"82 mm skis should be good enough for a foot of snow"

Folks thinking about making their first purchase of a wider than normal ski repeatedly make statements such as, "I expect that this 82 mm wide ski should be 'good enough' for foot deep snow".

The problem with this expectation is that soft snow skiing is a lot like water skiing. Specifically, for a given speed, you and your skis won't start lifting you up unless the upward force that the snow exerts on your skis exceeds your weight. So, a 200 lb guy going 25 mph may get 150 lbs of upward force from 70 mm wide skis, and his skis won't lift off of the hardpack base.

Assuming the lift force is proportional to area of the skis, on a pair of 90 mm wide skis (same speed, same angles, etc.), his skis will be generating 192 lbs of upward force and hence, they will still be sitting on the hard base under the soft new snow. However, as soon as he gets on a pair of 95 mm or wider boards, they will generate 203 lbs (or more) of upward force, and his skis will be able to lift off of the base layer and rise through the soft layer.

Thus, this skier will hardly see any change in float from 70 mm to 90 mm wide, then, all of a sudden, at 95 mm, he will start to experience a change in sensation and mode of skiing. This is precisely why many folks (particularly, those over, say 180 lbs) feel that mid-fats are "Jack of all trades, master of none", and why I object to the statement that mid-width skis are "good enough" for a foot of snow. If they really are "good enough" (ie, wide enough at your weight) to float in a foot of snow, they will float in a meter of snow.

This is also why many folks advocate bypassing the mid-fats and going directly to fats. It is also why lighter weight folks get enough float out of mid-80's skis and like them. To them, mid-80's are "fat" skis (at least in their behavior, if not actual width).

Now, obviously, the above description is a simplification. Skiers don't all travel at precisely one speed, and snow compacts, so new snow just above the hardpacked base will be more dense than snow near the surface, particularly if it has aged for a bit. This means that for real skiers in such snow, the transition from not-floating to floating will not be as abrupt as I described above. It will be more gradual with respect to changes in width, speed, and other variables, so mid-fats will actually float a bit better than 70 mm wide skis, just not as much as most skiers seem to believe.

Tom / PM

Just a few quick comments … more later:

Several people have brought up (directly or indirectly) the issue of vertical accelerations (eg, up and down unweighting, terrain undulations, etc.).

If you are a heavy guy on narrow skis, going sufficiently slow, and the new layer of snow is sufficiently light, your skis will always be riding on the base unless you execute a major unweighting move. OTOH, if you are on wide skis, and the new layer of snow is sufficiently dense, your skis will always be riding well above the base, unless you make a very strong extension and momentarily force the skis to submerge deeper. In the intermediate range of widths (ie, mid-fats), weight, speeds, g-forces, snow densities, etc., sometimes your skis will be riding on the base layer, other times, above that layer.

One of the reasons I think that I don’t particularly like mid-fats is the uncertainty described in the previous paragraph. I like my skis to be either (almost) always up in the new snow, or (almost) always down and well planted on the base layer. Obviously, it’s not the end of the world when you transition back and forth – you adapt to it – but it doesn’t give you that ultimate feeling of smoothness you get from always having your skis floating in the new snow. Nor does transitioning back and forth give you the consistency of technique you can use when your skis are always riding on the hard underlayer. Put differently, what’s hardly more than a “dust-on-crust” layer to someone on 65 mm wide skis may feel like velvet to another person on 95 mm fats.

Speaking of which, one of the things I most love about fats is this ability to turn 6 inches of fluff over a hard, miserable cut-up base into velvet.

SSH questioned how the effects of centrifugal force enter this discussion. These forces are in the plane of the hill, so neglecting some more complicated effects, they shouldn’t directly cause the skier to rise and fall in the (presumed) layer of new snow. OTOH, they most certainly will cause the skier to push a lot of snow out to the sides of the turns (as described in the old “Carve a Turkey” thread).

Comprex & Martin – For this analysis, I’m not considering the increase in drag when you are more deeply immersed in the new snow layer. It certainly is an important effect, but like all good physicists, I like to learn how to analyze the various physical mechanisms one at a time before I try to combine them.

Martin – I’m going to defer the discussion of the Rossi interchangeable links to some other thread. As you hinted, maybe the old “Carve a Turkey” thread needs to be bumped.

Posaune – you are absolutely correct that the density of snow has a major effect on the lift generated. In fact, for a given speed, angle of attack, ski width, the lift will be almost exactly proportionally to the density of the snow in grams/cc, ie, exactly as you pointed out.

As I pointed out towards the end of my first post, the existence of an abrupt transition is an abstraction for skiers who flex, extend, change speed, go over snow of different densities, etc. and thereby partially blur out the effects of increasing width, but my main point was that most skiers don’t even realize that the effect on float of increasing width can be abrupt. They assume it must be smooth (as mentioned by Martin).

