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Disadvantages of longer skis in powder

post #1 of 17
Thread Starter 
Obviously a longer ski will have better floatation and will be more stable at speed but what are the disadvantages, if any of having a longer ski in powder? Will a longer ski be more difficult to turn and more difficult to turn quickly?
post #2 of 17
Shorter is better in the trees too.
post #3 of 17
PhysicsMan, if the longer ski length leads to more force on the foot, then does force on the foot mean more force on the tip?

In other words, does one need, and get, more tip force out of the longer length?

The stuff you posted is really amazing to me, and I'm trying to sort it out.

Another thing, you wrote about going longer if you "need more float". Under what circumstances would that be?

In shin deep heavy snow, I found it a real chore and no fun on 160 cm shorty slaloms, which is what led me to buy my first pair of mid fats - what a relief! The longer and wider mid fats bulled right through it!
post #4 of 17
I am amazed at how good the float is on the newer Atomics.

My wife has this year's R10's and I have next year's MXI's and I float way better with the larger dimensions and the ski is only a 162 but has a huge surface area.
post #5 of 17
Quote:
Originally posted by oboe #45:
PhysicsMan, if the longer ski length leads to more force on the foot, then does force on the foot mean more force on the tip?

In other words, does one need, and get, more tip force out of the longer length?

If I understand PM correctly, then the answer (to "get") is NO. For the simple reason that the pivot point (fulcrum of the lever arm) is still directly underfoot. Your side of the lever is only as long as your foot/nature's side of the lever is as long as the ski from midpoint to tip.

The answer to "need" is actually already in PM's post- the shorter skis are easier to control at the tip.
post #6 of 17
Quote:
Originally posted by oboe #45:
PhysicsMan, if the longer ski length leads to more force on the foot, then does force on the foot mean more force on the tip?

In other words, does one need, and get, more tip force out of the longer length?

The stuff you posted is really amazing to me, and I'm trying to sort it out...
With respect to yaw movements, in some ways, a ski is little more than a long lever. If you push sideways on the tip with a force of 1 lb and the tip is 3 feet away from the center of your binding, the torque your foot will experience is 3 foot-pounds. If the ski was shorter and only extended 2 feet forward from the center of your binding, the torque your foot would experience will be reduced to only 2 ft-lbs. This is one of the reasons why I claim that shorter skis randomly torque your foot less in rough snow than longer skis.

Looking at the same effect in reverse, imagine you are trying to twist a flattened skis around through piles of soft thick snow. If you apply 2 ft-lbs of torque to the skis through rotation of your leg, this means you can apply a sideways force of 1 lb to snow located 2 feet in front of you, but you will only be able to apply 2/3-rds of a pound of force to snow located 3 feet in front of you with the longer ski (ie, 2/3 lb times 3 ft again equals 2 ft-lbs).

In other words, for a given rotary input (torque) from you, the longer ski has to exert LESS sideways force on the snow at its tip.

Quote:
Originally posted by oboe #45:
Another thing, you wrote about going longer if you "need more float". Under what circumstances would that be?...
IMHO, the circumstances in which more float is beneficial to most people is when they would like to modify their turns in deep / heavy-soft snow with a bit of rotary input, but their skis are mired down so deeply in the snow that this doesn't work for them. Floating higher gets them to less dense layers where the skis will once again respond to rotary inputs and not feel "locked in".

IMHO, the only circumstances in which I would go longer instead of wider to get more float are (a) if I already owned a longer ski of the same model (as is the case with the two Explosivs in our household), or (b) the ski needed to be used significantly on the groomers as well in the soft stuff, and going up to say 110 mm underfoot (say, to get significantly more float than the 95 mm Explosivs) would simply lead to really poor/odd groomer performance.

