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Calling Physicsman! - Spatula (Moved to Gear Forum)

post #1 of 7
Thread Starter 
Hey Physicsman - or any other contributers.

Have you seen that prototype McKonkey Ski in the Powder photo annual. I don't have it in front of me so forgive any inaccuaracies, but it has a convex shape and a negative camber. Tail narrower than tip.

Am I wrong or is this the snow ski of death. It seems that it may -sort of- work at high speeds in deep powder in open bowls -as long as you did not have to turn. Would'nt the convex shape give you a negative turning radius? The equivalent of banking a race track to the outside of the curve - centrifugal force eliminated. The negative camber giving you less edge - combined with convex shape - it seems like it would be hard to keep this in control. Your legs would have to work against rather than with the dynamics of the ski. However, it may set all sorts of pond skimming records during those end of season contests.

post #2 of 7
I've been trying to wrap my mind around how that ski would perform for a couple of weeks now as well.

McConkeys observations regarding the shape of surfboards might well have some merit, I'm just not sure. He apparently has found them fun himself, and Shane does have a strong technical background (racing) and of course is an excellent all-terrain skier as well, so I would expect there must be something to them.

However I do wonder whether they would only be good in very soft snow conditions? I think its an unusual enough concept that we can theorize all we want and succeed in nothing but wasting our fingers typing. We'll simply have to try them out and see!

<FONT COLOR="#800080" SIZE="1">[ January 09, 2002 07:05 AM: Message edited 1 time, by Todd Murchison ]</font>
post #3 of 7
When I first heard about this concept, I was tempted to get one of those ratcheting webbing hold-downs commonly used to secure loads to flatbed trucks, and use it to pull up the tip and tail of a pair of my old straight skis just to see how it would feel. It would be like a wildly exaggerated version of the "powder" settings on Marker and Tyrolia adjustable bindings.

After a few minutes I came to my senses and realized that I wasn't going to do this because (a) I couldn't stand the razzing and (b) was pretty sure I knew how they would act, at least on hardpack:

Even without the reversed sidecut, just putting negative camber into a ski will make them impossible to use to carve on firm surfaces. The ski will skid and pivot extremely well (even when on edge), but it will likely do these things *too well*.

If Bob Barnes was having a nightmare about not being able to "ski the slow line fast", and not having perfect control of line, these would be the skis he would be on in his nightmare.

On firm surfaces, they should be very unstable at speed, expecially when flat. In that case, the ski's contact area on the snow would be miniscule, so it would be like skiing on a pair of saucers. Now, you could certainly put this pair of saucers on edge, but with such a very small length of edge in contact with the snow, it will concentrate the force on the snow in a very small area, resulting in high local pressures, probably beyond what anything but rock hard ice could support, so the upper surface will give away and you will continue to skid. If you edge far enough to create a positive critical edge angle, they will of course try to dig in, and, on a positive note, I suspect they will make the transition from skidding to digging in more smoothly than if a longer length of edge was in contact with the snow, but unfortunately, since there is no possibility of carving and a great propensity to pivot underfoot in such a design, such smoothness is for naught.

Now, everything changes in soft snow conditions.

If the combination of the skier's weight, ski flex, negative initial camber, and compactability of the snow are such that a significant length of the ski is below the snow surface, the active load bearing area increases to a reasonable value, and I would expect that while they would be easier to turn than conventional designs. If the longitudinal compliance was not overly soft, they wouldn't be unduely unstable. Basically, they would start with a negative camber and just go to a somewhat larger degree of negative camber when loaded. The negative sidecut would, of course, increase their tracking stability.

That being said, the transition from pow back to packed would likely be a doozie!

My best guess is that this design could produce a decent pure powder ski, but of extremely limited versatility - the opposite of the goal of mainstream designs like the G4 and 10ex. However, as Todd said, they depart from the norm sufficiently so that the only way to know for sure if the theoretical expectations are true is to take a pair out for a test drive.

There's my $0.02 of speculation for the day!

Tom / PM
post #4 of 7
It's called a Spatula, and according to the few people who have ridden one of the 3 existing sets, in powder of course, they work quite well.
post #5 of 7
Ahh - Spatula. Thanks. I was told the name when it was first described to me, but my aging brain couldn't remember it. ... As I was writing the above message, I kept thinking spoon, fork, etc.

Tom / PM

<FONT COLOR="#800080" SIZE="1">[ January 09, 2002 06:49 PM: Message edited 1 time, by PhysicsMan ]</font>
post #6 of 7
Physics Man

I guess I disagree with your comments that having negative camber on a ski will make them impossible to carve on firm surfaces. I say that because when you do carve tight radius turns at higher speeds the ski is very much bent in reverse camber. Consequently, a ski that start out with negative camber just gets bent a little more as you get it into the turn. If you start the turn with good forward pressure, you can certainly get a reversed cambered ski carving. I have a pair of old tired Atomic 9.16 that as the day goes on ends up with quite a negative camber. At that time, those skis are very loose going straight, but still carves well.

One question about the statement of the shape of the ski in question. It was stated that the tip was wider than the tail. That is typical of the majority of the skis. Am I to understand that this ski also was wider at the middle than the tail? If so, sounds very interesting.
post #7 of 7
Excellent observations, Norefjell.

To answer your concern, lets first consider what happens in the case of a ski with a normal hourglass shape (positive sidecut) as you go from positive (conventional) to negative (unconventional) initial camber (aka, "rocker").

With a conventional ski (ie positive sidecut, positive camber) on hardback, when you edge it and push down on it with enough force so that its waist is touching the surface, it will definitely go into negative camber, just as you noted. Typically, there might be an initial 1 cm of initial positive camber that must be overcome, and then, maybe another 1 cm of flex in the negative camber direction till the waist bottoms out (on true hardpack) at say a 45 deg edging angle.

As long as you leave the sidecut and edging angle invariant, you can adjust the flex of such a ski from ultra-floppy to ultra-stiff and as long as there is enough weight on it, the amount of negative camber won't change by one iota because it is "bottomed-out".

For any ski that I know about, the amount of downward force required to bottom out the waist (say at a 45 deg edge angle) is *much* less than the weight of a skier, let alone the combination of weight plus centrifugal force, so you essentially never have to worry about *not* bottoming-out even the stiffest of skis (ie, as long as they are not suspended by tips and tails between two moguls - grin).

Now, if you start building in an amount of initial negative camber is greater than the amount of negative camber you would generate by a certain edge angle and pressing the center down, then at that edge angle (again, no matter how hard you press down on the center), the tips and tails can never be in contact with the surface.

At zero edge angle (ie, a flat ski), the tips and tails will be off the presumed planar hard surface by the largest amount. Depending on the sidecut and initial negative camber, they may not contact the surface until extremely large edge angles, or they may simply never be able to contact the surface.

If you now also introduce negative sidecut into the negative camber discussion, I think that it should now be pretty obvious that at ANY edge angle the tips and tails will not be able to contact the hard surface.

Now, as you go from rock hard ice to soft snow, more and more of the edge will be able to penetrate (ie, contact) the snow, and the ski will make the transition from skittery to stable (as I noted in my first post in this thread).

Hope this helped.

Tom / PM

PS - To help visualize what is going on in this interaction between geometry (sidecut & camber), flex, and downward force, I have found that playing around with a real pair of skiis in your hands on a hardwood floor (brakes locked up) is extremely useful. Don't let your wife see you do this unless the edges are really dull - grin.

PS#2 - I'm curious, which Scandanavian language does your alias come from? Icelandic? Does it mean North Mountain?
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