If you want the ultimate example of an abrupt transition to “float”, consider an airplane taking off. Overload a small plane with enough lardy passengers, and it’ll never get off the hard base layer (a.k.a., "the ground"). Put the same well-fed passengers in a plane with more wing area, and it will get off the ground just fine.

Tom / PM

I don't think this is the reason. On a normal ski edged at 2 degrees, your downhill edge may only be a few mm above the snow, whereas on a fatty, it might be 50% further off the snow. Fewer (mechanically strong) terrain irregularities are of the larger height, and so there are fewer opportunities for the aforesaid mentioned snow snakes to raise their heads and trip up wider skis.

Tom / PM

Hi Beyond - Interesting thoughts.

I’m sure it’s a factor. Just like the colloquial use of the word, “float”, encompasses many physical phenomena (cf. http://forums.epicski.com/showpost.p...38&postcount=9 , http://forums.epicski.com/showthread.php?t=2275 ), many factors contribute to tip dive. If there’s plenty of float you can get away with stiff low tips. If there isn’t much float, you’ll dive easier.

The above formula (with an appropriate choice of lift coefficient and exponent) is a very good approximation.

I’m not sure I would agree with you about (1) and (2). We ski in powder at velocities ranging almost a stand-still to making fig-11’s straight down the hill. The range of densities also goes over a wide range: cold smoke to slurpee.

Boot drag doesn’t directly influence lift because the two forces are at right angles to each other. If you keep your skis at a specified angle of attack, your skies and lower legs will generate the same amount of lift whether your boots are clunky or streamlined, so long as you can maintain the same speed.

Obviously, clunky boots will slow you down, but, in my experience, a much bigger factor is the drag caused either by the skier’s body, or by skis intentionally put at large angles of attack by someone who is worried about keeping afloat. This usually happens to folks on skinny skis when they hit the runout at the bottom of the hill on a megapowder day. They start slowing down and sinking, so they sit back to increase the angle of attack and lift. Unfortunately, this also dramatically increases their drag, so they start slowing down even faster, eventually leading to a stall and the skier finds himself floundering around on his back, tips up in the air.

In such a deep-pow situation, keeping centered and maintaining a low angle of attack doesn’t help either because as you slow down, you sink, and at the point when the snow is about waist height, there is so much frontal drag from your body, you’ll again come to a stop, except this time you will be stuck in the snow vertically, instead of laying on your back.

The analogy to a bird wing is not the best. Camber changes in a ski are fractionally fairly small. They will cause the angle of attack at the front of the ski to differ from that at the rear of the ski, but won’t make a direct change on the average angle of attack (ie, averaged from tip to tail of the ski). Thus, normal camber contributes to tip dive, while reverse camber (ie, Spats) helps dramatically in avoiding it, but neither changes the overall float very much. The main effect on float will come from the overall, average angle of attack, even if it is very small, only a couple of degrees.

Skis in bottomless powder will almost always be in reverse camber (assuming no unweighting moves are going on). To check this, just put your skis on a mattress and stand on them. Just don’t let your wife/GF/mother catch you.

Cheers,

Tom / PM

I agree about the need for versatility, but the performance envelope of modern fats is so wide that many *very* good skiers (e.g., the maggots at the recent Mammoth gathering) are quite happy staying on their fats on a bluebird day.


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(above from: http://www.tetongravity.com/forums/s...ad.php?t=52897 )

Tom / PM

Hi Ghost – Your experience on old 208’s matches mine. The first time I ever went to LCC was about 1975. I was on 207-ish Volkl Zebras and weighed a lot less than I do now. Some serious storms hit ( …maybe 5 ft in 2 days) and when they let us back out of the lodge, I got bogged down on the runouts just like you.

What I don’t agree with is your expectation that “length would have compensated for the narrowness”. If you stick the numbers for an old 208 cm Volkl Zebra ( 88/68/78) and the numbers for a 180 Explosiv ( 123/95/113) into my spreadsheet, I come up with areas of ~ 1520 and 1850 sq. cm., respectively. The shorter, more modern fattie has 22% more area than the old GS ski. The extra float you get from this is equivalent to a 200 lb guy dropping down to 165 lbs. It’s a BIG effect.

I don’t have a strong disagreement with your final statement that, “…a softer flex is very important for deeper snow…”, but you just can’t jump to that conclusion based on your comparison of skis differing by over 20% in area.

Tom / PM

PS – My sidecut numbers are from memory – I think they are pretty close. I didn’t bother looking them up, but even if they are off a bit, the same argument still applies.