cliff.

But on groomed or ice, have a feeling that the further apart the edges are, the less tendency to run straight. (Assuming here that edges have higher coeficient of friction than bases, on groomed constrain wander from a straight line by constant micro shifts from one side to the other. Faster the micro-oscillations from one edge to the other, better the correction.)

OTOH, if speed works anything like in boats, maximum speed is a function of the effective (running) length. So longer skis can go faster. At least in a fluid medium with drag. But if you also make long boats wider, all that extra SA in contact with the water actually slows them down from drag/friction. Bet this works for skis, too. Yes? No? :
post #13 of 25
Quote:
Originally Posted by KevinF View Post
I agree that the "stiff", "long" and "straight" aspects would certainly help make a ski stable. But why the width? I thought DH and SG skis were still pretty narrow, although they still have to turn.

What about speed skis? (i.e., the kind that Jeff Hamilton hit 150+mph on). Are those just super-long DH skis? They certainly aren't turning (except maybe to stop).
Well the answer to your second question will actually answer your first one. Speed skis are longer, stiffer, straighter, and wider than DH skis. I have never flexed a pair, but have heard they are pretty much unbendable by hand, they are typically 230cm or more, and in the 80-90mm range. So they're not super fat, but a bit of extra width just helps the ski plow through everything and get tossed around a bit less. I am honestly not completely sure why, but I can tell you its true from experience . I am also not sure of the turning radius numbers wise, but I can tell you that they are effectively shaped like old school straight skis.

Quote:
Originally Posted by beyond View Post
Yeah, I'd question the width assumption. Or at least qualify it: In pow or deep crud, only surfaces Maggot considers worth skiing, more width = more SA, probably = more stability. Ditto for landing air, whether off a jump or a cliff.


Just for the record, I think that ALL types of snow are worth skiing on. If I had my choice, I'd pick powder, but if I can't have that, I'll take whatever I can get. I even really enjoy maching groomers as long as they're steep enough to pick up plenty of speed, and aren't super icy, or crowded

Quote:
Originally Posted by beyond View Post
But on groomed or ice, have a feeling that the further apart the edges are, the less tendency to run straight. (Assuming here that edges have higher coeficient of friction than bases, on groomed constrain wander from a straight line by constant micro shifts from one side to the other. Faster the micro-oscillations from one edge to the other, better the correction.)
While that is probably true in a scientific sense, I am not sure the ffects would be big enough to matter when you're actually skiing.

Quote:
Originally Posted by beyond View Post
OTOH, if speed works anything like in boats, maximum speed is a function of the effective (running) length. So longer skis can go faster. At least in a fluid medium with drag. But if you also make long boats wider, all that extra SA in contact with the water actually slows them down from drag/friction.
Bet this works for skis, too. Yes? No? :
Yes, I think that thats true, but thats just what can go the fastest, a wider ski might be a bit slower (almost unnoticeable unless you're being timed) but I think it will be most stable at the same speeds.
post #14 of 25
Quote:
Originally Posted by MAGGOT View Post
I am also not sure of the turning radius numbers wise, but I can tell you that they are effectively shaped like old school straight skis.
http://cgi.ebay.com/06-07-Atomic-Rac...QQcmdZViewItem

100-81-88.
Quote:
While that is probably true in a scientific sense, I am not sure the ffects would be big enough to matter when you're actually skiing.
The biggest impact of width on stability as far as I can tell is where it puts the inside edge in relation to the skier. We don't need a ski to be stable in roll, the mechanics of the body are very strong for controlling that compared to yaw. Consider for instance how well speed skaters do at very, very high speeds with essentially no roll stability built into their skates. I don't think its right to assume speed skis are wider because they are more stable that way. It might be because they are faster. Or both. Or neither.

If you want to say for sure whether a narrower or wider ski is more stable in a straight line, you'll have to understand all the forces involved and the effect of width on them. I spent a few minutes on it this morning before class and didn't get very far...in fact I couldn't even convince myself that a typical ski is stable with a skier on it. Its obvious from seeing them run down hills that they are stable alone, but that is a far cry from what is going on when a skier is on them.

The more I think about it the more I contemplate that the skier/ski system might be a dynamically unstable system that we only keep from diverging with active control. I'm missing my copy of the Physics of Skiing, and I don't seem to be able to find any discussion of this on the interwebs at the moment.
Quote:
Originally Posted by beyond
OTOH, if speed works anything like in boats, maximum speed is a function of the effective (running) length.
Unlikely this has anything to do with skis. You can relate boat length to maximum economic speed for a displacement hull because wave making resistance is the dominant form of drag.

In a ski/skier combo, aerodynamic drag is dominant at non trivial speeds. The skier makes a wake as well, but we generally don't talk about "wave drag" in aerodynamics until we reach transonic speeds or portions of the body reach critical mach. Not even the dudes cruising the kilometro lanzado are anywhere near those speeds.

There certainly will be an optimum ski length and width for lowest friction, but what that is will probably vary a lot because snow varies a lot. We all know you can go pretty much the same speed down an icy groomer on SL skis as you can on GS skis if you point 'em, but on the other extreme we know that the big fat pow ski will let us go faster down a foot deep slope than a snollerblade.
post #15 of 25
Thread Starter 
Very interesting discussion ...

