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Photo Request - Page 2

post #31 of 47
I think Micheal A hit the point on the head that I was thinking of all night after reading this thread.

on a steeper powder slope I think it "looks" like most are weighted back but... most of us simply use the free braking of deeper snow on steeper runs to ski closer to the fall line and not have to cross back across the fall line as much to control speed.

I know I sometime weight ever so slightly to the back or simply point the skis up, at one specific point in a turn in deeper steeper snow simply to angle the skis up and get some free braking in order to stay on the fall line and keep the skis pointed downhill. I do this just at the end of a turn and just before I begin the motions to transition to another, all as part of bringing my legs back up underneath me.

the physics part lol yeah right, don't look for any help from me but it would be interesting to see a break down of that.

Great thread very interesting.

post #32 of 47
I have not read the last page of posts so someone may have already said what I am about to say...

to remain in dynamic balance in powder or any snow for that matter you must anticipate accelleration and decelleration.

Heavy wet deep snow in the Sierras vs. light dry powder in Utah will elicit different pictures of skier planes. It is all relative to the amount of resistance felt from the snow and the pitch which dictates the amount of accelleration and decelleration. Thus the skier, in order to stay in dynamic balance will have to make the appropriate adjustments.

Case in point: One deep untracked snow day at Mammoth Mountain in California I stood atop Lincoln Mountain looking down one of the "Avalanche Chutes" (pretty steep) and I thought to myself "I do not have to move aft on this run to ski it" so I took off straight down the fall line and allowed my skis to remain level. I traveled about a ski length or so and came to a screeching halt facing straight down Avi One in snow up to my waist. (It took me ten minutes to dig my skis out of the snow) Trust me, the higher the moisture content, the more you will need to adjust your balance aft at the turn completion. As you rise up out of the snow the feet need to be pulled back the anticipate the accelleration into the fall line then once again moved ahead to anticipate the deccelleration.

Now, in light dry Utah powder this movement is minimized because the resistance from the snow is much less. Those lucky bastards.
post #33 of 47
Marmot mb: Yep, I too notice a tendency for the front of a ski to ‘lift’ a bit when my speed in powder has gotten higher. Especially so for skis with a large/wide tip. I didn’t perceive it to be any effort on my part to weight backward or any attempt to ‘keep the tips up’. Seems to be more a function of the snow deflecting off the upturned tip and the fact that most skis are wider in the front than in the tail (flotation difference). Is that what you notice, or do you actually make an effort to reorient the skis?

Since the PNW rarely ever gets powder, most of the powder we do get is a few inches on top of a more compacted layer of firm or semi-firm snow. In such conditions I ‘ski the firm snow’ underneath. True, the skis don’t quite get there, but that layer relieves me of concern for tips going submariner.

I’m poking around my old physics books trying to wrap a few neurons around hillside accelerations. I already did the standard acceleration per slope-angle stuff. That was easy. The hard part (for me) is figuring out the impact of forward lean by the upper body mass.

Since the upper mass is at an angle forward (in relation to BoS) due to slope angle and rigid ski boot back support, how is the resulting torque applied? How much does the upper mass accelerate forward parallel to the slope due to gravity’s effect on the misaligned CM & BoS? How much does this tipping torque initially push the feet backward, resisting the acceleration of the feet? As the skier tips forward (using muscular effort) just ahead of anticipated acceleration, the feet are pushed back; how much does this impact a short radius turn? How much more for a Tall skier?

OK, I know this is infinitesimal detail for the Skiing Public to absorb but I’m in the instructor boat and like to teach honestly and accurately. Better to have a topic nailed than just to teach by parroting others. One gets boxed into corners that way by little Brainiacs in the thinker crowd. They keep asking ‘Why?’ Sorta like me.

Maybe exploration of the whole issue would be best in another thread. Could go fishing for input by PhysicsMan too.

Bud heishman: Here in the PNW, our experience mirrors your own with thick powder. It tends to compress into an unmoving wall in front of us. Must be a Cascade Range thing. Keeping tips up here is not necessarily a balance issue - it’s a survival issue to keep from doing a Javelins-at-the-Marmot move like the one you accomplished.