Quote:
Originally posted by oboe #45:
...In shin deep heavy snow, I found it a real chore and no fun on 160 cm shorty slaloms, which is what led me to buy my first pair of mid fats - what a relief! The longer and wider mid fats bulled right through it!
Yup. As we've discussed in the past, while the Bob Barneses of the world can make a pair of 63 mm wide skis work wonders in deep heavy snow, and, in the past, most of us had no other choices in width, so we had to muddle our way through on old straight skis in such conditions. Fortunately, we now have much better options in terms of mid-fats (for lighter folks like yourself), and true fats (for heavier folks like myself).

Take a look at my Equivalent Float Chart. You will see that at 150 lbs, your old 70 mm wide pair of k2 mid-fats gave you approximately the same float as the 180 lb'ers get with their 87 mm wide fat skis, so it's no surprise to me that you prefered them in the conditions you mentioned.

HTH,

Tom / PM
post #7 of 17
PM, is there any way to devise a simplified model for modeling different sidecut (holding waist size and overall length constant), with the idea of looking at rotary (yaw) force on an edged ski in bottomless snow?

For relative comparison between skis, can snow compression characteristics be left out?

(Taken to extremes: should we beef up the Gotama or build more Spatulas?)
post #8 of 17
If you are attempting to turn a ski in powder by twisting it around, either because it's your default turn method, or because you really need to do so (eg, breakable crust, hop turns on super steep pitches, etc.), a longer ski (in the same model line) will have a higher polar moment of inertia and come around more slowly for a given twisting input from your legs.

In addition, longer skis in the same model line are often stiffer than the shorter models because they assume that the longer models will be bought by heavier skiers. This extra stiffness makes carving on their bases in powder have a larger turn radius (essentially independent of the sidecut radius which is more important for edge carved turns on packed snow). Again, this means that a longer ski will usually carve turns more slowly than a shorter ski.

Finally, in uneven snow, if the tip of a ski gets deflected when it runs sideways into a hard lump of snow, the torque that this event puts on your leg can often be larger for a longer ski than a shorter ski (in the same model line) because of the longer lever arm. So, while a longer ski is "more stable" in the sense of lower frequency and lower average amplitude yaw and up-down perturbations, they can still put a lot of rotary torques on your leg. OTOH, you will experience higher frequency rotary "noise" from a shorter ski, but it will often be of lower amplitude and almost always easier to counteract.

While one can read about the mechanics of these effects till you're blue in the face, by far, the best way to get a feeling for the effect of length is to do direct, back-to-back A/B comparisons of different lengths (of the same model) in back-to-back runs in snow of your choice. I did this with 165 and 190 cm long Explosivs this season in slush, and was absolutely astonished at how stable and responsive the shorter version of the Explosiv was. I think I would only go back to the longer ski if I really needed the float. Even then, if I didn't already own the longer version, I would be tempted to go even wider, but keep to moderate lengths (eg, 180's) to get the float for powder days instead of simply going substantially longer in the same model line.

HTH,

Tom / PM
post #9 of 17
Hi Comprex - Interesting question. I don't think that the tip/tail width will have much effect on the ammt of yawing torque needed to twist a non-edged but decambered ski, neutrally buoyant and stationary in bottomless powder (with some reasonable density gradient). I know that's not the exact problem you specified, but its a place to start.

It sounds you think the tip/tail dimensions will have an important effect on the yaw angle slewing resistance, especially when edged, but this will depend on exactly which axis you consider the yaw to be around, ie, the usual perpendicular-to-the-ski axis, or a true vertical axis (say, on flat snow). IMHO, the first case will be similar to the case I mentioned in the preceding paragraph, but in the second case the extra projected tip and tail area will provide more resistance. Unfortunately, in the latter case, I think there will be a lot of other things going on simultaneously that may overwhelm this effect (eg, details of the snow density gradient, "bow-up, stern-down" angle, velocity, ammt of flex (decambering), etc.).

What exactly are you thinking of?

Tom / PM
post #10 of 17
The second case (true vertical axis).