I want to ask the same question in a different way:

What kinds of features or combination of features in a ski TEND toward directional instability??????

Thanks again in advance for any responses, and thanks for "spelling it out for me" where necessary.

By the way, where's PHYSICS MAN/Tom when you need him!?
post #16 of 25
Thread Starter 
Bump
post #17 of 25
I miss the channel (groove) that used to run up the center of my 210 cm straight Ptex ski base. I'm sure that's why we were able to tuck-it and "shush".

Kids today have no idea what they're missing. Always turning, carving and mamby pamby stuff like that. :

post #18 of 25
Quote:
Originally Posted by Trotski View Post
I want to ask the same question in a different way:

What kinds of features or combination of features in a ski TEND toward directional instability??????

As far as "instability", causes would be anything (besides skier input) that can create a yawing moment about the ski center. Zero yaw comes from absolute symmetry in the ski geometry and the snow (all the forces are in balance and cancel out). Anything that changes that balance has potential to yaw the ski. A stiff ski with a wide tip has potential to yaw because it's got a lot of beef sticking out there (with leverage about the ski center) to hook up (and this is true in experience). But a soft ski (regardless of sidecut) also has potential for yaw because even slight irregularities in the snow can deflect the ski and induce asymmetry (also true in experience). So basically, it comes down to any aspect of the ski that can make it asymmetric when perturbed, such that said asymmetry results in a yaw.

Some skis are more prone to perturbations/asymmetry/yaw than others. I find that good crudbuster skis are very resistant to perturbations from the snow, but all skis require some degree of skier input (edge set) to really perform well in crud. It's just that stiff super-sidecut skis hook quickly and soft bendy skis want to turn and ride over everything. Both require a little bit of conscious babysitting in my experience.
post #19 of 25
Quote:
Originally Posted by Cirquerider View Post
I miss the channel (groove) that used to run up the center of my 210 cm straight Ptex ski base. I'm sure that's why we were able to tuck-it and "shush".
Those channels were intended to reduce drag, by breaking suction between the ski and snow (where a thin film of water causes something called "creeping" flow). I have no idea how successful the concept was, because I have not seen it on skis in the last 5-6 years. I imagine that a good base structure has much the same effect.
post #20 of 25
Quote:
Originally Posted by Garrett View Post
Ski stability is actually a pretty complex topic. People tend to dramatically oversimplify these things. I think it would be difficult to describe the mechanics and aerodynamics that define a given ski/skier system's stability without a book. There are things all of us know from experience matter, and some of them haven't been mentioned yet. For instance, mounting point and camber.
The dampness of the ski makes a big difference in stability too. The more a ski dampens vibration, the more contact it is making with the snow. Skis are more stable when in contact with the snow.
post #21 of 25
Between Garret and 219, beginning to make sense. Question: If tip mass and stiffness have a big impact on yaw, what about tip shape? I'm thinking of another thread where Jer, SJ others mentioned spear point tips on modern pow skis (B-Squads, etc.) as producing better stability in deep pow/chop.

And if spear tips (which used to be the shape of ALL ski tips) are good, is it because they reduce mass, therefore less yaw potential, or because they part snow rather than ride up it (think Americas Cup vs. Amphibious Landing Craft) which reduces opportunity for getting knocked into a yaw movement? I'm also thinking about Legends here, obviously, which aren't a spear tip, but have that 3D prow thing going on.

Also assume that either/or both reasons will produce a more demanding ski, because they don't automatically rise in pow. But then, you could adjust for that with a longer rocker. So why are most pow skis very round/blunt tipped? :
post #22 of 25
Quote:
Originally Posted by MAGGOT View Post
Speed skis are longer, stiffer, straighter, and wider than DH skis. I have never flexed a pair, but have heard they are pretty much unbendable by hand, they are typically 230cm or more, and in the 80-90mm range.
.
I just so happen to have a pair of speed skis right next to me in my office

FWIW, they are from the mid-to-late 80's and are 240 cm and the waist is low 80mm's. Tip is in the 90mm's and the tail isn't much wider than the waist. Straightest ski I have ever seen. As for stiffness, they're nearly unbendable by hand. Fairly deep channel in the base of the ski too. With the bases held against each other there is a just a few mm's of camber tip-to-tail.
post #23 of 25
Thread Starter 
Quote:
Originally Posted by skier219 View Post
Some skis are more prone to perturbations/asymmetry/yaw than others. I find that good crudbuster skis are very resistant to perturbations from the snow, but all skis require some degree of skier input (edge set) to really perform well in crud. It's just that stiff super-sidecut skis hook quickly and soft bendy skis want to turn and ride over everything. Both require a little bit of conscious babysitting in my experience.
So let me get this straight - stiff skis can yaw, bendy skis can yaw, shaped skis can, straight can, in short -- everything can? Are you saying that there is NO WAY of predicting how a ski will behave based on common criteria? It's all individualized, all skier- and conditions-dependent, and it all depends on dozens of complicated interacting forces from which generalities are impossible?
post #24 of 25
The most directionally stable skis I've ever skied are a pair of 193cm Head m103's. I think it's because they're orange.
post #25 of 25
Really this is simple.
Longer.
Less sidecut.
Stiffer.
Everything else is minor in comparison.
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