You also refer to those ‘lucky bastards’ in Utah for their light, dry powder. I submit that such things are relative. Anyone with snow right now qualifies as a Lukyba to me right now as all our ski areas are now closed. GRrrrrr….

post #34 of 47
I like Rick's answer, but if the "gravity line" answer is the full answer, then consistency of the snow would not have an impact. Rogan talked about the path of the skis through the snow as part of the equation. In heavier snow, the skis take a higher path relative to the slope surface than they would in lighter snow. Is the angle of the skis relative to the angle of the slope different as well? How does porpoising factor into this?
post #35 of 47
Originally Posted by Bob Peters
Leelau - great pics.

any1 know wa atomics that guy is on?? sorry for the off topic post
post #36 of 47
Thread Starter 
Originally Posted by mcl116
any1 know wa atomics that guy is on?? sorry for the off topic post

(he's sponsored)

post #37 of 47
Originally Posted by Bob Peters
I can't quite figure out how to word this properly, but I'm confused about the whole issue of CM relative to the feet. The analysis of the photos posted by "the bag" brought this question to mind.

Could someone post a photo or two showing a powder or crud skier with the CM in the right place? I've looked through ski magazines and at all kinds of photos on the web, and practically every powder/crud shot I find would seem to show the skier with their weight further back than would be optimal.

Using the suggestion of therusty from that thread;

"To get a sense of where your hips are versus where they should be, look at pics 5 and 6. Draw a straight line perpendicular to the slope pitch starting from the middle of the feet. See how much of your body is behind the line?"

Given that as a yardstick, I'm very hard pressed to find photos of powder/crud skiers who have their hips where they should be. Even taking into account the effective "flattening" of the slope caused by deep snow forming a platform for the skis, most everyone I see in the photos - including practically every ski-flick rock star - has got their CM behind that perpendicular line.

Can someone post a couple of photos of what it *should* look like?



Oh - and racer shots don't count.
Next time you're out skiing powder in the back country have someone take your picture as you fly by. That's the only picture you'll need to see what it "should" look like.

And thanks again for the tour.
post #38 of 47

Geek Alert

Maybe it's something like this. There are two forces acting on the ski. The force perpendicular to the surface (physicists call this the normal force); see the green line beow. Second, there is the force of friction generated by the ski sliding; see the blue arrow below. The total force on the ski is vector sum of these two forces. In the picture, it's the red line. Now in powder/crud, the friction is greater, therefore the blue line is longer, therefor the red line angles up the hill more. The skier has to be in balance with the red line. Therefore, they look like they are back when in effect they are not. Or maybe I should stop drawing pictures and go skiing.

post #39 of 47
Don't forget that in powder, the "slope" is actually defined by the pile of snow you are standing on and that will be flatter than the slope that you are actually descending.

This also adds to the appearance of being "back" since what the skier stands on is more shallow than what the observer sees.

post #40 of 47
Learn2Turn, Wow! Few words, simple picture, direct to point. I like it. Any chance you have numbers on Coefficient of Friction for powder? I've not found any guesses anywhere on the 'Net. I'm messing with some extended ideas on it right now.

BigE, even is the skis are at an angle different from the hidden slope would it matter? So long as the center of the upper body mass and center of feet line up on his Red arrow, isn't all going well?

In the extreme one might envision a person sliding down a dry sandy slope in ski boots (which is what we have here in WA right now). I think the boot-bottom angle could be horizontal to the horizon on any slope at any acceleration rate and the person would still be in balance if their CM & boots are on his red line.

I'd agree perspective is a relevant, though misleading element of movement analysis.


[edit on spelling]
post #41 of 47
Originally Posted by michaelA

BigE, even is the skis are at an angle different from the hidden slope would it matter? So long as the center of the upper body mass and center of feet line up on his Red arrow, isn't all going well?

The drawing shows the ski riding on top of snow...

If this is powder, there would be another line, say a dotted line, which is the actual top of the snowfield the skier has sunk into. This line would be steeper. The tips of the skis would be just poking out.

So, if you now look at where the skier is standing in relation to the top of the snowfield, they look like they are way in the backseat!

post #42 of 47
Where's Physicsman when you need him? We need him now for this one.