Agreed, for a flat, decambered ski the yaw resistance is not affected by sidecut and we talk of float again.

Yes, I see that edge angle, flex, density gradient and amount of decamber will all be relevant to discussion of resultant moment about a true vertical azis (hopefully the skier's upper body is aligned therewith). I was hoping that a simple model of resistance to skier input would also provide insight into response to external changes (specifically changes of density gradient similar to slush bumps).

Reduction to practice: how much "twitchier" will a Phantom Ski be in 180/100/165 than a Fischer Big Stick in 135/106/123, encountering the same slush bump at the same speed, with the same length and flex?

Restated a different way: given the same float area, is () easier to stabilise than )(? (exactly the opposite from the way mass distribution behaves).

One probable easy simplification would be to talk of float change of an un-edged ski in snow with significant density gradient changes along the line of travel. I didn't want to start that horse directly from the float thread.
post #11 of 17
One factor that has not been addressed is fore/aft stability. When the Volant Chubbs first came out I demoed them at Loveland on a heavy powder day (heavy snow). I am 6'2" and the 180's the shop insisted I try did not do well when I hit transitions in snow density-windblown at the top, softer as a dropped from a ridge and sank deeper in the snow. After one face plant and another almost I went into the shop and insisted on a pair of 190's. These did fine in the changing snow and I ended up buying a pair in the 190.

I don't know that PM's torque analogy is applicable when making anything close to a carved turn. Even when you hit a hard lump of snow, if your momentum is mostly forward the extra length will stablize when the lump slows you down.iHopefully you going over the lump, not skidding sideways into it.

I do appreciate PM's experiments with the Explosivs, but my experience with the Chubbs was the opposite. BTW, I weigh about 175. Good discussion. Anybody else have feedback from demoing different lengths? Lew
post #12 of 17
Whoops, I skipped to the bottom too soon. Comprex did bring up changing densities of snow.
post #13 of 17
I would be happy with an analysis to look at net moment at midpoint in the pitch plane, for a flat ski. For four input values (length, shovel, waist, tail), consider four rectangles:
1. Shovel sidecut area (max(waist, shovel)-min(waist, shovel)*1/2 ski length)
2. Shovel to waist area (min(waist,shovel)*1/2L)
3. Waist to tail area (min(waist,tail)*1/2L)
4. Tail sidecut area (max(tail,waist)-min(tail,waist)*1/2L)

Suspended at the appropriate distance (dependent on the min/max outcome) from midpoint by a constant bending resistance (34 N/m to pick Odyssey's number). Look at the case for rho=const. and a "pseudo bump" drho/ds=const.

[img]graemlins/evilgrin.gif[/img] JavaGame: "SkiDesigner"- design a ski sidecut and powdery bump course to launch heroic skierdood out of his high-DIN bindings. That won't get anyone sued. MUCH.
post #14 of 17
Quote:
Originally posted by Lew Black:
One factor that has not been addressed is fore/aft stability. ... I don't know that PM's torque analogy is applicable when making anything close to a carved turn. Even when you hit a hard lump of snow, if your momentum is mostly forward the extra length will stablize when the lump slows you down. Hopefully you going over the lump, not skidding sideways into it...
Hey Lew - Thanks for your comments.

You are absolutely right that I didn't discuss fore-aft stability. The reason is that I was trying to address what I saw as the main concern of the guy who started this thread, "...Will a longer ski be more difficult to turn and more difficult to turn quickly?..". Accordingly, I just focused on rotary/yaw issues, even those seen when a ski is going sideways, because, lots of people are in situations and are at skill levels where they can't carve every turn in whatever type of soft snow they are dealing with and will usually have at least some sideways slip in their turns (particularly, when they get nervous in crud).

You are also absolutely correct that a longer ski will have more fore-aft stability, especially when carving.

