Originally Posted by ydnar
A quick question going back to the original topic. Which is more important in determining the amount of reverse camber of a ski the amount of sidecut paired with the degree of tipping or the pressure applied to the ski?
For the answer to this question appropriate to hard snow conditions, grab a modern ski with deep sidecut and lay it down on a hardwood floor, and tip it up on edge by some known, fixed ammt (say 45 deg). Get your eyes down at floor level, and look at the gap between the ski edge and the floor. At the tip and tail, the edge will be touching the floor, but in the center of the ski, the edge will be above the floor by some distance.
Next, press down on the center of the ski with say 5 or 10 lbs of force. Now, the edge in the center of the ski will be touching the floor, and if you look carefully down the length of the ski from an end (a helper is useful here), you will see that the ski is actually in reverse camber. Now, put all your weight on the center of the ski (while maintaining the 45 deg angle). The ammt of reverse camber didn't increase one iota, did it? The center is "bottomed out" and can't go anywhere. Now, imagine doing the same experiment on a frictionless, extremely hard and flat icy surface with the ski possibly moving, either sideways in a skid, or forward. Even in these conditions, more downward force at the center of the ski will not make the ski go into more reverse camber.
On the other hand, increase the edging angle up to (say) 80 degrees, and notice that the degree of reverse camber when "bottomed out" increases by a large factor, but that the downward force needed to "bottom them out" still is quite modest (ie, much less than half of your weight for most skis).
Finally, dig out an old pair of skis with little sidecut and re-do the entire procedure described above. You will see that the ammt of reverse camber for these skis at the same edging angle is less, and exactly as in the case of the more deeply sidecut skis, dramatically increases with edging angle, and can't be increased by adding more downward force once they have a minimal force on them.
The above hard snow scenario is one of the limiting cases for ski behavior considered by ski designers, and pretty well describes how the camber of your skis will change in real-world hard snow conditions.
For the opposite case of extremely soft snow, it's more difficult to construct an equally easy and obvious demonstration of the flex behavior of a ski. When your wife is looking the other way, one can put the skis on something like a deep soft mattress, but measuring deflections, keeping the downward force and edge angles constant, etc. all becomes more difficult. In any case, in the soft snow limit, it should obvious that increasing the pressure will continuously increase the ammt of reverse camber - the center of the ski never "bottoms out". Increasing the width of the tip and tail (while holding the pressure and longitudinal stiffness constant) will cause the tip and tail to float higher and increase the degree of reverse camber, just like in the hard snow case. OTOH, increasing the edge angle (while holding constant the flex, sidecut and applied downward force) will have much less of an effect on the ammt of reverse camber. (Don't forget that in real powder turns, centrifugal force is generated and the downward force that the skier applies to the ski increases).
Originally Posted by daslider
...I think it is the other way; it starts to curve because it is falling over as it slows and the gyroscopic effect lessens, so the tipping does cause the turn...
It's a chicken-and-egg thing. As in many things in physics, you can look at it either way - they happen simultaneously.
Tom / PM