This is a dynamic situation, not a static one. The skiier's CM should be over or ahead of the green line. When you're moving friction is pulling back on your feet, however, you need to be forward to pressure the fronts of the skiis to effectively use them.

Learntoturn, by your interpretation, if the friction increases, you need to get back more. Yet if I want to start moving from a start when friction is the greatest, I need to get forward, not back. Doesn't the skiier have to apply the opposite forces to overcome the forces the snow places on the skis? The only place that the forward force to overcome friction comes from is gravity. (Unless you have a rocket on the rear of your skiis and I don't think we want to go there. ) Anyway, if you are more forward, the vector componet parallel to the snow of the force applied to you by gravity goes up. If it is more than the friction force you will accellerate. (Boy are we starting to Geek out on this one!!! : )

(Moving friction between two surfaces is much less than static friction.)
post #43 of 47
Physicsman is busy teaching (both jobs). I'll see if I can get his attention.
post #44 of 47
Friction is not the issue, the change of speed is. If speed is constant the CM to foot balance line will be the gravity line supplimented with what adjustment is needed to compensate for wind.

As long as speed is constant the body doesn't care what the amount of friction is under the ski. On the same slope with varying snow conditions and ski base prep the speed a straight run can produce will vary greatly, but as long as the resultant speed remains constant the only difference in the balance line will be the amount of compensation assigned to account for the higher/lower wind speeds impacting the skier.

Once speed begins to increase or decrease then compensation for the influence of momentum has to be added to the mix. As you're speeding up you need to move the CM forward, as you slow down you move it back.
post #45 of 47
Originally Posted by T-Square
(Moving friction between two surfaces is much less than static friction.)
Sure, but once you are skiing, it's about variable conditions. Do you really think that being perpendicular to the slope in cut-up windblown is right? Get ready to go over the handle bars!

Everyone has experienced skiing from slippery into sticky snow. Ever get that lurched forwards feeling? What happens next? If you are lucky, you can pull your feet back under you. But what if you already have pushed the feet forwards in anticipation?

By pushing the feet forwards against friction, the skier remains in a neutral position, with respect to the slope AND the friction of the snow.

What learn2turns diagram does show is that the frictional forces are not visible to the eye in the way that the slope is visible -- these forces are felt.

You can't ski all types of snow holding the CM at the same point above the skiis and remain in balance -- the feet must be constantly moving fore and aft to maintain balance. To remain balanced over the feet in sticky snow, the feet must be pushed forwards against the frictional forces -- they simply cannot be ignored.

It's called adjusting for variable conditions: Feet closer together, further forwards, and more in the fall-line.

Hope that clears things up!
post #46 of 47
Just skimmed this thread again and one thing I have not seen is any discussion of ski shape. I wonder if part of the stance/balance issue is merely an artifact of typical ski design.

First a brief digression into what got me thinking about this. In a word, Metron. Don't run screaming "noooo, not Metrons again..." just yet... First time I had them in soft snow, I was struck by how readily the tips surfaced - with a 220 lb skier on 162cm skis! A few weeks later my family was in JH. Bob showed us some powder runs easily accessed by traversing Amphiteater Bowl just below the Expert Chutes & out below the Headwall. Not super steep, but steep enough. Such good snow for so little work, we ended up hitting these a bunch every day for the remainder of the trip. Because of snow, wind, people bailing early on the traverse, etc. - most of our runs were essentially fresh tracks in knee to waist deep powder. I skied my B5s every day but one. On that day I skied my 185cm PRs. Same runs under essentially identical snow conditions. Subjectively, I felt more comfortable in deep snow on the PRs. Not that the B5s didn't do OK, just that I felt the PRs were a bit more stable & mellow. The interesting part is that my family was unanimous in stating that my form and technique were massively better on the B5s and that I tended to ride the PRs in the back seat. (Kinda crushed me because I thought I was pretty studly on the PRs ) So...