In fact, I'll be the first to admit that the longer ski will even have more yaw stability (carving or not) in the sense that the average amplitude of the random yaw motions (say, in a skarved turn over a cut-up surface, and assuming the skier is applying no rotary input to correct these) will be less. OTOH, while it sounds really counter-intuitive, the force (torque, actually) exerted on the skier's boot for given sized angular (yaw) perturbation will actually be larger for the longer ski. (But in fairness, these larger angular excursions will be less frequent on the longer ski.)

The converse of this is that the skier will have to work harder (ie, apply more torque) to get the longer ski to come back to the direction it should be pointed.

It's basically the skiing / rotary version of difference between a responsive sports car vs a stable sedan. You feel every road irregularity more in the sports car, but the sports car allows you more control over its path.

One final note, while going long at a fixed width will certainly smooth out some of the fore-aft perturbations caused by snow of irregular density, so will going wider at the same length.

The extra width will allow your skis to consistently stay high in the lighter, top layers of the snow and not be constantly going up and down into every irregularity.

IMHO, some effects are a wash between going fatter and going longer. For example, the mass of the skis increases with both width and length, and extra mass smooths out the constant speeding up and slowing down encountered in rough snow. Extra mass also allows your skis to better displace clumps that are in your way.

Similarly, I think its also more or less of a wash between fat and long with respect to the benefits of having decreased average pounds per square inch of loading on irregular snow in terms of compression of the irregularities.

The one mechanism by which longer beats out fatter is that with a longer ski, the tip displaces or compresses snow irregularities, and the rest of the ski (assuming in a carved turn), can simply follow in the previously smoothed path. You don't get the same benefit by going fat instead of long.

Anyway, definitely an interesting discussion. Gotta run.

Tom / PM

PS - Comprex: Good stuff, but no time to respond. I've got to prepare for a trip. I'll be back next Fri.
post #15 of 17
Where the hell is my post? I know I did one last night betwixt beer 7 & 8 (or was that 9) re: the overanalysis of skis in certain snow conditions. Dp questioned the turning capabilities of a ski in powder but got a science lesson , the short and long of it all is "shorter turns easier and in most cases quicker" regardless of the snow your on.
Powder will let a ski get away with alot of mistakes , more platform does float better regardless of side cut but a narrow waisted ski will often require more speed than a wider one. The simple fact is most skiers worth their salt could actually ski powder on skis 20 - 30 cm longer than they are but would sacrifice some quickness in other areas . This results in the mindset of having a quiver with one ski , which for some may work but for a dedicated gear whore like myself is a kick in the face and could actually take business out of my beloved ski reps pocket!

Damn ,I almost did the same thing as some others..........re. longer skis being a disadvantage?
Only if you let them to be , ski whatever your comfortable on and don't let anybody talk you into something you can't handle , it happens all the time.
post #16 of 17
Quote:
Originally posted by PhysicsMan:
IMHO, the extra projected tip and tail area will provide more resistance. Tom / PM
If I understand the model, in the case where the ski is on edge, the yaw is no longer just a torque.

Since there is no pure motion in the ski's lateral plane, the pitch plane becomes the frame of reference. The sideways force we were calling yaw with ski flat is gone.

Or have I misunderstood something?
post #17 of 17
Nope, BigE you have it, which is why I would look at pitch-plane moment for an un-edged ski first.

The "pseudo bump" case is actually also relevant to yaw if there is a lateral gradient (impact on the side of the bump). I am looking just at the snow-density/surface area effect, because, in the most extreme case I can think of (the Phantom Ski above) the lever arm extension due to side cut is only 5mm. (Sqrt((160cm/2)^2+(180mm/2)^2)~80.5cm)

Right now, I'm just sorting out the flaws in the surface-area model.

For example, the position of the waist is irrelevant only for skis with identical tip and tail widths. Could we determine waist position from turn radius as a 5th variable? Probably not, so it looks like we'll have to work with idealised, center-mounted skis instead of real planks.
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