This got me thinking... I don't know if I'm heading off in the weeds or not, but one thing that struck me was that the B5s have a massive shovel and are incredibly asymmetrical front to back. The combination of surface area difference and where it is placed (levered way out front) has to make a difference in how the ski sinks in soft snow. Seems to me that you could get much further out over the front of a ski like the B5 without diving the tips (especialy at slower speeds). I mean, is there anyone here who hasn't pitchpoled in soft snow? So, my questions are: regardless of claims and theories about being out in front as much as possible, is it the case that everyone who gets decent at powder tends to hold a bit back (out of need) from where they'd be if their tips wouldn't dive? That their CM ends up, on average, a bit further back than the claimed "theoretical ideal" because they have learned to compromise between ideal balance + turn initiation/management vs. minimizing the risk of their tips diving? (note - by no means am I implying bad technique or skiing).Does a massive shovel and front-to-back asymmetry begin to "change the game" with respect to balance in powder? Is this not just a 2D problem, but a dynamic 3D one? If so, what would happen if someone did a real fatty with these kinds of surface area characteristics? Or am I just all wet?
post #47 of 47
Agreed, the visible surface of the snow lends the appearance of being back. I also agree the skier is ‘standing on’ a hidden platform of moving & compressing snow.

Still, to me it’s the change in momentum or acceleration to a new direction that determines the relationship between CM & BoS even if that new direction is in the *opposite* direction we are currently moving. To me, Deceleration is a potentially deceptive term because it can mean either accelerating-less-than-before or outright braking.

For clarity I’ d need to expand on Fastman’s statement snippet “…as you slow down you move it back…” (referring to CM)

As we come out of a turn we're still going downhill and therefore still accelerating - just not as much as a moment before. Since acceleration is not quite as much, we adjust our CM slightly back from where it currently resides but not to a position behind the CM. If we actually start Braking in some form then we need the CM to be behind the BoS (feet) to remain in balance.

Since we’re talking fore/aft balance in regular skiing, it seems like we’d never need our CM to be behind our feet unless we we're doing a Braking Wedge or going over a Very High Friction surface. - Which leads to BigE’s contention that sticky snow seems to launch us forward. Certainly does for me.

Sudden high friction under the skis can add sufficient braking to overcome the acceleration produced by gravity. If there is more braking than accelerating, the BoS needs to be ahead of the CM. Braking-Wedge skiers are actually in fore/aft balance for what they are doing when they are vertical or slightly back as they dig in.

Fortunately most sources of unexpected braking that we experience (like hitting sticky snow patches, rocks, crud, et al) are momentary. Our current rate of acceleration (even if Zero) is interrupted only briefly, then it resumes. Sticky and choppy snow provide repetitive interruptions to the ski’s forward motion but our upper body continues forward at essentially the same rate as before.

BigE mentions “…pushing the feet forward in anticipation…” of such occurrences. This is exactly what I try to do also if I see it coming. Ungracefully, I briefly thrust my feet forward then let the momentary braking push my feet back where they belong. Certainly not an exact science but I remain intact.

If conditions on the day include unending ski momentum interruptions, I choose to ski with a *slightly* more rigid body and legs rather than very much further forward. Maybe a tad more forward.

A flexible body encountering a sudden increase in resistance at the BoS will deform - our BoS will be shifted back while our CM continues ahead blithely as before delivering that lurching-forward feeling. A semi-rigid body will endure these conditions better because sudden resistances at our feet will produce torque on us rather than deformation. And since our true Base-of-Support is actually the entire ski, not just our feet, we can endure the considerable but momentary forward torque without experiencing the handlebar/ragdoll scenario.

My feet are constantly moving fore and aft on every turn regardless of conditions. Yes, my feet do migrate fore and aft depending on momentarily increased resistances, but I’m certain the *Average* angle my CM stays ahead of my BoS is consistent with that predicted by Learn2Turn’s Red line.

Taking this back to the question of fore/aft balance in powder I suggest concepts like proposing and allowing the tips to raise up will create similar momentary increases in friction where the CM/BoS angle must be adjusted. In powder, these moments of higher resistance last much longer affecting the average CM/BoS angle to a visible degree.

Thoughts by spindrift to fold ski-design into the equation do seem relevant to a degree. Ski flotation in powder will influence skier technique but will not change the fore/aft balance relationship WRT acceleration. PhysicsMan brought some light to powder skiing and fluid dynamics in another thread a while back.

Anyone know where that thread is?

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